Holding down work on Mill table

Hi, David. Do you have access to a bunch of random size aluminum and steel block cut-off pieces? The sizes need to be somewhat larger than the plastic boxes you are going to mill. The problem with milling any kind of plastic is its flexibility. The end-mill bit will flex the plastic as well as cut it. The result is an "L" shaped slot.

You need to design and mill a box to totally enclose the plastic box, except for the end you are going to mill the slot in.

The tooling I have done all have a pocket milled to hold the box with about .10 inch or less of the box exposed above the tooling. Then a steel plate is machined to size to fit the box. When the tooling and the plate are clamped in the vice, the plastic box is tightly held by the cover plate. Now, the plastic box is ready to mill.

I use a stop rod on the vice so the tooling is always returned to the vice at exactly the same location.

In one case I made the tooling out of a steel block to completely enclose the plastic box. The pocket is offset in the block so a rather thin portion is next to the place where the slot it to be made. The first box to be slotted also slotted the tooling.

The above steel tooling also has 4 holes in the side so it acts as a template and I can drill circuit board mounting holes in the box.

I think I can find the tooling and send you some pictures if that will help.

Paul Drahn, President Jodeco, Inc. Redmond, Oregon

Paul,

Thank you for your feedback.

I have not used a mill yet (complete novice). The idea of getting the mill was to be able to make small prototype runs with better results than cutting the slots by hand with a nibbler.

My experience is predominantly Electronic circuit design. It would be a little early to attempt milling out a holding jig.

For the metal end plate I was thinking maybe an aluminum block with 4 holes and screw the plate to the block.

The small plastic enclosure I thought could be placed end-on in the vice but not sure if I would get away with using tin plate between the box and vice jaws to protect it ? Maybe I could cut a wooden block to fit into the box to give support ? Really not sure what would work best here.

As for the handheld, that is more problematic as the sides are curved. I thought maybe put it lengthways into the vice so I could clamp on the square ends but I don't think this looks stable.

Any other suggestions how I might secure and protect the work without needing the experience of milling out a jig ? It is a bit of catch 22 until I get more experience. I don't mind messing up a few boxes as part of the learning process.

Thanks aga> Hi, David.

The end plate is easyest (and will come later). Unscrew, lay flat onto the table and clamp down. First put masking tape to where you will clamp. Second, put some plate between workpiece and clamp. It might be necessary to build some kind of stop that the part doesn't slip away. Expect this to happen!

Ob boy! You know, that the cases don't have parallel walls? They are conical. You can't simply clamp it down on one side and mill on the other. It will be tiltet and bend like mad! If you have a horizontal mill, you can proceed like with the first part. Putting the open end down on the table. If not, you need to weld some table to support the housing. I would pull out my power drill, bore hole near hole and file.

This is easy. Clamp the plate down onto some plastic (Styrol, ABS) that you can cut a bit into (mill into the plasic, NOT into your table). Also, use masking tape to protect the part and put some steel/aluminium flat between the clamp and the part.

You could think about casting some jigs out of plaster (reinforced).

Nick

It depends a little on how many you need to make...

The problem, of course, is holding an irregularly shaped of probably flexible plastic so you can cut it accurately without toooo much hassle.

You can put a lot of time and work into making fixturing. But the forces in cutting plastic aren't all that great, so maybe a different approach would be adequate.

You can also cast "blocks" that will fixture it nicely, and, although their useful life probably wouldn't be long enough for true production, they'd probably hold up for a few dozen/few hundred pieces. Go to a dental supply and get some material of the sort a dentist uses for making a mold for false teeth, etc. Then, make a fixture much in the same way as you would make a mold for casting with your "sample" piece as a pattern. Apply a little imagination and it isn't hard to visualize how to hold your little plastic box to make the cuts you need (if there are several, you may need more than one fixture...).

Jerry

Other folks have suggested fairly involved fixturing which I think is a bit more suitable for modest production runs. What I normally do for the one or two off projects milling plastic enclosures is to just cut a wood backer block to provide support underneath where I will be milling. Clamp the part down over the top of the wood block with the clamp strap/stud/step block setup and just put some masking tape on the surfaces the clamp bars will contact to prevent scratches.

Pete C.

Jerry,

Thanks for the suggestion. Maybe even body filler for cars would be good enough. I only have 1 of each to do for now. The largest run of prototypes would normally be 10 with maximum of 30. Beyond these numbers it starts getting viable in terms of waiting time and cost to contract the job out for someone with CNC facility.

Even a cast of the inside of the square box would help put some "meat" behind the end panel that I want to mill.

I suppose for the handheld unit I could make a cast in a rectangular box larger than the enclosure being cut, and then press the enclosure 1/2 into the mold to get a contour. Then use a clamp at each end and mill from inside the box.

The small plastic end of the handheld enclosure may be troublesome as there are no mounting holes. I could probably also use a mold to form a recess to prevent the plate from sliding and then come up with some way to stop it lifting up.

Jerry Foster wrote:

Use aerosol foam from the hardware store. Wrap your workpiece in plastic wrap.

If your only milling plastic then I don't see why you couldn't use wood to make the fixture/clamps.

AJS

Yes. The sides are sloped at 2 degrees, so the part will come out of the mold. Another problem I have is that steel tools cut the zinc-aluminum die metal so easily that it's hard to keep the drill from wandering unless everything is well clamped and perpendicular. Holes will wander even if one has done the whole centerpunch and pilot drill dance, because ordinary twist drills can easily cut sideways (like a mill) in such soft metal.

The box manufacturers (Bud, Hammond, et al) have mechanical drawings on their websites.

Using a 2-degree wedge from MSC, one can machine a set of clamp blocks that will fit the box precisely. Or, machine a filler block that will fit inside perfectly. For this, a hardwood like maple should be adequate for a short run.

Joe Gwinn