Wilsons Sandbox

#ygrps-yiv-1781528460 blockquote.ygrps-yiv-1781528460cite {margin-left:5px;margin-right:0px;padding-left:10px;padding-right:0px;border-left:1px solid #cccccc;} #ygrps-yiv-1781528460 blockquote.ygrps-yiv-1781528460cite2 {margin-left:5px;margin-right:0px;padding-left:10px;padding-right:0px;border-left:1px solid #cccccc;margin-top:3px;padding-top:0px;} #ygrps-yiv-1781528460 .ygrps-yiv-1781528460plain pre, #ygrps-yiv-1781528460 .ygrps-yiv-1781528460plain tt {font-family:monospace;font-size:100%;font-weight:normal;font-style:normal;} #ygrps-yiv-1781528460 a img {border:0px;}#ygrps-yiv-1781528460 {font-family:Tahoma;font-size:12pt;} #ygrps-yiv-1781528460 .ygrps-yiv-1781528460plain pre, #ygrps-yiv-1781528460 .ygrps-yiv-1781528460plain tt {font-family:Tahoma;font-size:12pt;} A problem with machining an automotive rotor is that the surface burnish area tends to be on the hard side, combined with the tough metal often used for better rotors. This requires some tool pressure regardless, but often more than ideal to cut in a reasonable time. The tool pressure causes the rotor to deflect but the deflection is greater the closer to the edge of the rotor. Machining a rotor on a standard lathe is difficult unless some provision such as a backing plate or pressure offset roller is against the back of the rotor. What results is that the rotor becomes tapered towards the edge. It becomes sort of like a hunting knife shape rather than for the sides to remain parallel as is wanted.   Disc brake lathes have a tool head which allows cutters to run against both sides so that the tool pressures counteract one another. One can see the effects by placing a dial guage with wheel against one side, then running only one cutter or simply making a cut to a true rotor and measuring.   Motorcycle rotors are, as others have stated, very thin and quite large in diameter making the deflection problem even worse. The effects to a thick automotive rotor make machining these one side at a time a difficult problem but doing a motorcycle one for someone is simply not economic. Using a flywheel grinder which places the disc onto a backing and grinding/tool cutting to one side is likely the practical way for someone to do this because it would omit the set up.   Place a dial gauge against one side of a bike rotor as mounted, then press the other side with fingers or thumb and see the deflection. Imagine that with the tool pressure required to cut burnished material?   As others have said, with so little margin of material which is allowed to be machined from bike rotors (made thin to reduce unsprung weight), one is liable to find that turning has taken one below the minimum thickness before the grooves or other damage is removed. This does not take into account that the braking heat and other factors combine to produce hard spots in the rotor which add to deflection problems and often require that these be ground out of thick automotive rotors. One doesn't usually discover this until one is machining and likely to find that the minimum thickness is passed. Problems with this include: people don't want to pay for a machining job which doesn't produce a useable rotor; and, what to do with a rotor which has been machined to below minimum? If one surrenders the rotor to the customer and an accident results, how likely is the customer to admit that they used the rotor despite that they were told not to do so? Does one bend or otherwise destroy the rotor then face the customer? Not a good choice so better to avoid the problem.   I ran a series of information seminars for mechanics when in colleges. One thing we did was to have presentations by lawyers who practiced in the liability field in order to put a stop to shops who would machine a rotor or drum beyond the limit at the customer's insistence and then believe that they had avoided liability on the part of the shop by noting that the machining was beyond limits and unsafe. They would state this on the work order and have the customer sign but of course all they were doing was admitting that the shop knowingly committed an unsafe act. Made their legal status worse.   A trick for checking to gain some impression of the limit of a rotor is to use some sticky grease to hold a pair of small bearing balls to the jaws of micrometer or calipers. The balls will bear into the grooves further to give a better impression of the actual rotor thickness. As someone noted, this isn't fool proof but allows measurements with better accuracy than a flat tipped tool. I've machined hundreds or maybe thousands of rotors, drums and such but the only way to be certain of what thickness will clean up a part is to actually machine, then measure. Same with crankshafts and such.   Automotive rotors have become so inexpensive that it's not worth machining most. Many shops don't even have a rotor or drum lathe these days despite that it was bread and butter in the 1970's and 1980's. Man, it's crappy to be that old....especially when the 1960's are so recent.    I recall relating an event in the automotive industry to one of my college classes back in the 1980's. I had to stop and laugh because I was not only the only one in the room who was working in the industry at the time, I was the only one who was alive at the time.