Floating Brake Rotor Hats and distortion after maching

Short back story, most of my experience is in sheet metal and tubular structure fab with little machining from scratch (although have been using a Mill for other work for probably 10 to 12 years now just not much machining complete parts from raw material especially round parts from round stock).

Anyways, I am well aware of the potential (and often occurring) warping / distortion or parts that are machined from flat bar / plate stock. However, in my limited machining experience (zero really when it comes to round parts from round stock), do dished round parts machined from round stock have the same tendencies or are they more stable because of having 360° of support?

I'm currently in the "planning stage" of making a pair of Brake hats / bells for a full floating brake rotor setup and I'm trying to figure out my fixturing strategy to make these. The raw material is 6061-T651 1" thick cut from 9-1/2" dia. bar stock. Other than the outer ring, the wall thickness though out the part(s) will be 6.5mm (i.e., the face flange and the 45° angled perimeter wall that leads out to the outer ring will be 6.5mm thick (the outer right will be a bit thicker at 9mm thick. The inside face depth is 16mm with a finish overall thickness of 22.5mm.

Please note that while I realize that this job would be best done partly on a lathe and partly on a mill, I do not have or have access to a lathe (big enough), just a CNC Bridgeport (clone) Mill.

I originally had a plan of attach for fixturing but then I started thinking about potential of warp / distortion along the way. My original plan of attack does not allow for accurate repositioning when I flip back to do the first side finishing cuts.

Since the part is round from round stock and essentially is fully supported around the full 360° (remember, "vertical walls" are actually 45° as noted above (i.e., cone shaped between the two face sections if that makes sense), is there really a potential for warp / distortion like parts that are substantially thinned in sections that are cut from plate / flat bar stock?
Part of me says that being there is still a high likelihood of warp / distortion but another part of me is saying, "no because the part is supported by the 45° (cone of sorts), and it being round from round stock that the forces should offset each other. The more I think about it, the more Im at war with myself in my head... UGH!!!! Someone please help! (and sorry for the long read... I just like to give as much detail as I can when asking a question on any forum).
Much Thanks in advance!!!

Oh and in case someone wants a better visualization of the final part, here is a picture of what I am trying to recreate (but to my own specifications / dimensions) and disregard the actual brake rotor itself.. Just attempting to make the center hat / bell.

Most of the guys around this neck of the woods use plate for one-off (or 2 off at least!) bells, seems to be more uniform in heat treat than bar. There's more stuffing around rough cutting to size than round bar so you can chuck it up, can't say I've noticed the distortion you mention being an issue in the 6 made recently this way. Wouldn't like to be doing them solely on a mill however.

few people around here make these only from round stock, never plate, haven't heard of any issues, one tip - do not hard anodize them whatever you're being told, hard anodizing is usually 2x thickness of typeII, and because the rotor is very hot during running, that heat is transferred to the bell via those steel slides, aluminum gets quite warm there, expansion differential between the anodizing and aluminum substrate is something like 5x, and that makes the anodizing flake off, which creates a considerable gap very quickly (because of the great hard anodizing thickness) - steel inserts then start to play in those gaps and hammer the bell every time you brake, which then wears out the bell quite quickly, with regular anodizing done on the bell this effect is reduced, it is not uncommon for the assembly to last nearly 2x the distance the hard anodized bell assembly has

anodizing flaking off of hot aluminum is a well known issue, there are no magic fixes for it

I've seen these made on mills, but normally "blanks" are turned from both sides, then holes and slots machined on a mill

I would think about the fixturing to allow for flipping the parts easily a couple times. If it's going to move around during the machining process it would be from hogging out the inside and outside. Rough cut that stuff first, then come back and do lighter cuts to get to the finished dimensions.

I don't know what your application is, but I have purchased blank hats from Wilwood that were mostly finished, just with a small center hole and no bolt pattern for the wheel studs. You just need to set them up, bore the center to the required size and add holes for the wheel studs. If they have something that will work for your application, it saves a ton of work.

For the reasons given in #3 regarding anodize and wear in the mounting slots, I've been really tempted to find some cheap offcuts of 17-4 in a hard condition and make bells from that. Lifetime part as long as you don't crash and/or burn. A little heavier than Al, but you can thin walls to 3mm or so except where the disk mounts (you can still lightweight a lot of the area if you want).

If it's a full-floater it won't matter if the carrier warps

I had some similar parts at one time, a little under 6 1/2" o.d., basically a big bowl with under 1/4" wall out of 7075 bar. They didn't warp enough to be a problem.

(I almost wrote "not enough to measure" but if the tools are good enough you can measure the diameter of a gnat's ass. I am sure they warped some but not enough to notice. Standard good machining practices were followed.)

Per Milland's suggestion, 17-4 would be prettier but a much bigger project. They'd sure look good on the car tho. Or polished 316 ....

in case of 17-4 you'd have to make these quite thin to match the weight of the aluminum part, I'm speaking from purely competition point of view (same for what I wrote earlier about wear, it might be a non-issue for blinged up street car), and thinness could introduce a host of other possible problems, aluminum has worked for this application for more than 40 years, I don't think it is worth experimenting here with other solutions

jz79 said:

aluminum has worked for this application for more than 40 years, I don't think it is worth experimenting here with other solutions

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I guess you'd be totally against my experiments in using silicon nitride for such parts. Or my ill-fated cast-magnesium Hibachi (in fairness, it was easier to pack when traveling).

Bah, a life without experimentation is one not worth living...

Graham Gott said:

I would think about the fixturing to allow for flipping the parts easily a couple times. If it's going to move around during the machining process it would be from hogging out the inside and outside. Rough cut that stuff first, then come back and do lighter cuts to get to the finished dimensions.

I don't know what your application is, but I have purchased blank hats from Wilwood that were mostly finished, just with a small center hole and no bolt pattern for the wheel studs. You just need to set them up, bore the center to the required size and add holes for the wheel studs. If they have something that will work for your application, it saves a ton of work.

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I am still playing around with ideas for the flip / reflip without loosing positioning. Even if it doesn't distort, I still feel that this would just be extra insurance since I dont like to throw away time or material.

Funny you mention the Wilwood stuff. The is actually to replace Wilwood hats. I've had problems (cracking around the disc to hat holes) with the current setup since they are bolted type hats. Wilwood has their T-Lock floating hats but nothing in the right offset that I need or I would just go that route.

Milland said:

For the reasons given in #3 regarding anodize and wear in the mounting slots, I've been really tempted to find some cheap offcuts of 17-4 in a hard condition and make bells from that. Lifetime part as long as you don't crash and/or burn. A little heavier than Al, but you can thin walls to 3mm or so except where the disk mounts (you can still lightweight a lot of the area if you want).

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Not really too concerned with wear in the slots since this setup will be using the Brembo slot shims. The Photo of the setup I posted is using the AP Racing bobbins where as I'll actually be using Brembo H Bobbins and hardware which uses a steel shim that snaps into the Hat slot and the float bobbins ride an that.

EmanuelGoldstein said:

If it's a full-floater it won't matter if the carrier warps

I had some similar parts at one time, a little under 6 1/2" o.d., basically a big bowl with under 1/4" wall out of 7075 bar. They didn't warp enough to be a problem.

(I almost wrote "not enough to measure" but if the tools are good enough you can measure the diameter of a gnat's ass. I am sure they warped some but not enough to notice. Standard good machining practices were followed.)

Per Milland's suggestion, 17-4 would be prettier but a much bigger project. They'd sure look good on the car tho. Or polished 316 ....

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That was part of my thought too. There is enough axial float (0.5mm to be exact) designed in that even if it were to bell / distort a few thou, it shouldn't cause any issues. I'm just concerned of it distorting much more than a few thou.

Milland said:

I guess you'd be totally against my experiments in using silicon nitride for such parts. Or my ill-fated cast-magnesium Hibachi (in fairness, it was easier to pack when traveling).

Bah, a life without experimentation is one not worth living...

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you could pitch the magnesium idea to F1 folk, they already mandated replacement of carbide skid blocks on front wing tips and car floors with titanium skid blocks, because carbide doesn't produce white hot sparks when it rubs on tarmac during heavy braking, so they might be interested in creating more show with an occasional burning metal fire...

and I suspect the performance gain from silicone nitride brake bells would be quite cost prohibitive, I'm quite sure there might be other places on the vehicle where that $$ could be better spent

CJMR2T said:

Not really too concerned with wear in the slots since this setup will be using the Brembo slot shims. The Photo of the setup I posted is using the AP Racing bobbins where as I'll actually be using Brembo H Bobbins and hardware which uses a steel shim that snaps into the Hat slot and the float bobbins ride an that.

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If for racing and not street use (especially in a Northern clime), that sounds fine. If significant moisture or road salt exposure was possible I'd want to check the Al to steel surface every so often to watch for corrosion.

jz79 said:

you could pitch the magnesium idea to F1 folk, they already mandated replacement of carbide skid blocks on front wing tips and car floors with titanium skid blocks, because carbide doesn't produce white hot sparks when it rubs on tarmac during heavy braking, so they might be interested in creating more show with an occasional burning metal fire...

and I suspect the performance gain from silicone nitride brake bells would be quite cost prohibitive, I'm quite sure there might be other places on the vehicle where that $$ could be better spent

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Heh, sheet magnesium chassis and no fuel cells provided plenty of sparks (and the occasional pilote flambe) during the 60's.

And si3n4 parts are fun to make, but it does take a lot of time with all the resharpening of the HSS tool bits I use...

CJMR2T said:

Not really too concerned with wear in the slots since this setup will be using the Brembo slot shims. The Photo of the setup I posted is using the AP Racing bobbins where as I'll actually be using Brembo H Bobbins and hardware which uses a steel shim that snaps into the Hat slot and the float bobbins ride an that.

Click to expand...

it is not abrasive wear that is causing the problem, hard anodizing is usually around 0,002" thick, can be thinner, but the photo posted above suggests at least 0,002" thickness if it isn't dyed, and this hard layer will simply rub off once the red/white hot rotor comes to a stop after some hard braking, shim or no shim - it won't matter, that layer going away will produce that 2 thou slop, which will kill the bell shortly after

I'm guessing those shims are some sort of a band aid they put there to combat rattling or whatever on some expensive road cars

I have a worn Brembo bell at work from an N group evo, can take detail pics and show you how they look worn

Milland said:

my ill-fated cast-magnesium Hibachi

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Do you have a video of the demise of the magnesium barbecue ? I presume it did a good job searing the steak ?

EmanuelGoldstein said:

Do you have a video of the demise of the magnesium barbecue ?

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Only in my absinthe-soaked mind. I could try doing a cranial download, but I don't have the right cable.

Milland said:

I could try doing a cranial download, but I don't have the right cable.

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You've been watching The Re-Animator again ? :P

jz79 said:

it is not abrasive wear that is causing the problem, hard anodizing is usually around 0,002" thick, can be thinner, but the photo posted above suggests at least 0,002" thickness if it isn't dyed, and this hard layer will simply rub off once the red/white hot rotor comes to a stop after some hard braking, shim or no shim - it won't matter, that layer going away will produce that 2 thou slop, which will kill the bell shortly after

I'm guessing those shims are some sort of a band aid they put there to combat rattling or whatever on some expensive road cars

I have a worn Brembo bell at work from an N group evo, can take detail pics and show you how they look worn

Click to expand...

Yeah, I get what you're saying. That photo is just something I found on via Google just to illustrate what I'm looking at making for anyone who is wasn't understanding my explanation. Also, I wasn't planning on Hard Coating them, just regular Type II. I do wonder if the fact that I'll be utilizing the Brembo H-Type bobbins rather then AP (or any others) T-type if this would help negate the effects you are speaking of?

The Shims according to Brembo are for Rub / abrasive wear and galling between the Bobbin and the Hat primarily for street cars. Rattle is kept down on street cars with spring clips (or Belleville washers in the case of some companies like StopTech and others that do Float in Disc rather than Float in Hat). In talking to Brembo, they run the wear shims in high end OEM (like Ferrari, Benz, etc..) and their aftermarket street setups just because of inspect and replacement intervals but race teams typically dont run them (or the anti-rattle springs for that matter) because everything is more often inspected and replaced and its just an added unnecessary cost for race teams. The only reason I'm running them is because I have a complete set that came with the H bobbins and while likely no significant benefit, still not detrimental either.

Milland said:

If for racing and not street use (especially in a Northern clime), that sounds fine. If significant moisture or road salt exposure was possible I'd want to check the Al to steel surface every so often to watch for corrosion.

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Haha, yeah, no road salt here in Florida!
While this car is still technically street legal, it is mainly a track car that is rarely driven on the streets except to car meets from time to time or in the rare even that its the only car I have to drive at the moment and have to get somewhere.