Optimum Ratio

[Allan]

Pardon me if this question has been addressed before, but I could not find an answer in previous threads.
Is there an optimum or ideal ratio between linear travel and stepper motor revolution, regardless of what is included in the driveline (gears, pulleys & belts variable speed gearboxes etc), between the stepper motor and the final object being moved.. Or at least some sort of guideline?

[ger21]

Not exactly what you're asking. First, you calculate how fast you need to go, and how much force is needed at that speed. Stepper motors will have a torque curve chart, showing the amount of torque it has throughout it's rpm range. You then need to gear your machine accordingly to provide the machine speeds required while keeping the stepper motors rpm where it provides the required torque to give you the force you need.

Now, what you asked is basically how far should the machine move per motor rev. Depends on your application. Steppers will have 200 steps per revolution. Some people can get away with .1mm of movement per step, some might want .01mm per step, or even less.

With steppers, speed and resolution is a balancing act. The faster you want to go, the less resolution you'll be able to get, due to the 200 steps per rev being a fixed number. If you want high speeds and high resolutions, you'll probably want to go with servos, which can change their steps/rev by changing their encoders.

[Allan]

Thank you very much Gerry, for your reply. In the meantime I have been reading "The user's guide to installation, configuration and operation for Mach3 users" and came across this paragraph (4.5.2) on page 33:

>>A gantry router might need a travel of at least 60" on the gantry axis and a ballscrew for this length will be expensive and difficult to protect from dust. Many designers would go for a chain and sprocket drive.
We might choose a minimum step of 0.0005". A drive chain sprocket of 20 teeth with 1/4" pitch chain gives 5" gantry movement per revolution of the sprocket. A stepper motor (ten micro-steps) gives 2000 steps per revolution so a 5:1 reduction (belt or gear box) is needed between the motor and sprocket shaft. [0.0005" = 5"/(2000 x 5)]

With this design if we get 500 rpm from the stepper then the rapid feed of 60" would, neglecting acceleration and deceleration time, take a reasonable 8.33 seconds.
The torque calculation on this machine is more difficult than with the cross slide as, with the mass of the gantry to be moved, inertia, during acceleration and deceleration, is probably more important than the cutting forces. The experience of others or experiments will be the best guide. If you join the ArtSoft user group for Master5/Mach1/Mach3 on Yahoo! you will have access to the experience of hundreds of other users.>>

This is in agreement with your remarks and addresses the root of my question, because I started comparing the consequences of a direct drive to a screw to that of a direct drive through a sprocket to a chain and realizing that resolution would be lost and speed gained by using a chain.

So it seems to me that in designing a machine the initial decisive factor is to determine what resolution you want and to gear for that. The speed can be regulated (within limits) by the software, but the resolution is mechanically built into the machine and governed by the choice of drive train although micro stepping is software controlled and affects resolution.

Your comment on my understanding will be appreciated.

[Al_The_Man]

There is a couple of references in post#5
http://www.cnczone.com/forums/showth...=inertia+ratio
Al.

[Allan]

Thanks Al, I'll check it out. Your response is appreciated.
Allan