chuck back plate thread

[tanii51]

im about to venture into making my own back plate for hercus 260
my chart says internal thread dimensions for 1 3/4 x 8 un
max diam 1.6297
min diam 1.6150
if i measure my existing back plate threads( burnerd and tos)
its 1.612
which one should i aim for
john

[cba_melbourne]

John, it does not really matter. The thread is not what is going to center backplate or chuck. Threads are designed to have some considerable slop, otherwise they would bind.

What matters is the registration flange. That is the short cylindrical section on the spindle, just left of where the spinle threas ends. This is supposed to be a tight fit with its female counterpart in the backplate. That is what centers your backplate and chuck radially.

A tightly fitting registration flange will make sure that your chuck will center in a repeatably fashion. That is to say everytime you mount the chuck, its runout will be the same.

If the registration is not a tight fit, then everytime you mont the chuck you will measure a different runout.

For this to work, the thread must be somewhat loose. A too tight fitting thread will make it impossible for the face of the spindle to properly mate with the face of the backplate. The result is a wobbling chuck.

Chris

[tanii51]

thanks chris exactly what i asked for
all the chucks i have were a very tight fit on the spindle even after cleaning all the threads
so i should aim for the the larger internal diameter range some where between 1.6150 to1.62

[pipeclay]

Chris can you elaborate on this TIGHT fit with its female counterpart in the backing plate.
I have made and sold a few and the tests that I run on my Spindles or Threaded mounts I have here show repeatability,the only part of the items that I make register on the FACE of the spindle and the FACE of the backing plate.

[HavinaGo]

I've only made one backing plate and one "miniature faceplate" and my skill is definitely in the "developing" category, yet I have been surprised at how repeatable the 3 jaw chuck and faceplate mountings are.

On the 9" I have there is very little parallel register on the spindle and my effort was not that close a fit. The thread form is not even that brilliant looking at it with hind sight.

I have read comments that the thread form effectively provides a taper that together with the flat face (which determines that the same piece of thread repeatably matches on the spindle) leads to a repeatable mounting of the chuck.

As to the size hole to bore for an internal thread, F.W Hercus in "Textbook of turning" second edition, 1970, page 58 says

"In preparing the work for internal screwcutting it is usual to bore the hole to a size slightly greater than the theoretical minor diameter of the thread to be cut. This greatly simplifies the fitting of the internal thread to its mating part and does not detract from the strength. It has been found by experiment that the depth of an internal thread may be reduced in this manner by up to 25% without its strength being impaired."

I concluded that bigger is better than too tight.... but there are more experienced members who will have a view.

All the best

[tanii51]

peter not sure if that "tight fit "was directed at me but i should have said the thread was a very tight fit the register part was fine once we screwed it on far enough.
how far should the bore to fit over the smooth part of the spindle before the thread starts be ?The other chucks seem to bind on to the spindle and can be hard to remove im wondering if its the end of the thread in the back plate binding on the end of the thread on the spindle where it stops closest to the register
john

[cba_melbourne]

Chris can you elaborate on this TIGHT fit with its female counterpart in the backing plate.
I have made and sold a few and the tests that I run on my Spindles or Threaded mounts I have here show repeatability,the only part of the items that I make register on the FACE of the spindle and the FACE of the backing plate.

Pipeclay, on threaded spindle noses there is always a short "register". Please refer to the draving here:
Page Title

Unfortunately this register differs slightly in size and shape from lathe manufacturer to lathe manufacturer. This is so, because threaded spindle noses are a very very old thing, and there was in the old days no "standard" that accurately defined dimensions and tolerances (nor is there now for threaded spindles). This is why it is so important that each backplate is either made for a specific lathe brand according to its particular dimensions and tolerances, or alternatively is made on and for the individual lathe it will be used for. If this register is not a perfect "sucking fit" to the backplate (or direct mount chuck), mount repeatability will be poor. This repeatability may not matter much for a faceplate or a 4-jaw chuck. It will matter for a (good quality) 3-jaw chuck, and it will certainly matter a lot for any collet chuck.

This problem only exists with threaded spindle noses (and maybe a few proprietary spindle noses for watchmaker lathes and hobbylathes) . All modern spindle noses (like the very popular camlock mounts) are wery strictly standardized and any backplate from a reputable maker will automatically fit any machine tool spindle regardless of brand as long as it is from a reputable maker that cares about tolerances.

If you do a Google search for "threaded spindle nose registration" you should get plenty of good additional information on the subject.

Chris


PS: lots of tips here too: http://www.armurerieduroi.com/pages/...kplate.html#12

[pipeclay]

Chris,all good info but still nothing in relation to the TIGHT fit in the FEMALE counterpart.

John, I generally give a around .160" to .200" length of under cut and around .005" to .010" clearance on the bore,these all seem to work for me on the 9 and 260,but I may be doing it totally wrong.

[cba_melbourne]

Chris,all good info but still nothing in relation to the TIGHT fit in the FEMALE counterpart.

John, I generally give a around .160" to .200" length of under cut and around .005" to .010" clearance on the bore,these all seem to work for me on the 9 and 260,but I may be doing it totally wrong.

Sorry, you are right Pipeclay. After some further reading I learn that there are two schools of thaught on this. There are the ones that like you completely rely on the threads to self-center and leave the register grossly oversize. Then there are others that like me were taught at school that it is bad engineering practice to rely on a thread for centering.

There are lathes that repeat well using a tight fitting register. And there are other lathes that apparently also repeat well, yet either have no register at all, or do not use this register for centering and leave it a sloppy fit.

Personally I still feel that a tight fitting register would be a guarantee for good repeatability, whilst simply relying on the thread being self centering may or may not always work equally well, depending on individual tolerances of the components.

Maybe it also depends a bit what the lathe is used for. Someone doing mostly smallish work and/or lots of second operation tasks will usually expect better chuck mount repeatability and probably insist on a tight fitting register. Someone doing mostly larger jobs may prefer to rely entierly on the threads centreing the chucks, with the benefit of less risk of the chuck getting stuck on the spindle if the mount is not at all times kept perfectly hospital clean. Chris

[tanii51]

well thanks guys for your input I wasnt too confident in my measuring skills so i made a gage from a peice if 1 3/4 shaft that the pratt bernurd would just screw onto
i bored the back plate blank to 1.62 as close as i could read it and commenced the threading( had to regrind the boring bar because it fouled on the thicker part of the tool)
i found there was a very small transition from "no go" to "go " using the gage
then i used the un threaded part of the gage to test for fit on the internal register( so it just slid in with a twisting motion)
it worked.
problem maybe???
with out the chuck on it seems to lock onto the spindle very easily is this going to be a problem when in use getting the chuck off again
these blanks i had caste for me as a trial from T10 grade maybe they are a bit course
the threads arnt as nice as id like but maybe due to going too slow using carbide tips

[morrisman]

I can see where you have ground the tool shank slightly to clear the work piece .

Don't pull yourself down at all... that's a great effort for a first go

MIKE

[pipeclay]

If you plan on leaving the chuck on the spindle for an extended period of time or any other threaded adaptors you have it is a good idea to put a paper shim or similar between the register faces,the 260 is prone to having hard to remove backing plates.

[tanii51]

how do you turn paper shims
this is my new paper shim turning tool ( use the back of the chuck as a template)
thanks for that pipeclay

[pipeclay]

Or a pair of scissors.