G-codes and V-carve question

[ryadia]

Hello again all,
I've been happily using Vcarve for a month or so until now when I need to level a new waste board. Seems simple enough — buy a flattening bit, create a rectangle, set the depth of cut, and flatten it. eh?
Well not exactly if you are still trying to find your way around. I've only engraved text and line drawings up to now.

So anyway, I created a rectangle bought a 22mm flattening bit and set the depth to 1.5mm (the change in levels I measured). Thats where I get into trouble. I selected the pocket toolpath. That was the only one I could find that came close to being a level. I set the vector outside the board dimensions. It all looked fine to me until I started to cut from the center out. It was noticeably off center. So far that if I let it continue to the edge it would have hit the limit switch on one side.

I put the board in the center of the machine. Set the X,Y and Z points to the left bottom (top?) corner. Bottom is where I see it on my screen. Immediately I started the cut (using Mach3) the spindle moved to what looked like the center and started cutting in a circular direction. At about 1/3 of the cut is where I noticed the whole cut was off center and likely to hit a limit switch. SO my question is:
What can I do to ensure I can actually design a rectangle to be cut using a 22mm Diameter bit that will not cut lopsided? Should I have set an offset?

All help gratefully accepted.

[Adrian]

What did the preview show? If that completes the cut inside the rectangle and the actual cut doesn't then that almost certainly shows a problem with the machine.

1.5mm is a reasonably aggressive cut for a large bit like that.

[sylvan356]

Mick Martin has created a free program to mill the spoil board. Assuming that your machine is set up properly use his little program. It works great! Get it at http://mickmartinwoodworking.com/cnc/downloads/.
Sylvan

[Leo]

Personally - I like to set "X"-"Y" zero in the center of the workpiece - NOT lower left or any of the other places.

You are experiencing one of the reasons I do that.

I draw a like from corner to corner - two lines - x, marks the center.

I also like to set my "Z" zero to top of workpiece.

I use a pocket routine and generally cut .032" (.8128mm) depth of pocket. Most dish, or bowl router bits of that size cannot plunge, so a ramp cut is needed. I generally ramp about 12" (305mm). I use an offset pocketing strategy in a pocket like that.

[ryadia]

I'll try to address each answer on one line each.
Thank you all for your excellent suggestions.

I downloaded Mick Martin's Spoilboard Wizard. I'll give that a try on a dry run before committing to using it. So far I'm comfortable with using an oversize pocket.

The aggressive cut is OK for my 2.5Kw spindle. It still leaves a reasonable surface finish and I'm not using huge direction speeds likely to bog down the bit. The spindle doesn't slow more than a few RPM.

I can see now why setting XY zero in the center makes sense for this type of cut. I have always set Z zero to top of the work. It fits in with my Vcarve use too.

The issue identified:
I'm not sure how it happened but the distance travelled Vs the actual distance moved had been altered in the software that drives the machine. I might have done it myself but its more likely the fellow I had using it was trying to get lettering to fit into a smaller area of acrylic than the design specified. It goes from one mistake needing corrections and the machine not being returned to its correct state. My calculations for sheet acrylic for best use of area get tangled up sometimes between imperial and metric measurements. I've previously used 8 feet x 4 feet sheets of acrylic (Perspex) but my latest purchase was an import from China. 100 sheets arrived in 2400mm x 1200mm sheets. I cut the pieces we needed slightly smaller, not thinking there would be any issues - but there was. We've had these problems in Australia for as long as we've been changing over to metrics — Nearly 50 years! I can remember my father (a house builder) ordering a bunch of doors that arrived in Metric sizes that didn't fit the doorways. So now the hungry bug of metrics has hit his son too!

Thank you all kindly,
Ryadia