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IndustryArena Forum > Mechanical Engineering > Epoxy Granite > Counter balanced spindle head.
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  1. #1
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    Counter balanced spindle head.

    I am by no means the first to think of this as a quick search will show but, after looking at the research paper of the FEA analysis in another thread it looked like the weight hanging off the Z rails would cause a lot of bending force to be exerted. Since EG is stronger in compression than all other directions I think it may be good to use a pulley attached to the top of the Z vertical and attached to the Spindle mount and a weight as counter balance riding on a track to counter this force.

    It works much like an elevator counterweight so the only power needed for the Z axis is for de/acceleration and no lifting of the head. This is more of a side effect though of this system because this should transfer the force down through the vertical into the base. Since there is now much less twisting (around the x axis) force and only the torque from cutting acting on the rails (around the z axis). It also helps with backlash since the weight will keep the spindle pulled tight against the ball screw.

    Other people have used springs or hydraulic dampeners but I think the counterweight is better because it keeps a constant force instead of providing more support at full extension.

    These advantages would be great for most mills but I think especially for a EG mill this could offer additional advantages.

  2. #2
    Community Moderator Jim Dawson's Avatar
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    Re: Counter balanced spindle head.

    My preference is an air spring. Compact, light weight, and constant adjustable force through the range of motion. With a counterweight you are still needing to move that additional mass.
    Jim Dawson
    Sandy, Oregon, USA

  3. #3
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    Re: Counter balanced spindle head.

    Counterweight is simple but as Jim says you will have additional inertia which impacts on motor performance / sizing.

    An air spring is probably the ideal solution as it does not add lots of inertia.
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)

  4. #4
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    Re: Counter balanced spindle head.

    Also remember that gravity limits how fast you can accelerate your Z-axis upwards....plus you can get nasty things like reasonans between the head and the counterweight, which most likely would tear itself apart very quickly...

    I'm making an EG-machine an i've opted for a pneumatic solution:
    https://www.cnczone.com/forums/verti...268-forum.html
    ...it consists of a cylinder on the back of the column. The chain/wire from the head is connected via a pulley, this halfs the speed which the cylinder must move with, but doubles the load. I've done this to minimize the effect of the pneumatic cylinder and airsystems "inertia" and make the spindlehead move faster...hopefully ;-) The pressure in the cylinder can then be adjusted to give the desired "counterweight"...

    I was considering if i should attach the cylinder to the bottom EG-part, instead of the column, to transfer the load down as low as possible...(?)

    /Thomas

  5. #5
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    Re: Counter balanced spindle head.

    You are counter weighting the Spindle not the Z Axis beam. The weights would cancel out giving you a resistance of 0. Inertia is also not a problem if you rotate a gear or pulley with the head and counter weight slung over it. The rolling force needed is your only resistance if your spindle and counter mass weight is equal.

    Imagine 2 buckets on a rope hanging from a stick. You turn the stick to move the one bucket up and the other down. As long as the weights are equal you have opposing forces. Look at how fast an elevator gets up to speed even though there is up to 8000KG of freight in addition to the 2 masses.

    I agree the spring will give more stiffness though and that can be a plus.

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