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Pockets on a manual mill?

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  • Pockets on a manual mill?

    Yesterday I was faced with making a blind pocket on my manual mill. I don't have a DRO (yet) so I was a little vexed. I drilled a hole, then touched off on the sides of the hole with an endmill and did it that way. However, in the end I ahd to cut the pocket 0.010" oversize due to a couple screwups.
    Is there are easyish way to do pockets manually? Should I just set up some indicators and do it that way to compensate for backlash?

  • #2
    Pre-DRO methods are varied.
    Two that I used were............

    1, set your dial's zeros in the middle of the lash and work your walls to corrected values.
    example, if you have .016" lash in 'X' lock down the Zero .008" from each side of the freeplay.
    adjust your target values by .008".

    2, work two adjacent walls to finish values that are 'Actual' both ruff and finish.
    pic the other two walls with corrected values during ruffing.
    and reset your zeros on the dial to finish the two odd adjacent walls last.

    Has to be a few other ways that might fit your mind-set better or not.
    I can't stand working against indicators, unless it's the only way.
    Last edited by Old Hat; 12-30-2014, 02:39 PM. Reason: adjacent... added for clarity

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    • #3
      I agree with Phil and want to add. When you have to do this be sure to check the gib adjustment ad as he said backlash in feed screw /nut. I would also conventional cut and if you have to climb cut keep tighten the lock a little and when the cutter is away from the end of the cut and slowly sneak up on it. if not the cutter will pull the work in fast. it will give you a thrill when that happens. I have also seen where some old timers would drill holes first and then finish by boring straight down after making the move with the feed dials above the work. Many machine feed screws have adjustments to eliminate the back lash. but if the screw is worn in one area and you move it too another area it will bind up.

      We were taught years ago to move the vise to each side now and then so the feed screw wore evenly and not just in the middle. :-) Rich

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      • #4
        Before I got my cheap iGaging scales, I used to write everything down. Calculate the distance accounting for endmill diameter, then write down how many full turns of the handle CW or CCW and how many thousandths after that. Then after the first two sides are done, lock the gib, take up backlash, unlock it and continue.

        It was a royal pain, but for what I was making, if I was within 0.01" I was fine. The scales make it a piece of cake now.

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        • #5
          On BPs and similar, I did like LWalker and locked the table to take up the backlash every time I had to change directions. Gauge blocks and indicators work well, too. My Kearney & Trecker die sinker actually has V's and built in dial indicators for using gauge blocks for precise distances.

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          • #6
            I am not sure how easy it is to do this on proper mill, but on my combination machine I made stops for both X and Y.
            I carefully set them up then just crank it along until you touch the stop then go back the other way.
            This has worked fairly well for the type of jobs I have done so far.
            Larry - west coast of Canada

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            • #7
              It seems to me that backlash when making a pocket should make it smaller than the dials indicate, as long as you move toward the edge of the cut. For instance, if you have the spindle centered in a 0.200" square pocket, with dials set at zero, you should be able to plunge into the work and then move 100 thousandths as read on the dial, at which point it would have moved less than that amount due to backlash. Then lock the table in that direction, and start cutting toward a corner, using conventional milling along the outside edge of the cut. When the dial reads 0.100 in that direction, minus the radius of the end mill, you should be just inside the true dimension. Now lock that direction and unlock the other, and continue until you have hit all four corners and back to the beginning of the cut. At this point you can hog out the rest of the pocket to the depth you want, and then stop the mill and measure the inside dimensions. They should be less than the 0.200" you want, and now you can sneak up on the specified dimensions by working around the periphery, using the dials to advance by the amount of backlash. If you are only taking a couple thousandths, and you have locked the table, you can probably use climb milling to get a better finish.

              It is also helpful to have the surface laid out with dye and scriber to make sure you don't make a stupid mistake, which is easy to do using a manual mill with dials. Also look closely at the dial ticks where they go through zero and then pick up at the measurement corresponding to one dial rotation. Most mills use a leadscrew that gives an integral inch value for each turn, such as 0.100 for 10 TPI, or 0.200 for 5 TPI. But my mill has a 6 TPI screw, which is 0.1666.. inches per revolution, so the divisions no longer correspond to 0.001" increments. For a small pocket, you can use a point midway on the scale, like 80 thousandths, but a pocket larger than 0.166" will cross the zero mark. So it may be necessary to calculate the dial readings you want within 0.001". That's why I got a set of DROs for my mill. If the leadscrew were not such a difficult thread pitch, I probably would not have bothered, especially considering the overall lack of precision I'm dealing with on my machine.

              The table stops are also a good idea, but you need to be careful not to move too quickly and slam into them.
              Last edited by PStechPaul; 12-30-2014, 12:19 PM. Reason: end mill radius, climb milling
              http://pauleschoen.com/pix/PM08_P76_P54.png
              Paul , P S Technology, Inc. and MrTibbs
              USA Maryland 21030

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              • #8
                Backlash is a very manageable thing, even if it is confusing.

                Phil has good procedures, I might add different explanations for 2 variations.

                If you can tolerate some possible overcut in 2 corners, it isn't too hard to rough the pocket and then finish the sides that lend themselves to the direction that you have the backlash set up for. Then when working the remaining two sides, overtravel into the corner slightly more than your best guess as to what the backlash is and then back out in the opposite direction to the dimension that you have calculated. This puts the backlash in the same direction as when you set the dial originally.

                Or, using the same sequence, raise the quill or lower the table to get the cutter out of the cut and do the overtravel in the air before adjusting the screw back in the same direction as when you set the dial originally.

                For many applications, you don't want the corners involved in the fit anyway. In that case it is a simple matter to drill holes for the corners and then mill from hole to hole. Just use a drill that is enough bigger than the end mill to allow for a small amount of overtravel, and then you will be able to dive a little deeper into the corner and then back up to take up the backlash in the proper direction.

                Or save a tree and all that paper and buy yourself a new year's Christmas present and buy a DRO.


                Dave

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                • #9
                  Way better Dave thanks!
                  I played with my wording and it just kept get'n worse.

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                  • #10
                    Originally posted by becksmachine View Post
                    Or, using the same sequence, raise the quill or lower the table to get the cutter out of the cut and do the overtravel in the air before adjusting the screw back in the same direction as when you set the dial originally.
                    Interestingly, this is the same procedure I use when writing control software for precision syringe pumps. In order to aspirate and then dispense an accurate amount of fluid (precision measured in single microliters) with a piston pump, while accounting for backlash I first have to pull the fluid from its supply, switch a valve to a waste position and push out the backlash in the drivetrain, then switch to the dispense position and dispense the desired amount.

                    Apropos of nothing, perhaps, but it was interesting to see the same concept in a different application!

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                    • #11
                      So my options are:
                      -DRO
                      -Locking, taking up lash and unlocking
                      -Just taking up lash by hand
                      -Doing two sides, then resetting dials
                      -Just move around in the air, and go back to the coordinate with the un-lashed side

                      I think the last method would work really well, so that I'm always traveling in my no-lash direction, but I will try taking up the lash as well.
                      Thanks for all the suggestions!

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                      • #12
                        I added stops to my mill in order to carry out jobs like this, see: http://mikesworkshop.weebly.com/mill...and-locks.html
                        Mike

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                        • #13
                          Originally posted by drmico60 View Post
                          I added stops to my mill in order to carry out jobs like this, see: http://mikesworkshop.weebly.com/mill...and-locks.html
                          Mike
                          Definitely stops, set them to cut the pocket just under finished size, deal with the backlash for the finishing cut only, much quicker than trying to allow for backlash on every cut.
                          Dave

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                          • #14
                            Originally posted by Daveb View Post
                            Definitely stops, set them to cut the pocket just under finished size, deal with the backlash for the finishing cut only,
                            much quicker than trying to allow for backlash on every cut.
                            Dave
                            Not when you are used to it.
                            But stops, give you a safety rail, so you don't need to be a good welder too, to get anything done.

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