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Bullet shaping

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klugesmith

Fri Feb 08 2013, 01:25AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Mr. Y:
Looks like you have made some exceptionally stubby (small L/D) bullets.
Have you tried launching steel balls?

Are standard steel dowel pins pointy enough for you? It looks like they can be purchased very stubby. 50 pieces for less than US $11:

[edit] Please let me know if the low-level McMaster-Carr links don't work.

Yandersen

Fri Feb 08 2013, 03:20AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Steel balls were in my mind once, but they will roll reversing the polarity of magnetization from stage to stage, giving unpredictable results each other shot. My experiments shown a little difference from reversing the insertion direction of a bullet each other shot (permanent magnetisation occur), but the effect of wrong coil polarity is much much stronger. Rolling magnetized ball will make the same huge negative effect.
Shortly, bullet must not be able to reverse direction during acceleration in a multicoil CG. Moreover, my simulations shown that for my current setup (17mm coil length, 11.1mm bullet caliber) the best efficiency may be expected with 15mm projectile length - shorter will not be good.

Links work, thanks, Klugesmith - good idea! "Hardened dowel pins â€” press fit" are very close to the desired bullet shape, howhether, the roundness of the tip is unmentionable. But I really want the fully rounded tip - simply to make reload mechanism working without jamming and crushing the front enter of an epoxy-paper made barrel. Smg like MDP20-7 DOWEL PIN can be cutted with my rotary tool, but 2pc of those will cost as much as a 1m iron rod, and 50pc will come to the 1/3 of what I can spend on lathe (and still, it need to be cutted, barrel and clip redone to the 10mm caliber...). And I want to play with lathe too, actually. I'm going to visit some workshops nearby next week - if I will not find a solution, I will try to make myself spend ~$700...

2Spoons

Fri Feb 08 2013, 05:10AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

We have a lathe in our workshop. But I'd use the drill + bench grinder in preference - much easier to control. Doing curves on a metal lathe is hard without CNC.

With the drill + bench grinder method it shouldn't take more than a minute or two to make a bullet. Just take care stray sparks don't ignite your apartment!

klugesmith

Sat Feb 09 2013, 01:39AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Couple more ideas for bullet-shaped (

) projectiles without lots of work.
1) start with thick nails.

2) connect round-head screw and threaded standoff.

The hexagonal standoff could be spun using a rifled barrel.

For the rifling, start by twisting a long hexagonal bar. That can be used as a mandrel for paper-and-epoxy composite barrel. Or it can be connected to a cutting tool that is pulled or pushed through a round barrel, while twisting. Here is how the pros do it.

Yandersen

Mon Feb 11 2013, 06:17PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Funny. After all, I bought a bench grinder (

:)
Hopefully the thing turned to be totally silent, howether, some vibration exists, so some rug underneath is required.
Let's see if I will be able to get the desired round bullet tip shape with it...

Overall I'm satisfied - process doesn't take much time, not too much noise, result is... as good as the hands are. Mine are OK. Will work on that, make some more bullets, and, maybe will post some pics.

Ash Small

Mon Feb 11 2013, 09:34PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Yandersen wrote ...

Overall I'm satisfied - process doesn't take much time, not too much noise, result is... as good as the hands are. Mine are OK. Will work on that, make some more bullets, and, maybe will post some pics.

Practice makes perfect

Yandersen

Mon Feb 11 2013, 10:00PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

So here is how I mounted the bench grinder: I took a piece of wood, screwed the device to it and put a sticky standoffs underneath to fasten the assembly to the table:

The bench grinder shown here has the following design flaws:
1) mounted lamp got powered only when the grinder is on, even though it has independent switch;
2) motor is very hot - no ventilation holes I found in it anywhere (dust prevention?).

Then I took the steel rod, rounded the tip, rotating the rod in hands (too long to put inside the drill, and actually if a piece is held in a drill, it jumps away from the grinder once touching the wheel).

Then I cut the end of a rod with a hacksaw (I use a rotary tool to make a notch so hacksaw doesn't jump out damaging the surface around the cut line), 16-17mm in length (15.4mm desired), mounted this little bullet in a drill, flat side upfront and grinded the excessive length to make it precisely 15.4mm (to make it fit into a clip which is done already). Then I put the bullet into drill, rounded tip out, and using a fine sandpaper made the tip smooth and shiny.

As a result, I have 3 bullets ready:

(~10g, 15.4mm in length, 11.1mm diameter).

It took me few hours, but the tips are just as round as I think they are - no precision in that part of a shape, which upsets me a little. Any ideas how to make bullets' tips precisely round?

Ash Small

Mon Feb 11 2013, 11:55PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Yandersen wrote ...

It took me few hours, but the tips are just as round as I think they are - no precision in that part of a shape, which upsets me a little. Any ideas how to make bullets' tips precisely round?

Practice

klugesmith

Tue Feb 12 2013, 01:20AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

If you were doing the work on a lathe, you'd want a Ball Turning Tool.
Here's one:

It includes pictures of the results, and you can easily find online videos of the process.

I just sketched a variant, for use with a bench grinder. The steel rod workpiece is kept long, while one (or both) ends are rounded. It takes the place of the single-point cutting tool in that ball turning link.
Workpiece is free to rotate around its own axis, but has a collar clamped onto it to control the distance from grinding stone.

This would probably work better with another pivot pin on top, so the swivel block has only one degree of freedom. In any case, the fixture parts need to be strong, stiff, close-fitting, and solidly clamped in place. Beware of setups where the fixture, workpiece, or your finger could be grabbed by the wheel and pulled into the gap.

Yandersen

Tue Feb 12 2013, 02:24AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Wow, you are The Engineer! Thanks! I'll think what could be used to make such a thing... But I still dream about lathe with ball turning tool as an ultimate solution - maybe, when I finish my coilgun I'll finally make myself buy the machine to make a surplus of good-quality bullets for the gun. Anyway, so far I got 3 bullets and it should be enough for test shootings and checking of reloading mechanism - hardly my gun will be finished earlier than in half a year... :)

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