What combination of speeds, feeds, bit, depth of cut, etc. will give a clean cut in 1/8" hardboard? Mine is just tearing like crazy. This is with a 1/8" single O flute upcut bit, speed around 2 on the Makita, feed 762 mm/min, depth 1 mm. Does it matter which side I cut from?
The only way I got anything looking nice in hardboard was to use a downcut bit, and then it depended on if it was down tight to the spoilboard.
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I’m thinking of redoing the struts in 1/4" plywood as that should be cleaner and a lot more rigid. I can’t imagine the hardboard adds that much rigidity, it feels practically like cardboard.
It adds a lot. It also should look better. 
More than you would think. Still, the plywood will be better.
I am using 1/4" MDF with a melamine coating. The melamine makes for a pretty good cut edge, though there were still some issues. I’d like to do something with 5.5mm acrylic, but it’s so expensive still.
I might suggest that you look at subfloor material. At the local Agent Orange big box, it’s cheaper than 1/4" plywood. It’s 5mm thickness, so a little thinner but significantly less money.
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I had a lot of tear out too. However, just a little bit of sanding knocked that fuzz off.
I started off thinking the same thing you are. I made an X gantry for LR3 using 1/8" hardboard, and then replaced it with 1/4" plywood.
I later found out that Ryan recommends, for the strut plates, use of more rigid materials such as hardboard, OSB, or MDF, all of them preferred over plywood. The 1/8" hardboard may seem flimsy but for its role in this, basically doing the same “skin” type thing that happens on torsion boxes, it works well.
I had saved the first gantry made with hardboard, and I went on to use it to make another LowRider v3 for carrying a plasma cutter.
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I got some cheap 2 flute 1/8" straight cut bits and they seem to cut a little cleaner than the upcut. I think with some careful cleanup and installation these will be adequate. I think it’s better to reinstall the screws completely rather than using the keyholes – I damaged my last set of struts by bending them that way.
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btw, flipping the hardboard to cut on the other side made no discernible difference in my small test.
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what about taping the area where it should cut?
New, sharp downcut or straight bit. Secured stock (double-sided tape or painters tape + super glue). A drum/belt/palm sander. And one of these:
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Got myself a random orbital sander, cleaned up the struts carefully and they’re looking pretty decent! Thanks everyone for your help.
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I went the harboard route. Yours look to turn out great. Mine did look rough but 320 grit and it cleaned up pretty easy. Like @DougJoseph said, just knock the fuzz off. I used the 1/8" Single Flute Carbide Endmill and happy with the results.
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