Brake Rotor Machining

Michael Mobile Technician Clinton, Utah Posted   Latest  
Out Of Parallel
Resin Deposits

This past week I did training for technicians to take the A5 Brakes Test for ASE. In the class most of the technicians and managers stated that they no longer do rotor machining. They stated that it was just not worth the risk of comebacks. Even the company owner liked the idea of replacing rotors because it added more to the ticket. The consensus was that when the rotors are machined and in specs that they easily warp or start pulsating. This observation does not align with what the part manufactures are writing in their articles. I would like to get some solid data on this subject. From the class I have learned that rotors were being cleaned with brake clean. The industry standard is soap and water. They were not adding a non directional surface. They were not breaking in the pads before delivering to the customer. From what I have read, the parts manufacturers are stating that most pulsation is not from warp or out of parallel, it is from uneven distribution of the pad resins on the rotor surface. They also expressed incorrect prep of the rotor and hub surfaces. The corrosion was not being cleaned off before assembly. I believe that if best practices are followed the machined rotors should perform on par with aftermarket replacements. What have you found? 

Chris Technical Support Specialist
Commack, New York

Using on car machining equipment provides the most true machining possible. The stack up of tolerances of rotor out of true and hub runout can lead to out of spec runout even with new parts. Many technicians feel intimidated by the machining process and typically use the cost of a poor quality rotor to justify replacement. Taking care of your customers is the best way to retain them. Forcing them to spend money on cheap rotors made of recycled metal is not always in the best interest of the vehicle owner even if it does make you a few more dollars.

Because we are talking about rotor warpage, how many techs still run the lugs on with an impact gun and a socket? Hand torqueing is slow, but most effective, Using torque sticks will maybe take an extra 30 seconds to use when installing the wheels. This will prevent most of the warped rotors people experience.

Machining rotors seems to be one of those things that will get lost on the next generation of mechanics due to cost. The time to machine rotors for most techs exceeds the amount of flat rate pay for doing the additional work. If you want to keep your customers, machining rotors (when in spec) will help keep the cost of brake service down.

+1 Ð Bounty Awarded
Randy Curriculum Developer
Raleigh, North Carolina

Technicians need to understand what causes most pulsation issues. "Warpage" (excessive runout) is commonly blamed but is only the start of the problem. Excessive runout will not cause pulsation by itself if caliper slides are in good condition. The fact that excessive runout causes the pads to contact the rotor in an uneven manor will create excessive rotor wear in those areas, hence creating a parallelism issue. That's what causes the pedal pulsation. How many times have we seen a "good" brake job go out the door only to return a few weeks later with a pulsation complaint. That being said, you are correct in saying that "best practices" will cure all of the issues that techs experience including washing correctly, non-directional surfaces, and measuring runout when completing the job. Unfortunately, "best practices" are not understood or not practiced in many shops largely because of the cheaper is better attitude of consumers and even a few shop owners. Good training and consumer education at the counter can alleviate many of these problems. I know I'm preaching to the choir here but your post brings to light how rampant poor practices can be in our industry. 

+3 Ð Bounty Awarded
Bill Technician
Rosetown, Saskatchewan

I feel it would depend on where you live as well as your shops labor rate to an extent. I will offer my opinion, based on my own experiences.

I am very familiar with pro-cut on car equipment as well as off car equipment. FNC rotors do not cut well and I feel cutting the outer layer is likely going to get down into inferior material. Even FNC rotors in my area are rotted on the outer edges most times, and continue to rust at an accelerated rate by the time pad replacement is required. The rotor fins are plugged solid and have a heavy layer of corrosion weakening and sometimes splitting the rotor in 2 pieces. Where I live I do not feel rotors could be "properly" cleaned and turned in a feasible amount of time to recover the costs of the equipment and in the majority of cases should not be attempted. 

If I lived in an area where the cooling fins were clear, the flange area was not heavily corroded, the edges were not typically rotted out and the rotor was not FNC, then I would consider it an option.

0 Ð Bounty Awarded
Randy Curriculum Developer
Raleigh, North Carolina

Agreed that the base condition of the original rotor should be considered first. My shop was in a saltbelt area so we saw a lot of those rotors that you couldn't believe were on the car.

0 Ð Bounty Awarded
Rudy Technician
Montebello, California

Agreed best practices are whats needed.

At the shop I am in now ,when I got here 3 years ago,there were quite a few comebacks for pulsation. What I found was that the techs here were not properly mounting the rotors on the lathe,resulting in an uneven cut. They would just continue cutting until the rotor cut evened out.

The thinking was that the rotors were not cutting evenly because they were warped,when in fact they were simply mounted incorrectly. 

+1 Ð Bounty Awarded
Ric Owner/Technician
Hooksett, New Hampshire

We rarely turn rotors these days. By the time we see vehicles that need brake jobs the rotors are very near their life cycle. Add salt and corrosion and it only makes sense to replace them. Proper mounting, cleaning and fastener torquing has led us to a zero comeback rate for vibration and noise. Replacement works for us.

0 Ð Bounty Awarded
Martin Instructor
Burnaby, British Columbia

I believe that there are many pros and cons in regard to consideration for machining brake rotors. First, geographic influences such as regions where rotors physically degrade, resulting in heavy corrosion around the non-contact areas and heavy flaking and buildup in the vents, may warrant replacement over service. I have seen many east coast rust belt vehicles brought in from auctions, where the rotor braking surfaces might even warrant resurfacing, but the structural integrity of the rotor was compromised in the mounting and venting system due to excessive corrosion.

Here in BC, we do not typically experience such severe weather extremes that result in such excessive corrosion, so machining brake rotors can serve well, depending on rotor thickness. During many years at a GM dealership, brake rotor pulsation issues under warranty, required measurement of the rotor pre and post machining.

Often, the measurement of the OEM rotor even after use was thicker than the vehicle manufacturer nominal specified thickness, enough that even after machining the rotor was still at, or within a few thousandths of the published nominal value.

That said, there were also rotors that would support no more than one refinishing cut under warranty. GM requirements during my time on the bench were that the technician must leave 50% of the allowable material that could be machined from the rotor for a subsequent retail brake repair. IOW, 50% of the material value between the specified nominal value and the machine to specification. If less than 50% remained following machining, rotor replacement under warranty was warranted.

Most often, poor preparation of rotor and hub mounting surfaces creates excessive lateral run out following brake rotor refinishing, resulting in pulsation resulting from thickness variation issues just a few thousand kilometers after brake repairs.

In training we ensure that students learn how to perform all of the measurements properly, inspect bench mounted brake lathe arbor run out and perform adjustments, properly set up rotors and drums for machining. Tool bit condition and thus the resulting surface finish must be within the specified acceptable range.

Post-machining is followed up with a non-directional finish, cleanup with soap and water rinse and drying with a non-oiled air source, thickness measurement, mounting and LRO verification. Re-indexing may be necessary or shim compensation to ensure no stacked tolerance issues result in out of spec LRO.

With the brakes assembled and final cleaning, wheel hand lug torquing and a road test with careful burnishing, is an absolute must final step for a quality brake repair. Poorly distributed resins from the pads can quickly cause brake pulsations in the same way that many years ago, poor metallurgic distribution in some rotors affected the friction coefficient of the rotors, resulting in pulsation.

Again, surface finish, post assembly cleanliness and procedures can play an important role in how well the resins transfer into the microscopic pores in the rotor friction surfaces.

We use both bench mounted lathes and ProCut on vehicle lathes and both can result in an excellent surface finish. Mounting of the bench lathe refinished rotors can take the extra steps of reassembly and LRO verification compared to on-vehicle machining and the results of measuring both lean in favour of on-vehicle machining.

There are other considerations and influences in the repair facility environment. The low cost of questionable quality new brake rotors from various big name vendors is often a more attractive proposition for a shop owner over purchasing a brake lathe. Training the technician to perform accurate inspection, measurement, use of equipment and the "rust belt" degradation issues, often results in replacement rather than rotor service being the choice.

It seems however, that reasons for justification of rotor replacement rather than refinishing is a result of shop policies, geographical influence and past experiences, rather than whether machining could be an effective procedure.

Some vehicles by design do seem more prone to issues with brake pulsation than others, so rather than be blamed by the customer for issues resulting from machining, rotors are simply replaced, time after time.

Issues noted firsthand in the workplace. Poor technician practices in rotor inspection, cleanup and a lack of measuring skills. Failure to prepare bench mounted lathes and follow up with best practices for machining, post-machining reassembly through burnishing.

FWIW, torque sticks have been a GM non-approved wheel torqueing method for several years. Results proved to be inconsistent, often from technician abuse or use of impact equipment that resulted in higher values due to the rate that the lug was secured.

We have the ability to provide the training necessary to restore brake rotors following best practices, yet cannot guarantee that repair facility equipment is present, in good condition or well-maintained. How many technicians actually bother to measure rotors? Does the technician or shop have measurement tools in good condition?

Does rotor replacement reduce the need to follow best practices, such as proper mounting surface preparation, LRO measurement etc? Of course not, but laziness or sloppy workmanship often prevails, causing premature thickness variation issues that result in pulsation. This can occur whether a rotor is replace or refinished.

So, in summary, it really depends on many variables whether rotor refinishing is practical or viable in some areas and in some shops, based on shop policies, equipment and technician skills.

I foresee that just as many facilities no longer offer in-house cylinder head and valve services or other processes that have become sublet or replacement only, that brake rotor refinishing is slowly heading in the same direction.

+2 Ð Bounty Awarded
James Owner/Technician
Pike, Pennsylvania

We quit machining rotors over ten years ago. The cost of labor vs. new rotor cost and possible comebacks is a no-brainer for us. Clean the hubs and install coated rotors w/premium pads is the way to go for us. Maybe if we did trucks or heavy equipment we would.

+1 Ð Bounty Awarded
Tim Educator
Québec, Quebec

Have you ever had rotors out of the box have unacceptable parallelism or LRO? We used to have to true up new rotors a lot, even with OEM parts.

Then there is the “brake job” itself - a lot of DIY’ers have done this operation and don’t see why it should cost so much to do correctly. 

Consumer education is the final frontier! 

+2 Ð Bounty Awarded
James Owner/Technician
Pike, Pennsylvania

Hi Tim

"Have you ever had rotors out of the box have unacceptable parallelism or LRO? " Yes, but very rarely and we exchange them along with another set of pads no questions asked. But again for the high line of vehicles we work with we will only use

a high quality manufacturer.

0 Ð Bounty Awarded
Marlin Technician
Estacada, Oregon

In-spec OEM rotors which have been correctly machined, washed, and mounted are typically superior to new AM rotors. There are so many variables involved that there are not many solid general statements which anyone is wise to make. 

One way to lower comebacks is to use low-quality, highly abrasive pads and low-quality rotors, and replace the pads with 25% or more thickness left; this way, wear is so rapid to both that vibration is unlikely to develop before everything gets replaced again. This is common at chain stores, but also occurs are general independent and dealership shops.

Correctly performed and high quality brake repair is neither gravy nor inexpensive, but it also provides much longer service intervals and better satisfaction than is typical.

0 Ð Bounty Awarded