RE: Low cost Servo Controller

Dave,

I thought that running with acme screws just plain sucked. The program
that I was running at the time- Dancam had backlash compensation, but my
stepper system at the time was underpowered and consequently lost steps and
ran too slow. I did program like it was a manual mill, but losing steps
made it impossible. One time, I was milling out a pocket in a piece of
cold rolled steel, taking 1/8" deep passes at a time. The machine lost
steps every pass and I didn't relize it- when it was done, the pocket was
out of shape and the end mill was chipped up. The next day, I called
Rockford Ballscrew and ordered a pair of 5/8" ballnuts and ballscrews.
They cost $275 ( rolled thread and double preload nuts). It was a
decision that I haven't regretted. I can set the preload for less than
.001" backlash by tightening up the knurled nuts. Everything is much
smoother now and the system is more reliable. Later on, I added a more
powerful stepper driver system.
Over a year ago, I bought Maxnc Deluxe to run the system. It seemed to run
okay. It's simple to setup and runs a lot of G-codes. It has backlash
compensation as does every other PC cnc program out ther. But, when I was
running some production parts, I would occasionally ruin a part with some
missed steps ( a bunch of missed steps at a time). It seemed really odd.
The error always seemed to come up at the same point in the cycle ( this
was a fixture with 7 islands loaded with plastic parts). Soon after that,
I loaded EMC onto the machine and never got that error running the same
parts. So, I don't know if some software can contribute to lost steps, but
it sure seemed that way to me.


Dave, right now I am retrofitting a new mill- a little RF-45 bedmill. It
will have ballscrews on it. The Acme screws are going in the scrap bin.
McMaster Carr sells a lot of different ballscrews and ballnuts for general
industrial work. These certainly aren't for toolroom applications, but
will work fine for most stuff. Currently, I have in the shop ballscrews
and ballnuts 3/4" dia. that are going in the machine. They sold for $30
each. The ballscrews go for $1.64 per inch. This is very reasonable.
Saturday, I made a yoke for the X-axis that holds two ballnuts and adjusts
for preload. Ballscrews aren't that hard to install and they aren't that
expensive anymore.

Software: I am a convert to EMC. There is much more potential to it than
with any Windows/Dos software. Fred is writing new code for it every day
and he's getting paid for it! That's the best value for our tax dollars
that I've seen. Go back to the beginning of the Archives on our list to
see all the reasons for me liking it.

Current shop setup:

Mill/drill
470 oz-in steppers
5-6 amp half step drives
15 amp 24 volt power supply (36 volts with no load)
5/8" ballscrews
EMC running the machine

Sherline for light engraving
65 oz-in stepper Y axis
110 oz-in stepper X axis
100 oz-in stepper Z axis
2.5 amp half step drives
24 volt power supply from DEC printer
regular Sherline leadscrews
Maxnc running the machine

Index Knee mill ( baby knee mill)
Acme screws
human running hand wheels and sweating profusely- Hmm, this needs to be
retrofitted!

Next machine in shop (within two weeks):
Rong Fu RF-45 mini bedmill
some sort of hybrid servo system from Dan Mauch
3/4" ballscrews
EMC running the machine

That's all I can type for the night. Time to make some more parts.

Dan


At 03:01 PM 9/14/1999 , you wrote:

>From: David Howland <dhowland@...>
>
>Dan (Falck), It seems as though you have a fair amount of experience in

the area I am exploring. Others may also have this kind of experience as
well. You have run CNC without ballscrews. You said you tried this and
upgraded to ballscrews after running without them for a few months. Aside
from the cutter sucking the table out of position, as it can in manual
milling, was a major part of the NO FUN aspect of working with the
backlash, the fact that you had to program in a manner to adjust for it.
That is, before changing directions, add steps to the program just to deal
with the backlash? Was that less fun than what you had to do to keep the
cutters from sucking the work into themselves? Your third experience of
loosing steps I suppose meant that you had to reduce the feed rates to
reduce the torque required from the stepper motors? Did you experience the
steppers loosing steps because of fast table moves, such as Home, jog or
power supply overloads?

>
>If you had more speed and no lost steps, in your case, how important and

how expensive is the ballscrew? Was the aspect of the cutter sucking into
the work a major or a minor issue in the attention required to work around
that problem?

>
>When Fred got EMC up for steppers, did you bring up EMC for your shop? If

not, what operating system do you run and what software and hardware do you
run in your shop? When you move into servo systems, what cost effective
options sound good to you? What does it take to make that transistion?

>
>Thanks for your help.
>
>David Howland
>
>
>