Help Identifying a chuck back plate

[Mark Davison 1Participant @markdavison1]

I have a threaded back plate for lathe that is looking for a new home. It came on a 6″ Pratt 4 jaw independent chuck that I bought off eBay a few years ago.

It appears to be a 2″ x 4 TPI 60 degree thread. It could be 6mm, it is hard to get my short thread gauge in and to see at the same time.

The plain register is 2-1/16″ (if it’s imperial).

Any ideas as to what lathe this make/model lathe it would fit?

[Nigel Graham 2Participant @nigelgraham2]

If it narrows things down a bit –

 

60º suggests metric or American thread.

4tpi X 60º suggests American (Unified) thread although the standard 2″ UNC is of 4.5tpi.

2.1/16″ = 52.39 mm, which does not seem a logical metric size. Even if allowing for wear, 53mm is not very logical either; but it would have a metric thread anyway.

So the original lathe could be of US manufacture.

[Nigel Graham 2]

On Nigel Graham 2 Said:

60º suggests metric or American thread.

Should your first question not be to ask the OP with what level of certainty he has determined it is 60 degrees as opposed to 55 degrees? What is the confidence level concerning the thread angle?

Is it possible that he is using 60 degrees as shorthand for ‘vee thread’, in order to distinguish it from a square thread (e.g. Raglan)?

As a general note, that diameter/pitch combination is unusually coarse. Would it be reasonable to say that lathe spindles generally use a finer pitch than a fastener of the same diameter?

[Michael GilliganParticipant @michaelgilligan61133]

As I have stated repeatedly … it is a simple matter to take a wax impression from a thread, and [at the scale being discussed] it  should be a trivial task to distinguish between the Whitworth and Unified thread-forms

.

.

MichaelG.i

[JasonBModerator @jasonb]

OP says he is using a thread gauge and at that sort of pitch the gauge should easily show it is 60deg not 55.

[Michael GilliganParticipant @michaelgilligan61133]

Agreed … but he specifically states:

it is hard to get my short thread gauge in and to see at the same time.

MichaelG.

[Mark Davison 1Participant @markdavison1]

I tried both 60 and 55 degree thread gauges.  It is definitely not 55.  I could see enough of the first thread to be sure but it is hard to get a good view of the fit with the 3rd and 4th threads to see a subtle difference between 4TPI imperial 6mm metric.

[Michael GilliganParticipant @michaelgilligan61133]

Then Nigel’s logic seems sound to me

It is most likely for  an American spec. lathe.

MichaelG.

[old martParticipant @oldmart]

There is a possibility that it is not from a lathe, some dividing heads have threads.

I will ask about it on the Home Shop Machinist forum over the other side of the pond.

[old mart]

On old mart Said:

There is a possibility that it is not from a lathe, some dividing heads have threads.

I will ask about it on the Home Shop Machinist forum over the other side of the pond.

That’s not a bad guess “old mart” if I recall my Brown & Sharp dividing head spindle has a large diameter very coarse thread. I will check it in the morning as I have to remove the adaptor plate which I made so I can take a chuck off the lathe and bolt it straight to the D/head.

[Mark Davison 1Participant @markdavison1]

Andrew, if it fits you can.have it. PM me.

[Mark Davison 1]

On Mark Davison 1 Said:

I tried both 60 and 55 degree thread gauges.  It is definitely not 55.  I could see enough of the first thread to be sure but it is hard to get a good view of the fit with the 3rd and 4th threads to see a subtle difference between 4TPI imperial 6mm metric.

Take an impression with some modelling clay, Platicene etc, or even some bread screwed up into a dough ball. Then compare that with your thread gauge.

[Andrew CrowParticipant @andrewcrow91475]

<p style=”text-align: left;”>Hello Mark, I think “old mart” has probably got the right idea, although not the same as my Brown & Sharp, having just measured it as 2.5 diameter × 4.5 tpi and almost certainly of American National form.</p>
Because of the awkwardness of it,  I made an adaptor to suit the chucks and face plate I used on my old Colchester Student (also with an adaptor to suit a D1 spindle taper) so I could transfer between the two  without disturbing the job.

[Mark Davison 1Participant @markdavison1]

Based on this thread

KERRY TYPE A.G. LATHE

by

@steveambler1

It sounds like it may fit a Kerry A.G. lathe.

[Author]

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