Newbie- Lacking some understanding

[vline5]

Ok, I'm building a moving gantry style cnc about 4'x4'. My problem is I don't know what reduction I need on the x axis. The gantry is going to weigh about 100-120lb. I'm using the g540 even through I know it could be a little small for the size of motor I might need to move this at a reasonable speed. Already bought the kehling 387oz and am planning on overcoming the small motor size by utilizing the 4th driver and running two motors on the x axis. I believe that would give it more power and not sacrifice speed. So what is confusing me is I'm thinking I need to have some kind of reduction for this but the 2000 step think is not making since. If I actually had 2000 steps I wouldn't need any reduction? 1.19in pulley*3.14= 3.7366/2000=.0018683 is close to 1/512 of an inch? Thats better than it would ever need to be for what I'm doing. wood, plastic and aluminum. Do I still think of it as 200 steps? How much do I reduce it? I know someone on here has the answer.

[ger21]

The main reason for reduction is to get more power. For example, 5:1 reduction will give you 5 times more force.

Also, without reduction, 300 rpm at the stepper will give you over 1100 ipm. Figure out how fast you want to go, and gear it so that you reach that speed at around 800-1000 rpm. (Provided you can get 800-1000 rpm. )

Design it so you can play around with gear ratios to fine tune it thru trial and error. You probably want somewhere between 2:1 and 4:1, I'm guessing. If you need more power, you'll need to sacrifice some speed and gear higher. If you have enough power, you can gear lower and get more speed.

Some quick math (If I did it right) tells me with direct drive you have ~40 lbs of holding force. But at 500 rpm, you might only have half that, or even less, as torque decreases as rpm's increase.

[LeeWay]

You do get more torque by gearing it down.
I have about 150 pound gantry on my router and just use a single direct drive 425 oz motor with 5/8" double nut ball screws and the gantry rides on skate bearings. I can get 300 IPM pretty easily, but with fairly low acceleration. Maybe 10. I am using 203V's and a 62 VDC PS.
I did put a timing belt and gears on it once, but chose the wrong type gears and belts. They left too much flex in it.

I now know the right type belts to use and may upgrade in the future, but for cutting lexan, I really don't need it now. I only cut at 80 and 150 IPM. I have reduced my rapids to 250 IPM just to be on the safe side.

[johnmac]

My gantry weighs close to 200 LB. It rides on THK style rails and is powered by a Kehling 425 oz/in stepper driving a ball screw. If I grab the frame and gantry while its moving and try to hold it back, I cannot stop it.

Don't make the mistake of trying to gear down the stepper to get more power. You will loose travel speed. It may not seem important now, but you will get tired of waiting and waiting while the machine moves into position.

Just remember, steppers produce their highest power at slower speeds.

John

[ger21]

Originally Posted by johnmac Don't make the mistake of trying to gear down the stepper to get more power. You will loose travel speed.

He's not using screws, it's a belt drive with almost 4" of travel per stepper revolution. He really should gear it down.

[johnmac]

Yes, I see that. Kinda comparing apples to oranges.

I still stand by my other statements. Select the "gearing" for the resolution you really need.

If you have to gear a stepper down to move at slow speed, you won't have power to run at higher speed.

I'm not trying to sound like an expert, but my first machine suffered from too much resolution and no speed.

John

[vline5]

How should I look at the micro stepping? In reverence to gearing. Do I divide by 2000? or do I build it like I only have 200 steps and count the 2000 as bonus resolution? Is there reduced torque when micro stepping to account for?

I think I'm going to go with a 2.5:1 ratio that will get me .0074732 per step for 200 steps. Yeah, I do have it setup so I can play with the gearing. Wouldn't do it any other way What size of belts would you recommend for the x axis drive belts and reduction gearbox belts? I'm looking at ordering from econobelt.

[johnmac]

I think a 2.5:1 ratio would be an excellent starting point. The ability to change the ratio sounds great.

My ball screws are belt driven. As soon as I was able to test run an axis I promptly changed the ratio. I found I had plenty of power and wanted more speed.

I can't help with belt size or type.

John