Creating a Wasteboard

The wasteboard makes up the table surface on which you mount the stock that you are milling. It can be somewhat sacrificial, meaning that you may occasionally intentionally or (more likely) unintentionally cut into it.

It should of course be flat, relatively millable in case your bit cuts into it, and ideally have some convenient means to rigidly mount your stock.

For my CNC Beta 01, I chose to use 3/4" MDF with a grid of threaded inserts to use with stock hold down clamps. Here's what the finished wasteboard looks like:

Here I am inserting the threaded inserts. I inserted these in the back of the board, and then flipped it over when installing it. This provides more MDF to accidentally cut into before hitting the metal of the insert.

Parts List

• 3/4" MDF (I purchased at the local big-box retailer in a 2x4 foot sheet) - about $10
• Threaded insert for M6 bolts: McMaster-Carr part# 95596A500 - about $10 for pack of 50

Build Steps

1. Cut the 3/4" MDF to 48.4 cm x 56.9 cm.
   1. The 48.4 cm width allows the wasteboard to overhang the Y linear rails, which I hoped would limit dust from falling onto them. The 56.9 cm length fits perfectly between the end aluminum pieces on my machine, but you may want to measure to make sure it fits for you.
2. Mark for the four bolts that hold down the wasteboard.
   1. For the 48.4 cm width I used, I simply measured in from the left and right sides by 4.1 cm to find the center of the bolt. The other direction is more flexible since the hold-down nuts can slide. I used the same 4.1 cm from the ends to the center of the bolt.

1. Drill the four hold down holes with a 5/16" drill bit, and then counterbore them with a 5/8" bit so the head of the bolt is just under the surface of the wasteboard. Note that the four bolts provided are a little long, and counterboring too far will cause them to bottom out in the metal track before binding down on the wasteboard. Shorter bolts could be purchased if needed.
2. Temporarily mount the wasteboard upside down (counterbores down) so we can drill the field of 36 holes for the threaded inserts. We will use the CNC to cut these.
3. Load the following gcode file into your gcode sender (in Chilipeppr, you can just drag it into the browser window):
   1. drill-threaded-inserts.nc
   2. (for those interested, to create this file, I exported the basic movements in Easel for one hole, and then hand-edited the file to create the other 35 holes at the desired spacing)
4. Load a 1/4" bit into the CNC, and use the manual Z controls in your control software (chilipeppr, for example) to move the bit down until it is just above the wasteboard (maybe a sheet of paper thickness above). Zero out the Z axis (G28.3 Z0, or by selecting the first "Zero Out Z Axis" in the Z axis menu in chilipeppr).

1. (if you want to test the file first, you can set this Z height to at least an inch above the actual wasteboard and then run the gcode without cutting.)
2. Power on your router and run the gcode program. It should cut the field of 36 holes for the threaded inserts. As always, keep your finger on the powerstrip your CNC is connected to in case you need to stop it in an emergency. Please also wear eye and ear protection!
3. (Note: when I ran the program, for some reason Chilipeppr occasionally paused the job no clear reason. I was able to click the resume button labeled "~" and it started again.)

1. Screw in the threaded inserts using the proper size of metric allen wrench.
2. Finally, remove the four bolts holding down the wasteboard and re-install it right-side up. That's it!

Hold-down clamps

You can now use any clamping method that uses M6 bolts to thread into the wasteboard. I made some out of wooden shims cut down to about 6" long, a couple M6 bolts, an oversize M6 washer, and another of the threaded inserts. I have to be a little careful not to hit the metal parts with the endmill but otherwise they seem handy.

I made these out of some surprisingly sturdy and dimensional shims I found at Lowes:

And yes, of course, I cut these on the CNC. :) I made a little fixture that bolted down to the wasteboard and held one cut-down shim in a repeatable position:

For a second batch of hold-downs, I used a 4" drill press clamp. I used two spacer sticks to keep the shim elevated from the clamp, and on the far end used a hold-down to act as a stop. This made the positioning easily repeatable for cutting out multiple hold-downs. This worked great, although extra care was needed to make sure I didn't do something dumb and run the bit into the metal clamp. :)

I first used Easel, and then Fusion 360 for the second batch. It is just a 9mm hole for the threaded insert and a 7mm slot running around 70mm long.

The gcode I used can be downloaded here: stock holddown v2.nc, although it may not be that useful directly since your shims or mount may be different.

If you do use this gcode, orient the shim's long axis along the Y axis with the least deep (tapered down) end to the left. Use a 1/4" bit. Start by setting the bit's Z height to just above the thickest part of the shim (right side) and zero out the Z axis. Then, move the bit to the lower-left corner of the shim (the skinniest part), and zero out the X and Y. Finally run the gcode. Note I have a movement in the Y direction at the end to provide access to the finished part.

As always, please wear eye and ear protection and keep your hand on the power switch in case things go bad!