Tailstock barrel keyway fit

[Ian PParticipant @ianp]

I have recently added a cut down Chinese caliper to the tailstock on my Harrison M250 and whilst it works OK the play in the keyway has a disconcerting effect.

There is nothing original about the concept but I wondered how the scale and head would be best mounted to allow for the barrel rotation caused by the key and keyway. This probably applies to most makes of lathe.

The scale and reading head are a close fit to each other so if the head is rigidly fixed to the tailstock body there needs to be some allowance for the fact that the scale bar shifts its position at right angles to the linear axis. (front to back movement)

On mine I have only one screw holding the reading head to the casting and at the end of the barrel I have the scale bar clamped between two Belleville washers. The washers grip the bar firmly so the bar follows the movement of the M4 fixing as the grip is firmer than the caliper slide friction, the read head rotates slightly as it self aligns.

I was wondering what other people have done with their adaptations.

Ian P

[Ian PParticipant @ianp]

[steamdaveParticipant @steamdave]

Here's how I did it on my M300:

http://i1149.photobucket.com/albums/o591/steamdave/Installed.jpg
http://i1149.photobucket.com/albums/o591/steamdave/Parts_1.jpg

Your rigid link to the barrel is what is giving you the problem. I solved this by using two ball joints normally used on R/C aircraft joined together by a piece of studding.

The ali ring around the barrel means that you don't have to drill into the barrel. The ring is stepped so that only half the thickness clamps over the barrel so that when it is fully retracted to eject a MT, the ring does not hit the front of the tailstock housing. It's been OK for the last few years.

Dave

The Emerald Isle

Edited By steamdave on 04/08/2015 22:56:02

[Michael GilliganParticipant @michaelgilligan61133]

Ian,

I have never had the need to fit such a thing, so this is hypothetical:

I would probably fix the read-head rigidly to the tailstock body, and replace your Belville washer assembly with a piece slotted at 90deg to the lathe axis … then replace that screw with a hardened pin, running in the slot.

Success or failure would probably depend upon how substantial the read-head is.

MichaelG.

[Roderick JenkinsParticipant @roderickjenkins93242]

Ian,

This is mine on my Super7:

The split collar is easy to remove if I need to use taper tooling without a tang; which won't eject without the barrel being fully withdrawn. The slot in the DRO end fitting has enough leeway for the slop in the barrel keyway.

HTH

Rod

[Ian PParticipant @ianp]

Posted by steamdave on 04/08/2015 22:49:55:

Here's how I did it on my M300:

http://i1149.photobucket.com/albums/o591/steamdave/Installed.jpg
http://i1149.photobucket.com/albums/o591/steamdave/Parts_1.jpg

Your rigid link to the barrel is what is giving you the problem. I solved this by using two ball joints normally used on R/C aircraft joined together by a piece of studding.

The ali ring around the barrel means that you don't have to drill into the barrel. The ring is stepped so that only half the thickness clamps over the barrel so that when it is fully retracted to eject a MT, the ring does not hit the front of the tailstock housing. It's been OK for the last few years.

Dave

The Emerald Isle

Edited By steamdave on 04/08/2015 22:56:02

Dave

I chose to drill and tap the tailstock just to keep the barrel as slim as possible as I felt it would get in the way with some machining operations, (I have done something similar on the milling machine quill).

Using the model aircraft ball joints on the link is a good idea as they have no lost motion when these low forces are involved.

What I have works well and its not a 'problem', technically any barrel rotation causes cosine errors in the measurement although its such a small amount its well beyond the resolution of the calipers.

Ian P

[Ian PParticipant @ianp]

Posted by Michael Gilligan on 04/08/2015 22:50:47:

Ian,

I have never had the need to fit such a thing, so this is hypothetical:

I would probably fix the read-head rigidly to the tailstock body, and replace your Belville washer assembly with a piece slotted at 90deg to the lathe axis … then replace that screw with a hardened pin, running in the slot.

Success or failure would probably depend upon how substantial the read-head is.

MichaelG.

I did start off with the read head rigidly mounted but but with the scale being a good fit the pair of them tried to act as the keyway for the barrel.

I think success or failure of your method would depend entirely on the fit of the pin in the slot, It needs to have low friction and yet make contact with the leading and trailing face of the pin without any play, well, less than 0.002mm I would say.

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ian Phillips on 05/08/2015 08:17:15:

I think success or failure of your method would depend entirely on the fit of the pin in the slot, It needs to have low friction and yet make contact with the leading and trailing face of the pin without any play, well, less than 0.002mm I would say.

.

Ian,

[just wondering] … Would you really need it to be backlash-free ?

Most tailstock operations are unidirectional [unless you are using some special 'repetition' tooling]

MichaelG.

.

P.S. … Rod has produced pretty-much what I had in mind.

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