Spoil Board Question

[Bill George]

On my Fine Line Automation I was going to use both a 1/2 inch sheet of plywood (first layer) and a 3/4 sheet of MDF but now just thinking the 3/4 inch will be enough. I am someday going to install the tracks I have for hold downs, which would involve routing out the slot for the track. Suggestions?

[Mark Bolton]

I remember you saying the 1/2" ply and then the MDF a while ago. I would be concerned with the ply. We have a UHDMDF plenum for the vac and then just run 3/4" spoil over top of it. Not sure about your machines construction details but I would think if no vac I would run a layer of MDF directly on the frame of the machine, deck it off flat, then either screw or glue your spoil board to that. If you glue'd it to the base layer you could run your spoil board down to zero and then just glue another one on.

[Bill George]

No I would put the 1/2 inch directly on the frame and then 3/4 directly on top of that for the spoil board and face that. The frame cross members are predrilled and tapped M6 and I have a bunch of flat head screws and a drilling pattern in gcode now. I thought first I would just drill both sheets together, and when the top 3/4 was shot I would replace. Now I am thinking I am wasting a 1/2 inch of Z height?

[bobby milam]

I would not use the plywood as it will move some with changes in humidity. The MDF is much more stable.

[Gary Campbell]

Bobby....
My experience has been exactly the opposite. Although mdf doesn't appear to warp like plywood does, it is by far the least stable of all the sheet goods with humidistatic changes

[Mark Bolton]

Im with Gary on the MDF changing with humidity but my issue with the PLY is that at least in my world it is has never been consistent enough that I would bother to put it on the machine in the first place. My point about the MDF base and MDF spoil is who really cares about the base layer. In my short experience I would seal the bottom and four sides of a base layer of MDF and counterbore to bolt it to the frame, then I would deck it off flat (or maybe even skip that step) and then I'd glue my spoil board to the base layer (if I had no vac). No fasteners in the spoil board. Do whatever you want, machine it down flat when needed, and when you get close to the glue line, deck it off and glue on another one and start over.

This would all be sans t-tracks as we never have much of a need for them. If you need t-tracks then there would have to be an alternate plan.

As far as only using a single layer of MDF as a spoil board, only you can determine if that would give you enough support over your frame. Id think yould need very closely spaced supports for just a single sheet of 3/4 but Im sure you feel it would work if your considering it.

[bobby milam]

learn something new. I've always been told to use MDF for that reason and never questioned it much because that is what I see from so many manufacturers.

[David Falkner]

On our Saturn 2x4 I screwed 3/4" MDF to the frame (prototype Saturn so I had to drill and tap 20 holes in the hardened frame - ugh!!). Then I put 1/2" MDF on top of that and attached with plastic screws. I thought about the loss in Z height but the Saturn has more than normal Z height anyway and there's only be one time since I finished the machine 18 months ago where I came close to needing more Z height.

I don't use T tracks or vacuum, fwiw. I just screw pieces directly to the spoilboard or use hold-down clamps.

David

[Mark Bolton]

learn something new. I've always been told to use MDF for that reason and never questioned it much because that is what I see from so many manufacturers.

We use it all the time, spoil boards, fixtures, and for rare work. It is definitely stable to an extent and is flimsy enough to be forced into submission. But we see the dimensional changes with humidity and time almost daily with regards to fixtures. Work holding fixtures where you deck off parts right close to the top of the fixture over and over you will start to see the tooling cut the edges as they swell, you will periodically have to deck off the bottoms and tops to keep them flat. And for a real mind bender try deep machining super thick MDF if you want a lesson in MDF stability lol. We run occasional parts in 1 1/8" that is machined 2/3 of the way through the sheet in certain areas. The tension in the sheet will overcome the vac in a heartbeat.

MDF door shops deal with warp/twist quite a bit when your cutting profiles in the front of the sheet and zero on the back.

We have to deck off our spoil board pretty regular in the humid months even when we seal the edges due to edge swell and even in the small areas where you cut into the sheet. If you dont clean them up for a period they will raise up at the edge of the cut.

My only suggestion for the MDF base and spoil is that its pretty much all sacrificial so keep it cheap and as stable as can be for a flat sheet which MDF is. 30 bucks for a sheet of 3/4 is tough to beat.

[Mick Simon]

We screwed 3/4" MDF to the M6 predrilled holes and another 3/4" MDF to it using plastic screws. Run a table mill on the top layer. We're going to redo the top layer to use T-tracks screwed directly to the bottom layer with 3/4" MDF between the tracks. That way the tracks sit lower so that there's more material to remove on tables mills to keep it flat.

[Bill George]

We screwed 3/4" MDF to the M6 predrilled holes and another 3/4" MDF to it using plastic screws. Run a table mill on the top layer. We're going to redo the top layer to use T-tracks screwed directly to the bottom layer with 3/4" MDF between the tracks. That way the tracks sit lower so that there's more material to remove on tables mills to keep it flat.

That may be what I do. Where do you get those plastic screws? I already have the T tracks.

[David Falkner]

They're showing as not available but this is what I bought - Nylon pan head screw

David

[Gary Campbell]

If I were going to use a wood product table I would bolt a layer of 3/4" MDO to the frame, sealed on the back and edges. Then I would glue a layer of 3/4 MDF to the top of it and surface that. Once it gets down to the last ~1/4" of thickness, I would glue another layer of 1/2 or 3/4 on top.

[Bill George]

My other thought since I will be using T tracks and clamps mostly anyway the need for re-facing should be a lot less. Correct?

[Gary Campbell]

Your ability to accurately measure material thickness and set up a job and toolpaths determines how "gnarly" (technical term) the spoilboard gets, little else has an affect, other than humidity swelling