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Fixture Dilemna???

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  • Fixture Dilemna???

    Help please. I recently made a fixture 10 inches square and 1 and 1/2 inch thick aluminumn plates/ There are two one upper and one lower plate. They have Vees milled into them so as to hold eight 5/8 inch precision ground air cylinder rods. The problem since they (the rods are all the same diameter) fit into the vees and i tighten the crap outa the bolts holding them in place there is ALWAYS ONE OR TWO RODS that are loose. Now normally all i woukld do is throw a piece of paper into the vee to tighten up the loose ones. BUT its for a new customer (only one i got so far since losing my job due to the chinese underbidding on all our die work) He said i need it idiot proof we cant be sliding paper into it. HM. Guys the first time i dropped the fixture off at there shop they really liked it. I spoke with the machinist there and assumed he would giver a chance. Well that same evening into my shop walks the owner with my fixture in his hands. He says three rods are seized in it and bernie cant get them out? I looked at it oh well he didnt loosen one of the cap screws holding it in place HM. That was the first indication i was in trouble on this job. I m not joking. Anyhow i indicated the entire fixture and it is within .oo1 on vee heights. Placing a dowel into the vees and indicating all over on everty vee entire fixtures within .001 The problem is how do i get to hold all eight rods tightly in place?? I ended up (temporarily sliding black rubber vacuum line with .080 weed wipper nylon string into the vacuun tunbing and gooping it into all the vees at the bottom. This way it at least holds the rods but its a **** fix. I nedd some good ideas if anyone has any .Thanx Mike

  • #2
    Can you drill and tap each vee groove on one of the plates for a couple or three nylon or brass tipped fine thread setscrews? If possible the setscrews should have jam nuts so once everything is set they won't vibrate loose when machining. That way he can adjust for the tolerance of his stock too.
    Last edited by Mike Burdick; 11-13-2006, 02:22 AM.


    • #3
      have you measured the rod that thier trying to hold? maybe its not precision after all?

      Ten inches is quite a span, are your bolt holes for tightening properly located?


      • #4
        If you mill the vee relative to the surface opposite the milled surface you are subject to warpage and surface irregularities in the stock regarding plunge depth of the cutter. This is the same problem with radial arm saws when cutting mouldings - the wood warpage and thickness affects the depth of the cut. To look at it another way - a router cuts on the same surface, a table saw making a blind cut does the same. An overhead tool like a mill or a radial saw cuts relative to the opposite surface. This came to mind because of the square area you're working with. It's hard to hold something that big flat to the table. It may be a factor or I may be fulla crap, I dunno, but I'd look into it.


        • #5
          Your measurements show the Vs are all be the same depth, but what about the spacing between them? If that's off, some of the rods will not be clamped tightly.

          Sounds like the Vs need to be cut in all three pieces at the same time to guarantee spacing AND depth. Mark the parts so they can always be assembled with the left end of the top bars over the left end of the bottom bar.

          Having said that, if the bars aren't really stout, they will flex enough for the ones in the middle to be loose. Can you put screws in the middle so you have two sets of four Vs or four sets of two Vs? Overtightening makes it worse if you only have screws on the outer ends.

          Why not use the fixture as it was and just lay a strip of paper, plastic, or rubber over the rods before the top bars are put in place? You just can't get equal pressure on all 32 contact points without having something that's going to give some.

          Any products mentioned in my posts have been endorsed by their manufacturer.


          • #6
            Here's what I suggested back at the beginning of October when this original question came up.

            Only a quick sketch and out of scale are regards the end half jaws as they need to be lower in hight than the bars so it clamps.

            The secret of this setup is it relies on the sideways clamping to make sure all the bars are secure.
            Clamping in vee's on there own won't work unless you can guarantee absolute flatness when clamped and precision of the parts.


            Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


            • #7
              Thanx Guys

              John they didnt like your design. It would of necessetated moving there mill which they wont do. I liked your design a lot though. Actually was my first choice. Winchman i cut all the slots at once started with two flute .250 cutter down the centers then a 90 degree homemade cutter i rough pass and i finish pass. Looked good all within a .001 good considering my ancient bridgeport. I am carefull when setting up tramming and such. The shafts are all the same size being centerless ground. I also think i should of cut slots or split the top clamp so i had some flex in the damn fixture. It being 1.5 inches thick isnt good. Anyhow sure learned a lot and had to pinch myself to keep from laughing while at there shop on my improvised rubber weed whipper inserts that seemed to fix the problem. I will see what happens ./ Thanx everyone you guys are great. Mike


              • #8
                I think cutting the grooves with a ball end mill would give better contact area and putting two clamp bolts between every two or three shafts should hold them down. The vee slots are subject to damage from clamping force and will eventually have indentations in the vee's making it hard to hold them all from spining or moving. You may have to put saw cuts in the top of the cover plate to allow it to flex over the shafts. The saw cuts should run the same direction as the shafts and cut maybe half way through the 1 1/2" plate. Use studs in the base plate and washers and nuts on top. Don't cut the round slots to deep, maybe just less than half the diameter. I assume they are sliding the shafts in from the end and if so then put some springs on the studs to lift the top plate so the shafts can be easily slid in and out. Also, put a stop bolt at each corner so they can only let the plate come up so far. It's hard to make a jig or fixture idiot proof. I found that out the hard way. I wanted to dope slap some of the production workers but kept my hands to myself . Be sure they know and understand that they have to blow the chips out of the fixture before reloading it.
                Last edited by Carld; 11-13-2006, 11:37 AM.
                It's only ink and paper


                • #9

                  I know about what your saying. I al=ready had to pick out many chips from my fixture and its hardly used. Also the guy couldnt get three rods out well whaddya know you gotta loosen ALL THE BOLTS FIRST HA HA . I think his boss reamed him out good. Also this fixture gets flipped 180 degrees with all the rods in it can get darn heavy. This is how they wish to do it. so ? Anyhow thanx for the reply. Mike


                  • #10
                    Re reading your first post it strikes me [ splat - sorry about that ] that your jig is too rigid and can't take up any slight differences.
                    On one plate can you mill some slots in lengthways between the Vee's but stopping short of breaking thru so each section can give a little.
                    A bit like the cross section of a comb ?


                    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


                    • #11
                      I've had to design fixtures like this before, and it's about impossible to do it the way you're trying. Rather than chase the problem with the solution, I'd offer it's better to address the initial problem.

                      What, exactly, is trying to be accomplished here? there six 5/8" air cylinder shafts being held.

                      What orientation? are they normal to the table, parallel to the table?

                      Are they being held for a machining operation on some portion of the shaft?

                      How much clamping force is neceassary?

                      I've only been succesful with this sort of thing by using seperate clamping for each part.

                      I'd say to make the lower block in one piece, and slit the upper block into 6 pieces. bond a sheet of rubber or urethane around, say Shore 75A to a slab of 1/2 CRS plate, and use that as a top clamping plate that will add some compliance.

                      But the shafts in the bottom plate, put the 6 clamping blocks on, put the clamping plate on, dog the whole mess down.

                      Bear in mind that an aluminum v groove will deform under load.
                      "Lay on ground-light fuse-get away"


                      • #12
                        Fixture Troubles

                        I am holding eight 5/8 ground rods. i1 1/2 inch alum plate top and bottom vees milled in all at once wthen the plate (20 1/4 x 10.5 ) is cut in half and mill to clean up saw cut. I have 5/8 dowels (8 of them) loctited (4 per side) to locate nicely into ythere milling machine table. The fixture indicated in at .001 max deviation anywhere on it. Mainly i was concerned with the v grooves all being equal . Its a pain i put the rods in and ones loose reclamp and two are loose. I was laughing to myself thinking you are a madman to be making this thing like this, LOL. Anyhow i ended up stuffing it full of rubber tube stiffened up inside y=the tube hole with weed wacker string hahaha. Then gooped it all together went in the house to let it cure. Surprise i delivered it and no calls yet its been running a day and a bitnow. Heres the funny part im supposed to build another one this one has 2 inch thick plates for larger rods. I inch and 1 3/8 inch. I think the slots cut or semi flexible top plate will cure this problem. Thanx everyone for the inpput. Mike


                        • #13
                          I would concur that some equalization between nests is necessary. This is the same problem as lining up several parts in your vise and expecting them all to be clamped equally and securely. In fixture designs I've studied holding mutiple parts, there are always yokes to equalize between parts, sometimes a seqence of them, so a single screw presses one yoke, which in turn presses two others which in turn clamps four parts. The vise trick I've used is to place a strip of emery cloth between the moving jaw and the multiple pieces to take up just a little and provide a high friction surface.

                          Flex slots in one side of your fixture, or the rubber compliance features ought to help you out a bunch.

                          "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill


                          • #14
                            I will add to the chorus..... the nests will NEVER be exactly the same, and if they WERE, the rods would not be.

                            You HAVE to have some sort of "give".

                            Essentially, one bolt/clamp/pressure point can hold at most two rods. That gives 3 points of contact, two on rods and one on the clamp.

                            A "strongback" or crossbar is pressed down at a point between two rods, and the two sides of the "strongback" each press on a rod.

                            Since you have 8 rods, you need 4 of these. Since the rods have some length, maybe you need two sets.

                            They don't need to be made with V-notches, they will work OK without, since the bottom V holds the rods.

                            With a fancy setup you COULD do it with one clamp.... each of two second-level strongbacks holds down two of the "bottom" ones, and a third holds down the two second-level ones. Sort of a pyramid....

                            If they want one screw loosening the whole row, that's one way, but it's a lot of "stuff".

                            We have this issue all the time, holding semiconductors onto a heatsink with even pressure on every one of a row of maybe 8 or 10 parts. One screw can only hold two parts, that's the rule.

                            Only in a perfect world will two plates hold evenly at all points.... and that is with no flexing, and zero tolerance on everything.

                            Keep eye on ball.
                            Hashim Khan


                            • #15
                              What tolerance are you trying to hold, and where?

                              If the rods need to be constrained relative to one another in x-y-z, the question is:

                              "assuming that there is SOME deviation in the diameter and length of the fixtured parts, which sides will be constrained by the fixture?"

                              I'll offer that a plate with the Vs machined in and stops at the bottom edge with 4 individual top clamping plates or 8 bars will work well and be customer-proof.

                              See image.

                              "Lay on ground-light fuse-get away"