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Dovetail jig

Started by Coastallogger, October 15, 2021, 09:09:16 AM

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Coastallogger

I am about to start a dovetail log cabin build.

Still wondering whether or not to use a jig or to lay out the joints manually.

It seems to me that a jig could only be used on milled timbers that are all the same size, is this right? This would mean they would need to be milled on four sides. Is this correct?

I am wondering if the time saved on laying out the joints would make up for the extra time I would need for milling the log on two extra sides to make them uniform size compared to just milling them on two sides.

Also I would probably need an extra course or so of logs to get to the same height due to the height milled off the logs.

I am also concerned that logs flat on the top and bottom will hold more water if any makes its way into the chinking, instead of running downslope and out when using logs rounded on the top and bottom.

Also, I understand if you mill on two sides only, your checks will all be on the outside where you can fill them as they happen, whereas if the checks are on the top and bottom of the logs, they can hold moisture and you not even be able to see it.

Are the old-timer's cabins only hewed on two sides? Or did they hew on four sides sometimes?

Just wondering if my thoughts have any validity as I decide which method to use on my cabin before I start.

Thanks.
Building 20X20 dovetail log cabin off grid.

jake pogg

I don't have any clear answers for you,but having just finished (the walls) of a cabin using half-dovetails and here's some thoughts,for what it's worth:

I didn't use a jig.
(it was put to me that i maybe should've but it was too late by then).
I laid out the top part of the joint once the log was already in final position on the wall,using a level.
The corresponding,the bottom-facing half of joint i laid out by measurement while the log was still on the ground.
Thus,each course's joints were "re-dialed" to the absolute,so to speak.

The joints came out tight and uniform enough,the next few years will tell how they'll fare in shrinkage and settlement and so forth.

In retrospect i probably would've been better off striking a longitudinal mid-line along each log when laying out the joint by math,vs using the sides/corners of timber as reference.

I've no personal experience using a jig,but would assume that the struck line is what you'd use to align it on a timber.
In that case you can just as easily fab a jig for 2-sided timbers as you would for 4-sided.

Flattening the top and bottom of a log,necessarily to the least common dimension,you may loose as much as 1/3 of future wall-height.

As far as the checking goes a sided timber will actually open to less width of check(the distance to heart being less),otherwise siding a timber has no bearing on checking in the sense you're bringing up.

Driven moisture running into checks is something that you cannot really control by the shape of a given timber,it must be prevented by other details such as overhang length et c.   
"You can teach a pig anything,it just takes time;but what's time to a pig?"
Mark Twain

Coastallogger

Quote from: jake pogg on October 15, 2021, 11:35:01 AM
I don't have any clear answers for you,but having just finished (the walls) of a cabin using half-dovetails and here's some thoughts,for what it's worth:

I didn't use a jig.
(it was put to me that i maybe should've but it was too late by then).
I laid out the top part of the joint once the log was already in final position on the wall,using a level.
The corresponding,the bottom-facing half of joint i laid out by measurement while the log was still on the ground.
Thus,each course's joints were "re-dialed" to the absolute,so to speak.

The joints came out tight and uniform enough,the next few years will tell how they'll fare in shrinkage and settlement and so forth.

In retrospect i probably would've been better off striking a longitudinal mid-line along each log when laying out the joint by math,vs using the sides/corners of timber as reference.

I've no personal experience using a jig,but would assume that the struck line is what you'd use to align it on a timber.
In that case you can just as easily fab a jig for 2-sided timbers as you would for 4-sided.

Flattening the top and bottom of a log,necessarily to the least common dimension,you may loose as much as 1/3 of future wall-height.

As far as the checking goes a sided timber will actually open to less width of check(the distance to heart being less),otherwise siding a timber has no bearing on checking in the sense you're bringing up.

Driven moisture running into checks is something that you cannot really control by the shape of a given timber,it must be prevented by other details such as overhang length et c.  
Thanks Jake,
I saw the scribing method on the BC School of Log Building's videos. That's another way of doing it. I hadn't really considered it yet. 
So do I understand you right that sided logs will have smaller checks because the distance to the heartwood is smaller?
Your comment gave me an idea: if I am milling off two sides, maybe it would benefit to mill a bit lopsided: take a bit more off one side than the other. The outside could have more wood left on it, encouraging the checks to form inside rather than outside where they can catch driven rain, dew, etc. I do have a 3' overhang, but still I expect some rain to hit every once in a while.
I checked out the pics. Ambitious project! Looks good so far! How long did it take you so far? Did you harvest and mill yourself?




Building 20X20 dovetail log cabin off grid.

jake pogg

Quote from: Coastallogger on October 15, 2021, 12:12:09 PMYour comment gave me an idea: if I am milling off two sides, maybe it would benefit to mill a bit lopsided: take a bit more off one side than the other. The outside could have more wood left on it, encouraging the checks to form inside rather than outside where they can catch driven rain, dew, etc. I do have a 3' overhang, but still I expect some rain to hit every once in a while.


It's an interesting idea...I'm not sure if it'd be worth it though,to deliberately unbalance a timber...
In considering the checks,as far as the driven moisture is concerned,i think an important consideration is the "timing",for lack of a better term,i.e. if the moisture that will inevitably find it's way in those will have the time to dissipate/evaporate before any biology has a chance to take place inside their deepest parts.

In the past,in the traditional wooden architecture of many cultures,that Timing was a very crucial part of the equation of longevity of any given structure.
As examples i could site the carved through-drainages employed in ancient Scandinavian construction,especially in the bottom dado of stave-work(something that inevitably will take in water).
Another is the use of aspen as roof shingles in a number of Northern and Eastern European regions.
The use of aspen,which is extremely prone to rotting in the moist environment,seems counter-intuitive,yet the deliberate provisions made to allow it to dry between the wetting episodes made such roof systems viable for periods ranging somewhere between 40 and 80 years of useful life.

So it's an Equation with many variables,the species,the climate,the possible chemical treatment,and that wetting/drying sequence all needing to be in some balance.

Quote from: Coastallogger on October 15, 2021, 12:12:09 PMI checked out the pics. Ambitious project! Looks good so far! How long did it take you so far? Did you harvest and mill yourself?


Thanks,and so far it's taken since the end of May,but only the one guy doing all the work(so in spite of that man working the maximum humanly-possible hours it does not amount to too many man-hours per se).

Initially the project was based on logs purchased from the nearby place further up the same valley.
As it turned out about 2/5 of these were not usable due to rot underneath the bark left on the unmilled side.
So starting from the 5th course i milled all the logs out of the locally harvested driftwood.
(the unusable logs from original batch i milled into lumber that i'm now using in the roof structure,what sound parts of them that is).   
"You can teach a pig anything,it just takes time;but what's time to a pig?"
Mark Twain

jake pogg

Here's a book that i,personally,have found very useful in trying to Understand the wooden structures in principle:

Amazon.com: Norwegian Wood: A Tradition of Building: 9780847809547: Holan, Jerri: Books

It's a detailed study of elements of wooden architecture that were built out of coniferous species,and have survived in a very lousy climate for a Very long time.

I think it illustrates the Dynamic nature of preservation of wooden buildings,something that allows it to work With the given climate,vs the more static ideas of "preservation" as in stopping all biological processes finally and completely(if it's even possible).
"You can teach a pig anything,it just takes time;but what's time to a pig?"
Mark Twain

Coastallogger

Quote from: jake pogg on October 15, 2021, 01:19:05 PM
Here's a book that i,personally,have found very useful in trying to Understand the wooden structures in principle:

Amazon.com: Norwegian Wood: A Tradition of Building: 9780847809547: Holan, Jerri: Books

It's a detailed study of elements of wooden architecture that were built out of coniferous species,and have survived in a very lousy climate for a Very long time.

I think it illustrates the Dynamic nature of preservation of wooden buildings,something that allows it to work With the given climate,vs the more static ideas of "preservation" as in stopping all biological processes finally and completely(if it's even possible).
Thanks Jake, 
I ordered it!
Building 20X20 dovetail log cabin off grid.

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