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 horizontal edges to provide clamping surface 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 (or cam lever) on each end into a t-nut.
5. They can be positioned anywhere along the side rails.
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Waiting for cam levers
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| With cam levers to permit quick positioning |
So far two have been enough, but a 3rd will 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 can 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 or T 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 2 or 3 lengths of t-track are needed as opposed to several to cover the bed area.
So far I haven't come across any flat work I couldn't clamp securely down to these bridges.
My strategy so far for clamping down flat cuts is to use the center of the board as the Home (start/end) point. Then I make sure the bit won't try and pass though a clamp during cuts. I can offset the start point to the center even if zeroing the bit on the board edges.
I have some thin cork sheets I bought that might be useful as shims to protect the bridges from through-cuts. A 1.5" square to put under boards where each c-clamp ends up should be enough lift to account for router bit penetration. I'll pre-cut a stack of them and keep them stored close to the CNC. 1/8" Baltic Birch plywood scraps can also be used as shims.
I used a self-centering punch to make sure the screws for the t-track would be centered in the screw holes. Their screws go through the top plate and into the top edge of the vertical rib.
Locking the bridges down doesn't take much time, but does require getting the Philips screwdriver out and putting it away when done. The cam levers are the magic bullet that makes the bridges easy and quick as possible to set in place and use.
Now I'm contemplating how best to accommodate thick/tall work that I could cut if I could sink it down into the frame. The bridges could be bolted into the bottom slots of the frame to gain 78mm save for the Y-axis limit switches that protrude into the 30mm width of the rails.
The day the cam levers arrived I cut a notch in the top end of the side rails to slip t-track nuts into for them. The levers came with a stud too long but none were available the size I needed. I cut the threaded studs down to the the length needed using my metal cutting chop saw.
So far I'm happy with this design. The cam levers make a huge difference in the time it takes to set up for a cut. When I can't use the t-track clamps small C-clamps do a great job of holding down the work. I just have to take care when setting up a job to not run into a clamp.
The black t-track came from Amazon.com:
The cam levers came from Zoro
Comments and questions welcome.
4D
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