New Chuck won’t screw on

Thank you for that but it’s way out of my capabilities at this time, I do believe the problem is the root of the thread and or the fact that the start is not fully formed. I did ask from which side the backplate is threaded from because I think the end I am trying to screw it on from is the finish of the thread and it has not been formed fully.

-Are you quite sure that the problem is with the thread? I agree that it is most likely the thread being tight, but I have seen situations where the problem was with the register on the backplate being a tinch (technical term) too small. In fact in the "old days" Myford used to supply chucks with registers which needed to be lightly scraped to suit individual spindles. Since it would be quick and easy to check by knocking up a gauge with exactly the same diameter as that of your spindle, why not try it, if only to rule this out as the problem?

Mike

Richard

Howard is absolutely right but it's possible to interpret his instructions as "The thread is a bit tight and if you can scrape it out a bit it will fit better"

The fit of the chuck, both alignment and concentricity, is determined by the register (The parallel part behind the thread ) and not by the thread itself. So the thread is only there to keep the register engaged and stop the chuck falling off.

So it does not matter much if the thread is a bit of a loose fit as it screws on, it will tignten up as the register engages.. So feel free to poke about and scrape about inside the backplate with whatever you have to see if you can remove a bit of metal and any burrs etc to enable the backplate to screw on. Try not to damage the register.

Failing that, return it to the supplier as not fit for purpose and see if that wakes them up.

Rod

Before jumping in & modifying a brand new item in the hope that it might work it would make more sense to find the cause of the problem first!

I would suggest good old fashioned marking blue to highlight exactly where its binding.

Surely it is better to have the chuck too tight than too slack. At least it can be made to fit. The fit should be on the register not the thread as has already been stated.

The Chuck would only go on half a thread, anyway I have sorted it no thanks to RDG who appear to have washed their hands of the problem. After reading Howard’s post it got me thinking as to wether in my dads box of taps and dies I had the relevant tap, but the nearest was 3/4 x12 tpi. So I rolled the chuck on the bench with the tap seating on one edge of the thread and after 1 1/2 hours managed to get the chuck to screw fully on. I had oiled the thread and this clean oil kept turning black, so I presume the root of the thread was full of Iron powder and the tap must have cleaned it out. Thank you very much for all the replies, nowto go and put a dti on it and check for any run out.

Thanks Richard

Your 12 tpi Tap probably took a little off the crest of the thread so that it longer interfered with the root of the thread on the Mandrel.

The main thing is that you have now managed to solve the problem.

Thanks for the feedback

Howard

Richard

Well done you!

Your situation was very much not your fault but your experience may act as a reminder to the rest of us about how important it is to keep the internal threads of chucks and similar fittings really clean.

Rod

Well done, you will find that in the real world, the fit is better when it is a bit on the loose side. You should remove the chuck from the backplate and screw the backplate on and off the spindle several times to settle it into place. Then check the runout of the front face outboard of the step and skim that part true if needed. Then refit the chuck.

Can't for the life of me see why machining the backplate on a Myford would predispose it to fit a Myford.

Then check the runout of the front face outboard of the step and skim that part true if needed. Then refit the chuck.

Steady on! You’re not suggesting that rdg would get that wrong as well? I am very selective when buying items, so don’t often buy from that source. I would guess that only s small proportion of parts are actually checked at the time of manufacture - and likely no further checks at all (unless a complaint arises? ).

The chuck fit certainly can’t be described as loose, in fact the thread will definitely be doing the locating. I did put a dti on the circumference of the chuck and found two high spots nearly 180 degrees opposite to each other, I am not sure that’s right,reading it it doesn’t make sense  I will check as you say when I have fastened the lathe back down as I am in the process of lifting it a couple of inches. I wear glasses and my focal point was making me bend down causing back pain.

Edited By Richard Jarvis on 25/07/2021 10:03:13

But the complaints fall on deaf ears they told me someone else had the same problem as well but when they tried the chuck back at RDG it was ok!. I have bought a few bits from them over the last 18 months but maybe not anymore.

Hi Richard,

Many years ago I had a similar problem with my then new ML 10. The actual register of the Grip-Tru chuck was too small, and that was down to Pratt & Burnerd, they were not cheap, even then.

As regards the supplier checking the thread against another machined part, then this is asking for trouble. Unless the "Inspection Parts" have been checked to see if the threads being used conform to the "Go" and "Not Go" sizes of a Screw Plug Gauge, then this approach is useless. Plus using the Myford spindle as a gauge is checking several conditions at once, and for one thing does nothing to actually check the true size of the Register diameter. The Register diameter on the Spindle, will always be smaller, it has to be to enter the hole. This hole could be 0.25mm or 0.010" oversize and the spindle will still go in. This size of Register would probably not do anything to help the concentricity though.

I suspect from what you have done with your 3/4" tap is restored the correct Crest Radius on the thread. No doubt the Tap, or screw-cutting insert used to produce this thread has become worn on this radius, and this is where the undersize condition actually was. This problem would have been found if a Pukka Screw Thread Gauge was used to check the part. As the "Go" gauge checks the form, pitch and size of the thread. The "Not Go" checks the maximum Effective diameter and looks like a severely Truncated thread, with the top half of the thread ground down to the Max effective diameter.

Lastly, you say you have put a clock on the Chuck. I have seen several posts where a clock has been put on the Body of the chuck. I for one would not expect this diameter to be "true", or "spot on". Nine times out of ten it has only been ground to make it look pretty. This is probably done on a centre-less grinder, and can be anywhere as regards concentricity, as its function is irrelevant to the working of the chuck

Gripping a ground diameter in the chuck jaws is the only safe test for concentricity, as this is where it matters. An old "Gudgeon" or "Wrist" Pin from an internal combustion engine is ideal for this, or a Hardened Dowel Pin.

I hope these notes are of help

Regards

Gray,

Edited By Graham Meek on 25/07/2021 11:43:49

Richard and others

I know how you feel.

I have a very eccentric 3 jaw chuck for my Myford. Thinking I would impove matters I bought a genuine new back plate. It did not fit the lathe so I just put it aside until I had the time and inclination to skim the internal thread with a single point tool.

The time came and I could not find the back plate. Therefore I bought another genuine one which, again, did not fit. At this point I put it all aside, feeling a fool and keeping my thoughts about the new Myford concern to myself.

Does anyone want a genuine Myford big bore back plate that will not fit, for free? When the virus has gone I, myself, might even delivery it for free!

JA

The parallel section of the register can only dictate the limit of concentricity as there must be a clearence. The Thread flanks however are contact faces and so will contre the chuck. If the threads are off and loose then the chuck will be off centre by the clearence of the paraleel part of the register. OK I know I'm splitting hairs and these limits are going to be smaller than the chuck run out anyway. The most important factor is the flange as this sets the centreline of the chuck parallel to the centreline of the spindle.

regards Martin

It is always a good idea to check the runout of the backplate when the opportunity arises, especially when it is only fitting properly for the first time. I would bet on the spindle threads being innocent of any faults unless there is visible damage. Unlike most of the experts, I have a choice of 12 items which screw on the threaded spindle of the museums Smart & Brown model A and the register fits are from 0.0005" to 0.020" and as if by magic, they all repeat perfect runout every time. Martin Kyte is one of the few people who understands this.

.

@ old mart,

I thought this was the evidence ^^^

MichaelG.

Don't ladle too much praise on me I'm only repeating the words of George Thomas.

regards Martin

With regard to run out of the chuck in either plane, unless the thread is integral with the chuck body, this will be determined by the backplate.

The OD should be a really snug fit in the register in the chuck. (ANY clearance will reduce concentricity. )

The front face of the backplate should be skimmed to clean up, to ensure that is as square as possible to the lathe Mandrel axis.

If we are still pursuing absolute accuracy reliance has to be placed on the jaws holding work absolutely parallel to the register in the chuck body.

The threads will provide a location in the form of two long tapers being brought together..

Howard