cutting query

[denmanmarine]

Hi all,

I was cutting a pretty simple file tonight - some 12mm ply panels with a profile tool path and a drilling tool path.

I know you can combine the tool paths into one file but I had an issue with that so I was playing it safe. Anyway the drill toolpath was with a 3/16 end mill (for 3/16 holes) and this completed as expected with holes in the right locations on the sheet. I then changed bits to a 1/4" upcut and proceeded to cut the profile.

About 1/2 way through I noticed some holes were closer to one edge of the sheet than the other which seemed odd as they were meant to be evenly spaced.

A check of the job showed that the profile cut was exactly 2.6mm off to the rear of the table (y-axis) but spot on with the x axis on all profile cuts which seems weird.

The material has not moved as it is clamped down. I have gone back into my design program (Rhino 4) and checked measurements there and all was fine. then I went into V-carve and checked there and all measurements were fine. No offsets were selected for the profile tool and it was told to cut outside the line.

Checked the shopbot files and the correct tools were in the files. The finished dimensions of the profile parts are spot on its just they are 2.6mm out of alignment.

It must be something simple but for the life of me I can't see what I have done wrong.

Running a PRS Alpha 120-60 with the latest version 3.88 software.

Any ideas?

Andrew

BTW, thanks to all on the advice on static electricity and grounding - it has made a huge difference.

[bleeth]

Andrew:

When you went home did it return to an accurate o,o physically or was it also off?
If it was off then somehow you lost position. Did you re-zero x and y after your bit change? It is possible to move the y slightly when changing bits, even with an Alpha. To avoid this I always set the wrenches so to do initial loosening or tightening I am squeezing the wrenches together with one hand, putting the least lateral pressure on the mechanism. Even then, you should always re-zero with c-3 as well as c-2.

[jTr]

I have gone back into my design program (Rhino 4) and checked measurements there and all was fine. then I went into V-carve and checked there and all measurements were fine. No offsets were selected for the profile tool and it was told to cut outside the line.

Any chance you've got an open vector on that cutout rectangle?

When importing dxf's, you should always analyze how vectric is interpreting your shapes. It is not unusual to import a rectangle and have it seen as a 3 sided vector with a single line as the 4th side, regardless how your external design program generated it. Vectric will cut that last side as a single line, which could explain why it seems to have shifted the location of the bit.

jeff

[denmanmarine]

Thanks for the replies guys.

It turned out to be a slight movement of the Y car during the tool change. Once I re-zeroed and cut the same part again it was perfect. Lesson learned!

Andrew

[rb99]

Thanks I learned from this too... (If I can remember it!)

[scottp55]

Thanks all. Never heard this before.

[jTr]

Once in a while, I'll get impatient in startup routines - changing bits, c3, c2, c5, and frequently I'll key in MZ-3 to raise spindle for good bit changing clearance.
If full remnants of previous routine have not completely cleared the screen, it will miss a keystroke. In this case, it's usually the "M", so program receives the message: "Z3" = zero all three axis where they currently are.

Would love to say it only happened once, but...:o

Just be aware - I would suspect an errant key entry over a shift in position due to bit changing - (unless of course, you are using a crowbar:eek:)

If you're regularly doing so and convinced it is the bit change, try a different technique for tightening/loosening. Try to position the wrenches so they can be grasped with one hand, especially for loosening. You should be able to squeeze the two together with opposing forces shared by the two wrenches, thereby negating most of the torque transfer to your stepper motors. Tightening can be done similarly, though usually w/two hands pulling wrenches in opposite directions. If you have a solid grip and need much more than a fist squeeze to loosen, you may be over-tightening.

Jeff

[coryatjohn]

If you're regularly doing so and convinced it is the bit change, try a different technique for tightening/loosening. Try to position the wrenches so they can be grasped with one hand, especially for loosening. You should be able to squeeze the two together with opposing forces shared by the two wrenches, thereby negating most of the torque transfer to your stepper motors. Tightening can be done similarly, though usually w/two hands pulling wrenches in opposite directions. If you have a solid grip and need much more than a fist squeeze to loosen, you may be over-tightening.

Jeff

On my machine (PRS Alpha), it's virtually impossible to move any axis by hand when the machine is turned on and reset. Am I missing something here?

[curtiss]

When you loosen or tighten the bit from the collet,

if you set the wrenches at a small angle and then squeeze them together (like a scissors) you will tend not to move the x y position... if that really can happen at all.

[adrianm]

I must admit my first response when I read this yesterday was that there must have been movement when changing the bit but when I saw that the machine in question is an Alpha (like mine) I discounted that.

It takes a huge amount of force to move mine when it's turned on. Far more than I can apply with the supplied spanners.

[srwtlc]

...and it will move back into position on an Alpha unless you fault the motor by keeping it there to long. Seen it happen, kind of weird.