I purchased a bench-top morticing machine, which is powerful enough for me and portable within my garage/workshop. However, I thought that I could improve the design a little, as registering the chisels square with the back post was hit and miss, as was aligning the fence.
The problem stems from the fact that most small morticers locate their chisels in a circular throat, using a pinch-screw arrangement. Mine has a handle on the front that connects to the screw thread and the machine came with various internal bushes to account for a range of chisels. It still needed something to register the chisel squarely with the back post of the machine, as without that you are likely to get a series of unsquare cuts which can produce a "sawtooth" pattern as the mortice is cut.
|The problem is made worse by the fact that the fence is controlled by two hold-down screws, one each side of the back post, so it is possible to rack the fence as it is moved, as shown in this picture.|
|I decided to solve the problems. First, I put in my smallest chisel and with it I gently marked the center point of the base. Taking that measurement from the front of the base with a marking gauge (once I had removed the base from the machine), I scribed two lines, one each side of the base. These gave me a front register point. I then routed two very shallow channels into which I glued small engineers' metal rules (with a zero edge and marked in both inches and millimeters), as seen in this picture.|
The rules were cut off to match the base dimension at the rear. They perform two functions, using easy math.
Firstly, if using a 10mm chisel, then the fence side of the chisel will be 5mm back from the center point. Move the fence until both rulers read a hair over 5mm, gently secure the fence. Now insert the chisel and gently pinch. Pull the chisel down (making sure it doesn't foul the fence the first time you try it!) until you can test it for squareness against the perfectly perpendicular fence. Lock the chisel after returning the machine to its rest position. The chisel is now square with the backpost.
Using, say, a 10mm mortice in a 20mm wide workpiece, this means that the fence needs to be 10mm back from the center point in order to center the mortice in the workpiece (10mm mortice leaves 5mm each side when centered. The far side of the 10mm chisel is 5mm back from the center - add the 5mm thickness of remaining stock = 10mm). Therefore, move the fence until both rules read exactly 10mm and all you then need to do is to mark the length of the mortice on the work and away you go.
If you don't want the mortices centered - change the math. It works really well.
|This concept can be applied to drill press tables as well:|