Co-axial Centring Indicators

[awemawson]

A long time ago I picked up a Co-Axial Centring Indicator at a boot sale - didn't pay a lot but it's so long ago I cannot remember how much.

When I tried it, there was a little bit of run out, which I never solved. If you turn the shaft relative to the body you get about half of one division of wobble. Never been able to isolate which bit is wonky or how to solve it. As far as I can tell, the culprit could either be the flat ground surface that the probe holder rests against, or the central shaft.

I thought during my enforced light duties I'd dig it out and see what I could do with it. Answer; nowt, nadda, nothing as I can't seem to dismantle it. Anyone been inside one of these things and can give me any clues? It looks that the cross piece that holds the swinging probe holder is riveted on - but that seems a rather odd construction for something that should be fairly precise.

Any pointers appreciated.

[BillTodd]

The run out is caused by the flat surface being out of perpendicular to the shaft/bore. This could be from poor manufacturing or perhaps the casting has relaxed after being machined.

The fix would be to reface the the flat surface while holding the part square with the bore.

As to getting it apart, i'm not sure, but typically it would require to removal of the rocker.

Bill


Edit:

Actually, there is one other possible source of the error : if the under side of the top-hat (the face that the indicator rides on) is not square to the shaft, it will also cause the needle to wobble.  Again, the fix is to reface while holding the  shaft square in a lathe.

[vtsteam]

I bet krv3000 will have some ideas for getting it open...

[Davo J]

I have rebuilt one of these myself and the above answer is what I trued up, l do have pictures on one of the computers but would have to chase it up.
To get it apart you need to tap the pin out at the bottom.
I mounted a piece of mild steel in the lathe chuck and trued it up to fit the bore snug, then locked it to the chuck and used a dremal tool to grind the base flat.

For the shaft I set it up it the ER collet chuck and ground under the top hat.

When I was finished it wasn't perfect but so close to it and does me fine for what I use it for. If I want perfect I use a indicator and follow it around. 
A lot of people will tell you even the blake is only good enough to get with in a thou.

Dave

[Davo J]

Here is one I found in my photo bucket account

[awemawson]

Thanks for that Dave. I'll dig it out again and have another look

Andrew

[Davo J]

No problem, I had nothing to loose as the US seller refunded 3/4 of my money when I told him about it. Like you mine sat for a long time before I did anything with it.

Careful setup and light grinding will make it a precision instrument again.
Make sure when doing the setup above the chuck is holding it nice and true.
The other half will be fine because you turn thr shaft down to size to be a snug fit in the bore. When you get close us emery cloth, that way it will only take a little off and will give a nice smooth finish that wont scratch the bore.

Oops I just started using a mobile to post and it looks like I mucked up when I was only trying to edit my post

Dave