Holding things down on a CNC can be the most frustrating part of the entire process. Luckily for us our main machine has a vacuum table and we use window sealant foam for the back of the wood. This works amazingly well for parts that are not cut through. We have had to get creative for parts that need cutting all the way through. Our large machine has an MDF table that has T-Slots cut into it and it requires hold downs. I have seen all sorts of solutions for this, but few solved my main concern of wanting to carve on the entire board unhindered by the hold downs.
The machine came with aluminum plates that secured the top of the board(see picture). Not only did this not hold it down all that well, but it had three other problems. First, if we hit the aluminum we ran the risk of breaking a bit. Secondly, It took a lot of hold downs to securely flatten a slightly warped longboard blank. Lastly, you could not cut on the entire top of the board.
We thought long and hard and some students came up with a design to hold down the side of the board if the board had a cut made into it. They made a computer model of the part and it was ready for 3D printing. It was going to take 9 hours to print it, when another student said, we could just make those out of HDPE plastic on the Bridgeport mill. 1 hour later he had 10 done and ready to use. We have liked the slight give on the HDPE plastic and think they are holding up way better than printed ones could have. I will include both the Solidworks model (found below) and a 3D printable model (.stl) (found below) in this post for your use. If you want to copy them exactly, we used a table saw and a mill (blueprint link found below). You will notice that the tab is not in the center of the part and that was purposeful. We wanted to be able to flip them upside down for other uses.
The issue then moved to making the slot on the side. Luckily for us, we have a shaper that is perfectly set up to do just this. We have a 1/4 cutting wheel set up on the shaper. It cuts in about 1/4 inch deep and is set about 1/4 inch high in the wood. We use this cutter set up this way for other things in the shop. We use it for cutting slots for the bottom of boxes and drawers. The same setup could be made with a router and they make a similar cutter for any router table. These little hold downs have solved all of our problems, but it created a new one. Your final project would have these slots on the side. Most of the time we have just taken it to the table saw, or if we are making longboards, the part gets cut out of the middle and this slot is of no concern. We have also added a wood edge to the board and covered up the slots.
Step 1: Our New Hold Downs in Use
We generally use this CNC for making larger pieces like our skateboards. The hold downs have been one of the best and most useful creations in our shop.
Step 2: Our Original Hold Downs
These worked, but were almost so frustrating, we found other tools to make our parts.
Step 3: Our Solidworks Part
Again, the files are all attached, but I should repeat that the tab is not in the middle. We have used both sides equally and like being able to flip them upside down for thin materials that can't get a slot cut in them.
Step 4: The Shaper All Set Up
It's nice having a dedicated tool for this, but it could easily be repeated with a router and similar bit or with a table saw making repeated cuts. If you have ever used the table saw for dado or slot work, it will work similarly to the shaper. I tend to use a regular table saw blade and move it over an 1/8 of an inch a recut till desired width of dado. It doesn't make the cleanest back end of the dado, but it is effective, easy and very customizable.
Step 5: Furthering the Process
some more ideas:
We have also had to create longer hold downs to hold smaller material.
We have also used them on the other edge and used the CNC as a large planer for some live edge cedar. Using the ends as a hold down location
We are also working on some improvements like having knobs or wing nuts rather than having to find that elusive wrench.