Or use the spin indexer but don't rotate it while cutting. Set the quip depth stop and peck away at it rotating a little bit and locking it down each time but with the cutter out of the work. Might also help in following the line as you can adjust position before reaching final depth.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-