Motors and leadscrews

[Gary]

You mentioned using leadscrews, is this for price?
A leadscrew is not that efficient and you should expect between 40-60% for a lead screw and up to 90% for a ball screw.
This means that with a leadscrew you will need up to 50% more power to get the same amount of force.
So, because of this you may be saving on the leadscrew but will need to spend more on the motor and driver.

Gary

It's time to start thinking about motor power and leadscrew pitches for my machine.

What's the best way to approach this?
I've seen people talk about target speeds, target accelerations (0.5g? 1g?), gantry weights, cutting forces, motor curves, etc.

I should say that I'll be using trapezoidal leadscrew, which will have a much lower efficiency than ballscrews. Before now I've been thinking about 3Nm motors, but perhaps I don't need all that power.

Practically speaking, having never done this before, where is the best place to start?
Acceleration seems more important than top speed for the relatively compact routing I'm going to do (many direction changes), but what should I aim at? What do you lot use?

Speed is much easier - I'd ideally like to achieve a jog speed of 120mm/s (~280 IPM), which is 900rpm on an 8mm pitch leadscrew. Can steppers do that speed? I'm thinking cutting speed will be less closely linked to motor power, more to router power, chip load, and machine rigidity.

Cutting force seems important too, but what figures should I use? Or should I worry about it less and just bolt some stuff together?

Kip, Smiler, Lee,
What do you think? How do you do it? :question:

[Tom]

Hi Gary,

I hear what you're saying. This build needs to be pretty cheap, so in the short term I'll go with leadscrews, and just take what performance I can get. My budget means I don't really have the option to upgrade the motors. As Kip suggested - just build it and see what the results are. If it's slow, that'll help me justify the extra cash next time around.

Since its only an MDF build it's unlikely I'll be using the router and screws years from now, whereas the motors will definitely find another job to do....