Adventures in Work Holding

One of the problems when using a CNC mill is holding on to the work piece.

It needs to be securely fastened to the machine in some manner, but the method of fastening needs to be out of the way so it does not interfere with the tool head.

Here are a few things I have tried. Some worked out better than the others.

The simplest method is to clamp the work piece in a vise that is attached to the table. I have a vise that is semi-permanently attached to one end of the work space. It has jaws that open to about 5″ and I have it indicated in so that it’s parallel to the Y axis. I also have a stop setup so that I can clamp multiple pieces  in the same location.

LH_BackZero for X and Y are referenced from the stationary jaw face and the stop, thus I don’t have to reset them for different work pieces.

Clamping the work directly to the table is another method that I use, but the clamps tend to get in the way. I have 1/4-20 Tee-Nuts inserted into the table from below on a 3″ grid, so I have lots of places to fasten down clamps and jigs.

IMG_5296This worked out fine for machining the pockets, but when I machined the outside profile I had to remove a clamp so it didn’t interfere with the tool head.

IMG_5298Note also that there is piece of MDF under the work piece so I don’t mill into the table, and if you look closely you can see the tabs that connect the part to the stock. This prevents it from moving around and being grabbed by the bit.

After making the part above I got smarter. Since I was going to be making a large batch of these parts, I needed a better way to hold them and line them up. I grabbed a larger piece of MDF, screwed a couple of pieces of 1/4″ plywood to it for locating stops, and clamped it to the table. Then I determined where I could put screws into the waste areas of the stock and drilled the holes on my drill press. I screwed the stock to the MDF and it worked out great!

IMG_5306No clamps to get in the way and all the waste pieces are fastened down. It’s hard to see, but there are still tabs in the slots to hold the parts in.

This worked so well that I ended up doing it for all the other parts.

IMG_5308

IMG_5311Another method I tried was double-stick carpet tape. This works okay for smaller parts, but I discovered that it also covered my bit with goo from the tape when I milled all the way through.

But what do you do when you need to machine both sides, and neither side is flat? I make several different styles of pistol grips now and they have varying amounts of detail on the back. Some have none, some just have a pocket or too, and some have multiple height levels.

The answer is jigs and fixtures. I start out by designing the jig in CAD as I’m designing the grips. This way I can get the holes and locating pins to line up properly. Then I clamp a squared up block in the vise and mill the back of the grip.

Next the grip gets attached to the jig. This usually includes locating pins and a screw or two inserted through the screw holes that are used to attach the grip to the pistol.

After it’s firmly attached, I mill the profile.

Here are some of the jigs I use.

The simplest one is used for 1911 grips. They are flat on the back so the jig just has locating pins in it.

Top surface profile.
Top surface profile.

Next up we have the jig for Smith and Wesson revolver grips. They have height two levels on the back. This is after the surfacing pass has been run. The large hole in the lower left is not part of the jig, it was already in the scrap of MDF I used. I have since added some locating pins to this jig, but don’t have a newer photo.

The jig after all the machining.
The jig after all the machining.

This is the most complicated jig so far. These CZ 75 grips have three levels on the back side. I have not actually finished a set of these yet, I’m still working on the surface profile. There will be locating pins in the four holes you can see here.

IMG_5319

You can also see the two dowels that I use to locate the jig to the machine. The dowels are in the same location on each jig and they enable me to swap the jigs in and out without having to worry about squaring them up again.

These are by far not the only methods to hold a work piece. I’m just showing some of what I have come up with in hopes that I can inspire you to solve the issues you run up against.


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