Archive for March, 2008


Introduction to Improper Machining Techniques

Mar 27, 2008 in MIT & Boston, Project Build Reports, Stuff

So I’ve used alot of “improper engineering techniques” while making some parts due to either a lack of proper tooling, actual experience/training, or desire to take a few short cuts (teehee), so I figure I’d document them here. Mostly it concerns lathe work, since the milling machines I have access to are rather well stocked. A running list, which will be updated as I find new and more unsafe ways to make my parts, is here:

1. Counterdrilling. Obsolete since we got real center drills, but before that time, I managed to start holes with a countersink. The multi-flute ones have a symmetrical pointed tip, often with remnants of the ground flutes running to the point, so it was easy to dimple the part with the countersink in the tailstock. Then I used a small drill bit (usually 1/8″ or less) and piloted the hole. Then I enlarged it to whatever size it needed to be.

2. Not-Boring-Bar. We still don’t have a proper boring bar setup, so I decided to grind one from a stock carbide turning tool. It had its issues, but once proper clearances and draft angles were ground, it actually worked great. Due to the nonadjustable tool angle, some times I have to fudge with how the toolpost is set up to have it actually cut (or cut well and not just make a loud screech), but that would happen with a real boring bar also.

3. Steady rest centering. Since I never took an actual machine tool class, I’m not sure how the professionals get their workpieces all centered and true (past using a dial indicator and selective mallet bashing), but I found a bit of solace in the steady rest. Any time a piece sticks out more than half an inch or so, I can squeeze the thing in there and center the piece. First, the chuck is tightened lightly, enough to not loosen when the spindle is turned on. Then the spindle is fired up and the little jaws of the steady rest are cranked down until they just contact the material. This almost always makes the workpiece run (decently) true, but I have to be careful not to push too hard with one, or else it will cock off to one side. Then the chuck is cranked all the way down, the spindle run again (to see if the aforementioned fault did occur), and if it’s good, the rest is removed and machining begins.

4. Amputee’s Cutoff Tool. The lathe doesn’t have a cutoff tool setup either, so the proper technique is to make half the part, hacksaw it off, then flip it over and make the other half.

Yes, that often means maniacally wielding a hacksaw with your arm hanging inches away from a giant spinning round thing with protrusions and a bunch of pointy steel bits. I’ve been told that other people cut further away from the chuck to avoid dismemberment, but I don’t have that much material to spare. Moving the saw back and forth does speed the cutting, but often I can hold it in one place and keep it there until it cuts through. Moving the carriage with the tool bit mounted close to the line of cut helps keep the saw steady.

Yes, the spindle is on. No, I probably won’t be allowed near any machine tool in the student shops again if the instructors read this post. No, MITERS does not have a portable trauma kit.

5. Differential gear oil mixed with automatic transmission fluid, knife honing oil, and some WD-40 actually make a pretty neat cutting lubricant mixture. Gives beautiful finishes with a slow power feed (that I subsequently smudge when I take the piece out). Just plain WD-40 also works, and has the added upside of making huge smoke clouds on large diameter parts which I use to frighten new MITERS members.

6. Face-drilling. No, this does not involve using my face to drive the drill bit. Due to (again) the lack of a cutoff tool, I find it easier and safer when working with small diameter rounds to take a giant drill bit and just drill down the round stock to the dimension needed. A slight waste of stock, but if I size the stock right, wouldn’t matter anyway. The picture linked was me drilling down a 1/2″ bolt to make the Extend-O-Shaft 2000 for TB4.5.

7. Ghettoedging. One of the only techniques I had to use on the milling machine before I 1) found the edge finder at the Media Lab and 2) got edge finders at MITERS. The technique is simple. Color the edge to be located with a Sharpie marker. Then load up your favorite endmill. With the spindle running backwards, slowly feed the material in, and stop as soon as the first bit of sharpie disappears. Raise the endmill by the quill/column/whatever suitable Z-axis part, account for the tool radius, and start on the next edge. Don’t forget to run the spindle right way when you’re done.

It worked great on the ML’s junky import mill-drill, since with every tool contact there would be loud vibrations through the whole machine (and the… filing cabinet… it’s sitting on). With the giant Bridgeport at MITERS, it was alot harder, since all the cast iron and concrete floor would absorb the tool vibrations.

8. Ghettocentering. Combined with the Steady rest, it’s all I need to make true parts on the lathe (okay, true within reason). Drill bits have shanks which are usually the same diameter as the rest of the tool. Shanks are solid, round, and very close to the chuck, which is mounted in a stiff tailstock. If I need to remove the part for any reason, I make sure to drill a center hole first (using as large a bit as the end part would allow). Then, when the part is replaced, I can slide it on the drill bit shank first, then clamp it down in the chuck. It keeps the part centered axially as well as radially (within reason). I can then follow up with the steady rest. Or, on occasion, I can manage both at once.

There’s probably more that I can’t think of at the moment, but rest assured that this post will pop back up on top if I break some new ground (or some parts, or me.)

Snuffles Reloaded: Update n(n-1)!/k^n

Mar 27, 2008 in Project Build Reports, Project RazEr

I forgot which update I’m on again, so pick a value of n and k of your liking which evaluates to the previous update + 1.

Finally got a chance to actually work on the wheelmotor… the days so far have been absorbed by the abrasive waterjet cutter at the Media Lab, among other things. I should be able to get some more work done soon.

Remember the aluminum biscuits I got a while back? Shouldn’t have gotten them supersize. The side plates are .393″ thick at the maximum and I got .75″ slices of aluminum. This meant I had to turn down almost half the length of each disc, which was sketchy enough considering the lathe’s limited spindle speed and feed options.  Worse, I still haven’t figured out how to activate the power feed for the cross slide, so it was hand cranking all the way…

I dug up the reversed jaws for the chuck in order to hold the 4 inch piece. This first facing cut was just slamming one bandsaw-finish side onto the chuck, since I need at least one flat, round surface to start.

The piece was then flipped over and spaced with milling parallels shoved between the chuck and the flat face. This gave me some reasonable accuracy and trueness despite the different setups (and the very frighteningly loose chuck jaws).

And violin, three trimmed biscuits. They’re both .393″ thick with a nonplanarity between the faces of at most .002 across the diameter, which is pretty good I think for ghettocentering. One piece was faced too far because of some strange force of machine nature which causes the tool to not cut until I crank the handle past my original target dimension, at which point it removes all material up to the new dimension at once. Oops.  The short piece is .385″, and has been designated as the practice piece.

Two pictures don’t quite convey the passage of time, but these three discs took two hours.

Because you can’t machine an outer diameter while holding onto the same outer diameter, I had to make my first metapart, a part which helps make parts. Dale clued me in on using a mandrel, or custom spindle mount, for machining the other features of each side plate.

This doohickey is made from 1″ aluminum round and has a .875″ section which the discs mount on. A large custom washer (not shown) fits the bore of the discs from the other side. The whole thing is retained by a 3/8-24 bolt. Like a giant inverse prop adaptor, almost.  Each disc has a center bore of .875 (or close enough to it) to mount snugly on the mandrel. The were, of course, made using the not-boring-bar.

Test cut! I threw the short disc onto the mandrel and tested everything. Seems to work great, and the runout is minimal once the mandrel itself is secured and centered. I called it a night there, as it was somehow 4AM.

Now that the raw outer-dimension parts for the side plates have been made, I can take another evening and actually make the final parts. More pics to come! Everything is actually round this time! Holy Robot Jesus!

I can has shenanigans?

Mar 26, 2008 in MIT & Boston, Project Build Reports, Project RazEr

So I fell back into a phase of “Oops, didn’t bring the camera” syndrome, but here’s some of the goings-on for the past two days. I’m pretty sure that my left arm is going to fall off on its own and beat me to death just by itself soon. There’s been alot more threading and tapping. It builds character….and muscle, since it allows me to try to equalize the arms a bit. Being right handed, my right arm has historically been a bit stronger than my left.

Anyways, onto pictures.

More progress on the steering arm, now with 99% more gears! They are all some ungodly large pitch (10? 8? Module 3?) and 3/8″ in thickness, and all waterjet-cut.

I have watched the pile of 50-pound bags of abrasive sitting outside the waterjet room slowly get smaller over the past week. I wonder how much the machine weighs with a full tank of water and sand?

A closeup of the geartrain. Yes, I know, two of the gears don’t touch and two more have very little contact. This was a design error that was corrected by recutting those gears.

The total ratio is around 40:1.

Random closeup through the Lexan mount. I suggested running the gear teeth in with polishing compound to smooth out the sandblast-like finish of the waterjet, which probably pitched the efficiency out the window. They haven’t taken me up on it….yet.

Switching gears a little bit, I did some more cutting and subsequent assembly work on this parking stand for the scooter. It does two things – allow the front wheel to lock against something for leverage while folding, and also allows the vehicle to rest vertically, leaning on the Wolverine-claw-like things (Which will actually have a plate mounted on top). It’s missing a few parts, but that will be addressed soon.
The big difference between this piece and the steering arm is that this has right angles and hence was easier to slam on the drill press and drill quickly and accurately.

Hmm, so all this fabbing has distracted me a bit from wheelmotor work. My magnets and bearings both arrived today.

Interesting thing about these bearings is that they’re tiny. 15mm bore, same as the previous, but only 24mm diameter and 5mm width. I’m slightly concerned about loading and bearing life. Whatever, I suppose. I should be able to get some work on the side plates done tomorow (today? what week is it!?.

28 magnets test loaded and…. HEY! What’s that giant gap?! It’s supposed to be a perfect fit!

Oh well. Nothing some index cards can’t solve. Did you know that a full circle of magnets like this can actually be detrimental to efficiency becuase of increased hysteresis losses in the core?

Stay tuned for the next episode. Meanwhile, bot on.

More shenanigans… and a Snuffles Reloaded update.

Mar 23, 2008 in MIT & Boston, Project Build Reports, Project RazEr, Stuff

Starting from now, I’m going to post mini-pics in 512 pixel-wide preview mode and then the linked pictures will be at 75% camera resolution. I realized that thumbnails suck, and a 512px preview didn’t do much for a 1024 pixel picture.

Anyways, the box of aluminum puzzle pieces turned into…

One of the steering arms for the 4-wheeler. The wheelmotor pod will sit on the end, and will be able to swing about 200 degrees or so. The steering motor mounts in the circular cutout, and there will be a reduction geartrain running through the arm itself (the Lexan plate is a placeholder, but may actually be used). The whole thing mounts to the chassis through the flat plates on the left.

Mating holes drilled and tapped. I took this one, and both regret it and am glad I did it. Some of the holes didn’t quite line up, and there were enough awkward angles that a drill press couldn’t be used. So it was hand drill, clamps, and alot of patience. The screws are temporary and will be replaced with hardcøre cap screws later on.

So now that I’m twice as buff because of all the hand-tapping, I should do the rest… hey, it’s Spring Break, after all. Gotta look good for the ladies.

Here’s a closeup of the steering gear. This is the waterjet on a good day (and on slow, ultra-fine finish mode). Even without a tilting head, it does great on 1/2″ aluminum.

Update: In fact, I like it SO much that I made part of it into another rotating site banner image. Hit refresh enough and you might get to see it.

And now for an intermission. After the great threading ordeal was over, I decided to head over to MITERS with my 4″ steel pipe and try to turn it into a 3.25″ steel pipe. I was a bit iffy about shoving such a large piece of stock onto the lathe, and tried a few ways. It ended up turning (teehee) out rather well.

First pass. Lots of chatter and earsplitting noise, due to two factors. One, I’m using the wrong bit for the job and the tip is far too angled. This problem was overcome by rotating the bit such that the left face was flush against the end of the pipe, which closer approximates how you’re supposed to do it. This solved the chatter issues.

However, the lathe has only 3 speeds: slow, medium, and fast. And slow is not slow enough to turn a 4″ steel pipe (surface speed-wise), so I was actually cutting almost twice as fast as I was supposed to. But it’s a carbide bit, which is supposed to allow a ~100% increase in machining speed…. so maybe it balances out.

Almost there. There’s still a bit of chatter, but most of it was overcome by a slow power feed, light cuts, and plenty of cutting fluid automatic transmission fluid mixed with differential gear oil. It took around 20 passes in all to reduce the diameter from 4″ to 3.245″. I probably could have done it in less, but wanted to tempt neither fate nor the robot gods.

During this time, I generated so much smoke from the cutting that when I finally turned around to look, the room was hazy. Naturally, not wanting to set off any alarms, I opened all the doors and windows as quickly as possible, and waited for the cloud to dissipate.

So, now that 10 years have been taken off my lifespan, the One Ring is done. Dimensionally, everything is within .003, which is freakin’ good enough. The ID, however, is a bit too large by about a quarter millimetre 0.01″. Which is actually beneficial, since that 0.2mm air gap was a really, really tight goal to hold anyway. I’d rather have some more space.

Now, onto the artifacts. For some reason, I find lathe-curl dynamics rather amusing, and have been in a contest with myself to generate the longest continuous lathe curl. My record with a 3″ aluminum round has so far been 25 feet, enough to stretch across the MITERS room and back a bit. The steel didn’t want to draw out as much, but was still cool to look at.

These were from boring out the ID of the pipe with my not-boring-bar. They would clump together inside the pipe, bob around, then when I pulled the tool back out, would roll out and fall onto the ground. They were nicknamed lathe hairballs by the other MITERers.

Here’s one of the ginorm-o-curls I’ve been looking for. Somehow, they find an island of stability in the clump that gathers by the bit, and then extend straight out and form very wide curves, which then proceed to get tangled in everything. I had to stop this cut because I couldn’t reach the levers any more. Without pushing aside razor-sharp, oil-covered smoking steel slivers, anyway, which doesn’t turn me on at all.

And now I present… the Mass.

This is the conglomeration of all the curls from all the OD-reducing passes from the night. Each pass resulted in a big clump of curls, so I just kept piling them on…. and on…. and on some more. Average 12oz WD-40 can for scale.

It has an enthralling iridescence that the camera flash drowns out. There’s all shades of colors, from purple to blue to blue-silver to plain silver and dull gray, but blue is predominant. Part of the reason, I think, is the sudden cooling of extremely hot steel slivers by the oil I dunked the whole thing in. I effectively heat-treated the curls. A process like this is used industrially (and more carefully…) to make hardened steels.

The only fitting fate for such a clump is, of course, as decoration. It now sits on the tool shelf, monitoring all in the room.

So, tomorrow (today!) is Sunday. Will I rest on Easter Sunday? Nah, not even that’s enough to get me to stop building things. The ML guys will probably be off, but I’ll be working on the side plates, hopefully. Magnets are due to arrive by Monday or Tuesday along with my bearings.

Something better move by the end of the week, or I’m going to be pissed

Media Lab Shenanigans continued

Mar 22, 2008 in MIT & Boston, Stuff

Even though neither the omnidirectional 4WD vehicle nor the Roboscooter are my own projects, per se, I still feel like taking a bunch of pictures and writing about their development as Media Lab Sponsors Week approaches, especially since I’m pretty heavily involved in the R&D.

And of course I feel like showing off all the shiny toys. Here’s some stuff from today.

40AH large-scale rectangular LiFePO4 cells. Yes, this is one cell. It’s roughly the size of a construction brick, but a little flatter. 3.2 volts per cell. These are from Thunder Sky, which is a big enough name in the R/C hobby world to make me consider them legit.

The 4-wheeler will use one 48v(15S) pack for drive and one 24V (7S? 8S? 7.5S?!) pack for steering and the rest of the electrical system.

Packs under construction. The shipment of cells came with a bucket of formed copper battery bars that mounted to the terminals with screws. Handy, since nobody had to wield a kilowatt soldering iron.

The chassis hasn’t changed much from last week, but that will not be the case soon.

Some waterjet-cut aluminum plate that will be eventually assembled into the steering arms and motor mounts. I love this method of construction and will probably be using it more in my own stuff – that is, 2D plates cut with slots and keys that assemble puzzle-like into a 3D shape, which is then retained by screws. With a 5-axis nozzle, I’m sure we could make even weirder shapes.

Here’s a shot of the (mostly) functional scooter in folded form. No, that’s not me fiddling with it.

When folded, the scooter is strangely aesthetic in a way, like someone in various yoga positions. It has a strangely organic yet still very mechanical look. But now I’m getting too artsy. The parking stand is only a mockup at the moment, as the above picture shows. A hardcøre metal stand will be made for the presentations.

There’s been both mechanical and electrical work on the thing done, since some parts weren’t made to spec or weren’t there at all (Hey, how much moving does it need to do at a show? None!). I’ve been working on a closed-loop controller for the folding process as well as doing some fabrication work.

Like this. This is the main seat folding control link. The 4-bar linkage which folds the seat in sync with the chassis was made with some links too long (I think it was a revision control issue). Fortunately, they were too long, becuase they could be shortened. The process was to slit each link, mill a bit off each end, then recombine them with a “connector block”. The links are C-channel in profile, so making this block was simple.

I also discovered that having the depth stop on the drill press work with your countersink is a wonderful thing. Those are the best countersunk holes I’ve made to date.

So, what on earth am I working on anyways? Well, sort of both projects at once. I’ve been rather general purpose, building whatever needs building, but focusing somewhat on the electronics. This is great, since I get the best of all the worlds involved, but get to work on a field which I don’t have as much experience in.

The whole of spring break will probably be filled with build pics, so stay tuned!