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Okay,
so I'm on a real budget. SS Disability pay. I can't afford some expensive Cam software. I do have Lazy Cam though. So I'm thinking that if I do a section view of both ends of the neck contour profile and plot the Z values from both and use constant velocity between the two on the X axis for a given distance, I should get a constant slope and Y+ and - angle as well between these, and thus a reasonable facsimile of the contour. Using a 1/16th ball mill and using say .010 offset this should be very close.

What say you? If someone has a better idea I'd love to hear it? Or maybe see some of your code?

Mike

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I'm outside looking in, just farther from the window than most.

Offset both of your profiles by 1/32" (ballnose radius) in CAD (or by hand, if they're arcs, use the same centre and change radius to be bigger) and then move them straight down 1/32" in the Z axis (ball radius again). This will give you a curve defining where the cutter end position should be to mill each curve.

Run passes between the two curves generated by that method and you've got a proper toolpath.

That said, doing it your way won't be off by a massive amount: it'll be off by 1/32" on vertical walls and less so the more horizontal the position on the curve.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Thanks Bob,
What you said is what I was envisioning, I just didn't quite communicate that as clearly as you did.

Also I will be using a 1/8in ball end with 1/16th cut depth and 25% to 30% offsets.

Again, thank you for the reply.

Mike

BTW, Here's wishing you a Happy Thanksgiving.

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I'm outside looking in, just farther from the window than most.

I don't understand the % offsets you're talking about? If you're talking about stepover then those will work (though I'd put them even closer with a 1/8" tool). If you're talking about the curve offset then both of the offsets I spoke of have to be equal to the cutter radius (to produce the correct geometry).

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Mike is a 1/8" bit as large as you can use? I was always under the impression you would get a better finished surface if the cutter was larger and the stepover was small.

Maybe Bob can shed some light on this, since he has experience with this.

Mike

On a basic level, scallop height is what matters. The scallops are the little sharp pieces between two passes of a ballnose (essentially, they're what the cutter misses). The shorter the scallops, the easier it is to sand the piece.

On wood, a scallop height of 300 micro-inches (0.0003") sands level very quickly, and a scallop height of 100 micro-inches is pretty much finish-sanding ready. A 100 micro-inch finish is something you only go for on the machine if you have very specific knowledge and technique; even the manufacturers do something in the 300-500 range (or coarser) and finish sand.

The Ra roughness, usually used in machining specs, is half of the scallop height. Also note that the finish, relative to stepover, changes based on the angle of the surface. A stepover that gives you a 300 microinch scallop height on a flat gives a 600 microinch finish on a 45 degree. Essentially, this means that the finish of your neck will be best on the back and worst on the sides if you use a uniform stepover.

Bit size - Ra250 Ra125 Ra50
3/4" - 0.039 0.027 0.017
1/2" - 0.032 0.024 0.014
1/4" - 0.022 0.016 0.010
1/8" - 0.016 0.011 0.007

As you can see, the stepover VS the finish can make a huge difference. Using a stepover that's 3/4 of what you were using theoretically cuts your sanding time in half (so far as material removal thickness is concerned). Using a 0.03 stepover (25%) with a 1/8" cutter will give you a Ra900 finish which will be doable, but very hard to sand. If you went with a 0.015" stepover, I think you'd be much happier with the amount of time you won't have to spend sanding, but it depends on your parameters. On a neck shaft, if you have to, you can use a pretty coarse finish and smooth it up with a file or scraper pretty quickly.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Great info Bob!

Thanks!

Anthony

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Anthony J. Lembo
Lembo Custom Guitars - www.Lemboguitars.com

Thanks Bob for clearing that up for me. It sounds like I need to change the stepover to about 5% or 10% at the worst. One thing I do have is Time so if it takes longer no problem.

That clears up more than you know for me.

Thanks again,
Mike

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I'm outside looking in, just farther from the window than most.