Testing Single Point Thread Fitment

[berwickParticipant @berwick]

Guys,

I have a question about [ being able to ] test a small, custom, single point thread for fitment.

The question;

How can I, ” pick up the thread” after having removed the male threadded work piece from the chuck, test it for fitment, and replace it back into the chuck for further single point thread cutting if needed?

 

Project;

Making and or modifying a new percussion nipple, to fit an old, unusual, early 1800’s Birmingham-gunsmith percussion cup thread form. As you know…. percussion nipples are tiny little beasties, and there are no commercially available nipples in this 30 TPI threadform. Further, it is of BSF 55 degree angle with rounded roots and thread flats, so yep, it has to be a custiom-made one-off nipple, made by me, fitted to the female threads in the percussion cup.

I have ground and honed the hss threading tool to fit as perfectly to the original nipple thread rounded root, as I could. I have made a “test” thread from brass, just to see if in fact the pitch actually is, 30 tpi [ or .0847mm ] and it is. I matched the major diameter of the brass test piece to the original nipple major, and made a test piece [ plug gauge ] to insert into the female percussion cap threads to determine the minor.

The theaded brass test piece was almost a perfect fit, no wobble, but only scewed in two threads with finger / thumb torque. It’s close but not perfect.. I have not used more than finger torque in attempt to swage the threads to possible better mate. Not yet anyway.

Now, to complicate matters, the original nipple is not truly round and I suspect neither are the percussion cap iron female threads truly round. It is .001-.0017 “out of round”, used, and has the expected corrosion and pitting that old gun iron has.

This is the very first thread that I have cut. So with my inexperience in single point threading, and any lathe-screw cutting, I don’t know if the fitment issue with this test piece is of major diameter origin, minor diameter origin, or because my thread was round and true, but the cup female threads were not.  Also, I am not 100 percent sure that my thread depth was cut deep enough on this test piece, thus my need to remove the next attempt [nipple] from the chuck to test for fitment. Can it be done? Can the thread be “picked back up” once the nipple has be removed and replaced in the chuck? I don’t dare attempt to test fit a nipple whilest still in the chuck and me holding the gun barrel up to the nipple thread, due to likely missalignment and to the “lack of feel”.

Major thread diameter; .215″-.216″

Minor thread diameter .195″-.196″

Thread depth approx .010″

ML7, hand Crank threading technique, 3 jaw, 4 jaw, 2 MT drill chuck, ER 32 collet [ the cap on the collet holder causes interference nipple / hss tool clearance and thus cannot be used ].

Thank you for your thoughts.

 

 

 

[Nigel Graham 2Participant @nigelgraham2]

Something I try to avoid having to do of course, but I have successfully re-set the tool to the thread on odd occasions. My method (which I assume is well-established, not only my idea!):

Wind the tool in until just clear of the thread crest.

Take the saddle down-lathe well past the end of the thread.

On low speed, set the lathe going as if for the next cut, then stop the motor a few turns along, keeping the half-nuts in. (Or rotate it by hand.)

Carefully use the top-slide and cross-slide feeds to engage the tool with its nearest bit of thread.

Withdraw the tool, start the lathe or better, rotate it manually, for a few turns, then stop again.

Test the engagement of tool and thread at this new position, to verify the first adjustment.

Withdraw the tool, move the saddle back clear, repeat the test just to be sure.

One happy, resume thread-cutting.

 

This assumes using the clasp-nuts and thread-dial indicator which your Myford should have. Without that you would have to keep the clasp-nuts engaged all the time, especially for a thread that is not an integer multiple of the lead-screw.

You would use the indicator whether using the motor or hand-cranking the lathe, of course, though at least with usinjg a handle you can keep the clasp-nut engaged throughout. (You need do that with some threads anyway.)

 

Points to note:

Using a single-point tool rather than fully-profiled one, starting with the nominal outside diameter and feeding to the theoretical depth will not produce quite the true thread shape and you may find it will bind. I normally thread-cut to close to what it ought be and finish with a die to profile it, but cutting a non-standard thread as you are, with no dies available, is a bit more of a challenge. I would suggest you shave a very tiny bit off the crest diameter, giving a slightly truncated profile, and cut a few thou deeper than specified. Where I can, I turn a little spigot to root diameter on the end of the work, so when a scratch appears on that I know I am close to finished size but may need cut just a touch deeper and take a skim off the crest. If necessary the spigot can be turned off afterwards.

Also, you will likely find a surprising amount of spring in the system. I repeat some intermediate, and the last, cuts at the same setting as “spring cuts” to shave off that left-behind material. That especially so when the work is as slender as yours. I use a half-centre in the tailstock if possible but the ML7 is not very co-operative in that regard, needing the top-slide swinging round quite a way to clear the tailstock.

===

I don’t understand your comment about unable to use the ER collet fully. Are you trying to thread a workpiece already of finished length? The normal approach with a very small item is to turn as much as possible of it, including the thread, on a stock bar long enough to hold and machine properly; then part it off.

Test all the clearances, approach space etc. when setting-up. For example, I find this especially important with parting-oof from the rear tool-post, as the parting-blade holder has a small flange not very visible from the front, and too easily caught by the chuck jaws.

If you are contemplating a lot of screw-cutting on very small parts I’d suggest grinding a tool off-centre so the tip is as far to the left as possible compatible with enough width of the triangular part. That will help working close to the chuck or collet. It will also help when screw-cutting up to a wide shoulder. If you consider using indexable tooling, the normal thread-cutting tools are quite wide, bringing you back to the same approach problem.

[not done it yetParticipant @notdoneityet]

Don’t forget that concentricity will not be retained if using a 3 jaw chuck.  It may not be far off, but if an old or cheap chuck it could be significant on a fine thread if close to finishing size.

Better, if possible, to check the fit by removing the chuck for that purpose.  That should allow simple replacement with no faffing about.

[JasonBModerator @jasonb]

make a small hexagonal bush with a shoulder to hold the stock. You can then remove the bush and stock to test fit and then provided you mark the bush so it goes back in the same way it will be the correct distance from the chuck due to the shoulder acting as a stop and the correct rotation as the hex will index it so the thread will not need picking up

[bernard towersParticipant @bernardtowers37738]

You can reset the tool by engaging threading and moving along the thread (taking up the slack) and the loosening the toolkit screws and pushing the tool into the part cut threads and tighten screws. This has worked for me several times on similar awkward jobs. Best of luck.

[larry phelan 1Participant @larryphelan1]

Some very helpful information there [as usual ]. I have come across this problem many times and sometimes I can pick up right away, other times more hit and miss.

It,s all about using the top slide to position the tool into the thread while the clasp nuts are engaged, not always easy to do if the thread is a fine one and your eyesight is not great [ask me how I know ].  That idea from Jason sounds good, must try it sometime.

[Clive FosterParticipant @clivefoster55965]

Re-engaging a thread is a right pest at the best of times. On a baby size one like that it is essentially impossible to sufficient accuracy unless you are set-up for micro work with a suitable lathe and magnifying gear so you can see whats going on. I like the digital microscope idea discussed on another thread for helping visibility.

Jasons replaceable bush idea will work well but, like so many other precision things it needs some practice to get the technique reliably sorted. I’ve always reckoned three goes are needed before I have a technique “sufficiently different to me for practice to be needed” but similar to what I’ve done before sorted.

I used the Zero-to-Zero thread cutting method which both guarantees thread cutting to what you set on the machine but also permits minor adjustments with absolute repeatability. Given that its very, very easy to start with a length of rod turned down to the correct diameter and make a series of test pieces chopping off each failed variation before adjusting the machine settings and trying again. Once you have the correct settings on the machine the result is absolutely repeatable down to the limits of your skill and the machines repeatability.

The only disadvantage of Zero-to-Zero is that it does need adjustable dials on the slides for rapid, easy, use. But it can be done with fixed dials by writing down a table of positions.

Short version of the technique starting with workpiece turned to size :-

1) Set top-slide to just under the thread half angle, I use 25° for everything.

2) Set tool perpendicular to workpiece.

3) Run cross slide forward until tool touches workpiece.

4) Set both dials to zero, or note positions if you have fixed dials.

5) Pull tool back a bit so it doesn’t scratch the job and move the carriage sideways past the end of the workpiece.

6) Run cross slide forwards from your start position, zero or noted reading, to the depth of the thread you wish to cut.

7) Re-set cross slide dial to zero or note new position with fixed dials.

8) Pull back top slide to clear workpiece and move up to start position.

9) Apply appropriate cut to topslide and make first cut.

10) Pull back cross slide, move saddle clear of workpiece and move cross slide back to zero on the dial or your noted fixed dial position.

11) Apply next cut to topslide

12) Rinse and repeat until both slide positions read zero, or your noted fixed dial positions.

13) Work out spring cuts without adjusting slides.

14) When spring cuts no longer remove material job is done and the thread you have is the thread you set.

Which may or may not be right.

If the fit is poor adjust the finish position on the topslide to suit. Either in if tight or out if loose and try again.

If I’m doing a batch I’ll get the first test piece right, so the topslide dial records the changes I’m making, then adjust the zero position on my topslide dial. All the rest done Zero-to-Zero will be as close to identical as can be measured in a home shop.

The explanation reads complicated but it’s fast to do and reliable.

As has been said earlier the common problem with home ground single point tools is incorrect tip shape. Getting the tip just so is hard without more gear than most of use have. Zero-to-Zero easily lets you compensate. With a full form insert the results are dead nuts straight off.

But that’s basically cheating for folk like us.

There is no special magic about Zero-to-Zero. The whole point is that “what you get is what you set”. The technique  merely makes it easy to introduce known minor changes until what you have set actually gives you what you want. Doing your thinking and setting before you start cutting makes life easier and less stressful.

Skilled folk can do the same thing on the fly with random dial settings, even down to doing it all on the cross-slide, but its easy to loose track. I can do it on the fly if seriously in the mood but prefer the easy life with Zero-to-Zero.

Clive

[FulmenParticipant @fulmen]

Just make a few test pieces from scraps until you get the desired fit, then make the part.

[Dave SParticipant @daves59043]

Why do people not just move the topside until the tool is back in the thread?

Thats how I do it on the occasions when I have to take the part to the female thread.

I tend to leave the workpiece in the chuck and remove the chuck from the lathe.

Dave

[Clive FosterParticipant @clivefoster55965]

Dave

Moving the topslide to regain registration is a reasonable technique with a decent size thread. Not so easy with a very small one such as that discussed by the OP. However carefully you are there will always be a small longitudinal error. Hopefully small enough that the extra cut(s) needed for a good fit will remove it. In general the absolute size of the error will be pretty constant depending on operator skill, machine quality and state of machine adjustment. But the smaller the thread the greater that minimum error will be in relation to the thread form. So for small threads the adjusting cut may not wipe it out.

Adjusting teh topslide to regain registration is another skill to learn.

A major reason for my advocacy of the the Zero-to-Zero method is that it is pretty much a universal technique. Accurately adjusting teh cut by measured amounts is an integral capability. Whether trimming one piece to size or making several test pieces of known variation the method cares not a jot.

The only thing it doesn’t do well is trimming an ACME or Square thread to as close to zero lash clearance in the nut as can be managed.

Clive

[egaParticipant @ega]

When examining the tool/workpiece thread engagement it helps to use a lens and position, say, a piece of white card behind the work.

With something like this I would first make one or more test pieces, carefully noting the settings for each.

Apologies if this has been mentioned above

[duncan webster 1Participant @duncanwebster1]

Geo Thomas details how to grind a flat on the tip of a tool to get a nearly full fit tool. It has a flat bottom instead of round, so not as good in fatigue, but I doubt that will worry most of us. Metric threads have flat bottoms.

Only problem is you need some way of accurately taking a little bit off the tip of the tool. Quorn owners will have no difficulty, bench grinder chaps like me will

[duncan webster 1]

On duncan webster 1 Said:

Geo Thomas details how to grind a flat on the tip of a tool to get a nearly full fit tool. It has a flat bottom instead of round, so not as good in fatigue, but I doubt that will worry most of us. Metric threads have flat bottoms.

Only problem is you need some way of accurately taking a little bit off the tip of the tool. Quorn owners will have no difficulty, bench grinder chaps like me will

Rub it on a bench oil stone rather than grind it to make the small flat.

[JasonBModerator @jasonb]

Looking at the numbers in the first post it does seem a bit shallow with a core diameter 0.020″ less than major.

If you take something close like 7/32″ x 32ME then the core is 0.040″ smaller so you would expect a bit more for a coarser 30tpi thread say 0.042″.

Given the OP says it has a radius to the vally, then there will be about 8 or 9 thou of flank to try and pick up on and a very flat crest.

 

[alecs]

 

1. On alecs Said:

   On duncan webster 1 Said:

   Geo Thomas details how to grind a flat on the tip of a tool to get a nearly full fit tool. It has a flat bottom instead of round, so not as good in fatigue, but I doubt that will worry most of us. Metric threads have flat bottoms.

   Only problem is you need some way of accurately taking a little bit off the tip of the tool. Quorn owners will have no difficulty, bench grinder chaps like me will

   Rub it on a bench oil stone rather than grind it to make the small flat.

   Define ‘small”. GHT quotes it in thous,  need something like a small slide with a screw feed

[alecsParticipant @alecs]

Thats where the craftsmanship comes in. Something that seems to have been more common in GHT’s day than in pur own digital era.</p>
All a bit moot anyway, as threads generally should be made a tad undersized on the OD and oversize on the ID so there is clearance between roots and crests. Thus the exact radius or flat width is not critical.  The main benefit to flatting the point of the tool is so it doesn’t snap off in use. I always fo it on an oilstone and it has always worked for me.

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