which gear ratio

[sempar]

Hi I am trying to design my x axis for a router,

The idea is to have an open ended toothed belt and pulley either side, they will share the same axle, be driven by a harmonic drive gearbox 1:50 or 1:100,
powered by a servo with max rpm of 4600.

using a pulley of 60mm diameter and 100:1 gearbox i think this will give a max linear speed of 8.6 metres a minute or 144mm a second? and 1.8mm travel for 1 rotation of the servo?

does this sound reasonable is that a good speed? will it have acuraccy to the fraction of a mm, does it depend on the encoder?

any info will help, thanks

[aerowright]

I suppose this all depends on the size of your machine. But really you need to look at what you will be cutting, spindle speed and tooling. Tooling manufacturers will provide the chip load for a given tool. Presuming your spindle is up to the task, has sufficient hp, you can calculate the feedrate thus, RPM x # of cutting edges x chip load. For reference the smallest cutter (with the smallest chip load) I use is 1/16" single flute. On an aluminum clad polypropylene at 21000 RPM it needs to travel around 1600 mm/sec to maintain proper chip load which is important to tool life.

Most servo systems I've seen are around the 3:1 ratio. You can always run the servo slower to get the feed rate you need. Running the 60mm pully directly or possibly 2:1 gives more reasonable speeds in my estimation.

Regards,
Mike

[apache405]

Aerowright brings up a good point, most CNC systems are between 3:1 and 5:1 because the motor is directly connected to a ball screw.

Are you doing the speed reduction for torque or to help with accuracy? If you need the torque a 50:1 or 100:1 reduction gives, are you sure that a gantry style machine is right for you?
Encoders basically a wheel with a series of notches where light passes through to a detector. So as long as the encoder you are using is rated to accurately measure the max rpm of the motor, in your case 4600 rpm, the encoder will have little effect on accuracy.

[sempar]

i thought i would need a gear ratio that high because if using a ballscrew that has a lead of 5mm it would be fine for a gear ratio of 5:1
I was talking about a direct pulley to belt drive, similar to a rack and pinion
so a pully circumference of 50mm at the smallest is giving 10 times the linear motion for a ballscrew of 5mm lead for the same single rotation, thats why i thought the gear ratio feeding the pulley that gives the linear motion would need to be higher,

maybe im way off? im new to this

[apache405]

Kinda like a printer's movement?

Having disassembled several printers for parts, I can tell you that the ratio on a printer is roughly 75:1 or so, but they are using little motors with little
torque.

A couple of questions:
What kind of torque can your motors deliver? What kind of acceleration can the other parts, namely the belt, sustain without losing significant accuracy?
(I am not 100% acceleration is the right term here...Why must I forget everything over the summer?)

After all the accuracy of the end assembly will be determined by the weakest link.

I'm new to CNC machines too; however, I've built, broken, and fixed more than my fair share of robots, sound systems, R/C vehicles, etc.

[aerowright]

Sempar,

First off my apologies, my calculations were off. I blame it on not being natively metric;-) The 100:1 reduction would work for a wide range of bits. Some bits I use, 1/2" diameter, require speeds about 30% higher than the 100:1 reduction will allow. You could accommodate this buy slowing down the router, but the 50:1 reduction would produce very respectable rapid speeds and speeds needed for things like surfacing bits.

Again, sorry for the initial confusion.

Regards,
Mike

[sempar]

Well im not sure about wat acceleration they can handle but two 15mm steel rienforced belts must be very strong for mostly routing wood.
The servos are 350 oz/in max or 50 oz/in continouos.
Yes maybe 50:1 is the way to go with a pully circumference of about 60mm giving the linear motion?

[apache405]

If you are routing wood, your system is fine. Wood expands and contracts with temperature, humidity, etc. so any accuracy issues your linear motion system introduces will be lost in the noise, if you will, of the wood expanding and contracting.