Re: [ATM] (not) grinding with diamond

["Scott Milligan" <starzkey@charter.net>]

Doug:

I suspect that the diamond matrix has become clogged with glass frit, and
needs to be dressed to return to the initial performance you say you had
gotten.  Pyrex is somewhat notorious for clogging up diamond tooling.
Dressing can be accomplished by forcing an abrasive dressing stick into the
cutting edge while rotating at low RPM (40-100 RPM).  An aluminum oxide
stick, 100-150 grit, bound in a LOOSE (i.e. friable) matrix works best
(don't get the sticks that are hard matrix bonds made for knife sharpening,
etc.)  Universal photonics in Hicksville NY sells good sticks for this
purpose.  Another method that I have used to advantage is to grind the
diamond edge (I am using cup wheels) on a flat cast iron lap using 120
carbo.  Slow RPM, friable grit, and ability to dress in both directions
(i.e. forward & reverse) seem to be the keys for a successful dress.


Scott Milligan
 
 

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Douglas S Angle
Sent: Monday, July 10, 2006 9:59 AM
To: atm@atmlist.net
Subject: [ATM] (not) grinding with diamond

I've built a curve generator using a cheap diamond blade, similar to what's
described on Dan Cassaro's page http://home.comcast.net/~cassarole/
and Peter Smith's http://www.users.bigpond.com/pjifl/page16.html.  It
started off well, but after a while the blade just stopped cutting.  I've
tried a few things differently, but now I'm stumped.

The project is a 16" f/6.  The glass is mounted on a rotary table built from
a worm gear reduction drive.  The tile cutting diamond blade is backed by an
old gear for rigidity, which is welded to a 1" shaft, which is turned by a
belt off a drill press pulley.  The whole thing is held together by a wooden
frame.  The glass rotates about 30 RPM, and the blade about 200rpm.  The
shaft is held by wooden blocks, oiled for friction. 
Pressure on the blade is just from the weight of the pulley, shaft and gear,
which amounts to a few pounds.  Not exactly a milling machine, but it does
seem to work.  The glass rotates, the diamond wheel rotates, it makes
scratching noises, in fact it seems to do everything I expect but cut the
glass.  Which was kind of the point to start with.

I started with a 10" tile cutting blade, mounted to a wooden disk, such that
the wood presses only on the rim, and the mounting bolts pull the center up
a little.  The shaft is tilted to the correct angle, and garden hose
provides water for cooling and dust control.  (if you are considering trying
this search the archives for diamond generation and read all the safety
warnings about silica dust first!)  The curve starts in the center, and went
well until there was a concave section about 5" in diameter. 
Things then went progressively slower until it basically stopped grinding at
about 8".  There seems to be some wear on the blade just inside the diamond
rim.  At that point I added the wooden backing disc, but no change.

I then bought a new blade that seemed more suited to the task from shars 
http://www.shars.com/Online_catalog.htm?sec=126&lang=1033  pg 82   cat# 
D1A1-2234.  I would have preferred a cup wheel, but didn't find one big
enough for the job.  The diamond is actually thicker than the aluminum part
of the blade, so it protrudes a bit.  There is a little lip on the inside of
the diamond edge, which I've trimmed off as much as I can. 
There's no wear patterns on the metal part of the blade.

I mounted this blade, and got essentially the same results. After an hour,
it hasn't even taken out the scratches from previous work.  The references
above talk about hogging in minutes, but I've had this running about 4 hours
in total now. 

Any suggestions, particularly ones that don't involve buying another diamond
wheel? 

Thanks
-Doug

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