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  • More mill design questions

    I am seriously designing my CNC mill. I have ordered motors and driver boards and managed to call in a favor from a friend for some nice 51,000 mfd, 40volt caps for the stepper power supply (35 volt).

    I intend to make the X axis ways from precision ground round stock, stationary on the bottom "layer" of the table. This seems to be the opposite of most mini mill designs which normally have the Y axis as the bottom axis.

    Am I missing something? My reason for doing this is because the X axis will have the longest travel and would benefit from heavy, large diameter ways, weight not an issue since the ways are not moving. I can't think of a reason not to do it this way.

    Second, I notice that the small micro and mini mills have tables much longer in the X direction than the travel in the X direction. What purpose does this serve?

    Third, I am probably going to use this as a spindle:

    I still haven't decided what to use as spindle bearings. Suggestions would be welcome.
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  • #2
    The reason the X is on top is to give a larger bed surface for work mounting.
    Same reason the X is greater than the Y is that it allows you to work on the longer part.
    If Y was greater than X you would have a problem with hitting the column with the work.

    Ironically if you make a gantry mill with two columns to get by this problem then Y then becomes X !!, X becoms Y and you still have the prblem of hitting the column.

    If you build as you say with X on the botton, Y on top and still use a long bed then a lot of it will be unsupported.

    Spindle is nice but it's still MT3. They can be a bitch at times to get the tooling out due to being a self holding taper.
    Is there any way you can get or make a spindle with a non holding taper like a R8 or even a special to suit you.

    John S.

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


    • #3
      The mini mill spindle is pretty short. It won't be easy to design a support and drive that will handle 1hp and have the ridigity you probably want. On the plus side, it is an inexpensive and readily available part. Haver you considered a part from Grizzly? They are good about stocking parts and many of their parts manuals are on line. You could get and R8 taper that way. I also agree with John - all too frequently it is necessary to whack the drawbar to get Morse taper collets to come out. Every time I do it, I worry about the bearings.


      • #4
        Have a look at this site for some idea's.

        This is designed to fit into an R8 bridgeport collet but there is no reason why you can't adapt this system to use a special collet as you only need one off collet and many off holders.

        Incidently this company Tormach are selling the ripped off Gecko drives under the Prodrive name. They know they are ripped off and defend their actions by saying "well there is no patent"

        "Well there is no patent on their toolholders" so feel free to rip the idea off them.
        What comes round, goes round.

        Here's one that a local CNC company has done for me. I have some initial figures for a batch run and believe me they are 'very' affordable.
        I just need a couple of nights to make some mods to the whole setup and this will be better as regards holding than theirs.
        Keep your eye on the tips page.

        John S.

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


        • #5
          If I had a mill with an MT #3 spindle, I would suffer with it. If I were building a mill, or otherwise had a choice, I would go for R-8 minimum, or something in ER series. I had a Benchmaster for a few years, and cringed every time I had to knock the collet out. If it was not tight enough, it would creep.
          The holding power of a collet becomes even more of a factor in a CNC application.
          It would not be at all difficult to make a spindle to accomodate a decent holding system. It would be a shame to go for cheap or convenient on such a key element of a new machine.
          Jim H.


          • #6
            I would go for the R-8 too,I got a buddy who has one of the little MT3 mills and occasionally it does get stuck and stuck good(read BFH)Plus collets are so cheap now.

            As for the travel,I think John is right,you may end up with less travel and lots of unused table.
            I just need one more tool,just one!


            • #7
              In a normal configuration,the saddle moves in the Y direction but the weight of the saddle stays centered over the knee while the weight of the table and the work piece thats on it is cantilevered over the edge of the saddle when moved near the end of travel, causing the X axis ways to wear like a rocking chair rocker.If you were to add the weight of the saddle to the X axis then this wear problem would be compounded.
              Having said that,if the X axis travel is short relative to the width of the knee,it wouldn't make much difference.
              As for the table travel(X axis) being short relative to the table length, it has to be that way or make the ways on the saddle longer. Otherwise the table wouln't have any support when moved to either extreme of travel.
              This is of course in addition to the attributes that John S. mentioned

              Jim W.

              [This message has been edited by drof34 (edited 06-25-2004).]


              • #8
                OK, ok... The same spindle is available in R-8.


                It's just that I have a bunch of MT3 tooling including a full set of collets by 1/64. I suppose I could use an MT3/R-8 adapter for that stuff. I don't like the idea of whacking it out either.

                On the X/Y issue, since I am using round ways it changes the consideration as to which axis should be on the bottom. Here is a preliminary plan for the table section of the mill. The X ways are 1 1/2" precision ground round. In this case putting the X ways on the bottom allows the use of very heavy stock with nearly zero deflection for the longer X ways. The system gives an X travel of about 11 1/2" and a Y travel of about 8"

                [This message has been edited by Evan (edited 06-25-2004).]
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                • #9
                  Evan, About a month ago I mentioned on this forum that I had purchased off E-bay a milling spindle. The item # is3804991869. this head is verry well built, using sping loaded tapered bearings. It used some unknown taper/ collet tool holder. But, I checked that an R8, or MT3 could be adapted to it. I paid #83.00 Cdn for it, and would be glad to sell it to you for that price. Shipping wont be a problem, it could be there in a few days by bus. I should mention that it has a built in swivel mount, and a right angle drive function.(purpose unknown). Doug


                  • #10
                    Thanks for the offer Doug. I don't think it would do as I intend to run the spindle at up to 10,000 rpm.
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                    • #11
                      Evan, word of warning about that Spindle.
                      My mini Lathe, has that MT3 spindle, well its not exactly the same, however it comes from the same exact factory. The internal MT3 taper is off by .001 from the outside chuck mount.
                      In any case, I hear that guy has very good customer service, he should exchange it if your not happy with the runout.


                      • #12
                        Evan --

                        Let me make a suggestion: Separate the ways as much as practical to keep the CGs of the saddle, table, and workpiece BETWEEN the ways as much as possible.

                        Keeping the composite above-ways CG between the ways will minimize the shifting of the workpiece relative to the spindle by keeping the gravity and cutting forces acting in the same direction on the ways.

                        Also, as a fairly trivial point of semantics, I am fairly sure that the (US) National Machine Tool Builders' Association defined the X Axis as the translating axis that is both horizontal and perpendicular to the Spindle Axis, with a conditional clause that
                        if both axes perpendicular to the Spindle are horizontal the X Axis is the longer of the two.


                        [This message has been edited by John Garner (edited 06-26-2004).]


                        • #13

                          I have seperated the ways as much as I can and still have acceptable range of table movement. I am constrained by the size of the parts I can reasonably machine. I am also constrained by the size of the material I have for the base of the mill which is two pieces of 3/4" 6061 aluminum fastened together to make a piece 14" X 20" X 1 1/2" thick. I ran a calculation for Young's modulus on the 1 1/2" steel ways 20" long with a 100lb load at the center supported at the ends. It predicts a deflection of 6x10^-7 inches. It should be fairly rigid. I could easily get away with 1 1/4" ways for the X axis.

                          As for the nomenclature, it seems I have it correct by the definition you gave. Also, in graphics work and math the X axis is left/right, Y is up/down and Z is in/out. This corresponds to what you see looking down on a mill from above.

                          [This message has been edited by Evan (edited 06-26-2004).]
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                          • #14

                            Hi Evan,
                            Here’s how I would do a new spindle design for morse tapers. The hexed bolt screws into the taper to hold it from slipping out especially when you are cutting with endmills that can chatter in certain stock. (I’ve seen morse tapers �pop’ out during this operation) When you want to remove the MT just back the draw bolt out till it hits the top of the collet and give it a twist.The collet screws snug to the top of the spindle and can remain in it.You can put two flats on it to hold when you have a stubborn MT. If you keep a �T’ handle hex wrench around it is a easy task to remove the MT.

                            Two questions, why are you using such a large capacitor in your stepper supply and what's the max current draw of the steppers?


                            • #15

                              I have decided upon the advice of the other members of this board to use an R-8 taper spindle. I know it isn't the best design but it seems to be the North American standard. I can easily and cheaply adapt it to my existing tooling. I did have a plan for removing the MT3 tool holders without pounding on the bearings but the non-locking R-8 appeals to me more.

                              Capacitors? Rule of thumb is ten thousand microfarads per amp. If I am driving all four axes at once then it may take up to 10 or even twelve amps of current. I am a bit light on the filter capacitance.
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