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  • Need help with air spindle

    Guys, I'm building a Tinker tool grinder, and decided to try and make an air spindle for it. I have the spindle and the bore so the spindle is a nice sliding fit, with zero play...as far as I can measure it. When I put the air to it, it gets more difficult to turn.....more pressure, more difficult. Tried it 2 lbs up to 90lbs. At 90 it's close to being locked. It turns easiest with no air...LOL.
    Any thoughts that might help me?
    Thanks,
    Jim

  • #2
    Posting a design drawing might be useful.

    Phil

    Originally posted by IdahoJim
    Guys, I'm building a Tinker tool grinder, and decided to try and make an air spindle for it. I have the spindle and the bore so the spindle is a nice sliding fit, with zero play...as far as I can measure it. When I put the air to it, it gets more difficult to turn.....more pressure, more difficult. Tried it 2 lbs up to 90lbs. At 90 it's close to being locked. It turns easiest with no air...LOL.
    Any thoughts that might help me?
    Thanks,
    Jim

    Comment


    • #3
      You have to have a slight amount of air escape or no bearing affect..You got it too tight..

      Comment


      • #4
        Air is not getting around the spindle. Instead of creating an air bearing it is pushing it against the other side clamping it in place.

        Air bearings need to be honed to fit. A turned shaft is probably not going to do the job.

        Comment


        • #5
          Originally posted by moe1942
          You have to have a slight amount of air escape or no bearing affect..You got it too tight..
          I have air coming out the ends, around the spindle. I had a a thought that the problem was uneven air pressure, as macona suggested, but I'm not sure how to proceed from here. I'm afraid I may screw up the fit by trying to hone it, but may have to bite the bullet and give it a try. I'm a long way from being a machinist, but understand I'll have to try, or I'll never learn anything.
          I'm using a spindle from one of those Chinese grinding jigs, and it measures pretty decent for parallel.
          I've got an axial air groove in the bore, that intercepts the air input, and radial grooves about 1/2" in from each end that intercept the axial groove.
          Thanks for the input...I'll keep you posted.
          Jim

          Comment


          • #6
            Originally posted by philbur
            Posting a design drawing might be useful.

            Phil
            Don't have a drawing to post, Phil. The bore is .982", length of bore is 3.125".
            Some of my plan was from Glen Wilson's article in Projects in Metal II, some from other's posts on various machining sites.
            Jim

            Comment


            • #7
              Originally posted by IdahoJim
              I'm afraid I may screw up the fit by trying to hone it, but may have to bite the bullet and give it a try.
              Glenn Wilson has a great article on building an air spindle in Metalworking 2. The general idea is that you have to maintain a small uniform air gap along the length of the spindle.

              Glenn's spindle has a 0.0015" air gap. The bad news is that Glenn specs the taper tolerance required for the air bearing to float at +.0002" over the length of the spindle: 4".
              "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

              Comment


              • #8
                Air Bearing

                On the air bearing on my Darex T&C grinder the air inlet is not direct to the bore. The bore is a sleeve with 2 sets of cutouts about 1/4 of the way in from each end. These cutouts are 120 degrees apart, looks like they were cut into the side with a woodruff key cutter. The 2 sets of 3 cutouts are indexed rotationally to match. There is a hole with a brass filter at the bottom of each cutout that lets air into the spindle. I would try another axial groove opposite the air inlet to equal things out.
                Kansas City area

                Comment


                • #9
                  Originally posted by Toolguy
                  On the air bearing on my Darex T&C grinder the air inlet is not direct to the bore. The bore is a sleeve with 2 sets of cutouts about 1/4 of the way in from each end. These cutouts are 120 degrees apart, looks like they were cut into the side with a woodruff key cutter. The 2 sets of 3 cutouts are indexed rotationally to match. There is a hole with a brass filter at the bottom of each cutout that lets air into the spindle. I would try another axial groove opposite the air inlet to equal things out.
                  Great idea! I was trying to decide between that method, or just honing the center slightly larger to facilitate the airflow around the spindle. I also thought about cutting an addition radial groove centered on the air input.
                  Right now, the shop is 22* so I'm sort of stooging around waiting for some sunshine. My shop is solar-heated, and gains 35-40* above outside temps...but only when the sun shines. Winter's a PITA
                  Jim

                  Comment


                  • #10
                    Originally posted by lazlo
                    Glenn Wilson has a great article on building an air spindle in Metalworking 2. The general idea is that you have to maintain a small uniform air gap along the length of the spindle.

                    Glenn's spindle has a 0.0015" air gap. The bad news is that Glenn specs the taper tolerance required for the air bearing to float at +.0002" over the length of the spindle: 4".
                    Yup..I've got that article, Robert. I don't even have the means to measure to tenths.
                    The way I did this bore is in sections. Not having the confidence to try the full-length of the bore, I thought I might do it by doing the bore in sections....rings, if you will. There are 4 rings about 7/8" long. I bored and honed each to a nice fit on the spindle. I then bored the housing about .003 larger, assembled the rings on the oiled spindle, and then epoxied the whole works into the housing using JB Weld. I was hoping to find a method that anybody with ordinary machining equipment could use to make an air spindle. I think my problem is more of design, than fit, but we'll see. I'm fairly sure that macona, and others, are right and the trouble is even distribution of the air. Whether, or not, I can fix it remains to be seen.
                    Jim

                    Comment


                    • #11
                      My Darex EM sharpener has several holes in the bore of the fixed sleeve. Also, you do not want any oil on any of the parts. It is best to clean both the cylinder and sleeve with alcohol. Makes a huge difference.

                      HAP
                      Who do I think you are...?

                      Comment


                      • #12
                        I don't think your on a starter unless the air supply is balanced around the circumference of the spindle.

                        Remember how air speeds up over the wing of an aircraft, and the pressure drops, holding the plane up ?

                        Well, if your spindle moves over to one side, and the air has to speed up to get out of a narrower gap, its pressure will drop, and the spindle will get pushed further in that direction.

                        Your supply must be radially symmetric, and preferably three supplies at 120 degrees at least. An odd number of supplies is probably a good idea.

                        If I understand you to say you have three air-fed chambers that are effectively individual rings around the spindle, then you aren't providing radially symmetric control of the air as it exits the annular gap. Maybe some narrow axial channels, and a higher flow rate, may be a temporary fix.
                        Richard - SW London, UK, EU.

                        Comment


                        • #13
                          Originally posted by IdahoJim
                          Yup..I've got that article, Robert. I don't even have the means to measure to tenths.

                          I'm fairly sure that macona, and others, are right and the trouble is even distribution of the air.
                          We're saying the same thing: in order to provide even flow around the spindle, there has to be very little taper along the spindle length. It has to be very straight. Any deviation in gap width is going to change the localized air pressure, and the spindle won't float.
                          "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

                          Comment


                          • #14
                            I believe you should think through the dynamics of what makes an air supported bearing. It should be like spotters around a trampoline so that if the spindle deviates from center, a force on that side pushes it back to the centered position and then the forces on all sides again balance. You can think of it either as a reduction in force at the large gap side, or an increase from the small gap. For those dynamic changes to take place there has to be a certain relationship between the air escaping around the shaft and the air being resupplied at a certain pressure and over a certain area. That is, it's a hydraulic problem with PSI x Area developing a force, and those forces have to change in response to a spindle offset. Simply blowing air around the spindle doesn't develop any balancing forces to force the spindle into any particular position.

                            Think of a series of holes around the periphery of the bushing. A force develops from the PSI x Area equation. Now, if the spindle is pushed to one side, you want a relative increase of force there. Since the feed air pressure is constant, this relative force is produced by the DECREASE in force at the opposite side where the annulus between shaft and bushing has increased creating a pressure drop. For the decrease to happen, the hole feeding air into the annulus must be small enough that it can't compensate for the greater area of air escape. In other words, the air being fed in is metered by the small hole so a differential force is created for recentering.

                            The other feature to include is the little divots Toolguy mentions in his Darex. This is to make sure that if the spindle is pressed all the way to one side covering the inlet hole, the feed there can still develop a centering force. If the small hole were blocked off completely, the PSI x Area equation says the actual force at that point becomes miniscule. The depression in the sleeve wall ensures that you maintain at least some nominal area for the force equation.
                            .
                            "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

                            Comment


                            • #15
                              Building an air bearing spindle is more difficult than most people expect, at least according to what I've read in the Quorn group:
                              http://groups.yahoo.com/group/quorn_owners/

                              It is worth searching that group for comments by Frank Kurda who made a successful air bearing spindle. I believe he's German so his English is good but it clearly is difficult for him. He seems to be a talented machinist who builds from ideas rather than drawings so there is no published plan for his spindle AFAIK. However he does describe it and it uses a number of VERY small (0.3mm) holes which would likely be challenging to drill. Plus the spindle is very accurately lapped. He linked to this video (which was posted under a different name?):
                              http://www.youtube.com/watch?gl=DE&hl=de&v=I9YgG_soRcE

                              Also see Kurda here: http://bbs.homeshopmachinist.net/showthread.php?t=28528

                              John
                              Last edited by GadgetBuilder; 01-04-2011, 03:52 PM.
                              Location: Newtown, CT USA

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