stepper motor question(s)

[Tony]

I'm not sure if this is OT, my apologies. but the end result will (hopefully) be a CNC machine, if that counts any.

a rep told me that stepper motors can NOT be used without reductions (geared/belted/etc). and told me, for CNC applications, the minimum is a 4:1 reducer.

is this true? and why.

I did some math.. and figured i needed 400oz stepper motors.

called a supplier and he tells me, that for my design, 400 should be okay but i'll need a 4:1 reduction.

for $30 more, i bought 700oz-in stepper. my numbers show this is more than enough to move my table. he calls me and tells me i STILL need a reduction or the steppers won't budge under start-up.

he talked alot, but didnt give me any good reasons why.

thoughts?
-tony

[BillH]

For one thing, it will be smoother, much smoother. Steppers move from one field to the next, why they are called steppers. Turn the motor by hand, you feel many little cogs, each step it moves to the next cog. Stop go stop go stop go stop go. If you gear it down, these steps become much smoother, not to mention you get more steps per inch. Any real reason why you dont want to gear them down?

[Mcgyver]

doesn't seem to make much sense does it? might have come from the fact that a lot of apps would have ball screws with corresponding longer leads and therefore he's never seen one less than 4:1.

whats the leadscrew? obviously you need to have the steps a fraction of the resolution you want. most would go further and say you should micro step it to avoid resonation. just saying reduce it 4:1 is kinda dumbass with factoring in the lead, forces, etc.

[John Stevenson]

Tony,
No reason at all to gear them down. Steppers develop the most torque at low revs.
Servo's OTOH develop most torque at high revs so need to be geared to be spinning at the optimum speed.

Many machines are direct drive on steppers.
Geraing down is done to either get better resolution or use a smaller motor for a given touque application.

My full size Beaver mill uses 900 oz/in motors direct onto a 5tpi ball screw with no problems but the software could 1/2 step tto give you 0.0005" resolution.

Bridgeports used the same motors and screws but geared down 2:1 to achieve the same resolution as their sofware wasn't as good in the day.

Given modern microstepping drives there is no reason to gear down if you have the available torque.

Running an axis in 1/2 step then in 10 microsteps is unbelievable in smoothness.

John s.

[Tony]

thanks all. being new to CNC (and all the fun and mayhem its bringing on), i wasn't sure.

i've gone with John's idea: timing belts. 12.7mm pitch (they're supposed to be "inch" belts? (i'm in europe)) on 14tooth pulleys.

if my math is right, i'm getting 177.8 steps per revolution. (12.7x14)

i bought two Gecko 210's per Ibew's suggestion.. they microstep. so.. 1/2000 of a rev per step.

that means a resolution on my plasma torch of 0.09 mm! (my torch alone cuts a 3mm kerf!) ..

hell sometimes i can't even get that on my Bridgeport.

the whole mess weighs about 20pounds. i think 700oz-in is enough to move it by itself.

should i go ahead and mount then direct?

i'll post pictures in a minute.
-tony

[Tony]

here is a rendering of things to come:
table is 1m x 1m
gantry is all aluminum construction except for 2 16mm ground rails for the carriage.

the gantry rides on 20mm rails.

this was drawn up before i upgraded to 700oz-in steppers, and it shows a 2:1 reduction on both axis. the lower axis has no belt, i got lazy


http://img.photobucket.com/albums/09.../CNCplasma.jpg

<img src="http://img.photobucket.com/albums/0903/knucklehead/CNCplasma.jpg" alt="Image hosted by Photobucket.com">



[This message has been edited by knucklehead (edited 05-03-2005).]

[hammerhead74000]

You don't need to gear (or in this case belt) them down; and 700 oz/in is more than plenty. A plasma cutter has no cutting forces to overcome; and the head is (compared to a mill table) quite lightweight. Geckodrives are a good choice for this; your machine should work just fine.