Alright CAD gents, help me out

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BoredCop@reddit

Damned good question, I have been wondering how to properly model a helical cam track as I've been on and off working on a weird bullpup design that would use an off the shelf AR-308 pattern bolt head but in a completely different carrier and receiver. So I need to design a cam track to rotate that bolt head.

I think I can figure out some ways to actually machine it, but the modeling in order to quickly churn out non-firing 3d printed test models for iterative development is tricky.

I tried extruding a profile along a helical path, but couldn't quite get the geometry to work correctly. The profile didn't rotate correctly around the central axis of the helix, it ended up all kinds of wonky.

This is a bit annoying for me, because I'm in a country where the only legal way to DIY a gun involves filing paperwork beforehand and this makes it hard to just keep trying until it works. I sort of have to nail the design first time, by doing all the development work in CAD and with non firing plastic models to test feed angles etc, before making anything that's theoretically able to go bang.

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ad895@reddit

What software are you using? I've found fusion is lacking control when it comes to complex geometry like this although I can usually get what im trying to do. Im a bit spoiled with NX at work.

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BoredCop@reddit

Fusion.

It may well be that other CAD packages are much better at this. Or it may be that there's some counterintuitive way to solve the problem, like, can I do it by defining a custom thread profile and model it as just a stub of a thread?

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kohTheRobot@reddit

I came up with a weird way of doing it if you check my posts

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ad895@reddit

Thats a possibility. Id have to try it in fusion but if I need to make some real messed up geometry at work ill surface model them then trim everything back into a solid model.

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concussedhummingbird@reddit (OP)

My first attempt for an 11pm “fuck-it”‘s sake was a lazy Boolean. Which, I still kinda think could work but would require a lot of tool-side modeling to make that twisting & turning of the track, and any result would be difficult to edit for iteration. Because hey, I want iteration too!

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BoredCop@reddit

Using tools and subtracting from the main part is very useful in Fusion, I often find it easier to model the negative of what I want. For iterative stuff, I use "keep tool" so the tool part doesn't vanish but can be made invisible so it's "gone" until needed. Then if a small change needs to be made, bring up the tool and join it with the main part again with "keep tool" checked. This gets you back to before the cut operation, without the mess of having to search back through the timeline. And then you can adjust the tool as needed, and repeat.

I'm in Norway, our gun laws are quite lax on some things and ridiculously strict on others. Suppressors are fair game, not regulated at all, and some gun modifications are also allowed but making a gun from scratch requires prior permission and a government-issued serial number.

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kohTheRobot@reddit

I did this recently for an AR bolt carrier. What I did was create a line along the central axis of the cylinder. I dimensioned that line at the start and end of the cam rotational path. At the start of the path in a separate sketch I created a cylinder in the “start” angle. I then loft t -> solid sweep with 100% length and then punched in my angle.

Then you simply split bodies with your cam body as the tool body.

For some stupid ass reason, using it to straight cut never worked.

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concussedhummingbird@reddit (OP)

Surprisingly, that made sense the second time I read it, usually takes more attempts. I think the biggest problem I foresee with your method is AR BCG is a cylinder with a through-slot, AKs and its derivatives are solid, and the lock/unlock points are designed differently.

What if I modeled the track like you did your AR, added the lock/unlock dwell points, and THEN modeled the rest of the carrier around it? I think I can pull that off.

And no, I don’t have a blueprint, just a couple of carriers themselves.

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kohTheRobot@reddit

I posted on my profile the images I worked up, hope this all helps. If you have a spare bolt and a carrier, you can probably derive the depth by measuring the stickout on the lugs and doing some mental engineering to guess how much clearance you realistically need in your depth.

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kohTheRobot@reddit

Re read this, so what I’d say is that those lock unlock points are “solid slots”, extruded from the center axis using off-axis angle construction planes. You use that pin-can-sweep method I said just to get accurate cam paths, like the cam path between your stops.

What I’ve noticed with the other methods of embossing the cut you always get a taper on your cam slot, this method I cooked up should get you a parallel cam slot, like how an endmill would.

I just banged a mock up out on my work computer, I can do a better one later if you want. But the big difference between this and the AR cam slot is that because it’s “reversed” you can get some weird geometries on the face of your cut . So what I did to get it all smooth is simply over cut.

Let’s say the depth of the slot should be .250 from the center axis. Instead I cut it at .375. I do this for the first stop, the exit stop, the revolve for that removal pocket, and the cam slot. Then I simply put a sketch plane perpendicular to the bolt-bore axis. I make a tube shape, with the ID being that .25 and the OD being whatever. I then join that section of the tube with what meets the bolt carrier, filling in the weird gaps and smoothing it out to be a consistent diameter based depth.

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kohTheRobot@reddit

Yeah! So if the Galil bolt is the like the AK there is a center cylinder, it’s the bore for the bolt. You’re gonna have to make that your origin. Or you can use the axis through cylinder construction function.

I’ve seen the ghetto way they cut these carriers on manual machines. They have a dowel pin they stuck that bolt bore onto perpendicular to the cutter, then have a complicated jig to rotate the cut around that by hand. So it should be based on that bolt-bore cylinder plane.

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concussedhummingbird@reddit (OP)

u/thatamericandude I may have changed my mind. I don’t want to pay $150 for a Galil carrier when I could spend a thousand hours iterative modeling and testing, along with hundreds of print hours. That’s a rational motivation, right?

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staykindasick@reddit

You gentlemen are my people

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concussedhummingbird@reddit (OP)

Give my condolences to your wallet and sanity

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staykindasick@reddit

Team creating problems for our “solutions”

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F1uffydestro@reddit

Sometimes the real reward is the micro plastics in my balls

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concussedhummingbird@reddit (OP)

I need you to know I just spat out my coffee at work because of you

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bertmern_@reddit

Same tbh

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i_can_menage@reddit

In fusion you use the emboss feature (under create in the solid modeling page).

If you're embossing onto a round face, create a planar sketch tangent to the surface, sketch your geometry, then emboss the sketch items you want onto the round face.

Its easier to get the dimensions and positions of the projected features you emboss on the surface if you make the length of the sketch the same as the circumferential length of the round face you're embossing onto. e.g. if you wanted to emboss a feature all the way around a cylinder, start your sketch by drawing a rectangle the height of the cylinder tall, and PI*d of your cylinder long. Then the position and size of the features you emboss can be set in ratio to that (e.g. if your cam goes 1/4 of the way around the cylinder, divide your sketch rectangle into 4 parts using construction lines and work between the lines).

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ad895@reddit

Id have to look at the model itself but if I had to guess its probably a swept cut. If you are having trouble figuring out how to model something think about the path a cutter would have to take to cut the geometry and work backwards.

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Emotional-Box-6835@reddit

Beat me to it. Working backwards is definitely an easier approach on stuff like this.

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ad895@reddit

Yep its actually kinda a good mindset to get into as well. If you are thinking about how some thing will be made as you are designing it, you'll be less likely to design something that cant be machined.

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concussedhummingbird@reddit (OP)

I’m totally on board with your rationale, the issue I keep having with sweep is I can’t or don’t know how make sweep it across three axes in an inconsistent shape while making sure the face starts consistent (“square” to the carrier), then angles (so the track will be “twisted” appropriately, and ends square again

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ad895@reddit

Again without the model im kinda guessing here but the "floor" of the pocket is curved correct? I would do that in a separate operation to the walls because that is just the inside of a cylinder which let's you have more freedom to just make the vertical walls without messing up the floor. Once I get off work ill load up fusion and see what I can figure out.

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concussedhummingbird@reddit (OP)

You would be correct in your assessment.

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Hazlllll@reddit

I am also curious.