An alternative strategy for clamping down flat boards.

[4DThinker]

 I previously made a couple of frame bridges to support an assembled student project for some CNC joinery cuts. Lately I've been using the bridges more often to support flat work. They have proven their worth in every case and these are the advantages they offer:

1. They are stiff and flat and remain level to each other and plane-parallel with the CNC's X/Y travel.

2. They have a horizontal side to provide a clamping edge for c-clamps to hold the work down.

3. They have aligned holes for pins to set the project board against for perfect alignment with the CNC Y axis. 

4. They attach quickly into the side frame rails with one screw on each end into a t-nut. 

5. They can be positioned anywhere along the side rails. 

So far two have been enough, but potentially a 3rd might be useful for long boards that may have a bow in them. The 3rd bridge could also be used to clamp a stop block on for repeated identical cuts using the same Y origin. Small C-clamps have been enough to hold work to them, but with a t-slot track embedded in their top any other t-track clamp could be used. 

MDF slats between t-track on another MDF panel that span the CNC frame tend to sag in the middle over time. Plywood bridges in an L configuration remain stiff and straight with no deflection under the weight of projects boards or flexing when clamped to warped wood.  With a single t-track on each bridge only 3 or four lengths of t-track are needed as opposed to several to cover the bed area. 

Even my thin bed vise for small parts that is design for sliding into t-slots can be simple held down by small c-clamps. 

When I get my new bridges with a section of t-track made I'll post another pic.   New t-track should arrive this coming week. I have a rotary axis on my person Probotix CNC that has a rail for the dead center tailstock to attach to so my bridges now have a notch in them to step over that rail. 

4D 

[4DThinker]

Clearly there isn't a great audience for this idea, but I do appreciate the likes!

Keeping the CNC bed flat has been the one irritating ongoing challenge using the Probotix CNC I own and those I oversee.  Mill it flat one Friday and find a project cut on Monday hasn't cut through evenly all around is common.  I've traced some of it down to the boards actually warping the MDF when a warped board is clamped down snug to the bed.  Adding cross beams every foot or so and screwing the MDF down to each one would lessen that problem, but add considerable hassle to change out the MDF when it needs replacing. Also the expense of the 3060 beams and corner brackets needed isn't small.  

 

My new bridges with a section of t-track running down their middle have been made. I've got some cam-levers coming that I want to use to quick-clamp the bridges to the rails. I'm currently using M5 bolts into t-nuts that have to be loosened and re-tightened each time I want to reposition the bridges.  Not hard, but getting the screwdriver or Allen wrench out each time is a slow-me-down step that shouldn't be needed. Cam levers should be here Wednesday.   I want to add t-nuts into the rails (I use drop in/twist ones now) so the bridges can just be slid up and down the rails then locked in place without lifting them out each time.  With the rail ends capped by end plates I either have to disassemble the frame a bit or cut an opening in the top of the side rail end to drop the t-nuts into. 

 

A side benefit I can foresee is that the bridges can be bolted into the bottom slot of the side rails.  For thick wood which I need more Z clearance for, this would allow clamping boards 78mm lower (60mm rail and 18mm plywood) in the frame.  The Y axis limit switches do occupy some of the underside rail space, but don't block the bottom t-slot. 

4D

[4DThinker]

Photo of bridges with t-track in the middle. Still waiting on cam levers. 

4D

[Cal]

Well thought out 4D

[4DThinker]

Thanks Cal.  While I can occasionally suffer a stroke of genius, designs like these bridges are more of a slow burn evolution of earlier ideas and years spent trying to get down to the real problems to solve.  T-track strips between MDF slats has done a relatively fine job of hold down work to cut with only a little post-cnc work required to clean up the parts.  What persistently nagged me over the years were the occasionally unexpected results that were hard to track down the reason(s) for.  I've known for a long time that MDF is not structural and more of a brittle plastic.  Plywood could be a better CNC bed if it was well made and inexpensive but those two qualities apparently conflict with each other.  It took a student project hardwood board that was severely bowed on an MDF t-track bed that had been milled flat but likely too thin to expose how the bed could be warped by what was clamped to it. Through pockets on one side cut through cleanly while pockets on the other side cut the same depth left 1/64" or so uncut.  I looked under the bed and could see it deforming as it held the warped board. 

 

As I've had better results when using my old bridges to clamp flat work to on my home CNC it finally clicked that their designed stiffness was the "secret'" that made the difference.  Refining their design a bit by contemplating just how useful they could be made led to this bridge design. 

4D

[4DThinker]

My cam levers came in.  Spent the morning trying to cut an opening in the top of the side rails so I could drop in new t-nuts.  I have a nibbler but the head wouldn't fit into the slot.  I thought about using a drill bit to make an opening big enough for the nibbler head.  Bad idea.  Got out my fancy dremel clone tool but none of the cutters I had were making good progress cutting the aluminum.   Ended up using my Milwaukee cordless trim router and a 1/8"  high speed steel end mill to cut out enough so I could tip in the new nuts.  

 

The cam levers are powder coated, and came with a M5 stud that was 1/4" or so too long.  Couldn't find any that came exactly the length I needed so I made a little jig to hold the levers and used my metal cutting chop saw to cut them to length.

 

I'm pleased with how well they work. Easy to adjust. With a little slop in the hole they drop through they were easy to line up with the t-nuts.  Shown with a jig I made to cut a student's parts with.  

4D

[4DThinker]

It seems three will be the minimum recommended bridges to deal with 99% of flat work. I found this out while cutting the parts for my 3rd bridge. I cut the top board first as it spanned between the first two bridges.  That included a shallow dado in the bottom for the vertical rib.  Apparently the plywood was bowed down some between the bridges as the depth of that dado was shallow near the middle and deeper at the ends.  Flipping it over to cut the slot for the t-track the board bowed up and the slot ended up deeper in the middle than at the ends.  

 

All is well now though as I've "fixed" the flaws.  Let the CNC mill flat the top before installing t-track, then also milling the slot to a consistent depth while glued to the vertical rib.  T-track arrived today but I'm still waiting on more cam levers. 

4D

[4DThinker]

It has been almost a year since last posting in this thread. I thought a one-year update on how well these bridges have served me would be of interest, so here goes:

 

A+.   There were no challenging CNC jobs from my students (or myself) that couldn't be held down and flat using these bed bridges.  It takes less time to set them in position and clamp the board down flat and aligned on them than it took me to insert/move t-slot clamps in my previous t-slot array bed to a good position, then square up the board with the CNC axes before clamping it down.  I've also done joinery work on my front end adjustable angle fixture, and rotary jobs on the 4th axis next to it,   Moving the bridges out of the way takes just seconds.  Set just out-of-the-way from where the CNC did its work they made for a nice place/shelf to park the clamps or shims or next part waiting to be cut.  The bridges have also come in handy as temporary support planes to hold an occasional odd thick lump of wood that might have been resting on the rotary tailstock rail precariously while a 3D surface was cut on it.  

 

These clamps take advantage of the t-slots in the frame rails my CNC is made from.  Not every CNC would work, obviously.   When I find a larger shop space I may purchase another CNC from Probotix.com and order it with NO top as I know my frame bridges will be far more useful than the MDF top they normally provide. 

 

4D 

[Cal]

Good deal 4D