Drilling new PCD in discs

[e21Mark]

Is it safe to drill a brake disc to a new PCD?

[PaulKemp]

From what pcd to what?
You'll nearly always have one new hole close to an existing one making the whole disc scrap
Discs are cheap your life is not

[e21Mark]

4x114 to 4x100

It's not for my car but it's another e21 where they're putting a big brake kit together. Ufortunately there nothing available off the shelf so they're using parts from another manufacturer. I take your point about discs being cheap but the drilling of a different PCD does seem pretty common. I'm just intrigued to learn what the specific potential risks are?

[stevieturbo]

Perfectly safe and very easy to do.

The holes are only to allow the bolts to go through, they do not locate nor prevent the disc rotating. The clamp force provides that.

[PaulKemp]

An I missing something?
If the bolts/studs don't locate the disc to the hub/wheel what does
The caliper would hold the disc and the hub would spin with no braking force to the wheel

[e21Mark]

I think stevieturbo is saying the clamping force of the wheel to the hub is what stops the disc from spinning, as opposed to the studs or wheel bolts.

If this is so, would drilling the new holes weaken the disc though?

It does seem to be a relatively common modification.

[stevieturbo]

PaulKemp said:

An I missing something?
If the bolts/studs don't locate the disc to the hub/wheel what does
The caliper would hold the disc and the hub would spin with no braking force to the wheel

they locate to allow you to fit the disc, nothing more. The wheel bolts provide all the clamp.

If the disc was able to turn at all, it would break those tiny little M6 bolts many use to hold the disc tight to the driving flange.

Neither this bolt or the wheel studs see any shear loads at all in operation.

Drilling new holes wouldnt cause any weakness I'd be concerned about.

[jimbob82]

only place I've ever seen a brake disc fail is right where it meets the edge of the mounting edge, as below:

[Mr2Mike]

PaulKemp said:

An I missing something?
If the bolts/studs don't locate the disc to the hub/wheel what does

Location of the disk is usually performed by the spigot on the hub being a good fit into the hole in the center of the disk (same as with wheels).

[PaulKemp]

With the comments so far you could then slot the stud holes without problem

[e21Mark]

WeirdNeville said:

I say find a disc that actually fits. I would not want discs with re-drilled holes or slotted holes anywhere near my car.

This is his problem. He has simply been unable to find any with the correct PCD.

It does appear that these are holes that are drilled in the manufacturing process though, as opposed to the holes for cooling/pad deglazing. As such is there any more reason why one drilling would propagate cracks over another?

(I'm just asking out of curiosity)

[Mr2Mike]

WeirdNeville said:

rilling locally heats the metal, and unless you know exactly what you're doing you can de-temper cast metal.

Absolute rubbish, you can drill through cast iron at very low speeds creating very little heat. In the very unlikely event of affecting the temper of the metal around the hole it wouldn't make the slightest difference, since it's taking none of the braking force.

WeirdNeville said:

You then have local areas of "softer" metal near the very hard cast metal, which gives differential properties. This leads to cracking over time, and could make a brake disc fail catastrophically. Don't forget, they are repeatedly heated and cooled, so you don't want differential expansion in the material.

Any holes drilled or milling done to a brake disc after casting is done in tightly controlled conditions, and the disc then re-tempered to prevent the structure of the material being adversely affected.

Put it this way, put a disc with additional (amateur) holes and slotting done to it though a manufacturers approval testing, and I'd be happy to use it. Since they won't undergo that testing, I'm not.

You clearly have no idea whatsoever about the stresses seen in a disc. The centre is clamped between the hub and the wheel, you would have to make the centers look like a teabag before there was a serious risk of failure. Re-drilling discs for different PCDs is an extremely common and risk free operation.

[Mr2Mike]

WeirdNeville said:

You say many people do it: that doesn't make it safe, or right.

I have never even heard of a failure in the center of a disc, let alone seen one. It simply won't happen.

[jfdi]

WeirdNeville said:

Ok, well, I'm not an engineer.

Obviously.

As an ex engineer, and one that used to do machining "in tightly controlled conditions" on brake disc castings. The tightly controlled conditions you speak of consist of setting the speeds and feeds on the cutting tool as fast as you can get away with without breaking the tool or causing excessive wear of the tool. Heat generation in the casting was never a consideration, it's only cast iron.

[mickrick]

And since when has a hole been a "stress riser" ? Drilling a hole in the end of a crack will stop it propagating.

[jimbob82]

WeirdNeville said:

Ok, so how much does redrilling holes in it weaken it?

not enough to be concerned about

WeirdNeville said:

How does it affect the mean time before failure?

discs would wear out before it failed

WeirdNeville said:

How does it affect the interaction of the wheel face and hub face, since clamping force is so important?

it doesn't

WeirdNeville said:

How does heat cycling affect the drilled holes?

they warm up then cool down again

[PhillipM]

WeirdNeville said:

Ok, well, I'm not an engineer. But I know people who design cast car parts and they say they would not entertain a drilled disc for the reasons I listed.

You say many people do it: that doesn't make it safe, or right.

Yes, drilled in the braking surface, where the massive heat generated at the interface makes the difference, redrilling in the hub, done sensibly, is fine.

Edited by PhillipM on Saturday 6th July 23:06

[PhillipM]

mickrick said:

And since when has a hole been a "stress riser" ? Drilling a hole in the end of a crack will stop it propagating.

Because the bigger radius of the hole is less of a stress raiser than the tiny radius of the end of the crack. It is however, still a stress raiser....

[static2010]

For the sake of 2mm, have them milled out on the existing holes.
If done properly, no probs.

[Mr2Mike]

WeirdNeville said:

Ok, so how much does redrilling holes in it weaken it?

Insignificantly.

WeirdNeville said:

How does it affect the mean time before failure?

0%. Discs don't tend to fail from cracks propagating from the bell. They wear out, distort or crack on the braking surface.

WeirdNeville said:

How does it affect the interaction of the wheel face and hub face, since clamping force is so important?

Insignificantly. The clamping force is exactly the same since it's determined by how tight you do up the wheel nuts (and ultimately the yield strength of the bolts/studs). Since you have reduced a clamping area a very tiny bit with the extra holes, the pressure on the bell exerted by the hub and wheel will go up very slightly.

WeirdNeville said:

How does heat cycling affect the drilled holes?

What heat cycling? This seems to be a term you have latched on to with no actual idea about what it means. The iron around the hole will see a small rise in temperature during drilling, but the braking surface, made from the same material, will be temperature cycled every time you use your brakes, to a vastly greater degree. Have you never seen disc brakes glowing red hot?

WeirdNeville said:

How are the holes drilled/slots ground? CNC? Pillar drill? Dremmel? hand held drill?

You simply have to make a hole of the correct diameter, and in the correct location, and it should be straight. A pillar drill is perfectly adequate (though requires manually marking out the hole centers), a manually operated milling machine with a rotary table would do a lovely job, and a CNC machine is overkill. A hand held drill could do the job, but places more demands on the skill of the person holding it. A Dremmel is simply an inappropriate tool for this job. The holes could also be water cut, or possibly even laser cut if you want to throw money away.