Lesson in humility: failed headset crown race press

Part of doing-it-yourself is failing to do it yourself. It's not a bad thing unless you refuse to learn from it. Up until now, I had made it a point to only post my successfuly hacks/tweaks/mods/etc to this blog. However, today's post is abouta well-planned diy tool that didn't work. Let's try to learn something from it.

While doing my annual bike tune-up, I realized that part of my headset was installed incorrectly. I designed and built this tool in an attempt to fix that.

First, a bit of terminology.

A press is simply a tool that works by pressing things together, in contrast to a jack which pushes things apart. There are lots of kinds of presses: simple ones use a screw, complicated ones use pneumatic or hydraulic pressure.

A bearing is a mechanical component that acts to reduce the friction between two parts which move relative to one another. A ball bearing is a bearing which uses balls to accomplish this (though many other types exist: the brass bushings on cantilever brakes are an example of non-ball bearings).

And now some bike terminology.

The fork is the fork-shaped piece of a bike which straddles the front wheel (n.b. the things on the back are called the stays, and are NOT a fork). The steer tube is the tube which extends up from the fork to the stem, which connects to the handlebars. The fork crown connects the steer tube to the two tines of the fork.

The headset is a pair of bearings which connect the fork to the rest of the frame, allowing you to steer without much friction. Those two bearings in the headset are cup-and-cone style bearings. These types of bearings are popular in older bikes, but are being gradually replaced with cartridge bearings. This is unfortunate, since cup-and-cone bearings can be adjusted for a prolonged life, while cartridge bearings must eventually be thrown out and replaced. Nonetheless, because the main force on headset bearings are thrust forces (parallel to the axis of rotation), and because of the size of the bearings, cup-and-cone style bearings remain the standard for headsets.

A cup-and-cone bearing consists of three pieces: two races, and a set of balls. Sometimes, those balls are held in a ring formation by an unnecessary, though helpful piece of metal or plastic called the retainer. One of the two races is concave (the cup), and the other is convex (the cone). The fork crown race is the cone which is seated on the steer tube just above the crown, on what is called the crown race seat.

The crown race is press-fit onto the crown race seat, which is to say that the crown race seat is slightly larger in diameter (1.185") than the internal diameter of the crown race (1.180"). This minor difference in diameter (only 0.005 inches) is enough to make it hard to install, though once installed, it may as well be a piece of the steer tube.

So, as I was stripping my bike down for its annual paint job, I noticed that the crown race was incorrectly seated, and I decided to remedy it. It only took a few seconds for me to decide that I wouldn't be able to press it on by hand, and so I went forth building a tool.

I tried two variations on the same design. The first attempt failed: as I tightened the drive bolt, it worked well until the steel bar bent, and then the plastic snapped, and ultem shrapnel ricocheted off of my luckily-closed eyelids. Although a failure, it worked for a while; the race was halfway pressed onto the race seat. I was encouraged to try again, using a heavier construction that hopefully wouldn't snap, crackle and pop.

I have annotated the picture on the left.




I used my lathe to turn a piece a ultem plastic (a metal replacement, similar to delrin) into a race guide. The race guide had a 1.125" hole bored through the center of it, so it could slide along the length of the steer tube, and had a larger recess bored at one end of that hole to snugly fit the crown race.




I then drilled and tapped two holes on either end of the race guide, and used bolts to fasten the race guide to a bar of steel placed between the two tines of the fork. A third bolt--the drive bolt--past through the center of that steel bar against an ultem plunger, which pressed against the other side of the fork.

Again, I have annotated the picture on the left.






I wrapped some fabric around the new paint on the fork, and began torquing the drive bolt. Just like the first time, it began working. In fact, this second attempt put the race close to the right position.

But alas, it too failed.




The failure was very similar to the first time. First, the steel bars bent. And again, since it seemed so close to being complete, I chanced tightening even further. Unlike my first attempt, however, the plastic never broke. Instead, the press tore the head off of one of the hanger bolts.

Disheartened, I decided I would need to try a different technique. But what? I couldn't easily make this design larger, since I was already working with the largest plastic stock I had available. I read and re-read Sheldon Brown's advice, until it came to me.

Duh! I have a lathe.

I chucked the crown race, and took a few (i.e. added) a few thousandths to the race's internal diameter. The race still had to be pressed onto its seat, but I could do that under hand power using the race guide I had already built.

So, as I mentioned earlier, there is a lesson hiding in here somewhere. I'll try to be thorough it down, but please let me know if I elide anything:

Lesson 1: Keep it simple, stupid (KISS). This is the prime directive of engineering. By elaborating the design, I added too many points of potential failure.

Lesson 2: Determine which problem constraints are true constraints, and which are only constraints in name / by convention. The crown race should be tight on the crown race seat so that the steer tube doesn't rattle within the bearing. But does it need to be press fit? There will be no motion between crown race and steer tube, so long as the bearings are doing their job. By increasing the internal diameter, I allowed easier installation, but without sacrificing holding power.

Lesson 3: Wear eye protection.

My minilathe, and several mods thereof

It's been a while. There are various excuses for that, but the main excuse is that I've been putting in long days at my new job.

The big news (on the diy/hacking/modding front) is the arrival of my minilathe. I had been dreaming about owning a minilathe for a long time. I had always been frustrated by the cost and accessibility of various machine parts; to me, it's torture when you have a great idea and no way to build it. I looked forward to all of the rights and responsibilities given to lathe owners.

This lathe is just one brand of a common chinese lathe, sold by Cummins, Harbor Freight, Grizzly, and others.
One benefit of this oft rebranded tool is that many online communities have sprung up about how to use or modify it, for example the right-wing 7x10 minilathe group. However, the Cummins model is in a class by itself. It is simultaneously the cheapest, and shipped with the most accessories, including a faceplate, a 3-jaw chuck with both internal and external jaws, 5 HSS tool bits, a tailstock chuck, a dead center, a set of change gears, a steady rest and a follower rest. It is a great value.

Let me first say that, despite the limitations inherent to small lathe, this lathe is quality. Every moving part has a gib or some other way to adjust it. Although each part may slowly wear out, the designers have provided the end user with a way to compensate.

Let's compare it to the famous Sherline Mini-lathes. Among hobby machinists, Sherlines have a good reputation for quality, and are comparable to my Cummins in size. However, Sherlines do not include change gears and a feed screw -- necessities for cutting internal / external threads -- nor do they include a compound slide, thus one cannot cut bevels. Oh yeah, Sherlines cost more too.

Enough about comparisons, on to the juicy stuff. I unpacked the lathe. The first thing you need to do with a machine like this is take it apart. I took it down to it's solid pieces, cleaned off the factory grease, re-greased. re-assembled and adjusted it. These preliminaries are critical, since they give you a good understanding of how the machine all works, and yield a certain confidence toward using or modifying the machine.






So, what modifications have I done? First the simple shit: I removed and recycled the two chip guards. Then, I removed the plastic do-hickey covering the emergency stop button. Perhaps I'm misunderstanding it, but it seems much safer to have a clear path between my left hand and the stop button. Additionally, I replaced the bolts that secured the hand wheels, since they tended to hit my knucles.


Next, I grabbed some strong magnets pulled from a scrap hard drive, and used them to keep track of the various chucks. I find that the use of a magnet is just slightly entertaining, perhaps only on the subconscious level, but that is enough that I never forget to replace the key. I don't imagine I'll ever accidentally leave it in the chuck.




I don't own a grinder with which I can grind my tool bits, but I can put a small arbor-mounted grinding wheel in my lathe. The beauty of this is that I can use my lathe's compound slide to get a precise angle onto the tool bits. Similarly, I realized that I can use fragments of hackway blades to shim-up the cutting tool.




My lathe came with a lot of accessories, and suddenly I had nowhere to keep them all, let alone work. So, I went to work building a drawer set. I collected a bunch of scrap wood and plastic from around the neighborhood (and trash day in Brooklyn contains enough wood to build a few houses, I'm sure). The drawer set is sturdy as hell. I added in a few solid brass drawer pulls that I found at the Park Slope Flea Market to finish the effect. So much of this sort of improvised building involves moments of inspiration. This simple allen wrench holder is one such example.

Most recently, I built a apron chip guard, inspired by the one that Varmint Al made. One failing of the minilathe design is that it leaves the gears behind the apron exposed, and poised to collect metal chips from the cutting action. As Jobst Brandt once said, "Commercial abrasive grinding paste is made of oil and silicon dioxide," and so I was concerned about these chips, even though the gears were otherwise greased. So, here is how I built it:

Cut a sheet of 1/8"-thick pastic to the rough contours of the lathe's apron. I used some plastic that used to be part of a printer's enclosure. Drill a hole large enough for the gear's shaft (I cut mine 7/8" since I had that bit available, though smaller would do). Drill and countersink five holes around the perimiter of the chip guard.




Transfer those holes from the plastic chip guard onto the back side of the apron using a center punch. Drill and tap those holes. I used a #36 drill, and a 6-32 tap, though anything of similar size will work. The apron is made of cast iron, which is a surpisingly soft metal. Nonetheless, proceed with caution so you don't break your tap, and use lot's of wd40.







This is a view of the back side of the apron after the five holes have been drilled and tapped.








Clean out all metal chips from the area, and re-install the gears.








Drench the gears in a heavy grease. I used an old hacksaw blade as a spatula to really muck it into the nooks and crannies.







Install the chip guard. It is secured down by five 6-32 machine screws. Re-install the apron onto the lathe, and rejoice in your apron's newfound ability to repel chips!

It's been too long

My appologies for taking a week or so off from the blog. I assure you there is more to come. Here are some of my excuses for slacking on the blog:

I have started a new job, where I work as a Java developer, though so far our work has mostly been assembling Ikea furniture. The office is awesome--on vibrant Franklin Ave in Crown Heights, Brooklyn. I like how every five minutes or so someone stops by to ask "so, what is this place?" Apparently, the office was an abandoned/condemned building for many years, and though people are glad it's no longer rat infested, the neighborhood really wants another restaurant, lounge, or other public space. The job has been keeping me busy, but in a good way.

I've heard from a few Graduate schools. What? I forgot to mention that I applied to a few PhD in CS programs? Yeah, I did.

So far, Princeton has made me a good offer, and CMU and UPenn have rejected me. More schools are pending. At the moment, it looks like I'll be moving to small town Jersey unless Columbia wants me. On a related note: does anyone out there live in Princeton? What do you think of it? Of particular interest to me are cultural events, bike-friendliness, a leftish-leaning populace, and urbanity (ha!).

I purchased this lathe today, and am looking forward to its arrival. As far as metal lathes are concerned, this one is dirt cheap (though still an expensive tool). I plan to fit this into the cheaphack mission by working with scrap materials. I anticipate my first project will be a crank puller, followed by fancy brass crank bolts and top bolts, working my way into Stirling engines. Don't worry, you will be posted.

I have two projects on the way: a stylish lamp made from recycled materials, and a bicycle generator built with an old air conditioner. Stay tuned for more absolutely cheap diy technology.

NOOO!! My wishlist lathe is no longer available

I had planned to buy this lathe when I had gotten up the spare cash. At $399, the price was great, the reviews were great, and it came with tons of accessories (like a faceplate, steady rest, a slew of change gears, a few tool blanks). It was almost within my reach.

But, no... Cummins Industrial Tools is now ToolsNow.com, and instead of this great lathe, they sell some inferior lathe for more than twice the price. The problem, I suppose, were all of those jerks over at ebay who were auctioning this thing, starting at $500. They basically told the CIT/ToolsNow folks to raise their prices.

This ruins my evening. fr0wnx0r.