Video: Cutting Mortises with a Mortising Machine

Skill Builder Video

Cutting Mortises with a Mortising Machine - Video Transcript

Speaker: Most tools in our shop can do more than one task, but in the case of benchtop mortising machines they have one dedicated job and that is to cut square holes. How do they get that job done? Well, a benchtop mortising machine uses a hollow chisel bit that's sharpened on all four sides and there's an auger bit that fits inside to clear out most of the waste.

The tool itself is actually pretty straightforward. It's got a drill chuck inside the motor head and that's what spins the auger bit and there's a bushing underneath that locks the chisel in place. When you pull the handle of the machine, you drive both cutters down into the workpiece and that's what cuts that square hole. I'll show you how to set up the machine and actually go ahead and cut a mortise, but before I do that I want to talk a little bit more about the dynamics of this bit.

In order for these bits to work properly, the auger bit actually has to go into the workpiece first. As I said it clears out most of the waste. The chisels follow behind the auger bit squaring up the walls of the cut and clearing out the rest of the waste on the bottom. In order for this to work properly, that auger bit needs to be out ahead of the chisel and you need to leave some clearance space between the auger bit and the chisel. That clearance space enables the bit to draw those chips up out of the cut and eject them through a slot on the front of the chisel. Now, the amount of that clearance will vary based on the power of the machine you're using, the sharpness of the bit to begin with and the density and type of wood that you're cutting. You have to experiment a little bit to get the chip clearance correct.

The space can be anywhere from a 16th of an inch to 3/16ths of an inch and it's best to just try it on some sample test stock to see how well it cuts. With that said let's get this machine set up to cut a mortise. First thing we need to do is install the chisel and bit in the machine, so I'm going to use a block of wood just to protect my fingers from the tips of the bit it's razor sharp, slide it up inside the bushing and just hold it in place there. Now I can actually start to account for that clearance space I mentioned before using this 8 inch thick piece of bar stock. It's going to establish about a 16th of an inch of clearance between my bit and the tips of the chisel. I take my bar stock and I insert it right above the chisel and hold that in position and at this point I can go up inside and tighten the auger bit in the chuck.

I like to tighten my bit using all three access points of the chuck to make sure I've distributed that pressure evenly all the way around. Now with that done, I can go ahead and take my spacer out of the tool and tighten my chisel just enough to hold it in position. Now we need to square the chisel in relation to our workpiece. I'm going to bring the workpiece up and I'm going to use the front face of the workpiece to square my chisel.

I'm going to pull the fence of the tool forward and I'm going to lower my chisel down and use the workpiece as my reference. I want to bring the two uptight to one another, so that the chisel is flush with the workpiece, loosen my screw and adjust my bit left or right until it's meeting that workpiece flush. Now I can actually take my chisel slide it up tight against the bushing and that opens up that important clearance space that I need.

I'm going to grab my screwdriver and I'm going to go ahead and I'm going to tighten that chisel the rest of the way. Now I'm going to want to double check to make sure that my chisel is still flush with my workpiece after I tighten it home and it is, looks pretty good. The next setup task is actually aligning our chisel to our layout lines. In order to do that, I'm going to bring the fence of the tool forward, bring the chisel back down and line this up by eye.

I need to come forward a little bit more and that looks pretty good. Now I can go ahead and lock the fence. Next step in the process involves setting the depth of cut and I've already marked the end of my work piece for that cutting depth. I'm going to pull my work piece back, lower my chisel down until the tips of the chisel align with my layout line. I can lower the depth stop on this tool which is a rod that makes a contact with the base of the tool and lock that in place.

With my depth stop set, I know that my chisel is going to stop at the appropriate depth for every one of my cuts. Now that I've got my depth stop set, I know that my chisels lined up to my workpiece, I can install the hold down. Every mortising machine comes with a hold down, it prevents the workpiece from lifting off the table when you're extracting the bit after each cut and I'm going to tighten that securely.

Now I want to make sure that once I've locked that hold down in place I can still slide my workpiece back and forth easily and I can, I think we're in good shape there. One last thing before we start to make our cut, you'll notice here that I've positioned this bit so that the slot, the ejection slot is facing forward. I'm stepping away from what the manual suggests which is to direct the chips to the left or right, but I find that when I set it up that way the chips end up clogging up my mortise and blocking my view.

I like to direct the chips out in front of the workpiece instead, it dumps them all in front of the workpiece and not inside the mortise and really makes the work a lot easier for me to see. It's a matter of personal preference but it's what I like to do. All right, with that said, I think we're all set to cut our mortise. I just have to plug my tool in here and line up the end of my mortise with my mortising chisel, make our first plunge cut.

Now you'll notice that when we're making these first cuts the chisel is completely surrounded by the wood. It's the point at which the friction is the greatest, so you need to give the chisel plenty of time to go in and extract those chip from the cut. Once you've made your first cut slide it down about a chisels width from your first cut and go ahead and plunge it in for the second cut.

Give the chisel plenty of time to clear this chips, you might even want to plunge it in and several passes to clear out that waste. Now the advantage to spacing your cuts out is that you're supporting the chisel all the way around. If it's only supported on three sides by the wood, we want to wander in the direction of the open end and if you're using a small chisel that can actually bend the chisel, so space your cuts out. Now that we've cleared out the first round of waste, we're going to go back through and make a second round of passes to clear out this space of slot. These are much easier cuts to make because there's only resistance on the front and the back of the bit and not on the side, so they go pretty quickly.

Now I like to make one last round of passes just to make sure I've cleaned up the walls of my mortise nicely and I find that this also clears out a little more waste from the bottom of the mortise as well. Just overlapping these passes, see we're still clearing waste. Wait for my bit to come to a stop, found out what little waste is left and we've got a nice clean mortise. That's how you cut a mortise with a bench top mortising machine.