## RE: [taigtools] Weight lifted by Taig Mill Z axis?

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• Hi Ken, First the easy one: Moving downwards, the mass has increased to twice so max acceleration will be only half which would significantly slow down the
Message 1 of 13 , May 5, 2008
Hi Ken,

First the easy one:

Moving downwards, the mass has increased to twice so max acceleration will
be only half which would significantly slow down the pecking since that is a
lot of acceleration back and forth.

Moving up:

I agree that we probably do not exceed 1G but that makes it an interesting
mental exercise:

The connecting cable will always be in tension since there is gravity
forces so we can consider the two masses to be rigidly connected and it
ought to slow down the up movement of acceleration by the same amount.

Another way to think about it that the gravity preload exists on both sides
so the forces cancel out.

Resonance:

Since we are not talking about very fast movements, the spring resonance is
a lot higher than our mills movements so I do not expect a problem with
that.

Of course an even better solution is to use a gas spring as Tony suggests.

An interesting thought:

If up and down acceleration could be set independently, a standard
configuration could be made to go down really fast with the help of gravity.
Of course going up is no improvement.

Bertho

From: taigtools@yahoogroups.com [mailto:taigtools@yahoogroups.com] On Behalf
Of Ken Cline
Sent: Monday, May 05, 2008 01:54
To: taigtools@yahoogroups.com
Subject: Re: [taigtools] Weight lifted by Taig Mill Z axis?

On 4 May 2008, at 10:35 PM, Bertho Boman wrote:

> A weight and pulley system will balance out the weight of the Z-axis
> which is commonly done. There is a drawback though: The mass of the
> Z-axis
> doubles and the acceleration will be less for the same available motor
> torque.

Is this a real drawback for a tabletop mill? The added mass will only
affect downward acceleration (you can't push a mass at the far end of
a cable) and then only for values greater than 1g (below that rate
gravity provides the force to move the motor and spindle downwards).
Is there a need for acceleration faster than 1g on the Z axis of a
Taig mill? For a graphic demonstration, drop something (I used my
pencil) a few inches to see how fast 1g is. If my calculations are
correct, the time it takes to go from 0 to 100 in/min at 1g is less
than 5 milliseconds. I don't see this affecting the sort of up and
down operations I do (peck drilling is probably the most demanding).

> There is a simple alternative that avoids this drawback: A long and
> strong
> spring connected to the ceiling or to some bracket above the head
> that just
> balance out the weight of the head. In other words, the head hangs
> from the
> spring. The added mass is insignificant.

What prevents a spring like this from developing large vibrations?

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• ... That misses the fact that adding the counterweight reduces the force needed for upward movement by an amount equal to mg. Thus, while mass has indeed
Message 2 of 13 , May 5, 2008
On 5 May 2008, at 2:44 AM, Bertho Boman wrote:

> First the easy one:
>
> Moving downwards, the mass has increased to twice so max
> acceleration will
> be only half which would significantly slow down the pecking since
> that is a
> lot of acceleration back and forth.

That misses the fact that adding the counterweight reduces the force
needed for upward movement by an amount equal to mg. Thus, while mass
has indeed increased, the force required for upward movement with has
not (assuming acceleration of no more than 1g). If you could
accelerate at 1g before adding the counterweight, you should still be
able to.

I did another quick calculation. It turns out that the axis will
travel .0036" accelerating from 0 to 100 in/min at 1g. That works out
to .005 sec to ramp up to speed (in a distance of .005"), or .02
seconds per peck. A cycle with 50 pecks takes a while on my mill, and
only 1 second of that would be acceleration time if I had really fast
rapids. In practice, I run the Z axis at 15 in/min, resulting in a
negligible .0023 seconds to reach maximum speed (0.1 second of accel
for the entire 50 peck cycle).

By the way, 1g is the value I actually use for acceleration.

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• Has anyone considered that the Z lead screw nut doesn t have any backlash take up and I was under the impression that the weight on the Z effectively
Message 3 of 13 , May 5, 2008
Has anyone considered that the Z lead screw nut doesn't have any backlash
take up and I was under the impression that the weight on the Z effectively
eliminated any backlash. Right/Wrong??? If it is right, then wouldn't the
balanced head result in some backlash?

Don

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• ... backlash ... effectively ... wouldn t the ... Yes, you are correct. The trade offs as far as I know are if you counterweight the Z axis you should be able
Message 4 of 13 , May 6, 2008
>
> Has anyone considered that the Z lead screw nut doesn't have any
backlash
> take up and I was under the impression that the weight on the Z
effectively
> eliminated any backlash. Right/Wrong??? If it is right, then
wouldn't the
> balanced head result in some backlash?
>
>
>
> Don

Yes, you are correct. The trade offs as far as I know are if you
counterweight the Z axis you should be able to get some more speed, if
that's what you want. Doing that with the "stock" lead screw nut will
most likely introduce backlash. To remove some of the backlash you
can install an adjustable lead screw nut on the Z axis. I believe
it's the same nut as the CNC Y axis. I bought one from Nick Carter,
haven't yet installed it. I haven't done any counterbalance work on