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9mm design feedback requested

Posted: Mon May 04, 2020 9:17 pm
by ddmm
Hi all,

Thanks so much for all the info on here, it’s been awesome to read through so many builds. I’m waiting on my first form1 to clear and trying to prep everything in the meantime. I’d love to share my designs and limitations and see if anyone out there has thoughts on it. I don’t have access to a mill or lathe, so I bought a solvent trap kit with k baffles, they're a little bit chunky, but the finish is pretty nice. Once Form1 clears, I need drill bore, skirt vent, and face vent. I will 3d print jigs to hold a steel drill bushing to guide the bit and hold the baffle securely. This will make my end result slightly different from what is recommended on here I think. More on that below, my biggest questions in bold.

General design intent: 9mm subsonic, 8" OAL, 1.375" diameter. Bore drilled to .406" (1)too small? could go up a bit to .422". (2)Should end cap be bigger?

SDTA booster

1" SS blast chamber/spacer

1 - SS blast baffle (3)should I drill an additional 4 holes around the face? If so, any recommendations on size?

5 - AL baffles

Vent hole drilling (bits visualized by blue and purple): both operations will be done with a 3/8” drill bit.(4) Is this the right size for 9mm?

The face will no longer be flat, the drill bit will leave a pointed indentation. (5)Does anyone think this will be a deal breaker? I could fill it in with something high temp maybe…

The halfmoon vent is .375” x .22” (6)does this feel about right or is it a bit too big?

(7)There isn’t any overlap between the two operations. Is this okay?

(8)Is it worth trying to come up with some alignment system? I could drill a 1/16” hole near the edge of each baffle and run a long hardened 1/16” pin through all to maintain alignment.

Pictures below, many thanks for anyone with the time and experience to comment on this!

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(note the sharp edges, I plan on softening these with the dremel)

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(note that the operations do not overlap, I read once on here that they should, but wasn't sure if this was widely accepted)

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(drilling jig models)

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Re: 9mm design feedback requested

Posted: Tue May 05, 2020 7:24 pm
by Historian
DDMM,

Compliments on your notional design
presentation. Clever innovative holder.

You are certainly going to have mega fun in the coming
years with the talent that can be inferred from
your drawings.

It brings a smile to our aged faces to see how the
new young wave is using technology that 50 years ago
was considered 'science fiction'.

Wait until you get a lathe and milling attachment.

Then you will know what it is to be greeted with an ear shattering
and resounding "Tool-Junkie_Alert". :)

Best.

Re: 9mm design feedback requested

Posted: Tue May 05, 2020 8:24 pm
by #40Fan
I like what you are doing!! I was trying to buy a couple K-Cup "filters" off of eBay to make drilling jigs for, but they were snagged by Customs.

Couple suggestions. Make the jigs slightly undersized. Warm up one of the cones to where it is just warm enough to deform the plastic, but not too hot that it will melt the filament into its liquid state. Drop the cone into the jig and put both halves back together. Should give you a tight and repeatable jig. Can even do the same with the drill bushings.

The press-fit bushings are the cheapest I have found and continue to use. If you're needing some, I'd be more than willing to help you out. They are all 5/8" O.D. with a 1/2" long shank. They do have a head on them though. I have 3/8", 25/64", 13/32", 10.5mm (letter Z drill bit), and 27/64". $13 is the most expensive one and $4 shipping will get up to 4 of them to your door.

Re: 9mm design feedback requested

Posted: Tue May 05, 2020 8:42 pm
by T-Rex
1) The 9mm Ks I make (home brew) have a .386 bore, but these are precision machined. In your case, I wouldn't go smaller than .415. Really hard to tell because your jig isn't in my hand to check. I head our 3D prototyping group so I'm versed in this technology. Be careful with your filament choice, printing temps, tolerances, machining temps, etc. Regardless of how well you design it, all these temps can and will influnce the jig's precision.
2) I usually cut end cap a hair larger.
3) The dater hole is for FRP reduction. You want to experiment, go for it, but it's a tried and true feature.
4)&5) 3/8 is ok, but use a ball mill, not a drill bit.
6) Size is ok, but use an end mill. (hard to tell)
7) Everything is dependent on the design, which you're stuck with. Yes, the 2 features should be cut deep enough to pass a plane perpendicular to the bore. Gas needs to jet across, through the vent, and into the coaxial volume.
8) Not necessary. Just stack them, as best you can, slip tube over, and close.


A couple points of note.
-You want the blast chamber small...small
-6 baffles is not enough, especially for an 8in 9mm can