Car Brake Lathe Myths

10 Brake Lathe Myths

Are your employees properly trained in the lost art of turning rotors? See how many of these myths your staff still believes.

Brake Lathe Myth No. 1: I don’t need to use a dial gauge or micrometer.

A dial gauge and micrometer are store’s best friend when it comes to brakes. These two tools can save you countless comebacks. But, it is amazing just how many stores do not use, or even own, these indispensable tools.

A dial gauge is used to measure runout in the rotor and hub assembly. It can also be used to check for runout in the arbor of a bench lathe. Also, it can be used to verify that you have a rotor or drum properly mounted to a bench brake lathe.

The dial gauge is a very robust piece of equipment that can last a lifetime. It is also a good idea to invest in a good magnetic base or vise-grip handle that can be setup quickly.

Always use a dial indicator to verify the amount of runout present in a rotor and hub assembly, both before and after the rotors has been machined. This must be performed even if you are using an on-the-car-lathe. By measuring the rotor on the hub, you can also check for play in the wheel bearings, corrosion on the mounting surface and other possible errors. Even if you are putting new rotors on a vehicle, runout should be checked. It doesn’t take much, only about .002 inches on some vehicles, to cause a noticeable pulsation. Also, if you are using a bench lathe, the dial indicator can be used to check to see if you have mounted the rotor or drum properly to the lathe.

Micrometers are essential to measuring rotor thickness. Always measure rotor thickness and drum inside diameter to determine the amount of wear. You need to measure rotor thickness at six to eight points around the rotor. Doing so should reveal the variation that’s causing the problem. A rotor or drum should be replaced if it is worn to the discard thickness or cannot be resurfaced without exceeding the “machine to” or “discard” thickness.

Myth No. 2: Every new rotor must be machined.

It is an automotive “urban legend” that a rotor could be damaged from the time it leaves the plant to the time it’s put on a car. Even if a rotor was dropped from the warehouse’s highest shelf, it would not have any damage that would cause a runout condition. The conditions rotors experience on the road are more violent than the conditions at a parts store or warehouse. But, if a rotor or drum was improperly machined or cast at the plant, it is their fault and it is not up to you to correct it. Send it back!

In most cases, you do not need to machine new rotors. Your customers should verify the amount of runout in a new rotor once it is mounted on a vehicle. If you are getting a large number of rotors with more than .003 inches of runout, contact your supplier as long as the runout was not caused by a mounting error.

Removing even .0015 inches from some new rotors reduces their longevity and introduces a process that could induce runout itself due to possible error by improper machining. If you do measure new rotors and find yourself having to machine more than .003 inches of runout, call your supplier. If you are not satisfied with the overall rotor finish and appearance, there are products on the market that can help you without having to remove large amounts of material from the rotor. For more information on that, see Myth #4.

Myth No. 3: Those cutting bits on the brake lathe will last one more turning.

The condition of cutting bits will have a great effect on the quality of the cut. This is especially important for one-cut capable on-the-car lathes. Some lathes can produce a finished rotor with just one cut, but not with a dull bit. Some manufacturers claim that their lathes are capable of removing .015 inches of material per cut. Do not try this with a dull bit.

Some technicians or store personnel say that they can tell a sharp bit from a dull bit from the sound it makes. But, sound frequencies are also indicative of the turning speed and the depth of some cuts. If you feel comfortable with this technique, use it, but remember to always perform a visual inspection and follow the lathe’s recommendations for arbor and cross feed speeds. If you do not have the experience, do not try using your ears as the guide.

The best advice is to develop a regular replacement interval for cutting bits. Whoever turns your rotors at your store should be responsible for the condition of the bits.

Myth No. 4: I don’t need to do any additional work after a rotor has been turned.

One of the biggest mistakes jobbers make after machining a rotor or installing a new rotor, is not addressing the new surface. After the rotor has been turned, it is covered in metal particles. In the case of a new rotor, it may be covered in metal particles and anti-corrosion coatings. Either material can contaminate the brake pad and lead to two conditions.

First, metal particles or coating can contaminate and become embedded into the brake pad. This could lead to longer stops, glazing of the pads and eventually overheating the brake system. Second, if the surface of the rotor is contaminated with metal particles or anti-corrosive coating, it could prevent the new brake pads from laying down a transfer layer of friction material onto the rotor. This can lead to longer stops and increased pad or rotor wear.

All rotors should always be cleaned before installation. The most effective method is to use a mild soap solution and then wipe dry with a clean rag or paper towels. Do not rely on a few squirts of brake cleaner and compressed air to do the job. There are also special soaps and cleaners to achieve a clean surface.

If you are not satisfied with the appearance of the finish on newly machined or new rotors, there are products on the market to help achieve a finish that is up to your standards. These devices are designed to remove the peaks from the grooves and give a non-directional surface while not removing large amounts of materials.

Some stores may use #150 grit aluminum oxide sandpaper on a block that is moved in and out as the rotor is on the bench lathe. Some of these devices can be powered by the brake lathe or drill motor. With some of these tools it is import to follow the manufacturer’s recommendations or you could be doing more harm than good. Most of these tools need to be used with drill motors that turn slower than 800 rpm. Also, these types of tools can “load-up” with metal particles that need to be removed, or the abrasive insert must be replaced.

Myth No. 5: On-the-car brake lathes don’t deliver a good finish.

This is one of the myths that is rampant in the brake industry among your professional customers. It is based on the assumption that a rotor can not be cut in one pass. Rotor speeds for just about every on-the-car lathe are between 65 to 110 rpm. These speeds are close to, or even below, most bench lathes’ slowest speeds. Yes, it is true that you can achieve spectacularly low Roughness Average (RA) numbers with a bench lathe using multiple passes and differing cross feed rates. But, these types of RA numbers should be saved for other surfaces.

When most OEMs approved on-the-car-lathes for use in their dealerships, they made sure that the lathes could machine and deliver a finish of 40 RA in most cases. This is well below the average industry acceptable RA finish guideline between 70-80 for most rotors.

Myth No. 6: Dont trust on-the-car brake lathes.

It is difficult for some technicians to surrender control over some lathe functions. The thought of allowing an on-the-car brake lathe to control how much will be cut from the rotor and at what speed the rotor is turning is difficult to comprehend. This is even further compounded by it being done in one pass and the technician possibly not having the chance to make finishing cut.

Most manufacturers of on-the-car brake lathes that are hub mounted have put vast amounts of research and development time into these machines. Also, some of the companies are willing to take the machine back if you are not satisfied with the results. This commitment from these manufacturers will hopefully let you trust the machine to provide a satisfactory finish. This is also a form of control.

Myth No. 7: I don’t need to learn how to machine rotors because rotor prices are dropping dramatically.

With the introduction “off shore” rotors, the prices for some applications have come down dramatically. This may make it more cost effective, in terms of labor and comebacks, to just slap on new rotors. In some cases, it is not cost effective or even ethical.

Some automakers are putting more “meat” back on the rotors to help eliminate runout problems within the first 40,000 miles. According to some sources, more new cars and trucks are using captured rotors or designs where extra disassembly is required. The front brakes on the new GM Colorado pickup truck is an example of the new trends in rotors where it may be more cost effective to machine the rotors with an on-the-car brake lathe than to remove them for machining or replacement.

Myth No. 8: Lathes don’t need cleaning or maintenance.

If you don’t take care of your store’s lathe, chances are you are doing more damage than good. Take the time to clean the lathe and verify that the spindle and arbor are within manufacturer’s runout specs.

Cleaning should be done with a brush. Compressed air may embed metal particles, for example, to get into bearings. The protective boots or bellows that cover the various spindles, cross-feed mechanisms and cutting heads should also be inspected daily.

Make sure the various adapters, cones, and hardware used to mount the drums and rotors to the arbor are clean and free of nicks and scars. Metal chips and brake dust on adapters increase the likelihood that a rotor or drum will be improperly mounted. If mounted improperly, the drum or rotor will be cut with runout included.

All adapters will rust slightly. Spray them with light penetrating oil and wipe them down thoroughly. Excessively oily adapters can attract dirt and metal chips. Penetrating oil will find its way into the microscopic valleys of the metal, preventing rust and corrosion.

Arbors and adapters should be inspected for nicks and other damage.

Myth No. 9: I don’t have to learn how to machine drums.

Brake drums will not go the way of the carburetor. Learning how to machine a drum properly is not a dead skill. Yes, the numbers have decreased in favor of rear disc brakes, but they will still be on the back of some economy vehicles and heavy-duty vehicles.

Myth No. 10: On-the-car brake lathes can damage ABS sensors.

Yes, it is possible for the metal particles from cutting the rotor with an on-the-car brake lathe to end up on the end of the wheel-speed sensor. But, this is becoming rarer with the OEMs placing the wheel-speed sensor and tone ring in sealed areas where contaminates can’t get to them.

An exposed wheel-speed sensor that has a build up of metallic particles needs to be cleaned along with the surrounding area. If the sensor has to be removed for cleaning, remember to reset the air gap.

On ABS-equipped vehicles, do not run any on-the-car brake lathe with the vehicle running or with keys in the “run” position. The differences between the front and rear wheel speeds could set a trouble code.

Rotor turning is still an important skill for jobbers to know. Don’t fall for these myths and your customers will thank you.

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