[JoshK]

So, I have a bit of a conundrum.  I bought a prototype slate plinth for my Teres.  The armboard is slate as well.  The armboard was drilled for my Rega RB300, however the mounting post is not long enough to reach all the way through the armboard, so I need to countersink from the back side so that I can attach the nut.

I've been told that slate drills relatively easy with masonry bits.  However, I have no clue what kind of bit can be used for such a counter-sink job where the hole is not going to go all the way through.  Any ideas?   I need the hole to be ~ 1.5-2". 

[Ericus Rex]

A shop that cuts/sells countertops could probably do that for you.  If not there then a machine shop could certainly do it, I just don't know what kind of bit/cutter to recommend for slate.

If you're hell bent to try it yourself then you could clamp the armboard to your drill press table (NO FREEHAND!) and drill down to proper depth with a suitably large hole saw at slow speed and chip out the guts with a junk chisel.  But I won't lie, you risk splitting the arm board that way, for one, and two; you would have a very rough surface for your mounting nut/washer.

I'd try the countertop thing first.  I'm sure they have to do this often to mount some types of fixtures.

Keep us informed!

[JoshK]

That is a good suggestion, but I ordered a masonry bit from McMaster that the seller suggested.  We shall see.