Folks and interested parties...
I'd started my own lengthy project update last night, but just saw
Dave's post. Darn if he is not quick! I'll rejigger this to be a
general update to reflect his post. All are welcome to follow the
project progress and join, participate, or watch. It is a DO-OCRACY
as Chris says.
Frame = We will use the kit of extruded aluminum rails for the frame.
I've started the 2D and 3D assembly drawing and have been designing
corner brackets and piece-parts for the structure that can be made on
my home CNC. I'll figure out how to post some static images and can
probably export and post the as-is working 2D as a DXF file and the 3D
as a STEP file.
Linear Bearings = Since this is a "quick-and-dirty" CNC learning
experience we won't have fancy linear bearings on the X/Y/Z linear
axes. There has been project discussion, but Jim has prototyped a
sliding journal bearing that keys into the central grooves on the
extruded aluminum channels. It will be easy to make and hopefully
good enough not to bind-up and get clogged with machining chips. Stay
tuned for a week or two to see working parts.
Motors = We plan to start with NEMA 17 frame size steppers (rather
than larger NEMA 23). Dave's post said he's ordered a test set of
drivers and motors. Cash and parts do have a way of challenging a
project to move along further and faster.
Electronics = Dave's order includes the four Spark Fun EasyDrivers
http://www.sparkfun.com/commerce/product_info.php?products_id=9402
Each takes the two 5 VDC STEP and DIR signals and sequences the power
to the four stepper motor wires to increment the motor armiture 1/200
of a rev (1.8 degrees) with each STEP. TP is prototyping that up with
Jim's prior test EasyDriver and NEMA 17 motor. Next week's discussion
will be on other electroincs, break-out board, laptop vs. Arduino,
etc. I'll make another post on this topic later in the week.
Mechanical Drive = Since the NEMA 17 motor has a 5mm shaft, I
recommend to use 5mm all-thread threaded rod for the lead screw and
put a shaft coupling between motor and screw. We'll just use the
motor bearings as the axial thrust bearing for the lead screw at the
head end and float the tail end of the lead screw in a loose journal
bearing. This is not the most rigerous mechanical setup but keeps
with the quick-and-dirty theme to be good enough for now. The 5mm
diameter all-thread has a 0.8mm pitch (0.031") per revolution. Doing
the numbers, one STEP = 1/200 rev (1.8 deg) converts to a linear
distance of .004mm (0.000157") which is too small to see and that is
good. Six STEPS equals .024mm (or about 0.001") which is a good small
nominal working unit. Realistically we will have reversal (backlash)
errors and a general level of precision likely to be 5 to 10 time
bigger than that. This amount will be noticable, but not too bad for
such a simple beast. Based on the above the resolution should be OK
fine. The next trick is to mock it up and confirm that the torque and
speed capabilities are OK fine too. The threaded rod will increase
the motor's torque delivered to the moving carriage. Assuming it all
works out, we'll use the same general setup on X, Y, and Z.
We'll also restart the dormant EGGBOT project. It will use similar
motor/drive technology and be a quick follower to this first DIY CNC
undertaking. Stay tuned for further posts on that as it has special
additional needs.
More later, bye for now... Jim
So many toys, so little time!
