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help ! i need machining advice, or a new rotor

4K views 29 replies 13 participants last post by  farmall 
#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.
 
#2 ·
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.
 
#3 ·
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.
 
#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.
 
#4 ·
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
 
#5 ·
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.
 
#6 ·
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.
 
#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👍
 
#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.
 
#10 ·
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
 
#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....
 
#11 ·
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.
 
#22 ·
First, im trying to learn something and do it myself, not just bring it somewhere unless i have to. Secondly, from what i read , motorcycle rotors are made of a harder heat treated steel than cars. Car rotors are much thicker which wouldnt warp as easily. Third, a machinist acquiantance sai the rotor is very hard and he would take care of it for me, and its perfect! I think ur misinformed about oriellys being able to do it but i have one near me and will ask and have a spare made
 
#21 · (Edited)
If you can saw off some of the excess, you're going to be ahead but at first you'll have to sneak up on it to avoid chatter or worse, snagging the rotor and turning it into a very heavy Frisbee.
It's best to pick up the inside diameter. If your rotor indexes with countersunk screws then an adapter you can chuck up in your lathe then bolt the rotor to it will help make the job easier.
Definitely use carbide cutters and as slow as you can run the lathe until you have a continuous cut all the way around. Otherwise an "interrupted cut" is a good way to start a chatter that's sometimes hard to get smoothed back out. Cutting should be almost noiseless, just a little metallic hiss. If you're hearing squealing, you're hearing a harmonic so slow your speed down.
Others may tell you you can turn your work piece faster but if you ever have a part jump out of the chuck jaws, NOT being in its path at that moment may be the only thing that saves you.

We round out cylinder fins on some jobs and I start with our Monach at 60 rpm, .002" carriage travel per chuck rotation, .010" per pass until the fins are round. Very slow but that way I don't have to worry about snagging and breaking a fin or worse, having the cylinder get away from me.
 
#23 ·
I agree with learning to do stuff yourself. Later, even if you choose to take a part to have a mechanic or machinist do the work, you'll know and understand the procedure.
One thing that's invaluable is learning set-ups. Knowing 6 or 7 ways of holding and machining a part instead of 1 or 2 will open up the ability to do lots of jobs that otherwise might seem impossible.

One detail everybody slid past was checking your rotor to make sure it's not worn beyond minimums or has excessive runout. If not, find a machinist to walk you through the procedure or let you watch as he does the work. If it is worn beyond limits, it's a still good piece to practice machining on.
 
#24 ·
"If it is worn beyond limits, it's a still good piece to practice machining on."

That's a good point, one of many in this topic. I am absolutely not complaining when I say it has attracted far more posts than the subject would have seemed to deserve. A whole lotta guys shared (nearly all) useful, (often) outside-the-box ideas, procedures, and knowledges in general. It is one version of what I think is the Ideal J.J. Tech Thread.
 
#29 ·
Calipers are designed to float from side to side to 'find' the rotor and where they need to be to do their job. Not so with drum brakes on a fixed backing plate.
Once you get your inner and outer diameters where you want them to be, you should be in good shape. Just to have a dependable starting point, it might be a good idea to make sure the center hole is the right size and is where it should be. Once that's done, you can clamp the disc in your lathe by the center and dial in the O.D.
The outer diameter isn't super critical as long as it isn't so far off that it messes with the balance, or the high spot(s) could saw the caliper in two. A spinning disc doesn't have the centrifugal forces to deal with that a spinning brake drum does with lots of its weigh at its outer circumference.
Truing the outside too lets you know it was done and is concentric. Good practice for future machining jobs.
 
#30 ·
I used half a scrap steel hub as a mandrel since my 14" lathe was too small to chuck the rotor and mostly because I didn't feel like installing a faceplate. Worked fine for boring late rotors for an AMF OD hub. However it makes more sense if ya don't have enough tool reach to visit your local machine shop who you REALLY should make friends with anyway. I used generic carbide insert tooling.
272577
 
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