1. Arbor runout
2. Adapter condition
3. Not verifying accuracy of setup
This article will cover a process called "Scratch Cut" which is used to verify accuracy of setup.
Solution
All rotors and drums machined on a bench lathe should be scratch cut before machining. This is necessary to verify the accuracy of the lathe setup. Skipping this step can lead to machining runout into the rotor. The process of verifying the setup is commonly referred to as "scratch cut". The list below shows each step of the process:1. Position the bits away from the outside edge of the rotor.
2. Bring the outside bit in until it barely touches the rotor. While holding the outer knob, place the inner dial on zero. On lathes without this feature, take note of the dial reading.
3. Back the cutting tip away from the rotor a small distance and turn the lathe off.
4. Observe the scratch cut. If the scratch cut is at least 50% around the rotor proceed with the machining process. (first image) If the scratch cut is less than 50% then the setup needs to be verified. (second image)
Steps 5-10
5. Two things will cause a scratch cut less than 50%. The first is that the rotor has runout and needs to be machined.6. The second is a problem in the setup. Adapter cleanliness, arbor runout and tightening of the arbor nut are the main causes of setup problems.
7. To verify the setup, loosen the arbor nut. While holding the inner and outer adapter rotate the rotor 180 degrees. Re-tighten the arbor nut. This process changes the relationship of the rotor and adapters.
8. Turn the lathe on and move the twin cutter in or out a small distance.
9. Make a second scratch cut by turning the dial to zero. On lathes without this feature, turn the dial into the same number as the first scratch cut. This process makes each scratch cut the same depth which makes comparing the cuts easier.
10. Back the cutting tip off a small distance and turn the lathe off. Compare the next two scratch cuts. If the problem is in the rotor, the cuts should be on top of one another. (See below)
11. A setup problem will cause the second cut to be in a different position from the first. (See first image below). If this is the case, disassemble everything and check for cleanliness, correct adapters and for nicks on the mating surface. (See second image below) Do not proceed with the machining process until the scratch cuts are in the same position. Doing so will include runout into the rotor.
Bonus Tip: EVERYTHING machined on a bench lathe should be scratch cut to verify accuracy. For additional info, see our article on bench lathe adapter service.
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