Milling speed question

They most likely will be aluminum calipers if for a performance model.

I would probably take a pass at about .070 depth of cut. Then measure it and make the next cut to get it to size. It sounds as if you have a very small amount of metal to remove. If that is the case just use a low speed and do not worry about the optimum speed to do the job in the least amount of time.

Yes carbide does give you a better surface finish if you have a higher speed, but he is doing this on a Mill Drill. So I would use a slow speed.

=20 Dan

I would probably take a pass at about .070 depth of cut. Then measure it and make the next cut to get it to size. It sounds as if you have a very small amount of metal to remove. If that is the case just use a low speed and do not worry about the optimum speed to do the job in the least amount of time.

Yes carbide does give you a better surface finish if you have a higher speed, but he is doing this on a Mill Drill. So I would use a slow speed.

Dan

An 1/8" I would use a grinder or file. Depending if it does not have to be a precision flat surface.

Play with some scrap first to get a feel of the machine.

A mill drill is going to have limited speed selections any way and if you are using an 1 1/8" mill you are probably going to be on the low speed side.

Another factor is the rigidity of the set up and the machine itself. If you are running too fast and taking a deep cut and something slips a bit you have just scrapped the part. When I am unsure of the set up I tend to have a finger on the part where it meets the table. That way if it starts to move you feel it.

Listen and watch the cut as it progresses. Be wary of changes in chip color or changes in sound.

Not sure of the lowest speed on this, but I suspect something around

300. Still OK with the carbide tool? I also have flycutters in 4 sizes, biggest probably 1.5" And HSS endills up to 3/4", probably larger, 2-flute & 4-flute.

I checked and it is cast iron. These came off a BMW 320I, std model.

Still OK with carbide, right?

It does have to be precision. This surface locates the caliper in relation to the rotor, so it has to be parallel and flat

Be sure to spend the time with a dial indicator to locate the part and ensure that the existing reference surface is fully in plane before making any cuts. The cuts will be the easy part, the setup will be the difficult part.

Good suggestion. I've given that some thought. As far as I know, the only reference surfaces will be the mounting ears themselves, both sides. I don't expect there will be much surface area to indicate on. Now that I think on it, there will also be a surface where the two caliper halves mate, probably on a difference plane, but parallel. That might be usable.

The plan is to support both ears (at opposite ends of the part) with spacers, bolt and Tee-nut through one ear mounting hole, also a clamp on the body, probably bearing on the bottom of the piston bore hole. I'll probably also support & shim under the caliper body beneath the clamp spot, taking care not to distort the body in clamping. I'm sure it's a super-stiff forging, as calipers have to be inflexible. We do have a worksheet (below) from an expert who has done this many times, and this is the recommended setup. I don't see any way to set this up one time to make both cuts.

Here's the instructions from the expert:

MACHINING THE CALIPERS (FOR LATER 914-4s ONLY)

The 320i caliper does not have the correct flange-to-center dimension (too small) for the later 914-4 struts, so you will have to machine the inside face of each mounting flange (not the face on the outside of the caliper, where the caliper bolt head is seated) to 0.587" +/-0.005" inches. The flange thickness "out of the box" should measure 0.712" +/-

0.002". This means that 0.125" of material must be removed from the flange. This will leave the caliper with a flange-to-center distance of 1.025" (this dimension is difficult to measure), matching that of the original 914-4 caliper. What you?re effectively doing is moving the center of the caliper away from the strut mounting towards the outside of the car. Before you have the caliper machined, convince yourself that machining the outside flange faces would do nothing to re-locate the calipers properly about the rotor?this way you won?t be confused as to which flange faces (the inside ones) need to be machined. Your calipers may have a recognizable casting mark that is just about at 0.125" from the face to be machined.

NOTE: Before machining the 320i calipers, you should verify (with an accurate measuring device, e.g. dial caliper or micrometer) that the two mounting flanges are equally thick. If they are, follow the instructions below which DO NOT involve "splitting the caliper open." If, on the other hand, the flanges are not of equal thickness, explain that to your machinist and he will come up with a method to guarantee that the caliper flanges are machined to the exact parallel of the center line of the caliper. One suggested method is to split the calipers open and use the center half as a clamping point. However, if you choose to use this method, you will need to replace the fluid O-ring seals between the halves, and retorque the caliper bolts properly upon reassembly. If you split the caliper halves and reassemble them you had better know exactly what you?re doing so as not to compromise your safety.

The recommended machining setup is on a mill, with the caliper?s outside mounting flanges resting on two (large) gauge blocks on the table, i.e. with the outside mounting flanges face down, and the inside ones facing up. Typically one clamp is pressing down on one inside flange, and the other flange is unobstructed and ready to be machined. Another, much taller clamp presses down on the caliper near the ends of the bolts that hold the caliper together on the same side as the flange to be machined. It is extremely important to ensure that the caliper is being held down "flat" against the mill table (via its outside flanges and the gauge blocks), otherwise the caliper will be skewed relative to the rotor when its mounted to the hub. Considerable clamping pressure is required to immobilize the caliper. Fill or cover the caliper orifices with something to prevent metal shavings from entering.

Use a large end mill (e.g. 1 5/16") at very low speed to remove the proper amount of material from the face of the inside flange. Be sure your cutter is big enough to maintain the large surface area of the inside face of the mounting flange. Do your work slowly and check it as you go. Try for +/-.005" tolerance, but +/- .002" would be better. Unfortunately each caliper has to be setup twice (once for each flange) due to the fact that the only place to clamp it to the table is the very surface you?re trying to machine. The whole job can be done in under an hour.

Yep, it does sound like a bit of a pain to clamp and indicate properly.

Yes.

If you have some cast iron handy, it would be good to do some practice machining is Roger Shoaf suggested.

Dan

I think I have several calipers and similar items laying around. I'll do that.

Well, that went just fine, at least the cutting part did. Cast iron is so easy to cut. The only issue we had was getting the mounting surfaces in plane with each other, and presumably with the CL of the caliper. Holding a parallel across one machined surface, you could slip a .020 feeler gauge between it and the other mounting point. I suspect the backside of the mounting holes may not have been in plane, as there was some thickness variation of as much as .008. The result was imperfect, but the car owner considered it acceptable.

I think if I were doing it again, I'd spend more time building a fixture. I'd also probably take a skim cut on the outside of the caliper where the piston centers, to give me a good base reference plane and a solid clamping area. It would also be parallel to the mounting surfaces we were cutting.

Thanks for all the help, folks.

Carbide itself is probably not a big issue here. Carbide really shines on production runs where seconds count and in cases where the material is hard.

Your project doesn't seem to be either so unless carbide is more convenient for you to do the job. then pick any cutter that makes doing the job easy even if you have to accept less chips per pass.

Also generally speaking, the smaller the cutter the less it costs and the less damage it will do if something slips.

It would be an entirely different equation if you had a thousand parts to modify and were paying someone to run the machine, than 2 parts for your buddies car.

Good report, enjoy the beer.

One thing you might want to consider next time is how to mount the irregular part.

Having pressure on only one point on a parallel gives you very little clamping force and things could go south on you really fast.

For jobs like this I have used wood shims, plastic shims, business cards, mach book covers, or what ever was available to hold an odd part firm.

Develop the habit of thinking "what if" and figure out what you will so if things do not work the way you think they should.

I think the clamping arrangement we used was secure. Bolt through one mounting ear, spaced up by accurate standoff spacers. Standoff spacer under the ear to be machined. Spacer stack under machined surface closest to the target area, clamped down with the usual bridge clamping components I would have liked one more pressure point, but nothing moved.

You can always "Pot" your part in Bondo or Mahogany filler to fixture it. Get some Greenseal to use as a release agent so the part will come out when you are done or use a couple layers of Saran Wrap between the filler and the part. You will also need a piece of wood to use as a base.