I’m about ready to reassemble my CNC. I need a solid flat baseplate to mount my CNC and to keep it true. Right now I’m planning on laminating 2 or 3 layers of 3/4" MDF sheet to make up this base plate. I’ll glue these together with wood glue and paint the exterior with shellac to seal it. I was thinking about screwing the MDF sheet together, but haven’t found any similar reported recommendations.
I plan to do this fabrication on a friend’s large table saw top to make sure the assembled base plate is as flat as possible.
Not having a great deal of experience with MDF, I’d like to ask ToolGuyd readers for comments as they likely have much greater experience. Thanks!
Are you going for absolutely perfect flat or live with .005+/-? if its the Second I’d say your on the right track with your build. When selecting your MDF be sure to get your sheets from the middle of the stack, not that MDF is prone to warp just a good rule of thumb, also there is almost no give to MDF but still be sure to torque your screws the same, I would clamp all the layers pre drill and counter sink each hole and use a (I can’t believe I’M saying this) drill set at low speed with a clutch set around 200 in lbs roughly half depending on the drill unless its a new one with insanely high torque, if so turn that baby way down. I would uniformly layout the screws at 8 inch (some might say 12 is fine) with even and low torque you would get flat results. Also DO NOT USE fine threaded screws you WILL strip out your holes in MDF, I would recommend 2’ construction or deck screws for 3 layers.
I would think that if you’re looking for dead flat and warp resistant, then a torsion box design is better than laminating sheets. This design is often used for assembly tables, the Wood Whisperer built one some time ago and provided instructions but there are many other examples out there.
@ crBondt: Thanks for the detailed reply! Ideally I’d like +/- 0.001, but that’s
unreasonable based on the project economics. I’ll accept what I can get, using MDF, and best assembly practices. Speaking of such, you suggest that I screw the layers together on 8 to 12" centers. I did some reading last night and found one MDF screw recommendation to pre-drill holes with a diameter of 85% of the screw. I was thinking it might be better to predrill with a diameter equal to the minor diameter of the fastener. I also read one recommendation to add some wood glue in the pre-drilled holes. Your experience?
If I use three sheets of MDF, you suggested a 2" deck screw. Would you use the same pre-drill diameter for all three sheets? Going that route, will that pull together the three sheets equally or adequately?
Something else to think about is maybe not worrying about how flat your base plate is and use a sub base on top of it.
You’re going to tear up your base plate. Do you want to rebuild an entire base plate every time you want to replace it. Rather build a stable base that you can easily attach a sub base to. Then use your CNC with a flat bit to level the sub base. It’ll be more accurate than anything you can laminate. (when I’ve seen other experienced CNC’ers really need a flat base that’s what I’ve seen them do.)
My MDF experience is primary in speaker cabinet building, I always wood glued the seams, I see no problem glueing the holes, makes sense. Whenever predrilling I go for tight, about 1/32 - 1/16 smaller, so 85% sounds good, and yes I’d do the same diameter for all layers, the key is torquing them equally and as often of 8". When building a speaker enclosure MDF is perfect for a whole host of reasons, but most important to you is how flat the factory surface is(for me its how flat the resonate frequency is which translates to low vibration which I’m guessing is good for you too).
Yes, I bet those screws would work very well too, in 2" length, 3 layers 3/4"= 2 1/2" with about 1/4" counter sink should put the screw 1/2" into the bottom sheet and be nice and snug. EDIT: if using these screws use a forstner bit to make counter sink holes.
After spending some time thinking about reader recommendations and laying out a (~ 8") screw hole pattern, I’ve come to a potential problem with my plan.
For my 2x4 foot base plate, I’m laminating three 3/4" layers of MDF and fastening them together with 18 screws which require pre-drilling. Since MDF is so thirsty, I’m thinking the glue might be set up between these layers to some degree before I can finish the pre-drilling and fastener installation. Therefore I’m thinking, I should omit the gluing, or pursue another option.
That other option might be a modified torsion box. Two layers of MDF, sandwiching a 2x4 or plywood frame (1-1/2" t) that does not require pre-drilling. With this option, glue could be easily included.
You could try a glue with a longer open time like Titebond II extend, Titebond III, or an epoxy. That brings up another issue if you don’t spread the glue evenly will it telegraph through?
I wonder if a contact adhesive like what you use for counter top laminate would work. You coat the two surfaces, wait until tacky and press together. Just be sure you get it right the first time. I would definitely think about going a little oversize and trimming it in that case.
When I used two sheets of 3/4" MDF for my 2" by 3" router table. I did not glue the layers together. I just screwed them. I don’t think it was nearly as strong without the glue. I had to put a section of angle iron to keep the middle from sagging under the weight of the router.
If I would have glued the two sheet together would it have sagged as much? I can’t say, but my gut says not as much.
Of course my recommendation would be the torsion box approach as per my previous comments, however just to play devils advocate a little, I think you could still glue and screw laminated MDF with one of the following approaches:
Use construction adhesive, this has a way longer open time and is not nearly as easily absorbed by MDF. You should have ample time to pre-drill and screw.
Pre-drill and screw the bottom layer to the middle and the top to the middle as separate exercises. You could use shorter screws, say 1.25" as you would only be going through two layers with each one. Just make sure the screw pattern is offset top to bottom.
Pre-drill and screw first without glue, then disassemble, add glue and screw back together. This is obviously more work, but has the advantage of being able to line up the holes perfectly rather than trying to keep the panels perfectly aligned when they are slippery with glue. Even doing this with a screw in each corner might be a good modification to any approach.
You could use contact adhesive as @benjamenjohnson suggested, but this is very unforgiving and is usally a bit easier with thin skins like formica. I’m not sure how easy it would be to stick 3/4" MDF together, depending on how large and heavy the sheets are.
If you do go with a torsion box, then even with MDF (on edge) frame you should still use glue and screws (and brads) from the top and bottom as two separate exercises. Put down one skin first, build the frame, put glue and fasten the other skin on top, then when dry flip everything over and glue and fasten the other skin.
Commercially, on our CNC routers, we always use 3/4" MDF as a spoilboard. This is a sacrificial piece, that is screwed down to the table, and then we flatten it using the router itself, just like @benjamenjohnson said. We have a program that we use periodically to flatten the spoilboard, so that even if the table isn’t perfectly flat, the spoil board becomes flat. Also, after so many programs, we end up with lots of routes into the spoilboard, so we can get rid of this, and then eventually the spoilboard gets too thin, and we replace it.
On a 5x12 CNC like we have, MDF just isn’t ever flat over that span anyway, so this is the only way to go.
I made a torsion box base for my CNC. To build a flat one I needed a flat reference surface. I used a steel straight edge on my heavy workbench (which was constructed of 4 plies of 3/4" particle board glued together) and was surprised to find an 1/8" drop in the center of it. How to make it flat? Well, I bought 2 straight 4"x4"x1/4" angle irons as long as the bench… clamped them to the sides so that they aligned perfectly and then used a router sled across the tops to router flatten the workbench. Once I removed 1/8" off of the high spots I had a dead flat reference. I then glue laminated my torsion box on this newly leveled surface. Here is a picture of my base
@Bill_K I just had a dumb idea… So I’m whipping up my wife a craft station… A work bench… And I picked up some remnant kitchen counter top… Why wouldn’t that be perfect… MDF, laminated, waterproof, cheap, FLAT!
Love the dialog, pictures, and suggestions, all based on real world experience. ToolGuyd readers are really special people IMHO.
My take away is there is no perfect (wood) constructed “baseplate,” but there are some workarounds for optimization.
@ cr8ondt. your last suggestion to use a countertop remnant, is worth looking into. I’ll have to look around with a straight edge and see what I can find.
I’ll post my completed base plate when I get there!
