Page 3 of 3 FirstFirst 123
Results 21 to 28 of 28

Thread: Precision Blind Hole in Aluminum?

  1. #21
    Join Date
    Dec 2015
    Location
    Chilliwack, BC, Canada
    Posts
    5,392

    Default

    Donf, you keep telling us you're not a machinist. OK, you were never trained as one. Fair enough. But you're well on the way to becoming one if you're willing to take on stuff like this and learn as you go.... Granted you're green as grass perhaps but you'll learn fast since it sounds like you're keen on doing so instead of relying on paying outside shops. Just the fact that you know enough to ask and understand enough to realize the factors shows that you're not as green as you might think.

    You mentioned blind holes but not how deep they would be. You also mentioned cup seals but then switched to the idea of an internal seal to seal against the shaft instead of the seal being on the shaft and sealing against the wall of the hole.

    Seals, be they fancy ones like cup seals or the sections you showed in that sketch you grabbed, generally have a fixed side and a running side. Aluminium is never going to be very good as the running side if it isn't somehow treated or unless you use a pretty fancy and specific alloy. So grooving the wall of the hole and making the aluminium side is going to have more longer term success.

    But depending on what you're doing and how much movement there will be it is possible that you might get away with a simple O ring as your seal. Or perhaps a square section O ring (think brake calipers on cars and motorcycles).

    The other concern is the side loads of the shaft against the side of the hole in the aluminium. If the load will be almost completely axial with the shaft then fine. But if the rod will have much side load on it this will wear at the aluminium bore at both ends of contact fairly rapidly if there will be regular movement. If this is an issue you would be better off boring oversize and fitting a suitable steel bushing which is finished smooth and also provides the seal groove.

  2. #22
    Join Date
    Aug 2014
    Posts
    206

    Default

    Common bores and hydraulic cylinders do not have exact corners, and work just fine.

    Common hydraulic cylinders, even to extreme pressures work just fine with the right finish, and typically never go near the end with the seals.
    Seals are usually extremely sloppy in machinist accuracy terms, and 0.02-4 mm is just fine.

    Reamers deliver about 0.01 mm in terms of size, good finish, consistency, and more or less follow the bore.
    Anything normal in hydraulics or pneumatics will work perfectly well with a reamed bore.

    Polishing the bore with abrasives cannot hurt, but generally wont bring any advantage.
    Both soft polishing (many kinds) and hard honing type finishing.

    I know of and have heard of zero cases of precision bores in alu.
    Auto alu blocks have steel inserts, diamon honed to better than 0.01 mm in tir, radial accuracy, and size, one op, 6-9 secs, hard hone.

  3. #23
    Join Date
    Apr 2012
    Location
    Medford Oregon USA
    Posts
    132

    Default

    I like the square o-ring idea, I may try that first! Its nothing fancy just a small homemade valve spring tester to use in my press. A few friends said they may want one too. Mostly its just a learning project and I may have to make a few to figure out what works. The bore will not be exactly 3/4, more like .4, half of a square inch. I have not looked at the math of it yet, but it should not be too hard.

  4. #24
    Join Date
    Dec 2015
    Location
    Chilliwack, BC, Canada
    Posts
    5,392

    Default

    So the body will extend to a pressure gauge? If so that's a pretty cool idea for a project!

    In effect this will be much like a brake caliper then. Very small and short movement of the piston. Just enough to move oil to the gauge. So the piston will be mostly seated almost fully home. So lots of support off the walls and no significant motion. Likely a fair bit of side loading though....

    Had you thought about the idea of simply adapting a small import car brake caliper piston to do this job? A lot of them only have the one piston. The odd size would require a touch more math what with the cross section being some oddball number.

    Of course then while simple I'd be taking away the fun of making and learning. So maybe just carry on and enjoy the project.

    If I'm right in how you'll be using this why not make it an honest square inch in cross section for the piston? That way there's no math to do to read the spring pressure for whatever flex. That would make the piston to be 1.128 in diameter. Which would be easier to work an O ring or square ring into the groove. And it would give you a little more support for the spring.

    The other nice thing is that with a steel rod as the piston and because you're using an O ring or some other ring (while I suggested a square seal ring they are not really all that common) the sides of the cylinder don't need to be crazy smooth. But the piston does because the seal will run on it by just that slight amount. So it has to be smooth and slick. But the walls of the cylinder in this case can be given just a moderate finish. The sort you get from a boring bar and some light polishing with finer sandpaper would be just fine.

    You'll want to play with the ring groove to make sure the O ring has just a light press fit on the piston. If it's too tight the "sticktion" it causes will be taken away from the reading. There's going to be SOME sticktion for sure. But the key is to keep it minimal but still have some light amount of tension so the seal can't weep oil when there's no load. And yes, if the groove is generous enough you can end up with a perfectly fine seal under load but for very light loads or simply sitting on it's side in the tool box the oil can migrate around the seal if there's no static pressure of the seal in the groove.

    Normally that static pressure would be on the OD of the groove. To set that size put the O ring you will use on the shaft you make and measure the OD of the ring in place. I'd suggest that the OD of the groove ends up at something like .003 smaller so there's some pressure. The width should be about .005 wider than the ring so it has just a touch of float end to end.

  5. #25
    Join Date
    Apr 2012
    Location
    Medford Oregon USA
    Posts
    132

    Default

    Thanks for the advise. I could just buy one, probably cheaper than what I can make it for but I figured I would probably learn something in the process. Maybe I could make some design changes that are not in the commercial versions too. The reason for the smaller size is most commercial testers don't look that huge to me. I had probably better look at the math now as I don't think psi and a direct lb reading are the same.
    Here is what a store bough version looks like.


  6. #26
    Join Date
    Jan 2014
    Location
    In the desert
    Posts
    1,322

    Default

    For the naysayers. Aluminum tube/pipe is used extensively in the fishing tackle industry for low pressure low speed injectors using a piston and an o-ring to make rubber worms. I am sure high speed and or a high cycle rate would damage the o-ring from heat and friction, but a seal in a "plane" aluminum cylinder does work in some limited applications. I've made a few of these injectors myself and used them sporadically for years before replacing the o-ring seals. Atleast a dozen suppliers make and sell them on-line.

    Now I admit I have not read this entire thread to see if the OP stated speed and cycle rate for his application.
    *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

  7. #27
    Join Date
    Jan 2014
    Location
    In the desert
    Posts
    1,322

    Default

    When I say low pressure I mean if well constructed upto maybe a couple hundred PSI. Actual working in the application I described is usually well under 100PSI. What you can exert with your hands without putting your weight into it usually.
    *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

  8. #28
    Join Date
    Jul 2017
    Location
    Buffalo NY USA
    Posts
    345

    Default

    Can't see any reason why not? Recall that the valve bodies in automatic transmissions are aluminum, and they mass-produce those.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •