Screw-cutting – Is There Something We’re Not Told?

[Nigel Graham 2Participant @nigelgraham2]

Such as how to make it work!

Single-point tool on a conventional lathe.

Task: a special Tee-bolt; a shank as close to 0.375″ as possible, and a 3/8″ X 20tpi BSF thread.

Material: free-cutting mild steel 25mm diameter stock, though badly rusted.

Machines & Tooling: Harrison L5. Myford ML7. Tailstock die-holder (commercially-made). Carbide insert threading tool.

Attempt 1: On the L5 because it was a lot of swarf to make and I judged the bulk of the machine would make achieving that tight diameter easier. Outer end held on a steady, augmented by a half-centre as the diameter reduced.

Also the tailstock die-holder would have some chance of success on this lathe. It’s almost useless on the Myford.

Result: Accurate shank. High-quality thread…. Err, too short. I’d mis-read the drawing.

 

Attempt 2: Same machine and method. High-quality thread. Correct length. Shank 0.004″ undersize and tapered! The taper might be related to the saddle now being on a more worn part of the bed as the stock bar becomes used up.

 

Attempt 3: The bar was now rather short. Roughed down to about 5/8″ dia on the L5, took a finishing cut that proved the taper. Reversed it in the chuck to turned a short clean area on that end. Transferred it to the ML7, holding it by the cleaned part in a 4-jaw SC chuck. The shank length was not concentric but I had expected that. Centre-drilled for tailstock support.

Result so far: Shank within 0.001″ on diameter, and a beautiful finish (HSS tool).

Cut to about half-depth with the parting-tool to delineate the head for milling the flats, to avoid removing the work from the chuck, then set up for the thread….

Back-gear, no more than 0.005″ depth of cut, eased down to the witness extension on the outer end. Tried it with an ordinary 3/8″ BSF nut: too tight. Took any number of spring-cuts. No better.

Eventually finished the thread with a die. Those taper-shank die-holders are hopeless. Their tanged MT2 shanks do not grip in the tailstock properly on a small lathe like an ML7, and their use is always a gamble for anything more than about 1/4″ diameter.  As now. It gleefully removed so much metal the nut almost shakes down the thread, which has a central patch with almost no thread at all. Although the work is still in the chuck but now on the mill for cutting the flats I am considering having to spend more steel and electricity on Attempt 4.

(The others won’t be wasted but kept as general-purpose Tee-bolts.)

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I have had this previously with these thread-cutting tools. They cut beautifully to about two-thirds depth then it becomes impossible to finish screw-cutting to a proper standard that fits the nut well, no matter how much care you take.

Though I am not sure I’d be any better with an HSS tool, despite having a T&C Grinder so can shape it properly.

Since I wish soon to cut two 1/2″ X 20 tpi UNF threads some 3″ long, for feed-screws, and possibly in stainless-steel, this does not bode well.

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Or is there something I am missing? Or we are not told?

 

[JasonBModerator @jasonb]

“I have had this previously with these thread-cutting tools.”

Can you enlighten us as to what “these” are

 

What profile was your threading tool, sounds like you are not allowing for a correct root and crest radius. A HSS tool ground to a simple Vee or  apartial form insert will not cut deep enough if only fed in to your witness dia assuming it was the core dia of the thread.

[bernard towersParticipant @bernardtowers37738]

As Jason says is your tool ground partial profile if so greater depth required and do you screwcut with the set over method or just plunge in?

[Bob WorsleyParticipant @bobworsley31976]

A side issue might be the lathes. The L5 could be well worn from its previous life so Vee ways not working, and the ML7 is notorious for saddle guidance problems. In either case engaging the nuts results in a less than adequate guidance to the saddle, and hence problems.

An easy test is to chuck a piece of 1″ or so mild steel with about 2.1/2″ exposed. Cut two grooves at least 1/8″ deep in the end, one about 3/4″ in and the other 1.3/4″ in. Fit a suitable Vee tool that will cut equally well to the left and the right. Now you need to cut two surfacing areas on the bar. Put the tip of the tool in the end groove, put on a cut of at least 0.080″ so to make the lathe work. Lock the cross slide. Arrange for power feed to cut to the left and at a suitably fine feed rate, at a suitable speed. Make the cut from the first to the second groove. Quickly move the saddle by hand from the second groove back to the first groove. Reset the lathe to power feed to the right. Make the second cut from the first groove to the end of the bar.

What you have done is made the lathe cut both to the left and the right at exactly the same cut depth, hence the locking of the cross slide. Therefore both the machined areas should be exactly the same diameter. Measure them and be very upset.

In both cases the power feed will twist the saddle on the bed, moving the tool in or out of the cut. On the L5 the saddle is rocking on the worn part at the front left of the saddle, on the ML7 the saddle guidance touching the bed has space.

You need the two diameters to be within a few tenths for the lathe to be usable.

 

[HopperParticipant @hopper]

You should make the OD five thou less than .375″ for clearance. Makes life a lot easier without weakening the thread one jot.

You rarely get a good fit at the theoretical thread depth, regardless of what formula or chart you use. (And never use tapping drill size to determine thread depth.)  Most often it is a matter of a series of light cuts and “try and fit” to finish off with. If you keep the topslide in the parallel to lathe axis position, you can carefully take a one or two thou skim first along one thread flank and then the other to create a bit of clearance and a good finish without increasing thread depth. And always run the corner of a large flat file along the thread groove to clean the burr of the tops of the flanks before trying your nut for fit.

The best tailstock die holders for that size range are the ones than have a handle sticking out for you to hold and take the torque with. The holder is a sliding fit on a parallel spindle sticking out from the taper that fits in the tailstock. No load transferred to the tailstock or the flimsy little key that holds the barrel in line.

For any threads under half inch diameter, you might as well just cut it with a die in such a holder. No point in screwcutting anything that small really, other than as an exercise. Making a mandrel crank handle makes it all sooooo much easier than wrestling with the chuck key etc in the traditional manner. And remember to back off ever half to one turn to break the chips up. Trefolex etc on the dies helps too.

 

[Neil WyattModerator @neilwyatt]

have had this previously with these thread-cutting tools. They cut beautifully to about two-thirds depth then it becomes impossible to finish screw-cutting to a proper standard that fits the nut well, no matter how much care you take.

This suggests you are simply increasing depth of cut, try advancing the tool towards the headstock by about half the amount you increase the depth on each cut, so that it chiefly cuts on the left-hand side.

[Nigel Graham 2]

On 17 March 2024 at 23:45 Nigel Graham 2 Said:

Or is there something I am missing? Or we are not told?

 

From the issues you have, you may be missing some guidance? Godd comments above, potential issues include:

• Single point thread cutting techniques.
• Sizing adjustable dies.
• Using tailstock die holders correctly.
• Thread shapes – especially root and crest clearances.

Time for a review of thread cutting techniques in MEW?

Neil

[Adrian Parker 1Participant @adrianparker1]

I have always struggled with dies on anything over about 3/8 inch diameter until I was taught to cheat.  Screw cut with a tool vaguely like correct to about 60% depth then put a die on it.  I use an Arrand Engineering (Unfortunately now defunct I think) die holder that has interchangeable heads held by 3 socket head screws to allow centring the die.  Centring the die is a tricky process but I usually leave the screws slightly loose to allow the die to centre itself.  I am sure that purists will decry this but it works for me.

[Nick WheelerParticipant @nickwheeler]

I find that suggestion very odd. When I screwcut threads it’s because I don’t have a suitable tap or die and can’t justify buying them either because of the size and therefore cost, or they won’t be needed again.

[JasonBModerator @jasonb]

I’ve certainly done it either because it saves having to grind a tool to the correct radii or buying a full form insert that may not get used again.

It also reduces the physical effort not only in turning the diestock for a large coarse thread but I find that holding the part without it rotating is the biggest issue when faced with the effort put into a large pipe threading die, it either slips in the vice or feels like I will rip the vice off the bench. By removing most of the metal first it takes less effort and the thread will be true.

[Andrew JohnstonParticipant @andrewjohnston13878]

I’m not entirely sure what the original question was about; threading dies, single point screwcutting or both?

Although I have a good selection of threading dies, from 10BA to 1-1/2″ BSF, I don’t use them that often. For the centre lathe I have a set of tailstock holders from ArcEuroTrade and a selection of special holders with dog clutches for the repetition lathe. But I find dies a pain to adjust, they always seem to cut a thread too tight or too loose. Coarser threads can be difficult to start and finer threads have a habit of tearing. Even with a tailstock holder the dies sometimes seem to cut a wonky thread. I much prefer to use Coventry dieheads. They produce excellent thread forms from stock size. I never turn stock undersize before threading.

In general I prefer to screwcut threads on parts other than basic fasteners. For external V-threads I use full form inserts as they form both root and crest correctly, like this 3/4″ BSW thread:

For larger internal threads I use full form inserts, for smaller internal threads I use solid carbide tools; these are for 32tpi and 40tpi Whitworth (ME) threads:

When threading it is all about fit and never mind the theory. For screwcutting I know what the theoretical thread depth is and machine close to it, but always check with a mating part, especially if the internal thread has been formed with a tap. As an aside I always screwcut with the topslide set parallel to the lathe axis. Then depth of cut is simply read of the cross slide dial. I always take spring passes before checking fit and give the thread being cut a wipe first. Even a tiny amount of swarf can make it seem like the parts don’t fit properly.

If the mating part is commercial then all bets are off. In general commercial thread tolerances are quite loose and there is no guarantee that the part meets them anyway.

For non-standard threads I grind a HSS tool and screwcut.

Andrew

[duncan webster 1Participant @duncanwebster1]

If the thread is of any length using just a die is likely to result in pitch error. Chewing most of it out with a single point tool and finishing with a die gives better results, as well as being less physical

[Nigel Graham 2Participant @nigelgraham2]

Thank you all for all these suggestions. I have examined them and what I was doing.

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The insert is the sort with three sizeable triangular tips, so when cutting a fairly small thread these penetrate the work only partially (just over 0.03″ for 3/8″ BSF). This type will therefore leave a sharp crest. So would a plain Vee-tool unless ground specifically for the thread.

I find the thread depth from the Zeus book, and make a small root-diameter extension as a guide as well as using the dial.

Usually I do cut the thread to near-depth than finish it with a die, but even with the die open in the holder, this time it still spoilt the thread.

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My die-holder is the type with the through-hole for a handle. Normally I use a rod bearing on a safe area of the lathe.

Its tanged arbor does not hold very tightly in the Myford’s tailstock so the action of the die pulls it out. I don’t know if the tang stops it going back just that tiny bit more. Other tools usually hold properly. It grips on the Harrison lathe, yet neither lathe has a tang-slotted tailstock barrel.

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The Harrison’s bed is worn at the head end, with a definite step onto the gap-filler. My first attempt gave much more accurate diameters, because to save material I used a fairly long piece supported in the fixed steady, putting the saddle a long way down the bed.

Perhaps I should not to try to turn the full length, but finish the thread section then the shank section separately, so controlling the depth over a much shorter length. It won’t remove the taper completely but will reduce its effect.

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The saga continued, concluding in finding an enormous amount of play in the Myford lathe.

I took the embryo bolt still in the 4-jaw SC chuck to the milling-machine and screwed it onto the dividing head with the tip of the bolt supported by the matching tailstock, to mill the two pairs of flats. At the point needing 0.04″ total removing, 0.02 a side, I raised the knee 0.015″. The result measured not 0.03″ gone, but well over 0.03″ still to clear!

I was holding the cutter in an R8 collet. It seemed fully tight but I gave the drawbar an extra tweak.

The next cut worked, taking it down to 0.503″ wide. I wanted to finish at ~ 0.495″. Raised the knee 0.002″, skimmed the two flats, measured…. 0.474″ .  That’s rubbish!

Thoroughly dispirited: this was the second time that had happened. I cut the two steps (the stepped profile is not strictly necessary on a one-piece Tee-bolt, but never mind), took the chuck and work back to the lathe to part off the bolt.

I had not cut the preliminary groove deeply enough, so the two flats gave the 3/32″ wide, HSS parting-tool a sizeable interrupted-cut. Despite the small size and using very low speed, this lifted the saddle and the work visibly on each bump, until the parting-tool had gone below the flats. Then it started playing up again, and I had to saw the last bit.

The flats were too tight. I had to ease them with a file.

This Tee-bolt finished the correct length with a beautifully smooth shank within 0.001″ of size… but a very poor thread, rough, inaccurate and slightly unsymmetrical flats that produced ugly scars on the shank, and still tight in the Myford Tee-slots it is supposed to fit.

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I had to stop there anyway as I was out for the afternoon, but I locked up the workshop knowing after over 50 years in model-engineering I cannot even make a simple Tee-bolt to Loan Section quality looks and function; let alone competition “Commended” level.

 

[HopperParticipant @hopper]

Sounds like you have a duff die. IT should not ruin the thread like you say. Some of today’s hobby grade dies are not very well made. Or is it old and blunt? It can be sharpened with careful running of a dremel tool down the front face of the teeth.

[Andrew Johnston]

On Andrew Johnston Said:

… But I find dies a pain to adjust, they always seem to cut a thread too tight or too loose. Coarser threads can be difficult to start and finer threads have a habit of tearing. Even with a tailstock holder the dies sometimes seem to cut a wonky thread. … I never turn stock undersize before threading.

Try turning the OD undersize before cutting your threads with a die (or screwcutting). It makes life a lot easier. Under half inch diameter I make the OD five thou under, then increase the five thou proportionally for larger diameters or decrease for tiny BA etc . That is the way I was taught to do it at engineering college and on the job in the toolroom machine shop, for general work.

Threads are designed to have clearance at the roots and tips so they bear on the flanks without binding. So if there is a bit of a flat left on the tips instead of the theoretical radius, no matter. And according to Tubal Cain’s research in his book on threads, you can reduce thread depth down to as little as 65 per cent engagement and still have 95 per cent thread strength. It is the base of the triangle that bears most of the load, not the tips.

Likewise, far too much fuss is made in some amateur circles over grinding screw cutting tools with the correct tip radius as per ISO standards or whatever. A few licks on the bench oilstone to remove the sharp point so it does not snap off under load is all that is usually needed for most hobby sized jobs. By making the OD of the male thread a bit undersize and the hole in the female thread a bit oversized, the tips of both threads are operating in clearance space so exact radius is not at all critical, unless it is made ridiculously too large.

 

[JasonBModerator @jasonb]

It is what I said you are using partial form insert so need to go in deeper than the core dia of the thread that is given in tables. It is the flanks of the threads that need to be right hence why they are often measured by wires which don’t touch the root or crest.

Taking Andrews Photo from above on the right I have drawn in green how a full form insert will correctly cut the thread going in as far as the core dia. On the left I have sketched a partial form insert with it’s more pointed end but that could also be an HSS tool. In red I have shown how much deeper it needs to go to achieve the same “depth” of cut to give equal flank depth in blue.

The partial form insert will also not round the crest so you will either need to do that with a file against the rotating work or reduce the OD as hopper says

As for the rest who knows one minute you say the flats measured 0.474″ which was 21thou under what you wanted then you say they were too tight and needed filing? Either you are measuring wrongly (one line of the mic would make it 0.499 which is 4 thou smaller than 0.503 which 2 thou off each face should have given) or something else is going wrong.

It is not so much a case of what you are not being told, more a case of what you are not looking for. You are reluctant to watch video and don’t seem to be able to navigate the net well to find things out in insert manufacturer’s data which will tell you these things. You won’t find threading inserts in an ancient old text book so not much hope unless you look where the info is these days.

[Andrew JohnstonParticipant @andrewjohnston13878]

A few notes, and some oblique comments in answer to those from Hopper.

One reason I use full form inserts when screwcutting is that the insert forms the crest, so one gets a nice looking thread. There would be no point starting undersize when screwcutting. Another, unrelated reason for screwcutting is the ability to thread to a shoulder, external or internal. The old school pictures of boiler fittings showed the internal threads on gland nuts going all the way to the end; how are you supposed to do that with a standard tap? Screwcutting means I can get within one thread pitch. Screwcutting also helps on short threads:

The external threads on the oiler caps (bottom left) are only 1/8″ long. They would be difficult to cut with a die. The threads on the bottom of the bodies were also screwcut.

The work by Tubalcain on thread strength was primarily aimed at determining tapping hole sizes, not reducing the OD of external threads. Although, to be fair, he does mention that reducing the OD of an external thread by a few thou may help. Not surprisingly I am an outlier on tapping sizes too. I use tapping drills that are larger than most people on here use. For instance I normally use 5.3mm for M6, or 5.4mm in tough stuff like stainless steel.

Coventry dieheads produce nice full form threads too, these studs are 1/4″ and 5/16″ BSF:

Just to prove I do use threading dies sometimes the threads on these screws (M2) were cut with die:

I’ve run out of time, got to go to the hospital for the follow up check after my second cataract operation.

Andrew

[Dave HalfordParticipant @davehalford22513]

This link to APT will give you an idea. I bought this one for a collet draw tube thread.

Note that as a partial form insert it can produce threads from 48 to 16tpi so the extra depth of cut will also vary, least ways I found that I needed an extra approx 10 thou to get a fit on 20tpi.

If you buy them on ebay it’s a bit Wild West what you get told and sometimes the vendor doesn’t know anyway.

Its tanged arbor does not hold very tightly in the Myford’s tailstock so the action of the die pulls it out. I don’t know if the tang stops it going back just that tiny bit more. Other tools usually hold properly. It grips on the Harrison lathe, yet neither lathe has a tang-slotted tailstock barrel.

Well, the die has to ‘pull out’ toward the chuck or you don’t get a thread so the die holder action is too stiff or it’s arm is  sticking on your ‘safe’ part of the lathe.

If the Myford tailstock has impacted swarf that stops the taper seating square then your drilling is off as well as any thread made by the die holder

Do you not have a saddle clamp? It’s there to stop lift, if not the saddle gibs have worn, that includes the rear.

[Nigel Graham 2Participant @nigelgraham2]

Hopper –

Nothing wrong with the die I used. It was not some ‘flea-Bray’ cheapie but a little-used one from a reputable supplier. It was my using it that failed, and I don’t know why.

I was also using very good, free-cutting mild-steel.

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Jason –

That is what I was effectively doing: cutting the thread deeper than the nominal to compensate for the sharp tool point. I could see it grooving the witness stub on the end, at nominal root diameter.

The test nut would screw on for a couple of turns then jam. Yet this bolt’s brother, differing only in length, had worked, perhaps because I had not tried to screw-cut the thread.

Hopeless because I need also make two threads some 3″ long and this rate I can see the only way is to use studding and modify the rest of the design to suit!

 

I could measure the flats in progress only with the work in place, so carefully brushed all the swarf off and moved the table clear to allow full access for the micrometer. I also measured the thing several times after I’d finished it, but whatever was happening doesn’t make sense.

It’s possible the cutter was slipping in the collet, I suppose, but there was not enough room for the Autolock collet next to a dividing-head.

Or something was springing, maybe.

I just don’t know.

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I’m afraid you mis-read me.

The nominal A/F size is 0.5″, to fit Tee-slots milled in the factory – when that was in Beeston, too.

So I wanted the head close to 0.495″ A/F – as on ones that worked.

Instead, despite all care to measure the head as I went, this specimen, and a previous one, came out at 0.475.

Yet I’d only raised the work by two thou to take that last stage down to that very gently. How does 0.002 suddenly become 0.01″ ?

””

I avoid videos because I can’t learn from them, even if they seem reliable. Most seem simply demonstrating skill, and in ideal conditions, not teaching it. That was even before Google etc. started ruining videos with constant advertising breaks.

While I am sorry but your last paragraph was rather hurtful. Of course I don’t rely on old text-books to tell me about things that did not exist in their day. I use them only where their information still covers the particular task, tools and materials – but a lot is still relevant.

””’

Actually it is becoming harder to find anything useful on the Internet. A company web-site name always works;  but otherwise unless you know exactly the right search phrases the system will use, it returns screenfuls of ads, usually not at all relevant (probably by weak word-matching).

I have just broken off to test this. Entering “indexable tool standard specifications” did soon reveal the goods. The mysterious ISO has its own web-site – but that only indexes information available only by sale only to they who move in the Right Circles.

Luckily I also found – and book-marked in my “Engineering” list – the Seco tool-makers’ site with comprehensive, clear tables of the ISO-standard designations etc. That’s mainly for the holders but does show the inserts’ codes too, at least partly. I’ve also a copy of a printed Sandvik catalogue with similar information.

Fine… doesn’t help though. I could have been using brand-new machines and fittings, and still used 18 inches of steel to fail to make a decent-quality Tee-bolt 2 inches long.

 

[Nigel Graham 2Participant @nigelgraham2]

Andrew –

I hope all went well with the eye examination.

Your samples show the standard I was aiming for, but everything is stacked against it. Stage A works properly, stage B goes wrong. Start again and this time B works, C fails…..

I have a Coventry die-head but no sensible way to mount it on the lathe. Sets of chasers too, though possibly not the ones I need or even right for the head itself! I’ve also some tool-post held chasers and those (if I have any the right threads) should give reasonable threads, though with no top-rake might be best on steel used only for finishing-cuts.

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Dave –

Both of my lathes have saddle-clamps. I had the Myford’s one tightened but it did not stop the saddle moving.

The tailstock tapers on the lathes seem fine as they grip centres etc properly. The Myford one won’t grip the die-holder very well though. I’ve not considered the possibility of impacted swarf though so will investigate. Wear and tear might mean they are no longer truly concentric and parallel to the spindle, of course.

[JasonBModerator @jasonb]

Nigel

 

I did not miss read you.

“The next cut worked, taking it down to 0.503″ wide. I wanted to finish at ~ 0.495″. Raised the knee 0.002″, skimmed the two flats, measured…. 0.474″ .  That’s rubbish!”

You said your initial measurement was 0.503″ and you wanted 0.495″

You then raised the knee by 0.002″ which should have reduced the 0.503″ to 0.499″

You then say you took a measurement (now known to be with a micrometer) which read 0.474″

The difference between 0.499″ and 0.474″ is exactly 0.025″ which is one division of a micrometer.

I am suggesting you misread the micrometer, something confirmed by you later finding the flats were oversize from your 0.495″ original requirement. Had it actually been 0.474 then it would have easily fitted as it was 21thou under your required 0.495″

You have now changed the numbers so have no idea what you are now on about

[Nigel Graham 2]

On Nigel Graham 2 Said:

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Dave –

Both of my lathes have saddle-clamps. I had the Myford’s one tightened but it did not stop the saddle moving.

The tailstock tapers on the lathes seem fine as they grip centres etc properly. The Myford one won’t grip the die-holder very well though. I’ve not considered the possibility of impacted swarf though so will investigate. Wear and tear might mean they are no longer truly concentric and parallel to the spindle, of course.

Nigel,

Welcome to my world, My lathe is a rockwell delta that’s seen better days with bed wear of 6thou on the prismatics which shows more on the hand wheel than power even with the saddle nearly locked stationary. The tail stock is packed as well.

The Myford sounds like it needs the wide bed mod . Look to the gibs at least.

Sometimes a tail stock barrel clamp needs tightening to bring it to centre.

There should be no load on the tailstock with a floating holder unless it’s cocked over.

[Nigel Graham 2Participant @nigelgraham2]

Sighs…

I have not changed anything.

Measured the blank, subtracted 0.500, halved the difference.

Took an initial cut, re-measured – it needed 0.040″ taking off.

0.02″ a side.

Set 0.015 cut, for safety…

That should have reduced the work by 0.030″. It had not!

So I examined everything to find why. No obvious reason but I did give the drawbar an extra tug with the spanner. I no longer believe that plain friction collets (R8, ER, etc.) are right for holding milling-cutters for accurate depths; but there was no room to use the Autolock collet.

Next cut worked, bringing the work to 0.503″, give or take a thou. (I remember because this was proving such grief to do!)

Table up by 0.002″ to leave the piece 0.499″ wide.

I was aiming for 0.495 as close as I could so expected one light skim more would do it.

So re-measured for that finishing skim: not 0.499 but that blasted 0.475″.

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The micrometer is one with a mechanical analogue display, so difficult to mis-read, and is reliable. That error suggests mis-counting a conventional 25/1000 scale but by co-incidence because I did not use that type.

Somewhere, somehow I lost 0.012″ a side – and I do not know why.

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Knowing now that these Tee-bolts do not need steps, I considered side milling the flats using either a Stevensons Block with an ER collet, or the jig-borer and a 3-jaw chuck; but the cutter would probably act like a gear and rotate the work-piece or chuck. Almost easier to file the blasted things! If I can’t make a simple Tee-bolt, I have little chance of making anything that really needs be accurate, like engine components.

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I am afraid that if anything I build works, such as that long-suffering steam-wagon I should have completed years ago despite no drawings, it will only be by a miracle of work-rounds, inaccurate parts, re-works, holes filed oval, surplus holes from changes to overcome problems not foreseeable, tatty threads, loose fits, poor finishes and plain old-fashioned bodges… Despite every care to get every part right first time, at every stage, too.

……

Just having a post-lunch cuppa and trying to motivate myself to have yet another go with the last bit of that length of steel. It’s just long enough with great care. I’ll search my collection of chasers but don’t think I have any of the right size.

 

[JasonBModerator @jasonb]

But if it did truely measure 0.474 or 0.475 why were the flats too tight and needed filing to fit?

Even more puzzling is where you get your initial sizes from the Myford tee bolts that I have are 0.530″ AF to fit the nominal 9/16″ wide tee slots of my Myford vertical slide. All from Beeston. So you should have had miles of room for them to fit whether 0.499, 0.495 or 0.475🤔

[Nigel Graham 2Participant @nigelgraham2]

I am following a drawing that calls for 1/2″ – no decimals – so didn’t think to question it. The two T-bolts (different lengths) are to fit the ML7 saddle, and I found slight width variations on the upper part of the slots on mine. The bolt enters one but is tight on another.

The tightness on the bolt was on the steps I’d given it – since I am making two things in parallel, I looked at the salient dimensions for the essentially similar components but failed to spot the ones to fit the Myford are plain head type. That would not matter if the thing had actually fitted – it didn’t because I could not make it well enough!

I did not want the head too narrow because that reduces what little contact area they have anyway.

 

What was really upsetting was being unable to achieve the result I wanted, and trying to make just T-bolt had become a nightmare of my mistakes, worn machine-tools and things just going wrong for no clear reason.

……

I finally succeeded this evening.

I had only just enough steel of that bar left so had to fit the spindle back-stop to help hold the material in place, and support the end from the tailstock. Unable to rely on turning the full 2″ length of 3/8″ parallel I roughed it down to under half-inch then took the thread section alone down to 0.371″ diameter.

Using the parting-tool I cut a run-out groove of .032″ deep from that so in theory probably enough to accommodate the non-truncated screw-cutting tool. It proved about right.

This was where I solved –

 Problem 1 – the loose-fitting arbor on the tailstock-die holder. It had spun in the tailstock at some time, gouging it just enough to prevent full engagement. I cleaned it with a small file and and that was sorted. However I was not going to use it with such a delicate set-up…

and….

My mistake 1. It does help to use the holder with the Whitworth-form insert, not the 60º one as I found I’d done. No wonder the BSF die had munched through so much metal. It was trying to trim a UNF thread!

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Then My mistake 2: I thought the developing thread looked a bit fine for 3/8″ X 20 BSF. I’d not set the gearbox back from the fine feed, and was now cutting a 28tpi thread. Fortunately the cut was still shallow and the error disappeared eventually.

Tickling the thread down and lightly running a smooth file over the crest eventually produced something for the nut to run on almost as it’s meant to! I have no idea how many passes and tests this process took but the tool seemed to find steel to remove every time.

Now turned the shank, which will eventually fit a reamed hole. We can but hope.

Experimenting while still well above finished size I found an insert tool that gave a slightly better surface than the HSS one doing most of the work, though itself giving a very good finish. Very gently, without letting the tool rub, I crept up on that 0.375″ figure, using the self-acting feed on its lowest rate.

It is too! Well, in the centre of the 1.25″ length it is.

The wretched thing is still tapered by 1 to 2 thou over just that length!

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So Problem 2 – Why can’t I turn an accurate parallel?

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I’ll just have to live with it. I turned the material round, reduced the head to thickness then cut the flats. I think I found the source of –

Problem 3 – weird widths. Cutting the wider flange for the stepped head seemed to have left narrow ridges, and I was probably measuring either on those or the lower surfaces without realising it.

This time I used a slightly different technique and anyway was milling a much narrower flange, and this time did indeed reach that nominal half-inch less five thou.

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So after 4 attempts I now have a Tee-bolt that is to drawing and close enough to work as it’s intended – but would fail any proper QC inspection.

What a ruddy palaver, and convincing me I’ll never create any significant project to decent standard.

 

 

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