Discussion About The Incra Dovetail Jig
I am not familiar with any dovetail jig except the Incra Jig. This discussion about the jig is to explain what I have found to be a problem and why I needed to find la way to fix it.
During the time that I have been a member of this forum I have not seen any long descriptions of woodworking issues. So, if my writing about an issue and my verbage is longer than what is allowed or exceptable, please feel free to inform me and I will refrain from continueing to to do so.
I am reluctant to attempt to explain in detail how the jig works, if anybody would like to see a demo on line they can do a search for Demos For Incra Toolsl. At that site there is a complete list of their tools and if you were to scroll down to the demo on the "Router Table LS Fence Positioning System" you would see the system and how it works, it is a very interesting demo, there are three parts to the demo.
Now let me see if I can explain the problem that I have found and how to overcome it. The issue that I am going to address is not discussed in the demo or the DVD that comes with the system that teaches you how to cut dovetails.
Because you cut the dovetails or box joints beginning on the edge of the board or boards and then move the fence to the correct position for the next cut and then repeat that untlil you have made all of the cuts across the end of the board or boards. I am saying board or boards because you can cut several boards at a time if you would like, normally you you would cut two at a time. Two for the side of a box and two for the ends of it.
Now here is the rub. If you do not set the center cut up so that it is perfectly centered, the cut on the first edge of the board will not be the same distance from the edge as will be the last cut from the far edge of the board. In other words the entire row of cuts will not be perfectly centered on the end of the board. Let's go further with why this becomes a problem. The first cuts that are made in the first set of two boards will not allow joining parts of the dovetail to fit together at 90 degree. This is remedied by placing the the parts face down on the router table and runing the bit into the intial cuts at the 90 degree angle that is created when the parts are placed face down. By the way these face down cuts are only done on two of the four parts of course.
These face down cuts are made in the same way as were the first cuts, that is the template in the carriage dictates where to set the fence for each cut. This will work find on one end of the board, but when the board it rotated flat on the table, the off set from center will not line up with the bit due to the off set or error created by the off center issue. This problem is eliminated if instead of flipping the boards end for end after making the cuts on the first end of the first set of two board, you rotate them from side to side, this keeps the faces of the boares facing the same way for the second set of cuts on the opposite end of the board. Now, when each of the boards is rotated on the table top to make the end cuts both ends will now match the position of the bit as dictated by the template and all is weil until it is time to put the four parts together.
Because of the off set of the centering issue it is impossible to get the top edges of the sides and bottom of the box to be flush unless the parts are properly orientated. Due the off center of the set of cuts the cut on one of the edges of each board will be a distance from the edge that is not the same as the distance from the cut on the opposite side of the board. So as you make the cuts in the boards you must mark which edge is against the fence with every cut. Then when doing the assembly, each corner must be matched so that the edges reamain flush. That is, a wide cut matched to a thin cut etc. When this is accompished you end up with the edges being flush and because they are flush you can cut the tops off of the box using the fence on the table saw or the fence on the band saw that is if your band saw is set up so that the blade does not drift.
Incra seems to avoid this matter and shows all of their demos as if the boards are all perfectly centered. My experience is that getting that perrect center is very difficult if not impossible and knowing about what I have just attempted to explain will fix the off center issue.
I believe that the majority of folks that read this will not be peticularly interested in the subject matter, but somebody that has been dealing with the issue will understand the problem and if they would like to discuss it further with me, just let me know, maybe I can explain it better one on one.
C City, TX