There are several different
ways to move the CNC axis. Most commercial machines use a leadscrew
or rack and pinion drives. Also used are chain and wire drives,
but these are not as accurate. More recently linear drives have
started to appear. The linear drive combines a servo motor and drive
system in one, its like a motor that been laid out flat, with the
electrical stator running the full length of the CNC axis, and the
armature attached to the axis to move.
All-thread, or threaded rod, is the cheapest way to build a drive
system for your cnc router. My first build used 12mm all thread,
which comes in lengths of 2 meters ( 6 feet ) and has a pitch (
distanced traveled per revolution ) of 1.5mm. The all-thread is
secured in bearings at each end of my axis, driven at one end by
a stepper motor, and a nut running along the threads length is attached
to the axis I want to more. Dont be tempted to go too big in thread
diameter, remember the drive thread acts like a flywheel, so any
change in direction or speed means more work for the stepper motors
if you choose a larger thread.
Backlash is the sideways movement in the nut along the thread, the
"slop", and its a problem. You want to remove as much
backlash as possible, without increasing friction too much. There
are a few ways to do this, one way is to cut the nut half way through,
and then squash it down onto the thread to take up the backlash,
another way I used is two nuts on the same thread, one nut adjusted
slightly away from the other untill the backlash is reduced without
creating too much friction.
I later changed the all-thread for Acme thread. Acme thread has
a much higher pitch, 3.5mm in my case, so this means the axis will
travel 3.5mm per revolution. When I was running the 1.5mm pitch
all-thread, my stepper motors were running at their maximum speed,
so by going to the 3.5mm pitch Acme thread I could expect to at
least double the axis speed. I bought my acme thread from United Fasteners in 6 foot lengths
for about $50 per length, cheap enough. The problem was the nuts
that came with the thread were rubbish! About 1mm of backlash and
poor finish on the thread, so I had to make my own nuts.
has some scrap HDPE ( you can use those plastic kitchen cutting
boards, same sort of stuff. ), 12mm thick. When heated HDPE turns
into a clear slimey liquid, can be shaped, and then cooled to retain
the shape. I cut some blocks as shown, the center hole is slightly
smaller than the Acme threads inside diameter. This block was then
wrapped around a scrap length of the Acme thread. Using a gas blowtorch,
I heated the Acme thread about 2 inches away from my HDPE block.
As the thread heated up, the HDPE started melting into the thread.
Using a big pair of multigrips, I squashed the block down untill
I felt the HDPE had fully melted into the thread, then been carefull
to hold everything nice and square, I dunked the whole thing into
a bucket of cold water until the Acme thread had cooled. Its then
just a case of un-winding the newly formed nut, seperating the two
halves and cleaning up the edges with a file and sharp knife. The
first few didn't work too well, it takes a bit of practice to get
it right. The result is a HDPE nut with no backlash ( the top self
tapping screws adjust the backlash ).
I use chain and sprockets for the connection between stepper motor
and drive thread, but I could have also used toothed belts or direct
drive, so why chain?
has a few advantages for the hobby CNC machine. First up, its cheap,
the chain I use has a 1/4 inch pitch, and comes in length of 10
feet for about $30. The sprockets to match are about $5 for the
smallest 11 tooth, to $15 for a 22 tooth from memory.
2nd advantage is its very easy to change drive ratio. I can swap
sprockets to play around with the axis speed and torque, and its
easy to shorten or lengthen the chain with connecting links.
The last big advantage for me is chain has a tollerance of incorrectly
lined up sprockets. My stepper motor may not be in perfect alignment
with the drive thread, but the chain drive will still work OK.
Dissadvantages? Chains need a drop of oil occasionally, they need
to be kept clean, and they can be noisy. Idealy, now that I have
set up the machine with the optimum drive ratio's, I should replace
the chain and sprockets with a toothed beld setup.
Direct drive coupling is best, if you can run a 1:1 drive ratio.
The coupling needs to allow for missalignment between the stepper
motor and drive shaft, and this can get a little tricky, but once
set up a direct drive will be reliable and quiet.
To add in the near future...
CAD drawings of the lead screw end layout, bearings and pre-load