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Removing and re-chucking a part whilst thread cutting on Myford Super 7

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C J02/09/2018 12:17:50
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I recently made the pictured screwed cap on my Super 7 and had some difficulty in getting the thread diameter right.

I have a DRO fitted to my lathe so I can add on more cut by known increments and to avoid un-chucking the part I initially backed off the saddle enough (with the feed still engaged) to remove the chuck containing the cap in order to try it in it's destined hole but the thread was binding and having the part in the chuck made the handling unwieldy, and after two goes at doing this I thought there must be a better way.

At the third attempt I just removed the part from the chuck, as the thread was still binding in it's destined hole, and then put the part back in the chuck, hand cranked the saddle towards the chuck until the cutting tool was positioned approximately half way along the thread, but away from and not engaged in the thread, and then adjusted the fit of the cutting tool into the thread using a combination of the slide (positioned in line with the spindle) and the cross slide, and when it fitted withdrew the cutting tool reversed the lathe and then restarted cutting the thread

I encountered no problem using this technique other than having to ensure that the part runs true on re-chucking and finally achieved a working fit for the thread after two more cycles of removing and re-chucking the cap!

 

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Edited By C J on 02/09/2018 12:31:55

Edited By C J on 02/09/2018 12:32:35

Edited By C J on 02/09/2018 12:33:42

Edited By C J on 02/09/2018 12:36:22

C J02/09/2018 14:58:49
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Correcton “..... and adjusted the fit of the cutting tool into the thread using a combination of the top slide... “

thaiguzzi02/09/2018 15:01:17
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Nice. M/C inspection cap?

C J02/09/2018 15:06:03
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Well spotted, yes a motorcycle inspection cap, in the pursuit of lightness in magnesium!

C J02/09/2018 15:06:04
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Addenda

Fitting the cutting tool into the thread in approximately the middle of a length of cut thread is far easier than trying to find the position in which the thread starts and once fitted and then the cutting tool withdrawn the fit is of course saved for the length of the thread after you reverse the lathe, take the backlash up, and then start cutting again. 

 

Edited By C J on 02/09/2018 15:06:54

Edited By C J on 02/09/2018 15:21:29

Edited By C J on 02/09/2018 15:24:34

Edited By C J on 02/09/2018 15:26:21

Edited By C J on 02/09/2018 15:30:01

Brian Wood02/09/2018 18:41:49
1942 forum posts
37 photos

Hello CJ,

I have done this sort of picking up on a part cut thread by combined movements as you describe many times and it is easier than one might think too.

Regards

Brian

Neil Wyatt02/09/2018 20:35:40
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I always run the tool under power for a bit to take up any backlash before using the topslide to line it up.

Neil

not done it yet02/09/2018 22:08:40
3246 forum posts
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Clearly not ‘best practice’, but when ‘needs must’ many of us have done it! Even Doubleboost has succumbed on occasions - not removed from the chuck, but having to make a new cutter half way through an internal threading job on one of his u-toob videos.

Personally, on a job like that I think I would make a plain thread to get the fit and then make the item. Not too bad with a coarse thread, as yours, but I would not relish rechucking with a very fine thread.

Marcus Bowman02/09/2018 22:53:59
161 forum posts

That's quite a short thread, but it looks as though you have 3 fully formed turns, so I would use thread wires to measure the pitch diameter, because that's the crucial dimension for fit. You do need a minimum of 3 threads to be able to position the wires. With care, that method should avoid the need for re-chucking. You should, in any case, be able to get a good indication of the required depth of cut by consulting tables. That might not get you exactly there, but you should get a good indication of depth, so that you can judge when to use the thread wires to work out the final cut(s). Thread wires are a bit fiddly to position, but worth the effort for a thread like this.

I guess the thread is too large for you to have a tap so that you can make a female thread to act as a gauge.

With that short a right-hand thread, I would turn it from the flange outwards, to minimise the need for a groove at the inner end of the thread. If you use the tool right way up, mounted at the rear of the thread; reverse the rotation, and feed the tool towards the tailstock, life becomes much less stressful. Grind the tool so that the point is close to the chuck end of the tool when it is in its mounted position, and you should be able to start it right up against the flange. Let it cut a vee groove for clearance, before engaging the half-nuts.

I like the nice crisp slot.

Marcus

not done it yet03/09/2018 07:34:56
3246 forum posts
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Of course, the re-chucking (the really naughty bit) could be avoided by cutting the threads between centres so that alignment is not a problem, leaving only adjusting the thread starting point as the much less of a problem, even for finer threads.

Nick Hulme16/09/2018 15:23:16
698 forum posts
37 photos
Posted by Marcus Bowman on 02/09/2018 22:53:59:

I guess the thread is too large for you to have a tap so that you can make a female thread to act as a gauge.

Or try it in the only part it's ever going to be screwed into?
Sometimes the simple solution is the solution

chris stephens16/09/2018 15:45:05
1045 forum posts
1 photos

For fine adjustment of threads you might find a hand held thread chaser of some benefit. No need for realigning part with a leadscrew or even running super true in the chuck.As another benefit you can also get correct thread form on rounded Whitworth threads. Do Tracy Tools still do their chaser sets?

chriStephens

Do

Neil Lickfold17/09/2018 07:22:18
562 forum posts
102 photos

Another way is to use a collet chuck, that has a radial index, and is placed in the same z position.The collet with the work piece can be removed, tried on the part, returned and carry on with the thread cut.On a 25mm shank holder, if you get it to a radial position with in 0.1mm, the pitch position error is negligible.

Neil L.

Clive Foster17/09/2018 09:22:37
1807 forum posts
59 photos

Standard way to deal with this sort of dilemma is to leave the half nuts engaged and work with the topside set the parallel to the bed. Exact replacement position becomes a non issue as the topside provides the necessary fine adjustment to give proper alignment between thread and tool.

As Neil says its important to use the threading feed to run up to the approximate position to take out backlash.

For final adjustment I prefer to cut on the front edge of the tool as its easier to set up a very fine skim.

Can be a pain in the backside keeping track of cuts. Best way is to start by touching the tool off the workpiece.

Set the cross-slide dials to zero.

Crank the saddle clear of the workpiece and run the cross-slide in by the book thread depth (plus an allowance for tool tip shape if yours is too pointy).

Set the cross-slide dial back to zero.

Cut thread as per normal using only cross-slide feeds. When the dial gets to zero thread should be the calculated book depth.

Remove, test fit and adjust as needed.

I normally use the zero2zero threading method where the lathe self generates the required in-feed with the topside set at close to half the thread angle so this is a simple modification to my standard process.

Got me a left hand 26 TIP thread wheel bearing retaining ring in the "Please fix this for me" box that will need this approach. On machine trial fits are a bit impractical with a 19" diameter motorcycle wheel. With a simple job like that its easiest to make a test piece then revert to the zero2zero method for proper job as the final cross slide dial reading give the extra in-feed needed beyond the original setting so the proper depth can be used.

Clive.

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