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  • Loosening work

    I'm sure all you long timers know this, so I'm throwing this out there for the hobbyists like me.

    I was milling a very odd shaped part (spindle from a VW beetle) and having a hard time figuring out a way to clamp it down. I decided on a method that had me clamping on the bearing surface of the spindle. So I put pennies between the clamping screws and the bearing surface to act as soft jaws. I settled in on a cutting feed and speed that gave me "comfortable" amount of vibration (my milling operations always seem to involve some vibration). About half way through the operation, I got to thinking about how those clamping screws could be digging into the pennies. So when I finished that pass, I grabbed the allen wrench, and was able to put about another 1/4 turn on each of the clamping screws.

    Last night I was boring a block of aluminum in the lathe 3" deep, with about 1/2 of that being an interrupted cut. Again, about half way through the operation, I grabbed the chuck key and was able to re-tighten one of the jaws.

    So my advice to the masses is when working on soft materials or using soft jaws, double check your clamping pressures occasionally, especially if there is vibration involved.
    Definition: Racecar - a device that turns money into noise.

  • #2
    A good point. I've had to tighten things like you suggest. But where I've noticed it the most is when working with soft jaws in a bench vise. Hadn't thought much about the same issue on the machines. I'll have to keep this in mind.

    A little side thought on using softer metal as the part or as padding is that such metal WILL deform. That's why we use it So it's best used with the pressure spread out. You mentioned the screw ends digging into the pennies. Perhaps something like a steel washer or some other sort of harder pad used with the soft metal would spread out the force and aid in preventing the digging in? Or at least reduce it so more of the often inevitable give is from the part instead of ?

    Your penny trick won't work up this way We went to copper plated steel back in the early to mid 90's. And we finally dropped the penny altogether a few years ago.
    Chilliwack BC, Canada

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    • #3
      I wouldn't use something as thick as a penny, assuming it is indeed copper, as a shim in a critical situation. The thicker it is, the more material to deform under clamping pressure and vibration. I keep some copper and brass stock around 0.020"-0.030" thickness to protect the workpiece from damage when clamping on the mill or chucking in the lathe.

      It also helps to have as wide a contact area as possible.
      Jim H.

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      • #4
        Ordinary photocopier/printer paper is what i use most often in a lathe chuck or milling vice. Its really strong in compression and provides good friction between smooth metal surfaces. Rough jaws or sharp setscrew need metal shim though. For metal shims I'm mostly using soda/beer cans..
        Location: Helsinki, Finland, Europe

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        • #5
          We need to look at the way we hold things too. If using the horizontal or vertical "V" in the jaws to level up round work soft thin shim stock can be cut by the pressure from the edges.

          Printer paper and file folder card stock is a pretty frequently used soft grip method in my shop too. These work well in chuck jaws. Same with the pop can side stock.
          Chilliwack BC, Canada

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          • #6
            Originally posted by JCHannum View Post
            ...It also helps to have as wide a contact area as possible...
            This is often the key to a secure setup. When working on oddball shaped parts in the mill you often have to get creative but you should never have to re-tighten clamps or the like--makes it much more likely that the part has moved from its original position. The trick is to have enough points of support to begin with and to get them really tight the first time. Use of a larger contact area means that you don't get "point" pressure of the kind that could deform any protective material you might be using between the clamp and workpiece. I might tweak my clamps and vise after making a couple of roughing passes just to make sure everything is holding but I'd be really concerned if I found 1/4 - 1/2 turn of play--I'd be re-checking some dimensions to make sure I was going to be able to hit my final numbers...
            Keith
            __________________________
            Just one project too many--that's what finally got him...

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            • #7
              One good thing about paper is it has a fair amount of friction vs. copper or aluminum.

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              • #8
                If you are really having trouble, 120 grit shop cloth works well. I had to use it a few weeks ago machining a casting that just didn't want to stay in the vise for long.

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                • #9
                  Tried aluminium cooking foil the other day, wraped round the part, worked ok, wonder if there's a thicker version. The cuts were really light btw, nothing severe.
                  Think the answer is there, not too heavy cutting.
                  Mark

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                  • #10
                    Originally posted by MattiJ View Post
                    Ordinary photocopier/printer paper is what i use most often in a lathe chuck or milling vice. Its really strong in compression and provides good friction between smooth metal surfaces. Rough jaws or sharp setscrew need metal shim though. For metal shims I'm mostly using soda/beer cans..
                    Woks well clamping to the table also. Mine was ground at some point, and is as slick as a hockey rink. "Paper wraps table"..... or at least keeps it from slipping
                    CNC machines only go through the motions

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                    • #11
                      I always save the backs of those 8 1/2 X 11" paper pads and cut them to size to act as an interface between a workpiece and whatever I am clamping it with.

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                      • #12
                        Originally posted by DATo View Post
                        I always save the backs of those 8 1/2 X 11" paper pads and cut them to size to act as an interface between a workpiece and whatever I am clamping it with.
                        It's the card stock boxes that my favorite Costco pizzas come in for me... Not only is it handy for padding in the same way but when cut into file card size squares it's superb for mixing up epoxy because it's not very porous on the glossy side.

                        I think the card stock backers from the old days are a heavier grade. But these days the card stock used for cereal boxes and my pizzas are about the same.
                        Chilliwack BC, Canada

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                        • #13
                          It can also be a function of the style and condition of the knurling/checkering on the clamping surfaces of the jaws.

                          When I worked at Ingersoll-Rand making centrifugally cast babbitt bearings, the finish boring of the babbitt, and finish turning of the O.D. could be a ticklish operation.

                          The process that we developed was to make the steel shell for the bearing extra long so that there was approximately 3/8" - 1/2" of extra length. This extra length was enough to chuck on and allowed us to finish both the bore and O.D. without removing the workpiece from the chuck. When both I.D. and O.D. were to size then the parting tool was used to cut the finished bearing to length, leaving the excess still in the chuck.

                          As long as the piece didn't move in the chuck jaws, this pretty much guaranteed average concentricity close to the limits of runout of the spindle itself.

                          These bearings were relatively thin shells, maybe 3/8" wall thickness of steel with .060" - .125" of babbitt lining for a 6" - 8" diameter bearing. They were also split lengthwise, with dowels theoretically holding things in alignment along the split lines. This process did supply a fair amount of pucker factor, taking fine cuts and preventing the finished ($1000.00 - $2000.00 at that point) part from falling into the chip pan when parting.

                          Because of the thinness and the split shells, using heavy chucking pressures would distort the assembly into a 4 or six sided polygon which didn't work too well as a bearing at 10,000 rpm.

                          So to help hold the assembly securely we would use aluminum shims between the steel shells and the chuck jaws. We would initially tighten the jaws very tightly but hopefully less than enough to cause permanent deformation of the bearing. This would squish the aluminum into the relatively rough finish of the bearing shell as well as into the checkering on the chuck jaws. Then using a dial indicator, each jaw of the 4 jaw chuck would be independently adjusted to .0002" - .0005" deflection from a fully released position. When removing the waste ring from the chuck, the ribs that resulted from the extrusion of the aluminum into the jaw checkering and the rough finish of the bearing shell was very evident. These ribs provided enough lateral stability to prevent the workpiece from shifting postion even with relatively light chucking pressures.

                          Dave

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                          • #14
                            Originally posted by LKeithR View Post
                            I might tweak my clamps and vise after making a couple of roughing passes just to make sure everything is holding but I'd be really concerned if I found 1/4 - 1/2 turn of play--I'd be re-checking some dimensions to make sure I was going to be able to hit my final numbers...
                            Fortunately, neither of these operations were tight tolerance. I was milling the steering arm off the spindle to weld on a new one. The previous owner converted the car from drum to disk brakes. I guess the steering arm was in the way so he heated it up and bent it. Did wonders for the steering angles.

                            Originally posted by BCRider View Post
                            A little side thought on using softer metal as the part or as padding is that such metal WILL deform. That's why we use it So it's best used with the pressure spread out. You mentioned the screw ends digging into the pennies. Perhaps something like a steel washer or some other sort of harder pad used with the soft metal would spread out the force and aid in preventing the digging in? Or at least reduce it so more of the often inevitable give is from the part instead of ?

                            Your penny trick won't work up this way We went to copper plated steel back in the early to mid 90's. And we finally dropped the penny altogether a few years ago.
                            Hmm.... I see what you mean. I'll try something thinner next time.

                            And pennies here are copper plated zinc. We're too damn stubborn to get rid of them.
                            Definition: Racecar - a device that turns money into noise.

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