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MrBean
10-07-2004, 02:40 PM
What do you guys think about belt driven axis?
I've had my machine running for just over a week now. The X axis is driven by a 30mm wide, 5mm pitch, HTD belt. The Y is Ballscrew, hardened shafts and bearings. The Z is ballscrew, THK track & skates.

The machine's a bit of a Frankenstein, but that's the parts I got for free, so I used 'em. It's going great. The only thing holding it back, is me. (still learning)

If I had to pay for parts to build a second machine. I'd like to go with belt drive for X & Y, as the cost compared to ballscrews is about 1/10, but performance still seems good (at least it does to me). Also, if the machine is moving gantry with a wide span, it's far easier to put belt drives both sides, and link with a shaft. I suppose it's a bit like rack/pinion drives.

Is there any reasons you would no go with belt drive. E.g. accuracy, wear, backlash...etc.


Thanks for any input.

sol
10-09-2004, 11:09 PM
Well, your question has been sitting dormant for a few days so this fool will jump in where the wise fear to tread.
From experience I know that racks can allow the pinion to jump and skip teeth thus losing position. A good belt system can be built to avoid this while still giving the advantages of a rack including no long whipping lead screw and more convenient placement. But you already knew this...
There have been positive comments about belt drive on other CNC forums...
I intend to replace my 5ft 1500mm lead screw with a belt when I get time....
Dean Couillard of Axxus made a number of tables that use belts for X and Y; I cannot find the pics on his site anymore but did happen to copy this one a while back which shows the gearing for a belt on Y.

ger21
10-10-2004, 10:40 AM
Dean Couillard of Axxus made a number of tables that use belts for X and Y; I cannot find the pics on his site anymore but did happen to copy this one a while back which shows the gearing for a belt on Y.

http://www.axxustech.com/Axxus4x8/axxus_4x8.htm


I think the reason you're not getting any responses is that very few people are using belt drive systems.

How do you like the system you're using. I think the belt drive has a few advantages, such as no backlash, and not having to worry about keeping a greased or oiled screw clean.

samualt
10-10-2004, 04:38 PM
The only downside I see in using a belt drive is that if you have motors with a fast rpm, say 2000 rpm, then your looking at a huge step-down to run the belt. All those extra pulleys, bearings, and belts cost money and time. Whereas a ball screw usually takes care of some of that by having 10 TPI (turns per inch). Ball screws and lead screws have a built-in step down.
If you have to have a large step down then your looking at about 2 pulleys for the step down and 2 more for the axis and one for the motor. 5 pulleys, 3 belts, and extra bearings. It's probably about the same price as a ball screw all said and done.

However, like ger21 said, there are advantages to belt drives also!

MrBean
10-10-2004, 05:56 PM
For me, my belt driven X axis is performing best out of the three. My machine is mostly made out of crap destined for the bin. The stepper drives are also crap. No microstepping & no current limiting, so steppers are running at rated voltage. (slow). Belt drive suits this situation quite well. Low revving motors, with some gear reduction, and bugger all amps * volts. I am getting 90 IPM rapids on X axis with no lost steps. Resolution is not as good as the ballscrew axii though. Minimum movement of X is 0.007" which is ok for me at this time. Another bonus is, it sounds really cool compared to the ballscrews.

This is my first machine and is a learning experience as much as anything. CNC is only a hobby for me, so I cannot justify spending huge sums of money on equipment. I do plan on building some PIC microstepping drivers I saw in the forums somewhere. That's my next step. More power to the motors.

balsaman
10-10-2004, 09:31 PM
what ratio are you running on the belt?

Eric

MrBean
10-11-2004, 05:11 AM
The main belt runs on 26T pulleys either end, 5mm pitch.
The reduction is 3.6 : 1

So:

1/(26*5)=0.0076923

0.0076923*3.6*200=5.53844 steps/mm

1/5.53844=0.1805562mm/step or 0.00710"

That's with a 200 step/rev motor. Microstepping would increase the resolution.

I'm going to try a 6:1 ratio this week, time permitting.

balsaman
10-11-2004, 09:00 AM
I want to cnc my lathe. I am considering a belt on the carriage. Mostly for no backlash. I am wondering about "strech" tho. Maybe it's not a good choice for metal working machines.

E

MrBean
10-11-2004, 05:40 PM
Not sure about stretch specs of belts. I do know that mine will lose steps well before the belt would stretch or snap, which ever would come first?

If you have a CNC router that can do aluminium, you could try machining your own pulleys. Tho' they're not expensive to buy.

I still like my belt drive and plan to build a small, entirely belt driven machine as an experiment.

Regards Terry...

Kookaburra
10-12-2004, 05:07 PM
Mr Bean,

From experience watching my customers come up with various ideas to drive their axes, there have been mixed opinions regarding belt drives. I have one customer in particular that is using a 25mm (1") wide belt driving an axis 3.5m (137.8") long and there was a noticable bounce in the belt as the machine axis started and stopped. It took a little longer to tune in the PID control of the servo drive but in the end it ran quite smooth. We have other customer that runs over a shorter distance without any problems at all. I personally think that the guy with the 3.5m travel should have used a much wider belt and used support idlers on the belt return side to eliminate some of the bounce. I guess what I'm saying is that belt drives can be good if they are engineered properly for CNC use in your particular application. Nice and cheap, I agree, and easy to install. Let us all know what you decide to do when the time comes I will be interested to hear the outcome.

JavaDog
10-13-2004, 02:44 PM
Hmm...I didn't even think of belt drive when starting to design my CNC router. I might have to go back and look at that - lower cost is always a plus!

Besides, my set-up will be fairly small (26" x 30")...so it sounds like belt drive will work well.

ger21
10-13-2004, 03:26 PM
Hmm...I didn't even think of belt drive when starting to design my CNC router. I might have to go back and look at that - lower cost is always a plus!

Besides, my set-up will be fairly small (26" x 30")...so it sounds like belt drive will work well.

Lower cost would depend on what you are comparing it too. Ballscrews probably. Acme with Delrin nuts, belt drive would probably be more.

JavaDog
10-13-2004, 03:30 PM
Lower cost would depend on what you are comparing it too. Ballscrews probably. Acme with Delrin nuts, belt drive would probably be more.

Well, I am looking for durability and accuracy within my budget (aren't we all!) and I like the idea of belt drive and no backlash. My particular needs would probably be best described in another thread however... :)