SpudFiles

wyz2285 wrote:How did you mill the spiral fins? Ball end mill and rotating table?

Yes set the rotary table axis at 45 degrees to the direction of travel of the mill, mill one groove, turn rotary table 72º, repeat, and you will have 5 spiral fins.

How did you synchronize the rotation speed and the mill travel speed?

You don't have to, give me a minute let me draw a pic to explain.

No need, I already got it, so they're just 45º slots

Pretty much - but because of the curved face of the ball mill you get a nice spiral groove.

But then the slot in the ends will be less deep then in the middle I think.

Yes, it's not a way to cut a "proper" spiral - but for the purposes of fins, good enough.

You could get a step motor for the rotating table and mill cross table

Now that it will become part of my livelyhood, I will probably invest in some CNC some time in the near future.

Final package for taofledermaus before I pack my tools up

jackssmirkingrevenge wrote:Now that it will become part of my livelyhood, I will probably invest in some CNC some time in the near future.

Not so therapeutic anymore is it

Gun Freak wrote:Not so therapeutic anymore is it

It still is... but CNC allows for things which would be nigh on impossible to machine manually.

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You've probably seen this paper before, JSR, seeing as how it's the first Google search result for "sabot design", but it has a neat little dynamic analysis that explains exactly what went wrong with your first batch of projectiles:

http://www.dtic.mil/dtic/tr/fulltext/u2/a056428.pdf

Basically, that configuration (contact patch aft of CG) is unstable in yaw. Obviously a longer contact patch will be more able to resist this, but that tends to work against the principle of reducing sabot mass.

There's also the more obvious failure mode involving the core penetrating the sabot owing to insufficient mechanical strength in the sabot. Have you investigated your safety margin against that happening? There's a good chance you may have been seeing a combination of the two failure modes I mentioned.

Yep, I should have known better, those first rounds were a combination of lazy machining and mindless optimism.

DYI wrote:There's also the more obvious failure mode involving the core penetrating the sabot owing to insufficient mechanical strength in the sabot. Have you investigated your safety margin against that happening?

That's what happened here:



On acceleration, the core slipped through the delrin tail (where it was only a push fit) and dug into the shot cup.

Both factors have now been taken into account though, using more supporting sabots to stop the projectiles yawing in the barrel, and delrin disks to reinforce the base of the shot cup.

Here's the latest batch for demolition ranch, I've been machining like crazy lately to reduce my material stocks before moving to Quebec

Here's the latest fabriaction, fitted with another set of find approximately at the projectile centre of gravity, so that it can hopefully be fitted directly in the shot cup without the need for extra sabot parts: