Keeping Slabs in Place with CNC

[Glenn1]

I just received a CNC machine for flattening slabs.  I was using the Woodpecker Slab Flattener which worked nicely but I've gotten tired of filling my pockets with saw dust.

The machine has 2x4's across the metal rails so I can use the skid steer to gently place the slab on the table and remove the forks.  
 

Initially, I used a clamp at each corner to hold the slab in place. As soon as the cutterhead engaged the wood, it was pushing the slab.

I am looking for ideas to keep the slab in place without drilling into the wood.  All suggestions are appreciated,

[Poquo]

I mount stop blocks on the ends and sides of my work table , then use spacers and wedges to hold slab in place .

[Raym]

I do the same thing.....I will sometimes flatten a slab using a 4x8 cnc bed. I screw a ledger board on all 4 sides and then use shims wedged in to secure. You should only be taking off 1/32-1/16" a pass so there shouldn't be a lot of force. 

[Haleiwa]

Can you set weights on the slab and move them as the head works across the slab?  Tedious I know, but it won't mark the slab.  That or screw down the very end and cut the end off after flattening. 

[Hilltop366]

Vacuum?

If you had some vacuum holders that were height adjustable for flattening the first side and some stops to reset to level for the second side.

[scsmith42]

Glenn, my first thought would be to use some conduit clamps or plumbers strap to mount some pipe clamps onto the top of your 2x6 slab supports.  Gently tighten the pipe clamps against the sides of the slab, and then screw a stop block at both ends of the pipe so that it can't move from side to side (or use a c-clamp).  

You will want the pipe clamps to be able to float from side to side when tightening them, but once tightened you will need to clamp a block.


Fast and inexpensive.  Let me know if you want a sketch of the concept.

[boonesyard]

What kind of CNC did you end up going with and how big? I've been looking at them, wow there's a lot. Been looking at the Slabmizer with the self feed option, but you're at 20K all said and done. You can do a new cnc for that, or close.

[Mike W]

Looking at that setup (nice by the way) the 2x4's look rather easy to replace at some point if needed.  I think I would just drill a series of 5/8" holes and buy a length of dowel, with a couple wedges / shims, you could just but in the dowels in a close hole and tighten up with the shims, best part if the dowels are too tall by a bit, no issue trimming with the slab.  Some day when I grow up I'll be getting some cooler toys like that, for now its the trusty router sled, which I don't mind, never in a big hurry when working slabs, to date they have been for our own use, and not any type of production.

[Glenn1]

Thanks for solutions to this issue.
Scott, can you make a diagram of your solution. I'm having trouble visualizing your idea.  
The dowels are also a good idea.

I had someone custom make it for me at less than the commercial units sell for.

[SawyerTed]

Would clamping the slab along the length with clamps let into the 2x4 frame underneath work?  With the 2x4s secured to the frame, the clamps could be laid into and attached in notches in the 2x4s to prevent side to side movement.  The clamps would prevent lengthwise movement. By clamping lengthwise you don't crush live edge or mar the edges.

[terrifictimbersllc]

Think about what most band mills are able to clamp with two side stops and a movable clamp on the opposite side.

[doc henderson]

there is some great double sided tape out there that might augment whatever you decide to do.  (gorilla ect.)

[red]

Google April Wilkerson River table . . there is a 23 minute YouTube video about using a cnc machine on a large slab 

[nopoint]

I have access to a similar machine and have flattened a few slabs with it. Typically we use it for more detailed work. Usually we have gotten by with screwing something like a one by one to the ends of the slab and then screwing that to the table. Sometime the slab won't lay very well or has really bad harmonics and we will screw some cleats to the table that rest on the edges of the slab. The harmonics/vibration in addition to being super annoying can make for a poorer cut.

[customsawyer]

One thing I have learned with the Slabmizer that I have is to not clamp it super tight. Some slabs move a little when you start removing material. I find it's better to let it move while I have it on there and can fix it.

[scsmith42]

Thanks for solutions to this issue.
Scott, can you make a diagram of your solution. I'm having trouble visualizing your idea.  
The dowels are also a good idea.

I had someone custom make it for me at less than the commercial units sell for.

Glenn, what I'm envisioning is using 3 - 4 pipe clamps as the base for your CNC router.  The clamps would attach to your cross members with strapping. You need to allow room for the clamps to slide from side to side for adjustment to live edge irregularities, and for adjusting to different widths.  The clamps could be attached to the wood cross members with some type of strapping that would allow them to slide from side to side.
I don't think that you need to worry about longitudinal  movement because the strapping will keep the clamps in place, and once clamped they won't be able to rotate.  The strapping should be loose enough so that the pipes can be slid through it to allow for adjustment.
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[Larry]

Make some eccentric cam clamps.

Drill 1" holes in your 2 X 4 every 1" or so.  Use a 4" long 1" diameter cold roll for the axle.  Cut 3" discs out of 3/4" plywood for the eccentric.  Drill a 1" hole in the disc about a 1/2" from true center.  Make a steel bar with two 1/4" pegs to fit into 1/4" holes in the disc to cam the clamp.

Fast, cheap, and don't take much room.

Lots more money would be pneumatic cylinders to clamp similar to bar clamps.  I think maybe the luxury solution.