Re: [ATM] High Speed grinding with a pottery wheel...???

[Mike Lockwood <melockwo@uiuc.edu>]

Brent,

Brent Russell wrote:
> Some weeks ago a my wife bought a pottery wheel that uses a simple
> mechanical drive to achieve a variable speed turntable. Upon seeing this
> I though 'brilliant just what I need' and I have proceeded to grind a
> small 6" cass on it much to my wife's disdain.
> However the results seem pretty astounding.
>  
> I rough ground by hand but did 150, 320 and 600 on the table. Speed at
> very slow at first to get good grit spread and then a zoom up to about
> approx 80-120 RPM until the grit is worn out. I am working both TOT and
> MOT and it works so fast it is incredible. I am using a tile tool and
> last night did 320 Sic and 600 SiC in 35 minutes total...WOW 
>  
> Has anyone else tried this? is there something that I may only pick up
> later as I get to polishing that may be bad about doing this? I have
> been careful to keep strokes and speed random and the surface looks
> great so I see no problems.

For grinding, a turntable really helps.  I built one to use when I 
made two Cassegrain secondaries.  I used similar speeds.  Hogging out 
was a breeze, and getting to the end of 220 grit was FAST.

Slow down after 220 grit, and really pay attention to contact.  By the 
time you get to the last abrasive (5 micron for me) you should be 
running much slower (30 RPM if you're hand-holding the top piece) and 
you should have excellent contact.

Be careful with fine grinding.  If you use high speeds, there's a 
chance the disks can seize together momentarily if you apply the wrong 
type of pressure, and you will then scratch the heck out of them.  If 
the machine is strong, this can also pose an injury hazard if you 
don't let go immediately.

As always, carefully inspect the surface to ensure a uniform grind is 
obtained.  Spend extra time with the largest Aluminum Oxide grit if 
you hav eany doubt.

Polishing with a machine is another thing altogether.  The lap acts 
differently, different things happen, and it can be confusing.  I used 
a manual overarm to hold the lap/mirror, and just moved it back and 
forth by hand, sort of like a manual "Mirror-o-Matic":

http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/machine1/machine1.html

Polishing the normal way with the mirror/tool slowly rotating on the 
turntable eliminates "walking around the barrel", and you can sit 
there and watch TV while you polish.

	Mike Lockwood


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