Has anybody used aluminum plate as the base for their spoilboard?
Currently my spoilboard/bed is one layer of 3/4" MDF as a base layer, with t-slot tracks bolted every 4" on top, and 3/4" MDF strips in between the tracks to act as the sacrificial spoilboard. The issue that I am currently having is that the MDF swells with temperature/humidity changes, which makes making consistent parts difficult unless I surface the top every day to get it flat again. I mainly machine acrylic panels that need to be a specific thickness and with recesses that need to be a precise depth, and having a warped spoilboard makes it hard to do.
I am contemplating ordering a piece of cast aluminum tooling plate to replace the first layer of MDF. Tooling plate in thicknesses of 3/4" and greater come pre-ground to a flatness tolerance of <0.005", so the piece I am looking into is 3/4" thick, 48.5"x60" (I have a 4x5ft bed). The plan in my head is to bolt the aluminum plate to the frame, then drill/tap it to attach the t tracks on top, and then still use the MDF strips which will be resurfaced. The tooling plate won’t warp like the MDF will, and although the MDF strips will still swell/shrink, they wont be able to warp bolted to the aluminum. The only variation will be in the thickness, which should be consistent across all strips and therefore still planar (within a reasonable tolerance).
I did seriously consider it. ALCA5, 583 cast aluminum is what you need to use, its dead flat and a pleasure to work with. You can get a 3/4" sheet cut that big but its going to weigh a ton. I have a 48x48" bed and quickly decided that 3/8" was thick enough and at a little over 100 lbs was manageable enough. The quote I got was about $1000 delivered. My hold down strategy is a grid of threaded holes I planned to thread mill into the cast aluminum. At the end of the day I back off to 1" thick super refined MDF which is much more stable than standard MDF and 6mm threaded holes milled into it has enough holding power for my purposes. I did set aside a 30"x24" area where I used the cast aluminum plate for working soft metals. I am still in the middle of the build so I can’t tell you how it is all going to work out.
Have you thought about putting a few coats of spar varnish on your MDF to prevent moisture absorption? My workshop benches are 3/4" Home Depot MDF with several coats of spar varnish and they’re hard as rock. My “technique” is to start with a thinned coat to help penetration, then keep dumping it on and rolling with a foam roller until the MDF won’t absorb anymore. Set it outside in the sun for a day or two. It’ll keep smelling for a week or three, but once it’s cured you’ll have a surface as tough as nails.
If you really wanted to get the spar varnish down into the MDF, put a vacuum on the opposite side of the panel for the first couple coats.
This is an old picture and I have a new design but this is what we did for our vacuum table. My new design integrates threaded holes and a vacuum table into one and is easier to install/remove.
I looked into HDPE and UHMW, but still worry about flex and warping, especially since the thermal expansion of the aluminum frame and the plastic wouldn’t match. Fun fact acrylic plastic has a thermal expansion 3X that of aluminum, which I also have to account for when machining my pieces.
I am hesitant to keep using MDF for the main bed after the problems I’ve been having. It just leaves a sour tasted in my mouth, even though I know there are better types to use which would be more resistant to humidity. I will look into using it for the sacrificial strips when it is time to replace the current ones I have.
The plate I am looking at is Vista Metals ATP5 5083 plate. The size piece I need would be just over 200lbs, and around $1500 delivered. I looked into thinner, but the flatness tolerance goes up to +/-0.015 instead of +/-0.005 when you get thinner than 3/4", and for the price increase I’d rather get a flatter piece that is also sturdier and heavier to help with vibrations as the machine moves.
That’s an awesome looking bed/vacuum table! Is that one big piece or a grid of smaller vacuum panels? I think I’m going to stick with blue tape/super glue for my hold down for now, but if I ever need a vacuum table I would probably design one similar to yours.
Shipping and weight get to be problems when you have a 5x10 bed. It is a bunch of smaller panels. An LDF spoilboard goes on top of everything. For smaller bed sizes there is also stuff like AirWeights.
That is a good price. I would say go for it if you have some help getting it moved up on the bed. The thinner sheets can be skim cut with a surfacing bit if flatness is all you are worried about. I put 3/4" plate in my limited metal machining area since the weight of the sheet wasn’t an issue. I can really tell how much more rigid and solid it is but it could be overkill. Make sure you have all the 40-80 extruded aluminum crossmembers aligned with side rails in height. I noticed on mine I didn’t pay enough attention to them when I was first putting it together and had some pretty misaligned joints in height. They needed to be clamped to remain square while tightening the side screws.
