Best way to make keyed hole(D shape)
I'm sure everyone has seen the D shaped hole that stops rotation on many types of panel mount or bulkhead mount electrical connectors, potentiometers, fuse holders etc with the threaded jam nut. What is a good way to make this hole on a manual mill? The holes I will be doing are .375 to .5 in ABS plastic that machines really nice. I know I could do it with a rotary table, but there has to be a faster way.
Would a hot punch work to just melt the shape in? This would work on the plastic ones, but I may have to do some in aluminum as well.
Probably the correct ( read expensive ) way is to use a chassis punch.
Here are some results from a Google search "punch d hole"
.5" plastic? Make a single stage D-shape broach.
Greenlee chassis punches are insanely expensive. I have a set for most of the holes I need to make. (D, double-D, serial, SCSI) Greenlees run about $400-$500 per punch. It is also quite slow unless you are doing one offs and prototypes. Another option is a CNC mill or CNC router with a small diameter bit. Most places I know that do limited production runs farm them out to a specialist.
On major production runs of plastic panels the holes are molded in.
You could also make your own broaches.
One other option (if you can find them) is special washers that go under the bezel ring. The washers have the right shaped hole and a little tab that goes into a small hole drilled near the main one to stop the washer from rotating. I often use those for toggle switches. They have a little "tooth" that engages the slot in the toggle switch shaft. They are also available in D and double-D shapes.
if you make a D shaped insert that drops into the parent material you're not making the D shape in a blind hole - might making broaching a possible.
overkill for plastic ..... but I've made these before via a built up assembly in brass. drill/ream and turn od of 'insert'. mill out a section and solder a flat piece there. back to the lathe to re-turn the OD & part off. now you have a round OD and D shape interior ready to loctite into a bore. the pic is from my old digital camera and is crap, but you can make out the outline of the insert and get the idea
David said, "It is also quite slow unless you are doing one offs and prototypes." So true. I now use an electric impact wernch on them, really speeds them up but lube on the threads is a must.
I think the punch is out of the question for sure as the bezel is very small, plus the price. Out of curiosity, how does one index the punch to make the hole location accurate within a few thousandths? I have never used one before.
Mcgyver, that is very interesting, I would have never thought of that, D hole stock so to speak!
You are never going to punch that in 0.375 or 0.5" plastic. Not practical.
I'd think you could make a nice broach to do that starting with a smaller round hole. Wouldn't take much for those plastics.
Milling is too much like work, and you can't get the sharp inside corner.
We drill 1/8 holes near as we dare to the 2 corners then drill the largest hole we can, than broach them with a broach made of drill rod and do thousands of parts with a vmc up to 1/2 thick aluminum.
Rotary Broaching the D
Plan D: Rotary Broach tools are available in D shape, but the maximum recommended 'tooth height' for this type of tool in steel is about .010. The one continuous chip on a rotary broaching application creates alot of pressure. With a rotary broaching tool, you would pre-drill to the largest possible diameter, then broach the remaining 'fingernail' or 'moon' shaped chip. You could only rotary broach a shallow depth though, because of the extreme chip accumulation.
if the abs is .5 thick, drill a round hole, glue in a spline or cross piece. or if you want to get fancy, start with a piece of round stock and slice off the section and glue that it. Universal pipe cement works very well on abs.
if the abs is .05 thick, you can get an arch punch, these used to be standard, I had a large set that I kick myself for not saving. If not standard, a die maker that makes forged cutting dies for leather, can make one, maybe 100 bucks.