re: Re: Re: Steel plate for hexapod bed

Another easy way to get holes in granite is to use a carbide glass drill.
These cost a couple of dollars, are spade shaped. Put in drill press, make
a little dam of clay around where you want the hole to be, fill with water,
and drill away. Only takes a minute or so. Use lots of pressure.

Threaded inserts can be bought, or make your own. I drilled some holes,
measured the hole size, turned some steel to that diameter minus a couple of
thou and threaded it. Make sure to thread all the way through, otherwise
when you put the insert in the hole, the epoxy will seal the air under it
and it will pop the insert up while the epoxy is hardening. Oops! Then you
have to go back and kiss off the tops of the inserts.

Cheap surface plates may cost more for shipping than the retail cost, but
Enco and others sometimes have specials on shipping. I got a 12 x 18"
surface plate for $22 with the special.

Flattening thin steel is very difficult because rolled metal has a lot of
internal stress. If you cut some of it off, the stress is released, bending
the metal. If you anneal the piece of metal maybe all the stress could be
released, but machining stress will bend it again to some degree.

The granite plate (or cast iron if you can get some) is much better:
flattness, damping, stiffness, stability over time and temperature. Also
consider the scrap value: if the project is retired for some reason, a
granite or cast iron plate has some value for other things, a piece of
sheet steel is just scrap.

regards-
Elliot B.




Date: Sat, 04 Dec 2004 16:29:08 -0600
From: Jon Elson <elson@...>
Subject: Re: Re: Steel plate for hexapod bed

Simon Arthur wrote:

>I thought about granite, but how do you attach fixtures?
>
>

You drill holes with abrasive slurry on a piece of pipe (core drill)
or a real diamond core drill. Then, you bolt on the fixtures, and maybe
even apply glue before tightening the bolts. That's how Excellon makes
their $100,000 printed circuit board drills. The use a HUGE granite
slab, and glue and bolt a granite beam to it as the X axis slideway. A
"cross"
fits over that, and carries another granite beam as the Y axis. The table
slides over the Y beam on air bearings, resting on the big table.
It is insanely expensive, but makes an incredibly reliable precision
X-Y table for light drilling. The gantry over the table that holds the
multiple drilling spindles is also granite, and the drill motors use air
bearings. (I have some of the pieces off one of these machines,
and some of that is now the X-Y drive for my CNC Bridgeport retrofit.)

Jon
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