Heat Growth on 52100 Material

[Butchie1946]

I have a Piston that I am holding tolerances of .000035 & .000050 on a .2500 Dia. Ball. The material
is 52100 and is Heat Treated to 56-62 HRC. In order to achieve a 4 rms Finish, I polish the Ball
using oil. I have a Sphericity callout to hold .000050 on Diameter. Question I have is during the
polishing of the Ball, am I generating some Heat on the Surface which would give me a false reading as soon as I check and when the part has a chance to cool to room Temperature, I would get a different reading. Looking to find out how much would this grow during polishing and is their a
tolerance band I could use for during manufacturing.

 

[AndrewFinAustralia]

Tolerance of 1.4 microns - 2 microns.

This is getting towards F1 tolerances - the mills that grind these are in a climate controlled room with coolant at a specific temperature.

If you know the heat profile in the ball, you could use standard coefficient of expansion, but to be honest, for repeatibility, I'd look to standardise the grinding/polishing process - standardised coolant (if any), standardised environment, etc...

 

[EdStainless]

What temperature is your coolant? 35F?
you need to minimize any temp change.
Better to grind cold, have they a little too large and have to remove more stock.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Tmoose]

How far off the mark are commercial precision balls as used in ball bearings?

The silicon nitride balls used in "hybrid ceramic" bearings are pretty nice.

http://www.bearingstockshop.com/docs/Marcas/Pdfs/barden_siicone_nitride_balls_us_en.pdf

Mighr be hard to drill thru, if that is how you are turning them into "pistons."

 

[tbuelna]

An AFBMA grade 50 ball sounds like it would meet the requirements noted.

https://www.precisionballs.com/afbma_ball_grade.php

 

[Maui]

Is a cryogenic treatment being performed as a standard part of the heat treatment process? If not, it definitely should be included.

Maui

 

[MagBen]

52100 has a thermal expansion coiefficient of 12ppm/C.
If the temperature increase is 100C, the linear expansion will be .0003''
If the temperature increase is 10C, the linear expansion is .00003''
One can see that even with 10C increase at polishing, the thermal expansion is comparable to your tolerance. It seems you need to take thermal expansion into consideration for oyur applications.