Knife-Setting and Grinding Tolerances

Given that nothing's perfect, how close is close enough? August 8, 2008

I'm new to corrugated back tooling - setting up a shaper cutterhead for a crown profile with a new cutterhead - not a hydro-lock head - and new knives. I have a simple jig made up with a dial indicator to measure the cutting circle. What sort of tolerances am I looking for? There are two knives in the cutterhead. I'm not expecting to see zeros, but not sure what would be realistic. At different points on this profile, I'm seeing about 3-4 thousandths in one place, and 10-12 thousandths in another. Does anyone have any recommendations on a knife-setting gauge or something that would help?

Forum Responses
(Solid Wood Machining Forum)
From contributor A:
If I am reading you correctly, your problem is with grinding, not with the head. If you grind the knives in the head, then they should be concentric to each other. If they are not, then the problem is in the grinding room and nowhere else. If you buy your knives you need to specify knife concenctricity to the supplier, or they will give you knives like you are describing. This will, of course, present itself as finish issues. I keep all my knives within 1 or 2 thousandths, unless I am jointing. Then I never go over 1/2 thousandth.

From contributor B:
What is the condition of your shaper spindle? Sometimes a slightly warped spindle could do that.

From contributor C:
I would not be too concerned about it, as only one knife is cutting anyways. Once you breakdown the head they were ground in and place them in another head you will see a few thousandths difference. There is a bore tolerance on your cutterhead of anywhere from .000 mikrons to .025 mikons which is .001". In reality you should be no more than about .006" once you have placed them in another head. If they were ground in the same head then I would agree with contributor A, .0005" to .001". It's not a perfect world, and there is only one true cutting circle because of the bore tolerance. I would be more concerned with perfect weight for balance than perfect cutting circle. This will give you the best finish and prolong spindle life.

From the original questioner:
Thank you for the responses, gentlemen, they were most helpful. The knives were made by a reputable vendor, and I was surprised at the numbers I was seeing, although I knew going in I would be getting a one knife finish. I checked some of my insert tooling on my little jig as a reference, and was able to get the new cutterhead within the tolerances mentioned in the replies, or in that ballpark anyway. I ran some moulding, and the finish is what I was expecting.