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  • repetitious operations

    An ongoing project has me at this point making about 140 pvc insulator discs. Each disc has three holes in it, one centered and all are in line. To help speed the process, I clamped a sheet of the plastic on the mill table and drilled all the holes, spacing them out simply by cranking the axis the right amounts. Then with a special trepanning cutter I just made, I enter each center hole in turn, grooving partway through from one side, then grooving the rest of the way through from the other side till the part pops out. Works fine.

    It's tedious of course. Because the swarf ends up wrapped around the cutter, I have to stop the spindle about every two plunges, sometimes every time. This is going to be a lot of on-offs for the switch and motor.

    I've done this type of thing many times before and there's never been a problem. But I'm wondering if there's a better way to run the motor under circumstances like this. I'm going to get pretty tired of hearing the centrifugal switch cutting in and out about ten times a minute for a couple or three hours.

    This is a round column mill with a 1 horse (I think) with starting cap and switch. My drum switch is also going to take a beating with this constant switching on and off. This isn't the last time I'm going to be doing a production run of parts- one job a few years ago had me trepanning more than 600 uhmw discs, with a lot of stops to peel the swarf away and remove each disc when it decided to mate with the tool instead of stay with the blank.

    I think you guys are going to say it's a job for a vfd and 3 ph motor. I would probably want a deployable control for this- maybe something with a magnet that I can temporarily stick to a convenient spot. The existing motor is very smooth and quiet, and comes to speed very quickly, but does take some time to stop. I'd like to keep using this motor if I can. I'm thinking though that the answer is a different motor with a controller. What are my options?
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    A clutch maybe?

    How many hands do you have? Could you slacken the motor mount as I assume you would do to change belt position or to put on a new belt? Can it be swung far enough to slacken the belt so the spindle stops?

    If you can meet your requirements (safely) by slackening the belt you could add a convenient lever to tension the belt when you want the spindle to run.

    There are various kinds of clutch pulleys you could consider for mounting on the motor including the electric clutch from an automotive air compressor but I have no idea if they are suitable.

    You might even be able to use something such as a bicycle brake cable and a simple foot board to give pedal control of a clutch.

    If it is just a matter of the swarf is there any way you could clamp something very close to the tool so that the swarf is deflected away for wrapping around the tool?


    • #3
      I like Artie's idea of a clutch but I'm thinking more in the area of the cutting tool and the holder itself. If you have an ongoing contract for these things you might consider making something like an auto-tapping head but for a flycutter.

      Long ago I made a clutch driven screwdriver head for the drill press and it was fairly easy to do. A male tapered side of the clutch extended to the shank of the tool (what went into the drill chuck) and opposite this there was a female tapered cavity which extended down to the screwdriver blade. The two tapers were separated by a short spring and obviously both were common to an external housing. When the drill press handle was pulled down the two tapers engaged enough to drive the screw but when the pressure on the handle was released the spring pushed the two tapers apart and the screwdriver blade would stop turning.

      Now, if this general concept could be used to make a "break-away" flycutter I would think that might solve your problem.

      Another option might be to just saw them out and then turn about a half dozen at a time while sandwiched over pins common to the holes on a fixture in the lathe.


      • #4
        You really do need to worry about blowing the start cap or even burning the start winding. It does depend on the type of motor but if it is a standard capacitor start motor then the usual duty cycle rating is only in the range of 20 to 60 starts per hour. Given it can be pushed somewhat that means no more than 2 to 3 starts per minute over an extended period of time.

        The only way to get around this is a different type of motor. For single phase a capacitor start-capacitor run motor will handle many more starts per unit time. These motors have two capacitors and they are both of the run type so they can withstand continuous use. They are both used for starting with one switched out for running. The motor is also much smoother as it has far less torque ripple.
        Last edited by Evan; 11-22-2011, 08:24 AM.
        Free software for calculating bolt circles and similar: Click Here


        • #5
          Just a thought but how thick are these?
          Could you punch them instead of machining each one?

          just trying to think outside the box.

          drill 3 holes in the appropriate round stock and then cut off in slices?



          • #6
            Maybe adapt an automotive AC clutch to the motor.

            Have you tried changing the grind on your drills?A 60* point will actually produce tight curls instead of long strings.
            I just need one more tool,just one!


            • #7

              Can you put a coil spring inside the trepanning cutter to push the slugs out?
              Maybe you could use a cordless drill with a wire brush on an arbor to clear the swarf without turning off the spindle.
              Last edited by Toolguy; 11-22-2011, 10:37 AM.


              • #8
                How about a hole saw? Cut to rough dimension, stack them all on a mandrel (threaded rod?), and take a finish dimension cut on the lathe?

                I usually clear a hole saw with a stiff brush while running.

                Lots of question marks. Just thinking out loud.


                • #9
                  It sounds to me like you have the wrong tool for the job. 140 of anything would force me to spend a few hundred on either a small turret lathe or production mill so I could knock them all out in short order.

                  I dont often put a price tag on my time, but considering their typical low price (~$300 for either machine here) I would simply save the aggravation and make the investment unless youre "out there" in a Yukon-esque area.
                  "I am, and ever will be, a white-socks, pocket-protector, nerdy engineer -- born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow."


                  • #10
                    Farm it out and have someone water jet them?


                    • #11

                      If these are simple flat discs with just simple holes, why not just use a pocket jig with bushings and do them in your drill press? Not much sense in putting wear and tear on your mill for this simple job. And a jig will tend to prevent mistakes and be quicker to use.

                      I might also consider using a different type of drill. Have you tried a spur tipped spade drill or maybe even a Forstner? Even twist drills could be modified at the point to provide better chip control.

                      If you think you understand what is going on, you haven't been paying attention.


                      • #12
                        Maybe with the right chipbreaker or use of coolant/lubricant for the plastic, the strings will fall out more easily.
                        Maybe also drop the speed and increase the feed.


                        • #13
                          First make a couple of steel templates the same as the part. Drill the holes in the sheet. Cut the parts out as squares with a saw or whatever works. Stack them all up (or as many will fit between centers on your lathe) on a piece of all-thread or better a rod the same size as the centre with a shoulder on one end with extra to hold in the chuck and threads on the other end also centre drilled.

                          When you stack them place one of the templates on first, then the parts, as many as possible, then the other template. Align all the parts with a couple of rods that pass through all the parts outer holes and the ends. It doesn't need to be a close fit. Use a nut to compress the stack as tightly as possible. Chuck up in the lathe with a live centre in the tailstock and rasp the parts down to a rough approximation of the diameter. Then turn them down to size. The rods will help prevent them from spinning.
                          Free software for calculating bolt circles and similar: Click Here


                          • #14
                            I haved turned PVC on the lathe, use a small shop vac to collect the string of plastic coming off. No more stoping to clear the bit.



                            • #15
                              As I read the OP, the problem isn't drilling the holes, it's cutting out the disks.

                              A couple of possible solutions have been posted that I think I might be inclined to try: saw them out as squares and mount them on an arbor in the lathe, as per Evan. Or, use a vacuum to catch the chip string as it is peeled off, as per dwentz. I've used the vacuum trick on stringy plastic, and it worked.
                              I question the success of a hole saw, unless you drill a "drain hole" along the cut so the chips can get out. Even then, I've found that some plastics don't saw worth a hoot.
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