Machinist question - How to make a hole

I'm modding a Shimano centerlock rotor to fit a scooter hub. Regen carries the bulk of my braking work, so I believe a bike brake on the rear will be fine and mostly used for very low speed stuff. With the centerlock portion off just a tiny bit of filing makes the rotor fit like a glove. Now I just need to make the new larger bolt holes.

Here's a pic of one of the existing 4mm holes, and the red circle is the 8mm hole I need to make. My problem is how to make that new hole since the holes will overlap slightly. I can't just drill the new one can I? or is there some special bit for the job?

You can cut a new hole while intersecting an existing hole with a typical end mill in a milling machine. A very rigid drill press could do it too but it would be difficult to keep it centered. With my CNC mill I can cut a .250 hole and then spiral out to whatever diameter it needed to be. A very handy thing when a properly sized drill is not available while working on my stuff at 2AM.....

Thanks Joe,

If the nearby machine shop doesn't have a milling machine, my other thought is to turn bolts into dual size threaded rods. Use Locktite to set the threaded 8mm portion into the motor bolt holes. Then the disc would fit on the outer portion that is 6mm with threading for nuts that hold it flat and tight, probably using a retaining ring. That way the new holes don't have to intersect the old ones and it's only a matter of proper placement.

You have a few options. A drill bit that size will want to walk towards the existing hole. You can use a drill bushing to guide the drill bit but those cost money so I will try to give you the basic idea and you may be able to improvise.

A drill bushing is a hardened steel or carbide sleeve that has the same inside diameter as your drill bit. In this case you need a bushing that is about 13mm or more tall and with an 8mm through hole. The idea is that a drill bit wants to cut at its tip rather than on its side so if the bushing is c-clamped tightly over your hole location it will prevent the drill from walking towards the other hole. In order to walk sideways the bit would have to cut the bushing sideways. You could probably accomplish the same thing by drilling an 8mm hole in a chunk of steel and using lots of oil and time to carefully drill through the rotor. There will be a very narrow sweet spot in the amount of pressure needed to bore the hole without the bit walking towards the hole and moving the bushing/guide plate. I suspect just from the look and feel of the rotors that they are hardened so conventional drill bits may not work very well. I have not tried to drill one yet.

If you cut most of the cutting flutes off of a drill bit by shortening it a lot, you could use the steel plate method that I mentioned above without needing a hardened bushing. The reason for shortening it is to get the smooth non cutting shank of the bit riding on the steel plate. That way it is not destroying your 8mm guide hole. This also assumes you are able to resharpen the drill bit after you cut it.

Another option would be a center cutting endmill. Endmills look like drill bits but are flat at the tip and a lot more rigid. This would require a very rigid setup like a milling machine or strong drill press. You could still use the bushing/guide hole method with and endmill. An endmill would be better suited for the job than a drill bit if you can support its side to side movement.

Yet another option would be a high speed abrasive tool like a dremel. Again you would need a small piece of something with an 8mm hole in it. Clamp it to the rotor right over where you need your hole and slowly grind out until you hit your 8mm guide plate. This would be really tedious and would probably get the rotor really hot possibly changing the hardness or creating a stress point for cracks. I have seen the way you ride through traffic so this option scares me.

Lastly, if its worth postage to and from Missouri you could mail the rotor to me and I can mill it for you.

take a center drill and cut the hole to its final size. They are very short and solid, which makes them incredibly rigid, and they have a very shallow cut angle, so they dont follow into other holes. Here is a link:

http://www.amazon.com/gp/product/B00AUBCF3O?ie=UTF8&at=&force-full-site=1&ref_=aw_bottom_links

However, they do not cut very aggressively, so use lots of force, and spin it slowly and in stainless, use oil. They are my go-to for starting holes at angles, and they also work really well on thin sheet steel/ally.

I assume there are 5 or so of these holes around the rotor ?
Can you reposition them slightly ( left or right) on their PCD to avoid overlap with the existing holes ?
If not try as Dan suggested by drilling through a thick steel plate clamped onto the disc to guide the drill as it cuts the disc.
Or , you could fill the existing hole with weld , grind flat , and redrill as normal.

John you have several options that will work in a pillar drill, you could use the flat ended drills designed for drilling spot welds out of car panels or maybe a rotabroach cutter can be got that small? either will follow a good centre punch without wandering due to the other hole.

http://www.sterlingtools.com.au/productimages/9L80.jpg

I can't match the experience/knowledge of the other posters in this thread, but based on recent experience I can offer a warning that bicycle discs are surprisingly hard... I'm not sure that they are stainless steel. I had to slot the holes in one recently and it was tough going with a Dremel and grinding stone, a file (admitedly an old one) would barely touch it. Even after localised heating with a propane torch to try and remove the temper I only just about got there. Several mini-grindstones turned to dust and the file teeth burred. No problem I'm sure for appropriate machine tools, but just thought I'd mention it if you go the DIY route Perhaps welding in the old holes would be a good idea as it ought to soften the surrounding metal a bit. I know MIG weld is "fairly" hard, but it's still readily drillable with normal HSS tools.

I won't speculate that the strength of the disc will be unaffected, but my reasoning is that the hardness is for the benefit of the wearing faces, not the mountings and they just harden the lot.

First choice I would clamp it down and use an end mill on the Bridgeport.

Second choice clamp it down and use the center drill method on a drill press.

Third choice clamp it down and use progressively larger drills on a drill press and finish the hole size by hand with a rat tail file.

Any choice needs to be clamped and not held by hand. But then again, none of us hold things by hand on a drill press, do we? ... Funny as I type this I look at two scars across the back of my left hand near the thumb... took that long to get my hand out of the way after the drill grabbed in some thin steel on the drill press 40 years ago. :|

hi,
You don't need anything fancy, just a drill and a few normal high speed drill bits.
Mark your holes accurately, then use a centre punch to mark the hole centres so you don't slip (or apply masking take and mark on that as that stops slipping.)
Now clamp your piece to a timber bench. Use a 4 or 5mm drill and drill an initial hole in the centre of each hole required BUT drill past and into the wooden bench. Drill a matching hole in a piece of timber or similar.(do a few)
Now swap to the 8mm drill.
Clamp your small piece of timber over the holes as you drill each one, using your previous size drill bit to line up the holes before drilling.
Press firmly and drill - the top timber will guide you straight and stop the drill migrating into the other larger hole.
You may find it easier to have a new top piece of timber for each hole, hence do a few, but you will prob manage with just one

simple
j

hi forgot to add -
to drill stainless is pretty easy and rotors are a pretty soft grade. You need decent high speed drills, cheap crappy ones are no good.
The trick is to drill at the slowest speed you can - you can slop some oil down the hole if you want, it will help.
Should you get bored and use a higher speed, the drill bit will become hot and get blunt instantly, that's where most people fall down.
Drilling a 5mm pilot hole first will also half the load as the material is already gone when you switch to the 8mm.
So patience, and be aware that the drill torque at slow speed is very high and will try to snatch, so hold on!
If it snatches, calmly stop and reverse the drill back out, and then start again. Keep doing that until through.
Im used to doing motorcycle discs which are 6 to 8mm thick with no problems whatsoever, a bike rotor is 2mm maximum!
Good luck

j

Give it a brush with a file to gauge how hard it is first, but I would of thought this was something I could do on my knee. Cordless drill a 6mm bit and a round file.

If I really had to I would make a jig, but that seems very unlikely. I certainly wouldn't need any help. Just hand tools.

Thanks for all the input.

I don't want to weld on the disc. I already have one I made a couple of years ago where I made an adapter to fit scooter motor disc mounts, welded a bike rotor to it, and cut off the extra that was at too small a radius to fit. I haven't used that disc, because I'm afraid that welding changed the metal properties and made it brittle. Bike rotors are so small and thin that they'd gotta be under real stress when braking hard, so that welded one scares me.

I'll see what the machine shop guy says. His shop is pretty primitive, so I'm still leaning toward smaller holes and fabricating 2 size bolts. As long as I can cut threads into a sufficiently tough bolt that seems like and easy route and I won't have a weakened hole. The holes are much further out than a common bike disc, just over 40mm radius, so I'm confident that three 6mm or 7mm diameter bolts will be fine, since bikes use 6 m5 bolts at a radius of about 22mm. With it being a rear and regen kicking in first, this disc won't see nearly as high stress that a front rotor can see. It's for my personal careful riding, not racing, and not for sale, or I'd just bite the bullet and install the moto caliper.

John

Hi John

The only way I can get a good smooth looking larger hole in the situation you are in if I can not clamp down the part real well on a mill, is to drill a small hole first and use a good fitting counterbore to get my final hole diameter. Works great even with a hand drill.

Ah, I've just seen the photo in the OP (had images off on the work's computer earlier) and I see it's nowhere near as overlapped as I'd assumed.

Hardness considerations aside, I don't think you'll have any trouble drilling the new hole. As the overlap is less than 180* the new hole itself will stop the bit from walking. It might snatch a bit. A guide, as suggested by others, can only help.

John in CR said:

I'm modding a Shimano centerlock rotor to fit a scooter hub. Regen carries the bulk of my braking work, so I believe a bike brake on the rear will be fine and mostly used for very low speed stuff. With the centerlock portion off just a tiny bit of filing makes the rotor fit like a glove. Now I just need to make the new larger bolt holes.

Here's a pic of one of the existing 4mm holes, and the red circle is the 8mm hole I need to make. My problem is how to make that new hole since the holes will overlap slightly. I can't just drill the new one can I? or is there some special bit for the job?

Click to expand...

What is the chance of using those lightning holes with a washer? Looking at the picture it may be close.....