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Discussion Starter · #1 ·
hello gentlemen, i have an issue here. i am not a mechanic or a machinist so please keep the comments constructive, thanks!. i need a 10" rotor with a 2 3/16 center hole. i cant find one , no one makes them. i have a metal lathe that i can barely use and want to turn a heat treated steel 11.5" rotor, which has the proper center hole (i have 4 of those) to 10". is this possible? i knoiw this stuff is hard and i dont want to overheat it and warp it. what type of tip should i buy? i have a smithy 1220 3 in 1 machine that should be up for the task, i just need the right tip to buy, or a connection to an experienced machinist in shittastic CT . ideally i would like to try this myself, the machine is just sitting there......but if you guys think its going to be a disaster i will mail it to a willing machinist. will pay in cash or parts.
 

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i don't know.
But what would occur to me is that there is a lot of meat on the part so i'd grab a carbide tool and have a go, SUPER SLOW revs. You're not looking for a super fine tolerance so a rough finish would be acceptable. If its not going to work you will know pretty damn quick then off to an equipped machine shop, I wouldn't have thought grinding was called for.
 

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Cutting a 11 1/2" rotor to 10"...
Does the rotor you want to cut down have vent holes in it, will they be at the 10" diameter?
I am no machinist by any stretch of my imagination but I do try now and then.
My old south bend lathe is to small to do rotors but have learned small machines have large limits.
I don't know your machine but most smaller machines are not rigid enough to work on large diameters.
Large diameters, improper cutting speeds (to fast) possible interrupted cuts (passing through holes) will bash carbide tooling to junk quite fast.

Maybe in an un conventional way it could still be done at home with simple or less than ideal tools.
First the want and the need.
You want a smaller diameter rotor cut from a larger one.
A good thing is that the outer diameter of a rotor need not be exactly precise down to a 0.001" run-out.
The outer diameter can be slightly off with no effect on braking, balance maybe.
I have and am sure most have seen rotors with outer diameters in all shapes but round.

Maybe you can mount your rotor on your machine and get it turning true to the center hole.
Then mark or even cut slightly into the face of it at the diameter you want to make a good reference line.
With much hand labor, cut the diameter very close to the line with a jig saw.
Saw off the bulk of the material.
You could then "maybe" turn the outer diameter the last little bit.
With great care, patients, skill and a bit of luck you might be able to turn the finish diameter in very close and as a last resort, finish it to the line with an angle grinder.
 

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what bike is it for ?... Harley shovelheads rear use a 10'' disc with the centre of 50mm, easier to get one & open the center hole up, they are not hardened so easy to machine & any brake shop will do it for you for a few bucks if you cant do it,.. DONT attack an 11.5'' disc with a grinder, it needs to be done properly on a lathe or on a milling machine
 

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Yeah, stuff that bitch on an old Accuturn brake lathe and go to town. An indie automotive shop near you will likely do it for the cost of a few bits. You can spin that thing way faster than you think you can. I used to turn rotors and drums and flywheels for years as a kid.
 

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Bring it to a local machine shop. Those 3 in 1 machines are just not rigid enough for heat treated material, plus the diameter is large for one of those. It can be turned or milled, just depends on what the shop has in terms of workholding, etc.
 

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Discussion Starter · #7 ·
Cutting a 11 1/2" rotor to 10"...
Does the rotor you want to cut down have vent holes in it, will they be at the 10" diameter?
I am no machinist by any stretch of my imagination but I do try now and then.
My old south bend lathe is to small to do rotors but have learned small machines have large limits.
I don't know your machine but most smaller machines are not rigid enough to work on large diameters.
Large diameters, improper cutting speeds (to fast) possible interrupted cuts (passing through holes) will bash carbide tooling to junk quite fast.

Maybe in an un conventional way it could still be done at home with simple or less than ideal tools.
First the want and the need.
You want a smaller diameter rotor cut from a larger one.
A good thing is that the outer diameter of a rotor need not be exactly precise down to a 0.001" run-out.
The outer diameter can be slightly off with no effect on braking, balance maybe.
I have and am sure most have seen rotors with outer diameters in all shapes but round.

Maybe you can mount your rotor on your machine and get it turning true to the center hole.
Then mark or even cut slightly into the face of it at the diameter you want to make a good reference line.
With much hand labor, cut the diameter very close to the line with a jig saw.
Saw off the bulk of the material.
You could then "maybe" turn the outer diameter the last little bit.
With great care, patients, skill and a bit of luck you might be able to turn the finish diameter in very close and as a last resort, finish it to the line with an angle grinder.
 

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Discussion Starter · #8 ·
some interesting comments, i never would have thought a sawzall could do what my lathe couldnt. I was also surprised at the anglegrinder suggestion. I had read about special super hard cutting inserts being neededAnyway, i reached out to an acquaintance in the local scene here to askwhat inserts to buy and he graciously offered to take care of it. Johnny rotten aint so rotten after all👍
 

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Discussion Starter · #9 ·
i forgot to address a few comments. 1)cutting it down will not interefer with the holes in the rotor, it has 5 round holes, lki an old centerline champ 500 wheel. 2) the bike is a completely handmade custom, i mean every detail ( i posted pics of it on here before and got plenty of thumbs down comments, but i think its amazing. google eastern fabrications lock baker fist of legend if you want to see a pic) and i dont want to switch the caliper mount for that reason so i need a rotor to fit.
 

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Right up front I mentioned doing it in an unconventional way.
I expect most all didn't fully understand what I was attempting to get across.
Even your comment about a sawzall tells me you had missed the point somewhat.
A jigsaw is not a sawzall.
Just guessing here but I'm from an older generation than most who look in on and respond here.
I have also forever had more desire to accomplish something than I had the proper tools to do so.
There are always more than one way to do things.
Experience will teach you your limitations.

A smaller lathe may not want to cut a 11 1/2" rotor down to 10" but it could cut an accurate grove in the surface.
With a saw, a hand held jigsaw or maybe a bandsaw a person could cut close to that line.
A band or disc sander could refine the cut to bring it to the accurately cut line, an angle grinder will work if that's all you have.
Is that the proper way, nope.
Will it work, yes.
Will it effect how the rotor would perform, not in the slightest.

Do not let the lack of the so called proper tools limit what you can do.

Just as an example...
I wanted to adjust cylinder lengths to alter compression heights on HD big twins.
I have a 9"X36" south bend model C lathe, it's old, very basic and limited.
A friend of mine has an old worn out Bridgeport.
I made a tool that will work on my lathe...
272482


That home made tool made with less than ideal machines can accurately adjust cylinder lengths within 0.001" with ease.

If you read and try to understand only one line in this response, read the one below this.

Learn how to look at projects differently, know there are different ways of doing the same thing.

Oh...
272483
 

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Can you purchase a rotor in the right diameter and cut the center hole to proper size? Far easier. Use carbide and take multiple small cuts/slow feed using cutting oil. I wouldn't use the lathe but a boring bar in the mill mounting the rotor to table. Far easier set up and accuracy.

At least that's my take on it.
 

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Discussion Starter · #12 ·
Right up front I mentioned doing it in an unconventional way.
I expect most all didn't fully understand what I was attempting to get across.
Even your comment about a sawzall tells me you had missed the point somewhat.
A jigsaw is not a sawzall.
Just guessing here but I'm from an older generation than most who look in on and respond here.
I have also forever had more desire to accomplish something than I had the proper tools to do so.
There are always more than one way to do things.
Experience will teach you your limitations.

A smaller lathe may not want to cut a 11 1/2" rotor down to 10" but it could cut an accurate grove in the surface.
With a saw, a hand held jigsaw or maybe a bandsaw a person could cut close to that line.
A band or disc sander could refine the cut to bring it to the accurately cut line, an angle grinder will work if that's all you have.
Is that the proper way, nope.
Will it work, yes.
Will it effect how the rotor would perform, not in the slightest.

Do not let the lack of the so called proper tools limit what you can do.

Just as an example...
I wanted to adjust cylinder lengths to alter compression heights on HD big twins.
I have a 9"X36" south bend model C lathe, it's old, very basic and limited.
A friend of mine has an old worn out Bridgeport.
I made a tool that will work on my lathe...
View attachment 272482

That home made tool made with less than ideal machines can accurately adjust cylinder lengths within 0.001" with ease.

If you read and try to understand only one line in this response, read the one below this.

Learn how to look at projects differently, know there are different ways of doing the same thing.
Can you purchase a rotor in the right diameter and cut the center hole to proper size? Far easier. Use carbide and take multiple small cuts/slow feed using cutting
Can you purchase a rotor in the right diameter and cut the center hole to proper size? Far easier. Use carbide and take multiple small cuts/slow feed using cutting oil. I wouldn't use the lathe but a boring bar in the mill mounting the rotor to table. Far easier set up and accuracy.

At least that's my take on it.
i was thinking the inside hole has to be much more precise than the diameter, its gotta sit perfect on the hub. plus my machine is small. i have to chuck it from the interior hole and cut the outside. im above my paygrade here but thats my thought....
 

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i was thinking the inside hole has to be much more precise than the diameter, its gotta sit perfect on the hub. plus my machine is small. i have to chuck it from the interior hole and cut the outside. im above my paygrade here but thats my thought....
Do you have a face plate for your machine? One could bolt a rotor to a face plate with spacers between. The inner diameter could be cut larger much easier than cutting down the outer... Think lower surface speed of the cutting tool.

If you are cutting the inner diameter of a sprocket or a brake drum, yes the diameter does need to be concentric. Disc brake rotors are not so fussy.

I know many are shaking their heads about now...

The rotor doesn't need contact with the hub center for location, it's a non issue. Rotors don't see a radial load like a drum or sprocket. They simply need to be tight to prevent lateral run-out. If you used flat head mounting screws for the rotor, the screws will locate it by the countersunk mounting holes in the rotor anyway.

Still not convinced? Look at the 10" dual disc front rotors on every 77-83 Harley. The rotor bore on all of those are close to a quarter inch larger than the hub.

At times it pays to step back and get a slightly different view...
 

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Doe your lathe have enough swing?
 

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$20
15 quid when where you last in the states ?
 

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Fuck it gets cheaper !
 

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its not a big, hard or a long job, they would simply put it in a lathe & make a couple of passes, less if they use a tungsten tipped cutter, if ya pay much more than that ya being taken for a ride
 

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I usually have better luck with tool steel than carbite, even on interrupted cuts in cast iron. Also the last time I messed with one of those 3 in 1 machines we could use the quill with an end mill to cut the od of a similar item.. YMMV..
 
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