cutting flutes on a big spindle (5" dia, 22" long)
First "real" parts -- the big bolt was fun, but just a test, and a bit of a joke. This is the first "real" part -- a base for a round table (I think). I turned the spindle by hand on the manual wood lathe, and modified the thread code to cut a series of flutes. It worked pretty well, in spite of a couple of errors in the code:
1. I intended to climb cut (feed with the cutter so the bit pulls with the direction of travel. Makes a cleaner cut across grain). This code runs the cutter against the direction of travel (conventional cut), which is more prone to splitting when cutting across the grain.
2. I rotate the part by one degree between the forward and reverse cut in order to make a slightly wider cut. But I didn't account for that when spinning between flutes, so each cut advances one degree around the spindle. I actually like the way it looks, so I didn't fix it, but it was a (happy) mistake.
bottom of x-axis carriage showing new nut and diy tap
Upgraded lead screw. The screw that originally drove the x-axis was a standard hardware store 1/2" x 13 threads per inch rod. It had the advantage of being cheap, easy to work with and with standard threads, easy to connect to. The disadvantage was that it was inaccurate, not straight, and really slow: it took 13 revolutions to travel one inch. I upgraded to an acme lead screw with 5 threads per inch, 2 starts. This gives me faster travel at lower motor RPMs, where they have more torque. It also speeds up the rapid traverse -- it no longer takes a full minute to return the carriage to zero.
The screw was pretty expensive so I just couldn't bring myself to spend another $40 on a nut, and decided to make my own out of UHMW. It only took about 5-6 hours to come up with one that worked... (how cheap do I work? You do the math)
First, I tried to heat form the nut but wasn't happy with the results, so I decided to make a tap and cut the treads instead. I ground a couple of grooves in a length of the acme rod, tapered the ends, made sure that all the thread starts were sharp. Next, a hole was drilled in the UHMW blank a few thou over the thread minor diameter and it was chucked in the metal lathe. With the home brew tap in a chuck in the tail stock, I started the threads by hand. After a few turns, it bound up and started to spin, so I took it out and finished with a vise and a pair of channel lock pliers. That actually worked well because I was able to flex the nut and squeeze the sides in, making the cut deeper. By the time I had gripped the nut by all sides to turn it, it had cut enough clearance to spin fairly freely. Then, I ran the tap back and forth through the nut with a hand drill a few dozen times until it was running smooth. A little teflon lube and it's running smooth with with very little resistance and almost zero backlash.
A couple of observations:
UHMW has a nasty habit of flexing away from the cutter and springing back, so your cuts and holes tend to be undersize.
This approach worked on UHMW because the stuff is so soft and easy to cut. It might work on acetyl, nylon or acrylic, but I doubt it would work so well. My "tap" was very crude -- I think you'd need to make something much more refined to cut a more rigid material.
I left the burr on the cutting edge, so when I was running it back and forth with the drill, it was shredding off a little bit of material with each cut. It did not leave a clean cut, but it helped to overcome UHMW's tendency to cut undersize -- each time through, it removed a little more material.
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.)
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:
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.
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.
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.
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.
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.
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.
