[cgallery]

Hi.

I have a small K2 prototype. I'm using Mach3 and a licensed copy of LazyCam (call me crazy, it is okay for my needs).

My "spindle" is a Bosch Colt.

I'm using a USB motion controller from CNCDrive (a UC100). Love that thing.

My question is in regards to drawing dimensions vs. the size of pieces my machine cuts.

For example, this last weekend I was cutting some small pieces of MDF.

My drawing called for the piece to be 3.75" wide and 6.1875" high. But it cut the piece to 3.764" wide and 6.2" high.

That is .014" and .0125" oversized, respectively.

I know this isn't a LazyCam offset problem BECAUSE even when I don't use offsets, the cut sizes don't match the drawing sizes by the same amount.

When I setup the piece with LazyCam, I did an OUTSIDE lead-in and CLIMB MILL. I always leave CLIMB MILL checked, as that is the default.

I think part of the problem is with the climb cut, as it tends to push the bit AWAY from the piece you want, instead of pulling the bit into it. I have done work w/ climb cutting unchecked, and my recollection (it has been a couple of months) was that the parts were within .001 to .002 of what I drew.

But I guess my question is, are there any tips for fine-tuning this stuff? Should I keep climb cutting but add some compensation somewhere? Should I not use climb cutting?

TIA.

[ger21]

THere are a lot of factors at play here. A lot of it has to do with machine rigidity.
Personally, I almost never climb cut in wood. You'll usually get a better finish with conventional cutting.
As you noticed, climb cutting will typically result in slightly larger parts, while conventional cutting will result in slightly smaller parts.

If the error your seeing is consistent, then I think your best option is to add that offset to your drawing. Split the difference, and you'll be within a few .001's.

[cgallery]

Yeah, I know if my machine were more rigid, this would be less of an issue.

I guess I will switch-off the climb cutting and take some more measurements.

BUT, when would I want to climb cut? I've done plastics (acrylic, polycarbonate, Corian-type stuff). Are there any rules of thumb in regards to when to climb?

[ger21]

Plastics and aluminum tend to cut better climb cutting.
With wood, the only time I climb cut is when cutting a rabbet along the edge of a board, with the grain.

[cgallery]

The climb cutting thing is pretty interesting to me. I've been an amateur woodworker for quite a while and am accustomed to referring to a climb cutting only when working at the edge of material.

It has taken me a while to wrap my head around how climb cutting actually refers to bit rotation direction, vs bit travel (releative to workpiece) direction.

I still think I may make-up a climb cutting quick reference cheat-sheet.

Thanks for all your help, Gerry.