Re: Blind hole technique
Originally Posted by
Peter Neill
A couple of points - if you're going to run a moving 'O' ring or other seal in there, then the finish you'll produce from a drill, end mill, or reamer, will not be nearly smooth enough to avoid any abrasion of the seal. For that you really need a lapped finish. A lap is easy enough to make, you can google for it.
Secondly, a reamer cuts on the end and not along its length, so the flute length doesn't really matter, as long as it will reach down to the end of your hole.
Thirdly, you can make a D-Bit, which is a home-made reamer that cuts very well indeed for the single hole or few holes you may need, and all you need is a bit of drill rod and a propane plumbers torch. Again, you can google for it.
However, neither will give you the finish you need for a sliding 'O' ring.
If I were making this, I'd make the entire thing with a though hole, drill a few thou' under. get it just-about-to-size with a D-Bit, then make a lap that I can pass through end to end to get the finish and final size, and then make a seperate end cap to close the blind end off.
To the OP. The advise to drill is spot on. Your biggest problem with a hole that deep is getting the chips out of the hole. A twist drill is going to do that better than any other tool. When you go back to ream or bottom mill or bore or however you bring it to near final size you still have the same problem. Getting the chips out of the hole. You won't have the bulk of chips you had to start with but packing up all the chips from reaming at the bottom of the hole can cause you to leave a ridge at the bottom. This does leave a conundrum. I hate to run a ream thru a hole twice. Atleast with the cheap chucking reamers I have the run out tends to slightly over size the hole from the reamer size.
Lapping and polishing is a good advise, but you have to take some care not to pear shape the cylinder, or bell mouth the hole. Maybe you can account for that to some degree by how long the piston is beyond the o-ring, and how much you engage the o-ring. Since this is a moving application and not just a static seal you have to consider how much engagement you need to contain the pressure you will be working with and still move as easily a you need it to. Remember that a piston ring is a wear part. Whether its a chromoly ring in a gas engine or a rubber ring in a "home built" device.
All is not lost though. An o-ring can make up for a lot. I sometimes make hand injectors for soft low (290-350F) temperature plastisol injection. (for my own use) My pressure is low and unless there are visible imperfections inside the tube I just use the raw extrusion's mill finish. I don't size. I don't polish. I just machine a piston (on the lathe I admit) to a rough dimension that easily drops into the tube. Then I measure the ID of the tube and cut a ring groove in the piston so that the o-ring has too much engagement. Then I slowly deepen the groove until I get engagement that "feels" good with the tube or cylinder of the injector. This is what I call "machine to fit" as oppose to "machine to spec." Its not suited to mass producing injectors, and depending on the source of the tube the parts may not interchangeable, but they all work.
I'm definitely not saying you should take either a sloppy or a machine to fit approach. Just that you should consider what you actually need to be able to do to accomplish your end goal. Is a precision exactly perfectly 0.5000 hole necessary. Is the cycle rate of your piston fast or slow. Will a slight variance in diameter of the hole matter. Will your piston bump on the cylinder walls or will the o-ring(s) be such as to prevent that. How much pressure are you dealing with. Is it necessary for the hole to be blind or does it just need a ridge to stop the spring. If the hole has to be blind how will you deal with back pressure or vacuum behind the piston. I agree the hole needs to be polished smooth, but how smooth. If it only sees a few cycles an hour or possibly in its lifetime that may not be as important to be perfect as if it see a rapid high number of cycles in an hour... or a minute. Is it more important that it seal perfectly or move easily.
They hardest thing I have had to learn in machining is that chasing perfection can sometimes cause more problems than it solves. The hardest thing I've found to design is acceptable tolerances. Sometimes I have to step back from a projecting and think , "How crappy can it be before it fails? How good can I possibly make it? What is something in between that will work and can be achieved with a reasonable amount of effort relative to the value of the project?"
Bob La Londe
http://www.YumaBassMan.com