Technique Required to Make Accurate Backplate

Good evening.

I am trying to make a back-plate for a lathe collet chuck and would like to ask how best to go about it. I am part way through doing so, but am not convinced that it is going to be accurate.

I drilled and threaded a disc of mild steel and spun it firmly onto the lathe spindle. I then faced it off to make it run true. Next, I cut a register and checked that it was a good, snug fit.

Next, I screwed the back-plate onto the spindle and started to face off the working surface. I then checked it with a DTI and confirmed that it had no run-out.

Next, I removed it from the spindle to clean it up and get ready to cut a register for the collet chuck, but when I put it back onto the lathe, it was running out of true a little.

I removed it again, cleaned it and the spindle thoroughly, replaced it and faced it again.

Removing it again and replacing it, it was out of true again.

Am I doing something fundamentally wrong?

Regards,

James

Not really a screw on chuck man myself but I would have said you should have drilled undersize then bored to tapping size, thread cut and then cut the reegister and faced all at one setting on the lathe.

The way you have done it the register may not be totally true to the thread.

Edited By JasonB on 26/08/2016 19:59:34

Silly question but when you cut the thread on the disc have you put a small undercut on the back end of the thread to allow the disc / spindle faces to mate up. Sometimes the run off at the end of the thread may leave the tip of the thread bent over preventing the faces mating up?

George.

I always make the thread a little loose to make sure that it seats on the registers with no interference from the thread. Did you make the thread too tight perhaps?

Russell

Did you turn a recess into the back of the backplate to mate up, size for size, with the plain spigot on the lathe spindle behind the thread? It is this collar/register that locates the chuck in position, not the thread. If you only threaded the backplate it will not always seat on the thread in the same position, necessarily.

From Lathes.co.uk

Thank you for the replies.

Jason: I suspect that this may be the problem. Unfortunately, though, I could not do the threading on the lathe. It does not have screw cutting and the back plate is too large for my chucks. It is a small Flexispeed lathe.

George: not a silly question and, no, I did not. I shall give it a try.

Russell: I used a tap, so am rather limited in how I can adjust the thread, but can try running it through several more times to try and make it looser, if possible.

Hopper: Thank you for the drawing, which is useful. Yes, I did cut a register.

I shall have another look tomorrow.

Regards,

James.

Frances.

I suspect that I might end up looking for someone with a larger lathe to do the job for me, or let me use their machine. I do have access to one, but it is quite some miles away and can only be used it the evenings. Also, as it is used by a business, I cannot really leave work in it overnight.

I like your second idea, but do not have a second back-plate, unfortunately.

I shall persevere.

Regards,

JAmes.

Threads cannot be used for concentricity, as George said he always makes his loose. Turn the spindle side of the plate first including part of the outside diameter if you cannot turn it all, including the thread. Mount on the spindle, ensure the fit on the spindle is good enough not to give a concentricity error, check on the finished diameters. Then finish the chuck mount face and diameter. You can only have one location diameter on a short piece, the thread is there to hold it in place nothing else.

Howard

I have another go at my back-plate and have got it to be as good as I think I can.

In the end, I ran the tap all of the way through the thread a few times and them ran a reamer through to take the tops of from the threads, cleaning it up again with the tap.

I then faced off the plate and cut the register for the collet chuck. There was no run-out when tested with a DTI immediately after the facing cut. I then removed the plate, cleaned it and span it back on. This time, there was about 2.5 thou run-out. Repeating this several times, I get between 2.5 and 0 thou run out.

I now just need to drill and thread the retaining bolts.

James.

In that case I would guess you have made the backplate ID rather larger than the spindle OD. Nowt much you can do about that now.

I looked up the dims for my lathe, which has two registers - one at the front of the spindle nose and the one like yours. The tolerance is zero thou on the rear and one thou at the nose. Generally, or subsequently, the front one has been demonstrated as unnecessary and makes no, or insignificant, difference to the operational run out.

After cutting the thread on backplates I have made I mounted the plate backwards on the spindle thread and skimmed the back face. This makes sure the back face does not have uneven contact that may cause runout if the backplate is screwed on more sometimes than others. Have you done this?

Martin

Uneven contact will only occur with a non flat face. I still would prefer to cut the spindle mating elements in one setting. The face contact area can be reduced to remove any chance of a convex face being created.

Howard

You can have swash on the back face even if it is flat. Since the thread in this case was cut with a tap not single point threading there is a chance the thread is not normal to the machined back face. Putting the plate onto the spindle reversed then skimming the back face ensures the thread is normal to the back face and eliminates the risk of swash. If there is swash then there is a possibility that the runout will vary depending on where the backplate is pulled up to.

Martin

Thank you for the replies.

I have decided to scrap this version of my back plate and try again. I made the front register perfectly for the collet chuck to fit onto, but having to test and replace the back plate each time to ensure that it is running true is not good. I suspect that my original snug-fitting register was, in fact, a tight thread. Now that the thread is looser, the whole thing seems to be slack. Instead, I am going to try and re-machine the plate.

Subject to careful machining, does the following sound like a method that should result in a good plate, bearing in mind that I have just the Flexispeed and no screw cutting?

1. Turn a stub of steel to be a snug fit into the register of the original Flexispeed chuck.

2. Screw the back plate that I have just made onto the spindle using the existing wobbly register to fit it.

3. Machine the face flat.

4. Cut a new register, using the plug gauge to check size.

5. Cut a slight angle on the inside of the register to ensure that there are no burrs or raised edges from the thread.

6. Turn plate around and mount using fresh register.

6. Machine face flat.

7. Cut new register for the collet chuck.

Should this work?

Regards,

James.

I'm surprised no one has mentioned that the spindle fitting aspects all need to be machined in one setting.

If the tap has a centre hole the tail stock centre can be used to keep it square, just the same as when the face of the tails tock quill is used to keep a die square. A bit of a pain as the tail stock quill has to be withdrawn each time the tap has done a part turn cut and then wound back to break the chip. I have been known to use a spanner to turn the tap but in that case the tail stock centre must be used to apply pressure so that the tap remains square on. Needs two hands. One on the spanner and the other on the tails stock feed wheel.

Sometimes work still runs true if the chuck holding it is removed from the lathe and then put back on. Easy to check and if so the tapping could be done off the lathe but the tap must be kept square. In training they used to make us check that every 1/3 of a turn with a small square. Once a tap is well in it tends to look after itself but the initial stages are crucial and easy to mess up. ( The end of the stock of a small toolmakers square - the blade was usually to long )

The initial hole in the blank might be drilled off the lathe. Fine but it needs to be bored when it is on the lathe. Same if it's drilled on the lathe.

The best way of finishing to size is to make a complete dummy spindle nose first. It needs to be precisely sized. Threads can be cut with a form tool but if done single point the sums have to be done to produce a flat topped thread with flanks in the correct position and the valleys too deep to account for the use of a pointed tool. It can be tried in another back plate. Ideally one that was made by the lathe manufacturer as they will have used a very precise fit. Measured the threaded portion of the spindle nose will probably be -0.002" max under size and the thread in the back plate very slightly over sized.

Normally the face plate blank would be mounted, faced, drilled, bored and screw cut to suit the dummy nose. Then the register cut. The threading will help keep the dummy nose square as it's screwed in to test the register. The usual answer to an accurate register fit is if it needs to be -0 +001" is to get close and then take 0.0005" cuts till it fits. I'd be hoping for a very slight interference fit which would mean very fine cuts. However if the flanks of the thread are very precise they will tend to centralise things when the back plate is screwed on.

Given no screw cutting I think I would still make a dummy spindle nose but one without the thread just turned down to what ever core diameter is going to be used for the tap and bore for that to fit first. Then tap and then cut the register. If the register is the same size as the thread or as they usually are very slightly smaller there will still be thread marks in the register when that is on size especially when a tap is used or a pointed tool. This can also happen when a form tool is used due to not aligning it precisely. The register is significantly larger on some lathes so that doesn't happen. The marks don't really cause any problems providing it is all accurately made.

There are some comments on making back plates here

**LINK**

They have changed since the last time I looked. Pity on some of them. As mentioned not using cast iron isn't a terribly good idea. It doesn't pick up and stick like mild steel and others can so a very close fitting register is probably not a good idea at all. The comments about the Boxford are interesting. The spindle thread nose is just over 0.0015" under the size of the register on mine. It is possible to make a back plate that appears as he describes and also be a perfect fit. More often there will be signs of the peak of the the thread in the register because the thread depth has had to be increased to account for flank angle errors or maybe the lathe turns a touch of taper. It doesn't take much of an error at all to cause this to happen.

John

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John,

Thank you for your detailed reply and I shall have a look at the link that you suggested.

I did try, during an early attempt, to tap something else held in the chuck whilst guiding the tap with the tail stock. The problem that I encountered was that the force involved in cutting a 1/2" thread overwhelmed the gripping power of the chuck unless it was tightened extremely firmly. Then, even with the back gear engaged, the spindle would turn and the belt would slip. There is no other way to lock the spindle on the lathe. For this reason, I have to do the tapping off from the lathe and then true up the plate on the machine.

I am going to turn a test spindle, unthreaded, as suggested, and try to rescue the existing plate. If that fails, then I shall probably look around to get the back plate made somewhere.

Regards,

James.

There may be another way of tapping in your lathe. Sometimes it's possible to hold larger taps in a drill chuck in the tail stock and rotate the chuck - shudder - using the chuck key as a lever. The tail stock is left free to move and can be held down and pushed by hand.

Usually it's not a good idea as tail stocks on some lathes are too heavy. It might work out well on a small light one.

The 4 jaw will hold parts far more firmly than a 3 jaw will.

John

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James,

May I suggest an alternative approach ?

It's a little convoluted, but should get you the right result without endangering the lathe:

Buy a commercially made nut to fit the thread on your spindle nose [we have already established that this does not need to be a super-precision fit] and turn the hexagon down to a cylinder ... You have now made a threaded insert.

Concentrate your efforts on getting the register bore, in the backplate, "just-right" and then bore a recess to accept the threaded insert, which can be fixed with Loctite 638.

MichaelG.

That might work if the nut is big enough to include the register. I suspect that the register might be the same dia as the thread or none existent. I don't accept the view that a very loose thread makes no difference either in practice if something needs to go on repeatedly with high accuracy. Also the manufacturers make the fit good enough to make sure that the register more or less lines up. A tap is very likely to provide a pretty loose fit anyway but at least the flank angles should be correct which will help. It's difficult to ensure that home cut thread flank angles are that correct even with a chaser 'cause it has to be set square. I've been wondering if the carbide tip types are better in that respect

There is a germ of an idea though. Say a piece of bar was drilled and threaded and then screwed onto the spindle. O/D trued up and the register cut in the end. If the lathe does have a register a washer would have to be turned up to space the piece of bar away from it.

There is still a problem though. Ok the piece of bar has been faced, bored and the o/d trued up. If the part is tapped off the lathe the thread may be slightly angled so wont be true to the faced end.

I have these problems as well. Fitting a Jacob's collet chuck. Making decent back plates wasn't too bad when I had a myford as I had a spare spindle. The dummy that came with my boxford and made on a cnc machine isn't too good. An RDG 2 morse Boxford adapter aught have been sent back. The register is 0.020 over size and the thread well under. I've heard of their pre made back plates being too tight as well so wont be risking that.

I did try making a Raglan back plate without a dummy nose as couldn't be bothered. I wont be doing that again if what ever it carries has to go back in the same place every time.

The chuck fitting by the way should be a rather light press fit when that part is done. That needs a very precise size on larger diameters. As the chuck fit on some mini lathe chucks is when they are bolted down on the plate.

John

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