RE: [CAD_CAM_EDM_DRO] Getting Started with Servo CNC

I think you may run into problems trying to install 600 oz in servo
motors with 500 cpr encoders on a shoptask.
1. The hole in the shoptask X axis case is too small for a 600 oz in
servo and I don't know of an easy way to make it larger without a lot of
work. If those are the motors that I am thinking about you will also
need an external power supply because those encoders draw more than the
Gecko's can handle.
2. The motors that you probably got have 500 cpr encoders which means
that 500 times 4 for quadrature =2000 per rev. Times 2/1 ratio = 4000
time 10 TPi= 40,000 steps to the inch. Master 5 is limited to ~8000
pulses per second so your max speed will probably only be about 12
inches a minute
3. The shoptask X axis as I recall comes with a 42 tooth driven pulley.
The shaft on your motor is 5/8 D so you will probably need a fairly
large diameter pulley on the motor losing some efficiencies. A better
solution would to have been to use the smaller motors that are C23 and a
have a .250 shaft from Jeff Davis on this list. I have used similar
motors and gotten over 135 IPM with a 3/1 ratio.
Master 5 is set up to run a mill/gantry system but by swapping the
motors cables for X and Z you would have a correct configuration then
all you need is the CadCam program that will give you the correct G code
files for a lathe.
The last question I can answer is that I don't own a shoptask but had
access one and have some pictures of my servo motors on that machine.
Email me and I will send you some pictures.
Dan

-----Original Message-----
From: erikreikes [mailto:ereikes@...]
Sent: Monday, September 09, 2002 2:15 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Getting Started with Servo CNC

Hello All,
I'm in the process of setting up my ShopTask for CNC with Servos.
I found some big servos (around 600 Oz-In) on Ebay for cheap and
decided to go with a servo system. I bought the power supply kit
from camtronics and assembled it and I bought Gecko G320 servo
controllers.

In the process of setting things up I realized that with 500line
encoders and a 2-1 reduction on the drive to 10 turn/in lead screws
that I would be getting (0.1 inch/turn / (2000 steps/rev *
2revs/turn) .000025" per step which seems might fine. By my calcs
this works out to around a max of 15IPM using Master5's max pulse
rate of around 10000 steps/s. Seems kinda slow for a beefy servo
system, but may be acceptable. I'm not sure because of my limited
CNC expreience what is a good number.

Question 1 : What is the recommended way to improve this? I've
noticed that gecko has a soon to be released servo driver with a step
multiplier. Being a computer engineer I could make something like
this fairly easily as a front end add on between the parrallel port
and the geckos, and I may go this route. I also was thinking of
going from a 2-1 reduction to a 1-1 reduction, but I'm not sure if my
torque is sufficient to handle this. Also, if I work up a custom 3
axis pulse multiplier board would anyone else be interested in
purchasing them to help me defray the cost of board spins? I would
estimate it would be in the $40 range with parts to build in low
quantity.

I've decided that as soom as I'm up and running my first project
will be a Stainless Steel chess set (well, first it will be aluminum
to test, but then SS). This involves mostly lathe operations. It
seems to me that Master5 is mostly set up for milling operations. I
use Rhino3D for CAD work and have been fooling around with importing
DXF's into Master5. It seems to always add Z movement even when I
inhibit z in the dialog box. I'm thinking I may have to import dxf,
export g-code, manually edit the code and re-import again.

Question 2 : What extra features does the paid version of Master5
support that would help in my endeavours. I plan on buying it
anyhow, if I like the way it works, but I'd like to know what extras
it has before I order it. Also, any similar programs that are sset
up for lathe CAM? Something like a dxf to lathe command program or
something. I am a bit overwhelmed at the dizzying array of programs
out there.

Vector CAD/CAM seems like a really neat package, but it is a bit
over my threshold of pain at $800.

Question 3 : Are there any used copies of people who have moved on to
something else? Any sales or things like that the list knows about?
Is it really worth the $800?

The servos I have do not fit the Nema 34 mounts on the Shoptask.
I've been trying to thing of a way to elegantly mount them. My first
thought was a servo bolt pattern to Nema34 adapter plate, but in
addition to being kind of kludgey it would be tough to figure a way
to provide access to all the bolts, plus I'd have to do machining on
the existing plates anyhow. My second though is to either modify or
fabricate entirely new mounting plates that fit only the servos.

Question 4 : Has anyone here done a servo conversion on their
shoptask successfully? If so, would it be possible for me to see how
you did the mounting? This is currently the last thing I really need
to get my CNC up and running, and I'd like it to be right.

Question 5 : What is everyone's experience with using backlash
compensation with Acme lead screws while CNC-ing? I know ballscrews
are the way to go, but I'd prefer to leave that sort of upgrade until
I ge t abit more experienced with cnc in general. Will I be terribly
dissapointed with acme cnc performance machining simple things with
moderate tolerances (.001"). Can I compensate by ensuring that my g-
code programs don't take out backlash? FYI the shoptask does have
backlash compensating nuts on the acme leads that gets me down to
aroun .005-.010" backlash on each axis (well, except the Z axis whic
sucks).

Thanks in advance for any and all replies.

-Erik reikes



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