Part processing ideas/ 3X mill

[dodgin]

This has been an entertaining job, to say the least. I don't know where our shop is headed with this but I'm curious to hear others ideas on how to process this piece. I'm shooting for clarity on the details while trying to remain as inconspicuous as possible.

If it's hard to see, what we have here is something of a gear in the shape of an ellipse - teeth all around the periphery. There's a 1MM tangential circle drawn in for the sake of perspective. These are plastic molded parts, and they come in about 8.5mm thick, which includes about .015" total on the thickness for cleanup.

There are bosses which are laterally central on either side of this piece, meaning on the face you can see from the "top" view and another on the "bottom" from where you're looking in the screenshot. The bosses are raised ~.0006" from each of the main faces of the piece. The total thickness tolerance on the bosses is ~.0005, and the total thickness between the main faces adjacent to the bosses is also ~.0005.

It may not be clear in the photo but the teeth on either end of the piece are rounded to a diameter, which must be held ~.0005.

The center hole is thru, comes to us already put in with ~.005 to cleanup to a diametral tolerance held to ~.0005. The holes on either side of that center hole are 5-40 threaded holes. They have come to us already in the piece at a dimension of .115. Not much for thread engagement.

Holding these parts square/flat enough to establish consistency hitting these tolerances has been an issue. We've tried to use the gear teeth to orientate the piece but efforts in doing this without marring the teeth have been fruitless.

Quantities in the 5 figure/year range.

 

[mhajicek]

If you want to hold those tolerances, don't start with a molded part. Nothing on a molded part will be flat and consistent within .0005". Lego molds are toleranced about .002"; .001" is a very tight tolerance for a mold.

Maybe ask the process engineer who decided to mold them, what his plan for finish machining was...

 

[DavidScott]

From my keyboard I would try soft jaws machined to fit those teeth, using some 2" double station vises I make with torque limited bit drivers to tighten them with. I would not try to engage the full tooth depth, somewhere around 50-70%, play with what material to make the soft jaws from, and minimal vise tightness. It looks like a fun job and enough parts to justify/allow getting it figured out.

There are many other things to do, like hold ALL temps to a set point, but so much depends on the details of the job that you haven't told us.

 

[dodgin]

From my keyboard I would try soft jaws machined to fit those teeth, using some 2" double station vises I make with torque limited bit drivers to tighten them with. I would not try to engage the full tooth depth, somewhere around 50-70%, play with what material to make the soft jaws from, and minimal vise tightness. It looks like a fun job and enough parts to justify/allow getting it figured out.

There are many other things to do, like hold ALL temps to a set point, but so much depends on the details of the job that you haven't told us.

This is the first path we went down. If we had more setup pieces we probably would have played around with it more, but we had to get things moving in the right direction. We had cut two teeth on each jaw, first in some jaws made of delrin and then jaws made from aluminum. First with about 50% length engagement and then we dropped down to about 20%. Numerous issues - jaws marring up the teeth, teeth marring up the jaws, pieces being thrown and most of all couldn't get the parts square enough to make in tolerance. By the time you dust one side in square you've got less than .01 to take the entire thing to thickness. Then it was requested to take more off of one side to remove ejector pin marks. So you spend .005 squaring it up but you need .009 to take off the ejector pin marks, then you better hit those .0005 tolerances straight off because you don't have any stock to play with.

We wound up making soft jaws to grab the majority of the profile of the part, but really low length of engagement. Do all the work from that top view, then make a bridge fixture pulling the part down with the 5-40 holes. We've had a mdoerate amount of success with that, but it's such an unreasonable setup with the kind of quantities they want.

I've spent the majority of my time working on the job thinking exactly what Hajicek said...the whole thing seems to be some sort of fugazi.

 

[DavidScott]

Yeah, work holding should not be as low friction as possible so Delrin isn't a very good plastic to make your soft jaws from, polycarbonate, PVC, or ABS would be better, in that order. Still, for accuracy, any plastic is a piss poor material for fixturing. Hopefully they will work with you, and grinding the ejector pins to +.001" proud, or flush, may be a start. What resin are the parts made from?

I wouldn't try holding these in a normal milling vise as you need far too much control over the clamp pressure at the low end of the spectrum.

 

[Orange Vise]

Wire EDM'd vise jaws would probably eliminate the tooth marring problem. This is an example of a simple concept (soft jaws) but more complex execution. Precision really matters here.

The EDM can use 0.004 or 0.006 wire to get the tooth profile fine enough for 80-90% engagement.

I'd use stainless steel for the jaw material. All metals will "overpower" plastic so you actually wouldn't be gaining anything with aluminum. In fact, aluminum sucks for WEDM.

 

[dodgin]

Yeah, work holding should not be as low friction as possible so Delrin isn't a very good plastic to make your soft jaws from, polycarbonate, PVC, or ABS would be better, in that order. Still, for accuracy, any plastic is a piss poor material for fixturing. Hopefully they will work with you, and grinding the ejector pins to +.001" proud, or flush, may be a start. What resin are the parts made from?

I wouldn't try holding these in a normal milling vise as you need far too much control over the clamp pressure at the low end of the spectrum.

Yeah, the delrin was a bit of a haymaker. Didn't land.

The material is something called Ryton. I did some googling but ultimately I don't know anything about it because I haven't worked with it. It seems to machine nicely.

At these quantities I doubt it works out that we take on this job. We're a prototype shop. There's plenty of shops out there that are well equipped now to do stuff like this at the price point the customer is asking for. I mainly just wanted to see how different folks would go about processing it.

 

[mhajicek]

Those parts would have to be really consistent to hold them by the teeth without marring. Unlikely.

Throw the molded parts away, put flat sheets of the same material on a vacuum plate, and cut the parts whole, leaving a few thou to the bottom to punch out. Then flip them into precision cut conformal jaws, which won't mar because they'll line up, and deck the back.

 

[dodgin]

Wire EDM'd vise jaws would probably eliminate the tooth marring problem. This is an example of a simple concept (soft jaws) but more complex execution. Precision really matters here.

The EDM can use 0.004 or 0.006 wire to get the tooth profile fine enough for 80-90% engagement.

I'd use stainless steel for the jaw material. All metals will "overpower" plastic so you actually wouldn't be gaining anything with aluminum. In fact, aluminum sucks for WEDM.

Funny you should say that. We have 4 wires and 2 sinkers. First thing I suggested was getting some jaws made. For some reason this idea never got any traction.