CNC Mill Spindle Positioning for ATC

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Thread: CNC Mill Spindle Positioning for ATC

  1. #1
    Member LBDesign's Avatar
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    Default CNC Mill Spindle Positioning for ATC

    Hi All,

    I wanted to get some advice on how to go about adding a rotary style ATC to my scratch build Milling Machine.

    I am looking into the feasibility of making an ATC. My plan is to buy an imported BT30 ATC spindle from overseas and rebuild my CNC Mill head and Z axis to suit it. I would then have to make the tool changer itself.

    I have not been able to work out how to get the spindle to index correctly so that the drive dogs will align with the ATC everytime. I see the new Tormach 1100MX has an encoder and it uses the VFD to position index the spindle correctly. (from 9:20 to 10:15 in their installation video. ) They use a m19 command. Is this possible to do with Mach3?

    I am currently using a 1.5hp 3 phase motor running from a danfoss micro VFD drive. The current ER16 spindle has an index pulse sensor that tells mach3 the rpms, but I imagine will not be sufficient to index the spindle accurately enough. Mach is controlling the spindle speed by a 0-10v output.

    The way I'm thinking, there are several options that might work....
    1) Buy a servo motor and drive to replace the current spindle motor (expensive option) and run the spindle through a spare rotary axis output.
    2) Find some way of interfacing an encoder to the mach3 system and work out how to program an index position into it and use the VFD to slowly turn the spindle until it reaches the programmed position via an M19 command.

    I am not an electronics person, I don't have the foggiest idea how to interface an encoder and have Mach read it, let alone program anything....

    Has anyone else come across this problem and solved it??

    Kind Regards
    LBDesign

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  2. #2
    Member Muzzer's Avatar
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    Default Re: CNC Mill Spindle Positioning for ATC

    If you have a decent power drawbar, you won't need to be driving the toolholder by means of the drive dogs. Do what Fadal did on my Shizioka machine and simply remove the dogs. Job done, no need for fancy spindle alignment.

    You have to wonder how many man years of time have been expended on this challenge, when in many (most?) cases it's not necessary. What is the maximum drive torque you expect on this BT30 spindle? And what torque will the pullstud system sustain with BT30 tooling? You can estimate it from the claimed pull force and the coefficient of friction of a 30 taper interface.



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    Member LBDesign's Avatar
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    Default Re: CNC Mill Spindle Positioning for ATC

    Hi Muzzer,

    Thanks for your quick reply.
    In short, I don't know the answers to your questions.
    Max torque is from a 3phase induction motor running at 240V 50hz (just shy of 3000 rpm). At the moment I have geared it to 6000rpm to drive smaller tools. so naturally, the torque will be half that of a direct drive 1:1 system.

    The specs for the spindle appear to state 280kg force and 4.5mm deflection for the retention. Do you happen to know the coefficient of friction on a 30 taper interface?



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    Member Muzzer's Avatar
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    Default Re: CNC Mill Spindle Positioning for ATC

    Must admit, I can't easily find anything that relates pullstud force to max torque although the static coefficient of friction between flat surfaces seems to be around 0.35-0.4. Relating that to a taper interface doesn't sound simple but 280kgf is the weight of 2-3 adults, so I can't imagine you'd be able to make that slip.

    1.5hp at 6000rpm is a mere 1.8Nm (16lb.in), so there is absolutely zero chance of that slipping. Sorry about mixing units. You could lose the dogs with no risk of slippage.



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CNC Mill Spindle Positioning for ATC

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