Trying to create an adjustable gas valve for semi auto rifle.
Posted: Tue Jan 03, 2017 3:52 pm
I'm looking for some help with the design.
I know there is an easy way of doing this, just copy the on/off only valve drill two holes for the gas flow, drill a center hole and thread it, screw a bolt in and out to adjust gas flow.
But I wanted to try something a little more fun to try and make.
The original gas flow valve is pretty much a simple cylinder held in place with a lock ring. Where the gas comes out of the barrel and into the gas tube the cylinder has a grove cut into it about halfway though and a little wider than the holes the gas comes in/goes out of. When an external lever is pulled the cylinder rotates 90 degrees blocking the gas port, keeping any gas from entering the gas tube.
My first idea was to drill two holes in a cylinder at 90 degrees. The cylinder would not move to adjust the gas flow, so the holes would always line up with the gas ports. Instead I would drill a hole length-wise through the cylinder until it intersected the 90 degree holes. I'd then make a thumb screw that was threaded only on the end. I'd make a threaded cap that would thread onto the end of the thumb screw and be the same diameter. To keep the cap from just spinning along with the thumb screw I'd cut a groove in the cap and put a pin through the cylinder into the groove so it could not rotate. Then when the screw was rotated it would screw the cap in and out. Another pin or two would keep the screw from backing out.
I've encountered two main issues with this idea, the size of the valve is fairly small (the cylinder is 0.275'' in OD and the thumb screw/shaft is only about 0.185'' in OD though that may be able to get slightly bigger) so doing some of this is a bit delicate and may end up with too delicate parts that will break easily in use or as I make them. And if I do what I'd like and have the cap go from full open to full close in one turn of the screw I would need to use a 7 TPI thread. A 'bit' aggressive for the small size I would think.
If I could come up with a way to tell if you were on the first/second/etc. rotation of the screw I could use whatever TPI I'd like and that would solve the thread problem nicely but I can't think of a good way to indicate this.
The second idea I had was to eliminate the cap and the threading on the thumb screw, just making it a shaft. I'd cut a groove a little wider than the gas ports and have it go from very shallow to cut almost halfway through the shaft within about 3/4 of a rotation of the shaft.
The issues with this are that I'm not sure how well you could do any fine adjustments with it and I don't know if cutting through that much of the shaft would weaken it too much.
Any ideas or advice? 'Just do it the simple way stupid' is acceptable as well .
Since I'm not the best at descriptions (and I'm operating on 0 hours of sleep, been up for 26 hours so far with another 4 or so to go at least) here are some quick sketches I made up. Not to scale (or correct threading) of course.
This one shows the two holes for gas flow and three holes for the pins.
The upper left and right are badly done end on views.
This one shows the thumb screw and cap idea.
The upper left is a cutaway though the shaft where the groove is cut in it.
I know there is an easy way of doing this, just copy the on/off only valve drill two holes for the gas flow, drill a center hole and thread it, screw a bolt in and out to adjust gas flow.
But I wanted to try something a little more fun to try and make.
The original gas flow valve is pretty much a simple cylinder held in place with a lock ring. Where the gas comes out of the barrel and into the gas tube the cylinder has a grove cut into it about halfway though and a little wider than the holes the gas comes in/goes out of. When an external lever is pulled the cylinder rotates 90 degrees blocking the gas port, keeping any gas from entering the gas tube.
My first idea was to drill two holes in a cylinder at 90 degrees. The cylinder would not move to adjust the gas flow, so the holes would always line up with the gas ports. Instead I would drill a hole length-wise through the cylinder until it intersected the 90 degree holes. I'd then make a thumb screw that was threaded only on the end. I'd make a threaded cap that would thread onto the end of the thumb screw and be the same diameter. To keep the cap from just spinning along with the thumb screw I'd cut a groove in the cap and put a pin through the cylinder into the groove so it could not rotate. Then when the screw was rotated it would screw the cap in and out. Another pin or two would keep the screw from backing out.
I've encountered two main issues with this idea, the size of the valve is fairly small (the cylinder is 0.275'' in OD and the thumb screw/shaft is only about 0.185'' in OD though that may be able to get slightly bigger) so doing some of this is a bit delicate and may end up with too delicate parts that will break easily in use or as I make them. And if I do what I'd like and have the cap go from full open to full close in one turn of the screw I would need to use a 7 TPI thread. A 'bit' aggressive for the small size I would think.
If I could come up with a way to tell if you were on the first/second/etc. rotation of the screw I could use whatever TPI I'd like and that would solve the thread problem nicely but I can't think of a good way to indicate this.
The second idea I had was to eliminate the cap and the threading on the thumb screw, just making it a shaft. I'd cut a groove a little wider than the gas ports and have it go from very shallow to cut almost halfway through the shaft within about 3/4 of a rotation of the shaft.
The issues with this are that I'm not sure how well you could do any fine adjustments with it and I don't know if cutting through that much of the shaft would weaken it too much.
Any ideas or advice? 'Just do it the simple way stupid' is acceptable as well .
Since I'm not the best at descriptions (and I'm operating on 0 hours of sleep, been up for 26 hours so far with another 4 or so to go at least) here are some quick sketches I made up. Not to scale (or correct threading) of course.
This one shows the two holes for gas flow and three holes for the pins.
The upper left and right are badly done end on views.
This one shows the thumb screw and cap idea.
The upper left is a cutaway though the shaft where the groove is cut in it.