Fine Points of Side Clearance and Draft Angle
Grinding a rabbet knife for accurate square cutting as well as good tooling life can be tricky. March 10, 2008
Question
We have been changing our knife designs in an attempt to find the best way to do various types of cuts to prolong knife life. Most recently I am cutting a 3/4 by 13/16 rabbet on both sides of a piece of s4s, using the left and right side heads. Should I use a 2 degree relief angle on both the top of the cut and the side of the cut? Most people refer to this as a draft; should it be on any cutting surface?
Forum Responses
(Solid Wood Machining Forum)
From contributor J:
The side clearance is the most important, as you already know. I have read many debates whether it is necessary on straight knives or not. I just got into the habit (good or not?) of tilting my wheel 10 degrees for the side clearance and sharpening the whole rabbet knife that way. I'm interested to see what others have to say.
From contributor R:
There are both pros and cons to using draft angles. I use a 2 degree or more if I can get away with it, and 5 degree side clearance on non-jointed knives, 10 degree side clearance if I have no draft angle. On jointed knives I use no less than 3 degree draft and 10 degree side clearance. The deeper you go with the cut, the more it becomes out of square on the product. The problem is some customers need that area to be perfect 90 degree. If it is a rabbet knife, it can be made 90 degree and it will put 10 degree side clearance, but they must be aware the knife can not remain axial constant; their axial will shift each time they grind and the knife will burn easier.
If it's a dado knife, and they still insist on 90 degree both sides (cabinet door guys), then I try and direct them toward insert tooling or braised on tooling where the knives are ground from the face or the insert is thrown away after use. I have made a couple of dado sets with 90 degree on both sides, but after the first use, must start to offset the knife to get the correct width on the dado. It is a real pain doing it this way. It works, but they will burn up quick also. It gets harder to maintain the steel's integrity once they have burnt the knives up. Maybe approaching the rabbet from a different cutterhead will make the area you need 90 degrees on burn less. Or maybe the draft angle is not a problem for your product.
From contributor J:
Could you describe this draft angle? I guess I mistook the term for side clearance. What is a draft angle?
From contributor R:
Draft angle = out of square anywhere there is supposed to be 90 degree, typical 2 to 3 degree. This is required to properly resharpen the knife, keep the knives from burning, and keep it axial constant. The side clearance allows the wood chips to escape from the side of the cutting edge. This allows the knife to breathe and also keeps the knives cooler.
From contributor J:
Thanks. I know what you are talking about now. I guess I just never heard the term for it.
From contributor C:
I grind 90 degree rabbets in HSS knives at depths up to 1" by tilting the grinding wheel (on a Weinig 960) 15 deg left/right. I typically grind from start to finish with the wheel tilted. If being jointed, the operator shapes the stone so as to avoid jointing that edge, leaving a sharp edge to scrape the wood much like the action of hand-held cabinet scraper. This has been successful even on hard maple. By the way, the WKW Opti-Knife has proven the best in these challenging areas.
