What causes this (extended corners)...

[jglazer]

I notice that when I carve a simple rectangle with (in this case) a 1/4" endmill, the corners drift outside the vector. What causes this. See photo below inside the box (rough carve):

[4DThinker]

Backlash is the only thing that I can think of that might cause what I see in your photo. When the bit changes direction, the drive shaft/belt/whatever slips until the threads/tension/whatever catches up, allowing your bit to slip back a bit causing the bulging corners.

4D

[ger21]

Not just backlash, but flex in your machine. Which I guess may be considered to be a form of backlash?
When cutting inside corner, the bit is actually pulling itself into the corner.

[jglazer]

What to do to fix? Tighten belts maybe?

[Bonch]

You can try reducing your feedrate and depth of cut to reduce the stress on your machine. Bits deflect based upon how much stress you put on them. The more stress the more bend. When too much stress they either break or transfer that stress to the next weak area which may be your rails. At the end of the cut, those stresses get removed by either the rail or bit moving back close to their normal unstressed position which can cause over cutting or undercutting depending on what you are doing. I see you are using an X-carve. If it is an unmodified one, they have this problems alot according to their forums. Most people say the cut at a feedrate of 60 and DOC at .08. It appears those are maximums depend on the material. Check your settings against these and see where you fall. Experiment with scraps by varying these settings and see if it helps.

[martin54]

Not familiar with your machine & there are a number of things that will cause backlash or bit deflection due to flex in the machine, first thing would be to try a less aggressive cut. It may be that you are trying to cut to quickly or to deeply per pass for your machine. Try slowing things down a bit &/or taking a smaller DOC

Never hurts to check the machine for anything loose that can cause a bit of free play either, be careful tightening belts as you can cause more problems if you over tighten them

[ger21]

What to do to fix? Tighten belts maybe?

Get a better machine. There's not a lot you can do to make machines like an X Carve more rigid, without replacing components with bigger, heavier, and stronger ones. And every time you do that, the next weakest link flexes more.

Try doing a roughing cut a little inside your finished cut, so the final toolpath is removing a lot less material.

[ElevationCreations]

You may want to post at the Inventables User Forum for feedback on what to check for on your machine.