Laminating Plywood for Bench Top
#11
Question 
I have a Kreg 44" x 64" universal bench for which I need a flat top for my CNC.  I don't want to make a torsion box, so am thinking of laminating 2 3/4" sheets of quality plywood.  

Titebond 3 spread with a roller and 1 1/4" screws every 6"; sound reasonable?  Anyone ever do this?

Thanks.
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#12
Guess it depends on how flat the plywood is before you start.  I did it for a router tabletop.
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#13
(06-06-2022, 06:36 AM)Bill Holt Wrote: Guess it depends on how flat the plywood is before you start.  I did it for a router tabletop.

Exactly.  Plywood isn't totally flat so it depends on how flat you need it to be.
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#14
(06-06-2022, 08:19 AM)Kudzu Wrote: Exactly.  Plywood isn't totally flat so it depends on how flat you need it to be.

Yes. Most of the time, plywood sheets will have a slight cup or warp. When you glue and screw two together, place the cupped faces together, and hopefully, they will come out flat.
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#15
(06-06-2022, 09:19 AM)Willyou Wrote: Yes. Most of the time, plywood sheets will have a slight cup or warp. When you glue and screw two together, place the cupped faces together, and hopefully, they will come out flat.
Cabinet quality mdf is very flat if that wlll serve your purpose.   Roly
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#16
(06-05-2022, 08:33 PM)APZ Wrote: I have a Kreg 44" x 64" universal bench for which I need a flat top for my CNC.  I don't want to make a torsion box, so am thinking of laminating 2 3/4" sheets of quality plywood.  

Titebond 3 spread with a roller and 1 1/4" screws every 6"; sound reasonable?  Anyone ever do this?

Thanks.

A torsion box will be flatter and stay that way.  The Kreg table isn't all that beefy.  If you use plywood or built up MDF you will need to add some cross ribs to keep it flat over time.  Several folks on the 1F forum have commented about this issue.  Keep in mind that many CNC users are doing crafts, where tolerances aren't as important as with making cabinet/furniture parts.  You will quickly find issues with cutting out full depth parts, or dados and rabbets, etc, if the table isn't dead flat, or doesn't stay that way.  

I made a torsion box for my CNC.  It's larger than my bench and I was too lazy to put some longer ribs on it.  Consequently, the torsion box came out with a slight hump in it because the plywood I used had a slight cup, and that's not going to go away.  But the business surface is dead flat after I surfaced the MDF spoilboard, and it will stay that way.  

If you really want to go the plywood, or MDF, route, I would use 3 layers, plus the ribs on the table.  And then an MDF spoil board on top of that.   

John
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#17
(06-05-2022, 08:33 PM)APZ Wrote: I have a Kreg 44" x 64" universal bench for which I need a flat top for my CNC.  I don't want to make a torsion box, so am thinking of laminating 2 3/4" sheets of quality plywood.  

Titebond 3 spread with a roller and 1 1/4" screws every 6"; sound reasonable?  Anyone ever do this?

Thanks.

That is an awful lot of screws and drilled clearance holes. Also, depending on where you buy the "quality plywood," 1-1/4" screws may be longer than the 2 layers of 3/4" ply are thick. I assume that the screw heads will be on the underside of the top.

Instead of most of those screws, you might want to look into using a vacuum bag for your glue-up.
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#18
(06-06-2022, 12:54 PM)Roly Wrote: Cabinet quality mdf is very flat if that wlll serve your purpose.   Roly
I am going to use mdf for the spoil board; but wanted to use plywood for the top.  I prefer it for holding screws for attaching other things.

(06-06-2022, 02:47 PM)jteneyck Wrote: A torsion box will be flatter and stay that way.  The Kreg table isn't all that beefy.  If you use plywood or built up MDF you will need to add some cross ribs to keep it flat over time.  Several folks on the 1F forum have commented about this issue.  Keep in mind that many CNC users are doing crafts, where tolerances aren't as important as with making cabinet/furniture parts.  You will quickly find issues with cutting out full depth parts, or dados and rabbets, etc, if the table isn't dead flat, or doesn't stay that way.  

I made a torsion box for my CNC.  It's larger than my bench and I was too lazy to put some longer ribs on it.  Consequently, the torsion box came out with a slight hump in it because the plywood I used had a slight cup, and that's not going to go away.  But the business surface is dead flat after I surfaced the MDF spoilboard, and it will stay that way.  

If you really want to go the plywood, or MDF, route, I would use 3 layers, plus the ribs on the table.  And then an MDF spoil board on top of that.   

John
I've been looking through the 1f forums and seems like many use plywood and it's working alright.  I don't feel like spending the time to make a torsion box, but now I'm questioning myself.  If this doesn't work well, then I probably will.  I will add on to the table to make it more rigid.


(06-06-2022, 07:13 PM)iclark Wrote: That is an awful lot of screws and drilled clearance holes. Also, depending on where you buy the "quality plywood," 1-1/4" screws may be longer than the 2 layers of 3/4" ply are thick. I assume that the screw heads will be on the underside of the top.

Instead of most of those screws, you might want to look into using a vacuum bag for your glue-up.
I'm getting the plywood from a local lumberyard; not Home Depot or Lowes.  I went for a look and what they had seemed far better than what you can get at the big box in that it was flat and smooth.
Laugh  I don't have a vacuum bag setup and was hoping to keep it simple.  Yeah; that is an awful lot of screws and was wondering if anyone has done this before to see if that many were necessary or if I'd be better off with construction adhesive.  I'd probably remove the screws once the glue dried.

Appreciate the responses.
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#19
I really don't know your specific needs for this perfectly flat top, so I probably should just keep quiet, but that's just not my style. If you have a "Bargain Store"
in your area, they sell new solid core door slabs for $20 each. You can buy two, 1 3/4" thick, 36" doors and put them together with joint connector bolts. Similar to what is used on a formica counter top. These doors are new, VERY heavy, and flat, but for what ever reason, did not pass final inspection. You can also sometimes find them at "Restore Stores"
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#20
I guess since you've got a spoil board, one would assume you are going to flatten that with the CNC, but you still want to start with as flat a sub top as possible.

I've made torsion tops this way, but I think its applicable to your situation:

Joint and parallel rip a couple 2x4's and fasten them to sawhorses. Stretch a string diagonally and ***** until they are level and co-planar.

With a torsion box you stop there, but what I would do in your case is put cross supports on top of the 2x4's to use as cauls (again, jointed, parallel ripped & the bottom fastened to the 2x4's).

Spread glue and clamp the upper cauls down before screwing.

It's probably overkill for what you're doing, but it will be a good bet for a flat top. But doing it this way, you don't need perfect material (good luck finding that anyway!)
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