SpudFiles

Well I think it may be theoretically possible if you delayed the return of the control valve with a shock or something, but that's another level of complication.

Safety/fueling/venting needs to be controlled by chisel position. If it was the nail gun style of a separate piece, it may not touch the rock if you're by a corner or something. I think what I've drawn here will work. It isolates the downward movement of the chisel on impact, but still allows it to control upwards movement of the valve.

I'm also not sure about hammer resetting. You're right, if there was a separate valve as pictured here, it would make it a lot easier for the spring to return it because it's only lifting the weight not fighting the pressure, even with a stepped piston. Trying to picture how an air spring could be used instead because it would allow for more control and tuning... perhaps external connected through a slot in the cylinder?

Also eliminates the need for orings on the hammer, which would prevent an air spring from forming above the hammer on return.

The function of the control valve here could be duplicated with commercial valves as you showed, but I think with this you wouldn't need check valves because there isn't any way for pressure to pass back upline? If I was making it could make sure it could withstand hybrid pressures, whereas air control valves wouldn't be rated that high, hydraulic would though I suppose.

Anyway my point is I'm concerned about being able to make a check valve for the fuel line that holds itself closed but is weak enough to open at the low pressures the fuel will be at. So if I can eliminate that check valve...

Agreed should be able to change out chisels easily, was thinking about using wingnuts or some sort of screws you could do by hand to hold the chisel in place (loosely so it can slide). I believe the brand they use has a common shank width on the different sizes which helps. Attached a picture of one. Hex shape could be good or bad depending if being able to swivel it is a desired trait or not. If it needs to swivel I would have to make collars instead of just using three screws.

The hammer part looks good, but I don't really get how that fuel meter is supposed to work.
I recommend two separate cylinders, one for air, one for fuel.

Two would probably be easier to make, I was just seeing if I could get it to work with one, in keeping with making it compact.

The wider parts in the piston are supposed to be holes drilled to the hollow part in the center. Fuel/air are regulated in, this is just on/off control basically.

The extra orings are just to prevent crossflow between fuel/air as the piston moves, and of course stop either from flowing out the ends of the cylinder.

@daccel
I still think that pneuamtic would be more practical but if you insist on a hybrid..

nice design... but do you really think that

Safety/fuelling/venting needs to be controlled by chisel position

?

just use a timer, 555timer or a PLC board...

I think the gas will not have enough time to flow into the meter unless you do it manually (slow).

I may be being a bit stubborn because I want to buy some aluminum to machine and play with a hybrid, and it's easier to justify the cost for this than a cannon . I suppose I should separate the two if I'm serious about finding the best solution for the chisel. Damn logic.

I did some research today on local sources for pneumatic and hydraulic cylinders, and will assess that route for a pneumatic before going ahead with this. Searched high and low but couldn't find anywhere in town with seamless pipe, except for some aluminum but he said it wasn't pressure rated. One more place to check Monday.

Anyone know if hydraulic pistons/cylinders will seal with gases, and if they have a higher friction than pneumatic ones to gain their higher pressure rating?

I know there are other ways to control it, I was just referring to the suggestion of a nailgun style safety, and the problem I saw with that.

It is manual. When the tool is set on the rock, you push down on it, then the chisel moves up pushing the ring and rod up which pushes the valve up. When the chisel gets pushed out on impact, either into the rock or staying in place while the cylinder recoils up, the ring/rod stays in place and moves back down with it's own spring so there is no shock transferred to the valve. Fuel/air will be wasted as the valve returns because it vents at rest, but I don't see a way to avoid this. I suppose if you had separate valves.

oh yeah now it makes sense

I'd say go for it if you want to