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“The vast majority of human beings dislike and even actually dread all notions with which they are not familiar… Hence it comes about that at their first appearance innovators have generally been persecuted, and always derided as fools and madmen.” –Aldous Huxley 1894-1963
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Before we begin with this weeks drivel, here is yet another totally professional and FREE cartoon:
I suppose one benefit of being an insomniac is that I have plenty of time to think about tools–and last night at around 3 AM, it occurred to me that we could use the KM-1 Kerfmaker to quantify the fit that defines a well made joint.
I am no scientist but today I am going to play one on this totally awesome and worthless blog. Here is what we did;
We cut a couple of strips of poplar and chopped them into 3″ lengths. Then using the KM-1 Kerfmaker and feeler gauges, we made several cross lap joints increasing the kerf .001″ at a time.
The first joint used the KM-1 as designed, and the fit was perfect. Next we added a 0.0015″ feeler gauge to the width of the stock and cut the joint. This was repeated with 0.002″, 0.003″ and 0.004″ feeler gauges.
So what did we learn?
We are reporting the following to every major science journal with cash prize awards; joints that fit tight the male portion is 0.001″ to 0.0005″ larger than the female and continue to feel “professional” in fit up to about -0.0005″under, which is a darn small number. For a real tight fit, the male is oversize 0.001 to 0.003″ and will require force to assemble. Aren’t you glad you asked?
A cross lap made with a clearance of .0015 to .002″ needs zero pressure to assemble and wiggles slightly. The joint with .003″ clearance clearly is a sloppy fit and the .004″ joint appears to me to be perfect for puzzle makers–or future Sh!t Bunny recipients. Here’s a brief movie if you insist on remaining an accuracy apostate;
So now you know.
The next time you run into a woodworking crackerjack that waxes poetically about how wood moves, or shrinks, or swells, or how you never need a ruler with grads finer than 1/16″, or worse yet, leave plenty of room for glue, you can confidently share that a well made joint has an overall tolerance of about plus or minus 0.0005″ for a total range of .001″. This of course were our results in poplar, a relatively soft hardwood. As your material gets harder, the allowable tolerance for a tight fitting joint shrinks even more.
Drivel Starved Nation go forth and speak–rest assured the scientists at Bridge City are up all night making sure your future joinery will be gap free. And for this we think we should win a Pulitzer or a Heisman trophy, or at the bare minimum, a case of wood filler.
–John
This is a really cool demonstration. Did you cut these on the JMP or a table saw (just curious)? It would be interesting to “cut in summer” “check in winter” and vice versa. Wood type would show its true colors then. You could accelerate this by starting with say 10% wood and “hot boxing” it down to about 7% to test it. With this extrordinary tool I could envision a chart showing wood type vs moisture headings showing a winter / summer tolerance setting. this would earn you a wood jointery phD!
It doesn’t matter when we cut it, or on what device as the results (fit) are relative. Granted, green woodworking has a set of rules, as does dry woodworking, but relatively speaking, we are demonstrating what a “pro fit” should feel like and this little diddy quantifies how a few thousandths of an inch makes a mile of difference. In other words, if you cut the joint today, and you intend to assemble/glue in within a couple of days, our demo is relative.
Basically, and most interestingly, good joiners, are working to tolerances that are really close to machinists. We find this amazing and worth sharing.
In short, to be human, and a maker–a good maker that is–is as good as life gets!
Lastly, we cut these on a 45 year old Delta table saw.
–John
PS: For those that have a KM-1, table saw or JMP, replicate our demo. We wouldn’t have posted this if we couldn’t (or the legions of others who own this tool) repeat the results.
PPSS: I already have a PhD in cartooning. Can’t you tell?
Fascinating topic. My uncle calls me “Friction-fit Chris” and I’ve earned it.
Many of my joints are cut too tight. Sometimes it works out, like when I put up a half-dozen shelves between the open studs in the garage without using a single fastener or interlocking joinery. Just a 5-pound dead-blow.
Then there was the 5-foot sliding dovetail I cut in soft maple. I should not have made that a friction fit – after I got one foot in, I couldn’t get it out so I spent the next half hour pile-driving it in.
It would be interesting to see the looser joints tighten up with the application of wood glue. I don’t know how you’d test that without actually gluing them together which would of course prevent any wiggling. Perhaps you could apply a thin layer of glue to each independently, let it dry, and then put the joint together?
“…joints that fit tight the male portion is 0.001″ to 0.0005″ larger than the female…”
PS: John, you gave me an idea for another box!
Wow. You insomniacs have all the fun! That’s a really interesting study you did, John. It really quantifies the tolerances that a top-notch woodworker works to. It’s pretty amazing how well people do on their joints, considering how many opposing faces have to be perfect (like a bridle joint, or a dovetail).
If you find a journal that has cash prizes (or even offers to publish free) let me know. You may not be aware, but in science we actually have to PAY to get our papers published. Sometimes A LOT! It’s pretty ridiculous when you think about it: the government pays us to do research. We then use the government’s money to pay a journal to publish that research. And then we have to pay again to read it! Why, there oughta be a ballot measure!
Sorry. Off topic. Awesome cartoon. I like Zins with wallpaper paste.
– Peter
P.S. I bet you got to choose the music in the video…
@Peter;
I cannot tell a lie, Michael composed the music–and I really liked this one.
Interestingly, when we post a YouTube video and the music source is original, after a certain number of hits, we receive money from YouTube. So far we have earned $7.62 over the past couple of years. If you would tell more of your friends about our videos, I might be able to retire someday…from what I don’t know, but it sounds good.
Did not know you were in a Pay to Print business model. Where is the impartiality?
Lastly, one of the coolest stories to come out of your neck of the woods is Zenyetta–what an amazing animal. I am going to watch my very first horse race this weekend!
–John
@Chris;
Glad you were struck by the muse…don’t forget to post pics in our Design forum!
–John
Wow! Go Michael! Nice job on the music. You guys are all such multi-talented people.
The impartiality in the scientific publishing world is the anonymous peer review. You submit a paper to a journal, and they send it off to 2 or 3 reviewers, who get to say nasty things about you because you don’t know who they are. If your paper’s good enough, the journal may let you revise it for another review, or perhaps accept it after some minor revision. Then you pay to get it printed.
There’s almost no step in this whole enterprise (having an idea, proposing that idea to a funding agency, having your proposal anonymously reviewed, doing the research, and publishing the research) at which anybody says anything nice about you or your science. It’s a lot like making high-end, innovative tools. You have to have a big ego to take all the abuse.
And then you have to deal with the public who think that global warming’s a hoax. Sigh.
Yes, that Zenyetta story is a great one. 19 wins! Impressive. My first thought was to place a bet…
– Peter
Good demonstration. The amount of slop is also dependent upon the width of the two pieces of wood. Do the same exercise using five inch wide pieces and I would think you could increase the amount of allowable slop. Of course tight is tight and loose is loose no matter the width.
@Paul;
You are absolutely correct–we were constrained by the size of our video photo booth and the capacity of the KM-1. That said, as you said, loose is loose.
–John
As much as I’m brimming with half-formed questions about the nature of precision in a hygroscopic material, I’m more curious about potential tricks for dealing with insomnia 🙂
For most, the hygroscopic nature of wood is only an issue if the moisture content of the stock is greater than 10 percent. I NEVER used any material that had a moisture content of greater than 8% back when I was a furniture maker.
One notable exception is green wood construction (separate set of rules).
When making joinery the fit is relative at the time of cutting. If your result is sloppy, then it is a safe bet that over time, it will be unmasked as a poorly constructed joint.
What we are demonstrating is that the quality of fit falls within a very narrow range. The difference between “acceptable” and “unacceptable” is literally a few thousandths of an inch. Wedged tenons were invented for a reason…
I have had insomnia for so long I actually look forward to the ideas that are chalked out on my brain between 1-4 am every night. Part of getting old I guess…
This is a fun thread.
-John
Hydroscopy: Hn. One of the reasons I was in Atlanta was to look at modified wood, the stated virtues of which include greater dimensional stability.
JMP: I just had a potentially dumb question: If making a lap joint, would the upward slope of the blade make cuts higher on one side than the other?
Insomnia: Me, since a baby.
As long as your stock complrtely passes over the last tooth, your cut is square bottomed.
-John
I sometimes exploit the hygroscopicity of wood when I’m making some kinds of joints: I heat the wood to be inserted before inserting it. That shrinks it a little. Then when it swells with the glue, it gives a good pressure fit.
– Peter (sleeps like a baby)
Couldn’t you take a sloppy joint and throw it in the lake once a month to tighten it up?
I’ll have to try this [S]if[S] after I get my JMP and KM-1.
John,
As a fellow insomniac, clinically proven no less, I must say that I get quite a kick out of your musings. So, what is the story with your worst joint at .005 when glued? The glue companies go to great lengths to show the outstanding strength of a glued joint and that the wood will break before the joint will. However, what I do not hear from them is the length of time that the glue will hold so maybe that throws a kink into this.
How can you not love a horse who does nothing but wins, loves people and is named after a “Police” album. 😮
Fred
I’m back – busy week.
John, You are correct – making the joint is done in a relatively short period of time. I had in mind early american furniture when I asked my question about wood movement. I use all air dried wood as the early Americans did. You may have met Alan Breed at Marc Adams along the way, and we had a very interesting discussion on this topic. Alan has spent a lot of time in museums restoring these pieces and his observation is that the pieces that survived all these years had the ability to “move” seasonally. Many great pieces broke up over the years because of too tight of joinery / incorrect wood orientation, etc. Air dried wood does not have crystallized pitch in it and does tend to move a little more than kiln dried wood. Some of the “masters” did things like slightly elongating holes for movement, etc. your (great) demo caused me to think about this…a dovetail joint pounded together will more than likely result in a long term crack down the piece if there is much seasonal change in humidity. Theoretically both pieces should absorb water the same, but often they are different species and may not. A “great fit” – now defined as less than .001″ slop would probably be what you want. I have the tools to play with this, so I should learn from your demo and do some experimenting myself to see if there is a significant “sweet spot” for different woods. I thought it was “food for drivel thought”..
John: between hygroscopic and anisotropic, my mind’s eye holds a torsion box curliqeued like a dried pepper with sheared pins (term?) of lap joints rattling around inside like seeds. My mind-box failure is geometric, as the number of joints along the axes multiply the force differential shearing off the pins.
Essentially, I think an understanding of why this isn’t an issue–or an issue that matters–is why you are a pro and I am a con.
Dave
I’m curious, John. Do you do all your lecturing with your arms out to the sides? Your photorealistic drawings seem to show that…
– Peter
Dave,
You need some sleep.
John
John,
Cool test, it’s always great to be able to quantify something rather then just guess.
Since you mention the Sh!t Bunny and wood putty in the same post, I may have to nominate the first piece of furniture I built; it’s a small cherry coffee table, that suprise, I’m using as a temporary entertainment center!(I have the stereo sitting on it.) Just to give you an idea; It has a solid wood top with solid wood mitered edge banding, guess what the corners have in them!
I have a cure for insommnia: Most people think that they should try and create a pleasent and tranquil environment in which to sleep. I think that is not the best way, in fact I beleive that the opposite is true. I always seem to fight falling asleep while I’m bouncing over potholes with blinding headlights in my eyes, sitting upright in an uncomfortable seat, stereo blaring and cold air in my face while driving my car! I just need to invent a mattress that will do the same things and I’ll be stinking rich! I think I’ll call it the “Sleep Discomfort System.” But I won’t let you buy one, because I’m enjoying all the great tools you invent at 2 am!
-Rutager
🙂