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Cutting brake rotors any techs???

Discussion in 'The Hokey Ass Message Board' started by gotwood, Apr 10, 2011.

  1. gotwood
    Joined: Apr 6, 2007
    Posts: 264

    gotwood
    Member
    from NYC

    Ok I was changing pad compounds on a car that had rotors that were never cut. They are low mile but had some surface rust from sittting, just flash nothing crazy.

    I took a red scuff pad to one ad put a Snap On(if that matters) dial indicator on it. Spun it and it was .005 out. The bearing itself was approx .0015 so I figured for a single piston style floating piston they would be fine after the cut. I checked all on that hud and between .003 and the above .005.

    I dropped them off to get cut and he only had 3 ready. The 3 that were ready were now out .007. The 4th which was done later was .003.

    I take them somewhere else where the guy questions my gauge and states they were out more like .011???

    I say hey there are tolerences in all gauges and spindles so lets just get them to a more acceptable level.

    I get them back after paying $80 and they are now off the chart at .012-14???

    Best part he challenged my gauge and he then explained he was using a $30 Harbor Freight dial to test them.

    What can they be doing wrong here??? I always buy new rotors and haven't had a set cut in 15 years. There lathes were dirtier than you could imagine.

    What is wrong here?
     
  2. 50styleline
    Joined: Apr 23, 2010
    Posts: 360

    50styleline
    Member

    First, what are they on. If its an OT late model anything you need to get them cut while they are still on the car. There are a lot of variables when putting them on the lathe. Most of the time it's the user or the jigs are not being set up right.
     
  3. gotwood
    Joined: Apr 6, 2007
    Posts: 264

    gotwood
    Member
    from NYC

    It is a general question what it is on has really nothing to do with it? Bearing play is the reason for supposedly cutting on a car correcting lateral movement. By the way if you buy into that you can find about 20 of those $8000 machines for sale for about $800 on EBAY at any given time. You can easily measure bearing and then rotor, cutting on the car just saves the time but in my case time is not a factor, getting it correct is. Yeah, I know they also correct parallel issues.

    I explained I took bearing play into consideration. Hey, just trying to get some interesting tech on what can cause a cut to create further lateral play. I am not discussing how to put flares on my new Honda.
     
  4. He may not hace had clean adapters or his spindle was bent some
    i put them pn the machine put a scratch cut then loosen it and turn 180* and re scratch cut again
    if it is in the same spot spindle is good
    dirty or badly dinged adapters make a diferance
     
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  5. 50styleline
    Joined: Apr 23, 2010
    Posts: 360

    50styleline
    Member

    Just perspective-----I didn't mean to get your panties in wad,,,,,,,,,,DAMN
     
  6. DR JAY
    Joined: Mar 27, 2011
    Posts: 37

    DR JAY
    Member

    I have worked @ many shops & used many lathes and have found many variables in cutting rotors: age and condition of the lathe
    condition of the adapters-have they been repeatedly dropped on the floor or do they have crap on the surfaces
    how tight did the guy crank the spindle nut on the lathe- I have seen cases where the lathe was old and abused and the shaft would wobble if the nut was cranked tight beyond reason
    how true was the spindle shaft to start with? I've worked with alot of weaklings over the years and have witnessed COUNTLESS heavy truck drums being dropped on the shaft with a desperate grunt as the tiny apprentice's vegan muscles gave out...

    I prefer to make sure everything is clean by dressing all surfaces with a Roloc pad-MOST importantly the center hub holes- and inspecting and cleaning all adapters prior to installation. In my whole career I've never made a drum or rotor worse than I got it- hope this helps some
     
  7. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,140

    HemiRambler
    Member

    I did a pair on a regular machinist lathe once. They ran out a couple thou and were parallel (sides) within .0015". I tossed them on and they were great. Few years later I set them up again - same lathe - for whatever reasons (hard spot who knows) I couldn't get them better (parallel) any better than .003" I thought surely I should be able to repeat the .0015 I had last time, but gave up (it was a beater car) and tossed them on - figuring it's ONLY an "extra" 1 and a half extra than the last time - shit you could feel the pedal pulse!!! Bottom line - IMHO it's more important to have them measure the exact same thickness (I checked 4 places) than having them EXACTLY true. The calipers will float a little so you don't feel that as bad BUT if the thickness is "off" you WILL feel the pedal pulse pretty quick!!! YMMV
     
  8. Scott K
    Joined: Oct 17, 2005
    Posts: 826

    Scott K
    Member

    When I worked as a brake engineer at an OEM, our target for total indicated runout for the mounted system was just over 0.001" (30 microns to be exact) on factory new components. Our spec for parallelism between the inboard and outboard brake plates was approximately the same.

    Initially , the parallelism between the plates is more important to avoid brake roughness (pedal pulsation, wind-down noise, etc), when new.
    If the overall system runout is large, brake roughness will surface over time, depending on how worn the suspension is and how agressive the pad material is. Something on the order of 0.003" of total system runout will probably result in brake roughness for many modern cars over time.
    Older cars can tolerate more runout because their suspensions were not tuned as tightly as modern cars are.

    .012" runout sounds like junk to me.
     
  9. budd
    Joined: Oct 31, 2006
    Posts: 3,478

    budd
    Member

    i tried turning a rotor off my f250on my lathe, it only had one side that was a little scored, i made an adapter that i mounted in the chuck, took a fresh cut on it to make it true, bolted the rotor on and cleaned it up, the inside of the rotor was fine and i had no way to reach it without fliping it around, when i checked the back side it was like .003 out, the front was now dead on, i figured it would be fine, no way, i can feel the pedal pulseing, when i have watched my rotors being turned in a shop they cut both sides at the same time, or thats the way they did mine, i think ill try to make a cutter holder for my big lathe so i can cut both at the same time.
     
  10. Mr48chev
    Joined: Dec 28, 2007
    Posts: 26,543

    Mr48chev
    ALLIANCE MEMBER

    I think DrJAY and I may have worked in the same shops at different times.

    Sometimes those brake lathes get abused in way that they shouldn't. In one dealership I worked in I caught another mechanic who was the local circle track hot shot at the time using the brake lathe to straighten out his race care wheel by spinning the wheel on the brake lathe and hitting it with a three pound hammer.

    As he said, some of those lathes have been abused and others are worn or have damaged adapters. But there are just as many well maintained brake lathes in perfect condition that turn out perfect work every time if the operator knows what he is doing out there too. You just need to find the right shop to do the work.
     
  11. Scott K
    Joined: Oct 17, 2005
    Posts: 826

    Scott K
    Member



    0.003" = 75 microns. Not sure what your F-250 sensitivity level is, but some cars / trucks are sensitive down to 15 microns (0.0006").

    Critical for some vehicles to turn both sides at the same time on GOOD equipment.
     
  12. gotwood
    Joined: Apr 6, 2007
    Posts: 264

    gotwood
    Member
    from NYC

    See that is kind of my question?? What set up is capable of delivering a .001 cut?? I mean the lathe itself would have to have almost zero movement itself? If you cut that rotor and drove the car the corrosion from just sitting in a perfect climate would probably exceed that number. I have always shot for .003 which again is not always achievable without shims.

    I would say 95% of rotors cut are ever checked? I have never actually cut a rotor but I look at the lathes compared to the prep I do to the rotor mtg flanges and I can't see how it is possible to obtain those numbers?

    On a side note I have decided to just shim them as they do appear to be the same thick as measured in several areas across the disc.

    I was on the hunt for shim stock and some old timer over heard me and told me why not just use foil duct tape??? I had zero luck finding shims local so why not at least use the tape to get things set up? It actually works so easy and fast I don't want to take it off. I tried a piece at 300 deg and besides the loss of adhesive it stood up. Anybody ever hear of using as a shim?? I ordered stock from MCCARR but they are .002-.003 as mtd incl bearing so I would love to just leave on rather than cut shims in same thickness.


    THANKS
     
  13. gotwood
    Joined: Apr 6, 2007
    Posts: 264

    gotwood
    Member
    from NYC

    Sorry for the double post but... If the surface of the cut itself has .001 ridges wouldn't that also effect a reading?
     
  14. HemiRambler
    Joined: Aug 26, 2005
    Posts: 4,140

    HemiRambler
    Member

    I suppose in theory you could shim it - I mounted some ground rotors on a spindle mount wheel once and I shimmed those - except in that case I could shim each mounting point to get it "perfect" at the brake surface.

    Not too easy to shim a rotor with a bearing in it so I assume you're talking hat rotors - in that case remember that the shim needed at the flange is a fraction of the amount you want to change at the brake surface. Not sure I'd want to just toss in one shim and call the rest good - what happens when you torque up the lug nuts? Also you'd have to be sure to index the rotor any time you had it off. All in all - I suppose it's possible, but not so sure it's all that practical.
     
  15. str8axle55
    Joined: Dec 19, 2006
    Posts: 355

    str8axle55
    Member
    from MA

    I cut rotors weekly at my work, all hubless. As stated adapters need to be clean/true, as due the rotor hub surfaces. It does depend on the user, I never measure runout, I eyeball it. If you see it wobble to much, loosen, rotate, re-check. The hat style rotors don`t cost alot to replace, some Euro makes are not meant to be machined, dealers don`t heve lathes, parts under warr are replaced w/new for excessive runout.
     
  16. gotwood
    Joined: Apr 6, 2007
    Posts: 264

    gotwood
    Member
    from NYC

    STR8AXLE Not to throw you under the bus but explain how you can eye ball .001-.002 of movement? That is the problem. If you did put an indicator on before and after you cut you would have hard evidence to provide the customer of exactly what was done. Instead you are doing exactly what was done to me. What is the runout of your machine?

    It has nothing to do with them being Euro, Japanese or US it has to do with the make up of the rotor. Is it cheaper to replace??? in most instances and do to the fact that most don't want to bother cutting and finding someone who can do correctly is a coin toss.

    As far as dealers not having lathes? Most Japanese makers insist they cut on car so I do not follow that argument either. If cut on car it actually speeds up dealers work and reduces comebacks as it takes bearing tolernces into account.

    You read tons of posts all over the net about shitty pads and shitty rotors without actually measuring. It isn't always the products fault. A little blueprinting and one can get a better than factory working brake system it is for some crazy reason just not done.

    As for shimming a hat style rotor Raybestos and Brakealign both make them but at $18 ea for a full tapered shim it is expensive. I have made out of brass shim stock and isn't that hard of a job, spin rotor with dial indicator and find lowest point. Most hat style rotors are retained with a set screw so swapping around isn't easy.

    Anybody think the foil tape would last??? It just makes it so easy rather than cutting and moving around shims???
     
  17. Scott K
    Joined: Oct 17, 2005
    Posts: 826

    Scott K
    Member

    Before you start shimming, have you indexed the rotor to try to match mount the rotor to the bearing/hub??
    When I was running my brake testing, we had a target for mounted runout. We could usually hit the taret by indexing the rotor and remeasuring. Although, occasionally we did have to resort to using shims...I think the shims were stainless.
    The testing I was doing amounted to running the vehicle 20,000 miles on a prescribed route without touching the brakes for maintenance. Any shims we used were installed and lasted the duration of the test.
     

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