Table rails for slab flattening

So have finally got to the point where im going to be attaching the cnc to the table. I plan to do the “run of the mill” cnc stuff i.e. cutting plywood and MDF. But I also want to be able to flatten slabs and use the LR3 as a jointer for bigger pieces. My plan is as follows

Table with top made of 1/2” OSB
Two layers of 23/32 OSB that are 3” wide running the length of the table to create rails for the CNC to ride on for a total height above the top of the table to be 1.43” that should give me the ability to that the LR3 be able to lift the remaining .57” to level the slabs I want to. For the times I wish to just work with plywood I figure on either a 3/4 or 1/2 spoil board or build it up more if I am working with thinner material.

That’s what I’m planning but I am sure I’m missing something. I would love to hear what all of you have to say about it.


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So you’re moving the LR3 up 1.5" so you can flatten 2" thick slabs?

I would have thought there was plenty of Z available for that unless you’re planning to use a particularly long bit?

It does I thought it went only 1.5”? If I understand it correctly, the higher you go the more room for error in your accuracy. My thinking was if I lift it up to near the slab height it would increase my accuracy.

I don’t think I need a long bit as I’m not going very low and when I’m not slab leveling I can build up the the cut area with a thick spoil board.

I don’t have an LR3 so I’m just running from ‘theory’ here. This is similar to something I was wondering.

The documentation says 80mm of Z height (3.15") for which is what I was remembering, but with the callout of 1.5" of cut depth. I suspect the difference there is from tool clearance. Hopefully someone might clarify what all this is in practice?

The rigidity of the machine will decrease with height, so there’s a definite logic to raising the entire machine to allow big jobs like surfacing to complete faster. The ideal would be something where the table height can be easily raised/lowered without adding its own rigidity problems.

For my personal situation (flattening slabs maybe once or twice a year, tops), I’d be tempted to try find a short bit to do it with (I’m currently using an utterly terrifying 3" carbide insert monster in a 1/2" router) and just have it take twice as long at high gantry height, given how much effort would be going into finishing after flattening anyway.

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Agree, raising the rails so gantry doesn’t need to raise as high will let you cut more accurately and/or faster. Should be able to use shorter/cheaper bits too?

Originally had lofty dreams of making a “Drop Table”. However, am currently planning to raise rails with stack of sealed MDF strips so only a 1/2" wide dovetail bit is needed to speed through slab flattening using Jamie’s Surfacing gcode generating web script. Will use (register?) bolts to enable quickly adding/removing middle MDF layers without having to figure out belt holder locations each time.

Counter intuitively for me, was learning from folks here that surfacing with a smaller diameter bit will help with overall speed and quality, especially if router isn’t perfectly trammed (consider using layers of tape). Ridges requiring post sanding will be larger with larger bit. Hitting knots with larger surfacing bits seems to not help too. In general, maximizing material removal rate with LR3 seems to involve cutting deeper and slower, rather than shallower and faster.

So, I think (without having tried… ) consensus here is to use 1/2" dovetail bit for surfacing with a LR3?

Thanks for starting this topic. Have a bunch of slabs I need to flatten this summer. Hoping someone who’s tried out different configurations with their LR3 can share what’s working for them? @cnckitchen :grinning:


Drop table is exactly what it sounds like to me, but from the dropped perspective.


Big fat spoilboard for regular sheet goods, remove the spoilboard for big stuff vs built for big stuff and add a fat spoilboard for thin stuff?

Back on topic, I think it wouldn’t take much to add the side strips, so it might make sense to at least get the gantry built, add your tool of choice, and measure how much room you have. Don’t forget to add some travel clearance and/or whatever room you need for clamps/fixtures.


I think the 1.5" limit applies to thru-cuts. With maximum 3" clearance the thickest piece you can cut is 1.5" where your cutting bit extends 1.5" below the gantry.

A workpiece thicker than 1.5" and you can’t raise the bit high enough to clear the top of the workpiece unless you reduce bit length, but if you reduce the length of the bit length then you can’t cut all the way through.

For surfacing you could use a shorter bit and the workpiece can occupy more of the 3" range. You’re not limited to 1.5". I don’t know to what degree stiffness would suffer. It might be ok.

Right, that makes sense.

So there’s 80mm/3" of overall Z travel, with the Zmin point being basically contacting the table.

If you want to be able to cut through a 40mm/1.5" piece, you need the tool sticking at least that much out below the gantry and then to be able to raise the gantry up that much to get the tool out.

From there, I guess it depends on where the lowest point of the toolholder ends up relative to the gantry. Assuming a best case scenario of that lining up with the gantry then something like a 16mm height straight cutting router bit could allow you to surface something ~60mm thick, with appropriately careful feeds/rpm for the rigidity at that point. Does that sound right? I guess the shank of the bit may need to be trimmed to fit the router collet depth etc.

That would actually be a fun test, try to do the same surfacing job in the same material at 60mm and 10mm heights and see what the actual practical difference is.

While not related to this topic specifically, I realize now that I have built my table 1” narrower than the recommended minimum (59.75”). How is this going to impact the assembly? Should I just trim the conduit down an inch or just proceed as normal?

The conduit and strut length need to fit your table. I don’t know what you mean when you say the recommended width.

If you mean you are trying to build a 48" wide LR3 and your table is 1" shorter than you need to do that, then you will have 47" cutting width. The tube length and strut length need to be fit for the size of machine you have, which will be determined by that table width size.

Looks like I have to build out the one side of the table by an inch or so :upside_down_face: