Belt reduction questions Please help!?

[Bird_E]

Looking for some help. I'm going to be converting a X-3 Desktop mill to Cnc using servo motors. This mill is a little larger then a mini-mill and about the same size as a small bench top mill like an RF-25 or RF-30.
I'm going to be using a 2:1 belt reduction from the servo to the ball screw.

First Question: What belt types are people using to do this type of conversion? XL, MXL, etc...... How wide of a belt? 1/2", 3/8", 1/4 etc???

Second Question: My servo motors will be mounted on plates with slots so I can tension the belt. I don't know how much these belts strecth, How much adjustement do my motors need in order to tension the belt? 1", .5", .100" ?
I'm a little limited in space in how much I can adjust the motors, base upon my current design.
Thanks for the help!
Bob

[Torsten]

Seams like there are so many choices?
Well not really if you start drawing them up in a cad program and then
print them out at a 1:1 scale you will see how different they are.
A finer tooth profile will result in much smaller diameter pulleys then
a course profile at them same tooth count ratio.
The larger diameters will require less force on the belt to transmit the same torque.
Consider building some flexibility into this setup.
For example make sure you can change your ratio later to 1:3 or 1:4 or 1:1 by having
to change only one of your pulleys to a available different size.

Consider these facts before making your choice.

Other things are often a matter of personel preference.
Here are some of mine.
Square Tooth profile verses round, my choice is square it engages better.
Pulley diameter, as large as possible within reason.
Beltwidth 0.5 up to 250 oz\in motors, 0.75 up to 1000 oz\in and 1.0 over 1000 oz\in.

Timming belts are designed not to strech much allowing 0.1 over hand tight is all
you need. To remove the belt you need a little more to get over the spur rails
so about 0.5 should do it. This will leave you with about 0.6 movement at least.
Hope this is helpful to you good luck.

[RotarySMP]

I have a minilathe and used 2:1 reduction with 9mm wide HTD belt. I did the maths and came up wiht a belt length that gave 45mm between shaft centers.

Gave one shaft adjustment between 44 and 46mm, and it has been been perfectly adequate. You have to slide the belts on both pulleys and then slide both shafts at once, but you only do this occasionally.

After a year I have never needed to retension the belt on X (can't say for Z as I removed it a couple of times for other reasons.

When I asked the question of which belt form is "best" I got a pretty clear, "doesn't make a difference " answer on a couple of boards. Choose HTD because the most convenient vender had alloy pulleys and a belt to give minimum shaft offset (45mm).

Hope this helps.

[DareBee]

The round tooth belts are designe for high speed use, they engage smooth and gradual into the pulley where is the "not quite" square belts engage much more harshly. Bear in mind that the belts come in set lengths and design your system to work off of a common length.