Monday, May 21, 2012

Router lathe: Full size test

Newly acquired shop space in the basement

After spending most of the weekend building a bench for the lathe, a table for the computer and a stand for the air filter,  I moved the lathe to its new home. Then I was finally ready to make a big cut as a road test, to test accuracy and speed. I found a 30" long oak 4x4 to use as a test. The lathe can actually handle up to 11" diameter, and about 36" in length, but I figured this was big enough to expose most major problems or weaknesses.

rounding the stock

Everything worked pretty much as expected -- at least the arduino/stepper motor part. No dropped steps -- the cutter followed the exact same path each cut. In my previous test, the pitch was off, but I'm pretty sure it was a loose coupler on the spindle.  I did find out that the tailstock is not aligned so the lathe cuts a pretty significant taper, and after 3+ hours of continual operation, my shop vac sounds like squirrels hid nuts in the motor. Once I deal with those issues, I'll start testing how fast and deep it can cut in a single pass.

3.5" x 2.5 threads per inch

Here's the arduino code:

Saturday, May 12, 2012

Dust control: Shop air filter

Here's a simple shop dust filter* -- a copy of the one that's been working great in my wood shop for several years. It's basically a 20 x 20 inch box with a blower inside, pulling air through a couple of 20" furnace filters. I cut "L" shaped strips to create slots for the filters (aluminum angle would work great too). The fit on the filters is fairly loose, but the suction from the fan pulls them in for a tight seal.

The whole thing cost about $70-$75 and took an afternoon to build.

The blower is from a Jen-aire range hood that I picked up at a local rebuilding center for $15.  It's a little big for the 20 x 20 box, so I mounted it diagonally. The previous one I built used the motor and blower from a window-mount air conditioner salvaged from an appliance recycling depot -- for free.

Ugly but effective. Here's the first filter I built, that's mounted in the rafters of my garage. The slots on this one are big enough for 2 filters stacked, and I put a cheap fiberglass filter on the outside to catch the big chunks. I'm not sure how much good that does, so I left it off the new design. The exhaust port is the angled aluminum flashing on the left. I typically leave this running the whole time I'm working in the garage, and unless I'm doing a bunch of sanding, very little dust settles out on surfaces.

1 --- 1/2" AC fir plywood  -- $29
1 ---2x4 kiln dried fir -- $3
2 ---20x20 furnace filters -- $20 ea. (you can get much cheaper ones, but I wanted as close to HEPA as I could find.)
Misc --  Switch, electrical box, wire, wire nuts, glue, screws, duct tape, aluminum flashing (to extend exhaust port)

Please note: this design is only as good as the filters you choose. Most furnace filters only filter out large particles. I used 3M's Filtrite filters with their highest proprietary MPR rating of 1900. They claim to filter out 90% of particles down to .3 micron. From what I can tell, they achieve this by relying on the physical filter to trap particles in the 10+ micron range, and an electrostatic charge to get the smaller ones. I'm skeptical of the longevity of the electrostatic charge, so I'm only going to assume this filter will trap the larger dust particles and also use dust collection at the router and ventilation to control dust.

*years ago, I taped a furnace filter to a box fan for a dust filter. It worked, but not well. A radial fan doesn't do well with restricted air flow. The the airflow over the blades stalls and pressure drops off significantly. A centrifugal (squirrel-cage) blower operates much better under conditions with resistance to air flow.

Friday, May 11, 2012

Router lathe: first threads

First threads. They came out looking great, but the pitch is a little off. I found a loose coupler on the spindle motor that I hope is the reason. Otherwise the stepper motor is losing steps or my math is wrong.

Threads look chipped, but that's just the wood grain showing,
the cut itself is very clean.

Wednesday, May 9, 2012

Router lathe: electronics

For electronics, I went with 280 oz, NEMA 23 8 wire hybrid motors, and drivers based on the Sanken SLA7078MPR chip. The power supply puts out 40V and the drivers are configurable up to 3A current. The drivers have step and direction inputs that run great off the Arduino -- just toggle a pin HIGH and immediately back to LOW, followed by a few hundred microseconds delay to control speed. The driver boards also have dip switches to select among full, half, quarter, eighth and sixteenth steps. Mine came set to eighth-step which gives a nice balance between power and smooth motion. Given the low gearing of the lead screw and the 4:1 reduction on the spindle, I'm still experimenting with the best step setting to use on each axis. I may go with quarter or even half step.

There's a 12v/5v switching power supply from a defunct external hard drive zip-tied behind the big transformer, and it breaks out to power the Arduino, the cooling fan and the logic circuit on the stepper drivers. That leaves the big power supply to run only the motors.

12v goes to the Arduino and fan, 5v to the driver boards. I put a diode on the 12v to the Arduino -- when USB was plugged in, but main power off, the computer's USB port was spinning the fan. The diode fixed that and drops the Arduino voltage a bit as well.

I've been pretty happy with the hardware so far, and the initial support was good, however the purchase experience was not fun -- sadly, I just can't recommend the company I bought them from.

More recycled this temporary build, the aluminum the electronics are mounted to is a shelf from the telecom rack that supplied the base and rails for the lathe.

Sunday, May 6, 2012

Router lathe: motor mounts and drive mechanism

Here's the feed motor mount and feed screw. The stepper motors are the yellow blocks. The screw is stock 1/2"-13 threaded rod. I picked up a flex coupler at a local surplus store, and put adapters on each end to convert from 1/4" at the motor to 1/2" at the feed screw. the black bearing material the rod runs through is UHMW. It was tough finding a piece of rod that was straight enough. Measuring over 42", the pitch is at least a half thread off (I didn't count, so it could be more than that). My intention is to replace it with 2-start 1/2" acme threaded rod, once the mechanics are all figured out.
 Tailstock end of the feed screw using same UHMW bearing.

Spindle drive. My auto mechanic gave me a selection of discarded timing belts. This one's from a Honda, I think. They are huge, but work great. I figured out what the gear circle would be for a 60t gear and drilled holes in a piece of particle board using a rotary table. Then cut & sanded the corners off till it fit. The particle board was just a test, but it works so well, I'll keep using it for now. The black adapter attaching the gear is acetyl turned to fit the spindle.

Spindle motor mount. The small gear is UHMW. This was cut the same as the particle board gear -- close guess on diameter, drilled holes and cut the diameter -- x-acto knife to round the corners. Not terribly pretty, but it runs well with very little backlash or slop. I plan to replace all these "Land of the Giants" parts with much smaller .2" pitch XL belts and pulleys, but this gave me a way to test whether a 4:1 ratio was good without buying a bunch of parts. There's a 5/16" shaft through the small gear and skateboard bearings pressed into the pulley housing, and one end is turned down to 1/4" so a simple coupler could be used to connect it to the motor shaft. This allows the belt to be tight without putting lateral stress on the motor.

(the 5/16" coupler was loose, causing inaccuracies in the first part cut. I thought there was a problem with electronics or code, but so far, all my major mistakes have been mechanical)

Router-lathe: Prototype

First off, I had to see if it would even work. I built a prototype out of acrylic with a rail from an old printer, a couple of little steppers and an Adafruit motor shield. Using a couple of nested loops, I was able to draw all kinds of spiral and geometric patterns on a pvc tube. Here's the code:

I learned a lot. First of which was that none of it could be used on the big machine. The Adafruit motor shield is a great little device and a wonderful learning tool, but it will only handle little motors (600mA). It also has its own Arduino libraries with step, speed and direction commands, that don't translate to bigger drivers that have step and direction inputs. That being said, for small projects, it's a very cool device -- if you're working with motors with Arduino, absolutely get one.

The next thing I learned was what odd devices stepper motors are, and how inconsistent the available data seems until you understand how they work. I'd salvaged steppers out of several old printers, but was never able to make them work. They would miss steps, rattle back and forth, get hot and smell bad. And I was mystified why, when if lucky enough to find a motor with an actual datasheet, it would say something like "2.6 v, 3.4Ohms", but the power supply in the printer it came out of  would be putting out 36v, 42v or some other crazy voltage. Huh? (anyone that actually knows something about this may be rolling your eyes right now, but this was all new to me.) I wish I'd found this simple explanation -- it would have saved me lots of time: Stepper motor specs typically give resistance and current values. Voltage, if given, is to calculate current. As long as the current is kept at or below that value, the voltage can increase. A lot. (eye-rollers: please correct me if I got that all wrong, I'm still working on it...) In fact it's assumed that steppers will run at many times the spec'ed voltage. For instance, the steppers I ended up using are rated at 3A, .92Ohms. That comes out to 2.76v. Seems low, right? It is, my power supply puts out 40v, but the driver restricts current to the rated 3A.

The Adafruit motor shield doesn't have current limiting, so you need to use motors that pull less than 600mA. The little NEMA 17 motors at Sparkfun and Adafruit are great for this.

Router Lathe: Rails and carriage

Here's the Sketchup model I used to work out the mechanics. The lathe bed is the angled piece with the t-slot on the right. The two rails on the left are 1.5" x 3" aluminum rails from a discarded telecom equipment rack. The big angle pieces on the bottom are the base of the same rack.

The assembly in the upper left is the feed carriage. The whole unit runs on skateboard bearings by a 1/2"-13 feed screw (not shown in the model, but visible below). On top of that is a z-axis-like router carriage that slides on 1/2" rails, driven by a 1/4"-20 screw to move the router in and out. Stepper motors will rotate the lathe spindle (not shown), and the 1/2"-13 feed screws. The router's depth of cut will be manual, for now at least.

After I got it all assembled, I discovered the angle I used for the base wasn't square, so I have to shim everything that attaches. Re-using the aluminum network rack seemed like such a good idea, but I've made a series of compromises in strength, wear and accuracy, in order to use it. Once I get everything figured out, I may rebuild the rails using something better suited.

Router lathe: arduino controlled

Delta Homecraft lathe circa 1940's (?)

I found an old lathe at a garage sale and decided to turn it into a router lathe for cutting spirals, flutes and threads. Initially, I considered using a strictly mechanical mechanism to synchronize spindle rotation and lateral travel, but after playing with an Arduino and some stepper motors, I realized that could be a better solution, possibly even converted to a full CNC system sometime in the future. Now that I know it's going to work, (I made the first cut today) I'm sharing some notes on the build. I'll be posting over the next few days to get this blog up to where the project is currently.