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Member postings for Andrew Johnston

Here is a list of all the postings Andrew Johnston has made in our forums. Click on a thread name to jump to the thread.

Thread: Cura 3.4.1
17/09/2018 17:22:11
Posted by Neil Wyatt on 17/09/2018 16:38:02:


Windows 10 Version 1803 here

The alternative at the time was W8, which is a complete POS. I bought a pukka copy of W7 before it was discontinued, and before I bought the computer.

I use W10 at work; sort of ok, but rather gets in the way of doing useful things. And the default PDF viewer is worse than a POS!

Even though W7 is a bit outdated I'd expect Cura to run on it, or at least not come up with an incomprehensible error box that you can't get rid of, whichever option you click.


17/09/2018 07:55:49
Posted by Neil Wyatt on 04/08/2018 08:29:18:

I just wanted to suggest that anyone who uses Cura upgrades to this latest version.

Well that was a disaster! The latest version of Cura won't run on my old computer with XP because it's 32 bit. Neither will it run on my new computer with W7 - never starts properly, just complains that an obscure hard disk is missing. Totally useless. On another matter I can't get basic information on my printer from the Ultimaker site. Sadly it seems they've been so seduced by fancy graphics and twiddly stuff that the pages never load properly unless, presumably, one is uaing the latest wizzo computer and OS.

So I've reverted to my old (V15) version of Cura.


Thread: Best type of material to use for beginners
15/09/2018 08:00:24
Posted by Kevin Murrell on 15/09/2018 07:00:18:

In something I read, there was talk about normalising stock like this: heating to 800C, then letting the stock air cool.

Is this something one should do as a matter of course?

Generally no. Bright drawn steels are stressed, particularly the outer layers, during manufacture. If you machine away parts of the surface the stresses can become asymmetric and cause the part to bend like a banana. For turning I never anneal as the machining is symmetric to the part and so the problems don't arise. Milling is different. If I need to machine BDMS then I normalise first. It's one reason I use a lot of hot rolled steel for milled parts, as it's a lot less prone to looking like a banana.

Going back to the original problem, it's the same old questions. I'm not sure I'd trust the material to be EN1A. What lathe are you using? What drills; manufacturer, old or new, where did they come from? Speeds and feeds? A picture of the setup may help to determine the issue.

I've never seen this problem when turning and dilling on the lathe. I've only seen it once when milling EN3B flats, where I broke two carbide cutters on the same part. Actually I didn't break the cutter, but the material took all the teeth off, converting an endmill to a ballnose mill. All the other parts from the same length of material were fine. Since I was milling spokes for my traction engine models these parts were normalised in an electric furnace before machining.


Thread: Backlash Allowances for Gears
13/09/2018 08:59:06

Personally I'd forget pootube videos and unknown spreadsheets and go back to basics. thumbs up

The conventional way to calculate the OD of a spur gear is number of teeth plus 2 all divided by the diametral pitch (DP). Although given that the addendum (J) is one over the DP it's the same as the PCD plus 2P. In theory the dedendum (K) is also 1/P. However from a practical point of view there needs to be clearance at the bottom of the tooth for the crest of the mating tooth. The clearance is arbitrary, but is often set as 0.157/P, or pi/20 for a 1DP gear. The clearance needed varies with number of teeth and pressure angle. The "standard" total tooth depth is 2.157/P. But for small numbers of teeth it can be 2.25/P. Any proper involute cutter should have the total tooth depth engraved on it.

Backlash allowance is different to clearance, but is also arbitrary. The value of 0.04/P is an average. For minimum backlash the figure can be 0.03/P, or for a maximum it can be 0.05/P.

To get the calculated backlash you cut the spaces slightly deeper. For 14½° PA the excess depth on each gear is the same as the backlash value. For 20° PA and on both gears the value is 0.73 time the backlash.

When I first started making spur gears I spent a lot of time fannying around with gear verniers and tweaking the depth of cut. Turned out to be a waste of time, now i just cut to the calculated depth without backlash and the gears work fine.

For changewheels it's a complete waste of time mucking about with backlash calculations, as you can achieve the same effect by twiddling with the centre to centre distances. In theory gears with different PA will not mesh, but if they're sloppy enough then of course they will do so. I don't know what the PA is for Boxford gears, but I'd guess 14½°. I suspect that the tumbler gears are 20° PA to avoid undercutting on relatively small gears. The need for undercutting is dependent upon PA and gets less as the PA gets bigger. There is a formula for it, which I can't remember, but I think for 20° gears under about 12/13 teeth may need undercutting.

To summarise: forget backlash and overcutting on depth, cut to calculated depth and the gears will work fine.


Thread: Possible 3D Printing Needed
09/09/2018 12:09:28
Posted by JasonB on 08/09/2018 18:39:39:

Is there any reason why you can't use the pantagraph to machine away the areas around the letters rather than the letters themselves? That seems to be how our CNC owners do it and a pantagraph is the same basic thing moved by hand rather than stepper motors.

I used to have a Taylor Hobson pantograph engraver. They're very good at engraving letters following a narrow path in an oversize pattern. Of course one could make an oversize pattern with protruding letters. It would be fairly simple to engrave around the outline of each letter. But the problem comes with the spaces in between. Of course it would be possible, but you'd be running completely freehand so it would be difficult to control width of cut. Ideally one would need to use a small milling cutter rather than an engraving cutter. To get any sensible depth you'd also need to make several passes, adding to the tedium.

Now that I've got a CNC mill the engraving machine became surplus, so I sold it. The only caveat is that you really need high speeds for engraving and small milling cutters. I had limited success on the CNC mill with small cutters, <1mm, until I fitted a high speed (24000rpm) spindle.


Thread: Boring bar size
08/09/2018 23:36:15
Posted by Neil Wyatt on 08/09/2018 23:02:10:

It has flats on it so the insert 'droops' at about 20 degrees which seems all wrong! With a hefty bit of packing (about 2.5mm) it's cutting rather below centre height in the SC4 toolpost. Not sure how the mini-lathe would handle it.

That's so you get clearance in a small bore with an insert primarily intended for external turning. Hefty packing equals 2.5mm? Dream on! My 10mm boring bar requires nearer 12mm of packing to get to centre height.


08/09/2018 19:42:05
Posted by Michael Cox 1 on 08/09/2018 13:08:27:

You can also buy blacksmith drill with a 12 mm shank that will drill to 25 mm or more.

Not my favourite tooling; I've got a cheap set but try not to use them unless absolutely necessary. On the lathe it's much better to use Morse taper drills. I've built up quite a collection, mostly from Fleabay, over the years. They range from around 3/8" up to 1-3/4". If you buy odd sizes, like 63/64", 1-1/64" or 1-5/32" then they're pretty cheap. Nothing like a drill for ripping out metal. With the bigger drills I'm limited by how fast I can wind the tailstock handle.


Thread: Lubing my new mill
08/09/2018 08:57:37

I use an ISO68 slideway oil in the one shot oiler on my CNC mill. It gets operated every time I power up and then every few hours when running. But of course a CNC mill sees more action per hour than a manual mill.

I also use ISO68 slideway oil on my manual Bridgeport. The nipples are for oil, never grease. The only place on my Bridgeport that gets grease is the varispeed plates. And that's not a nipple but a threaded hole into which you screw the tube of special lithium grease.


Thread: Speed Controller - Mini Lathe
07/09/2018 10:26:20
Posted by Martin W on 07/09/2018 09:45:54:

In this state the lathe will not comply with basic standards and certainly it will not comply with european/UK regulations. I would demand a full refund and get the seller to collect the lathe. If UK based then I would also raise this with the relevant trading standards authority.

That's a moot point in countries like Germany where the mains plug can be inserted either way round, so the equipment doesn't actually know which is LIVE and NEUTRAL.


07/09/2018 09:11:12
Posted by Derek Leggott on 07/09/2018 08:55:35:

As a newbie to this is a Potentiometer the knob that adjusts the speed?

No, it's a preset pot. It will be used during manufacture to set a limit (minimum looking at the silkscreen) that the actual speed control pot will work between.

To blow the preset apart like that means something is seriously wrong. I suspect it's had mains, or the output voltage up it, which is worrying. One would hope that it's an assembly fault rather than a design flaw.

Don't even think about getting the board repaired; the supplier needs to replace the whole board.


Edited By Andrew Johnston on 07/09/2018 09:11:29

Thread: how to machine an internal curve
06/09/2018 19:51:37

I wouldn't grind a relief angle, I'd make the relief curved and of slightly smaller radius that the hole. That way the cutting edge is stronger. thumbs up


Thread: Wadkin surface grinder.
06/09/2018 19:48:54

So not a surface grinder as we know it? In which case changing to a single phase motor probably won't matter. If it was a proper surface grinder, as opposed to a cutter sharpener, then changing to a single phase motor would be a bad thing due to motor vibration.


Thread: how to machine an internal curve
06/09/2018 10:28:00

The grade of stainless steel is important too. Grade 303 is lovely to machine, 316 is nice to machine too, but work hardens at the drop of a hat, don't even think about peck drilling! And 304 is horrible.


Thread: milling insert damage
05/09/2018 07:28:13

So why is the back of the insert broken and the bottom of the toolholder damaged to what looks like the same depth as the slot being cut?

Depending upon the supplier I wouldn't be in the least bit surprised if the toolholder rubbed. You've only got to look at the grinding on some of the cheap milling cutters to realise that they'd never cut properly.


04/09/2018 22:29:11
Posted by Dave Halford on 04/09/2018 21:03:08:


The whole clearance angle has flaked off. Are you saying don't go full width? But how else do you start a slot?

No, what I'm saying is that if the width of the slot you're trying to cut is too narrow compared to the width and thickness of the insert then the back edge of the insert will not clear the slot; leading to interference and a broken insert.

It's why boring bars for the standard CCMT inserts present the insert at a much steeper angle than the external holders so the bottom of the insert has clearance.


04/09/2018 20:04:35

TPUN inserts are intended for turning, not milling. It would seem likely that the insert simply doesn't have enough clearance when trying to cut a slot not much wider than the insert width. That may explain why the back of the insert at the bottom is chipped and holder is scored.


Thread: The size and shape of drill holes
04/09/2018 10:17:16
Posted by pgk pgk on 04/09/2018 09:32:37

I misread the mike of the drill shank. It was 5.9mm not 5.8mm

Still seems a little undersize. The shank on my Dormer 6mm drill is 5.94mm, ie, 0.06mm undersize. The taper is proportional to drill size; a 3mm drill shank meaures 5.97mm, ie, 0.03mm undersize.

For close fits for 6mm pins I'd drill 5.8mm and then use a 6mm machine reamer. If that's too loose you could try a 5.99mm reamer.


Thread: Mass Production
03/09/2018 23:17:39

Hmmmm, we seem to have a fluid specification. Not so good for a production part. smile

In the OP it states the overall length is no more than 26mm long with a 17mm deep blind hole in one end and an 8.5mm deep thread in the other end. In my mathematical world the following is true:


If that's not the case it would appear that Godel has been at work. wink 2

As an aside plunge milling will not result in a flat bottom hole, as endmills and slotdrills are slightly hollow ground.


Thread: Is there a new standard for taps?
03/09/2018 17:37:30

If at all possible I tap dry. It saves all the faff of getting the goo out of the hole and off the tap. If I do use a tapping fluid I use Rocol STD; works well when using larger taps by hand.


Thread: Best type of material to use for beginners
03/09/2018 17:34:05
Posted by Kevin Murrell on 03/09/2018 16:56:35:

........... two lengths of 1/2" BMS free cutting. Quite whether that is the right term I am not sure, but nobody laughed. It mills OK, but drilling into it is very very hard work. The centre-drill was OK, but then a 3mm drill wouldn't touch it. First I assumed the drill was blunt and tried another, then somehow thought the mill was going backwards (it can't BTW), but persevered. Eventually I broke through the surface and the rest was what I would expect. Is this surface-hardened?

If it's proper free cutting steel it will most likely have a small proportion of lead in it. It's a low carbon steel so there's no way it's going to work harden. Should be easy peasy to drill. So either it isn't what it purports to be, or there's something wrong with the drill. Given it milled ok I'd suspect the drill. What is the provenance of the drill?


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