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Automated CNC grinder

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Oznog

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I have a very odd but important task.
The needs take a bit of explaining. I participate in LARPs, Live Action Roleplaying, which essentially plays Dungeons And Dragons like paintball. Now one of my specialties is safe live action archery, and "light" crossbows are a very difficult question. Unless you're willing to use a dumb-looking pistol crossbow, there is nothing commercially available in the safe power range I require. Cutting solid fiberglass bars into a bow is a rather inefficient solution, without a recurve shape the shot is not as efficient and it doesn't look nice either. Curved aluminum is not a bad solution, but it's a bit heavy (makes the bow end-heavy), hard to shape, and somewhat prone to metal fatigue.

The best thing ever is these solid fiberglass, well-shaped prods on "flea market" crossbows. They're far too powerful as-is, but since they are not made of laminated layers, it can be carefully sanded into a new thickness & power. However, this is a very difficult job! A lot of material must be removed and if at any point you go just a bit too far it doesn't flex right anymore. The end thickness is not constant, it tapers thinner as it gets towards the tips.

So I dreamed up this idea of a CNC griding rig to do the job. I've got plenty of microcontroller experience. But there are quite a few hitches to that. The bow is bent even while unstrung, it curves back from the center and then recurves forward at the tips. Now that curvature is a bit of a problem- the thickness would probably by metered by spacing a roller and a griding wheel a specific length apart. If the span between then is not perpendicular to the material, it will bite in further than designed. I could use two rollers in back but wide spacings between them to solve the perpendicular problem will start to encounter issues with the curvature of the prod. Also I'm not sure how to move and measure a curved prod like this past the grinding head (or maybe the grinding head is a moving shuttle?)

To complicate things further still, the prod is not totally symmetrical. That is, if the curvature could be taken out and it laid flat, there would be a very slight "V" to its shape.

I'm almost thinking the thing to do would be to make a shuttle grinding head that is still hand-drawn across it, but have it continuously read out its physical location and adjust the thickness accordingly.

Any ideas?
 
G'day Oznog,
This sounds like a real complicated task but with my engineering experience I'd like to offer a few suggestions.

Firstly to gauge where the grinding head is in relation to the bow you could hook a probe to a sliding pot and read the resistance which was pre calibrated and secondly I'd make a travelling head with rollers positioned so it followed the contours. Another way supposing all the bows are the same is make up a jig to hold the bow and pre-bend a bar to suit the profile of the bow so the grinder can follow the path but still using the pressure head using the sliding probe. Hope you get my drift but I can draw a few designs of the head if you need it.

Cheers Bryan1 :D
 
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