V-Grooving for a Miter-Fold on a CNC

Cutting a V-groove with a CNC's rotary bit has inherent limitations because of bit speed. But there are solutions. March 25, 2007

I am trying to make open faced (5 sided) mitre folded boxes. The components are Baltic birch 3/4" ply. My test run worked fine with MDF, but when I run the Baltic birch, the mitres are no longer 45, but around 46 degrees, and the cut edge is kind of lumpy. The edges are also coming out with some excess fuzz, making it so that I have to go and carefully sand the outside edges before glue up. Can anyone offer advice? My MDF run was perfect. My Baltic birch BB/BB is a disaster.

Forum Responses
(CNC Forum)
From contributor D:
Don't mean to state the obvious, but would it be because the plywood is cutting rough versus the MDF cutting smooth? I would expect the plies of the Baltic to be inconsistently cut - this is typical when you use a router bit to cut it. The fibers sort of push out of the way rather than cut clean. When you fold it up, the cross grain is probably holding the joint open. Are you using a vertical router to cut the joint or a horizontal blade? Perhaps the tool people can suggest the best way to attempt this? I wouldn't expect it to work very well in a plywood.

From contributor P:
It sounds like the main problem you have is that the material is not being held down completely flat to the spoil board and there is movement.

From the original questioner:
Thank you for your responses. Am I wrong to think that this is a pretty routine job? Has anyone successfully made nice mitre folds with this material? By the way, the material is Baltic birch b/bb 3/4". I am thinking I should CNC the panels and then cut the mitres on a panel saw.

From contributor U:
If you consider the geometry of a V-bit which is used for V-grooving on a CNC machine, there is one major drawback - the tip of the bit has absolutely no rim speed. As rim speed is critical for quality machining, you might say that the tip of the V-bit is literally bludgeoning the material. The optimum method for V-grooving with a CNC machine is to use a single sided right angle aggregate head with a V-grooving cutterhead. We (Benz Inc.) can offer you a specially designed head which is equipped with a gear reduction system. This allows your operator to program the motor spindle to run at 9,000 RPM to 10,000 RPM, which is necessary in order to achieve maximum torque and horse power from your spindle motor with an output speed to the tool of right around 5,000 RPM to 6,000 RPM. This is typically the ideal speed for the V-grooving cutterhead.