If you’ve ever worked on brakes, you’ve likely used, or at least seen a brake lathe. This handy tool helps mechanics to machine rotors to the right specifications ensuring a smooth braking experience. However, there are times where the the brake lathe can end up causing more problems instead of solving them. This article will go over some of the issue that come up and how to prevent/solve them.
Causes of Brake Lathe Runout
Typically, there are three main reasons that brake lathe with cause runout:
1. Arbor runout
3. Not verifying accuracy of setup
The good news? Each of these issues can be helped via the steps below.
Measuring Arbor Runout
Essentially, proper care mixed with periodic measurement will eliminate the arbor as a cause of brake lathe runout. For example, follow the steps below to check and correct arbor runout.
Lathes using tapered arbors use witness marks to indicate the most accurate fit for the arbor to spindle. These marks are made at the factory after the arbor is matched to the lathe. For example, verify that the marks are aligned as shown in the image below.
2. Position the Dial
Next, you’ll need to set up the dial indicator . For example, see the image below.
3. Rotate and Measure
Finally, use the draw bar nut to rotate the spindle while also watching the dial. It is best to use a socket and ratchet when performing this step. Use a smooth 360 degree rotation for the most accurate measurement. If arbor runout is within specifications (typically nor more than .002 with the closer to zero the better.) No service is needed. If outside of specifications follow the steps below.
Correcting Excessive Arbor Runout
Once the issue has been found, steps should be take in order to correct it. The following section will go over the necessary steps to correcting the arbor runout.
1. Inspecting the Arbor(s)
Begin by removing the arbor. Inspect the arbor and spindle taper(s) for chips or rust build up. (See below) The tapered surfaces of the spindle should be cleaned using a fine to medium steel wool. The arbor’s tapered surface(s) can be cleaned using a wire wheel.
Note: Do NOT use sandpaper, emery cloth, or anything that could remove metal. The taper surfaces determine the arbor’s fit and changing them in any way could result in permanent damage to the accuracy of these surfaces.
Use a clean rag to wipe all mating surfaces before installing the arbor. Put a light coat of WD40 or similar treatment on the tapered surface(s) before installing. This will prevent rusting.
Next, align the witness marks and tighten the arbor nut or draw bar to specification. Typically, it will be around 50-60ft lbs.
Finally, repeat the steps found under “Measuring Arbor Runout”. If the measurement is now less than .002″ you are finished. Otherwise, follow the steps below to determine the cause.
Determining the Source
Even after completing all of the steps above, there are times that the specifications don’t match up. When this happens, it is important to figure out the cause, so that it can be fixed.
1. Greatest Runout
Rotate the arbor until the dial indicator is at its highest reading. This is the point of greatest runout. Use a magic marker to mark the point on the arbor as shown below. This mark will be used to determine if the arbor is the source of the excessive runout.
Next, loosen the arbor and rotate it 1/8 of a turn clockwise then tighten the drawbar. Continue by measuring the arbor runout.
3. If Less Than .002"
If runout is less than .002″, make a mark on the arbor at the spindle witness mark.
4. If More Than .002"
If the runout is more than .002″, loosen the drawbar and rotate the arbor 1/8th of a turn, then retighten th drawbar.
Now you are ready to re-check the runout.
6. Still Greater Than .002"
At this point, if the runout is still more than .002″ repeat steps 4 and 5 until the runout is less than .002″. Next mark the arbor to the spindle witness mark.
7. Check Mark
Finally, if the runout is still greater than .002″ after the arbor has been rotated 360 degrees, rotate the arbor until the maximum reading is shown. Next, check to see if the magic marker reference mark from step 1 is aligned with the dial indicator plunger. If the two are aligned, the arbor is bent and will have to be replaced.
Note: Some older lathes may not have a witness mark. Dial indicate the arbor and mark both the arbor and spindle.
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