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Designing Inlays and Neck Pockets for CNC: Gap Amounts?
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Author:  BobK [ Sat Jan 29, 2011 11:31 pm ]
Post subject:  Re: Designing Inlays and Neck Pockets for CNC: Gap Amounts?
The answer is it probably depends on who is machining your files. A guy with an expensive machine and a spindle will machine to much higher tolerances while a guy running a router might not need any additional clearance due to runout in his router. For inlay I use .005". It's not a press fit, but the gap is almost invisible. CAM packages allow one to resize vectors if necessary, but I've found that mine can produce less than predictable results. Now I create two separate CAD files for inlay and pocket and can inlay complex shapes with no problem.

I've never machined a neck pocket but I just picked up a pretty nice figured maple billet from Woodcraft today, so I'm curious about that as well.

Bob
Author:  Bob Garrish [ Sun Jan 30, 2011 2:29 am ]
Post subject:  Re: Designing Inlays and Neck Pockets for CNC: Gap Amounts?
I've done pockets on size before, but it's a little scary having inlay as a push fit where thin CA won't even wick in. I do them with a 0.001" to 0.002" gap now, depending on complexity of the design and production rate. If it's a bunch of boards with simple designs, then the extra thou of clearance makes the installation faster.

On something like a neck pocket, the dynamics of the tool probably have more to do with it than what the CAD says. I use the same clearances as inlays, but different machines can be all over the place. What makes things even more unpredictable is that the 'proper' offset is a combination of the machine's issues along with the real required clearance. A low-stiffness machine with a router for a spindle might have pockets coded right on size when the deflection of the gantry evenly offsets the runout of the tool. If running slower, it might need them programmed undersize.

I'm curious as to what happened with your headstock? It doesn't seem like something where a thou or two either way should make much difference? If you got it reverse engineered, that might be tough as there are all kinds of places that things can get distorted there. If it was from CAD then odds are that the problem's a machine/machining problem rather than a CAD problem.
Author:  npalen [ Mon Jan 31, 2011 12:08 pm ]
Post subject:  Re: Designing Inlays and Neck Pockets for CNC: Gap Amounts?
Thinking a little on the lighter side brings visions of variable gaps to simulate hand cut inlay.
Someone should write an algorithm to simulate hand cut inlay.
One of the parameters should be a choice between beginner, intermediate and expert <g>
Nelson
Author:  Mike Kroening [ Mon Jan 31, 2011 6:07 pm ]
Post subject:  Re: Designing Inlays and Neck Pockets for CNC: Gap Amounts?
Bob Garrish wrote:
>snip<

I'm curious as to what happened with your headstock? It doesn't seem like something where a thou or two either way should make much difference? If you got it reverse engineered, that might be tough as there are all kinds of places that things can get distorted there. If it was from CAD then odds are that the problem's a machine/machining problem rather than a CAD problem.


Bob, what Chris is referring to is, he was not aware that the vectors needed to be closed. He sent me the CAD files and I had to do a lot of clean up on them :)

Just a lesson for him as he progresses with using CAD and CAM software. :)

Mike
Author:  Bob Garrish [ Wed Feb 02, 2011 9:51 am ]
Post subject:  Re: Designing Inlays and Neck Pockets for CNC: Gap Amounts?
npalen wrote:
Thinking a little on the lighter side brings visions of variable gaps to simulate hand cut inlay.
Someone should write an algorithm to simulate hand cut inlay.
One of the parameters should be a choice between beginner, intermediate and expert <g>
Nelson


You can go analog on it. Use a dental burr on a grinder to match tools, and use one of those really cheap hardware-store CNCs to simulate the inaccuracy of the meat :)
Author:  Stuart Gort [ Wed Feb 02, 2011 12:42 pm ]
Post subject:  Re: Designing Inlays and Neck Pockets for CNC: Gap Amounts?
It depends on the materials but for my wood on wood inlays I cut the males net and ususally end up with the female cavities being between .003" and .005" larger. Often the concentricity of the tool and tool holder accounts for these numbers. I think a perfectly concentric tool would give me the fits I like at .002". So, I have to sneak up on these fits and then, once established, I can run all the parts without worrying about wear.

It also depends on what glue I'm using. For little wooden inlays I use a 50/50 water and Titebond solution through a syringe. It's so thin it goes into pores as the inlay is pressed into place and there is usually no need for a clamp once the inlay is in its proper position. If I make the glue thicker, I have to make the fits looser or I will create a "piston/cylinder" situation and the inlay will press outward from the pressure.

On my neck pockets .005" will usually do it. I dont' dare make it tighter because the neck pocket on my bodies are not surrounded by very much wood. Forcing a fit might crack the body so I get it to where it barely slips in, and then compress it in a form fitting jig to press out the glue. I also have a stud set into the neck tongue that allows me to tighten a small nut inside the pickup cavity which draws the neck into the body as the glue dries. This is an effort to really avoid having to make the neck body fit as tight as is conventionally preferred. I'm trying to apply direct pressure on all the gluing surfaces of the tongue and cavity doing it this way. It seems to work well though I don't intend to ever make a direct comparison to another method.
Author:  Parser [ Wed Feb 02, 2011 8:23 pm ]
Post subject:  Re: Designing Inlays and Neck Pockets for CNC: Gap Amounts?
I think about a .003 oversized cavity is typical for a single piece inlay cut on a good quality mill (Fadal, Haas, etc.). You'll probably need to allow more gap for a multi-piece inlay. The tolerance stackup will really get you on these...if you do ever design inlays with many parts, do yourself a favor and keep the individual parts big. It's a real pain in the you know what to cut really little parts...!

For a PRS style neck pocket - I would recommend machining the neck pocket undersize by about .005 and hand fitting the neck. When you cut the neck pocket, glue a neck in there within a day or so - even in a humidity & temperature controlled factory the bodies move around within a day or two. Typically, the neck pocket will "open up" and you'll have to clamp across the horns in order to get the neck pocket to close at that point.

Hope this helps...have fun!
Trev
Author:  Mike Kroening [ Wed Feb 02, 2011 8:35 pm ]
Post subject:  Re: Designing Inlays and Neck Pockets for CNC: Gap Amounts?
Parser wrote:
>snip< When you cut the neck pocket, glue a neck in there within a day or so - even in a humidity & temperature controlled factory the bodies move around within a day or two. Typically, the neck pocket will "open up" and you'll have to clamp across the horns in order to get the neck pocket to close at that point.

Hope this helps...have fun!
Trev
+1 on glue up. I personally when doing glue ups make the cuts only on the same day as the glue up.
As far as tolerances, I have found somewhere from 0.003 to 0.005 works with most woods.
MK
Author:  Fleck [ Mon Feb 21, 2011 3:59 am ]
Post subject:  Re: Designing Inlays and Neck Pockets for CNC: Gap Amounts?
Quote:
Typically, the neck pocket will "open up" and you'll have to clamp across the horns in order to get the neck pocket to close at that point.

Yup!
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