RE: [CAD_CAM_EDM_DRO] Small Problem

| -----Original Message-----
| From: Weyland [mailto:weyland@...]
| Sent: Friday, August 03, 2001 6:11 PM
| To: CAD_CAM_EDM_DRO@yahoogroups.com
| Subject: Re: [CAD_CAM_EDM_DRO] Small Problem
|
|
| Hey Jerry~!
|
| From: "Carol & Jerry Jankura" <jerry.jankura@...>
|
| > I'd strongly suggest that you dig into your pockets a little
| deeper and go
| > with the bipolar drivers. Years back, I used some of the unipolar drives
| in
| > an industrial application. In order to get any performance from the
| motors,
| > we needed a relatively high voltage power supply and some really large
| power
| > dissipation resistors which were put in series with each of the motor
| > windings. The resistors ran really hot.
| > Contrast that with some projects I've worked on more recently that were
| > implemented with bipolar drivers. First, no series resistors
| were required
| > so the entire system ran cooler. And, the motors were really
| "stiff." That
| > is, they really ran well and did not want to stall or drop steps
| (important
| > in CNC where you are essentially "counting steps" to know where
| you are).
|
| Okay, understood, but I have to ask - (no, I'm not trying to squirm outta
| the money)
| For a home/hobby type CNC mill or plasma cutter,
| is the difference *that* great?
| Obviously, we know it *can* be done by uni's for
| the type of stuff I want to do, but I'm not talking about
| running them in 8 hour shifts, 5 days a week.
| (Remember, I work as a Tool & Die maker all week)
| These are mainly for me to play with and knock out ideas
| and prototypes, or otherwise complicated parts.

I can't say for certain for a hobby system. At work, I replaced an
automation/control system using some of the old Superior motors with one
using Compumotors. The difference in performance was amazing. Now, the cost
of either of those systems was quite a bit more than anything we're talking
about on this list. Compumotor cost about $2500 an axis, plus the cost of
the controller.

Bipolar drivers and motors will allow higher speeds for slewing to a
starting point, for example. How much higher, I can't say. I know myself
well enough that, if I didn't like the performance of the unipolar system,
I'd rework the system to be bipolar. So, I just went with the bipolar
drivers to start.

| > I just went through the same exercise that you are going through and
| decided
| > on the Camtronics 2 amp board. My decision was made based on a couple of
| > things: First, the motor current requirements matched the driver's
| > capabilities.
|
| Okay, this got my attention.
| Am I supposed to make the motor decision first, and then find a suitable
| driver?
| That *does* throw a twist in the works.

Well, first you should figure out the torque required to accelerate and move
each axis at rates that you want. I took the easy way out and assumed that
the motors that Sherline sold for their equipment would be suitable for my
application. This assumption was strengthened by comments from a few people
on this list and/or on the Sherline list. Once you determine the torque,
then you start looking at motors. Things to consider are single ended or
double ended shafts. Double ended shafts allow you to reconnect your hand
wheels; single ended shafts do not. I selected double ended. Another thing
to consider is winding voltage drop and current. In general, for a given
holding torque, you want to select motors with the highest current and
lowest voltage drop to get the best performance. However, you must also
balance this against the capabilities of the drivers that you can buy. So,
it's a bit of an iterative process. But, it does you no good to buy the
Camtronics 2 amp driver board and try to run 3 amp motors with it. Another
consideration is the physical size of the motors. Size 23 fit on a Sherline
quite nicely, while size 34 motors would be a stretch.

| >Second, I don't mind building a kit and adding the "extras"
| > required to make a "stand alone" controller.
|
| Absolutely agreed.
| Besides having more time than money,
| I like putting things together.

So do I.

|
| > Third, both Gecko and
| > Camtronics have received several favorable comments on this list and no
| > negatives. I have already received the Camtronics toys and
| assembled them,
| > but have not tested them because I'm still waiting on delivery of the
| motors
| > and mechanical coupling to the Sherline mill.
|
| Hmmm, okay...
| *Which* Camtronics didja get?

I bought the three axis two amp board, since the motors sold by Sherline
require 2 amps. It would not have been a bad decision to purchase their 5
amp board either, but it would be overkill. I also bought his tachometer,
but I don't plan to put any speed feedback on the system at this point in
time. I'll simply dial it in before I start the CNC program, or insert a
"pause" in the G-Code to allow me to change speed.

| And will a bipolar "chopper" driver run a unipolar motor?

I don't think so. a Unipolar motor has one end of each coil connected in
common while a bipolar motor brings both ends of the coil out.

-- Jerry