Here is a list of all the postings Cabeng has made in our forums. Click on a thread name to jump to the thread.
|Thread: Gas Blowtorches|
Not relevant to the thread, but...
Paul, my eyes popped out of my head when I saw your photograph!
Many moons ago I was in the Nelson area of NZ, we passed an antiques place on some back country road, my wife couldn't resist, so we went in. Where I found this item:
Sievert Type 28. I sent my photograph to Sievert, who said that they had no record of the device, they didn't have one in their collection, and that it must be quite rare. And now there's another one, right in the middle of your picture!
A complete aside - my wife was talking to the owners wife, who was also English - turned out they both came from Upton on the Wirral, and both had attended the same convent school.
|Thread: Super 7 Headstock Set Up|
You may already be aware of this Alan, my apologies if you do, but over-oiling the angular contact bearings on either S7 or big bore lathes can cause them to make 'orrible noises as the races thrash around in the excess oil. Dificult to describe a noise, what you describe might fit the bill. But then again, it might not!
It's not a sealed system so they will drain eventually. Running at low speed will help to speed up the draining process.
|Thread: Parting Off MEW225|
Yes, rake angle does affect matters under discussion, but it’s not simple. So I’m deliberately staying away from rake angle at the moment, as I lent my copy of Sandvik’s ‘Modern Metal Cutting…’ out some while ago and haven’t yet got it back. Should be here by Saturday, when I’ll be able to consult the oracle about rake effects.
Chris: GHT recommends (p. 60 of Model Engineer’s Workshop Manual) “Use a top-rake less than that normally used on a turning tool for the same material”. The Iscar tip has, as far as I can measure it with a loupe with a measuring graticule,, 6 degrees of top rake, so it is relatively shallow.
Backlash – not sure I go along with that. I’ve just done a test on the Connoisseur: skimmed a 22mm bar round, set a 2.5mm depth of cut at 0.1mm/rev., and then backed off the feedscrew so the cross slide was ‘floating’ half way into the backlash. Then noted the reading on the DRO and set it cutting. A decent cut, although not as much as the lathe can handle, but the DRO never budged, even though it reads to 0.00025”. Can’t do this test with a parting tool, of course, but it should serve to illustrate my reticence re backlash, at least on its own.
Oh, the cross slide weighs 3.1 kg, equipped rear tool post (Dickson block + toolholder & tool) is 2.15 kg, equipped top slide is 3.4 kg, total 8.65 kg to be shifted by the tool.
You’ve brought out an important point re lathe condition – I only started off to consider the arguments re ‘up and out’ versus ‘down and in’, so other factors are, by definition, excluded from my postings! But lathe condition is of course very important, such as spindle & bearing condition and adjustment, chuck condition, are the jaws bell-mouthed, is the work held only by the outer parts of the jaws (e.g. 25mm bar in a 4” chuck that won’t pass it through), how much overhang, how stiff is the bar being parted, is the tool width appropriate to the diameter of the bar, etc., etc.. Any or even worse, all of these could completely override a perfect tooling set up. So yes, the envelope can be tighter than for normal turning. But probably the biggest factor is the chip clearance problem, I reckon, which is really the big difference between turning and parting - the forces are directly comparable!
Neil: all the forces disappear if you stop feeding, not just Fa! It’s not as straightforward as saying Fa is just the reaction force, or that Ft should be not be perpendicular. For example, there’s also a frictional force that pushes outwards, caused by friction between the chip and the tool rake face, but I’ll not go further until the Gospel According to St. Sandvik is back in my hands.
Whacking the tool in too fast? Yes, that would be a recipe for disaster, but would a model engineer with parting problems wind the handle fast enough? I found 0.008”/rev was ok, even though the lathe stalled because of the power requirement. But it didn’t dig in, it just ground to a halt.
Ah, now, feeding too fast, together with Chris’s comment re machine condition - the Connoisseur has a big spindle, big bearings, and carries a good chuck, in as-new condition. For the parting tests the bar was through into the spindle bore, so held over the full length of the jaws – if that had not been the case, the work might have tried to climb atop the tool, and that would have given the impression of a dig-in.
Will: jump in as much as you like, but given what we’re talking about here, wouldn’t chipping in be a more apt expression?!
Blowlamp: good point. I’ve tried a parting blade (not the integral-shank tool shown in one of the photographs) to reduce the diameter of a bar by pointing it at, and feeding towards, the chuck – worked well, no problem.
How do I get past this mental block about the direction of forces on the tool tip? OK, let’s see if this works: this link will take you to Sandvik’s home page.
Click on the ‘Knowledge’ tab, at the bottom of the next screen you’ll see a globe, scroll down to ‘register for our free e-learning...’ and click on that – you won’t need to register. Next page click ‘Introduction’ in the left panel, click the yellow ‘Training Manual’ panel to down load the manual.
When you go into the downloaded zip file, open MCT.pdf and scroll through to page A57, where you will find a diagram of forces acting on the tip of a boring tool (for some reason they don’t show the forces on an external tool anywhere that I can find, so you’ll have to imagine what it would be like). Note how the forces are directed – tangential force Ft perpendicularly down, feed force Fa, opposite way to the feed direction, and radial force Fr, AWAY from the work surface, it is not directed into the work surface.
Now think parting tool on the outside – the forces will be directed in the same directions relative to the tip of the tool, so we end up with:
Fr isn’t present now, it’s effectively part of Fa. F is the resultant force on the tool tip, down and out, exactly as I showed it.
The hardness of the material, in conjunction with the feed rate and depth of cut, determine the relative magnitudes of Fa and Ft, and therefore the magnitude and direction of F…oh look, another sketch…
HEALTH WARNING: much simplified re the forces at the tool tip, intended only to be illustrative, and to get your thinking into a different mode!
Sketches show a single tooth hacksaw blade sitting on a piece of mild steel, A. At B, you push the blade forward only, it does not cut. Similarly at C, you push downwards only – again, it does not cut.
Since you’re pushing, you’re exerting a force, so the arrows also show the direction of the force, with the length of the arrows representing the magnitude of the force that your applying. More force, longer arrow.
At D you’re pushing along and in, it cuts. The resultant of the along and down forces is the angled arrow pointing down and to the right. At E, the forces are moved to the cutting edge, as that’s where they are being applied.
As my good friend Isaac pointed out in 1687, every action has an equal and opposite reaction, so as the tip pushes down on the steel, the steel pushes back on the tip with the same magnitude and direction, F.
G – I is a softer material (e.g. aluminium), less down force needed, so the down arrow is shorter, the resultant is shorter and at a reduced angle, as is the reaction force on the tip.
J – L are for a harder material (stainless steel?), more down force needed to push the blade into the cut, reaction force on the tip is larger, but this time at an increased angle to the surface being cut.
G – I could also represent steel with a shallower depth of cut, J – L steel with an increased depth of cut.
Note that Sandvik make no comment with regard to any possibility of arriving at a force that would drag the tool into the work, surely they would have done so had it had the potential to cause disaster? The Training Manual mentions with all sorts of machining problems, so why ignore that one, if it exists? Or perhaps it’s because they never, in their wildest dreams, thought anyone would try to cut brass with a tip designed for aluminium, as Chris did? (Sorry Chris, I couldn’t resist that one!)
Back to the boring tool – can you think of anything that would better demonstrate that there isn’t any force trying to pull the tool in to the work? If there were such a force, wouldn’t the boring bar (probably the most flexible tool in our armoury) get pulled in? It would indeed get pulled in, and viciously. And if somehow it were to be pulled in in a ‘non-vicious’ manner, it would bore oversize. But that doesn’t happen, if anything, it bores undersize - it gets pushed OUT, so that we have to ‘work the spring out of the tool’ to get the bore to size. So the boring tool also demonstrates that the force is ‘out’ and not ‘in’.
Oh yes, another point about the boring tool system – the downward force Ft tends to INCREASE the depth of cut as it pushes the tool down, which would be a perfect situation for dig in to occur, if the tool were not being pushed out of the work. And not only is it pushed out, but as you can see from the diagrams above, the deeper cut results in more force pushing the tool out. Hence, no dig in.
How does that float yer boat?
Thank you for your kind comments MichaelG - the folding money will be in tomorrow's post!
Re the sprung tool - I have a problem with the descriptions of how this works (I do not deny that it does work!), and am surprised to see it in the link that MichaelG posted. My difficulty can be put simply:
The cutting force applied to a rigid tool is stated as pulling the tool into the work, whilst that for a sprung tool is stated as pushing it out. For this to be the case, the direction of the cutting force must change - but how does the workpiece know that a sprung tool is in use, and what action does it take to change the direction of the cutting force accordingly? Spooky action at not very much distance! (With apologies to Mr. Einstein.)
A speculation on my part, without any evidence at all and applicable to chatter, not dig-in - chatter is vibration, vibration of the tool and its support structure which will occur at a certain frequency, when what happens at the tip resonates with what happens in the support structure. Putting some spring in the system between the tip and the support inserts a different resonant frequency that avoids triggering the support resonance. It could be acting as a mechanical filter, effectively. I stress - pure speculation... Discuss!
Neil: I admit to an inherent distaste for sprung tools, as they would introduce variables that would be difficult to control, and inherently introduce non-linearities and dimensional inaccuracy. Dimensional accuracy obviously not important for parting, but as I described re the potential problems during normal turning caused by gib flexibility (a.k.a. a sprung gib!), improving rigidity (if possible) gives improved results all round. And since improving tool support rigidity improves parting as well - why bother with spring tools?
Will: would I be right in thinking that your now cutting faster also includes feeding faster? And that chatter (if any) only occurrs on first contact with the work?
Chris: I've never experienced what you describe, other than once when I had left the gearbox in screwcutting mode and tried to turn at near 0.040"/rev feed. It didn't like that.
By which I mean that I don’t know how many parts it had done at the time, but I knew immediately it had happened because the shape and form of the chip changed. Instead of short curls visible under the bed the chip form changed to the spiral chips that can be seen on top of the pile in this photograph that I posted earlier:
Perhaps surprisingly, it continued to cut nicely, with no tendency to stick to the sides of the cut. As you can see, I did try several times to create a jam, but it wouldn’t, presumably because running at 1500 rpm meant that it was above the ‘sweet spot’ temperature range for steel against steel. Or it could have been below it, I don’t really know, but it flatly refused to jam on the sides of the cut. I guess a more severe BUE, right across the cutting face, would have caused more trouble.
So take note of the chip form during parting, and if it changes in any way, withdraw the tool and have a very close look at it, to see if it has formed a BUE. If it has, change the tip, or you could be courting disaster, you might not be as favoured by the Gods as I was.
In my experience, trying to rescue a tip with BUE by cleaning it up with a diamond file doesn’t work. You might get rid of the BUE, but there will be some material residue remaining on the tip that will initiate formation of the next BUE as soon as the tip touches the work.
Also note that a change in the chip formation can also indicate a tip that’s worn past its sell by date, or worse, broken, so check for those as well. Take Andrew Johnston’s advice, accept that it came with a limited life, that life has gone, so throw it away and fork out some beer tokens for a new one!
What about use of coolant? I can’t recommend use of coolant with carbide tips, and I don’t use it. Yes, it works, keeps the temperature down, reduces friction between the chip, tool and work piece, and inhibits welding. But with carbides it must be flood coolant or nothing for two reasons: 1) dripping or intermittent coolant can cause thermal shocks in the carbide, leading to cracking and premature failure, and 2) the temperatures involved at the tip result in a lot of fume production from a meagre coolant supply, even if continuous. I don’t know whether or not this is potentially hazardous, so I avoid it entirely, just in case, and cut dry. Which also avoids the mess.
So there we are – how to part off without parting becoming not-so-sweet sorrow:
Two photographs to end with. Rear tool post cutting 5/8” fcms at 2105 rpm, manual feed, note chip formation and clearance:
And here’s the full photograph, showing that if you get everything right, parting really does become child’s play!
He was 9 at the time, and had to stand on a perch to get high enough.
I did say when I started that this would take some time and quite a few postings – I didn’t realise how much time, and how many large postings. I really did try to keep it down, but failed miserably. That was because I felt that some of the things I wanted to put over might not have been accepted without the background information, so it was preferable to put it in now rather than get into long discussions about it later.
My apologies for that, I do hope that you found it worthwhile, and that it might be of some assistance to you.
Thank you for you interest and attention, and most of all, thank you for your perseverance!
Here we go again, you ain't got rid of me yet! Chip jamming this time.
The following comments are for inserted tip parting tools. The only thing I know about parting with HSS is that I haven’t done it for perhaps 25 years – and when I did, I was absolute rubbish at the job!
A chip jams by welding itself either a) to the sides of the cut, or b) to the tool tip.
It’s a simple matter to avoid welding to the side of the cut: as often recommended on this forum, use an inserted tip cutter with a precisely formed chip former and chip breaker built into the tip. Chip former to ‘fold’ the edges of the chip away so that they don’t come into contact with the sides of the cut, and chip breaking to curl the chip up tightly so that it doesn’t run across the top of the tool, but breaks off as small curls. Probably #1 Reason why so many model engineers have what can only be described as instant success when they buy one, even though they’ve done nothing else to upgrade their working arrangements and practices.
So that seems to have sorted that one out, doesn’t it? Unfortunately, not entirely, for two reasons:
1) The first is that you can adversely influence the effectiveness of chip forming and breaking, by using an unsuitable combination of feed rate and, to a lesser extent, cutting speed. The manufacturer will have designed the tip for a specific material’s chipping characteristics (think of steel and brass), over a particular range of feed rates – your mission, should you choose to accept it, is to find the speed and feed values that suit the tip you have, cutting your material, on your machine.
Start at a feed rate that gets you just above the gib chatter region (it’s pretty much independent of cutting speed), then work up until you get something like short, curly chips. If you don’t have power cross feed, just wind the handle faster & faster – probably faster than you ever believed you would wind! Then up the speed to improve finish on the parted surfaces – but don’t expect as good a finish as you can get during normal turning. Neither will you get the chip production perfect across the whole diameter, because the cutting speed is reducing as the cut progresses. On the tip I used, 0.004”/rev and 1500 – 2000 rpm gave best results, as low as 250 rpm was ok, but the finish was noticeably poorer. Higher speeds also give better chip clearance – the chips get flung out much faster, but you’ll soon learn to duck quickly!
Remember that different materials (including different steels) will require different speeds and feeds – one size does not fit all, so you will find different performance on different steels, unless you adjust the cutting parameters. It might sound complicated, but you’ll soon get used it.
2) Everyone is familiar with the fact that aluminium can stick to the tip of a turning tool. This is because aluminium will weld itself to HSS very easily and at quite low temperatures. Steel and HSS are also quite good at it, steel and tungsten carbide are somewhat less enamoured of each other, steel and titanium carbide (cermet), or coated tungsten carbide, have most resistance to chip welding. #2 Reason why inserted tips are so successful in amateur workshops.
But none are totally immune, all will join up to some extent if the temperature is right for them. Operate above or below the temperature range and it won’t happen, hit the sweet spot and it can. This means that you have to play your part by operating at a suitable cutting speed, because cutting speed is a major influence on the temperature.
As mentioned above, the cutting speed inevitably reduces as the cut progresses, and it is possible that somewhere part-way through the cut the temperature falls into the ‘sweet spot’ zone, and something welds to the tip. So having the ability to increase the rpm as the cut progresses can be very useful, another plus point for a VFD.
Two things happen if something welds to the tip forming what’s called a Built-Up Edge, or BUE:
Number 1 – the cutting force and tool deflection increases. The effect is measurable by comparing the tool deflection for a new tip and a used one under the same cutting conditions. I was able to do this, and can confirm the effect.
Number 2 - the form of the cutting edge is changed by the BUE. This has the undesirable effect of changing the chip forming and breaking characteristics of the tip. This photograph shows a bit of BUE on the tip that I used for the parting tests:
This after something like 80 parting operations, including stalling the lathe when pushing it at 1500 rpm and 0.008”/rev feed rate. I don’t know when it formed, but I did know exactly when it formed!
We must first answer the Chicken & Egg question – which comes first? Dig-in then jam, or jam then dig-in?
Well, since I haven’t found any reason re tool post behaviour as to why a dig-in should occur when a parting tool is cutting normally, I reject that sequence of events, and choose the jam comes first, followed by the dig-in, option. So can I suggest a mechanism for this sequence? Yes, I can.
The following sketch shows a section through a parting off operation – let’s assume 1” bar, 500 rpm, tool half-way through the cut, so tip at 0.25” from the centre line:
The chip is flowing over the rake edge, then being curled up a bit by the step to full tool height. The tip experiences the cutting force, which is pushing the tip down and out, but as we have seen, this is ok, the tool support structure can hack it. But should that chip jam by either seizing to the tool, or seizing to the sides of the cut, we enter a very different situation.
So what does the motor do? First, let’s assume there’s no belt slip to think about, i.e. we’re working on the world’s only direct-coupled geared-head S7! The gearing is set to produce 500 rpm at the work, and it’s using about 0.25 h.p. Then the chip jams – the motor tries to maintain its speed, so provides more power, with the slip angle increasing, until it gives up the ghost and drops out. For a single phase motor, that will happen at about 33% overload, so it drops out at about 1 h.p. The situation as this is just about to happen now changes to that shown here:
Note that the cutting force is still there (it hasn’t quite stalled at this moment), an additional force is now applied to the tool by the attempted rotation of the bar, driven by the motor, and this additional force is most definitely down and IN.
It also has a positive feedback characteristic, in that it tries to pull the tool further into the work. Which increases the depth of cut, increases the cutting force (it still hasn’t quite stopped rotating!), pushing the tool further down, which changes the direction of the rotation force to slightly more inwards, and off we go to a complete jam up and dig-in.
How much down & in force from the rotation, I hear you ask…. 1 h.p. at 500 rpm results in 10.5 lb.-ft. of torque, which is being resisted by the jammed chip and tool., centred at 0.375” radius (half way between the tip position and the outside of the bar), giving a force on the tool top surface of 336 lbs, on top of the 70lbs or so of cutting force for a 2mm tip at 0.002” feed/rev.
A 1 h.p. three-phase motor driven by a VFD has the potential to develop something like 2 h.p. before it drops out, so that motor could apply an additional 672 lbs to the tool.
Plus, it will all happens so fast that it’s effectively shock loading the tool, which would have an even more dramatic effect than applying that load as a static load, and gradually increasing it to its maximum value.
Feedscrew backlash then comes into play, and the tool, tool post, top slide, and cross slide jump forwards. Followed by the Big Bang, and either the motor stalls completely, or something breaks – fortunately that’s usually the tool or the chip, and not the lathe. Or if it’s belt drive, then thank Heaven for belt slippage!
So I offer the foregoing as the explanation of what happens to cause a tool to dig-in. The chip jams, the motor pushes harder, the load builds up to a very high level, everything jumps about, something gives.
Which means that the $1000 dollar question is what causes the chip to jam in the first place? Because if we can stop that happening, we can avoid triggering dig-ins
I've done the tests applying loads to the gib side of both the S7 and Connoisseur top slides. I had to use a slightly different set up, so I can't be sure that the results are strictly comparable. That said, the tool deflections reduced by about 50% when loading the gib side, so perhaps Atlas knew a thing or two that other makers didn't!
Q1) You take a ‘decent’ cut down a bar, then wind the saddle back without retracting the tool, and get a left hand screw thread formed on the work.
A1) On the ‘decent’ cut the tool tip was deflected down and out, and cut a larger diameter than you expected, on the return path the flexure had gone, and the tool tip was able to form the pretty (or should that be ‘pretty annoying’?) pattern.
Q2) Making say six parts to the same diameter, taking say 0.080” off the stock. You work down to diameter carefully, frequent micrometer checks to see where you are up to, final cut to size say 0.003” deep, spot on, set the micrometer dial to zero, or the DRO to diameter, and make the next one using those settings, but going down to size in two equal cuts of 0.020”/0/020” – and the size comes out wrong!
A2) The DRO or dial thought it knew where the tool tip was, but the top slide had flexed more with the heavier cut, and fooled you by shifting the tool tip away from where it ‘should’ have been. The solution is to always take the final cut on the subsequent parts at the same depth of cut used to bring the first part to diameter, so cut at 0.020”/0.017”/0.003” instead of 0.020”/0/020”.
Q3) Having cut the stock to near finished size without any problems, you take the final cut at two or three thou. and the d**n thing chatters and wrecks the job!
A3) The roughing cuts had the gib in ‘heavy’ mode, the light cut let it drop down into the ‘chatter’ region.
So what can be done about it? Well, if you’re using the top slide… nothing that I can think of! You’ve just got to live with the behaviour, understand it, and work around it. Or use something more rigid to support the tool system, such as the rear tool post – and by now it should be obvious why this works better: it’s a more rigid structure. It’s as simple as that. A bit if chatter is still possible, because of lifting of the cross slide, but in my experience it’s significantly less of a problem than with a top slide. And, of course, it’s lifting against a block gib, securely bolted to the cross slide
But I do think that the front position is the best option, if the flexing behaviour can be overcome, because the forces are directly down into the bed, and lifting of the cross slide is also eliminated. For that you need something like Tubal Cain’s Gibraltar tool post, or Neil’s solid support block. I’ve no experience of those specific units, but this is my version on the theme:
It works very well, and I can be 100% confident that the Gibraltar would work at least as well. You will be surprised at the cutting capability of an S7 with such a tool support system. And in addition, you will eliminate the Q1, Q2 and Q3 problems described above.
A further improvement to mine and the Gibraltar would be to extend the base in and out to cover the adjacent tee slots, and provide two bolts in each ‘wing’ of the base, thereby eliminating any weakness connected with the dovetail spigot. Just like John Radford did, heaven only knows how many years ago. Have a look on the Hemingway site, Tool Holding & Positioning/Myford Specific Items/Improved Top Slide to see what it looks like – it’s beyond me why Myford never took this up!
Oh, if you do look at that, you’ll see that Radford fitted his top slide gib with a locating dowel and slide lock, exactly as GHT did – but who had the original idea?! I have that on my S7 top slide, and it certainly makes a difference to adjustment and smoothness of the top slide, but does it solve the gib flexing problem? Well, I’ve tested it, and unfortunately the answer is a definite no, not a bit of difference, I’m afraid. Locking the gib at one point only, roughly in the middle of the gib, still leaves the end under the tool post free to move.
A solid front support does work better than the rear tool post. But despite that I still use the rear post, because it has a significant advantage: the front position is where all the ‘normal’ tools go, and their support system (4-way turret, Dickson tool post, Aloris, etc.) frequently gets twisted around, so every time a parting tool is required, it has to be clocked true. And if that’s not done, and it’s even a little bit off-square, you end up with additional unknown/unpredictable behaviour. With the rear post the parting tool can be set accurately to centre height and accurately SQUARE to the work, and then just left there, ready when needed.
An obvious difficulty of all solid front supports is that there’s no feed screw with micrometer collar, but if you have a DRO fitted, who needs a micrometer dial? Put the top slide away, and keep it for turning short tapers. And if you haven’t got a DRO… well, you can work out for yourself what needs to be done!
I hope that’s explained chatter adequately, and provided some indications of how to deal with it. In the next chapter I’ll look at my conclusions re. jam-ups and dig-ins (yes, they can still happen), how they happen, why they happen, and how to avoid them. Most of the time, but not always!
Here we go again, but no sketch this time, just verbage & one photograph!
It should now be apparent that we are dealing with a complex mechanical system, with many variables, any detail of which can adversely affect the results of parting… and I’m only dealing with tool support systems here. each variable got right, in a logical and methodical manner, whilst and avoiding false assumptions.
So first eliminate everything you can – slide adjustment, obviously, but also little details like swarf trapped under a tool post, or dings and burrs on the top slide. They prevent metal-to-metal contact over the full available area, and add more flexibility and unpredictability into the system. Make sure the tool post bolt is vertical in both planes so that the tool post sits flat, and the tool post is either a good fit on the bolt, or sleeved to fit, so that it can’t shift around on the bolt.
Then understand what’s going on when the top slide and tool post come under stress, so to that end let’s think about what might be happening with that gib strip under a range of loading conditions – let’s designate them as light, intermediate and heavy.
Light: small depth of cut and feed, low tip load, gib strip very lightly loaded, virtually undisturbed, it’s life is comfortable and all proceeds normally. Probably only applies when scraping a thou or two at low feed, with a sharp cornered tool – it wouldn’t take much of a tip radius to increase the load sufficiently to get the gib into:
Intermediate: moderate tip load, gib under some stress, operating in what we could call its ‘squashy’ mode. Any variation in tip load will disturb it one way or another from its mean position, the tip will wobble about a bit and not be stable relative to the work. An alternative designation for intermediate mode could be ‘chatter mode’, for reasons that I think you will find obvious.
Heavy: some serious cutting work now, the gib is fully disturbed, by which I mean that all flexibility has been taken up by the forces at the tool tip, and it’s locked up solid. The tip position is stable relative to the work, variations in the cutting load do not result in its position changing. Please note that here I’m only considering the behaviour of the gib strip, not the other components of the tool support system.
I suggest that getting the gib into the chatter mode accounts for the chatter part of the parting problems, because due to the width of a parting tool it would be very unlikely that light mode would ever be experienced – one touch from the wide tool and the jib goes straight into chatter mode. Then it all gets exacerbated by the driver, who hears chatter start and backs off. Then re-approaches gently, and it starts to chatter immediately, as the tip gets rattled about by the previously engraved chatter markings, and it just seems to go from bad to worse.
So the answer to chatter when parting is to load the tool up more so that the load takes the gib strip into ‘heavy’ mode, where it’s stable. If it chatters, don’t back off, but speed up. This provides the rationale for the success of two techniques that have been described in this topic:
1) Use power feed – manual feed will inevitably result in variations in the depth of cut, and therefore the tip load, giving unstable conditions and risking the gib going into chatter mode, perhaps when the driver is changing hands. Power feed will be constant, much more stable, and will typically feed at a minimum of 0.002”/rev (which I find to be a satisfactory rate), which is equivalent to:
2) Feed faster – greater depth of cut, higher tip load, forces the gib strip into ‘heavy’ cutting mode where all play has been taken up, so stable cutting conditions with no further deflection. If no power feed, just wind the handle faster, and keep it as smooth as possible.
Now it must be said that the gib behaviour I’ve described is NOT proven beyond doubt, for that one would have to do a lot more detailed work on the gib itself, and such work is beyond any facilites available to me. I would like to think that it’s not just mere conjecture, perhaps ‘rational explanation’ of what could be happening might be the appropriate terminology. There is further support for the theory, as it also explains three other situations that you will probably have come across during normal turning. These are:
If there was zero flex in the whole system, we'd be picking up broken bits of parting off blades everytime there was a jam (belt slippage not withstanding)
Not if we avoid jams in the first place! Here's the residue of about 50% of the tests, there's something like 40 cuts there (some of the bits have multiple grooves), all using the same tip and not a single jam up for the whole 100%:
P.S. - can we have a whip round for some replacement 19mm bar, please?!
Martin & Andrew - I agree with Andrew, positive feed back from the cutting forces, if it occurs, would be well outside our performance envelope. However, the jam-up scenario could be described as positive feedback, but it has specific causes, which will come later. But it's not from the cutting forces.
MicahealG: some comments re. solid front posts in the next chapter, which is nearly ready for posting. They work very well indeed, better than a rear tool post, and do indeed overcome the problems discussed to date, as well as others that result from gib flexing during normal turning.
Russell: that's an interesting design, and would overcome any lifting of the gib side. But... the solid side of a slide can be considered to be the 'master' side, the one that provides the location in two planes. With an outboard gib the cutting forces push outward on the tool, and on the slide, forcing the 'master' faces into contact, but easing pressure on the gib. With an inboard gib, the forces would push the gib into harder contact but would ease the now outboard 'master' side, and allow it to lift until the 45 degree surfaces came back into contact. Would this be better or worse than the conventional outboard gib? The unhelpful answer is that I don't know the answer! But it should be easy enough to check, by turning my top slide round and applying some load. I'll have a look at that, it could be interesting!
Muzzer: that's an absolute bargain, bite their hand off! Most of my parting is done with that type of tool, albeit the Iscar version.
KWIL: A 3mm tip will produce 50% more cutting force than a 2mm tip. I suggest keeping the width to the minimum required, usually determined by the diameter to be parted off - the wider the blade, the more overhang it can accomodate. 2mm is the widest I find need of, most parting uses a 1.6mm tip, and I have some 0.75 and 0.5mm wide tips for small work. Those little ones are an absolute delight to use but unfortunately they have limited diameter capability (about 6mm), and are very delicate - I break more than I wear out, usually by knocking them with some tool or other. But great for making run-out grooves on small threaded components, or parting off a drilled component of around 3mm wall thickness.
Andrew: cutter life - seconded, 100%
I've been reluctant to respond to other postings on this subject up to now, for two reasons:
1) It's taking enough time up to re-do the tests, prepare sketches, and take photographs - I personally didn't need to do this as I did it all ... oooh... twenty years or so ago, but lost the notes and didn't take photographs at the time! And I thought that as some of it might turn out controversial, I needed to present the whole story, together with real evidence, so I ended up re-doing everything. But the time being taken is actually the secondary reason, the primary is:
2) It's all going to come out in the wash, so I don't want to pre-empt what's to come, and risk presenting information in an out-of-sequence and disjointed fashion.
However, I think it's now safe to respond to Kiwi Bloke, Chris Trice & Martin Kyte - but it will have to be this evening, as I must now take wotsername for yet more blood tests, and then I have someone coming round for the afternoon.
And if anyone else has a question... get it in before this evening!
Thanks for the edit Jason.
Would it not be a GOOD IDEA to completely remove those bloody smilies from this editor, preferably using EXTREME VIOLENCE!?
Summary to date:
For a machine in good condition, with well adjusted slides (and if it doesn’t fit that description, all bets are parted off):
1) The conjecture that saddle lift helps a rear tool post can be dismissed – at the tool loadings we’re looking at it doesn’t (o.k., shouldn’t!) happen.
2) The ‘up & out’ behaviour isn’t confirmed in my tests, so I’ll consign it to the WPB (Waste Paper Basket).
3) The top slide does deflect more than the rear tool post, but the ‘down’ movement does not result in any ‘in’ movement, it is in fact ‘out’ movement, because of a) the complex nature of the top slide assembly and its correspondingly complex movement in many directions, and b) the direction of the force on the tool tip pushes it out. So this one’s headed for the F+F file (File & Forget)
Which leaves just one other ‘reason’ for rear parting off, the one about the rear tool post pushing the spindle down into the strong part of the headstock, whereas the front post pushes it up towards the weaker bit. Well, if anybody designed a lathe where an upward load on the headstock bearings was a limiting factor, they’d be a very poor designer of lathes! That’s obvious enough, but additionally, consider the following:
The 92lbs. load I applied to the tool posts corresponds to parting with 2mm (0.080" ) wide tool at a feed of 0.06mm (0.002" ) per revolution. But that is exactly the same tip loading that would be experienced when TURNING at those values, and an S7 wouldn’t even work up a sweat doing that! The 164lbs. load corresponds to 2mm (0.080" ) depth of cut, with the feed rate increased to 0.13mm (0.005" ) /rev., still within its capabilities. Those loads would be applied upwards at the headstock, just as parting off from the top slide does… and since it’s perfectly capable of taking the turning loads and surviving for years, why should a bit of parting off in the front tool post cause it to get its knickers in a twist? So:
4) Spindle load down instead of up? Definitely one destined for the LTU (Laugh & Tear Up) file.
So where do we go from there? Tune in next time to find out! And find out why the rear tool post IS better for parting off. And what to do about the topslide. And.. other things as well.
Edited By JasonB on 11/02/2015 16:37:57
Given that my loading system was vertically down on the tool, and not inclined as in reality, the static measurements of inward tool movement D must be regarded as marginal as far as coming to any firm conclusions is concerned. To resolve the matter I did some dynamic tests, measuring the in/out movement whilst actually parting off. The photograph shows the position at which I set the indicator to ‘measure’ this movement.
Now before I go further, I am well aware that some might find some of the information, some of the measurements, and some ‘details’ of the cutting tests that follow, to fall somewhere between surprising and unbelievable.
The measurements etc. might not be ‘accurate’, due to their rather crude nature, but they are certainly … what shall we say… indicative?… of the right order?… in the right ball park?…. representative of the situation? Choose whichever one you like! But they are REPEATABLE, plus or minus a bit, and that’s important. Because I’ll be more than happy to demonstrate them, and the turning tests, to anyone who wishes to view them. So if anyone lives convenient to the Chester area and wants to come round, have a look, and verify them on behalf of other Forum participants, just let me know.
Right, dynamic (cutting) tests using the front tool post on a top slide that flexes as shown in the above table, using power cross feed. The saddle was NOT locked to the bed, the indicator was mounted on the cross slide so that it could track with the tool…
The tool moves OUT from the work at lower loads (0.002”/ rev) by <= 5 tenths. And why should we not be surprised by this? Look again at my second sketch at the start of this series – look at the direction of the force applied to the tool tip – it’s down & OUT.
Does a higher feed rate change the ‘out’ movement? Yes, it does, but with increased feed, the OUT movement increases, 0.003”/rev shoves it out by about one thou.
Can it be made to go ‘down & in’ at higher loads? No. OUT at 0.004”/rev, OUT at 0.006”/rev, OUT at 0.008”/rev, but not for long at that feed because the lathe spit out it’s dummy, stalled, and ran home to mummy, whilst wiping the tears from its eyes! Well, taking that cut at 1500 rpm does calculate out at 2.6 h.p.!
So yes, that’s front tool post, Iscar parting tip, 19mm bar, 2mm wide cut, 0.006”/rev, 120 metres/min, 1500 rpm, 2.14 h.p. (calculated, not measured), using the top slide and front tool post, power feed. No chatter, no ‘down and in’, all ‘down & out’.
And perhaps more surprisingly, I normally part of 1” bar at c. 2000 rpm, 0.002”/rev, but had to slow down a bit as 2000 rpm didn’t give me enough time to observe the indicator reading before the end dropped off the bar!
Parting off problem? What parting off problem would that be?
OK, could it be the chatter, jamming and dig in problems? Unfortunately, in trying to resolve these problems, model engineers have been looking in the wrong place for decades, and consequently haven’t found the real answers. They’ve been led up the garden path by the ‘Four Theories’, which my tests have examined in more detail, and shown to be false premises – I hope you agree with that claim, so let’s just recap on where we’ve got to!
Apologies for being a bit tardy with Part 3, but wotsername got badly hit by an infection a couple of days ago, so it's been a round of doctor & hospital visits. Which continue, and may delay further postings a bit.
Yet another sketch to start off with:
Once again not to scale, but the top slide etc. are sitting in about the right position relative to the saddle.
The force at the tool tip now pushes the tip down, and since the tip is out-board of the top slide base, it exerts a turning moment trying to tip the top slide over. In this case, the pivot point is obvious, it’s at P3, the in-board edge of the top slide. We now need to look at what resists this turning moment, attempting to keep the tool tip stable relative to the work.
The obvious answer to that question is the gib strip, i.e. not very much! Just a floating strip of steel, held lightly (it has to be lightly, or the slide wouldn’t slide) against the inner vee of the base, located laterally and vertically by four round-nosed grub screws pushing into shallow dimples in the gib strip. Since the assembly bears the same relationship to rigidity as blancmange does to concrete, one might expect it to deflect when a load is applied to the tool tip. And it really does deflect:
Units for deflections are again 0.0001”, for A, B, C & E negative numbers are down movement, positive numbers are up. D is inward movement of the tool tip. I tried to keep the loadings the same as for the rear post, but they’re probably a bit different, perhaps slightly higher.
Some of the results might not seem to be consistent with each other, that’s for two reasons: 1) the readings for A, B, C & E are all vertical, but they’re not coplanar, and 2) it’s a complicated situation with things bending, tilting & twisting all over the place!
As far as the inward movement of the tool (D) is concerned, it's very low, but I'll deal with that later, if I may.
E is the back end of the top slide. The cutting load, being outboard to the left of the circular base, causes it to lift. Quite possibly this is not just down to the gib strip, but also by lifting of the right hand side of the dovetail spigot securing the top slide to the cross slide, pivoting about the point where the front edge of the top slide base meets the cross slide.
This upward movement of the back end, and the corresponding downward shift of the tool tip, and both are BADLY affected should the top slide be extended off the base, towards the headstock. My tests were undertaken with the top slide arranged as shown above, i.e. NOT overhanging the base.
So what have we got? The tool load causes the slide part of the top slide to pivot inwards about P3, movement allowed by the gib strip allowing the operator side of the slide to lift. The cantilever load on the tool on the headstock side of the base pushes down and lifts the tailstock end, gib strip flexure again, probably augmented by movement in the circular dovetail spigot. The Dickson block moves differently to the top slide on which it sits, presumably as a result of stressing of the holding down bolt. Oh yes, let’s not forget the tool holder and the tenon bolt that holds it to the main tool block.
<I have right clicked on the table and selected table properties and set the width to 512 - Neil>
Edited By Neil Wyatt on 11/02/2015 17:53:13
Hello Muzzer & Frank. Clarification...
Muzzer: the results I've shown are for the loads that I applied as given in the table - I didn't detect any saddle lift at those loadings. It does lift if enough load is applied, e.g. by leaning on the lever, when it suddenly 'jumps' up by about 2 tenths. It's an example of one of the non-linearities to which you refered in an earlier posting.
My heaviest chuck on the end of the lever (equivalent to about 240lbs on the toolpost, which is as far as I can go and still be able to determine the loading) doesn't move the saddle when applied as a STATIC load.
240lbs. corresponds to the tip load when feeding a 2mm tool at 0.13mm (0.005" per rev.
Shock loading causes the saddle to behave differently, it can be made to lift suddenly by 2 tenths at what I think are lower loadings on the toolpost, but I have no way or applying shock loading in a repeatable manner. Shock loading can however be important during parting, jamming of the chips and all that, I'll be discussing that later... but not in a 'calibrated' manner, I'm afraid.
Re the direction of the applied force - you make a valid point . I know the general direction of the cutting force, but not the specific angle. So I decided to work with the vertical component only, as that was repeatable. Further tests with horizontal force only wouldn't be realistic, as that would merely end up causing lots of backward movement due to backlash in the feedscrew.
Frank: nothing is pivoting on the saddle, the lever pivots on the bar between centres.
I suppose I should have said something about the machine and parting tips - the lathe has been in service for about 3 years, and is in as-new condition. My S7 is 40 years old, but more about that later. The parting tips are Iscar GFN2/IC20, no other identification on the box re the specific form of the chip control features. It's the one with the dot against it:
Part the Second, and another sketch:
Although the above sketch is not to scale, the relative positions of the cross slide, rear tool post and rear saddle edge are about right when parting with a rear tool post. With a solidly bolted tool post, the post itself isn’t going to pivot about anywhere, the cutting force at the tip must lift the entire tool post and cross slide assembly as one unit. Either lift it vertically, or pivot it about some point… but where?
I suggest that the tool tip can only be pivoted about either P2 or P1. It wouldn’t be too difficult to calculate this to determine which one it was in reality, once the weights of the rear tool post, cross slide, top slide and tool post were known, together with the cutting force applied at the tool tip. As far as the cutting force is concerned:
About 70 lbs for a 2mm parting blade with an infeed of 0.04mm (0.002” approx) per revolution when parting mild steel.
Knowing the sort of loads we’re talking about, it’s not too difficult to apply a load and determine how the tip moves. I first did this many winters ago on the S7, and have now repeated it on the Connoisseur, ostensibly for the purpose of taking a photograph of the set up, but really because I couldn’t find my results from years ago and had to re-run the tests. The good news is that they confirmed what I’d found previously. Here’s the set-up:
Something of a jury rig, admittedly, but nevertheless it works well enough for present purposes. Re. the indicator – it’s a Verdict, markings at 0.0005”, which means that tenths can be detected &estimated, but I would not claim to be able to actually measure tenths accurately with it! The results are interesting, units for the deflection readings are 0.0001”.
Saddle lift – zero, I couldn’t detect any lift at all, so that disposes of the ‘saddle lift’ theory as to why/how a back tool post works well. At least on my lathe, dunno about yours!
Vertical lift – the figures are relative to the bed. The vertical lift has two components, i.e. for the 72/92/164lbs. loads total lift was 8/16/23 tenths, of which the cross slide contributed 5/7/16 tenths, the remaining 3/9/7 tenths coming from the Dickson tool block atop the tool post itself, presumably because of loading on the single central hold down bolt.
The lateral figures were INWARDS towards the work, not out and away from it. So the pivot point must be at P1, and the tool movement is definitely ‘up & IN’, as per the blue lines on the sketch above, not ‘up & OUT’ (red lines).
The results are, of course, for static loading of the tool post. I’ve done some dynamic tests as well, the vertical results were in reasonable agreement with the above, the lateral results showed no movement one way or the other, but I’ll discuss those further when we’ve looked at the front tool post.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.