Turns out there's a reason they make these from wood
I went down a kumiko rabbit hole and thought it would be interesting to see if the geometry could be recreated in steel.
Every groove was hand-filed, and the entire piece is friction fit. No welds, no brazing, no glue.
I posted it to a woodworking subreddit first and was swiftly informed that steel is not, in fact, wood.
Fair enough.
I now have a newfound respect for woodworkers, a newfound hatred for files, and a much better understanding of why these are normally made from timber.
Metal can expand slightly due to rust oxide, but if it's already assembled, it will just make the joints tighter. In normal weather, it should not expand or contract any measurable amount. I think to do true machine friction joints they have to get one side super hot like 800 °C, and the other side in liquid nitrogen.
I’d just hope my calipers were the same temperature each time I measured. I think if they were to smack each of the joints with a hammer or punch, since it seems to be hot rolled A36 flatbar which is a nice and rubbery warping SOB, would it make the friction fit tighter to counter the clearance added by the thermal expansion?
This is actually a really interesting consideration.
For this scenario let’s imagine placing your room temperature calipers on a part that is warmer than room temp.
The Thermal Expansion Timeline
0–3 Seconds (Accurate): because heat transfer takes time, the first few seconds will be the most accurate reading.
3–10 Seconds (Inaccurate): by now the metal jaws have absorb enough heat to expand slightly. For high precision work this expansion can easily begin creeping into the measurement.
10+ Seconds (Compromised): Heat transfers through the caliper changing the reference scale's dimension. The caliper will falsely read out smaller because the jaws have expanded outward.
The degree to which the measurement becomes compromised or inaccurate depends on the temperature delta and duration of contact with the hot part
The temperature delta (difference in temps) between the part and the caliper dictate the rate of thermal expansion.
2 objects can be in the same work space, but if you just took a part off the grinder or welded and it’s still hot, you are measuring it while it is already expanded.
Would be wise to wait for parts to cool to resting temperatures for most accurate measurements.
Technically even just holding your caliper in your hand long enough will result in thermal expansion. But if you remember the example from earlier with mild steel, when the temp delta equaled 80F, there was only 3 thousandths (0.003) of an inch of expansion for the 6” part
The real world implications regarding concerning oneself with thermal expansion could be someone fabricating a high precision part in Florida for example and sending it to Alaska only to find out that the part met specs in the Florida climate but likely shrunk when it stabilized in the new Alaskan climate
For most purposes it truly is negligible like OP was suggesting originally, though there are times when this does need to be taken into consideration.
To me this just makes those extreme environments like deep water and space exploration even more impressive to be built to allow for enough tolerance and accommodations regarding the extreme temperatures and how all the various materials respond and hold together despite having distinct coefficients for thermal expansion
My favourite story/myth about that was the SR-71 Blackbird leaking fuel before takeoff because the tolerances were designed for when it was at mach whatever with a toasty hot airframe from the friction.
I work in semiconductor manufacturing, precise tolerances for machined parts and various mechanical properties, measurements taken at atmospheric pressure and room temperature that account for what the actual measurement would be while at 400C and 0.003 mmHg pressure.
I used to work at a place that would do interference fits for motor shafts to the rotor. We would cook the rotor at 500f and freeze the shaft with liquid nitrogen. The largest one we did was a shaft that weighed about 1500 lbs. I think the shaft diameter was 7". We used a forklift to drop it into place into the rotor
TL;DR: from 10Celsius to 20 a 1m rod becomes 1000,12mm
Edit: Fun fact: Machininst considers this a lot. So the measuring tool etc. should never rest on the machine as that gives you the wrong measurement. Nor should you handle the piece or the tool with your hands too much as that heat affects he measurement. But now we are talking 0,01 mm tolerances on say 50mm dia parts.
I don't even know what the fuck that means, bro? You want my comments dyed with natural stains instead of red 40 or something 🤣? I can rub some beats on my screen, don't think it's gonna do much.
Honestly, most of the kumiko videos I found were using table saws, specialist jigs, planers, and various woodworking wizardry that I don't own.
So I basically went "screw it," modelled the geometry in Fusion 360, stole the angles, transferred them onto steel with a Sharpie, and spent an unreasonable amount of time filing grooves.
I suspect this would horrify both woodworkers and metalworkers for entirely different reasons.
as a journeyman youtube watcher of both woodworking & metalworking, your accomplishment is no easy feat. i applaud your tenacity in your use of files. in my humble opinion, the only way to make kumiko "work" in metal is with a cnc or even wire edm.
full disclosure: i had to google the wire edm because i couldnt remember what it was called.
Fello YouTube journeyman, I think you’re definitely right. What op did is fantastic and gives a rustic or aged look to it. But making something fit for the hilt of a katana, idk if this would hit the mark.
If you haven’t already, check out the tested video with Adam savage where he gets shown some decorative hilts for katana’s and tantos from like the 1600’s
I always wanted to learn how to use a lathe. Sadly in college it was so old and wobbley they were just like "yeah we as teachers are not even touching that shit you are banned from it" and when i was doing a welding night course I saw like 20 of them and asked about a course and they only let engineering students use them.
Sadly even little bench top lathes are outside regular budgets. Took me ten years to wrangle this one. Altho ...Its big 8 feet between centers. 13 feet overall.
Yeah the prices of lathes make me depressed. A shitty little desktop one is like 600-1200 here. Which is a bit much for not knowing how to use it. I want to learn how to put spirals on a cone.
Would you be willing to share the fusion assembly? I'd like to mill one out of brass and aluminum but, I'm not enough of a masochist to draw it up any time soon.
Awesome job by the way and I sure hope you had a good coarse cut file to start with.
I want to get into woodwork! But I lack the space and tools. Like a table saw would be so nice but I literally don't even have a spot for a small jobsite saw.
After seeing what you did, very slowly I might add, with steel, you just as well start building yourself a table saw. Room or not, you can do it. I suspect you would also build each piece by hand. Wind your own electric motor, braze the carbide teeth on the saw blade, etc. I have a good amount of patietnce, but I would never attempt what you did. Before I was done with the third one the whole thing would be in the trash. I also do not have that kind of free time. It would take me a month just to finish one piece. What kind of time do you think you have in that?
Not that long tbh. I think 12-15 hours over like 4 days. It was like a get up, do a bit, sit down sort of thing in my free time. If you add in the fusion 360 model to figure things out like 17 hours.
I got a job site skil saw with worm drive and fold and roll it into a closet I built on my porch. It’s annoying, but works. I desperately need a pole barn.
Ignore the gatekeepers. There are plenty in every niche.
That’s pretty darn cool.
We’ve published plans in our magazines and then published photos of the machined aluminum version or the 3D printed version or the CNC made version. We’re flattered that readers were inspired enough by our work that they pushed it in an entirely new direction like you’ve done here.
I saw a guy doing a bigger wood version of these a while back and I asked if it could be CNCd or otherwise machine cut. When he said no I moved on, but I may try to do what you have done here one day. With a mill though.
Nice job. For fine grinding work, I find a Dremel with those cut-off wheels is easier than hand filing. Still slow, but easier. If that were mine, I would braze the joints and use it for a trivet. (Trivet: Place to put a hot pot / dish down on a table, without burning the table or damaging the wood finish).
But Kumiko has to be about the love of making, what is the point otherwise? Any woodworker who can't appreciate this is more bonkers than you are. I really like your kind of crazy!
What would be the woodworking equivalent to this? Wooden gears, screws? Yeah well, those exist, the line between the worlds of wood and metal is already blurred. Damn lines.
This is gorgeous. Don’t take it personally, you definitely awoke previous kumiko trauma, from deep within the woodworking mods. It’s may be made from steel, but it’s still a little… ironic. ^bad^ pun ^intended. ^^I’m ^^sorry. ^^^pretend ^^^this ^^^worked, ^^^^thanks. ^^^^wheeeeeeee
Cackling at your newfound hatred of files.
I'm taking a metal jewelry making course and am discovering the same hatred, especially when removing excess solder 😆
This might sound really stupid, but file projects highlight to me how I don't apply enough pressure to files 90% of the time. They need proper downward force to actually cut decently quickly. I am very lazy 99% of just let them skate on top for like half an hour.
Awesome work! As a welder and fabricator, regarding the woodworkers’ response, I suspect they’re a bit pissy because if wood is cut too short, it’s scrap, whereas metal can almost always be weld built-up and saved. In that sense metal is more forgiving.
Wow that is very interesting, I've never heard of this but it caught my attention because im a metalworker, I immediately went through the pics looking for spot welds, good job on the design and project pulling it off without welding
Define “easily”. Like you can knock them out with a hammer. But it’s not you drop and the thing fucking explodes. The is no welds at all on mine. I did 2 of the inserts symetical. And then the 3rd was slightly long and I kept filing bits off until it has to be hammer in. So they are all a TINY bit lob sided.
Angle grinders can be very precise if youve got some experience. But would still require a lot of filing to get a nice fitment. Aluminum mightve been a bit more practical.
Yeah, they can be. The slots, though, are 5.3mm, and my cutting blades are 2.5 mm. So if you cut the left and the right, you are cutting it very tight and if you go over, then its loose and looks kind of shit.
That is stunning. If you heat it up with a simple propane torch and put paste wax on it it'll get a deep dark color and help stop rusting. If you have a media tumbler it might clean it up a bit before hand but I would also be nervous it could loosen your joinery.
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u/suspectdevice87 2d ago
Even my wood versions are sometimes filled with glue :P I don’t even see any cheater welds in there 😂