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Milling question

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  • Milling question

    I have a 9x20 lathe and am making some t bolts for the cross slide using a mini mill. I am pretty brand new to using a milling machine. My question is how do I accurately mark out the steel so that I can mill it to size to go into the t slots on the compound? Is it best to use digital calipers and just keep taking of very small amounts of steel and measuring on each pass or is it best to mark it out ?

  • #2
    It depends on the accuracy of your mill, which you need to get a feel for. When I first get a new machine, I mark it out, and then I check things constantly. I keep a little notepad around in the shop where I'm noting that stuff. Work goes slowly, but I know if I turn the wheel how much the resulting cut is and how reliably I can make that machine produce. Eventually, I get comfortable with just turning the wheels and perhaps checking just before the finish cut.

    The other thing to keep in mind is this particular application doesn't have to be real precise. The T-Nuts need to be small enough to fit the slots comfortably, and no smaller.

    I'd still recommend marking out and taking some frequent measurements so you can get the measure (pardon my pun!) of the machine.


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    • #3
      Not knowing your setup this may not all work for you, but I'll give it a try.

      First I'd mill the stock to its larger dimensions height/width. Measure what the stock is, then figure out how much you have to take off. Say it's 0.020" oversize. Put the stock in your milling vise, set up on a parallel (or two) so it's properly horizontal and protruding maybe 1/8" above the top of the vise jaws. Bring the end mill down (with power off), touch the top of the work, and zero your Z (vertical) dial. For this job, that will be close enough. Then move off the work, lower the end mill the required amount (here we're assuming it's 0.020"), lock the Z movement, turn on the machine, and mill the top of the work at that setting. Repeat for the other dimension.

      Now you have to make the "T" part of the T-nut. Once again place the work in your vise, this time blocked up on parallels so enough protrudes so you are holding only on what will end up being the "base" portion of the T-nut. Use an edge finder to locate an edge of the workpiece, then figure out where you need to mill to based on that location, factoring in 1) the diameter of the edge finder) and 2) the diameter of the end mill you're using. Zero your Y dial appropriately, move over (possibly in several passes) as required, milling one side of the stem of the T. Repeat for the other side.

      In other words, I wouldn't mark out at all -- use the milling machine dials to measure how much you need to cut, although marking out can be a good idea to help make it obvious if you're going to do something drastically incorrect. Remember to always take up backlash in the same direction.

      Does that help any?
      Last edited by SGW; 11-21-2006, 10:11 AM.
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      • #4
        Z axis stop

        Hello Jim,
        It is a good idea on the mini-mill to slide the stop up on the column so that
        you will not have it drop down more than you intended for it to do.
        What I do is to bring the tool down until it touches the work (not Running) and if I wanted to take off .025" I would place a .025" feeler gauge between the stop and slide the stop up and tighten it. Now you can make your cut removing say .010" at a time without worrying about removing too much.
        There is a lot of slack in the z axis down feed and this helps prevent accidental drops.I believe that hold downs are 12mm.
        Chuck Marsh


        • #5
          Thank you one and all for the replies, its answers like these that make one hell of a difference to beginners, much appreciatted.


          • #6
            I learned here from one of the guys you can by these really large magic markers. I paint the surface in the vicinity of where I want to mark with magic marker fluid and scribe my lines. I used to use DyeKem but it's awful messy. It comes off with denatured alcohol you can get at the hardware.

            By the way, where your lines meet, if you scribe them with more pressure you can then take a sharp punch and it will find the intersection if you pay attention to the feel as you go over the area with the punch. It's easier to find the intersection by feel then by looking once you get into the immediate neighborhood.
            Last edited by Your Old Dog; 11-21-2006, 06:39 AM.
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            • #7
              other ways

              Jim, rough and ready T bolts can be made by using standard bolts. Turning the bolt head diameter or milling two of the hex flats to just under the T slot width then cross drill and pin with silver steel to prevent rotation when tightening.
              If the hex flats fit well you`re sweet but if the shank diameter is too large just 'waist' it in the lathe to accomodate the slot width.

              If you are milling flats and you do not have a dividing head etc; you might try getting some square/rectangle stock,face off the ends, drill a hole through the centre to take the bolt neatly. Tighten your T bolt blanks firmly through said stock.
              Hold the stock (with the T bolt attached) firmly in the vice then machine one flat, turn the block through 180 and machine the other side. Done.
              To calculate how much material to remove.
              Measure the T slot major width minus, say, 20 thou, subtract from the T bolt blank diameter and divide by 2. get some roll your own papers, a bit of spit and put it on the surface you wish to machine then bring the tool down to the work slowly. When you are within coo ee start her up and slowly lower `till it picks off the paper. You`re near enough so start machining from there.

              If it is possible always use spacers with T bolts, you do not want to tear the guts out of the T slots.

              I hope that is clear enough?
              Last edited by speedy; 11-21-2006, 04:24 PM.


              • #8
                I would mill all your outside dimensions to size (overall dimensions) and leave your part in bar length that will fit in your vise. You can then set it on parallels as suggested. You can figure out what you have to mill off each side of the neck portion of the "T". Mill this off one side, zero your "y". Turn the part end for end and mill the other side returning to your zero on the "y". Now the neck is on the center of the "T". Cut off your lengths, finish the ends, drill, tap, done.
                Probably easier done than said!
                Just my $.02.



                • #9
                  Originally posted by JimDobson
                  I have a 9x20 lathe and am making some t bolts for the cross slide using a mini mill. I am pretty brand new to using a milling machine. My question is how do I accurately mark out the steel so that I can mill it to size to go into the t slots on the compound? Is it best to use digital calipers and just keep taking of very small amounts of steel and measuring on each pass or is it best to mark it out ?
                  if you read older machining books, there is an emphasis on marking out. i think this is a legacy from the days when a lot of work was done by hand, filing, cold chisel etc or maybe machine tools didn't always have graduated dials. its done less frequently nowadays because work is done differently - how many apprentices spend their first year at a bench vice?

                  still, laying out can be very useful, layout out a hex hole to file, sheet metal work, castings, or milling an angle to a scribed line. For simple part like a T nut where dimension aren't critical, it may even be faster to quickly lay it out and cut to the lines rather than measure and work of the feed screws, but either will work