Grinding knives to match mouldings
Why knife templates don't match the finished product. January 24, 2001
Q.
I am trying to grind moulding machine knives using a York 400 profile grinder. The problem is that the knives produced by the grinder do not match the template. Consequently, the moulding made with the knives does not match the original molding.
Could the angle of the template be the source of the problem?
Forum Responses
I am not familiar with the York, but grind knives all the time. The knife profile will always be deeper then the template. This is because the hook angle of the knife, when mounted in the head, produces a shorter profile height.
Do you know what size of grinding wheel you are using--3/16-1/8-1/16? Make sure that the stylus that runs against the template is of the same thickness. The wrong thickness will cause the problems you describe.
The profile on the knife will always be elongated due to the cutting angle machined into the head. The greater the angle, the greater the elongation.
If the dimensions differ across the axial face, there is a problem with either incorrect dressing of the grinding wheel width, incorrect stylus, tool rest height improperly set or other alignment factor.
I’ve had this problem. It was from a template made by a manufacturer of carbide inserts. The head they were using to hold the inserts while grinding had no hook angle. I tried to use one of their templates to grind the same knives in a head with a twelve degree hook angle. The only problem was the depth of cut. If you grind the knives in the same head you plan to run on the machine, you should not have any problems. If you are unable to do this, the depth of cut will have to be scaled to fit your desired hook angle.
When using a template to grind a knife, the template should match the wood sample or the profile exactly. The knife will not match due to the rotation of the tool. The longer parts of the knife will contact the wood first. This is why the knife does not match. The longer the knife, the more noticeable the problem.
In order to transfer the accuracy of the template, your grinding wheel and tracing pin must match in size and shape.
Another often-overlooked problem is the height of the tool rest. If the tool rest is too high, the profile knife will be oversized compared to the template. To check this, grind a bullnose profile and add side clearance. The side clearance should form a v-shape at the exact center of the tool.
Dave Rankin, forum technical advisor
