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  1. #1
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    Power Draw Bar Help for PCNC

    I'm trying to make my own Power Draw Bar for my PCNC 1000 and I already have the air cylinder and solenoids. Can someone let me know the size for the standoffs? Tormach doesnt sell all the Power Draw Bar parts

    Thanks for the help!

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    Quote Originally Posted by mendez View Post
    I'm trying to make my own Power Draw Bar for my PCNC 1000 and I already have the air cylinder and solenoids. Can someone let me know the size for the standoffs? Tormach doesnt sell all the Power Draw Bar parts

    Thanks for the help!
    Is this what you need

  3. #3
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    Re: Power Draw Bar Help for PCNC

    Quote Originally Posted by Karlwinter View Post
    Is this what you need
    Thanks but I need other dimensions. Please see attachment
    Attached Thumbnails Attached Thumbnails dimensions.jpg  

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    Quote Originally Posted by mendez View Post
    Thanks but I need other dimensions. Please see attachment

    3.7000" for the PDB legs

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    Re: Power Draw Bar Help for PCNC

    Quote Originally Posted by Karlwinter View Post
    3.7000" for the PDB legs
    Thanks!

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    Re: Power Draw Bar Help for PCNC

    I spend a few hours today designing and making a quick mock-up of my Power Draw Bar using some pictures of the existing Tormach design. Can someone explain to me how the Eccentric Mount Post work? Could someone share some pictures?
    I will be making some 3D printed parts as proof of concept and see if I could start machining this weekend.

  7. #7
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    Re: Power Draw Bar Help for PCNC

    The idea is that there is no force (except gravity) downwards on the bracket, so that doesn't need much to hold it.
    But, when the PDB is engaged, the bracket slides in under a flange on the spindle that can retain it when the PDB actuates, which will attempt to pull the legs/brackets up, away from the spindle -- this is the same force as the release force pushing down on the drawbar.
    The "eccentric mount" is just a hole through a bolt, that lets you pivot the thing out of the way if you need to change belts, and a pin on the other side, to hold/release it. There's very little precision needed -- just jam that cylinder down on the head of the drawbar!
    The bracket has a crescent-shaped slot cut out of the backside such that it can slide in under the flange on the spindle, and it has enough clearance that there is zero risk of it scraping against or catching on the spindle while it's in position but not being actuated.

  8. #8
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    The eccentric bolt is used to center the slotted bracket on the bottom of the drawbar cylinder on the flanged collar on the end of the spindle.

    Do not allow the slotted bracket to drag on the spindle flange. Not only will this be noisy, if the bracket hangs on the flange,it will unscrew jamming drawbar and spindle into a single mass. And I know this how?

    Gary

    Quote Originally Posted by mendez View Post
    I spend a few hours today designing and making a quick mock-up of my Power Draw Bar using some pictures of the existing Tormach design. Can someone explain to me how the Eccentric Mount Post work? Could someone share some pictures?
    I will be making some 3D printed parts as proof of concept and see if I could start machining this weekend.

  9. #9
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    Re: Power Draw Bar Help for PCNC

    Thanks guys. OK so I have a gap between the spindle nut and the slotted bracket (what holds the pneumatic cylinder).

    How much gap is OK between slotted bracket and spindle nut?

    Please see attachment. The eccentric bolt & Pin (other side) will stay fixed but allows the slotted bracket to move up. Does it look OK? I installed the 3D printed parts and it seems to be fine. I will post more pictures later.

    Attachment 442510
    Attachment 442508

  10. #10
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    Re: Power Draw Bar Help for PCNC

    Stupid question. Can I use Aluminum 6061 instead of steel? I don't see why not, since it should hold up well too; plus reduce significantly the weight in the head.

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    Re: Power Draw Bar Help for PCNC

    Here is installed. Last thing to check is the standoff but it should be OK. I will try to start cutting chips this weekend or next week.

    Click image for larger version. 

Name:	3D Part.jpg 
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ID:	442514
    Attached Thumbnails Attached Thumbnails 3d Parts.jpg  

  12. #12
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    Re: Power Draw Bar Help for PCNC

    Did you use a rougher on the profile of that bracket? It looks almost 3D printed :-)
    As long as that bracket can move up when the changer engages, that looks good!

    I would not use aluminum, as the load bearing area between the collet and the bracket isn't that large in surface area, and there's significant forces involved in the pneumatics. Aluminum is just slightly too squishy for me to be comfortable with that. (I bet if you actually do the calculation, it'll probably be fine, though... but still ...)

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    Re: Power Draw Bar Help for PCNC

    Quote Originally Posted by jwatte View Post
    Did you use a rougher on the profile of that bracket? It looks almost 3D printed :-)
    As long as that bracket can move up when the changer engages, that looks good!

    I would not use aluminum, as the load bearing area between the collet and the bracket isn't that large in surface area, and there's significant forces involved in the pneumatics. Aluminum is just slightly too squishy for me to be comfortable with that. (I bet if you actually do the calculation, it'll probably be fine, though... but still ...)
    LOL, those are 3D printed parts. I wanted to make sure everything fits before I start machining. I did stress analysis and buckling and it seems to be fine with aluminum bracket (5/8" thick) and steel standoffs.

    In regards to your comment about the load bearing; the forces are still the same if I have steel vs aluminum?.....I guess, if aluminum fails due by shear or buckling...then it will translate that load to the load bearings?

    Does anyone know the amount of load the pneumatic will produce at 90-120psi? 1,000 ft-lb OK to assume?

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    Re: Power Draw Bar Help for PCNC

    Yeah, that sounds plenty. I would actually be more worried about the contact area between the bracket and the flange on the spindle -- it's a somewhat narrow segment. You may end up getting "impressions" of the collet into the aluminum, which will weaken it, and eventually basically "strip" it.
    Given that these drawbars are also operated by normal non-gorilla human beings, though, the math seems to work out. And if we can't trust the math, then what can we trust?

    By the way, if the aluminum fails, what will happen is that the pneumatic cylinder will just lift itself against the drawbar, and the next stop for the bracket is the pin and the screw. Once those are stripped, the cylinder will simply just push itself up, and the tool won't release. I don't think anything more dramatic than that will happen. (Maybe the cylinder will jump half a foot in the air if it happens while it's under compression?)

  15. #15
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    Quote Originally Posted by jwatte View Post
    Yeah, that sounds plenty. I would actually be more worried about the contact area between the bracket and the flange on the spindle -- it's a somewhat narrow segment. You may end up getting "impressions" of the collet into the aluminum, which will weaken it, and eventually basically "strip" it.
    Given that these drawbars are also operated by normal non-gorilla human beings, though, the math seems to work out. And if we can't trust the math, then what can we trust?

    By the way, if the aluminum fails, what will happen is that the pneumatic cylinder will just lift itself against the drawbar, and the next stop for the bracket is the pin and the screw. Once those are stripped, the cylinder will simply just push itself up, and the tool won't release. I don't think anything more dramatic than that will happen. (Maybe the cylinder will jump half a foot in the air if it happens while it's under compression?)
    I agree! Thanks for the feedback. On anoder subject. I dont think tormach balances the spindle nut right?

  16. #16
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    Re: Power Draw Bar Help for PCNC

    I dont think tormach balances the spindle nut right?
    I have no clue, but for a 10,000 rpm spindle, I'm thinking it would be best if they did ...

  17. #17
    Does anyone have the dimensions for the spindal flange nut and the base plate by chance? Mine got lost in a move before I could put them on my machine.

  18. #18
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    Re: Power Draw Bar Help for PCNC

    mendez - Would you care sharing your CAD files when your done?

  19. #19
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    Re: Power Draw Bar Help for PCNC

    There are PDB files for the 1100 on Grabcab.com.

    https://grabcad.com/library/diy-powe...tormach-mill-1

  20. #20
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    Re: Power Draw Bar Help for PCNC

    Here is a video of my PDB (You can skip to 13:40). I uploaded the CAD files and parts list with a source for the air cylinder to Grabcad a couple of years ago. The PDB is still working perfectly. I made all of the parts on my Tormach

    https://www.youtube.com/watch?v=4YOtbSDSXNA&t=1s

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