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Ok, I am thinking of cnc'ing some moulds for my guitars, both for support and for the bender. I have Rhino 4 and was wondering how I take my drawing (I use my own shapes) and transpose that to CAD? Should I just trace the outline onto large graph paper and us x,y coordinates to make the outline? How many points would be appropriate..any other (good) ideas?

Thanks

Shane

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Canada

I am not a Rhino user but regardless the most acurate way is to create a point map based off of a given location as the Zero datum (center line at butt or neck joint). 1/4" incraments will do the job pretty well. Do this on graft paper and give and X and y dimension from this datum and creat your point map. To put it into cad recreate the points then create a curved spline that contains all of these points. The smaller the X distance betweem each map point the greater the accuracy.

Also the small units you use to measure each of the points from the zero datum to the X-Y coordinate for each point when creating your point map the more accurate you will reproduce it in cad. This is a place to use MM as your units when measuring the point locations. But what ever unit you use to physically measure be sure to use the same unit of measure when you set up the units in cad.

You could digitize your molds with a probe in your CNC, Shane, but may not be worth the trouble.
The methods described above will give good results and you can always tweak the CAD drawing to match the actual mold.
Laying the mold on a full scale drawing will tell you quickly how close they follow each other.
A series of plots on small format and taped together aligning witness marks will suffice in the absence of a large format printer.
Just make sure the printouts are to scale. Another thing to watch is humidity change as it will quickly change the scale on the paper.
Nelson

Tracing and scanning into a tiff file then importing the tiff file into cad then tracing over the scanned image is another method. That said I donot like this method. The problem I have with scanning a tracing is that the scanned image is subject to the accuracy and of the scanner being used. If scanned in on a flat bed scanner the results are usually pretty good but this large of an object you not likely to find a flat bed scanner but rather a feed type scanner and the gearing and tension of the feed mechanism can significantly alter the scanned image compared to the actual tracing.

Shane,
I've had good luck working off of scans. The best way to do a scan is to run to Kinko's and have them scan it in for you on a large format sheet feed scanner. They can burn the resulting tiff file to a CD for you as well (or they could probably load it onto a zip drive as well). It costs a little under $20 to do this, but it is as accurate as you are going to get with a scan.

A less accurate method is just to take a digital photo and use this is your raster image. In both cases you can import the image into Rhino and then trace over it. Rhino has some nice features that let you detect "bad spots" in the profile.

Here's a tutorial I did awhile ago that shows how to do this:
http://www.luthiersforum.com/forum/viewtopic.php?f=10106&t=24094

Using a touch probe to digitize the part and then "connecting the dots" in CAD is also a great way to do this, but even with that you have to clean things up/reinterpret them in CAD.

Trev

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http://www.PeakeGuitars.com

Trace it on graph paper and either manually enter the points or scan it and then scale the image to correct for error. The scanning 'stretch' error will be pretty consistent, if present, and a simple Cartesian scaling will be more than sufficient to correct it.

At first just take a bunch of points, but as you get more used to reverse-engineering you'll be able to figure out how many you need and where. Your goal will be to eliminate as many as possible, once in CAD, while retaining your shape as minimal control points create the smoothest curves.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services