Sharp internal corners

[Sanghera]

Hi,
I was wondering how sharp internal corners are cut with CNC. I have seen parts that have basically 90degree edge in internal corners. How are these machined? I want to know how to do something like the center hold for this RC car wheel.
Thanks.

[cadman]

On those particular RC wheels the D holes are probably cast in. If not, then they were done with a broach. Processes that produce sharp internal corners that can't generally can't be done with normal milling include broaching, wire & ram EDM. We have quite a few medical and areospace parts that we send out for wire EDM.

[Sanghera]

Have any of these methods be done by a DIY? I guess the best way would be to cut it out roughly with a very small bit, then use a file to get a sharp corner.
Thanks.

[miljnor]

Granted, this part probably doesn’t have enough stress going into it to worry, but sharp corners are stress riser areas, and since it sounds like you are reverse engineering/copying, why not make it better than the original?

Or make your own design entirely?

If you are making something to scale then just ignore me!

[Mcgyver]

edm or some parts are sintering in industry. it would be a complex broach as material intrudes on the bore - ie the opposite of say cutting a square hole where material is removed outside of the bore, but i suppose a broach could be made.

and bench work with a file as mentioned but this has the same challenge as a broach for this shape. fabrication is probably the easiest diy methods.

here's a shot of a D shaped hole i made via a fabrication. a cylinder was turn, part milled away, a chunk solder to it, returned to the lathe and final soldering into bore in the finished piece. P&S, or should that be POS camera would not focus where i want it to, but you get the idea

I also put up some shots of some homemade broaches, one hex and one square

PS, agree completely with miljnor, change the design for better functionality and easier production often makes a lot of sense and is a luxury of being a diy'r

[cadman]

Broaches come in standard shapes (round, square, hexagon and D) and are not cheap. The smaller the broach, the less expensive they are but that is a relative term. If you need that D hole then try to find a used broach. Some shops that make broaches also stock used ones, or try ebay.

Another type of broach are punch/blind/wobble broaches. These are smaller and mount into a collet or spindle. You can use these in arbor presses, milling machines, drill presses, etc.., and these are the cheapest type of broach. We do use these type of broaches in our machining centers and they work great.

[Sanghera]

Thank you all for your posts. So Mcgyver, you just soldered a piece in the round hole? To make the D shape? Is that what you are saying? If you did, as this piece probably withstands a lot of stress how does it hold up? Could you explain the process a little further. Also, I am making this piece to fit an RC car shaft, so I'm not sure if I can change that shape.
Thank you.

[Mcgyver]

here's a close up with some lables. take a piece 1, brass in this case). Bore the ID to the radius of the "D". in the mill, cut into the bore and then solder on a flat piece, piece 2 - it forms the straight leg of the "D". back to the lathe, turn the od, you're now turing the soldered fabrication of 1&2. turn to, and install in a bore in 3. If the bore and OD are to titght tolerances, it will be fairly accurate.

This piece is the handle for tapping tool for very small taps, the fit is very good but its obviousl low speed/force/duty cycle application. I'd have to check temp charts for soft solder, but there's a danger the soldering the brass is annealing it, you could do the same in steel and solder or press fit it into the wheel?

[Sanghera]

Cool. Thank you very much. I understand now. How would a press fit work in steel?
Thanks.

[miljnor]

I miss understood the question! I thought you were trying to emulate the spokes of the wheel not the center hole!

By all means use a broach.

I have done something similar to this on a bench top drill press. You can actually mill a D shaped slug from a round and then press it thru a slug of aluminum. For this particular op you might need a small arbor press (HF sells these).

I would drill a hole as Mcgyver says but make it very shallow and then you have a starting point for your punch. This is how we do it on an ironworker. You’re just going to have to try it on a more Labor-intensive piece. But then that’s the beauty of DIY projects!

[peter@slater]

I agree with miljnor, why not make it 'better than the original'. Take a look at rotary broaches for a wide variety of standard hole shapes. For $50-100 you can get a stock broach with a hex, square, serration, etc. and wobble it through. If you don't have a broach holder, some guys try to push the broaches through in steps as a test.
www.slatertools.com

[miljnor]

50-100.00?

no thats just for the broach cutter . The Actuall tool holder is like 1000.00 dollars.