Tolerance For Knife Stock Thickness

      A discussion of thickness variations in moulder and planer knife blank material, and the attendant problems for custom-ground knives. April 27, 2007

Question
Do those of you who make molding knives have any trouble with varying thickness of the knife stock? How do you handle it? I use a planer style molder and I have a bit of trouble with the differing thicknesses. At this point I have ground about forty different profiles and made a good selection of gibs with various lengths and weights. Because of the selection of gibs I have on hand, I seldom grind a new knife that doesn't match one of my existing gibs within tolerance for the specified knife/gib weight for the head.

Unfortunately, the tolerance for variation in the knife thickness with the gib stock I have is only about .005+/-. With the knife stock I have purchased in various widths, I have about .020" overall variance.

Is there another type of gib design that allows for this much tolerance in corrugated knife blank stock thickness, or are you making custom gib thicknesess for each thickness of knife bar stock? These are all corrugated high speed steel made in Japan and purchased though International Knife and Saw.

Forum Responses
(Solid Wood Machining Forum)
From contributor R:
I do see a few thousandths difference in knife thickness from different manufacturers. I use three different manufacturer's products, all 5/16" thickness. But all the customers I have use a conventional style moulder head that can take a thickness of 3/16" through 3/8" by simply adjusting the gib screw in or out. I know that doesn't help you solve your problem, but there are slight differences between manufacturer's products.



From the original questioner:
Okay, that's interesting about the .1875" through .375" tolerance in knife thickness for your customers! If I take even .010" off of a gib meant for .25" in my machine, the gib will raise considerably upwards out of its slot when I tighten the screws. I guess that's the problem with my RBI gib design. There is no means to handle difference in thickness. The screws drive straight down and wedge the gib upwards in its slot. Your customers must have a gib design similar to jointer gibs with lateral adjustment.


From Dave Rankin, forum technical advisor:
Knife stock thickness is one of the areas of tolerance requirement that I have for the knife material that we sell. The reason is simple: balance. Knife stocks that vary within a bar or between bars can cause a balance issue. I also require that all corrugations be the same from the flat bottom of the bar. Using a few other standards, it is possible to use the end cuts off of two bars of stock and maintain a good quality and balance knife. Otherwise, you will experience problems of some sort. I agree with contributor R that different manufacturers have different tolerance allowances and there is an American 5/16" and the European 8mm.


From contributor R:
To the original questioner: Yes, that's correct. There are some moulder cutterheads that are called centro lock and the have a tighten screw on each side. As you tighten the screw, a series of wedges push on the bottom of the cutterhead, forcing the gib into a wedge lock. These types of cutterheads and gibs must be changed to fit the size steel thickness you're using. I don't see many of these anymore. Costly to change gibs sizes and requires a lot of cleaning. Knives have also been known to move microns on jointed machines, causing a great difficulty keeping the knives jointed.


From contributor C:
The need of tight size tolerances, as well as vacuum heat treating (to better stabilize the consistency and end-resulting weight variances) led us to settle with high end manufacturers such as Wisconsin Knife Works. Their corrugated knife thickness tolerance is + or - .002". Crush ground corrugations and other attention to manufacturing details make it easy and accurate to use.

What we found with some other steels is that we were consistently fighting with knife balance issues. The added costs in time and effort far outweighed the initial cost savings in the raw bar stock.

IKS should be able to provide you tolerances on their product.



From the original questioner:
I run single knife only so I don't have a balance problem.

When I grind a new knife, I weigh it on a triple beam scale and then I can easily match it to one of my existing gibs that it will pair with and be in tolerance for the knife/gib weight requirement for the cutterhead.

My problem is mainly that I am often forced to either modify existing gibs to allow a knife that is .010" thicker than the knife the gib was made for to begin with, or else take the time to cut, bore, and fit a new gib for the knife.

I never thought to specify thickness when I ordered bar stock from IKS other than to ask for .25" thick stock. I will see if they can provide a specified thickness plus or minus .005 along with the width next time I order.



From contributor D:
The RBI incorporates the wedge design, because it is safer. If the gib screws were to loosen, the centrifugal force holds the gib in place. Coupled up with the corrugated heads it is almost impossible to loose a knife while running. The tradeoff is you have to go with a high end knife manufacturer like RBI does (Wisconsin Knife Works). This is just my humble opinion.


From the original questioner:
Yes, it is nice not to have to worry about a knife flying out of the cutterheard. I am the profile grinder in my shop and the Japanese made corrugated bar stock I get from IKS is extremely high quality. I am just interested in how outsource profile grinding companies deal with knife thickness requirements when it comes to supplying users of several different models of planer style molders.

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