I’ve noticed that the parts coming off my machine are bigger on the bottom of the part than they are on the top. They’re dimensionally correct on the top then things just gradually grow. I think in the 3d printing world this is referred to “elephant’s foot”? (obv different issue in this milling context) And it grows out in all directions (+/- x & y) almost as if the bottom side were scaled up. Pockets are the opposite problem, the shrink slightly as they get deeper into the material.
Anyone see this issue before and now what’s causing it?
Using 1/8" bits and dw660. Cutting 3/4" oak. All my tool paths are using a finishing path. As far as I can tell the z travel and bit are perpindicular to the work bed.
If your tool was not perpendicular you would end up with a very obvious step on at least one side before (sometimes even after) the finishing pass. Have a look for that, if that is not it I can not imagine any other way for that to happen other that too heavy of a cut and flexing the entire build.
I saw something similar when looking for a good fit in my box joints. When test fitting with parts laying flat, it would bind in the bottom (near spoilboard) while the tops of the fingers still had clearance, so the fingers were evidently a bit trapezoidal, even with a finishing pass.
I am not sure of the true cause but my best guess is that the spindle axis of rotation is not exactly parallel to the shank of the tool. This makes the effective tool radius (about axis of rotation, not shank center) slightly off and dependent on “azimuth” relative to the collet.
With a single flute bit, the cutting edge is at a different azimuth at different z height, so depending on the orientation of the bit in the collet, you can get a swept area thats wider or narrower at the bottom.
It seemed like a long shot but I did two tests: one, I inserted a plain rod and tried to measure if the collet was coaxial to the rotation. It was off a little but hard to measure. Two, I marked the bit and the collet and tried different orientations. This did make a difference in the fit of the box joints, and I got the fit to be pretty similar across the 12 mm or so of z.
I also examined the bit itself but it looked perfect as near as I could tell, and I observed it on two different bits, so I doubt the bit is the problem. And anyway if the bit were tapered and the collet were coaxial to rotation, changing orientation of the bit in the collet wouldn’t have made a difference.
Anyway, try rotating the bit in the collet and see what happens.
To my untrained self, it sounds like your spindle is flexing on the tool mount, which would cause it to “lean away” at the tip when under pressure from your material.
That would make it lean when doing finishing passes on all sides. So you might try tightening the tool mounts up a bit.
I just posted a new topic with a way to measure this angle. (This is for advanced users, you will probably need to recompile Marlin and it may be a little hard to use if you don’t have any programming experience.)
The script I made only measures this angle using two rotations of the bit in the collet (0 and 180 degrees), but you can run it multiple times with different starting rotations.
Ok, I am a beginner here. Please sanity check my response for understanding.
Your method measures how far off plumb the tool is.
Because the OP is seeing the effect in all directions, he might be perfectly plumb under zero load, but something is deselecting it has some elastic properties.
Yes. It measures how far off plumb the axis of rotation of the router is, and additionally it measures the angle between the axis of rotation of the router and the center of the bit. These numbers are measured under zero load (or a static load if you attach some weight to it somehow).
Yes, that might be the case. What Jamie meant is that the bit may be “bent” (relative to the router) so it is not perfectly parallel to the axis of rotation. When you would drill a hole with a bent bit, you get a cone shape instead of a cylinder shape. Even when your Z axis is perfectly plumb.
My reply was not intended to try to solve the original problem of the elephant’s foot, only to suggest a way of measuring one of the possible causes.
Okay, how thick is your material to leave for the finishing pass? Another thought, try a different endmill. Though you haven’t said how much the difference is. It could just be a badly ground endmill that’s a smaller diameter at the bottom.
I think the measurement is a good one that can help. There are multiple things possibly happening though and even if they can’t be measured separately it’s important to identify the concepts because the measurements are a mixture:
Direction of z movement is not perpendicular to plane of x and y movement.
Axis of rotation is not perpendicular to plane of x and y movement
2b. Axis of rotation is not parallel to direction of z movement
Axis of tool is not parallel to axis of rotation
Axis of tool is offset from axis of rotation (can be parallel but not collinear)
For surfacing a spoilboard for example, only item 2 is important, and Ryan’s tramming tool measures item 2 and only item 2.
Item 4 is interesting because with a single flute bit it can produce a tapered cut if the tip of the tool is closer to the axis of rotation, and at higher z levels the cutting edge wraps around the tool and is farther from the axis of rotation. In my case I rotated the bit in the collet and saw a difference, which 1, 2, and 2b alone cannot account for.
I would also suggest doing deeper, and slower. 1mm DOC isn’t much, so I’m surprised if it is bending the bit. But deeper is easier for these machines than faster.
Also, make sure you’ve got a good sharp bit.
I don’t know how an alignment issue would make the pockets smaller and the parts bigger, so I’m skeptical it’s a perpendicular issue.
I agree on the perpendicular issue. If there is too much tool deflection on your roughing passes your finish pass will not be able to correct it. See if you can check it before the finish pass and you may have to change the way the roughing passes are made.
Okay I made a mistake and I figured I should update. The fingers of the box joint were coming out barrel shaped but not because of the off-center axis like I thought. I made a dumb mistake and the depth of the workpiece was greater than the length of the cutting edge of the bit.
While it’s theoretically true that an off-center single-flute bit could produce tapered fingers or curved vertical surfaces, the discovery of this error has cast an entirely different perspective on the past experience and I am now skeptical that there could be a significant effect unless the runout or skew were extremely severe.
