Failed Miserable At Cutting A Disc Brake Rotor

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jmarkwolf

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I'm fabricating a desktop motor dynamometer to characterize automotive seat gear-motors.

It's basically a 1" solid steel shaft with the gear motor at one end, a shaft encoder at the other end, with a torque transducer and a brake in the middle. The whole shooting match is suspended on 4 bearings, a pair on either side of the torque transducer. With flex and solid couplers at the "appropriate" places. All sitting on 6" heavy aluminum channel for rigidity.

The object is to be able to "dial in" some repeatable and constant "drag" on the motor with the brake, while capturing torque and shaft-angle data from the transducer and encoder.

The first attempt at a suitable brake was a 6" diameter mechanical disc brake setup for a go-kart, actuated with a thumb wheel. The caliper is a big clunky affair that is free to "wallow" on its' "keeper" while finding it's own center. It offered little in the way of sensitivity and repeatability. The braking action was very unstable, and torque reading was all over the map.

I decided next to try a mechanical disk brake from a bicycle. Rather than fabricating a hub for the new disc, I tried to remove 0.050" from the 0.125" thickness of the go-kart disc on my new Grizzly 12x36 lathe, so it would fit in the new caliper. I failed miserably (see attached pics). The resulting finish is so rough it resembles the surface of an abrasive disc for a hand grinder.

I tried a couple different rpm's (360 & 600), and 2 different cutting tools, at least one being carbide. Taking cuts no deeper than 0.010" at a time.

While cutting the outer portion of the disc, it would chatter and sing like crazy. The closer to the hub the better it would cut, and it would peel off some nice curls. But at the out portions of the rotor it generate just small "chips" that came off like bullets.

I'm the first to admit I'm no machinist and am teaching myself as I go.

I know that disc rotors are turned all the time, which leaves me wondering why my feeble attempt at it turned out so bad.

Assumptions (perhaps mistaken) so far:

1. The outer portions of the disc is very hard and need special tooling.

2. The disc needs to be cut from both sides simultaneously, as I think they do on automotive brake rotors, to nullify resonance, and produce a better cut.

Is it possible to make such a cut on a lathe with good results?

Advice and post mortems gratefully received.

Dyno_rotor_disc_s.jpg

Dyno_rotor_disk_closeup_s.jpg
 
Basically, it looks to like the set up has to be more rigid. I have machined cast iron disks on a lathe using
a 4 jaw chuck in the lathe machining one side at a time with no chatter.
 
I would say that the disc has way too much flex to it. You could try mounting it to a flat backing plate and using a sharp HSS cutting tool. The disc it self shouldn't be very hard if it's anything like an automotive unit. Mike
 
Those disk are very hard. You need sharp carbide . And you are turning way to fast. I would start at 125rpm. And like said you need a backing plate


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With it cutting better toward the middle indicates your feed and speed is not correct. As stated, slow down your spindle and see about making it more rigid.

Brake rotor lathes turn slow. The brake lathe I've used years ago had two speeds, 105 RPM and 180 RPM with a feed rate of .003"/rev IIRC. Carbine tooling as well.

Just looked at couple of specs, found one very similar at 115 RPM, .005"/rev and .020" max DOC.
 
jmarkwolf said:
Assumptions (perhaps mistaken) so far:

1. The outer portions of the disc is very hard and need special tooling.
Click to expand...


don't forget that the speed that the tool sees at the "outer portions" is much faster than closer to the centre...

pete
 
It's been said already that feed/speed differs from inside to outside.

It's been said already that the disk is thin and needs a backing plate.

I agree on both counts and would like to tie those two together; on something thin and wide like that, the closer you get to the center, the less flex you experience. Probably that contributes to why your cut was better toward the center. You will see the same thing if you try to turn a long rod, out far away from the chuck, with no steady/follower rest. The further you get from the chuck in either axis, the less stability you have.

I second (or third? Fourth?)the motion to use a solid backing plate.
 
Be hard to mount to a face plate, but if you have a three jaw chuck with 2-piece reversible jaws. I would get a set of "pie jaws", bore out the jaws to grab the disk on the OD. When you bore out the jaws, make the ID of the jaws on a back taper of 15 degrees. This will force the disc against the back of the jaw set to stabilize the disc from vibration and keep the sides parallel to each other.
 
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