Direct Drive Vs Belt Drive for a Mill/Drill Retrofit?

[pfeist]

I'm plotting and scheming my retrofit...
And one question about mounting my X and Y steppers has occured to me...
I see most retrofits are belt drive with toothed pulleys rather than the motor being directly mounted.

Why?

Is there some advantage to belt drive rather than directly mounting the motors to the X and Y shafts?

The motors I have are dual-shaft, so I had planned to direct mount, and since I have two handwheels on my X axis running manually won't be a problem there,... my Y axis I had thought of mounting a handwheel to the portruding shaft of the motor.. (with a cut-off switch so I don't do bad things to my driver...).

So other than access to the shafts and handles, is there an advantage to pulleys and belts that I'm not aware of?

Paul F.

[CNCadmin]

Yes torque.

[balsaman]

I am with Paul. It depends on your setup tho as to what kind of torque/speed you need.

Eric

[HomeCNC]

If your steppers are strong enough then direct drive them. I used servos, and they turn at a higher RPM. I did not need that kind of speed on a small Drill/Mill so by placing them on a 2:1 ratio I get double the torque and 1/2 the speed. My speed is still over 150 IPM.

[pfeist]

Jeff;
Thanks!
My 680 Oz/in Steppers should be plenty for my Mill/Drill... especially once I add ball screws.

I'll probably TRY it direct-drive with the Acme screws, and expect to take light cuts... I can't afford to do the ball screws AND controller at the same time :-)

If direct drive doesn't have enough torque with the Acme screws, I'll wait until I have ball screws...

Paul F.

[vacpress]

pfeist - i bet you will be fine with that setup..

at my school they have a little minimill2 and its got 300oz/in steppers directly driving 5tpi acme. it gets scary above 70 or 80ipm, but it works....

[Greg Rutter]

I am in the same position. I have bought both pulleys/belts for the stepper motors and antibacklash couplings and now that I am designing the mill I am tossing up which way to go. I know that gearing was one benefit but being a beginner I was wondering if there were other issues. ie the couplings are simpler to fit but would the belts make it easier to make minor adjustments during construction and repair. Also I read something somewhere which could be complete BS about resonance created from vibration though direct connection, with the ballscrew potentially causing damage to the stepper motor in the long term. Is there any truth in this and if so would connection via a belt dampen this.

[RICHARD ZASTROW]

Greg, I'm the one who posted the "BS about resonance". In my case, it definitely was the problem and switching to a belt drive solved the problem. That was on a servomotor driving a ballscrew. The same thing happened on a servo driving a spindle. I finally was convinced when it happened on a servo driving a high speed rotary table being used as a 5th axis on a milling machine. In every case a belt drive was installed to replace the direct coupling.

These were all high grade rotary encoders. Maybe the problem is less critical with feed back devices that do not have as fine a resolution. The servo manufacturer claims resolution as fine as 2,097,152 counts per revolution. Since I'm not the electrical guy, I don't know if that's some computer multiplying or?

I can't fathom how the control can keep track of all those "counts" at 4,000 rpm. I'll leave that to the "whizz kid" that writes the software on the 8-axis gear hobbers, he's only 35.

Dick Z

[Greg Rutter]

Thanks Richard,
No offence about the "BS" comment it wasn't even on CNCZONE that I read it which means that other people are also having this problem. This has convinced me to go with the belt connection option just in case. I'll use a 1 to 1 ratio as I don't need to gear the motors down but the belts will give me some added flexibliity.

[Pres]

Belt drive provides:
Easier adjustment in axis inertia/resonance and gearing variation.

Much more flexibility in motor drive placement -e.g. less overhang, if desired.

Less critical axis alignment -compared to coax coupler.

Slightly more expense than direct drive. Making components is quite easy - compared to coax coupler.

It is a good idea to cover the belts - to keep out swarf/chips/fingers/coolant/etc.

Pres

[Greg Rutter]

Thanks Pres, I will make sure that I add a cover for the belt drive system.

[wizard]

The belt drive gearing effectively allows adjustment of steps per revolution. Depending upon your lead screw this can be very important with steppers. So sometimes this is a reason in and of itself to go with a belt drive. With steppers you have some pretty hard limits on what can be resolved reliably.

As to resonance you can get that with any system if the components aren't matched correctly and also tuned properly. I've seen DC servos with issues with belt drives where simple belt tension adjustments solves the problem. Tuning isn't always turning pots or tweaking registers. In any event belts are just another variable in your setup they are neither better no worst than direct drive.