View Full Version : A question about measurement tools

07-02-2010, 06:16 PM
Finished my 5c collet holder for the lathe and was measuring runout. Chinese dial indicator shows a definite .001"(measured on the 10 degree taper inside the holder where the collet seats) Had it mounted on the cross slide with one of those magnetic holders. Moved it over to the taper that the chuck mounts on and all looked good. That's a ground smooth surface. Pulled out a Brown&Sharp best test indicator and repeated the test, except this time I mounted it in the tool holder. The needle didn't move. The B&S still has a calibration sticker thats up to date when I bought it from a pawn shop. The 10 degree taper is not ground smooth. Was the "cheap" indicator picking up a ridge due to the bigger ball point? I also ran this test with a rod out of a printer up next to the collet with the same results. I'm :confused: Close enough for my beat up old machines?

07-02-2010, 06:31 PM
Some of the cheap import dial indicators have slop between the stem and housing. Any surface roughness can cause the stem to wobble a bit and slightly affect the reading.

In general, a larger ball end should show less variation due to surface roughness.

07-02-2010, 09:11 PM
I guess in answer to the 'close enough' question, run out is only going to matter if you either want to turn something, take it out of the chuck and the remount it and have things concentric, or turn something from stock that is a finished size (say some cold rolled) and have the turned features concentric with the non-machined surface.
As soon as you take a cut to get 100% clean up on your material, you are going to be concentric. From memory Schlesinger expects run out of 1/10ths of thou but we amateurs don't necessarily need that sort of accuracy (although it dosen't stop us trying to get it).


07-02-2010, 10:40 PM
I DON'T recommend this unless you really know what you are doing,but I most times have taken a tool post grinder,and taken a very light grind over all the surfaces on the spindle of an import lathe. They are usually not perfectly concentric,and may be a few tenths out.

You can ruin your spindle if you do not know how to keep the tapers right,so do be careful. Also,your tool post grinder had better be an excellent one.