Back plate modification

[Martin of WickParticipant @martinofwick]

I have a lathe with spindle nose dimensions of 1 7/8 inch 8TPI (why 1 7/8 inch….? even in 1950 trying to tie purchasers to proprietary accessories was a broken business model).

I also have a number of backplates machined 1 3/4 x8 TPI that I think could me modified to suit. There is sufficient depth to the plates to accommodate the nose length and sufficient on the collar thickness to accept the increased 1/8 inch of diameter bore for the register.

Simple enough to centre and re-bore the register, threads seem a bit of problem to me.

What is the best way to re-machine the threads? Presumably the tool point would have to be located at precisely thread dead centre at the point of positive drive to exclude all backlash. Does anyone know how this could be achieved reliably?

Assuming the tool can be repositioned correctly, is it then just screw cutting as normal? (calculating tool advance from the difference between parameters for 1 3/4 and 1 7/8 8TPI)

Thanks.

 

[bernard towersParticipant @bernardtowers37738]

I did write in a previous post of a method of picking up threads if I remember correctly there were several choices and the thread was titled ER 40 collet chuck from memory, perhaps michael can help find it, he’s good at that sort of thing

[JasonBModerator @jasonb]

mentioned only last week

[Martin of WickParticipant @martinofwick]

Thanks for that Jason…..

Hmmm….looks doable, but couple of assumptions in that approach, mainly use of pre-formed carbide tips and that the tip shape exactly matches the tip shape of the thread cut in the backplate. Also need to rig something up to ensure tool/ toolholder is perfectly parallel.

I assume it is possible to get W8TPI inserts with small toolholders to fit, grind say 15 to 20 thou off the tool tip and hope it is close enough to the previous threads.

 

[JasonBModerator @jasonb]

You could actually be a bit out with picking up the thread, draw it out and see how much by, I expect 10thou either way won’t matter.

Just put a dti on the tool holder and run the carriage up and down the lathe as an easy way to get it straight. HSS will do just as well as carbide. You can get a 16NR W8 which is not that big. I’d leave the tool tip alone so as not to risk the crest of the spindle thread fouling

[Andrew CrowParticipant @andrewcrow91475]

Hi Martin, I am making the assumption that your lathe can be set to cut 8 tpi.

Bore your back plates to give a truncated thread form at this pitch at least 20 thou per side and with an internal undercut if possible also I would suggest you make a thread plug gauge for the size you require much easier for checking the finished thread (always useful for future projects).

Set up your screwcutting tool on centre etc.

What I do then is set the top slide to 27 1/2 degrees (assuming it is whitworth form) but with the hand wheel at the back of the lathe engage the leadscrew and wind the spindle forwards by hand to remove any backlash until you are just about to enter the bore.Then by adjusting the top slide and cross slide until the tool tip is in line with the existing remains of the thread, preferably just to the rear, this then ensures that you remove any mismatch when you feed with the compound slide (top slide)

Set the scales to zero and start cutting the thread nice and slowly as with the coarse pitch it will move down the bore very quickly.

On my Myford I tend to use the mandrel handle to re-wind for the next cut (sometimes for cutting as well) but if you’re cutting imperial threads on an imperial machine you can of course disengage the leadscrew.

Best of luck,

Andy

[JasonBModerator @jasonb]

Ignoring any rounding you could be 0.82mm  (30 thou) either way and still be cutting solid metal.

 

[old martParticipant @oldmart]

A very easy mod because the thread pitch is the same. There should be sufficient thickness in the rear boss to allow a small increase. Check the angle of the threads on the lathe spindle first, and hold the backplates backwards so you can bore the register and the face which contacts the shoulder on the spindle in the same setup. Commonly, the register bore would be the nominal thread size or about 0.002″ higher, and the threads are cut until they just show on that diameter. After setting the pitch up and removing all the backlash, you can line up the threading tool in the original thread before clamping it in the toolpost. The fit of the register will not make any difference to the repeatability of the backplate fitting, I have some that are 0.020″ loose without any trouble, it is the self centralising of the thread and the rear face of the backplate which matter.

If in doubt about things, just turn the spindle by hand, there is not much metal to take out between 1 3/4 and 1 7/8, and safer if you forget some aspect of threading.

I have got er16 internal laydown inserts in 8 W and the holder looks rather like a boring bar. There is a huge ammount of room inside a 1 3/4″ thread. I had to go bigger to er22 for a 6tpi thread size.

As for tool advance, the thread is almost finished before you start, so if you start by just rubbing and zero the cross slide, then proceed with the first few passes at about 0.003″ cuts, getting less as you get bigger. You only have to take a total of 0.064″ out altogether. But do not forget to back off right out of the thread each time you withdraw for another cut.

[Chris CrewParticipant @chriscrew66644]

I always cut internal threads on the rear face of the bore with an inverted tool. The advantage being that you can still use the set-over top-slide method but you still withdraw the tool at the end of the cut by turning the cross-slide handle anti-clockwise just as you would when cutting an external thread. It’s so easy to momentarily forget which way to turn the cross-slide hand-wheel when retracting the tool with the saddle. Also, to a certain extent you, get a better view as to what is happening in the the bore when the tool starts to cut.

[Chris Crew]

On Chris Crew Said:

I always cut internal threads on the rear face of the bore with an inverted tool. The advantage being that you can still use the set-over top-slide method but you still withdraw the tool at the end of the cut by turning the cross-slide handle anti-clockwise just as you would when cutting an external thread. It’s so easy to momentarily forget which way to turn the cross-slide hand-wheel when retracting the tool with the saddle. Also, to a certain extent you, get a better view as to what is happening in the the bore when the tool starts to cut.

Good suggestion Chris, I should have mentioned that, as in hindsight its how I usually do jobs like this. Got carried away with trying to describe the whole process.

Andy

[Martin of WickParticipant @martinofwick]

Thanks gents,

Some very good advice and info. I was wondering if I put a bag of ice on my head and dug out my trig tables whether I could calculate how precise the pickup needed to be to avoid cutting a bastard thread, so thanks Jason.

So the plan I have is :

• Purchase or grind tool!
• Mount the work piece on parallels on the face-plate, spigot out (so there is room for the tool to run out towards the faceplate)
• Centre and re-bore register, checked to 1.875 in plug
• Engage leadscrew and pick up threads as suggested (having made a decision on whether to cut back or front side)
• Install spindle crank for cutting (it is only 4 or 5 threads after all)
• Cut…….

I still need a bit of help here. All backplates have been cut with a true Whit form (rounded or squared off troughs and crests). If the cutting tool was a matching whit form then the total theoretical tool advance would be 0.063 to get to 1 7/8 in diameter. All the inserts I have seen commonly available for up to 8TPI are  partial form (ie have a sharp point rather than the full form.) I suppose I could try to grind an accurate form tool, but I know my limits!

So if I have to use a sharp tool, would the method be to touch off in the trough as  picked up (accurately we hope),  then incrementally advance the pointy tool 15-20 thou. or whatever, until the flanks are just engaged? (might use some markup blue for this).  Once the flanks are contacted, do I then expect to advance the tool a further 63-64 thou in say, 3 thou increments to finished diameter (using the 1 7/8 spindle to check fit as size is approached)?

Sorry for the questions, but I have never modified a backplate in this way and it is usually a fine margin between success and scrap.

[RobCoxParticipant @robcox]

Make yourself a replica of the spindle nose thread before you start. Use thread wires or a thread mic ideally to match the pitch diameter, failing that use a faceplate or other chuck backplate to match the feel of the thread engagement. This replica can then be used to gauge when you’ve turned your new backplate thread to the right size.

[Martin of Wick]

On Martin of Wick Said:

I have never modified a backplate in this way and it is usually a fine margin between success and scrap.

Think through what margin is available to you. It is not quite so fine as you might think (unless you are a woodworker who thinks 1/16″ is ‘fine’).

If you are bolting the faceplate to be modified to an existing one, it is easy to remove the two and try the new thread on the spindle after every cutting pass if necessary. Being 8 tpi, it is unlikely you wil have any issues with thread pick up, meaning you can disengage the half nuts and drive the carriage back towards the tailstock to give plenty of space.

When you try the faceplate, it will either fit or it will be too tight. Assume when you try it, it does not quite go on (i.e. exactly zero clearance). Further assume you then advance the tool 0.010″ for the next cut. That makes a maximum clearance of 20 thou. which is not unreasonable. Backplate threads do not need to be tight as the register assures concentricity. The threads only provide axial restraint, and loose threads will do this just as well as tight threads.

You could gauge the inside of the new thread with a pair of calipers (modify the legs with a file if needed) and compare to a chuck that fits the lathe. The amount of rattle when comparing old with new will tell you how close you are. When the rattle is as small as you can detect by this method, then you can remove the faceplate sandwich and try on the spindle itself.

You are not cutting to a measurement, but cutting to a fit and that is by definition an iterative process so numbers do not help very much.

Rotagrip will sell you a 16IR 8W full form insert (https://www.ebay.co.uk/itm/292444606720). Sometimes 16IR are alternatively described as 3IR.

[Ian PParticipant @ianp]

By coincidence I’ve just read this thread after packing for posting tomorrow a part in which I have screwcut an internal 60mm x 0.70mm pitch thread. The thread is to accept the male ends of a lens ‘Extension Tube Set’ which AIUI is useful for close up and macro photography.

I bored a 59mm hole in an ali plate and used on of the rings as a gauge testing for fit frequently as I fed the tool in stages. As is my normal method I do not disengage the half nuts but reverse out for the next cut. Once I had got a satisfactory fit I did disengage and moved the carriage out of the way to slightly chamfer the thread start. Unfortunately I then discovered that two of the five rings would not fit the ‘nut’ I had just created, presumably because of loose manufacturing tolerance.

I needed to slightly increase the thread I had just screwcut and with a magnifying glass was able to reposition the tool tip in the existing thread (using the top slide for fine adjustment) and once satisfied could then take off a few thou and get the larger rings to fit.

I have easily picked up large diameter 2.5mm pitch threads before but I expected this 0.7mm to be a challenge, the only hard bit is due to aging eyes.

Ian P

 

 

[Andrew CrowParticipant @andrewcrow91475]

Hi Martin,

Before starting on the actual back plate make a test piece that is like the spindle nose of your lathe including the register. I’m assuming you have a chuck with the backplate which already fits your lathe which you can use to test your external thread.

Reduce the thread outside diameter by about 25 to 30 thou as that will avoid any clashes between the top of your thread and the root radius in the chuck thread.

You can then use the test piece to check your modified backplate. So then you don’t need to do any calculations and by including the register on your test piece it will ensure that it will fit your spindle nose when finished.

Also remember to make the bore diameter of your internal thread at least 30 thou bigger than nominal, again to avoid root radius/crest interference.

Andy

[JasonBModerator @jasonb]

Re the 0.82mm I mentioned, that is based on cutting straight which is what I do, angling the topslide will need you to get closer to the right position so the back of the tool runs down the slope of the old thread

[Martin of WickParticipant @martinofwick]

Thanks, my inclination was to cut with parallel top slide and tool holder advancing with the cross slide only to be able to see that the tool is cutting evenly off both flanks of the existing thread (or as evenly as I can get).

DC3, thanks for the W8 tip vendors, ‘spensive aren’t they! Still, I expect one insert will cut all the backplates I will ever need at that size.

Since I have the spindle out of the subject lathe for bearing replacement, I will use that for trial fitting. Probably didn’t make it clear I will be using another lathe for the work.

[HopperParticipant @hopper]

BEFORE you even start screwcutting, you need to bore the inside diameter of the existing threaded hole out to the inside diameter of the new thread, plus about five thou clearance. If you don’t do this and try to just enlarge the existing thread, you may well end up with grief. The thread may well bind on the tips and give a false fit reading. Then you keep taking cuts until all of a sudden, the thread is a too slack fit.

The ID of your new 1-7/8″ x 8TPI thread can be determined in three ways.

1. Measure the size of the hole up the middle of your existing 1-7/8 faceplate. Easier said than done on a threaded hole and is assuming the old thread is correct.

2. Subtract the thread depth x 2 from 1.875. Your thread depth formula for BSW is .64 x Pitch. That is .64 x .125 = .080.  Double that is .160″. So 1.875 minus .160 = 1.715. BUT, I would bore it out to 1.725 to provide five thou essential thread crest clearance each side.

3. Calculate the ID bore using the formula OD minus 1.2/TPI which is 1.875 – 1.2/8, = 1.875 – .150 = 1.725. Exactly the same as above! Fancy that. (This formula comes from Martin Cleeve’s book Screwcutting in the Lathe. )

Once you have bored the existing threaded hole out to 1.725″ diameter, then you can start screwcutting. As only about a third or so of the original thread will be left after boring out the ID, your job will be so much easier. You will not be cutting full length along the tool cutting edges right from the first cut.

You will need to set your tool bit to match the remains of the old thread, but alignment as you can see from the partial thread that is left is not super critical. As long as it is close enough, all will be well.

Then you will need to touch off your tool bit on the freshly bored ID and start working on your thread depth from there. Finished depth should be THEORETICALLY the calculated .080″. But as Yogi Berra said, theory is the same as practice in theory. In practice it ain’t. Factors including the extra clearance we bored in the ID hole, radius (or lack of) on the cutting tool tip and boring bar spring come into play, as well as slop and flex in the machine.

So when you get down to about .070″ thread depth, or even a bit before, you will need to start the “try and fit” process, checking how your male thread fits into the female thread being machined and take light cuts until it fits. Make sure to remove burrs from the crest of the new thread with a file etc before trying the fit.

 

[Ian P]

On Ian P Said:

 

I have easily picked up large diameter 2.5mm pitch threads before but I expected this 0.7mm to be a challenge, the only hard bit is due to aging eyes.

Ian P

 

 

The trick is, and I think someone mentioned this above, loosen off the clamping bolts so the tool is loose in the toolpost. Set tip of tool into existing thread groove. Tighten up clamping bolts.

[Martin of Wick]

On Martin of Wick Said:

..‘spensive aren’t they!

Or £8.66 for a pack of ten, including Express post, from a man called Ali.

If you ever need anything unusual, it is worth downloading Posithread’s catalogue. If they do not make it, it probably is not made. The challenge is finding someone whoi will sell them in singles.

https://posithread.co.uk/threadturningcatalogue.pdf

[Martin of Wick]

On Martin of Wick Said:

Thanks, my inclination was to cut with parallel top slide and tool holder advancing with the cross slide only to be able to see that the tool is cutting evenly off both flanks of the existing thread (or as evenly as I can get).

DC3, thanks for the W8 tip vendors, ‘spensive aren’t they! Still, I expect one insert will cut all the backplates I will ever need at that size.

Since I have the spindle out of the subject lathe for bearing replacement, I will use that for trial fitting. Probably didn’t make it clear I will be using another lathe for the work.

Hello again Martin, the only problem with cutting on both flanks is that the swarf can jam as it’s coming off both sides of the tool which may cause a rough finish particularly as it is quite a deep cut. I wouldn’t recommend it for self ground HSS tools, you may well be OK with modern carbide tips as they are designed to cut this way on production cnc machines.

Andy

[JasonBModerator @jasonb]

You can wind the topslide a few thou either way on alternate passes then you are only cutting on one side. See third image down incremental feed

[old martParticipant @oldmart]

https://www.shop-apt.co.uk/snr-internal-threading-bars-apt/snr-0016-m16-internal-threading-bar.htmlAPT have the exact tooling you need as long as your lathe can run 8tpi and hold a 16mm diameter internal threading bar. One insert,16NR 8W T350 @£4 and an SNR 0016 M16 internal threading bar (looks similar to a boring bar) @ £ 22. These prices are ex vat and postage.

Threading a backplate is easy as the threads are short and a through hole. Start by boring the register and then a facing skim before picking up the thread which is already part finished. Copy the register diameter and depth from your existing 1 7/8 backplate.

[HopperParticipant @hopper]

If you advance the topslide by half the amount you advance the cross slide, it will , by happy coincidence, move the tool in a 26.5 degree path, just perfect for screwcutting either 55 or 60 degree threads. It will cut mostly on the one side with the second side making a very light scraping cut, which eliminates any step marks you would get if using exactly half the thread angle.

 

[old martParticipant @oldmart]

I have more than once got away with unscrewing the chuck or faceplate holding the backplate being threaded, turning it round and using the spindle as a thread gauge. It will screw back on if another cut is needed. Dont be tempted to make the fits too close with a screw thread backplate, I tried that once and that one is a pig to fit and remove. I never rotate the compound when threading, that might be helpful with very lightweight lathes, but cast iron backplates are easy cutting, just the dust is a bother. The thread depth is simpler to calculate, just 0.080″ or 2.03mm

I did the calculations of the thread core diameter working from several Whitworth and BSF 8tpi sizes and got between 1.715″ and 1.741″.

What is your lathe, by the way?

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