Choosing Stepper motors oz/in...

[mrbaseball]

I am building a DIY Router that uses acme rods for movement and the overall size is roughly 36 inches long, 24 inches wide with roughly 3-4 inches depth, I was going to purchase 166oz-in stepper motors but was recently told by a salesman that they would not have enough torque to get the wanted IPM. S he suggested I get the 400 oz-in motors the price difference is roughly $45.

Is this a good assumption or will I be fine with the lower oz/in?

[Jawno]

I'm certainly no expert so don't take this as gospel but if it were me I would go with the bigger motors. 166 sounds a little small and I'd rather be overpowered than underpowered. Forty five bucks isn't much difference and you'll be money ahead if it turned out the 166's couldn't handle the load. On the other hand you might do some tests to determine just what your torque requirements are. I haven't seen a lot of builds with 166's but that doesn't mean they won't work.

[robe_uk]

ahhh now now, we need to know what feed rate you're after, the type of acme rod ie size and number of starts, what are you going to be cutting?

Might have seemed like a simple question, one I have asked myself before
but you'll do well to get a simple answer, in saying that there seems to be loads of builds using 380 oz/in motor with great results so I would go with the 400 oz/in motor you mentioned.

[mrbaseball]

I'm only looking to cut hobby stuff right off the bat, i.e. - balsa, light 1/8 plywood and such. The acme rod I have is 1/2 x 10 not sure of the start.

this has been an ongoing project for the last 5 years and I am finally getting around to purchasing the electronics from probotix.com

Will I need the same size motors for all axis or can I use a smaller oz-in for the z axis(depth)?

[robe_uk]

well if its single start then the lower power motors would do, but as all steppers run out of puff with higher rpm you'll never get high feed rates even with the larger motors.
But having said that you'll quickly tire of slow feedrates and look to upgrade to mutlistart rod at which point the higher power motors will come into play.

The 400 oz/in motors will work with single and multistart rod easily, the 166 may struggle with multistart. If the cost for both is similar then go for the 400,well thats my opinion and am sure others will have theirs

cheers

[ger21]

You'd probably be better off with the 260 oz motors. They'd still be good enough for 2 start screws as well. I use 250 oz motors with 1/2-8 2 start and get pretty good performance with a Xylotex drive.

I wouldn't get the 400 oz, unless you go with 5 start screws.

[ger21]

Will I need the same size motors for all axis or can I use a smaller oz/in for the y axis(depth)?

Z axis is up and down, and yes, it should have the same size motor. Most people think it wouldn't need as much power, but it can actually need even more power, as it has to fight gravity.

[louieatienza]

well if its single start then the lower power motors would do, but as all steppers run out of puff with higher rpm you'll never get high feed rates even with the larger motors.
But having said that you'll quickly tire of slow feedrates and look to upgrade to mutlistart rod at which point the higher power motors will come into play.

The 400 oz/in motors will work with single and multistart rod easily, the 166 may struggle with multistart. If the cost for both is similar then go for the 400,well thats my opinion and am sure others will have theirs

cheers

The finer the pitch on the screw, the smaller a stepper you can use. The problem of course, like most mentioned, is you'll lose some torque as you speed up. To note, a lot of mini mills use 166in oz and smaller steppers, but they have 10, even 20, tpi leadscrews, and the machines are small where rapids and high feedrates are not much concern.

Generally the coarser the screw pitch, the more torque you'll need. You won't have the same "resolution" as a fine pitch screw, but you probably won't need it, especially cutting wood. As for wood cutting, which require high rpm spindles to cut cleanly, the feedrates are faster, to prevent twood burning and premature bit wear.

Check out the geckodrive website; Marriss of geckodrive has some great articles on motor sizing, and steppers in general; and it may help you make a better decision.

[PaulRowntree]

Just to be pedantic, I believe that the units of torque should be oz in, not oz/in. The difference is important especially when doing conversions.

Another consideration : The high torque motors (eg 400 oz in) have higher coil inductances, which means that you need a higher voltage power supply to get maximum performance. To get this higher performance your controller must also work at higher voltages. My Gecko G540 is limited to about 50 V, so that means the maximum inductances would be ~2 mH, which means it can push the under 200 oz in motors to their performance limits. I think the Gecko formula was Vmax( in volts) =32 x (Inductance in mH)^0.5, but check the Gecko site before making any decisions. There is little point in having strong motors with underpowered drivers.

Cheers!

[jsheerin]

I have 425 oz*in motors and wish I'd bought the 376/380/387 oz*in motors like those available from Keling. They have a better torque curve - they have higher torque at high speeds than the the larger motors. These work really well with the Gecko G540 controller. I think Gecko even sells a motor this size now.

[ger21]

I got a PM from the original poster saying that he's thinking of purchasing the Probotix Unipolar drives, which are similar to the HobbyCNC.

With a Unipolar drive, the 400 oz motors are actually 300 oz motors with a much lower inductance, so they are a much better match for a Unipolar drive.

With a bipolar drive, however, the 400 and 425 oz droves are not the best choice.

[PaulRowntree]

I have 425 oz*in motors and wish I'd bought the 376/380/387 oz*in motors like those available from Keling. They have a better torque curve - they have higher torque at high speeds than the the larger motors. These work really well with the Gecko G540 controller. I think Gecko even sells a motor this size now.

The Gecko motor is actually 280 oz in