From: John Stevenson <john@machines.fsnet.co.uk>
Peter,
Please forgive me for emailing back on someone else's post
but I think
my
experience will help you. Unfortunately it may be a little long.
About 5 years ago I was going to do the same thing but with a
larger
machine. I researched for controllers and found two within my
price
range.
Ahha and one made by Millatronics. I sent off for info and demos
and
decided on the Ahha as they had better software and seemed to
handle it
from a commercial more than a hobby angle. Next came a cost concern
I
already owned the machine so that didn't come into it but it would
need
ball screws on the X and Y axis to get some form of accuracy [see
note
1 at
bottom ],
Three stepper motors [ note 2 ], couplings [ note3 ]
I then costed all this up with the price of each controller to
get a
price,
this didn't include labour etc.
I then looked at second hand CNC millers and found that the older
ones
fetch very little with their old style controllers on them.
I costed up buying one of these and fitting an Ahha unit to it
and it
worked out cheaper than converting a manual machine.
For a start it had all the ball screws, stepper motors etc on
it and
more
with the Ahha system you can use the machines original drive boards
so
all
yo have to buy is the Ahha card.
I then sold my original machine for almost the same as I paid
for a 15
year
old ex college Beaver, [ like a Bridgeport only just a big larger
]
I then bought the card from Ahha and literally within one morning
we
had it
working.
I realise that my position may not be the same as yours. You
have a
mill/drill and possibly that's all you have room for. I accept
that, I
started off very small as well. In fact I still have my original
Mill/Drill.
If you are forced to go the mill drill route then the following
may
help;-
There are now three player in the controller market within
a decent
price
range, Ahha, Millatronics and Flashcut at www.flashcutcnc.com
I still think Ahha is the best for milling as it has more commands
and
can
be programmed with macro's etc. millatronics is still hobby gear
and
flashcut lacks quite a few commands like G41/G42 tool offsets
etc
Now to the notes.
Note 1
Ball screws are essesantial to overcome backlash. I know that
you can
program for backlash but that will not stop the cutter grabbing
and
pulling
into a cut. When you do a circle you will get an ellipse. the
shape and
quality dependant on the state of the X and Y screws.
You can get away with a ball screw on the Z as the weight of the
quill
keeps it in the down position and you don't need to interpolate
in that
axis.
I believe that cheapish ball screws are available from MSC.
Note 2
450 oz in steppers are quite sufficient for a mill drill
Note3
Helical couplers are usually alloy tubular tubes with radial slots
that
allow some misalignment of the shaft.
If you go to ball screw though you might consider using toothed
timing
belts and pulleys to reduce the gear ratio. On the original screws
they
are
usually 10 tpi so with a 200 step per rev motor you will get 2,000
steps
per inch or 0.0005" resolution. Ball screw are always much
courser
pitch,
usually 5tpi so you will get 1,000 steps per inch or 0.001"resolution.
With
a 2:1 gear reduction to the screw this will get your resolution
back to
a
smaller number. Another advantage of a gear reduction is that
it takes
the
snatch out and you can mount the motors at the side of the table
and
reduce
the overall length.
To answer a couple of your original questions
>What limit switches do you use and how repeatable is your
homing
accuracy?
>
The limit switches I use are just ordinary micro switches but
I don't
home
with them.
With the Ahha system you can designate any point to be a home
point.
You can have an absolute home and a relative home. As it's a bit
confusing
I always set these to be the same.
When I set a job up I designate a reference point. Usually the
corner
of
the part. I then zero on that point and tell Ahha that that is
0,0,
from
then on it works from that point exactly, every time.
>Have you used the Ah-ha system for contouring? If so how well
did it
>perform?
>
It performs very well, we do curved fan blades and such. It does
tend
th
pause slightly as it changes from one contour to another but with
milling
this isn't a problem. Routing in wood or laser cutting could be
as it
could
burn.
Her in the UK the Ahha agent sells a demo disk and book for
about 20
UKP
I believe that it's also available in the US. It's well worth
it as its
a
full working demo and the book is exactly the same one that you
get if
you
buy the system.
Attached to this email is a file called manedit.txt or something.
This is the help text file from the program.
--
Regards,
John Stevenson
Special Purpose Machines
Nottingham, England