Brake Lathe Basics

Brake Lathe Basics

Resurfacing drums and rotors is a machining process with its own specific guidelines.

There was a time when our brake lathes never stopped, humming their metallic tune almost all day long. If rotors or drums could be turned, that’s what we did, but it’s a rare occurrence today. We almost always replace them. Most shops still have their lathe because it’s valuable equipment, and while it’s silent most of the time, when you need it, you need it.

Resurfacing drums or rotors is a machining process with its own specific guidelines to follow. So, the next time you need to blow the dust off the machine, here’s a refresher on some brake lathe basics to make sure your lathe is humming in the right key. If you’re new to the process, that’s more than just a bad joke. You can always tell when your cut is going well by the sound. If it changes, so is the quality of the cut.

The basic operation of a lathe consists of mounting a drum or rotor onto a spindle. The spindle shaft is turned by a motor, which rotates the mounted component. When a cutting bit contacts the turning component, metal is removed.

There are two different feeds on a brake lathe: spindle feed (the distance the spindle moves per revolution) and cross feed (the distance that the cutting tool moves per revolution of the spindle). Spindle feed is used for brake drums, and when engaged, the spindle moves to the left, which moves the surface of the drum across the fixed cutting tool. Spindle feed rate is adjustable on most lathes.

Cross feed is used when turning brake rotors. In this case, the rotor is turning but the spindle remains fixed, and cross feed pulls the cutting bits across the rotor. Cross feed speed has a fine and course adjustment. The key to successful resurfacing is based upon the combination of spindle rotation speed, spindle feed or cross feed speed, correct component mounting and vibration elimination.

Mounting hubbed drums or rotors is accomplished by using tapered cone adapters. Choose the sizes that are closest to contacting the middle of the bearing races.

Mounting hubless drums utilizes hubless adapters, along with a spring and cone to center the drum. Hubless rotors also utilize a cone and spring to center the rotor, but a different style of adapters. Be sure all mounting surfaces are free of rust or debris. Use spacers as necessary to reach the arbor nut, and always be sure to use the self-aligning spacer next to the nut to prevent diagonal thrust on the adapters.

When turning a drum, first install the drum silencer band, making sure it is wrapped up to the right-hand edge. Position the boring bar so it is close to the drum and tighten it. Turn the drum by hand to make sure everything is clear, then start the lathe. Using the cross-feed handwheel, bring the cutting bit toward the drum until it just contacts it and makes an initial scratch cut.

Back the bit away from the drum, turn the lathe off, then loosen the arbor nut and rotate the drum 180 degrees and retighten the nut. Start the lathe once again, rotate the spindle feed handwheel a ½ turn, the bring the cutting bit toward the drum once again to make another scratch cut. Turn the lathe off and look at the scratch cuts. If they are on opposite sides of the drum, you most likely have dirt or a burr on a mounting surface. If they are side-by side, the drum is properly mounted and you can proceed with resurfacing.

As a general rule, rough cuts for a drum should be no deeper than 0.020”, and finish cuts no shallower than 0.004” (adjusted using the cross-feed handwheel). The spindle feed for cutting a drum should be 0.006” to 0.020” (adjusted with the spindle feed dial while the lathe is running).

brake lathe

When turning a rotor, the twin cutter is used to cut both sides of the rotor at one time. Install the rotor silencer band, then center the twin cutter around the rotor. You can use the spindle handwheel to help position the rotor but be sure to lock the spindle position afterward. Here’s a step many people miss: Adjust the spindle speed to match the rotor size.

On the most common lathes, this is done by moving the spindle drive belt between grooves. The outer pully groove is for most passenger car and light truck rotors; the inner grooves for larger and solid rotors. The important point is, you are changing the speed based on the size of rotor, slowing it down for larger rotors.

As with the brake drum, rotate the rotor by hand to make sure everything is clear, then start the lathe. Cutting bits are adjusted by using the knurled knobs. Use the cross-feed handwheel to position the bits midway on the rotor surface and make a scratch cut. These are almost always an incomplete circle, but it can be caused by rotor runout or by incorrect mounting. Similar to the procedure with a drum, turn the lathe off, loosen the arbor nut and rotate the rotor 180 degrees.

Make a second scratch cut. If they are side by side, the runout is caused by the rotor, but if they are opposite each other, there is most likely a burr, rust or debris on the mounting surfaces. Once you have confirmed the rotor is mounted correctly, you can proceed with resurfacing.

Rough cuts should be taken with the cross feed speed lever in the fast position; finish cuts in slow. Rough cuts should be 0.006” to 0.010” per side, and finish cuts should be 0.004” to 0.006”. Cuts shallower than 0.004 tend to reduce the life of the cutting bit, because the heat generated isn’t transferred to the rotor. Enjoy the music! TS

You May Also Like

Don’t Forget the Basics

Every now and then, I get tunnel vision during diagnostics, and forget to think about the big picture.

2000 Jeep Cherokee

We’ve all made the mistake, at least I know I have. Every now and then, I get tunnel vision during diagnostics, and forget to think about the big picture. And this last episode even violated my cardinal rule: don’t leave a malfunction indicator light (MIL) unresolved.I always recommend fixing every problem that causes the MIL to illuminate. I hear the excuses all the time about why people would rather live with the light, but I always tell them, even if the immediate cause of the light doesn’t affect the dependability of the vehicle, if something else happens that does, you’ll have no idea of knowing. That’s the beauty of computer controls. You always know when something is going on.Just a couple weeks ago, a friend called me because they were having trouble with the transmission in their 2000 Jeep Cherokee. It was shifting erratically, and it occurred after driving on a very hot day. The fluid was dark in color, and when it last had been changed was unknown.This particular vehicle was located in Arizona, so phone-only troubleshooting was the only option. I decided to focus on the condition of the fluid and the hot climate. We’ve all faced neglected and burnt transmission fluid and had to make the decision of change or not to change. Some say change it, but some say that may cause the transmission to completely fail, so leave it alone.I decided to roll the dice, and, in this situation, I decided the fluid should be changed. Then, knowing the climate, I decided to dig in and focus on the other factors that could affect the fluid. After asking questions, I learned that the cooler in the radiator had been bypassed and the only cooler was a small aftermarket unit. I also discussed the fact that if the fan clutch was not operating properly, it could cause a problem with cooling as well.Taking my advice, the Jeep’s owner changed the fluid and filter, installed a new radiator, reconnected the transmission lines to it, installed a new aftermarket cooler in line and installed a newfan clutch.After performing all the work, the transmission continued with its erratic shifting. No better, no worse. The next day, unexpectedly, the engine stalled when coming to a stop. This had never happened before. The Jeep’s owner decided to check for trouble codes and found a code for the throttle position sensor (TPS).He didn’t think about looking for codes before because the MIL was already illuminated for another problem. He was “living with the light.” After replacing the TPS, not only did the engine run better than it had in a while, but the transmission shifted perfectly and has ever since.Of course, it all makes perfect sense. I wasn’t there to see the MIL, I didn’t think to ask if it was illuminated, and on or not, I didn’t even think to check for codes. My tunnel vision in this case only made me think about the transmission fluid, not the information the transmission needs to know in order to shift properly. Luckily, the outcome was good and the owner is happy knowing the transmission is serviced and the fluid is being properly cooled. TS

Refrigerant Oil Has to Be Right

Oil type is just as important as oil capacity.

Three bottles of refrigerant oil
Top 5 Tools: Steve Coffell, Auto World, Hazelwood, MO

Steve Coffell, a technician at Auto World in Hazelwood, MO, says his Top 5 Favorite Tools are:   OTC Genisys Touch – Quick scan, bidirectional control Snap-on VANTAGE Pro – Easy-to-use waveform views. Power Probe IV – Power and ground control PicoScope – Live wave viewer OTC – Low Amp Probe   WHAT ARE YOUR TOP 5

Wheel Bearing Adjustment Tools & Equipment

A wheel bearing that’s out of adjustment can reduce bearing life and can affect more than just the bearing. It’s important to adjust the wheel bearing endplay to the proper specifications. If the bearing set is adjusted too loose or too tight, it can cause the bearing to fail prematurely. There are a few types of assemblies, so using correct procedures and tools will ensure a comeback-free wheel bearing installation.

What R-1234yf Means for Service, Equipment, Safety

For shops, R-1234yf means several new procedures, a certification and new equipment in order to properly handle these new systems. Why the difference in handling and servicing R-1234yf systems versus R-134a? Flammability.

Other Posts

Finding A/C Leaks: Is Dye the Answer?

Electronic leak detectors are much more sensitive and accurate than they were years ago.

UV Dye
How To Prevent Rust on Hand Tools

Cortec suggests these simple options to prevent rust and ratchet hand tool maintenance up a notch.

The Next Gen of Tools and Equipment: We’re in for a Ride

The TechShop team visited Orlando, Florida last month to attend ISN’s Tool Dealer Expo.

Electrical Circuit Diagnostics

Your diagnostic approach needs to be well thought out, and it starts with your equipment.