The 'parallel' he is talking about is a machinists set up tool. It's a rectangular bar of steel with precisely parallel edges and faces. Normally it is also ground very smooth.
Laying the indicator tip against that smooth face is easier than trying to get it to hit the knife edge. That's all.
You are right, TDC is irrelevant. What IS relevant is that the relationship (angle and distance) between the cutters and the indicator tip not change between measurements.
Bill
I don't know what you mean by parallel.
The parallel is a machinist's parallel. Usually sold in pairs, typical size might be 6" long, 1" wide, 1/2" thick. Precision ground parallel and flat on all faces, hardened steel.
Most dial indicators have a convex contact. If you want to accurately set the jointer knife, you have to make sure that when it is exactly at TDC it is hitting exactly the lowest point of the contact. Pretty tough to do unless, as with Ed's jig, you fix the knife at TDC and run the contact over it.
If you put a parallel in between, you don't even have to make sure that the indicator is on a wide, flat stable base. You can use it on a magnetic base. It may not read the same when you set it up on the other end of the knife, but that doesn't matter. The only thing that matters is how much the knife lifts the parallel when it rotates.
John Martin
Hi Bill,
I'm not sure what you mean by this. Are you referr> Use a mag base to keep the relationship between cutting tip and
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