How to figure rapid speeds

[Hack]

I am thinking of building a new gantry router. THK Slides, ball screws, aluminum construction all the goodies. My question is how to figure potential rapid traverse speeds.

I want to use some exeisting steppers that I have. Some of the Automation Direct 276 oz. in models. Planning 5 tpi ball screws and some sort of grear reduction to keep rapids up and stepper rpm down.

I am figuring in my head I want 100 IPM rapids on at least x and Y axis.

So My formula is this. 100 ipm x 5tpi screw = 500rpm the motors need to spin... Way to fast according to the torque curve of the motors.

So I figured I would add 4:1 reduction to get the motors down to 125 rpm. should work nicely. 100 IPM is nice, but would settle for slightly less say 80 or so. Using 100 so I can error slightly and still be pleased. I know I can cut at these speeds.

Question is ---- Is my thinking (calculations wrong) or am I forgetting anything? How will microstepping or the weight of the gantry affect my calculations? How can i figure how much torque will be needed based on the weight of the gantry to push these speeds? How is the torque affected by gearing like this?

Thanks All

Hack

[kong]

Your 4:1 gearing will decrease the torque by 4 times unfortunately. So instead of gearing, just using a slower accelleration on the steppers can lead to increased max. speed, the only drawback being an increased distance required to attain this speed.

[ger21]

Read this starting at message #82. K2 Review, KT-3925

If you're using Gecko's, more voltage will change the torque curve and let you run at higher rpm's.

[andy_ck87028]

Gerry,

From that last example I can only conclude that when purchasing steppers always buy the largest possible stepper otherwise the 1:1 option might get forced out of the equation.

By denying 1:1, your machine could consequently suffer from performance problems that eminate simply from the simple fact that the motors are undersized.

Is that a fair summary?

Andy

[ger21]

There are formulas for figuring out force required to accelerate loads, and how much force a leadscrew provides for a given motor size. That should get you in the ballpark. "Buying the largest possible stepper" would be costly, and overkill for most applications. There are so many variables involved, there is no universal answer.

Hack is trying to go a certain speed with certain motors and ballscrews. I just tried to steer him in the right direction.