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Grinding Brake Drums on a Brake Lathe

dalmatiangirl61

Diamond
Joined
Jan 31, 2011
Location
BFE Nevada/San Marcos Tx
Wondering if anyone here has experience or knowledge about the whys and where this is done. I know people talk about grinding out hard spots, but according to mfrs of my lathes the grinding procedure is done after the turning, and neither mentions hard spot, or why. I'm guessing it has to do with surface finish, but I need to know where and why.

Google search only brings up every post since the dawn of the internet on people complaining their brakes are grinding, searching for "brake drum grinding machine" just gives me mfr's selling machines.

I'm not interested in the "is it profitable" aspect of brake turning, I'm just looking for information on the grinding procedure, and where and why it would be done. I've had a few BL's over the years, none had a grinder attachment, picked up 2 big BL's in past year and both have them. These are big tool post type grinders, not something that would be used by hand.
 
Yes the grinding is done after turning to remove hard spots ,all drums do not need to be ground. More common on big truck drums than car size stuff. spent many years working in/around HD truck parts and repair. The hard spots will be proud of the rest of the drum face after turning,that's why you grind to remove the high spots.
 
with modern ceramic tips they usually cut through the hard spot so its not proud but you can still see where they are. This is in reference to surfacing flywheels similar deal to brake drums.
Try a modern ceramic tip when turning and skip the old braze on carbide, meaning no grinding required.
Hard spots show up as shiney areas. Hope that helps.

Some systems use grinding to resurface flywheels as this was before ceramics and the only way then to overcome this hard spot problem in cast iron. Maybe your thinking of this.
 
As was brought up in another thread about disk brake rotors, there may be a 'screw-on' effect left by the helical path of a lathe tool acting on the pads when the pressure is on. So grinding would eliminate that effect, I would think. But I think if you are using proper tools and feeds, that there won't be much 'lay' to the surface that you couldn't remove with a few moments with a sanding block applied to the drum.

On a brake rotor, you can get a nice finish with face grinding the faces of the rotor, creating a cross hatched sort of 'lay' which the pads would not follow. This might possibly help seat a new set of pads quickly.

I've often wondered about grinding the hard spots on flywheels: I don't do it, I turn them with ceramics and maybe a bit of hand honing with a diamond hone on top of each hard spot. But regardless of how you go about reducing the hard spot, it's still there, and it shouldn't wear at the same rate as the rest of the drum/flywheel. But I never see them come back with the hard spots sticking 'proud', they seem to wear like the rest of the surface.
 
My local Automotive machine shop uses ceramic inserts on some clutch (flywheel) and drums/discs. My truck flywheel was done that way. Yes, you can "see" the hard shots but they do not measure proud.
 
The automotive shops around here grind flywheels and most use a diamond wheel to take care of hard spots. Maybe someone here turns them with a ceramic cutting tool; I just haven't seen it locally.

jack vines
 
I guess that explains my experiences. Way, way, back in shop class using what was an antique Ammco BL, we were taught to grind hard spots with a hand grinder. Working in real shops I always had better and more robust machines, and using indexable carbide inserts with low feed rates would cut right thru the hard spots leaving a shiny surface. Sold my last BL, a WVN204, about 12 years ago so been out of the game for awhile. When you guys say ceramic, are you talking carbide or do they now have pure ceramic inserts?

Guess I'll keep the grinder attachment with the machines, I've been known to use BL's in ways they were never meant to be used, it might come in handy.

If anyone ever comes across technical data on the process, I'd be interested in reading it.
 
back when i was a puppy i used ammco machines that had an electric grinder that mounted in place of the cutting arbor and the last pass was made with that. my kwik way did not have a grinder option and kwik way said it wasn't nec with carbide bits. i have not had any "proud" hard spots in the 30+ years i've owned it.
 
My dad worked for Ammco Tools for about 20 years. I worked for him during the summer doing new machine setups and repair work. The grinders were used primarily for drums and flywheels. Disk brake rotors typically didn't get hard spots. Today, auto brake drums are so cheap that it's not worth it to grind them. Truck drums and flywheels would still be a good application for grinding.

The grinding wheel had to be dressed before grinding, so a diamond dressing tool was attached to the drum or flywheel for this purpose.

Drum Grinder Dressing Tool.jpg

This dressing tool had a pin that was inserted into a hole in the switch panel that enabled the lathe motor to run. You had to remove it from the switch panel, disabling the lathe motor when dressing the grinder wheel. Then you put the attachment back into the switch panel while grinding the drum.

I've still got a brake shoe grinder with the Fixed Anchor head and the attachments for different size drums:

Brake Shoe Grinder.jpg

Most people that work on drum brakes don't understand the reason for this head. It is designed to simulate how the shoes actually mount to the backing plate and really should be used on Bendix-style brakes.

The Cam head was actually intended for use on Chrysler Center Plane brakes:

Brake Shoe Grinder 2.jpg

It does not give a proper grind for Bendix-style brakes.

Nowadays, I don't know if anybody grinds brake shoes anymore. When I worked for my dad, 98% of brake linings contained asbestos. The EPA and OSHA would have a cow if you tried to grind asbestos linings today.
 
do you think its actually neccessary to grind the shoes after the drum has been reconditioned?
 
Steve
I've only used a brake shoe grinder a few times, and that was in shop class, in working shops we never had one. I get the idea of getting shoe profile to match drum profile, but like you said "nobody does that anymore". I'd be interested in a repeat of that dissertation.
 
You raise an excellent question, dian!

Ideally, the answer would be yes; but in today's world, it may not be necessary.

First, let's look at how the Bendix duo-servo brake works. There are two shoes, primary and secondary. The purpose of the primary shoe is to engage the secondary shoe, which does the majority of the braking. The secondary shoe is on the back side of the backing plate and has more lining than the primary shoe. The top of the shoes engage an anchor pin located at the top of the backing plate. When the wheel is rotating in a forward direction, the primary shoe pulls away from the anchor pin and rotates, forcing the top of the secondary shoe against the anchor pin. It is this wedging of the secondary shoe against the pin that applies pressure against the brake drum. (Braking power in reverse is less because there is less lining area on the primary shoe, which is now doing most of the work). Notice the different lining length in the photo below:

Brake.jpg

When a brake drum is turned on a lathe, the internal radius is moved away from the anchor pin. This means that when the brake is applied, the top portion of the secondary shoe lining is farther away from the drum--i.e. it doesn't make contact. As the drum is cut to larger sizes, the amount of area that doesn't contact the drum gets larger. This results in several problems: less braking power, overheating of the smaller amount of lining in contact with the drum, and brake fade. This can also result in uneven braking from side-to-side, and can even cause the car to swerve into oncoming traffic in the event of a panic stop. (Younger drivers that have only driven disk brake vehicles have no idea how much better they are than drum brakes).

The Fixed Anchor Head for the Ammco brake shoe grinder simulates how the shoe mounts to the backing plate, and grinds more lining material off of the bottom portion of the lining to make it fit better at the top of the shoe. Although this was a much better way to grind the brake linings, Ammco held the patent on the device, so manufacturers (and the government) wouldn't specify this method. Other brake shoe grinder manufacturers sold a simpler design that cut the same radius on the lining, but it didn't compensate for the anchor pin to drum distance (Cam grinding). Notice how the end of the brake shoe fits into the simulated anchor pin in this photo:

Brake Shoe Grinder 3.jpg

Under heavy braking, the drum is forced into a more elliptical shape by the shoes. They conform to the shoes to a degree, but there is a limit. This is why brake drums are marked with a maximum size of .060" over the original size. Brake lining manufacturers supplied shoes in standard size, as well as .030" oversize as a compromise to solve this problem. A much larger oversize doesn't really hurt anything, if the shoes are ground to match. When I was young and broke, I turned the drums on my car to .120" oversize with no problem because I ground the shoes to match. Granted, there wasn't a lot of lining left on the bottom of the shoes, but at the time, drums were much more expensive than shoes. I've even removed drums from a car that were worn all the way through, they came off as rings!

At that point in time, most brake linings were made of asbestos, and we didn't have an EPA or an OSHA to worry about dust from brake shoe grinding. Brake service personnel were told to collect the dust from grinding in the bag on the grinder, then dump the dust it in a paper bag and dispose of it in the trash. I saw several shops that simply ran a flex hose from the grinder to outside the building. You couldn't do that today!

So much for the history lesson. With disk brakes, the problem of brakes causing a vehicle to swerve during a panic stop was virtually eliminated. Drum brakes, if used at all, are on the rear brakes. With the cost of brake drums from China being so cheap now, turning brake drums on a lathe is becoming a lost art. It's cheaper to buy new drums than to turn the old ones. New shoes, new drums, no need to do anything but wash the inside of the drums with solvent before installation. Oh, yea, the advent of anti-lock brake controls has greatly improved the ability to control a vehicle's direction during a panic stop.
 
When I worked at the Chev dealer in the 60's we maintained and leased the Duval county road petrol cars.Before the disc brakes became available we installed metallic brake linings on them when they arrived from the factory.

The cars were 396 cu in BelAirs with a few 427's.The cars came with ground brake drums necessary for the metallic linings.There was a part no that you could order ground drums from the factory.

GM would not install the linings from the factory because the emergency brake wouldn't hold if applied when hot,as the brakes cooled down the car would roll of if not in park.The new brakes were shipped in the trunk to be installed at the dealer.During a brake job we ground all the brake drums.

When the 396 cars came in they would only run about 95-100 mph.Just by installing dual exhaust they would run in excess of 130.A hard braking with a 4500lb car at those speeds and standard lining would smoke the brakes and probably not bring the car to a complete stop.The metallic brakes stopped that however when cold they wouldn't stop right away.Every time a new porter was sent to the back lot to bring a car up for service they would invariably hit the fence or another car!

The hard spots in drums and flywheels can be ground flat with the surounding material but not really removed,they are like an egg cut in half with the flat side up.There is generally not enough material to safely cut underneath the hard spot.
 
steve, i admit to have a limited understanding of drum brakes. i always thought grinding the shoes was weird, because the are of relatively thin material and would conform to the new radius easily. but what you are saying is the other way around: the drum get distorted by the shoe?

also, why does the secondary shoe take most of the force, since the pressure is equal on both shoes? its also not much larger that the primary in your picture.
 
Servo action forces the secondary shoe into the drum . Way back in the 60s quailty lined shoes had different friction co-efficient material on primary and secondary shoes to equalize lining ware. Look at high performance motor cycle and sports car brakes ,they used twin leading shoes per wheel to take advantage of the servo effect of the wheel rotation pulling the shoe into the drum.
When I first got into the automotive parts/service trade the parts store owner ran the shop and would even take a light skim cut out of a new drum to make sure it was round. Back then many of the boxes drums came in were marked "store flat'. And yes we always ground lining to fit the OS drums. That was so the brakes would have the correct performance with in the first few miles .
 
Thanks Steve, that is pretty much the way I remember it being explained decades ago, nice to know my brain is not mush, yet. As for profitability, doubt I'll get rich doing it out here, but market is far different than in a big city with parts stores on every corner. We have 1 parts store, it is the brand that has never been known for being a discount store, and they know they are the only game in town and price accordingly. General automotive might be hard to beat on pricing, but there are probably more light to medium duty trucks out here than people, and there is heavy industry and ag equipment too. I'm setting up a little shop out in BFE where its 250 miles in any direction to a discount auto parts store, or a machine shop. Picked up my 2 modern BL's so cheap that if I can't make them pay for themselves there is something wrong with me, not the machines. I'm not where the sig says I am anymore.
 
Chrysler cars of the 50's used double leading brake shoes,one wheel cyl at the top and one at the bottom.I used to have Ammco and Kwick-Way lathes.The Kwick-Way featured and interrupted feed to stop the threading effect.Back in the day my brake,rotor lathe and flywheel grinder were the most profitable machines I had,stayed busy all the time.
 
ratbldr427, as a teen, I was hard on brakes. I also ran the VelveTouch metallic linings. I wanted to see how good they were and made a series of hard stops from about 100 MPH. Melted the grease on the backing plates and turned the drums blue, but they stopped quite well (except when cold!). You mentioned the Chrysler Center Plane brakes, I've worked on one or two of those but don't remember much except the twin wheel cylinders. That is an application for Cam grinding, as I recall. When my dad turned 55, Ammco let him go (no such thing as age discrimination back then). He was their number 3 salesman in the nation and sold thousands of lathes. He then went to work for KwikWay for a number of years. I've never used a KwikWay machine. I did a lot of repair work on Ammcos. I even designed a new crossfeed gearbox for their disk/drum lathe that did away with the plastic gear nonsense. My dad was hoping we could market it as a retrofit after Ammco ran him off, but that never panned out.

dian, I never thought about the shoes flexing to conform to the drum, just repeating what I was taught. Very likely you're correct, I don't know if anyone really knows. As others have mentioned, you normally want a little clearance between the drum & each end of the lining to compensate for flexing. Actually grinding different radii and testing to study wear patterns is how they determined the best drum/lining clearance.

Also, dian, do you understand how a band brake is self-energizing in one direction? You have a fixed end of the band, and a moving end. Drum rotation toward the moving end will cause a tangential friction force that causes the lining to apply pressure by itself. The automotive drum brake is similar, only the lining is inside the drum, rather than outside.
 








 
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