Harrison Universal Miller Conversion


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  1. #1
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    Default Harrison Universal Miller Conversion

    I started the CNC conversion of my new Milling Machine in a rather random place, with a pneumatic drawbar. This was rather prompted by this thread:
    http://www.cnczone.com/forums/showpo...37&postcount=1
    In fact I started off posting in that thread (http://www.cnczone.com/forums/showpo...&postcount=124), but have now decided that I should start a new one, as I am not actually making a spindle, just the drawbar. The intention is a full CNC conversion running EMC2. (I have already done one machine, which is working as well as can be expected for the starting machine).

    The drawbar fits in place of the standard drawbar with no modifications to the spindle. It uses a finger-style gripper:
    Harrison Universal Miller Conversion
    From Gibbs

    which is released by a specially-made pneumatic cylinder:
    Harrison Universal Miller Conversion
    From Gibbs

    and is held clamped tight by a stack of Belleville washers in the conventional way.

    [nomedia="http://www.youtube.com/watch?v=pxrzJ_KfcQ0"]YouTube- Drawbar[/nomedia] shows the drawbar in action.

    Similar Threads:


  2. #2
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    That is fantastic!

    Very nicely mace, simple and even quiet in operation!

    I'm really looking forward to seeing the rest of your conversion



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    Sectional view of the solid model:
    Harrison Universal Miller Conversion
    From Gibbs




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    Question Question

    Andy - is there any way you could show in that cut away view which parts rotate with the spindle and which are stationary. Some hatch lines or ??? Also is there any sort of bearing between rotating parts or just an air gap?

    Looks great!



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    I can describe it in words, if that helps?
    The orange/brown part is the top of the milling head, comprising the bearing cap and the top of the rotating spindle. I didn't bother modelling it as separate parts as it didn't matter to me.
    The red/pink parts and the dark green part all rotate with the spindle. As do the collet segments and the actuating rod.
    The grey parts and the spring are stationary. There are no bearings, just an air gap of at least a couple of mm. It is intended that the spindle should always be stationary when the drawbar is released.

    When air pressure is applied the piston moves down until a hardened steel plug (not shown) presses on the top of the drawbar central rod. Then the whole cylinder assembly floats upwards until the lower plate contacts the underside of the collar on the top of the drawbar, and then the drawbar is "squeezed" open.
    I have seen many bits of advice that the spindle bearings are "not designed" to take axial loads so decided to use this "pinching" action.
    As it happens, I am totally unconvinced about that "fact": The taper rollers in my spindle are rated for something like 50kN static axial load according to the manufacturers.



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    Default Z-Axis

    I found that steel (or Alu) blanks of sufficient size for the Z-drive conversion were quite expensive, so I decided to go for iron castings like the original machine builder did.
    I made the patterns by gluing together 18mm MDF into blocks, then machining, hand-carving, filling, sanding etc. Then a couple of coats of spray-filler and red-oxide paint.
    Harrison Universal Miller Conversion
    From Gibbs
    I then sent the patterns off to a foundry in Birmingham and the castings came back a week later, for £25 each.
    Harrison Universal Miller Conversion
    From Gibbs




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    Wow, your lucky.

    I have not been able to find a decent foundry that will do small lots in iron.



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    andy: great work. We have looked at getting local castings and they don't seem too expensive.

    what is your plan for cnc'ing the axis?

    sam



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    Quote Originally Posted by samco View Post
    what is your plan for cnc'ing the axis?
    I am keeping the Acme screw (I think a ballscrew would need a brake, and I am sure the weight of the knee is more than any conceivable upwards cutting force).
    Currently the machine has a rotating screw and a static nut on top of of the machine base.
    I am converting to a rotating nut in the same location, with the motor in the machine base rotating the screw through a drive tube coaxial with the screw. I will then fit a locking pin to the existing manual feed (and possibly one to the CNC axis too, to retain the possibility of manual drilling)

    The casting that there are two of is a motor mount, it clamps round an extended bearing housing that pokes into the machine base. The motor fits into the part-circle on the top of the casting.



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    After a few months distracted with writing drivers for brushless servos in EMC2 I got back into the workshop today.
    I have finished the machining of the castings, and the Z axis is assembled.
    Harrison Universal Miller Conversion
    From Gibbs

    The design might require a little explanation.
    The original design for the table up/down is a rotating leadscrew and a fixed nut mounted in a casting on the base of the machine. The screw passes through into the machine base. I wanted to retain the option of hand-operating the table for those quick jobs you sometimes have, so my plan is to simply lock the screw in the table when in CNC mode, and to lock the CNC axis in manual mode (which is what the boss in the leftmost casting is for) I have therefore replaced the nut housing casting with one which has space for a double angular contact bearing in which the nut can rotate. The housing is extended downwards (to the right in the picture) through into the space in the machine base. The motor mount clamps onto this housing extension. Down through the middle is a machined tube, the upper end of which houses the leadscrew nut, and the lower end of which has the pulley fastened on to it. The pulley allows enough offset of the motor from the axis for the leadscrew to pass past it.

    I suppose I ought to paint it.



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    can you see much of an issue with balancing the spindle bits your making?
    We were looking at making a spindle but the grinding and balancing to finish the parts seemed to put it in the too hard basket.

    www.vapourforge.com


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    Quote Originally Posted by Valen View Post
    can you see much of an issue with balancing the spindle bits your making?
    The max speed of that spindle is about 2000rpm, and none of the actual rotating parts I have made is more than 25mm dia. or so. Time will tell, but I hadn't even considered it, I just saw my new bits as a modification to the existing drawbar.



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