Junk Physics

[flakker]

I was too cool to pay attention in high school so here i am now.

How do you convert 600 oz/in to linear power via a 5tpi (0.2) ballscrew?

i am thinking along these lines. a 600 oz/in stepper will produce 600 oz/in of torque if it was rolling with a 1 inch radius wheel. Each revolution of the wheel rolling it a distance of 6 inches (pi * radius *2).

So with a ballscrew at a 0.2 pitch the motor needs to turn 30 times (6inches/0.2pitch) to cover the same distance so it is 600*30= 18,000 ounces of force.

The head of an industrialhobbies mill weighs about 200 pounds, give or take. would a stepper of this size be able to lift the head on direct drive?

what is the minimum amout of linear force recommended for driving the head.

Is there a website i can go to on this?

[unterhaus]

people write oz/inches but torque is force * length or oz * inches, so your units are wrong. Your logic is along the correct lines. Don't think they really teach this in physics, you need a engineering class in statics.
The formula I have is Torque = Force*pitch/(2* efficiency) -- some people define pitch as revs/inch, so you may see Torque = force/(2 * pitch * efficiency)
So you have torque = 200lb * 16 oz/lb * .2 in/rev/2 = 400 oz-in using beer math (non-beer math ignoring efficiency and friction of the ways gives 320 oz-in). You should be good to go, although you may have to add an air spring to make yourself happy with the vertical acceleration. It really depends on how much friction there is and how bad your lead screw is.

[flakker]

i was talking about translating rotary to linear power. i am not very good at articulating thoughts.

i was looking at the TORQUE TO RAISE 1lb. (in.-lb.) column in the nook ballscrew catalog and i found that using the number an tpi figure i was able to gain a figure at 90% of what was theorized, which corresponds nicely to that efficiency of ballscrews.
ballscrew catalog
http://www.nookind.com/ball/BallInch.cfm

Using the spreadsheet i found that hypothetically a 200 oz/in stepper has 350 pounds of lifting power through a 5tpi ballscrew.

I have a fe of these lying around so should i be able to use it to lift the head (z axis) of an RF45 (clone)?

[philbur]

You will also have to allow for the friction of the z-axis ways and the fact that the head has to be accelarated to your choosen ipm.

Regards
Phil

Originally Posted by flakker i was talking about translating rotary to linear power. i am not very good at articulating thoughts. i was looking at the TORQUE TO RAISE 1lb. (in.-lb.) column in the nook ballscrew catalog and i found that using the number an tpi figure i was able to gain a figure at 90% of what was theorized, which corresponds nicely to that efficiency of ballscrews. ballscrew catalog http://www.nookind.com/ball/BallInch.cfm Using the spreadsheet i found that hypothetically a 200 oz/in stepper has 350 pounds of lifting power through a 5tpi ballscrew. I have a fe of these lying around so should i be able to use it to lift the head (z axis) of an RF45 (clone)?

[philbur]

The Tormach has a lighter head than industrialhobbies. The ways have low friction coating and the z-axis has 1200 oz.in stepper. No air spring or counter weights. Rapid is 65 ipm.

So be careful with a calculation that does not take all the forces into account.

Regards
Phil

Originally Posted by flakker i was talking about translating rotary to linear power. i am not very good at articulating thoughts. i was looking at the TORQUE TO RAISE 1lb. (in.-lb.) column in the nook ballscrew catalog and i found that using the number an tpi figure i was able to gain a figure at 90% of what was theorized, which corresponds nicely to that efficiency of ballscrews. ballscrew catalog http://www.nookind.com/ball/BallInch.cfm Using the spreadsheet i found that hypothetically a 200 oz/in stepper has 350 pounds of lifting power through a 5tpi ballscrew. I have a fe of these lying around so should i be able to use it to lift the head (z axis) of an RF45 (clone)?

[fpworks]

Flakker,
Your calculations are correct. However, in addition to losses from ballscrew/ways and additional force required for acceleration, you also have to look at the amount of torque you're actually going to get from your steppers.

Your stepper motor is going to make a LOT less torque than 600oz*in at 300rpm. (only 60ipm for you)

Stepper are rated at stall torque, and this isn't really useful since zero rpm is not doing anything. I would get a power/torque curve for your stepper motor, and consider how much torque your steppers will make in your particular application. (power supply, driver cards, speed, pulse/sec, etc.)

[dcprecision]

With a 5 tpi ball screw, I would use a minimum 3:1 reduction and possibly even 4:1. If your motor can do 2000 rpm 3:1 gives 133 ipm, plenty fast. The theoretical stall load should be around 480 lbs at this ratio. I think ???

[fpworks]

The proper method to size steppers/drivers/power supply is not for stall load, but the load that will cause the stepper to lose steps AT SPEED, whether this is is rapids or cutting feed. (application specific)

[flakker]

"The Tormach has a lighter head than industrialhobbies. The ways have low friction coating and the z-axis has 1200 oz.in stepper. No air spring or counter weights. Rapid is 65 ipm."

i speculate Its for the detent torque as well, inertia assumes about 100 pounds of the head weight on the tormach, with the rest of the weight to be assumed by the gibs.

suppose i was to tighten the gibs on my z axis so the head of my machine doesnt go up or down without power, that means the gibs would bearing the whole weight of the head so for a 200 pound head i would have 200 pounds on the gibs, if i were to push down on the head with my finger it would go down so therefore =

if: acceleration = force/mass

the force would equal 200 (gravity) plus my the force i place on my finger.
the mass would equal 200 (dovetail gib friction)

if i wanted it to go up:

the force would equal any force in excess of 400 pounds, if the figure is under 400 pounds the figure equals 0(because the dovetail gibs assume the weight of the head to equilibrium so there can be no negative acceleration)
the mass equals 400 (gravity + dovetail friction)


i believe that is how to calculate dovetail friction with head weight in a vertical position.