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Use of the Mortise Attachment
on MiniMax combination machines

Background on the MiniMax Mortiser

One of the components (or at least options) of a MiniMax five function combination machine or jointer-planer is the mortiser. It consists of a heavy cast unit that attaches to the side of the machine, so that a special "birds mouth" bit can be driven by a Wescott chuck at the end of the jointer/planer cutter. This gives me a 4.8 hp mortiser!

The work piece is firmly clamped to a moving table. The table manually moves left and right, in and out, with stops to limit the left and right movement of the table (the length of the mortise) and another stop to limit the depth of the cut. You might want to set the depth first... measure from the edge of the table where the workpiece will be aligned to the end of the bit, and set the stop that limits the depth of cut. Then I start with the four sides of the mortise drawn on the first workpiece. The height of the table that holds the work can be precisely adjusted with a crank, using the top/bottom lines you have drawn, aligned with the points of the bit. I set the left and right stops by aligning the table so the bit is next to the left-side mark, and then sliding the left stop in place and tightening. Same process for the right side stop. The birds mouth bit looks like a long router bit, and cuts with left-right movement, so it is gradually moved in (perhaps 1/8 to 1/16 inch per pass) while maintaining a left-right motion - it does not plunge. Different diameter bits are used for different width mortises. (See the notes on bits below, including end mills.)

It does a superb job, making mortises so easy and fast that I now use floating tenons, with mortises on both parts. Time to do the actual cut of a typical mortise is about 10 seconds. Setup time including selecting and putting the bit in the chuck, etc. is a few minutes, but a change in setup (e.g. for an apron not flush with a leg) is well under a minute. I had one project that required 252 large mortises in extremely hard ipe wood, but that took several hours.

This is a description of how I use the unit to do a typical table leg and apron.

Preparation of the machine

I found it very convenient to make marks on the table at the left most position and the right-most position that the bit can reach - see little marks by the left and right red arrows in the picture. In addition, on my machine the stops that limit the left and right movement cannot go past the center, so I made an additional mark there, by the middle red arrow ... that center point will always be included in the mortise. Don't ask me how many times I have set up a mortise, then had to set it up again to include that point. Some of my friend's (newer) machines don't have the restriction in the middle.

Mortiser setup 2617

Shop built guide block 2619

Since the center of the table must be part of the mortise, the guide provided must be removed, since it attaches in the center of the table. I invested in a sophisticated replacement guide - a scrap of MDF and an old C-Clamp ... with the screw on the lower side, it easily attaches to the cast iron table, with the screw part avoiding the cast iron ribs. This guide stays in position to align both the aprons and legs in a typical use.

Breaking news... I have upgraded the jig to a scrap of hardwood rather than MDF! I marked a depth scale on the wood (¾, 1, 1¼, 1½) so that I don't need a ruler to set the depth of the mortise.

Cutting Mortises

Either the leg or apron can go first, here I started with the leg. The leg is mounted against the sophisticated shop built stop described above, and along the machined edge of the table. It is clamped firmly in place with the eccentric clamp provided. The left and right stops are adjusted to the distance the mortise is from the top of the leg and for the overall height of the mortise. A third stop is set to control the maximum depth of the mortise.

The actual cut consists of moving the table in until the bit just touches the wood, them moving the table left and right to cut the mortise. As you move left and right, apply a slight pressure to make the cut deeper... like a plunge router without a plunge bit, you will gradually work your way to the bottom of the mortise.

Leg mortise 2618

Apron Mortise 2620

The apron uses the same alignment - the shop-built guide still marks the distance from the top of the workpiece to the top of the mortise (the same for the leg and apron). The length of the mortise is the same. The table height is adjusted for the amount the aprons are set back from the front of the leg.

The eccentric clamp cannot reach the apron, so a second sophisticated shop-built jig is called into play. This one consists of a scrap of plywood, with some sandpaper glued to the underside to prevent slippage. A scrap cut-off from the apron holds the far end of the jig, with the eccentric clamp pushing down in the middle.

The floating tenon stock is cut in advance using the thickness planer to create stock the size of the mortising bits (usually whenever I have some scrap wood of the appropriate size). The edges can be quickly rounded on a sander, ideally leaving a small area on the top and bottom of the floating tenon for the glue and air to escape. On other pieces I round over the edges with a roundover bit or bead cutter on the shaper (I don't have a router table, but that would work, too). Note that with the router or shaper you need to use a fence, since you are cutting away the part a guide bearing would follow. Then I cut the individual tenons as needed, a couple inches at a time, off this prepared stock.

Floaing tenon stock 2621

New machine alignment

When the mortiser is first attached to the side of the jointer/planer, it will seem too high. It can be raised and lowered with the screws it rests on at the bottom. Don't change it like I did. The normal position of the mortiser is fairly high on the mount, and if it is lowered, you may have trouble centering the mortise in thinner boards.

To check the alignment of the mortiser, I put the largest bit in the chuck, and raise the table until it almost touches the bit (separated by, perhaps, the thickness of a sheet of paper). Move the table back and forth, in and out, so this gap is constant. No special alignment procedures... the obvious screws are adjusted for left and right slope. I haven't had to perform the "slope away from the machine" alignments on any of the four machines I have helped set up.

Sources for bits

Birds Mouth bits

The bits distributed by MiniMax when I got my machine were fractional inch bits apparently made by Onsrud - 1/4 inch= part 24-084 $18.25, 3/8" = part 24-120 $18.25, 1/2 inch = part 24-160 $18.25. However, these parts have disappear from their web site - it appears that Onsrud is discontinuing them.

A friend bought a MiniMax machine with mortiser, and his bits were metric - for example 7 mm rather than 1/4 inch. Not a big difference, but I have never found them in the Onsrud parts list. I have heard that Clico made some bits, and perhaps these, but have not found a distributor or web site for Clico. Some have four points rather than two, but that doesn't matter for basic use.

I am still using my original bits - they are trivial to sharpen (just grind the inside of the mouth on a corner of a grinding wheel). I have found bits that look identical to mine at Morris Wood Tool Company, see the Style 6HS bits, about $12 each, in imperial sizes. Their web site has not worked recently and they are not answering their phone at 423-586-0110. Doesn't look good.

These bits available in carbide, metric sizes, from Rangate. The regular price varies from $68.95 for 6 mm to $84.95 for 16 mm. I suggest you call them at 888-810-2522 since you cannot purchase directly from their web site, there are occasional special sales, and their default shipping can be expensive.

In my opinion there should not be a concern about metric vs. imperial sizes, since either thickness tenon stock can be easily created as needed. Since the HSS bits are so easily sharpened without impacting the cutting diameter, I went for 10 years without wanting a carbide cutter, but finally have a 3/8 inch (10mm) carbide cutter on order.

End Mills

Some people wonder why you couldn't use an end mill with a half inch shaft.

Beware of the too-long bit. One user was looking for a ¼ inch diameter birds mouth bit, 4-5 inches long, so he could cut through tenons with a single setup. I hate to think of the possible deflection on a 5 inch long 1/4 inch diameter spinning shaft cutting sideways.


I need your help. If you have additions or corrections to information on this or other MiniMax products, please send it to me by eMail.

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