advantage to using a timing belt/pulley?

[phantomcow2]

Well I was looking around my work area today and I see unused goodies which cover most of what i would need to conver my lathe to CNC (if i ever decide to go that route after the mill).
The motors I have that are sitting there have a timing pulley pressed onto the shaft of the motor. If i decided to convert, I could possibly remove those. BUt this leads me to think, I have seen several mills and lathes which use a timing belt and pulley type system, rather than a direct link between motor shaft and leadscrew.
So what is the primary advantage to this? Is it the ability for reduction and increased resolution? It seems like it would complicate things

[Al_The_Man]

Less motor torque required, smaller motor can be used (inertia ratio is reduced by the Square of the reduction), cheap form of reduction over precision gear box, but above 5:1 limited in the practical sense.
Al.

[wizard]

Another thing to consider is that the timing belt does allow registration between the spindle and the motor. Given that the motor has an encoder, this means that the exact spindle speed can be determined. It may also be usefull for spindle indexing but that is not likely to be a big concern on home grown hardware.

Some people will claim that timeing belts are a disadvantage due to periodic error at the work piece and vibration. This may or may not be an issue, frankly I think it is more of a question of the quality of the components. Considering that many high quality lathes, in the past made, use of flat belt drives there may be an honest conscern here.

If timing between the spindle and motor is not of concern to you, you might want to consider the modern day approach of the multi-V or Poly-V belt. This is a belt that is close to flat with tiny vee's to help grip the pulley.

Dave

[RotarySMP]

I think Wizard has confused the spindle drive, with Phantomcow2's question about leadscrew CNC drive.

I used timing belts on mine because I used a reasonably coarse leadscrew and needed to gear down to get enough force from my smallish motors.

Other advatages:
Easy alignment (you only need to assure that the two pulleys are in a single plane, rather than aligning shafts in two dimensions.)
Compact (I folded the Z motor down under the apron)
Accurate (modern toothed belts are extremely accurate)
Cheap.

While a reduction increases resolution, it will not increase accuracy. On a minilathe conversion, the accuracy difference between direct coupling and a timing belt reduction is not going to be a significant.

If you are going to drive the standard 1.5mm pitch mini lathe leadscrew with a modern double stack NEMA23 stepper, you probably don't need any reduction ratio.