Resolution for a 4th axis

[brw0513]

My cnc router is coming along.  I built it as a moving table design seeking stiffness, so hopefully it will prove suitable to also cut gears and pulleys with a 4th axis.

I'm thinking about hardware requirements for the 4th axis now.  My plan is to use a Sherline ER20 industrial spindle belt driven (T5) to a 200 step NEMA23 stepper.  My CNC controller allows up to 8 microsteps per step.

Is there some rule of thumb regarding resolution of the mechanical drive to give acceptable results?  I see some dividing heads offer a 40:1 ratio, but I wouldn't have room to provide this with a single stage belt drive and I'm trying to keep things simple.

I'd be targetting:

* Mod 1 spur gears in straight and helical designs - teeth numbers would be 16 and larger.
* T5 timing gears - 22 teeth and up.

Thanks.

[ArtF]

Whats the stepper turn to index turn ratio you have?

Art

[JustinO]

It depends are what you are doing with the 4th axis.

(I don't include the microstepping I get from my Gecko203 drivers in the following.)

I use a standard Sherline stepper head setup for CNC threading. The system gives me 200*20=4000 steps per axial inch, and it gives me 200*2=400 steps per rotation. For a 2" diameter work piece, that 400 steps per rotation becomes 400/6.28=67 steps per arc inch. For a one inch diameter workpiece it would be twice that = 134 steps per inch.

I use a standard Sherline rotary table for most positioning. The system gives me 200*72=14400 steps per rotation. For a 2" diameter workpiece, that 14400 steps per rotation becomes 14400/6.28=2293 steps per inch.

I like the higher speed I get from the spindle stepper for my threading, but I get lower resolution, and  lower torque. I got a longer nema23 stepper to increase the torque, and I will probably go to a 3:1 belt instead of the standard 2:1 belt, to get a little more resolution.

So, do you want speed, torque, or resolution? What are you doing?

--Justin

[John S]

Careful if you are just using belt drive from motor to spindle.
cutting forces can make the spindle jump position, on a 4th axis it's best to run thru a work reduction such as a rotary table or dividing head.

[brw0513]

Thanks for the replies all.  I have added some photos to the original post and hopefully they might help to explain the intended setup.  The TeknoMotor spindle and Sherline spindle (for 4th axis) are shown.  The size of my machine will probably mean the 4th axis will need to run parallel to the router's Y axis.  I don't have all the components yet for the 4th axis.  I will use an ER20 collet in the Sherline spindle, a mandrel to carry the work piece and a tailstock with dead centre to locate the far end of the mandrel.

My ultimate goal is to cut spur gears (maybe helical one day) and T5 timing pulleys to support my other hobby - designing drivelines for radio controlled helicopters.  But for the moment, my question is about building an appropriate 4th axis for this duty.  I'm really trying to decide on an appropriate gear reduction between stepper motor (not shown) and Sherline spindle that will become the 4th axis.  It is possible a single stage belt reduction won't be sufficient.  

Art,

I'm not really sure what you mean by "stepper turn to index turn ratio".  My candidate stepper motor is 200 steps per rev and my controller allows 8 micro-steps per steps - so 200 x 8 = 1600 micro-steps per turn.  The Mod 1 pinions and wheels I have in mind to make would be would be 16 and 65 teeth respectively; the T5 pulleys would be in the range of 14 to 46 teeth (this is a change from my original idea).

JustinO,

I'm not sure what you were trying to teach me - my problem, not yours :)  I had thought about a Sherline rotary table, but I am trying to keep my costs down.  It's definitely resolution I'm after though.

John S,

Thanks for the advice.


Any further comments most appreciated.

Thanks.



 

[ArtF]

Hi:

>>I'm not really sure what you mean by "stepper turn to index turn ratio".

  What I meant was the how much the spindle rotates for one rotation of the stepper motor.
Consider you have a 1600 steps per rotation of the stepper. That 360/1600 or .225 degrees
per step. Thats not really high enough to ensure high resolution at a high torque. John was refering
to the same idea. On a rotary there are times you want higher torque to ensure the stepper doesnt skip a step or get jammed up. Id recommend a reduction of some sort if it were one to one. But if the stepper has a timing pulley thats 20 teeth ( as an example and the spindle head has a pulley with 100 teeth, then youd get
a 5:1 ratio..which woud be for example 360/(1600 * 5) = .045 degree's.. which woudl be much better.

  Mine for example has about 72000 steps for a full rotation of the rotary head. So about 360/72000 = .005 degrees per step..much better for torque.. Its all a tradeoff with speed, but generally you want to have enough
speed with enough torque, so aim a bit high for the ratio of stepper motor turns to spindle head rotations..

Art