Timing Belt Drive Accurace and Price

[drewgarth]

Hi Folks,
I am looking at building a new table axis. Travel is to be 8ft
Max speed is say 30ft / min
Load of say 40lbs.

The desired accurace is about 0.004" over the 8ft travel.

From practical experience are timing belts an option?

If they are can someone indicate some ballpark costs for a setup like that.

Im trying to compare it to the cost of ballscrews.

thanks in advance
cheers
Andrew

[Jason Marsha]

There are some members in the zone who have used timing belts with success. As far as I remember one of your countrymen (forgot the name) uses timing belts on his router so you should be able to source them in Australia.
I am also planning to use timing belts for my large router. I have already bought some parts from www.sdp-si.com.

Jason

[KEYTEEM]

i purchased thru sdp-si.com and they where about 45 dollars for
2 pulleys and a belt.
you can go to their website and look up the exact parts you want
and get the price and availability at the same time.

[garagefela]

Hey Drew,
Look for pictures by ynneb, (Melbourne) he made this awsome gantry router and used toothed belt. Top bloke as well by the way. ( pay you later Ben )

Cheers M

[andy_ck87028]

Andrew

I'm in Melbourne and am taking the belt option for X and Y. (Thanks Benny)

I am using 4 to 1 reduction from the stepper motor to the drive pulley which has a diameter of 2.5cm.

Each stepper revolution has 200 steps, so that is 800 steps to move the circumference of the drive pulley (7.85).

Precision is therefore .0001 metres per step.

Andy

[tekno]

lets see some pictures.....

Originally Posted by andy_ck87028 Andrew I'm in Melbourne and am taking the belt option for X and Y. (Thanks Benny) I am using 4 to 1 reduction from the stepper motor to the drive pulley which has a diameter of 2.5cm. Each stepper revolution has 200 steps, so that is 800 steps to move the circumference of the drive pulley (7.85). Precision is therefore .0001 metres per step. Andy

[drewgarth]

Hi Andy,
Thanks for your input, what source of belts and pulleys are you using in Australia?

Your practical advice is useful. One thing i am a little hung up on is the impact that the accuracy of the placement of the teeth on the belt will have.

I understand that the belt is going around a pulley of fixed diameter, so theoretically if the belt was infinitely stiff and the teeth on the belt meshed perfectly with the pulley then the only source of innacurace would be the step resolution.

That said, many belts seem to have a positional accuracy rating on the teeth of 0.2mm / 300mm.

Does this have any impact on the positioning accuracy in practice?

In either way, a good source of belt drive technology would be useful

cheers
Andrew

[andy_ck87028]

Andrew

Thanks for your comments. I do need to make it clear though that I am not an engineer and believe me I am very envious of their skills.

Your first point re. teeth and belt accuracy. I have established that there are standard profiles which means if you get pulley of profile X and belt also of profile X, you will not have a problem. I also learned that M profile is likely to have greater backlash than T profile so I went T profile. In terms of the distance between teeth, I was told 2.5mm T has least backlash but it was only available in a very skinny width (5 mm if I remember ??) so I went 5mm T profile where the belt width is 15mm. Stretch should not be an issue as there are multiple metal strands running lengthwise through the guts of the belt.

Also I said to myself that inaccuracies are more likely to come from machine flexing than belt backlash so I chose to ignore belt design as a factor. I reckon that the belt system I have chosen maximises bang for buck and if I have inaccuracies then I resolve them through G code and tightening the machine itself rather than spending humungous dollars on high quality belts.

Also, I observed that a belt system is good for wood and aluminium and plastics and generally large scale cutting. If I had high level precision needs then a) its likely to be for small stuff and b) I'd be better served manufacturing a small CNC mill with ball screws enabling me to also mill harder materials (eg steel)

So my guiding premise is that the belt system is well suited for large scale work where ultra high precision is less likely to be a factor.


If you want excellent design criteria for belt and pulley systems, the PDF in www.sdp-si.com is fantastic.

For pulleys, I went to Naismiths in Heidelberg Rd, Northcote and for belts I went to a great supplier near Tullamarine Airport. I think the belts were $20 per metre for T5 of width 15mm. I have checked the yellow pages and can't find them. If you draw a blank, PM me and I'll look further. Naismiths certainly knew them. (Titan Ridge is ringing a bell somewhere but I might be wrong)

(Fyi, others use T5 15mm belt so I am not alone)

Have fun

Andy

(NB Pictures when I can scrounge my wifes' camera... my wonderful camera went sick)