fig 1. the machines, an imported asian mini lathe and milling machine. They
arrived covered in red goo and the mill table was disassembled for cleaning.
many of the internal edges were sanded smooth. the goo was removed and
replaced with lithium grease (leadscrews) and oil (ways).
|
after using the mill for a bit it quickly became apparent that it is pretty
hard to cut accurately because not only do you have to do all the math mod
0.0625 but you have to remember which direction the leadscrew is pushing since
there is so much backlash. a scale on each axis solves both these problems.
fig 2. 12" digital scale mounted on x-axis. the mounts included didn't
fit; the bottom part of the mounting bracket has been replaced by a 1"x1/4"
aluminum bar with the end milled flat. the readout is attached to the saddle
with a brass strap bent 90.
|
fig 3. 8" digital scale on y-axis. a 6" would probably have sufficed.
the mounts are reversed. again, 1"x1/16" brass is used to connect the
readout to the saddle. 1+1/4" wide strap would probably work better, though,
since the two holes in the back of the scale are 20mm apart.
|
fig 4. 12" vertical digital scale on z-axis. original scale mounts
attached directly to the side of the existing scale with 1/4"x1" aluminum backplates. it fits perfectly under the torsion spring.
|
instead of bothering to find the matching connectors i just soldered wires to
the contacts. i found that if the scale isn't grounded it malfunctions, so
the shield is connected to the brass strap which is connected to the metal
backplate of the readout.
update: only the 8" scale had the scale electrically isolated. the 12"
horizontal has the scale connected to v+. (?) as such, plugging in both
caused a power supply short and neither worked. fixed by connecting each
of the scales to v+.
fig 5. scale -> rs232 converter with atmel avr. scales are powered by a
resistor bridge off +5v. lm339 comparators convert 1.5v signals to 5v.
74hct573 input/output latches. ttl logic to detect input change and trigger
cpu interrupt. rs-232 output. schematic and source code to follow once i
clean them up such as i am no longer embarrased.
|
decoding three separately clocked synchronous streams is tricky. bit time is
1/80khz = 200 instructions on a 16mhz avr. in the worst case, though, when the
scale outputs happen to all be in phase, you only get 1/3 that, or about 66
instructions. minus a few for saving/restoring registers. also, the interrupt
handler actually runs twice per input bit because of the positive clock edge.
that case is trivial but still costs time. loading / shifting by 1 / storing
an int32 costs like 20 instructions. my solution was to have the interrupt
handlers write the output on a 64 byte buffer with just one bit per byte. that
means it just has to load and store a 1-byte bit count and write 1 byte to the
array. 3ms after the last clock pulse another interrupt with lower priority
converts the byte array to two int32's.
fig 6. dro display on computer. now, to find something besides the computer
to display the numbers. ideally something cheaper or less prone to being
destroyed by swarf.
|
fig 7. modified pulley. i started with a 2" xl timing belt pulley, chucked
it into my lathe, (carefully and poorly, as is the case with flanged pulleys)
and bored it such that it has the same dimensions as the rotating dial, at
least to the radius of said dial, which it will replace. by this point you've
probably realized that the 150 watt halogen desk lamp isn't for illumination
while machining but rather for making machined parts look nice when i take
digital photos of them.
|
even without having added a "real" display to it the dro has been very helpful
in making the parts for the next step: attaching pittman servo motors to each
axis.
fig 8. y-axis servo drive
|
fig 9. x-axis servo drive
|
todo:
- make x and y axis servo drives swarf-proof
- procure ballscrew for z axis
- design and build servo driver electronics
update, 2006-08-06.
i haven't updated this page in a while. the x & y axis servo mounts are still there but i never got around to adding the z-axis servo. the dro has been quite useful, however - despite never finishing the cnc conversion i have used the mill to manufacture several custom robot parts.
|