Big Holes In; Little Holes Out

[GrandpaB]

I’m cutting 16 car bodies from a piece of yellow pine that has been planed to 1.4”. The bodies have two windows and two axel holes that are cut with a profile tool path. The router bit is a ¼”, up-cut end mill and the profiles were designed so that all the material is removed from the center. For example, the axel holes were designed to be 0.320” in diameter. Now the question, why are the holes on the exit side of the car about the correct shape and size (~0.315” x ~0.317”), but the holes on the entrance side larger and misshapen (~0.315” x ~0.408”)? The front window was not a circle, but the rear window design was a circle. I'm including a picture of the 16 car body project and a composite picture of a body showing both sides of the car.

[BillK]

Do you cut the cut out profiles first? Or the holes? Seems like something is moving on you. Why use profiles when you can use pockets?

[Eagle55]

I'm gonna guess a little bit, but it might have something to do with the flex of the gantry assymbly and feed rates and pass depths. What I have experienced is that if I plunge into the material to get a pass depth that I would like to us.... lets say cutting and 1/8" or 1/4" pass, the router tries to plung into the material but has enough resistance that instead of forcing into the material with the router bit, it is meeting more resistance in the wood than the gantry assy has to offer, there by flexing the gantry framework. Then when the bottom is reached, that flex catches back up to where it should be... Like I say I am not sure this is your problem but you could determine it by using a very low plung rate and seeing what the hole shape looks like then. I have had misshaped cuts that was due to this situation.

Roger

[Consultingwoodworker]

Roger is on the right path, something is flexing.

It might be your gantry, router mount or it could even be the bit itself.
I always try to use the shortest bit possible to minimize the flex.

Since this is a deep pocket, your bit needs to be fairly long. I would look to solve this with a program change.
Use a pocket tool path for the holes rather than a profile if you are not already.
You should also try using an "offset". Mill the waste out of the window holes first making them a little smaller (1/16" or less) than you want them to be, then mill the outsides of the windows to final shape/dimension. I've needed to do this even with full sized industrial CNCs to overcome bit flex.

Ralph

[thepuchman]

I agree with the others. Try running a sample with the feed rate over ride dropped down to see what happens. In metal working I have had to use solid cabide endmills to reduce taper in deep holes/pockets.