Thermal Insulators on Micrometers...

Seems to me that for hobby machining, one could drive oneself wacko chasing thermal expansion from just handling stuff. Not only will the mic in one hand expand, but how about the work piece or gauge block being held in the other hand?

I work a lot on the ignorance is bliss principle. ;) ymmv

Tom
 
Seems to me that for hobby machining, one could drive oneself wacko chasing thermal expansion from just handling stuff. Not only will the mic in one hand expand, but how about the work piece or gauge block being held in the other hand?

I work a lot on the ignorance is bliss principle. ;) ymmv

Tom

Yeah, we can measure it but cutting it is another thing! Then again, we're hobby guys and have all the time in the world to split microns. In a job shop ... I suspect not so much.
 
How about when we are turning something on the lathe, for example, we stop the lathe and measure the diameter etc. of our part and, at least me,
never even think of the expansion of the part even though it's quite often too hot to touch? Those of you that have coolant flowing over the part
are, of course, not included in this thought experiment. The expansion of the part is far greater than the expansion of the measuring device warmed
by our hand, and yet I have never found a part that measured correctly while in the hot state that changed after it cooled. This of course is actually
false, but for any of our needs, is a moot point.
 
The bottom line is that, as hobbyists, we make parts to fit together in assemblies. The only surfaces that are anything close to critical are mating surfaces and very few of them require measurements to tenths of thousandths.
 
Skf catalogue calls js5 tolerance for standard bearing sit. On a 15mm shaft it's +- 0.004mm. If you want "machine tool" quality (for example for small high speed spindle) its js4 +-0.0025mm - almost exactly +-tenth.
Maybe holding this tolerance is not necessary (maybe even futile if all you have is a minilathe) but im not a pro - i don't have experience to tell where it's safe to skimp.

Wysłane z mojego GT-N7100 przy użyciu Tapatalka
 
Skf catalogue calls js5 tolerance for standard bearing sit. On a 15mm shaft it's +- 0.004mm. If you want "machine tool" quality (for example for small high speed spindle) its js4 +-0.0025mm - almost exactly +-tenth.
Maybe holding this tolerance is not necessary (maybe even futile if all you have is a minilathe) but im not a pro - i don't have experience to tell where it's safe to skimp.

Wysłane z mojego GT-N7100 przy użyciu Tapatalka
Not futile, if you have enough time on your hands. I have been known to sneak up on a critical OD of a round matching part by using a strip of emery cloth to buff off the last few nanomillimicrons while spinning it in a mini or bench lathe. Can get a spectacular finish like that, too.

Tom

P.S. I didn’t measure the final OD though. It was trial and error fit. I don’t have a measuring tool with that fine of resolution.
 
Many a good workman has brought a part in on size with a lathe file and sand paper. You're in good company, Tom.
 
My luck usually is. It mic's .001" over, put some emery to the surface, get it looking pretty. Measure again and it's .0005" under! Hello Loctite. And I keep three different flavors of it, too!
 
My luck usually is. It mic's .001" over, put some emery to the surface, get it looking pretty. Measure again and it's .0005" under! Hello Loctite. And I keep three different flavors of it, too!
Been there, done that....more times than I care to think about. It is amazing how high those tiny little ridges can be.

I'm a little smarter now. When I have a critical diameter, I will cut slightly oversized and do my polishing to get a sense of how much material will be removed. Then I will make any correction and make my final cut.
 
(pocket posting )
 
Last edited:
Back
Top