The Cool Grips Project, Part 4

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Freshly poured dies with alignment pins, waiting to cool
The ultimate in extremely low-volume metal stamping.

In Part 3 of “The Cool Grips Project” I showed the very roughest prototype of a grip pressed in fixturing-alloy dies.  Based on the damage the dies sustained, this s not a viable production method.  However, it is a viable prototyping method, so the twin problems faced at this stage are the development of an accurate flat pattern and the alignment of that pattern in the dies.

The flat pattern was made by taking the CAD model of the completed grip, projecting it onto a plane, and then moving the corners around a bit.  I measured (with CAD) the distance along the curved surface and then stretched the projected drawing to the width corresponding to that measurement.  The bottom, where there is a curved crease, was much dicier.  I used multiple measurements, drew a curved line that I hoped would come out straight after bending and stretching, and just hoped for the best.  The resulting sketch was turned into a toolpath in CAM so that the mill could mark the outline for cutting.  Actually cutting remains a pain; probably production will require sending raw material out to a waterjet company.

Alignment is actually much easier.  Conveniently, the raw material for the grips has holes in a very regular pattern.  Working again with the flat pattern sketch, I drew a circle near the top front of the grip to serve as the reference point for the pattern.  That circle corresponded to a hole in the perforated stainless, and could be used as part of the flat-pattern cutout, as well.  I was then able to draw a series of circles along the front edge with appropriate spacing, and then another series towards the middle of the grip.  Since I know the hole size and spacing, ti is simple to work out where the middle row would be.

Now I had a reference point for cutting the flat pattern AND a series of holes down the middle to force the pattern, once cut, to sit correctly in the dies.  The first attempt to modify the existing die molds was a mess; I failed to place the origin in the right place on the CAD model, so the holes were drilled all wrong.  It was necessary to start over by cutting all new molds.

Having done that, I placed 3/32 music wire into the holes of the female side of the die, and on the male side, I put small brass tubes around the wire to receive the music wire in use.  After pouring, the dies were immediately tested on the new flat pattern, and the initial result is promising.  It remains to measure springback–after extracting the prototype from the die it is currently trapped in.  Probably it will also pay to redesign it so that such capture is no longer likely.

The latest prototype, so firmly fixed in the die that it will have to be melted off (and modified so that it doesn’t happen again!)


Part 3 | Part 5

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