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sharpening pvc

A place to ask general spud cannon related questions.
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sharpening pvc

Unread postAuthor: mikespahn » Mon Apr 05, 2010 5:14 pm

is sharpening more effective for breech loading potatos if you chamfer the inside, outside, or both?
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Unread postAuthor: mark.f » Mon Apr 05, 2010 5:40 pm

I usually use a long outside taper and a short, shallow inside taper.
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Unread postAuthor: geniusbomberman » Mon Apr 05, 2010 11:04 pm

a file is all i needed, but it took a while. i filed it a lot on the outside and a little on the inside.
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Unread postAuthor: jor2daje » Mon Apr 05, 2010 11:12 pm

I find a shallow outside taper like mark.f said works well because its the inside diameter you want cut the potato to. An inside chamfer would cut larger than your barrels bore making a tighter fit.
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Unread postAuthor: mikespahn » Mon Apr 05, 2010 11:33 pm

jor2daje:
that makes sense, i don't know why i didn't think of that.

geniusbomberman: no need, i run a machine shop, i can make just about any shape on anything. it's just a matter of deciding on the shape. i'll shape it on a lathe.
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Unread postAuthor: gabeb123 » Tue Apr 06, 2010 4:36 pm

i do both it on the outside but then you should do a little on the inside so you get a tight fit.
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Unread postAuthor: jimmy101 » Tue Apr 06, 2010 6:52 pm

A small amount of inside bevel will usually increase the performance of a combustion spud gun. The increased friction delays the start of movement of the spud by a little bit and allows the chamber to reach a higher pressure. It is kind of counter intuitive but that's generally how it works.

Edit:
HGDT is great for this type of question. A quick calc suggest an increase of about 10% in muzzle velocity comparing a very small amount of friction (as from no internal bevel on the knife) with the optimal amount of friction (from a small inner bevel). The friction force needs to be pretty large. Figure something in the vicinity of say 30 pounds of force in a 2" diameter barrel (~10PSI friction in HGDTs way of defining friction). In other words, ramming the spud home won't be easy.
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Unread postAuthor: psycix » Thu Apr 08, 2010 10:05 am

Some inside taper makes it tighter.
However, too much inside taper will cause the spud to be mashed when inserting it into the barrel.
Don't taper it too much on the inside.

So: a little tapering on the inside, and the rest outside.
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Unread postAuthor: maxa1 » Tue Apr 20, 2010 5:03 pm

yes i find it easier to get the potato when it is sharpened unless i have tendency to smash the potato
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Unread postAuthor: Gun Freak » Tue Apr 20, 2010 5:17 pm

maxa, I'm sorry, but that post was useless.
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Unread postAuthor: maxa1 » Tue Apr 20, 2010 6:30 pm

i know i was just giving my opinion on to sharpen or not
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