Router lathe: Big bolt

big bolt and nut

The first finished part to come off the router lathe. I figured when building a machine to cut big spirals and threads, what better test than to make a great big bolt and a nut to fit on it.

The entire bolt is about 17" long and 3.5" in diameter. The thread pitch is 2.5 threads per inch. The head and the nut are about 6.5" in diameter and the nut is 2.75" thick. 

The threads work

The original idea that started this project was to adapt a wood lathe to cut great big wood threads, both internal and external. And here they are. In that regard, I guess I'm finished. However, this project has presented a bunch of new exciting possibilities to explore.

With wrench and screwdriver for scale.

(side note: the wood looks like it has been stained, but it's not. This is its color with neutral Watco oil. The wood was some salvaged asian oak of some kind from shipping containers at the port, and came out much darker than I expected.)

Here's the code: Final Arduino Code

< Internal threads  -- New lead screw >

Router Lathe: Internal threads

Internal bore cutting spindle

I always knew that the big challenge would be cutting internal threads. What cutter? How much extension? Speed? How to manage chatter and deflection? The criteria was that I wanted to cut threads a half inch deep, at least 3 inches into a bore. For this I needed a 60 degree side cutting bit that I could extend 3 inches past the end of the router. Good luck finding that router bit…

My solution was to create an offset spindle with a half-inch shaft that holds a fly cutter. The spindle is made up of a couple of bearings and a .515" ground stainless steel shaft, turned down to a half inch on the ends to fit the bearings. The spindle frame and router clamp is made of birch ply. The spindle is hinged so it can be swung free to check the fit of the bore. The spindle is driven by a vacuum cleaner belt off the router with a 4:1 speed reduction. I bought a cheap Harbor Fright trim router to run it, just in case this is hard on router bearings. (better to trash a $30 tool than a $130 tool). I also got one of their router speed controls to slow it down. (I seem to be buying a lot of cra stuff from those guys lately)

speed control

spindle

.

 In order to keep the rotating weight down, I used 1/4" drill rod for the fly cutter, inserted at an angle through the mandrel, and ground so that each end only cuts one side of the thread groove.

cutter layout

 Grinding the tool bit on my other wood lathe. It was actually much easier than expected to get the angles and correct extension on the cutter. The only problem was the drill rod I was using turned out to be stainless, not heat-treatable tool steel. It holds an edge - kinda. 

Grinding the cutter.

To grind the cutter, I slowly spun the lathe spindle by hand while cranking the cross-feed vise back and forth. The cross-feed vise is set at 30 degrees to the cutter. It took a different setup for each end of the cutter. This fixture only ground the angle. All back relief and rake on the cutter were ground by hand & eyeball.

You can hear the harmonic vibration from the stretchy belt, right before the cutter exits the end of the bore. This is a problem I'll have to confront before I can increase depth of cut and cutting speed.

Here's a better video made after the outside of the nut was formed.

Lessons learned:

1. Needs mass. There's a fair amount of vibration that would probably be dampened by more mass in the router carriage and spindle. I was planning to rebuild anyway after this initial test, I'll just beef things up a bit more.
2. Stretchy belt -- not so good. I'm getting a significant harmonic vibration, especially when cutting across end-grain that I think comes from belt stretch. (you can hear it in the video, right before the cutter exits the end of the bore) I can dampen it somewhat by putting my finger on the belt. I'll look into a better drive belt to replace the $2.99 Hoover belt I used. More mass in the router carriage may help this as well.
3. Cutter needs rake. I just ground a flat cutting face on these cutters, so there's zero rake on the cutting edge. I think if they had a little rake ground in they would cut better -- slice the wood rather than scrape.

< Full size test  --  Big bolt >

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:

< First cut -- Internal threads >

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.


materials:
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.

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.

< Electronics -- Full size test >

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 parts...in 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.

< Motor mounts and drive -- First cut >

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)

< Prototype  -- Electronics >