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Problem:

Bench lathe induced runout is a leading cause of runout induced DTV pulsation comebacks.

Cause:

There are three main reasons for bench lathe induced runout. They are:

  • Arbor runout
  • Adapter condition
  • Not verifying accuracy of setup

This tip will cover checking and correction of the lathe’s arbor. If the arbor has excessive runout (> .002”), then everything machined on that lathe will have some runout.

Solution:

Proper care of the arbor and periodic measurement of arbor runout will eliminate the arbor as a cause of lathe induced runout. Follow the steps below to check and correct arbor runout.

Measuring Arbor Runout: (Ammco lathe shown, others similar)

  1. Lathes using tapered arbors use witness marks to indicate the most accurate fit of arbor to spindle. These marks are made at the factory after the arbor is matched to the lathe. Verify these marks are aligned as shown in Figure 25.1

    Figure 25.1 Witness Marks

  2. Position the dial indicator as shown in Figure 25.2.

    Figure 25.2 Dial Indicator Setup

  3. Use the draw bar nut to rotate the spindle while watching the dial indicator. 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 (generic no 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:

  1. Remove arbor. Inspect arbor and spindle taper(s) for chips or rust buildup (Figure 25.3). 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.

    Figure 25.3 Rusted Taper

    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.

  2. 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 help prevent rusting.
  3. Align the witness marks and tighten the arbor nut or draw bar to specifications. This is usually around 50-60ft lbs.
  4. Repeat steps 1 to 3 under “Measuring runout” above to check arbor runout. If less than .002” process completed. If not, follow the steps below to determine the cause.

Determining source of excessive arbor runout:

  1. Rotate arbor until 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 in Figure 25.4. This mark will be used to determine if the arbor is the source of the any excessive runout.

    Figure 25.4 Arbor High Spot

  2. Loosen the arbor and rotate it 1/8 of turn clockwise and tighten the drawbar. Measure arbor runout.
  3. If run-out is .002″ or less, make a mark on the arbor at the spindle witness mark.
  4. If more than .002″ runout is indicated, loosen the drawbar, rotate the arbor 1/8 turn in the spindle and retighten the drawbar.
  5. Recheck the run-out.
  6. If it is still more than .002” repeat step 4 and 5 until .002” or less runout is achieved. Then mark the arbor to the spindle witness mark.
  7. If the runout is still greater than .002” after the arbor has been rotated 360 degrees, rotate the arbor until the dial indicator shows the maximum runout reading. Check to see if the magic marker reference mark placed on the arbor in 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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