Vintage Denon Direct Drive Turntable

Hi Lewm
These are all interesting challenges as you are no doubt aware. Coupling the chassis to the plinth is the trickiest challenge. I like your idea of bolting the chassis right through to the bottom where it can be firmly anchored underneath - good one! The bore holes will need to be precisely aligned and ideally drilled through both layes in one go, which could be tricky. I had considered glueing 3 cabinet inserts into boreholes in the slate that would accept a small machine bolt as I'm not convinced that a good enough thread can be cut into the slate for a bolt/screw to bite into.

I'm not convinced that the two facing layers will be completely flat and smooth, so I wont be glueing them together, also I would like to keep the plinth modular because of the sheer weight. I'm more inclined to try sorbothane between them.

The tonearm hole also presents the same challenge, do you think a threaded hole can be cut into slate that will be good enough for screws to bite into?

(1) I have done the thought experiment as regards threading the slate. I don't think it would work. I am not going to try. I think the only choice with slate is to just drill a hole thru and then use a nut and bolt. There IS some appeal to the idea of threading the slate, because it would make for better coupling in theory. But I think the slate would just crumble if you tried to tighten down the screw. You might experiment with a scrap piece of slate, though.
(2) If you have the slate slabs honed, the sides would be plane parallel to each other and there would be a close fit between the two pieces. Most mills can hone the slabs, at least here in the US. (I had it done in Vermont and in Pennsylvania, by two different companies.) I don't think you need adhesive, but I do think that sorbothane is a bad idea. It would essentially de-couple the two pieces of slate, when what you want is good coupling. Just my 2 cents.

I've had a look at a similar thread on another forum, and agree with you Lewm that bolts are best for this job! I have decided to bolt both chassis and tonearm through the top layer only, I will drill a rebate/countersunk hole into the underside in order to secure a washer and nut, that will remain flush when placed on top of the base layer.

It turned out that in my stack of un-plinthed TTs, I had a DP-80 that I had picked up a while ago. This morning, on a whim, I put it in a solid wood DIY plinth that someone had built a number of years ago with a couple of SAEC armboards, and sold on the net for $50, and I installed my SAEC 407/23. Using the old "eyeball-it-and-guess" set-up technique, I got a wonderful TT in about 10mins. It has great stability, and I was reminded yet again how good the SAEC arm is. Upon returning home this afternoon, I spent a bit more time on set-up, and it turns out I was really lucky the first time. After several records, I continue to be surprised at how good it is. I think I need to look into a better plinth, but I am going to keep this TT in rotation for a little while.

T_bone, Raul used a minimalist plinth like that with both an SP10 and a DP80 and liked the results. It would be interesting to use your first effort as a reference, if and when you try to build a heavier/denser plinth for the DP80. I think the DP80 and other top line Denons are under-appreciated in the recent upsurge of interest in DD tables of yore.