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  1. #1
    Join Date
    Apr 2008
    Posts
    38

    Milling machine design and build

    Here is a small mill I intend to build. The rectangular tube 8 x 4 inch hot rolled steel with a 3/8" wall. Rails are THK HSR20. Travel is 24" X, 12" Y, and 12" Z. The table is a piece of 1.25" thick MIC-6 aluminum plate. Table size is 22" x 10". 3/8" 16tpi holes will be drilled and taped on 1" centers. The spindle will be a minimill spindle with two angular contact bearings and a custom housing. The motor is a 2.5 HP DC treadmill motor. The spindle will be driven with a timing belt from the motor with a 1:1 ratio. Maximum spindle speed is 4000 rpm. I plan to use 2005 series Chinese ballscrews from LMB2008 on ebay.

    The rails are mounted in pairs 5" apart. Do I need more space between the blocks on the X axis? The X axis is 33" wide and weighs about 70 pounds. The outside edges of the blocks are currently 8" apart. Does the head look beefy enough? It is 14" long and made from 1/2" steel plate. I intend to add a counter weight so the ballscrew does not backdrive under the weight of the head.

    I plan to weld everything together and then have it ground or milled flat. The joint between the base rectangle tube and the vertical rectangle tube will be filled with epoxy after it is squared. The whole structure will be filled with concrete after is is assembled.

    Any thoughts on this design or things I have overlooked?
    Attached Thumbnails Attached Thumbnails millversion1.png  

  2. #2
    Join Date
    Feb 2012
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    0
    The motor mount and its axis is too flimsy in my opinon and would benefit better if mounted rail was put either side of the 8x4 tube,so that the motor mount would slide up&down the mounted rail.

  3. #3
    Join Date
    Apr 2012
    Posts
    71
    As always the first step in machine tool design is build it rigid,Even the professionals stuff that one up.
    Where I worked a number of years back we were given an Italian Boring machine for Engine boring,prior we used an older strap on style boring bar,This machine on a six month free trial proved worthless as the boring head was so long the chatter it induced meant either very light cuts or leaving around 3~4 thou for honing in the Sunnen machine to clean up the finish and not leave any boring marks.
    Totally unacceptable for a Au$55,000.00 machine the time cost alone with method one was a bad joke and with method two the cost in replacing stones on the Sunnen Hone became quickly out of control.

    Therefore when designing any machine put in a factor above what you envisage for Strength,Stiffness and the size of work you envisage,A bigger machine can always do small jobs but a smaller machine can't do big jobs,Design it right build it right and do it once.

  4. #4
    Join Date
    Feb 2012
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    0
    Any thoughts on this design or things
    A milling machine doesn't necessary have to have the vertical column it can also have the cutting head mounted and moved horizontaly across the bed,the probem here though the bed has to be rigid when lifted up or lowered for height.

  5. #5
    Join Date
    Sep 2005
    Posts
    1195

    DIY steel cnc mill all bolted together

    christofer,
    Depend on what you want to cut, more weight more rigid. Your milling head IMHO, not too rigid as horid said. If you DIY your cnc mill, why not using thick plate for base and z axis. Your time and money spent not so much different. Aluminium is more expensive than steel, but al easier to drill. I also DIY my cnc mill using all steel (thick) and bolted together, now weight 500 kgs. When done, maybe 1 ton. Size about BF46. Mechanically almost done. Here picture w/o milling head. I will post current picture c/w milling head when I got time. I am waiting for electronics. I am using belt drive 1:3 ratio Toshiba motor 3 hp 3 phase for milling spindle motor.
    Attached Thumbnails Attached Thumbnails DIY CNC milling.jpg  

  6. #6
    Join Date
    Sep 2006
    Posts
    6463
    I would just like to say one thing....the further up the pole you go the more the pole deflects.

    A boring bar is considered to be at it's maximum practical rigidity at 4D, or 4 times it's diam stick out......you can just get away with 6D, but deflection becomes a factor in accuracy of the bored hole, no matter how fine the final cut.

    In the design in post #1 the column is approx 10 X it's base area in length.

    The same applies to the design in post #5.

    A column of 1200mm height should have a base at least 300mm in width and at least 400 mm in depth (back) to resist deflection, and the sides sloped to resist deflection.

    I refrain from commenting on the use of the square tube for the column and base in post #1.

    The design of a mill drill, with the round column is a typical example of the large mass on a small column approach that does not work.

    The Empire State building sways in the wind....the pyramids do not even move.
    Ian.

  7. #7
    Join Date
    Sep 2005
    Posts
    1195

    Dimension.

    handlewanker,
    My baseplate is 450mm X 650mm (I should have 450mm X 750mm), it is 1" plate all on bottom. Z is 2 ea slabe 200 mm X 50 mm X 950 mm and 1 block in the middle of slab 100 mm X 100 mm X 750 mm. Quite heavy, man. See attached my milling head including spindle. This milling head are spindle 18 kgs, box 17-1/2 kgs, motor 18.5 kgs.
    Attached Thumbnails Attached Thumbnails Image0229.jpg  

  8. #8
    Join Date
    Sep 2006
    Posts
    6463
    Hi, in post #5 you state that "in the end maybe 1 ton"....you can't compensate for design rigidity by increasing the mass.

    The Bridgeport mill weighs about 1 ton, but it has been designed around the body/column area to withstand deflection.

    In the design in #5 post the column attached to the 1" base plate is just nonsense.

    As this is primarily a CNC machine, the cutting forces will be much lower than for a conventional vertical bench mill, where large roughing cuts and fine finishing cuts are normal, whereas in CNC work the cuts are just lighter and many, so design limitations will probably not be so apparent.

    BTW, weight in machine design is not an ideal design factor......when you reach resonant frequency the weight becomes a stored reactive force operating in both directions, like a heavy pendulum.
    Ian.

  9. #9
    Join Date
    May 2005
    Posts
    3920

    Important concepts here, mass does nothing for you if in the wrong place.

    Hopefully many will read this. There is this idea that floats around that all you need to do is to add mass to make a good machine. Mass isn't a bad thing if it is in the right place but just throwing in mass won't correct basic mechanical problems.

    Quote Originally Posted by handlewanker View Post
    I would just like to say one thing....the further up the pole you go the more the pole deflects.

    A boring bar is considered to be at it's maximum practical rigidity at 4D, or 4 times it's diam stick out......you can just get away with 6D, but deflection becomes a factor in accuracy of the bored hole, no matter how fine the final cut.

    In the design in post #1 the column is approx 10 X it's base area in length.

    The same applies to the design in post #5.

    A column of 1200mm height should have a base at least 300mm in width and at least 400 mm in depth (back) to resist deflection, and the sides sloped to resist deflection.

    I refrain from commenting on the use of the square tube for the column and base in post #1.
    Well I will. There can be useful machines built around such arrangements, after all most drill presses aren't much more than a skinny column. Even a milling machine can be built this way for specific needs, but is hardly advisable for machining steels and other harder materials. In the end it is the application that dictates the suitability of such a machine and if your application is a general purpose mill such an approach is highly questionable.

    The design of a mill drill, with the round column is a typical example of the large mass on a small column approach that does not work.

    The Empire State building sways in the wind....the pyramids do not even move.
    Ian.
    This is a very good analogy. While it might not be practicle to build a mill in the shape of a pyramid, building in the shape of a pencil is to be avoided.

  10. #10
    Join Date
    Sep 2005
    Posts
    1195
    handlewanker,
    On my build, behind the column, I put 1" steel plate to hold the column more rigid, and also there are 2 ea angle steel (5" X 5") on the side of column. The bolt size is min 5/8" 8-unc. Ballscrew on column is 2505 THK. I plan to mill steel w/this cnc mill. Sorry, I am not hijacking this thread.

  11. #11
    Join Date
    Sep 2006
    Posts
    6463
    Hi Asuratman, your design is quite interesting, but in my opinion a bit on the overkill with mass.

    I would like to see the design criteria that you designed from, ie the type of machining the mill would be undertaking and the design approach to offset the forces encounted in the milling cutters.

    If'n you are going to use end mills and slot drills only in R8 collets, then the design is 100% overkill with little to gain from such massive members in the column.

    Going to larger size shell mills with R8 shanks would mean greater side deflection forces, and this machine would never handle a shell mill of say 75mm diam with any degree of confidence.

    I hesitate to think of the inertia forces that come into play when the combined weight of the head and 3ph 3HP motor are wound up and down while cutting at speed.

    CNC is all about many cuts of a light nature, which is totally different from manual milling where a few heavy roughing cuts are made and after measuring a further few light cuts to get to finish size.

    Manual milling needs a very rigid structure due to the nature of the machining process, and a light manual mill could also be capable of the same amount of metal removal but with a lot more lighter cuts, even though more time consuming.

    CNC falls into this category too, but the time factor is cancelled out with the program controlling the repetitive machining cuts that would have driven a manual machinist to drink.

    In my very humble opinion, a mill comprising of a fully welded and braced steel plate box section framed body would make a far more interesting design, closely approaching the sections of a casting in nature and very DIY capable for anyone with welding capability and equipment.

    In that design I would venture to suggest that the column and box base be made to form one piece, welded up from several pieces of 10mm steel plate and braced internally to attain the desired shape, but that's just my preference......circumstances dictate the method.

    Thr pyramid shape I mentioned earlier is totally practical in that if you have a column in the form of a cone with the top cut off, you have the Bridgeport column/base design, where the base casting is a box form with the column rising in a cone to the ring seating for the ram at the top.

    In a cone, at any one time, the sides are under either tension or compression depending on where the deflective force is applied, and reinforce one another.

    A straight tubular or square column acts like a parallelogram and tends to bend at the sides without resisting the side loading forces.....the full pyramid form has the ultimate strength to resist deflection, and although that is not a practical shape for a mill column, a frustrum of a cone shape it is almost as good.

    BTW, Wizard, the drill press with the slender steel (solid?) column does not have to resist side forces, only vertical thrust from the rack and pinion driving the quill, and most, if not all, drill press tables deflect downwards under load if measured, (a scissor jack under the table cures this).......the mill/drill heads deflect upwards and back when under drilling loads, and sideways when milling.
    Ian.

  12. #12
    Join Date
    Apr 2008
    Posts
    38
    Thanks for the advice. I plan to increase the size of the rectangular tube to 8 x 6 inches. I also plan to use a 1/2" thick wall tube rather 3/8" wall tube. I can't imagine a 8 x 6 x 1/2 inch steel tube filled with concrete will deflect much. My mini mill has a tube that is 5 x 2 x 1/4 inches and it works ok for light cuts. I imagine the design of this larger mill will be many times more rigid.

    is 5" spacing between the HSR20 rails ok?

  13. #13
    Join Date
    Aug 2009
    Posts
    291

  14. #14
    Join Date
    Sep 2005
    Posts
    1195
    christofer,
    is 5" spacing between the HSR20 rails ok?
    I am not machine designer, but I have 2 jap cnc mill (rokuroku and mazak), the gap between rail on the base is about 1/3 +/- of table length, so I follow this.

    rocketflier,
    That milling head is almost the same as mine. If not incline on bottom, my milling head machine will hit table too much.

  15. #15
    Join Date
    Feb 2010
    Posts
    119
    i have only made the z axis

    here's what i used. 6 inch square box tubing with .5 inch wall 36 inches long 1 inch plate at the bottom. put some .5 plates front and rear.


    most of my cutting is done with in 6 inches of the table. the rest of the z axis seems to be used for changing tools.


    just double up your z axis to make it 8"x8" and that should be ridged enough
    Attached Thumbnails Attached Thumbnails mill install 001.jpg   enclosure 053.jpg  

  16. #16
    Join Date
    Apr 2008
    Posts
    38
    bozidar22,

    Did you use hot rolled steel for everything? How about the supports for the rails? Did you machine the rail supports flat after welding everything together? Did you have any trouble with warping or inaccuracy of the rail supports?

    I very much like your design.

  17. #17
    Join Date
    Sep 2005
    Posts
    1195
    In my very humble opinion, a mill comprising of a fully welded and braced steel plate box section framed body would make a far more interesting design,
    Handlewanker, christofer
    If you welding together, using thin plate (maybe <0.5 in) you need to stress relieve.

    you have any trouble with warping or inaccuracy of the rail supports
    That is true, I experienced like that on my previous build. Use thick plate or steel block and bolted together is cheaper and faster way and alot heavier.

  18. #18
    Join Date
    Feb 2010
    Posts
    119
    i used hot rolled that was scrounged from the scrap yard. there was some warpage from the welding. the rails sit on .75"x 2" hot rolled. it weights around 200 pounds.

    the maching was done on a 20" shaper and was hand scraped.

    it was alot of back breaking work.

    use thick plates
    Attached Thumbnails Attached Thumbnails diy mill pics 022.jpg   diy mill pics 026.jpg   diy mill pics 031.jpg  

  19. #19
    Join Date
    Feb 2010
    Posts
    119
    do a search for this document, the Pdf file was to large to add as an attachment. hope it helps

    Principles%20of%20Rapid%20Machine%20Design

  20. #20
    Join Date
    Apr 2008
    Posts
    38
    Here is my latest design. The rails are 7" apart. X travel is 22", Y travel is 12", and the Z travel is 12". The column and base are 8 x 6 x 1/2 inch rectangle tube with rectangle bars added for rail support. I intend to use hot rolled steel, weld everything together, then take it to a shop to have it machined flat and square.
    Attached Thumbnails Attached Thumbnails millversion2.png  

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