View Full Version : Paging Forrest: 4-Jaw Question

11-03-2004, 11:47 PM

Thank you for the excellent article on 4-jaw chucks in the current HSM.

I have a question on dialing in the face of the work piece. I had always assumed that if I had enough length of the work piece in the jaws, the face would automatically be perpendicular?

Is there a minium work piece to chuck jaw length ratio or do you always dial in the face, even if your stock is several inches long?



Forrest Addy
11-04-2004, 12:51 AM
Thanks for the good words.

I haven't got a good answer for you. Everything is proportional and conditional.

In my article I tried to make clear that dialing in on widely separated diameters is preferred technique for long work whereas on short work one should use a diameter and a face. Then I went on to paint in weasel words that these techniques aren't engraved in stone.

There's several different ways to dial in stuff. Sometimes shoulders arent square, diameters concentric or whatever. You have to make do with the references you have, decide the references to dial to, and balance out the remaining errors to minimize their effects.

Let's say you have to dial in a large expensive turbine case worth maybe a million bucks to replace and thanks to 7 years of 1000 degree heat is as crooked as a crumpled coffee can. You have to skim the bearing bracket face square to the axis as represented by the existing interstage diaphragm diameters between the crush pins. The solution is to weld repair the case and re-machine it but not in our new cost conscious Navy. Make it work but don't take off any material.

Can you resolve all the part feature geometry conflicts and make everyone happy? Impossible. But can you do your best and only holler for managment and the engineers when the problem is insurmountable? Most assuredly but that's where "judgement" kick in but you gotta practice to develop it.

I guess what I'm saying is there's no hard and fast technique that will resolve indicating in machine parts where existing features are not quite in concentricity, planarity, square, or alignment. It's kinda like an joyless marriage - there's no hard rules or easy solutions; the best you can do is work with it and learn from your experience.

[This message has been edited by Forrest Addy (edited 11-04-2004).]

11-05-2004, 06:50 PM

You couldn't just lie to me could you? http://bbs.homeshopmachinist.net//smile.gif I appreciate the great info, thanks!