Problem?? Smaller leadscrew Dia. same pitch?

[lepton]

I am running 3 axis on turbocnc.

I had a 5/8 - 10 acme leadscrew on Y-axis and had to replace and used what I had on hand a 1/2 - 10 acme leadscrew.

Under SCALE in tcnc I had to change to a much bigger number?
(.0000625 to .00013) why, from what I understand this diameter change should not be such a big difference with overall proformace.

What difference does reducing the diameter of the leadscrew have on mechincal advantage if the TPI are kept the same?


Thanks

[ger21]

Originally Posted by lepton I am running 3 axis on turbocnc. I had a 5/8 - 10 acme leadscrew on Y-axis and had to replace and used what I had on hand a 1/2 - 10 acme leadscrew. Under SCALE in tcnc I had to change to a much bigger number? (.0000625 to .00013) why, from what I understand this diameter change should not be such a big difference with overall proformace. What difference does reducing the diameter of the leadscrew have on mechincal advantage if the TPI are kept the same? Thanks

Is the 1/2-10 screw multiple start? That's the only way anything would be different. If it's a single start 1/2-10, all settings should be the same.

[Pres]

Seems like the reduced inertia (increased response) of the 1/2" screw could have a bit different acc/dec values.

[RICHARD ZASTROW]

If the lead is the same and the diameter decreases, doesn't the lead angle change? If the lead angle changes, doesn't the mechanical advantage of the inclined plane (screw) change? Just a thought.

[Pres]

Yep, that's true (the pitch angle changes), but the effective radius that the thread slope is at also changes an opposing amount.
For a specific pitch, net effect is the same displacement when given the same rotation.
Friction, screw deflection(twist), and its inertia will be the main consideration when varying the screw diameter.

At least, it WAS that way BEFORE Bush became president!

[handlewanker]

Hi all, there will be no change. The lead of the screw is the pitch times the number of starts, and in this case you have a 10 tpi pitch single start on both, which means for one turn of the screw it will travel 1/10" no matter how big the diam is.
The only difference will be as the screw gets smaller in diameter the core of the screw gets smaller and could lead to twisting under load.
Ian.

[RICHARD ZASTROW]

handlewanker I was refering to the lead ANGLE. My question was if the lead angle changes the inclined plane angle changes. Then the mechanical advantage of the screw changes and the force required to move the nut changes.

[Pres]

Originally Posted by handlewanker Hi all, there will be no change.... The only difference will be as the screw gets smaller in diameter the core of the screw gets smaller and could lead to twisting under load. Ian.

Cool, thanks for setting me straight on this.

I would have thought that, say, going from a screw diameter of 0.5" to, for example, 3.0" would represent an inertia increase more than 10x what the smaller screw was.
That would require alot more energy to accel/decel the screw each time it starts/stops.
But now you've stated; "The only difference will be as the screw gets smaller in diameter the core of the screw gets smaller and could lead to twisting under load"

I guess I really have trouble learning the "new" math & physics!
Pres

[handlewanker]

I agree with your revelations but just going from 5/8" to 1/2" will not make a difference that you would notice.
It could be measured but it is minute and wouldn't figure in any equation that is significant.
The helix angle of the 5/8" thread compared to the helix angle of the 1/2" thread, both with the same pitch, can be got by using trig tables, and the tan of the angle is the lead of the thread divided by the circumference, which for 5/8 X 10 tpi is 2 deg 55 mins, and for 1/2" X 1o tpi is 3 deg 39 min.
Now if this makes sense in a torque requirement then you must calculate how much force a wedge having an angle of 2 deg 55 min will need to push it against a fixed load as opposed to a wedge with an angle of 3 deg 39 min.
As I have already stated, and been corrected, the only difference in this case in point is the weakening of the core.
The pressure on the sliding face is different but who cares?
Ian.

[handlewanker]

By the way Pres, apart from the increased friction of the 3" diam screw, the turning force will be less as the helix angle is less and hence the wedge effect of the screw is greater. Helix angle = lead /circumference.
Ian

[ger21]

Originally Posted by Pres Cool, thanks for setting me straight on this. I would have thought that, say, going from a screw diameter of 0.5" to, for example, 3.0" would represent an inertia increase more than 10x what the smaller screw was. That would require alot more energy to accel/decel the screw each time it starts/stops. But now you've stated; "The only difference will be as the screw gets smaller in diameter the core of the screw gets smaller and could lead to twisting under load" I guess I really have trouble learning the "new" math & physics! Pres

Going from 1/2" to 3" dia. would be a huge inertia increase. This would afect accelration and deceleration mainly.

[handlewanker]

True, something like carrying 500lb on your back while doing 100mph on rollerskates, pure fantasy.
The fact is, if you were to attempt to drive a 3" screw, then you would have a correspondingly larger stepping motor, which takes care of the inertia and braking loads, which will be a big mother, and I think a three phase set up would be in order.
Have you contacted your electricity supplier to advise them that there will be some fluctuations on their power lines from time to time?
Ian.