Holding down spoilboard

Looking for different ways to hold down spoilboard. In the past, I actually used nylon bolts buried in the first layer that engaged t-nuts in the t-slot frame. It held quite tight, but was problematic in that it was inaccessible.

I did switch to using some screws through angle iron on the side of the t-slot. But I was conservative on the screw depth, as I was worried about deep holddown screws from above striking them randomly, especially as the spoilboard thins out. However, it looks like there's a problem with the screws being *not* deep and numerous enough and pulled out of the MDF.

Any magic answers here I'm missing?

I find brass screws sufficient.they need to be set a little below the spoilboard and if you do accidentally buzz a little off them it isn't a disaster.

I have a steel grid frame underneath.

Dust layer of 19mm MDF is bolted to the underlying grid.

Then I just glue another layer of MDF on top. Surface every now and then. Glue new layer when getting thin.

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Originally Posted by pippin88

I just glue another layer of MDF on top. Surface every now and then. Glue new layer when getting thin.

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Exactly this...

Why would you want to screw down a spoil board? You loose usable depth, will hit a screw once in a while with the associated loss of a bit and the potential of some movement of the spoil board together with the material to be cut.

Well how else do you hold down a spoilboard? That's what I'm asking. You don't just lay it on the extrusions. It's not really accurate and surfacing would not help, as it's going to be springy and inconsistent.

You bolt first a layer of MDF or plywood on top of the extrusions, usually the same size as your spoil board. Bolt it well, countersinking all bolt heads to be just below the top of this layer. Then glue on top of that another MDF or plywood as the spoil board layer. Use generous glue for that. When it gets thin with usage, at some point, just glue on a new spoil board on top of the old spoil board.

I drilled, countersunk and screwed my first layer thru the t track and into the steel frame below. I marked the side of my mdf for each row of screws so I would know where they are in the future if I needed to know for any reason. I then used long brass screws and counter sunk then screwed the top layer onto the bottom and since I had the markings, I knew I would not hit another screw below. I am impatient and wanted to use the machine as soon as I put on the new spoilboard and did not want to wait for glue to dry. It lasts me quite a while before it gets low and when it does I just unscrew, slap a new one down screw in and back in business with no delays. I could go a few months longer before putting a new one down if I glued but I get my moneys worth out of it so I don't mind. Been doing it this way for years and never had an issue. Save your file and let the cnc do the counter sink holes when you need them again.

Well, the bed is t-slot. What I'm trying to avoid is burying the bolts again that engage the T-nuts like I did on my first rev. If something is adjusted or a bolt loosens, then you are kind of screwed. I did already have that problem when I wanted to reduce the span between supports by adding more crossmembers.

So, I figured if you're going to say "nothing can go so deep as to hit the first layer, since it may strike a bolt", well, it would make more sense to install an aluminum angle iron on the side of the extrusion with screw holes and screw upwards. Well now nothing can go so deep as to strike those screws' points, but that's ok.

Difficulty is I don't think I went deep enough and I think the screws are pulling out of the MDF. The MDF weighs quite a lot already so I didn't expect a problem. I think what may have happened is people using the nylon Raptor nails yanked super-hand and lifted the edges, but I'm not sure.

Started to raise questions like "wouldn't it make more sense to glue a block of something to the bottom of the MDF only where it's attached, something that goes beside the supporting frame not between it, so there's no screws/bolts over the supporting members and the whole depth of the spoilboard is usable?" Because that makes a very significant difference in the machine's performance and usefulness. But, I haven't done it quite that way yet.

Sorry meant to say the supporting frame under the spoilboard is t-slot. Yes it's easiest to bolt a base thickness of MDF from top down and forever bury the bolts, but the layer is unusable thickness. That is done, but that's actually pretty bad for machine dynamics overall and I want to avoid unusable thickness. In addition to making the bolts inaccessible.

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Originally Posted by milo30

I then used long brass screws and counter sunk then screwed the top layer onto the bottom and since I had the markings, I knew I would not hit another screw below. I am impatient and wanted to use the machine as soon as I put on the new spoilboard and did not want to wait for glue to dry. It lasts me quite a while before it gets low and when it does I just unscrew, slap a new one down screw in and back in business with no delays..

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I did something like that, but it was not good. I used the Raptor nails to hold down the top layer. But, there was an air gap underneath the top layer at some points, which became more problematic as the top got thinned. It was spongy, both because of the flexing of the top layer and because the span between supporting members was not as stiff as it would be with two boards glued together.

Then as people drove more into it- especially screws- the MDF was getting forced apart by the screws and nails deep enough to reach the bottom layer. Fust would creep through screw holes into the gap and jack up parts of the board between the attachment points.

Once I pulled off the top board to replace it, then I saw you really could not just screw on a new board onto the base layer. Every screw that had penetrated to the base layer, since they weren't predrilled, had created a bit of a mound around the hole from the MDF displaced by compression around the hole. At that point the base would still not be reusable without resurfacing anyhow, so there really was no benefit to doing it this way to avoid the glue.

Hi - I find the creeping fust to be a huge problem. Anyone know of a good fust remover? Prefer something in a spray can :) Peter

I use screws for my hold down on parts. I always predrill the hole slightly smaller than the screws that I am using to prevent them from forming that layer between the work piece. What size machine do you have? Many won't use an entire 1 piece spoilboard but rather mount it in sections to leave the t slots accessible for clamps. Best solution would be to have a vacuum table.

5x8 machine, I already have 2 pcs of cut MDF per layer.