demonchild

well since i sold the thirdgen about a year ago i just kind of lurk around here. However, the other forums i hang out in the people arent as helpful.

can you machine slotted rotors?

BretD 88GTA

Yes.

I had my old cross-drilled rotors machined a couple of times. Never had a problem.

Guest

Yes and no.

It all depends on what they are using to cut the rotor with. Even the best of typical brake lathes use a triangular shaped cutting bit that no matter what you do will skip off of the front side of the slot/hole and slam into the backside, leaving a low spot on the backside of the slot/hole. Never did measure it myself, but its not much of a low spot. However for this reason, they may tell you they cant cut it. It also dulls the bits faster. Some rotors are worse than others. GM rotors like on the thirdgen generally cut fairly well though. Jeep and Mercedes... ugh. Best you can ask for is ask them to cut .001 or .002 with a slow cut on the last pass and that should eliminate any surface anomalies besides giving you a better surface. Most places I have seen though try to cut .020-.060 in a single pass with the speed cranked so high the rotor looks like a record when they are done with it. Repair shops are really guilty of this, cant count how many badly grooved rotors I saw just singing away on the machine and nobody cared. Basically the more they are trying to cut off, the more pressure on the bit, and you can just picture what happens when it hits that open space.

If someone is using a surfacer instead of a cutting bit (usually only manufacturers use this type of machine), they can surface just about anything. Where I was had a single side surfacer, not exactly ideal for things like rotors where you really need both surfaces parallel. I did have to cut a motorcycle rotor that way once though.

BretD 88GTA

It could also depend on how the slots and/or holes are finished.

My cross-drilled rotors had chamfered holes so there was no sharp edge for the bit to snag on. My rotors always came off the lathe looking good.

The machine shop I use never complained about machining them either.

Guest

It'll still gouge it if they are trying to cut too much, chamfer or not.

sixtyV6

A good set of rotors will generally not have the slots cut that deep so 1 or 2 turns at the most will remove them entirely. The racing rotors used in such cars like TransAm Racing Series will cut the slots into new rotors after they have been turned and heatcycled.

As for everyday street rotors- I have been using any and every type of drill, or slotted, or drilled and sloted, or dimpled rotors surfaces for years. Any shop will turnthem but many will try and give you a surcharge and claim the cutting bit wears faster. If they take the proper time and cut them in several passes like any rotor should be turned, then the bit will not wear faster.

Guest

Yes it can and will wear the cutting bits faster. Take any lathe, even the oh so special Accuturn, and cut a rotor at any amount of cut setting you want. Make another pass, same cut setting. You'll find additional material is removed when you do this. The arms on the brake lathes, like anything else, are just not that strong. When the bit is placed against the surface, a load is placed on the arm and bit. It doesnt just move in the amount you turn the dial and not have a load on it. When the first pass is made, it cuts off to the point of the cutting capability of the bit itself (depending on material, wear, and size) and the load placed on the bit. On a second pass, same setting, it will remove more material although a small amount. I've even had material removal on third passes. This is why, if you know what you are doing with a brake lathe, you dont turn the wheel to place the bits back at the center of the rotor without moving the bits away from the rotor first or you'll have yourself a record. Even something so minor as harmonics when cutting a rotor will leave you with nasty grooves and worn bits. If you go to a shop and hear rotors singing, take your parts elsewhere.

So whats this got to do with wearing bits? As I said, the bits have load on them. In the slot or hole, the load is released. Once the bit clears the gap, it slams into the rotor. This creates excess wear over a rotor with no slots or holes, just as cutting too much material at once or having the adjustment wrong will tear them up. If you spend/spent much time cutting things like rotors, heads, etc you'd know a lot about what works, what doesnt, and what tears up your machinery. Many people, however, just dont care about that sort of thing because they arent footing the repair and maintenance costs.

I always made a .005, .001, and .0005 pass on my rotors to finish them up. Most guys cut .020 and call it a day.

demonchild

this is why i come to this forum when i need a definite tech answer...

good job with the knowledge.