Getting my LR3 table dialed in and not sure if I made a mistake. My table is the “janky” version, a 2x4 frame with a 3/4" OSB skin on top. I then put a 3/4" MDF spoil board on that. The gantry rides directly on the OSB on Xmin side and the Y rail is screwed / mounted directly to the OSB on the Xmax side. Here comes the issue. There are spots on the table that are definitely low and high but doesn’t appear to be on a slope. This is causing issues with through cuts as some areas get cut very deep into the spoil board and others don’t get cut all the way through which makes lots of hand cleanup to get the parts out. Ok, so I need to surface my spoil board at least (maybe the OSB as well). But…I’m wondering do I have an issue with my Y rail or perhaps the OSB surface on the near side not being smooth or flat enough. OSB tends to have surface variations due to the strands/chunks in the board. I’m wondering if that is a source of my issue.
So the question, do I need to replace the OSB at least where the gantry runs for a more flat surface? Or should I just surface the OSB in the work area to get things flattened out? And then of course the spoil board as well.
All of this is in prep for making it a vacuum table. My current plan is to cut the plenum into the OSB then use the MDF spoil board as my bleeder board.
The larger a table is, the more of a massive job flattening by surfacing bit becomes. Also, the larger a table is the harder it is chasing zeros to get flatness. If you can figure out where the low spots are, you can possibly shim the low area to help correct it — requiring less deep of a surfacing cut. Taking off a mound is not as hard, but taking off practically the entire top half of the MDF sheet, trying to get it down to match a low place, is where you wind up with a huge amount of problematic, dangerous-to-the-lungs MDF dust generated by the surfacing. Good dust collection is really important for surfacing, and wearing a ventilator type mask to protect you from the dust is important. MDF dust is mean stuff on your lungs. Consider using my floating-Z dust shoe mod to keep the dust collection happening down close to the surfacing cut.
Also was thinking that running the rollers/rail on the OSB with 3/4" MDF spoil board means that you are lifting the Z axis up further than necessary to do your cutting and losing some Z travel. I have read repeatedly that the LR3 is most stable and rigid when the core is at the lowest Z point, and loses stability/rigidity as it rises.
Maybe you might consider laying down a narrow strip of 1/2" or 5/8" or 3/4" MDF over the OSB. That would give you a nice smooth running surface, and would let you do your cutting closer to the lower Z travel limit.
Very good points. I had been thinking about using 1x boards as “rails” to run on and get the z at its minimum. I know MDF is more dimensionally stable so maybe I’ll look into that
That could work as well (1x3 for example), I think that they are actually about 3/4" thick, so probably close to your MDF spoil board. As you are mounting them securely to a fairly stable surface (3/4" ply), I think that they would be pretty stable. But if you are chasing zeros, MDF may (or may not) be marginally better.
Edit - cupping and twisting may be an issue with most softwoods, so MDF or something similar (trim boards) may be a better choice.
I did some probing and found that the surface on my table is about 2mm out of flat with a small section a at 3mm difference. So about 50% the table is at one level, about 40% of the table is 2mm lower and about 10% of it is 3mm lower than the “baseline” half of the table. It does not appear to be an issue where one end or side is lower as it kind of dips then comes back up and has a smaller variance along the X axis. Does that sound like a reasonable amount of variance and should I just surface it to flatten it out? Or is there some other issue going on here? I still plan to address the “smoothness” of the rails by adding a MDF strip on top of the OSB (which I realize will mean things may well change). Just wanted to get opinions on the current state.
This is probably what I would do. I’d put 3/4" MDF runners on both sides of the machine. One for the rail to be mounted to and one for the rollers to run on. You’d still be able to slide your 3/4" MDF spoilboard between the rails for running your cuts on.
I’d attach the MDF to the OSB from the underside of the OSB up into the MDF.