Is tempered hardboard / masonite a bad idea for spoilboard? I already want to re-do my OX’s bed with a design that exposes the extrusion grid for clamping, so I was trying not to cut the 3/4" MDF that I intend to re-purpose into lower pieces with 1/2" MDF spoiler pieces on top of it. I put a piece of masonite on as a spoiler, and then wondered whether I was doing violence to my solid carbide endmill…
I doubt masonite will dull good bits.
I only cut wood and I have moved away from clamps and now use double back tape.
Only this tape works [https://amzn.to/2UENRCq] I had been using it for work holding during turning.
It is easy to set up, no clamps in the way and it comes off easily.
I have an MDF spoil board and then a 1/4 fiberboard clamped over that. I routinely cut through about a 1/16 and replace the upper layer frequently leaving the MDF as a repeatable base.
This approach may not work well if you need depth cuts like dato’s to better than 1/32, I do not :).
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Masonite is, unusually among processed wood products, pure wood. It should be better for carbide bits than MDF, and way better for HSS.
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That exact tape is what I’ve been using so far, and I agree it’s awesome. I might well have bought it from your link earlier… 
You can always use the super glue between tape method as well.
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@HalfNormal I’m more likely to use that on metal though. The spectape is just about perfect for wood IMHO.
I’ve been using MDF spoil board for years and never had any appreciable wear on my carbide bits. Granted a lot of times I’m cutting aluminum so, slower speeds then someone might be traveling trying to do wood. The thing I like about it is how it’s so flat, real wood is going to have some variation and so unless you cut more deeply below your product you might end up leaving a paper thin sheet on the bottom if you don’t have your heights set right.
I’m thinking of surfacing the non-gloss face with it sitting face-down on MDF, but first I’ll just set it on MDF glossy-side-up and run an indicator across it to see how closely it squares. My next project will be cut from 3/4" hardwood plywood that I’ve measured between 18.1 and 18.2 mm everywhere I’ve checked, and almost everything is just contouring through the stock, so cutting to -19mm will be fine as long as the bed is trammed within .5mm total deviation.
I don’t think masonite makes sense for large thin pieces with upcut mills, because it is so flexible and will make it too easy for the bit to suck up the work. But I think it’s fairly consistent, and using is as a spoil layer on top of MDF it seems like getting the benefit of the flatness of MDF with a lower replacement cost.
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I ran into another problem today. I keep my shop quite dry (protecting the machine tools) and that might be why some plywood was somewhat warped by the time I went to cut it, about a week after I bought it. Tape wouldn’t hold the corners down, and I had to jury-rig a clamp system because I haven’t set the bed up yet for clamping. So that reinforced for me the idea that even if I depend primarily on tape, I’m still likely to want to have an integrated and working clamp system.
(Ultimately, it didn’t matter very much this time because I didn’t notice that I lost my X zero. Oops.)
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Cutting wet or warped wood is problematic anyway. You can constrain it but as soon as it comes out of the constraints it will usually re-form close to its pre-constrained shape.
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This wasn’t wet, before or after exposing to my 35% relative humidity shop after sitting for a while in the probably 85% relative humidity home depot. It’s just a few mm of warp in 18mm ply, which won’t really be a problem when assembled. I need to do 3mm and 4mm pockets that need to be locally correct. But only in one place, so that’s where I’m setting Z home; the rest is only cutting contours.
(I’m cutting a Valoví chair.)
Just FYI, most wood at big box stores are very wet (>20%).
Drying wood in open air down to <10% takes many months especially if it is a laminate.
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I had the 4x8 sheet cut into three pieces when I bought it (4x8 doesn’t fit in my tiny car, so might as well take advantage of their plywood saw). Tonight I cut the adjacent piece from the same sheet to cut out the rest of the parts for the Valoví chair, and there was hardly any warp at all, much flatter. I have no idea why, but I didn’t complain.
I still screwed it down to keep the whole piece still as well as using tape so that the individual parts I was cutting out wouldn’t get sucked into the endmill, and the one part that was in a location not held down well with tape did get a small ding from the endmill, so I was right to use the spectape even with the whole piece screwed down.
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The other day, I was cutting a shape out of ½" MDF. I put spectape on the back, laid it on the (fresh, as it happens) masonite, and cut my part. It worked fine, but I was surprised how easy it was to peel the taped part back off the masonite. I thought that spectape held better than that.
When I removed the spectape from the part, I understood why it had come off the masonite so easily. It would have held more more securely if I had removed the backing so that the adhesive was exposed.
This proved that using a downcut mill was a good idea.
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