aligning parts question

[sdantonio]

I'm looking more for an idea of how to approach a problem rather than a specific solution.

I will be working on a numbr of 2 sided parts on a CNC router based on Joe's 2006 machine. Lets take a complicated example first, that of a 2 side PC board. Second example would be a violin neck where you have a left and right side that are mirror oposites.

Cutting one side is a no brainer in either example. How does one go about aligning the part so that the cut in the second side mirrors the first (in the violin neck example), or so that everything actually lines up correctly (as in either case) and isn't offset?

I realize that "very carefully" is a legitimate answer, but I am looking for ideas that are a little more in detail than that .

Attached is a finished neck to gove you an idea of what I want to do. Immagine this starting from a maple 2X4, cutting one side, then flipping it over and cutting the other. Once you cut the first side you loose any flat surfaces for laying on the router table top, though a cradle can easily be cut in a scrap piece that would approximate the curve and allow alignment with respect to the z axis.

Thank you
Steven

[lakeside]

your thought of nesting the part if fine add a few dowel pins for location with circut boards you can can just go off a corner

[ger21]

Make a fixture with a matching carved recess to hold it in place. A vacuum fixture would be ideal. Didi you watch the videos someone linked from Taylor guitars? They showed an aluminum fixture for holding carved tops face down while routing out the backs. Same principle. Make sure you use some alignment pins or something to accurately locate the fixture.

[ger21]

Try this. http://www.taylorguitars.com/video/f...=14%3a+Tooling

[pastera]

For the PCB - you punch alignment holes in the board before you start to cut.

For the violin neck (or any 3d) just cut a negative for the fixture - cut one coat with release and pot into a bed of fixturing metal (cerrobend 160) or plaster.

Aaron

[Geof]

The peg holes must be through holes. Do these holes when you machine the first side; this means you know the location of these holes relative to your work coordinate location. When you flip the part for the other side use these holes to place your work coordinate in its correct location.

This is a general way of doing it; choose a feature that can be created during the first operation and which is accessible when the part is set up for the second operation. Sometimes it is practical to simply drill two holes that are just used for determining position and alignment. This is the case with PCB's or things were extra holes do not matter. The only important thing is to know the position of these holes relative to the work coordinate.

The Cerrobend suggestion for fixturing a 3D shape is very practical. You might also look into using Typemetal (if you can still get it these days). Cerrobend shrinks during cooling but some formulations of Typemetal are very low shrinkage or they even expand. The melting temperature is higher than Cerrobend but still okay for wood; this is what they were originally cast against scores of years ago.

[One of Many]

Your approach will depend on the format of the geometry that you prefer to use to create your tool paths.

One method might be to draw the mirror images side by side with a known offset. This same offset would be included in your fixture, tooling points etc., in reference to stable datums on the part, outside of the part or on the fixture itself. A repeatable nest can utilize part features, points and or holes that can be taken advantage of, from previous operations. Even if a portion of those features are to be removed later, such as tabs, lugs or gripping tongues. This is all part of planning ahead, yet not letting otherwise net material costs corner you into difficult if not impossible to produce operations. 2" of excess material to facilitate holding a good part will yield better results than dinking around with net material turned to scrap.

Another method may be as in a 4th axis setup that maintains the proper alignment throughout the paths as each side is presented to the cutter access about a common centerline.

The main concept is to use the machine to create or reference the nest, then reference the tool paths based on that included location to cut in the remaining features as associative geometry in subsequent operations.

DC

[ironDigit]

you didn't say you couldn't use/get a 4th axis but that seems like the ideal solution for the violin's neck. the labour invested in stopping and "very carefully"turning around the stock would sure justify the purchase of a 4th axis if you're doing this on a commercial basis if its a on off part any of the above mentioned methods would probably work

good lukk m8

[Walt@SGS.Inc]

What if the material used were to start out being 3 inches too long and the
extra length was used to clamp down the part. When the first side was
completed, turn the part over, clamp the 3 inch extentionlike it was before.
To help elimanate chatter, have an adjustable pin come up to wedge the
unsupported end of the part.
Maybe even make the part 3 inches too long on each end, clamp the
ends down, machine one side, slip it over, machine the other side then
cut the ends off .
Just some thoughts
Regards Walt

[sdantonio]

Walt,

Thats a good idea. On parts I have complete control over then I can do that (and was thinking along those lines myself). On parts where I buy a matching neck that goes with the back, most manufacturers don't give you anywhere near that much extra (maybe a half inch if your lucky).

[Walt@SGS.Inc]

What if you start out with a block of wood that is 1/2 inch longer than the
finished product. Just clamp it in a vise end to end the long way of the part.
When done with the one side, flip it over and run "mirror image" to make the other
side.
I made a slip in the last post should have said flip not slip.
Of course, you would need a vise or a clamping system to span the length
of the supplied blank part.
Is this router equipted with a vertical spindle or is it horizontal?
I worked on a router from France or England that had 3 spindles (vertical)
and the table was something like 20 wide by 65 or 70 long.
O yes, did I mention those dimensions were in feet.
This machine was in York Penna.
I wonder how many parts the type you are thinking about could be set up on
that table????
Regards, Walt

[ger21]

One thing to keep in mind when working with wood. Holding just the ends in a vise will result in a lot of vibration. Our router at work uses a vacuum pod hold down system. When the router is cutting directly over a pod, there is practically no noise generated, as that portion of the work is held securely against the pod. However, as the tool moves off the pod to an unsupported area, there is sometimes an incredible increase in noise, as the wood is vibrating while being cut. This is much more pronounced as the tools get dull.

For best results, a fully supported vacuum jig would be best.

Just an observation from someone who routes a LOT of wood on a daily basis.