Newsletter Article: Eight Secrets to Using the Cosman Offset Method for Through Dovetails
Several years ago, we introduced Rob’s offset method to transfer the tails to the pin board. It was a real game changer in hand cutting dovetails. So much that 80% of our students are able to cut perfect dovetails on their first or second try.
The key to the offset method is offsetting the tailboard to the right and left by the exact amount of your saw kerf and then marking that spot. Now you have a kerf in the perfect spot to set your saw and make a cut. All the guesswork of where to cut - on the line, to the left, or to the right of the line was gone.
Still, we get questions from folks using the offset methods who have problems with too loose or too tight dovetails. This issue almost always can be traced back to one of the following eight problems.
#1 Setting the marking gauge. If you are using a marking gauge (verses a Shawn Shim), you set the gauge for the exact thickness of your saw kerf. You do this by setting the marking gauge on your saw, dropping the cutter, and locking it. You must get this setting precise. The secret is to make sure you do not inadvertently angle the marking gauge when picking up the setting. Make sure you firmly register the face of the marking gauge against to saw, DO NOT angle it by pushing down on the opposite side. If you angle the gauge then your setting is not enough and your dovetails will be too tight.
#2 – Picking up the saw Kerf. When picking up the thickness or your saw, set your saw on the side of your plane not a piece of wood. If you use wood, especially soft wood, you may push the saw slightly into the wood giving you a false gauge setting and your fit will be too tight.
#3 – Choose one reference edge. When doing the offset you must choose one reference edge, either the right or left to place your marking gauge or Shawn Shim against. DO NOT use the right side for one direction then switch to the other side for the other direction. Any amount that your pin and tailboard are not perfectly the same size will introduce that error into your joint. Pick one side and stick to it.
#4 – Moving the tailboard (A). When setting your marking gauge on the reference edge start by firmly holding the reference face of the gauge against the reference edge of the pin board then set the cutter down on the board’s end. Keep pressure against the gauge’s reference face to keep it parallel to the pin board’s reference edge. DO NOT tilt the gauge. Now move the tailboard over until it touches the cutter. Same goes if you are using the Shawn Shim.
#5 – Moving the tailboard (B). For the opposite offset you do the same thing, on the same side but in reverse – kind of. Now hold the gauge’s reference face against the same reference edge (same side) of the tail board. Bring the cutter up until it touches the tailboard, now set it all on the pinboard and move the tailboard over until you touch the cutter. Same idea when using the Shawn Shim.
#6 – Securely hold the tailboard. Once you have offset the tailboard in either direction you must set down your gauge or Shawn Shim and transfer that hand to the tailboard and hold it so it WILL NOT MOVE. Having a rabbet cut into the board or using our tape method significantly helps registering the tailboard against the pin board so its less likely to move. As you let go of the marking gauge / Shawn Shim and then transfer hands watch the board and DO NOT LET IT MOVE…not even a little bit. If it does redo the offset.
#7 – Move right, Mark left; Move left Mark right. If you get this backwards your joint wont fit, it will be way too tight. If you need to, mark the tails with an “L” and an “R” so you don’t get mixed up
#8 – Gently mark with the Sawtooth blade. When using the sawtooth blade don’t jam the blade into the tailboard or you could hit the bottom of your saw kerf and move the tailboard, even if you are holding it tightly. Just place the sawtooth blade in the kef and drag it out, paying close attention not to bump or jiggle the tailboard
Ok those are my eight secrets to perfect dovetails using Rob’s offset method.