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Thread: Power draw bar idea - Feasible?

  1. #13
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    Yeah, I've seen parts rated for 3000psi, but the system I described and imagined in my head would be made up of parts made.. by me At this point in my "career", there's only so much trust I'm willing to put in my own parts!

    I guess there is very little to be invented anymore*, but I'm glad what I "came up with" isn't as far fetched as I had feared!



    *ok, that's not true. But boy does it ever feel like it!



  2. #14
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    Quote Originally Posted by TiagoSantos View Post
    After exchanging a couple of PM's with Air_GuNNeR (love his power drawbar setup!) I had some ideas as well..

    I'm thinking of a pneumatic cylinder, about 2" bore and maybe 1" travel.. This would be mounted on the side of the head, and would push on an head mounted hydraulic cylinder, something with a small diameter but fairly long travel (well, about 1" to match the pneumatic cylinder), let's say a 0.25" piston. I'd connect a stainless braided brake line (I have a few lines I removed from the race car as preventive maintenance that would be perfect for this) to this master cylinder and connect it to a large diameter, small stroke cylinder on top of the drawbar, with a scissor type setup similar to what most people do with the pneumatic drawbar actuators.

    2" bore pneumatic cylinder at 50psi = 157lbs. For 3000lbs at the belleville stack, that's a 19:1 ratio. The 0.25" piston has 0.049sqin of surface area, so I'd need a big piston with 0.9359 sqin. Let's make it 0.940, which gives me a diameter of 1.094"! And for 0.050" of movement to collapse the bellevilles and push the drawbar down, I'd need.. 0.955" of travel at the small piston! How about that.. I swear I was making it up as I went.

    Hmm..

    If you are going to mount it on the head anyway; why not just get the right size pneumatic cylinder right off the bat and skip all the hydraulics? I just don't see the advantage of mounting all those things on the head.



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    I'm limited in height. I can mount anything to the side of the head, just not on top.. There are plenty of solutions, I just thought this was an interesting option.



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    Thanks for all the great info!

    After reading that a smaller machine like my X2 only needs about 600-800lbs of force, it makes the pneumatic solution a little easier for me.

    I'm thinking of using this guy Aluminum Single Acting Spring Return Cylinder | Princess Auto which should be good for about 120lbs of force at 100psi. Then a 8:1 lever, which puts it at 960lbs, but sill allows for 1/8" travel.



  5. #17
    Gold Member neilw20's Avatar
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    Consider not just enough movement to squeeze the stack, but you need say 0.030 either end of the stack for clearance plus some extra to pop the collet.
    This extra movement means considerably more force than the holding force on the stack is required to pop and eject a typical TTS tool.
    Doing it manually, you undo a turn or two, and have to give a firm rap on the drawbar to release.
    You also end up with some lost travel from the drawbar elastic elongation under tension. Quite measurable!!
    A single 2" cylinder at 80 PSI needs close to 2" travel convert to about 0.25" travel at the stack to release AND eject.
    1" travel at the cylinder just won't give the extra pressure required to release and overcome the extra bits of 'lost travel', practical clearances, etc.
    If the pressure on the stack was constant as the travel changed all would be fine, but the pressure must increase considerably past the required holding tension in practice.
    At this point consideration must be given to ensure the fatigue life of the belleville washers are not exceeded.
    If you squash past about 75% they will fail during cycling, as many have found, and how many of the 'real professional' packs need their broken washers replaced?
    Fatigue life should be taken into account if you want to keep the maintenance low.
    Looking at washer data, much over 75% compressed from the free state is asking for problems, and you probably need them at 65% to get the required holding force.
    Any comments?

    Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.


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    Quote Originally Posted by neilw20 View Post
    "how many of the 'real professional' packs need their broken washers replaced?"
    Actually, the Bellevilles on a VMC are considered a "wear item". The spring stacks, often 100-150 Bellevilles, DO need to be replaced regularly.

    But, everything you've said is absolutely true. The Belleville force increases almost exponentially once you go beyond the rated compression, which can easily make the required release force 2X the working tension. And, when working with such large forces, mechanical multiplier, like levers, can get VERY inefficient, due to high frictional losses.

    The many headaches associated with Belleville drawbars are the reason I went a completely different way on mine - I built a motor-driven drawbar. This has the added benefit that I can swap out an R8 tool just as easily as a TTS one. You can't do that with Bellevilles! Since I have several tools that I use all the time that are just not suitable for TTS (like a 4" face mill), this is an important feature. Drawbar tension can be set almost arbitrarily high - My PDB is capable of up to 75 ft-lbs, but I run the equivalent of 25 ft-lbs of drawbolt torque, and it is absolutely consistent. Never had a single issue with TTS pull-out, even on the most aggressive cuts, and the tool has never once failed to release.

    Regards,
    Ray L.



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    edit - nevermind, move along



  8. #20
    Gold Member hoss2006's Avatar
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    Quote Originally Posted by PsyKotyk View Post
    Thanks for all the great info!

    After reading that a smaller machine like my X2 only needs about 600-800lbs of force, it makes the pneumatic solution a little easier for me.

    I'm thinking of using this guy Aluminum Single Acting Spring Return Cylinder | Princess Auto which should be good for about 120lbs of force at 100psi. Then a 8:1 lever, which puts it at 960lbs, but sill allows for 1/8" travel.
    Yes the lever type does work very well on the X2.
    I've done thousands of cycles over several years with not one bit of trouble.
    It takes very little drawbar travel to actually open the collet and release the tool, around .025 iirc.
    The bellevilles I used had a working force of 654lbs and a flat force of 858.
    If your setup can exert 900-1000 at min shop pressure you have plenty of headroom to account for any losses.
    Proved this with many videos showing tool holder retention tests, plenty strong for the X2.
    Mcmaster.com carries a 1.5 inch bore cylinder for $35, pity surplus center doesn't have any more of the $4 ones I used.
    They do have a 2" bore for only $17.
    http://www.surpluscenter.com/item.as...29&catname=air
    Hoss

    http://www.hossmachine.info - Gosh, you've... really got some nice toys here. - Roy Batty -- http://www.g0704.com - http://www.bf20.com - http://www.g0602.com


  9. #21
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    - Inverted mount
    - Not too sure about hinge forces. Using 8mm pin might not be enough. 1/4" aluminium plate might also be too weak.
    - Trying to leave room around the pulley for the Stirling belt drive, in order to change the belt without removing the plates.
    - Cyl. size and mounts unknown for now. Probably about 9" long with 2" stroke.

    Huge thanks to all the guys who have "been there and done that". Saves me lots of experimental aggravation.



  10. #22
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    No reason it wouldn't work. You'll want to make sure the curve on the end of the lever is such that the contact point remains very close to the center of the spindle, so avoid any unnecessary side-loading. I suspect though you'll find a lot of friction at that point, and a rolling contact (ball bearing) would be better. I did one years ago with a straight plunger riding in a sleeve bearing that pushed on the spindle, and a ball bearing riding on the lever arm. That eliminates the friction and wear that comes from having a high-force sliding contact, and keeps the force on the spindle purely axial, and always acting on-center.

    Regards,
    Ray L.



  11. #23
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    Ray,

    Did you use a servo or stepper to change your tooling over? Do you have a link to your build? I am setting up another new mill and would like to try something other than pneumatic if possible and if I recall from other posts your system sounded promising.


    Quote Originally Posted by SCzEngrgGroup View Post
    Actually, the Bellevilles on a VMC are considered a "wear item". The spring stacks, often 100-150 Bellevilles, DO need to be replaced regularly.

    But, everything you've said is absolutely true. The Belleville force increases almost exponentially once you go beyond the rated compression, which can easily make the required release force 2X the working tension. And, when working with such large forces, mechanical multiplier, like levers, can get VERY inefficient, due to high frictional losses.

    The many headaches associated with Belleville drawbars are the reason I went a completely different way on mine - I built a motor-driven drawbar. This has the added benefit that I can swap out an R8 tool just as easily as a TTS one. You can't do that with Bellevilles! Since I have several tools that I use all the time that are just not suitable for TTS (like a 4" face mill), this is an important feature. Drawbar tension can be set almost arbitrarily high - My PDB is capable of up to 75 ft-lbs, but I run the equivalent of 25 ft-lbs of drawbolt torque, and it is absolutely consistent. Never had a single issue with TTS pull-out, even on the most aggressive cuts, and the tool has never once failed to release.

    Regards,
    Ray L.




  12. #24
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    Default Re: Power draw bar idea - Feasible?



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Power draw bar idea - Feasible?

Power draw bar idea - Feasible?