Haven't we all heard this from a customer once or twice, but in fact how may customers find they have an arbor ontheir brake machine that is really bent?

Let's look at the correct way to determine if a customer does have an arbor shich is not "true".

With a dial indicator and magnetic base, position the indicator in such a way that the plunger on the dial contacts the arbor approximatley 6" from the spindle.  ( See Illustration)

Turn the machine spindle on and observe the dial indicator, the reading should not exceed .002 of an inch.  If it does, this can mean the arbor is bent or it could be caused by the surfaces of the arbor flange and or spindle being possibly damaged or just simply dirty.

Mating Surfaces 

(for arbor and adapters)


Mating faces of the arbor and spindle MUST be free of nicks or burrs.  If the faces are nicked or burred, they can be de-burred, the arbor cleaned then re-installed and tested once again as above.  If the run-out is still present, the arbor is bent and will require replacement.

What triggers this question or concern in the first place?  It usually is from a run-out problem in rotors or drums AFTER turning them on the lathe.  Customers will return with a pedal pulse, or the technician finds the condition during the road test.

If the arbor is straight, then what else can cause the condition besides the arbor?

We find that inmost cases, the problem is in the set-up or the adapters.  How many technicians do you know peform a "Scratch Test" prior to turning a rotor or drum?  Even better, how many even know what a scratch test is and how it is performed?

Lets look at the scratch test and how it is peformed, but better yet, how it can help discover problems before the customer does.  We will cover a rotor, but a drum is performed basically the same way.

Carefully clean and mount the rotor onthe lathe using the correct adapters.  Tighten the spindle nut.  DO NOT OVER TIGHTENNote:  Have the machined face of the nut facing in, contacting the adapters.

  • Traverse the cutter in until the tool bits are in the approximate center of the friction pad surface.
  • Turn the spindle on then slowly feed in one of the tool bits until the bit just contacts the friction surface.
  • Back of the tool bit, then turn off the spindle.  Loosen the arbor nut and rotate the rotor 180 degrees, then re-tighten the spindle nut.
  • Advance the took bed in about 1/8th of an inch, and then bring the same tool bit in as used to scratch the rotor as described above.
  • If the second scratch is located next to the first scratch, it is OK to cut the rotor.  If the scratch is 180 degrees away there is a set-up situation.  Dismount and check the adapters and arbor for any nicks or burrs.  DO NOT TURN THE ROTOR until the problem is found.

Yes, scratch tests do require a few minutes extra to perform, but think of the potential problems that can be avoided.

If your customer is still having pedal pulse (run-out) problems,  have them contact Technical Service for further assistance.