Design Question - Resin Case Floor

[groswald]

Bob,

I hope you don't mind these, "why didja ..." questions. I find your thinking quite enlightening. I had wondered about the studs versus bolts issue. Your answer made me think about the problem a different way and made perfect sense.

So, on the topic of the resin floor - I priced the resin locally and about had a heart-attack at the price. Nearly $70.00! I'm sure I can get it for less online, but I am cheap.

It did get me thinking though (always a bad thing) and caused me to wonder if the resin serves any purpose other than providing a level base from which to do the needed alignment?

If alignment is the only purpose I am thinking about getting a piece of 3/8" tempered glass cut to fit in the case bottom to use as a surface plate of sorts. It should be at least as flat as the resin, and can be shimmed during alignment to deal with any irregularities in the wood floor.

The glass would be removed during use of course, and replaced with a spoil-board which would itself be screwed down and then trued to the machine by a flattening pass. The glass would be used for various purposes in the shop, replacing the one I broke a few months back.

Thoughts?

Thanks,

Randy

[momus_cnc]

Yes, it is just there to provide as flat a surface as possible. Your idea of the glass sheet is a really good one. Like you suggest, as long as you can place it in a way that it can't rock around due to any irregularities, it should work great. It might actually be better than the resin in some ways. I'm waiting to see how the resin holds up over time. While I painted the base completely, with many layers, it is still a wooden base and can potentially move around if changing humidity levels can reach the wood. And the epoxy would move with it.

I also like the glass idea because it reminds me of a technique that Dave Gingery used in his "Make your own Metalworking Shop from Scrap" books. If I recall correctly, he attached two plates of glass to pieces of plywood by bedding them in auto body filler. He then lapped the two plates together to create a super low budget surface plate. I remember ordering those books when I was about 12 years old. I haven't looked at them in decades. Good stuff.

-Bob

[momus_cnc]

By the way, I don't remember where I ended up buying it (online), but I think I bought the resin for around $25-30.

[Michael In Cali]

I plan on skipping this step and working directly off the wooden base.

Accurately machined ply/MDF should be just about as flat as you need to get the rails aligned. If you cut all the internals of the torsion box on the table saw without moving the fence, they will all be exactly the same height and the torsion box will be as flat as whatever substrate you build it on and its small enough to be asembled on the ground top of your table saw (which should be sufficiently flat).

Now, if your using a straight edge and circular saw, all bets are off.

[groswald]

Michael,

I suspect you are right, especially depending on what you want to cut. My base is reasonably flat, and probably would be fine for most of the work I will do. But, it will be better with the glass (or resin). Even better would be a "real" polished granite surface plate, but now we are talking expensive!

Randy

[Michael In Cali]

My understanding is that it only needs to be flat enough and for long enough to set the rail height.

During use, one would add a spoil board and then use the router to surface plane the spoil board flat. Then, whatever you jig up on it, will be held to as flat as the spoil board is.

[momus_cnc]

My understanding is that it only needs to be flat enough and for long enough to set the rail height.

Correct indeed. And to do the rest of the setup adjustments. But yes, it is a temporary situation.

I plan on skipping this step and working directly off the wooden base.

This would be the ideal situation if you can build it nice and flat. Or if you have a way surfacing the top skin after assembly to true it up.

Unfortunately, even by cutting the parts perfectly exact, it isn't a guarantee that the base will come out flat once it is assembled. It is possible to twist the assembly of internal ribs slightly, even if the slots between them are nice and snug. If the skins are then attached to this twisted internal frame, the twist is going to be there forever. So you might only be able to assemble it as flat as the work surface you are using. Basically you would end up with a surface that is a very shallow hyperbolic paraboloid. The tricky part is that this is a "ruled surface" in two directions, so if you put a straightedge down on it in those directions it will misleadingly seem flat. But if you put the straightedge diagonally it will reveal the twist. If you do a quick Google image search for "hyperbolic paraboloid" you will see what I mean. A surface built out of lots of straight lines, but is globally saddle shaped and obviously not flat. Technically, it is an "anticlastic" geometry.

-Bob