RE: [CAD_CAM_EDM_DRO] cnc novice questions? (read as: "HELP")

Gene:

Let me take a stab at some of your questions, with the caveat that I'm also
fairly new to this game and have recently added CNC to my Sherline mill.

First, understand that stepper motors are primarily current devices. Holding
torque depends on "amp-turns," the number of amperes of current flowing
through the number of turns in the coil. Thus a motor with one turn and one
amp has exactly the same holding torque as a motor with two turns and 1/2
amps. The only places that voltage (across the turns) comes into the
equation is heat generation and response time. It is for this reason that,
all other things being equal, you prefer the motors which have the higher
coil currents and lower voltages.

Unipolar vs bipolar. One ways to make a current source for a motor is to use
a high voltage and a voltage dropping resistor. If the voltage across the
motor is small, compared to the voltage drop across the resistor, the
current flow is essentially independent of the motor. That is basically how
a unipolar motor works. You take a high power supply voltage, add a series
resistor to each coil, and toggle the coils, in order, to ground through a
switching transistor. It works, but there's quite a bit of power dissipation
in the resistor. Plus, motor response is poor because the inductance of the
motor tends to maintain current flow when you'd really want to change it.
Another way to drive the motor coil is to take the same power supply, but
connect switches to the motor coil in such a way that you can feed current
through the coil in either direction. (A transistor bridge does this). Now,
you can rapidly change the direction of current in the coil. What happened
to the series resistor? It went away. Instead, you simply continuously
measure the current in the coil and switch the transistors off when the coil
current exceeds the value you want. Thus, your power dissipation goes down.
And, performance increases because you are actively dealing with the
tendency of an inductor to resist changes in the flow of current. Of course,
there are neat integrated circuits that manage most of this for you so, to
the user, it's not all that complex. Given the choice, pick the bipolar
motor.

Full/half stepping - Stepper motors are rated for the number of required per
revolution of the shaft. Two hundred steps per revolution is typical. A half
stepping driver manipulates the motor coils in such a way that the number of
steps per revolution is doubled. A half stepping drive often minimized
resonances in the motor system, making performance smoother. If possible,
choose a half-step controller. With microstepping motors, you can get up to
25,000 steps per revolution (Compumotor, for one sells this type of system)
but prices are a tad high - I paid $2500 per axis for this type of drive
(NOT for my hobby, but for my job at the time) and used three axes

Suppliers - It depends on how much you're willing to do yourself. I have no
fear of building and testing kits, so I chose Camtronics. I can recommend
them both from a standpoint of quality of the product and from a standpoint
of support from the vendor. I chose to "roll my own" because I have some
experience in the area and wanted to sharply limit my cash outlay. I'd
suggest that you look at Sherline's site for CNC vendors who use their
equipment as a base. Many of these vendors could also provide the motors,
drivers, software that would work with your home-brew router system. As far
as the router is concerned, you'd only have to assure that you can specify
the number of steps per inch, which you can calculate from your leadscrews
and motors.

Home CNC for Dummies - We're all busily writing that book as you read this.
It won't be done until well after you've retired. :)

-- Jerry Jankura
Strongsville, Ohio
So many toys, so little time....

| -----Original Message-----
| From: egjameson@... [mailto:egjameson@...]
| Sent: Thursday, October 18, 2001 11:24 AM
| To: CAD_CAM_EDM_DRO@yahoogroups.com
| Subject: [CAD_CAM_EDM_DRO] cnc novice questions? (read as: "HELP")
|
|
| Hello, i have been lurking for a while and am confused enough now to
| ask for help. Currently i am trying to build a 3 axis, medium size
| cnc router. While i am still working on the structure of the system
| i am researching the electronics. i am greatly confused by the
| electronics' specifics. i have no desire to assemble my own board, i
| simply want to purchase a controller with drivers and power supply
| for 3-4 stepper motors. I say 3-4 motors because i may put 2 y axis
| motors on my next cnc project (thats right i still haven't figured
| this one out and am already thinking about the second).
|
| i simply am confused by all of the specs for the various controlers
| by various makers. uni v. bipolar, amp/phase, amount of motor torque
| needed to drive a cnc router (i plan to rout wood, plastics and soft
| metals), how to use limit and home switches, full/half/micro stepping
| are some of the areas about which i dont understand. basically, i
| dont want a lack of electronic know-how to keep me from being able to
| do this project.
|
| does anyone recommend one driver/controler supplier over another or
| are the components and techniques essentially the same? also, is
| there a "home CNC for dummies" :)
|
| Thanks,
| gene jameson
|