Using a CNC to solve unusual woodworking challenges.

Do a search for what a 3-axis CNC is capable of, and generally you'll find people cutting flat work on their CNC bed. Some have a radial 4th axis to do simple or complex cylindrically confined projects. Most come with limits describing the width, length, and vertical cutting range projects should be confined to.  Many CNCs are capable of solving far more woodworking challenges, but you'll need to literally think outside that cutting limits box.

In 2004 or so purchase my own 3-axis CNC Shark. I cut flat parts for student furniture designs in my garage for the next few years. Bumping into the area limits of this small CNC, I purchase a larger Probotix CNC for my home shop. A desire to cut tenons on the ends of long stretchers led me to open up one end of my Probotix CNC for vertical access down to the floor. Once opened up, I realized with some jigging I could clamp parts at angles other than 90 degrees. Four or five revisions/updates of my compound angle clamping jig have led me to what I know can be done today.  What a CNC can do doesn't stop at the spoil board plane. 

Opportunities arise when you think about using more than just the flat bed of a CNC to clamp your parts to.   Many come with 2 motors pushing/pulling the gantry, which may mean there is nothing running under the bed.  Once you consider using the space under the bed, the next step is to make access to it. In my case I initially left open the front 1/4 of my CNC's bed, and mounted the CNC on an open frame with no top on it.  I now have access to all the vertical space between the bed of my CNC and the floor.

With access, the next step is to come up with a way to clamp parts under/inside the frame to hold them rigidly where the CNC can reach the areas that need to be cut.  

If you clamp your parts or jigs to one of the frame rails, make sure the rails are securely mounted and can't be rotated or racked or bent or flexed when under load. I made a jig that clamps to the front rail as I knew it couldn't rotate the way the frame is assembled: My Jig 

Bridges that span the frame, clamping jigs, accessory rails, and even the frame rails themselves can contribute to the potential woodworking challenges you can solve with a CNC. Bridge support.

T-slots, either in track strips you can buy or the frame member extrusions themselves come in very handy for attaching jigs/bridges/cross rails to.  One example is my bridges that screw into t-nuts in the frame rails and themselves have t-track down their middle to use for clamps that hold your project boards down. Another is my advanced compound angle clamping jig that bolts into the inner face and top slots of the front frame rail.  Sections of t-track could be installed on such a jig, but my current version has holes for c-clamps to hold project boards and reference edges/stops to. 

It is below the bed where opportunities for creative CNC cut solutions live.  To take advantage you'll need creative jigging to clamp your project parts down there.

4D

Comments and questions encouraged and appreciated!

CNC Frame Bridges as an Alternative to T-Track rows or Vacuum Beds.

I previously made a couple of frame bridges to support an assembled student project for some CNC joinery cuts: Here

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. 

6. With a little modification, two bridges and some all-thread turn into a bed vise that can clamp parts vertically for joinery cuts on their ends.  Link:  Bridge Vise

7. Since the bridges can slide anywhere up and down the side frame members, only three strips of 30" long t-track are needed to cover the entire X/Y cutting range of the CNC. This compares to six strips of 60" long track needed before. 

Waiting for cam levers

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. 

My Probotix Meteor CNC has one design flaw.  The backplate that the router bracket attaches to comes down with the router.  This can collide with clamps behind where the router is at. Switching from t-track that runs up and down the bed to bridges that run side to side keeps me from clamping down anything from the front or back. Since changing to bridges I have yet to see the router bracket collide with a clamp. 

  

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. 

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