i have been reading CNCZONE up and down.....and have started to built. I want to router wood, MDF and foam....for hobby use.

Now i need some advice and have questions on stepper motors and motor drivers.

Discription of my machine:
- Welded out of 1 1/4 inch steel pipe, can be dismantled and transported elsewhere if need.
- The table size 28" x 72". Gantry is moving without friction on skate bearings.
- Gantry weighs 37lbs and will carry a full size router or laminate trimmer
- Router bit moves along the 72" side within a vertical accuracy of 0,008 inch due to the fact that my steel tubes are unmachined and not 100% straight.

X-AXIS
Due to the long table i am going to move the gantry with a belt drive on each side. A dual shaft stepper motor will be fixed in the middle of the gantry (under the tabel) and turn over an axis to each side a timing belt pully directly. The timing belts (one on each side of the ganty) are fixed on the ends of the table and set under tension.

Y-AXIS
Motioned from side to side with single belt drive. The timing belt is also fixed on the ends and set under tension. The stepper motor is sitting on the backside of the Z-axis.

Z-AXIS
Conventional screw drive.


STEPPER MOTOR & DRIVER
My stepper motors on X and Y axis will drive the timing belts direct. 200 steps for one revolution of the pullys will move the axis 4.19 inch (106.4 mm).

4.19 inch / 200 steps = 0.02095 inch (0.532 mm) at one step (1.8°)

Resolution of 1/8 = 200 * 8 = 1600 steps for one revolution
4.19 / 1600 steps = 0.002619 inch (0.0665 mm)

So 1/8 step would move my X or Y axis 0.0026 inch. This is more accurate than the 0.008 inch tolerance of the level of my tabel. (If i reach this accuracy at all due to fibrations...)



My questions are:

1. Do i need more resolution than this? Maybe to run stepper motors in a better movement?

2. I understand that the belt drive limits my machine to do metal parts but i should not have problems with speed vs. torque. One revolution of the main stepper motors = 4.19 inch
72 inch tabel length / 4.19 inch = 17.18 revolutions to run gantry side to side of the tabel. If i set the motor speed to 200 rpm i get 3.3 revolutions in 1 secound. 17.18 / 3.3 = 5.15 sec. To drive the gantry over the 72 inch long side of the tabel would take at 200 rpm 5.15 secounds.
So i will run the stepper motor more in a low speed level and get easy the full torque out of the motor. Rigth ?

3. The gantry runs without friction on her bearings but the timing belt is fixed on each end and set under tension. The metal timing pully is accompanied by an idle pully on each side to keep the belt around the turning metal pully. This belt set up will be on each side of the gantry and create a force for the single gantry moving motor. How much motor torque do i need? Can i use an 425 oz-in bipolar stepper motor like KL23H286-20-8B ? Or is an bipolar stepper motor with 282 oz-in strong enougth (like KL23H276-30-8B) ? Or do i need more power?

4. More expensive stepper motor drivers have an automatic idle-current reduction to reduce motor heating. Makes sense when you move your axis with a screw. The screw fixes the position of one axis while other axis's are moving. But how would a belt drive react? Reduced holding torque and the axis slides away while other axis's are working?


Any ideas and comments are welcome.....
I want to order motors and drivers.

Georg