cutting a scarf joint

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didit
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cutting a scarf joint

Post by didit » Thu Nov 02, 2017 3:47 pm

When cutting a hook scarf in something like a keel or joining two timbers where both sides will show then care has to be taken that the joinery of the surfaces are flat and square. One way to accomplish this is to use a mill and machine the joinery. This is not really cheating because it takes skill as a machinist to cut a perfect joint, however you do not develop the skill of using a knife or the feel of cutting wood. Another thing to consider is the $700.00 to $900.00 investment in a mill as apposed to the less than $75.00 investment in a box of 100 scalpels and a small vice. Once you mastered the sill of a scalpel and the method the results are equal to the best machined joinery. I do not have a list of hints and tips for cutting joinery so i will just describe in detail how i go about the task.


To begin, a piece of electrical tape is used where the scarf will be cut. I use the electrical tape because it gives me a clear and sharp edge.
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You can draw the scarf freehand or actually draw it out. On the electrical tape i use pin points to mark out the lines and a scalpel to cut out the shape of the scarf. For clarity i will go through the steps in a drawing. I like drawing the scarf out because it gives me a nice even looking shape. To start i will mark the center of the timber and the center of the scarf like shown in the first sketch. Next two lines are drawn on either side of the horizontal center for the length of the hook and two lines equal distance from the vertical center. Finally the lines are connected for the shape of the hook scarf.
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Once the shape of the scarf is cut out of the electrical tape i will rough cut the scarf with a scroll saw.
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I line up the edge of the vice with the upper edge of the joint so the tape line is right on the top edge of the vice. You can see the edge of the black tape just below the edge Using a scalpel i shave down the surface until it is level with the top of the vice.
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Before removing the piece from the vice i check the surface to make sure it is flat and even. I will check the surface by eye and also use a flat edge and run it along the top of the vice. It is critical all surfaces and flat and no high or low spots

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Just to make sure I will give the surface a final check on a flat surface.

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satisfied with the first surface of my scarf joint I now will move on to the next cut

didit
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Re: cutting a scarf joint

Post by didit » Thu Nov 02, 2017 4:37 pm

Moving on to the second surface the same is done and the piece is clamped in the vice even with the tape edge and i begin to shave down the surface. If you slightly miss your tape edge mark it is no big deal because the second part of the joint will be cut to match.
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Starting out I will take big cuts and hog out the extra wood. If you are a beginner at hand cutting joinery it may be better not to take big cuts because you might chip out something you didn't mean to do or spit the wood then you have to start all over. There is a certain degree of finesse which comes with practice. You can actually feel the cut and know if it is to much .
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As i get close to the finished surface i take small shavings until the blade glides along the top of the vice. This way i know the surface of the wood if flat and even from side to side.



as you can see the final shavings are small and much finer

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Finally the finished surface is flat and even. A sharp knife edge will slip right across the top of the vice as if it were on ice. K
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Brian077
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Re: cutting a scarf joint

Post by Brian077 » Fri Nov 03, 2017 1:59 am

thats a great tutorial.

didit
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Re: cutting a scarf joint

Post by didit » Fri Nov 03, 2017 9:27 am

finishing of the horizontal surfaces notice I did not cut all the way to the ends.

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For the vertical surfaces of the scarf the same is done and the piece is clamped in the side of the vice right to the edge of the tape. Be sure the piece is sitting on the bottom of the vice to insure once the two parts of the scarf are together the timber will sit flat.
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Cutting the end grain is a lot harder than shaving along the grain. This is the reason i use scalpels because they are extremely sharp. Exact-O blade also will do the job but always use a new sharp blade for each surface you cut. The blades are not wasted because you can use them for less critical cutting. Cutting the end grain i take shallow cuts producing very thin shavings. It will take a few cuts but slowly creep up to the surface of the vice.
This is not a rush job, make each cut deliberate and inspect your work after every other cut. The cutting is not straight downward force, the cuts are more of a slicing from side to side. Start at an upper corner and make a slicing cut down and to the side.
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The final surface will be perfectly square and flat.

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didit
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Re: cutting a scarf joint

Post by didit » Fri Nov 03, 2017 10:05 am

Finally all the surfaces are cut. The goal in cutting the first half of the hook scarf is to make sure all the surfaces are flat. The horizontal ends are at a 90 degree to the bottom edge, this is important because when the second half is being cut and fit you will want it to slide straight down along the end surfaces so it does not bind. If these ends were slightly at an angle you will get a gap at the top or bottom when the two halves are put together.

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The second part of the hook scarf is set up basically the same as the first by using a piece of electrical tape. The two pieces are clamped together and the shape is traced on the second piece by using a new blade with a fine sharp point. Because the first surfaces were cut flat and square you can rest the knife on the surface and cut the tape. From the photo you can see the second part of a scarf is an exact mirror image of the first.

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Like before the first step is to rough cut the scarf. Once that is done i will skip cut the vertical surfaces so they are at a 90 degree to the base. The base i refer to is the bottom of the vice pointed to by the red arrow.

When i skip cut that is cutting the high spots off a surface or lining up a surface as in this case i am lining up the verticals to a 90 degree. to do a pre surfacing it makes the final cuts easier to judge because you want to take the same amount off with each cut when cutting across the surface.
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the black tape edge is the final surface so i moved the piece out to the blue line to even out the surface and square it up, i will also do this to the other two surfaces.
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With a very sharp blade i will take fine cuts until i very close to the vice surface. You can see the reason for the pre cutting or skip cutting, if you starting with a flat surface it is much easier to judge your cut to be even. The shaving being cut is pretty even across the surface.

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didit
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Re: cutting a scarf joint

Post by didit » Fri Nov 03, 2017 10:19 am

Once the verticals are true square i start to fit the original half on to the second piece. As you can see by the slim amount of wood at the tape ends i am still a little tight with the fit. I will put the piece back in the vice and take another thin slice. At this point you can use a file or sanding block to remove the extra wood. One possible issue with filing the ends freehand is you might tend to rock the file and take more off the edges than the middle, or file it out of square. On the other hand the work is so close to finishing it only takes a few passed with a file.

Being careful when cutting the vertical surfaces clean, flat and square you know by just the slightest test fit at the very ends the two pieces will slip tight all the way down.
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i will put the piece back into the vice and continue to shave down the lower surfaces until the top horizontal surfaces meets the blue line.

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And now the hook scarf is done

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didit
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Re: cutting a scarf joint

Post by didit » Fri Nov 03, 2017 10:42 am

What could possibility go wrong and why?



As you are cutting down the horizontals, which is not really horizontal but more like angles, you notice on the left side the gap is not even. The vertical edges lock the two pieces together so all you can do is go straight down. This happens when the piece is not tightly clamped in the vice and the pressure from cutting moves the piece ever so slightly, you slightly missed when cutting the tape line, or you left one very thin slice on one end when shaving down the surface. you still have room to adjust so go back to the vice and make adjustments and trial fit again and again until the gap is even.

Remember in the photos everything looks BIG. Actually with macro photography we are looking so close the crooked gap may be only .009 thousandths of an inch out of whack. Not to panic take a file and file down the bottom surface at the inside of the scarf next to the hook or clamp it back in the vice and adjust the angle.
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In this photo the problem is simply not shaving down the surface and leaving a bump which you can see on the left part of the scarf. On the right side the problem here is not taking your cuts past the end of the piece leaving a tiny bump at the end. Again don't panic this is so close a quick swipe with a small file will fix it or putting it back in the vice for another ever so slight cut.
When making cuts while the piece is in the vice I never ever file or sand the wood surface because you end up sanding the surface of the vice as you go gouging it or creating ripples. Using only a knife you cut the wood and the knife will not cut into the metal vice surface and at the correct cutting angle the cutting edge will slide right across the surface without nicking or damaging the vice.
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The time to panic is when you realize these final adjustments are taking wood away and if you look at the ends of the scarf the timbers no longer line up. The more you try to adjust the scarf the more out of line the top and bottom become. This problem is caused by the material you start with. Lets say your timber measurement is .250 x .375 and it is cut to a finish of say .005 oversize. This teeny tiny margin is all the margin of error you have to build your entire model. So cutting, fitting, fabrication of parts and finishing all have to fall within the .005 or you might end up small. To avoid this never ever cut and mill the material you plan on using to a finish the trick is to "dimension" it or as the old timers would say cut it proud. Finish comes after fabrication not before.

didit
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Re: cutting a scarf joint

Post by didit » Fri Nov 03, 2017 10:53 am

final recap

1. it looks easier than it is. It may take some practice and a few tries so don't give up if you mess it up the first 10 times.

2 select a wood that is workable with hand tools, fruitwoods are the best. If you are a beginner at this type of wood work start with softer woods like Basswood or Poplar to learn how to do it, then progress to the finer and harder fruitwoods. Wood such as Boxwood or wood of this hardness is difficult to cut it is more of chipping at the wood or taking extremely small shavings, these hard woods work better with files than knives. If you choose hard woods the joinery by hand is going to be very slow going.

3 Use very sharp blades, once you feel resistance to the cutting force get a new blade. Practice the technique of cutting very thin slices of wood.

4 learn how to use a knife, this sounds simple but there is more to it. The direction of a cut, how you hold the tool, pressure of a cut etc.

5 If you select dark woods the joinery will blend into the dark wood and will not be as noticeable. If you want to show off your joinery use a light colored wood. The joint line will stand out against the light color of the wood and will show as a sharp, crisp line. If you really want to show off the joinery darken the joining surfaces with a pencil. Do not use black paper because no matter how you finish the piece the paper edge will fray and show as a fuzzy line. Also test things like paint or markers sometimes they tend to bleed into the wood and again show as a fuzzy line.

DocBlake
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Re: cutting a scarf joint

Post by DocBlake » Sat Nov 18, 2017 8:38 am

Great tutorial. I like the idea of using electrical tape.
Dave
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