Re: [ATM] Grinding Spindle

[Mike Lockwood <melockwo@uiuc.edu>]

Scott,

scott wrote:
> That looks like a nice setup.  I have focused on overarm machines and  
> have not set up a high RPM machine for secondaries.

Thanks.  I've used overarms, but not for small mirrors (yet).  I will 
likely make a small manual overarm, possibly for polishing the 
secondaries and test plates that I'm currently working on.

You can tell a lot from the feel of the lap, so I am reluctant to give 
that up, even in polishing, and definitely not in figuring.

> I have been told that variacs can be a shock hazard, but I don't know  
> enough about it to discuss it.  You might check variacs in the archive  
> to see if anything interesting comes up.

The variac is fully enclosed and grounded, as is the motor.  This 
should eliminate any shock hazard.  Additionally, I'm plugging it into 
a GFI outlet in my kitchen.

> Does it throw slurry at these high RPMs?

Yes, it can.  It's all caught by the pan at the speeds I work at (<150 
RPM, typically).

It can also throw blanks if you are not careful!  If you get the mix 
too watery with fine abrasives you can have the disks seize without 
too much warning, causing nasty scratches.  Best to reduce the speed 
as the abrasive size goes down.  60 RPM at 5 micron seems to be safe.

I spend most of my grinding time at 25 micron, at around 120 RPM.  220 
grit is the previous abrasive, so I spend lots of time making sure ALL 
the 220 pits are gone.  This can take 1/2 to 1 hr, but it saves a LOT 
of frustration with pits down the road.  This is still MUCH quicker 
than walking around the barrel.

(Yes, I could buy an intermediate grade abrasive, but I haven't been 
inspired to do so thus far.)

After a careful inspection with a loupe finds absolutely no 220 pits 
and a uniform 25u grind, I go to 12 micron and 5 micron, both of which 
finish up very quickly.  Even just 15 minutes appears to give a VERY 
good uniform grind for small mirrors, based on my inspections with a 
10X loupe.  After a good 12 micron grind is achieved, I test the ROC 
with a flashlight (after applying vegetable oil to the test plate) and 
tweak if I need to.  The same is done with 5u.

Note:  I'm using MicroGrit 25u, 12u and 5u, so the abrasive is of high 
quality and thus consistent size, saving lots of time.


Bob,

Bob May wrote:
 > A bit nicer job than my llittle spindle that I've used for up to 6" 
 > optics.  Small spindles with no overhanging stuff do really make a
 > lot of optical work a lot easier to do.

I agree wholeheartedly.  Your little spindle has served you well, it 
sounds like.  I built mine to handle up to a 12" mirror being figured.

They also make beveling the mirror/tool a breeze.

	Mike Lockwood



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