Thank you Liam! This is exactly the information that I was looking for.
HDPE looks like a good option. So far, I’ve just used pieces of MDF because I’ve been doing more experimentation, engraving, wood cutting so far to get my tool chains in place.
Now I’m wanting to start cutting on aluminum with the ShapeOKO so I’m upgrading parts to get there. (HDZ, torsion box base, rubber mat, replacement for the MDF base plate). As you pointed out, unless build a new machine, the ShapeOKO is a nice jack of all trades but not suitable for precision work. I enjoy modifying this thing as much as I do using it.
Jumping in here–I’ve been trying to address sag in the middle of my XXL per some of the previous discussions on this forum.
So far, I got the rubber mat from Harbor Freight and installed that on top of my torsion box, and removed the feet from the ShapeOko itself.
I still have the stock MDF spoil board that came with the XXL, and sacrificial 3/4 in piece of MDF on top of that with threaded inserts.
The sacrificial piece is ready to be replaced–my original thought was get another piece of MDF about 32" square, 3/4 in thick, drill it for threaded inserts and run a surfacing pass over it to make sure it’s level. That’s what I did originally, but I still got sag in the center with the bed off the torsion box.
If I get a piece of HDPE–would this go on top of the original spoil board? Or do I replace the original spoil board with HDPE, and then I put the sacrificial MDF on top of that? Recommended thickness of HDPE?
I’ve found that it’s easier to consider sag / rigidity and the spoilboard material separately.
Also, it’s not just sag to worry about but pull-up and vibration during clamping and cutting, there are upward forces too. Just propping the middle of the baseboard up only addresses one of these three problems.
The way I’m looking at the machine;
Structure
Supporting the baseboard of the machine such that the key working parts are held in position WRT each other is first, to whatever level of rigidity and vibration damping you feel is required. This can be done with minimal surgery by attaching or bracing the steel straps of the machine to a base such as a torsion box.
This is literally the foundation, if this isn’t sufficiently solid and rigid you’ll just be putting band-aids on symptoms not treating causes further up the stack.
Baseboard
Then you want some layer which allows you to fix the spoilboard and workpieces in place, this is the Shapeoko baseboard. This can be MDF or there’s various extrusions that can be used. Whatever this is it needs to be strong enough to bolt to the base and resist the clamping forces you expect to apply to it as well as holding on to the threaded inserts or whatever else you plan to use for fixing.
Shapeoko uses this baseboard as part of it’s structure because it’s a free-standing machine so these boards bolt to the steel frame and mostly provide anti-racking for the steel base in the plane of the baseboard to stay square. They are not able to provide any useful rigidity in the Z axis but being MDF they have a good loss coefficient and damp vibration modes in the steel. If you’ve bolted down your steel frame you no longer need to rely on the baseboard for anti-racking or vibration damping.
Depending upon your intended use MDF may not be the material of choice for the baseboard. If you want to run coolant sprays etc. then something not MDF that can cope with water would likely be better.
Spoilboard
Finally you want a spoilboard surface whose job is to be skimmed flat periodically and allow the mounting of your workpieces or jigs. Acetal type materials let you use adhesives (I think), HDPE may be less accomodating here. Holes cut in the right places to access the clamping points in the board below etc.
Great information. As I’m considering the 80/20 extrusions for the new base material, I’m wondering if I should put extra slot nuts in the extrusions to allow me to fasten them to the torsion box from below.
Also, @Gray do you mind referencing the HF mat that people are using? I want to make sure that it is the type that I’m thinking about. I have closer places that I have bought ‘horse mats’ from for various purposes. I want to make sure that I’m using a known good solution.
I was going on a couple of observations (including the guy with the over-oiled moustache) about double sided tape sticking to it well for machining, so not so much about permanent gluing, rather workholding.
I’ve ordered some for testing so I’ll find out for myself soon.
