What I'm going to describe is so useful, I'm not sure why Carbide3D doesn't offer it out of the box! Our FIRST robotics team decided to purchase a Shapeoko 3 XL to allow us to machine larger parts, but also fit it through doorways in our school. Our team has limited space and needs to pull everything out of a store-room to start working on any project. This version of the Shapeoko has a pretty hefty 3/4" MDF bed that bolts to the steel frame. It came pre-drilled for the M5 socket-head bolts. We saw videos from Winston Moy and others about the finer points of Shapeoko 3 ownership and were especially impressed by the hold-down system he showed for his Shapeoko 2. It involved threaded inserts and slotted wood pieces that were used to hold down material being machined with the help of bolts into the threaded inserts arrayed around the MDF bed. I've linked to the video to help illustrate.
We decided to tackle this and were able to get the insert part # by pausing the video and ordered this from McMaster-Carr. Further reading about these inserts indicated that they would be strongest if we inserted them from the bottom of the MDF. The flange provides additional strength that way so the inserts won't pull out. We wanted to be sure many students on our team would be able to use this machine for years to come, without damaging it.
We had to decide how many inserts to install and how to arrange them on the board. We use SolidWorks for all of our CAD and decided to use the LinearPattern tool to lay out holes in the MDF board. It was a huge help since the starting offset and spacing could be tweaked very easily. We ended up centering the pattern left-to-right (along the machine's X axis) and because of how the Z axis hangs off the front of the X axis gantry, we aren't able to reach the far back of the MDF, so we left a row blank on the back edge. It was easy to generate a dimensioned drawing from SolidWorks which helped when machining the holes.
Next was the hole profile. We had a little trouble generating g-code using the tools Winston used (probably due to lack of experience), so ended up designing the hole profile in SolidWorks and using HSM Xpress to define the tool paths. We chose to use 1/2" inserts in the 3/4" MDF, so there would be some room to level the bed by machining it with a fly cutter.
Since the Shapeoko cannot reach the extents of the bed due to mechanical limitations (except the front edge), we knew we'd need to position the MDF a few different ways to allow the hole pattern to be machined in. We also knew if we were going to machine all the way through, we'd need a waste board underneath to prevent us machining into the steel frame. We also got some 2" M5 bolts to reach from the top of the MDF (which is upside down) through that and the waste board and into the threads in the frame. After securing the MDF by shifting it to overlap the middle set of holes with the existing holes in one end of the MDF, we were ready to start.
Using a ruler, we measured the offset of the 1st hole. This would be the anchor point for the entire grid of holes, so measure carefully! We used an 1/8" flat end-mill to machine our holes. Using Carbide Motion, it's pretty simple to position the bit over the hole. Bring it pretty low over the MDF, adjust X and Y to line up with the marks. We use the paper trick to adjust the Z axis. Drop the Z by small increments while moving paper underneath the bit. Once the paper sticks, raise the Z 1 step, remove the paper and drop the Z back down 1 step. Now, set zero for all axis and you're ready to start!
After running the code for the hole, the bit ends up off-center, so before moving to the next hole, it's a good idea to use reset X-Y (through Carbide Motion). Then, you can jog the machine in either X or Y direction 3 inches to start the next hole. Once the X-Y are set, repeat the Z setting procedure with the paper and reset zero for all axis. Rinse and repeat till you need to reposition the board to reach more holes. Eventually, you'll end up with a board that looks like this.
Pretty slick! Now, you can install that on the machine and use your hold-downs!