Strength of a 1200oz/in stepper

[bigvinney]

I have a 1200oz/in stepper from homeshopcnc.com and was curious what kind of weight this stepper could lift with a 8tpi lead screw?

I have a small knee mill from grizzly and I am converting the knee for the z axis. (I am only doing 2.5d not 3d machining and for hobby use). The table itself weighs 75lbs and the knee is an additional 100lbs or so. I want to use the 1200oz/in. stepper that I have but was wondering if this would be sufficient or should I add a counter weight setup to make it work? Im trying not to have to add a counterweight due to the complexity required to make it and have to drill holes and other things into my new machine.

Thanks Vince

[fyffe555]

If you say the weight = the force produced by the leadscrew and motor and ignore friction and inertia entirely then usually Force= pi/8 * TPI * Oz. In.

With 8tpi and 1200 OzIn Force = 3.142/8*8*1200 = 3770 oz or ~235 lbs F.

The screw will have an efficiency. If it's an Acme screw its about 30% so you will get 30% * 235lbsF or 70lbsF. If its a ball screw its about 80% so you will get 80%*235lbsF or 188lbsF.

If there's friction in the ways or the bearings then knock some more off...

Andrew

[bigvinney]

Ok so it may work then, if all else fails I can put spring under the knee and try that or maybe a gas strut rated at 100lbs or so.

[ger21]

Keep in mind that 1200 oz is holding torque. The faster you want to lift, the less force you'll have. If you look in the catalogs for ballscrews and acme at www.nookind.com , they list the torque needed to lift 1lb with each of they're screws.

[fyffe555]

A crazy thought occurs.. You're currently lifting the knee by hand? get a torque wrench and try and measure what the actual torque required might be. Then work it backwards...

Also, theres a lot of weight in the knee table and workholding. If thats your Z axis I wouldn't have thought you'd get much speed out of it without a Very large motor. Or is the Knee 'Z' in addition to the spindle Z?

Oh yeah, Gerry's spot on with the torque curve. See if you can get a chart for the torque curve of your motor. It will drop off quite rapidly with speed and Larger motors usually have relatively low max speeds too. Otherwise you might have been able to gear it 2:1 or whatever..

[BobWarfield]

FWIW, Industrial Hobbies uses 600 oz in servos on the Z-axis of their square column mills. I know (having lifted the blasted thing!) that it weighs about 200lbs. Seems to work fine without counterweights, but could benefit perhaps with some according to some sources. From that, I would think your 1200 oz in should be fine.

Best,

BW

[fyffe555]

FWIW, Industrial Hobbies uses 600 oz in servos on the Z-axis of their square column mills. I know (having lifted the blasted thing!) that it weighs about 200lbs. Seems to work fine without counterweights, but could benefit perhaps with some according to some sources. From that, I would think your 1200 oz in should be fine.

Best,

BW

True Bob, but doesn't the IH conversion use a ballscrew and a reduction belt drive?

Do you have a cnc IH square column mill? if so what do you think of it? I'm working up the excuse to go buy one..

Andrew

[BobWarfield]

It absolutely does use a ballscrew, and you would need to factor that in. Still, one does not see the 1200 oz inchers very often in the home conversion world. The suggestion to take a torque wrench to it is not a bad idea. Al the Man will also come along shortly to direct us to some free software for sizing your steppers. If not, do a search to find his similar posts in the past.

Andrew, I do indeed own an IH square column mill. I am quite happy with it. Service from Aaron during the purchase cycle was excellent. I haven't done terribly much with it yet, but I did make an indexable dovetail cutter for making QCTP holders as well as a vise stop similar to a design by Widgitmaster. You can see both here:

http://www.thewarfields.com/MTProjects.htm

I have been surprised at the ability of the mill to take what I am dishing out so far. I like to use a big ole' corncob rough cutter for squaring blocks, for example, and it is working fine. Occasional chatter if provoked, but really, the faults there have all been on my side, not the machine's. It produces a nice finish with good quality tooling. I have not subjected it to any Asian endmills, preferring to buy name brands at half price from eBay. These are working out extremely well for me.

I definitely would rate this mill at a high end hobby range. It isn't a Haas VMC, but it beats the socks off round column mill drills.

I did order the IH mechanicals only CNC kit while it was on sale, so I will be interested to see how the conversion performs. I want to get to know this machine a bit better manually before attempting CNC however. You lose all the feel once CNC'd, so on these home conversion situations I think it's important to have a pretty good understanding of the machine's behavior before just diving into CNC operation.

Best,

BW

[rexstep]

Vince, Even the RHT34-1780 needs to be on a 2:1 ratio to power a full size knee. Since you have a smaller knee mill, you'll want to do the same with a 1200.

A good design tip would be to run at least at 60V to get better running torque. The earlier post is wise to point out that 1200 is only the holding torque. Running torque will be lower and the more so as you increase speed.

Rick LaLonde
www.homeshopcnc.com