Biscuit Joiner From a Grinder

For the first time I recently needed to make some wood joints with biscuits. I had borrowed a biscuit joiner and later decided I would like to have my own for the few times I need one, but I wanted to spend as little as possible. I decided to make a conversion attachment for my 4 1/2" angle head grinder, but I still want to use it as a grinder.*

Materials

• 1/8" x 3/4" steel bar (3.2mm x 19.1mm)
• 1/8" x 1/2" steel bar
• 3/4" steel angle iron
• 5/16" steel rod (7.9 mm)
• 3/8" threaded rod (to fit the handle thread in angle grinder's head)
• 3/8" nuts (9.5mm)
• 1/8" steel plate
• 6-32 x 1/2" machine screws and nuts (about 3.2mm x 13mm)
• Carbide tipped blade for a biscuit joiner (Harbor Freight)

Tools

• Hacksaw
• Wire feed welder
• Drill press, drill press vise, and drills
• Vise-Grip pliers
• Square
• 90 degree magnet for welding
• Angle grinder with cutting wheel.
• Grinding wheel
• Wrench
• Hammer

See the second photo for the project that required biscuits to join the pieces of 1/2 inch red oak end to end. I wanted precision, a quick method, and stong invisible joints. The photo shows a processional cross our church wanted. It is made from 7" cut nails manufactured by Tremont Nail. Earlier I did an Instructable on how I make 5" (12.7cm) wall crosses from cut nails. The process for this cross was nearly identical, except that I did not have any jigs for laying up the larger nails, and had to use spring clamps placed by hand.

The third photo shows the biscuit joiner cutting wheel (or blade) I got at Harbor Freight for $11 US. It operates safely under 12,000 rpms. Angle head grinders like mine run at about 11,000 rpms. Note the rotation arrow. My grinder has a rotation arrow cast into its head. Naturally, these need to match when installing the cutting wheel.

* A German company makes an attachment for an angle grinder to convert it to a biscuit jointer. The price is nearly $40 US. And, router bits are available for cutting biscuit slots.

My angle grinder is a Ryobi and I was surprised to find the threads on the screw-in handle are 3/8" coarse threads very common here in the United States. If they had been something less common to my home area, I would have taken the handle to a local hardware store and used a wall mounted thread checker. I found a 6" (15.3cm) carriage bolt that was threaded its entire length. I decided to cut it in pieces as an inexpensive alternative to threaded rod.

I cut two pieces of 1/8" x 3/4" steel bar 1 3/4" long (45 mm) each.I drilled a 3/8" hole near the end of each. I bent them into an "L" shape. (The attachment point for the grinder's handle angles upward a little for user comfort. That means my "L" has a bend a little greater than 90 degrees. See the second photo. Later I found I needed to tweak these bends just a little to make my attachment align with the cutting wheel.)

I also screwed the threaded rod fully into the holes in the head for the handle. I marked the threaded rod to allow for the thickness of the "L" pieces and a nut. I cut the threaded rod and ground away any sharp edges.

I had some old 3/4" x 3/4" angle iron with a couple of holes already drilled near each end. I chose simply to enlarge one of these holes to 3/8" and ignore the other hole. I needed two pieces of angle iron for the attachment framework, and cut them 4" long.

The attachment framework pieces should be parallel to each other and parallel to the cutting wheel. I used a small Vise-Grip pliers to position and hold the angle iron to the "L" pieces for tack welding in place. In the second photo the top "L" and angle iron have been welded. The bottom pair are aligned and clamped with the Vise-Grip pliers for welding.

I had a 1/8" steel plate. I cut a piece 4 3/4" x 5" from it for a base plate (12cm x 12.7cm). I cut a "U" shaped piece out of it to fit around part of the grinder spindle.

The length of the threaded studs mounted on the baseplate can be determined by placing the cutting wheel on the grinder and positioning the base plate for the thinnest wood likely to be joined with your biscuit joiner. In my case, I decided 1/2" wood is the thinnest I will need. Measure up to the framework piece and allow for the thickness of a nut. In the case of my angle grinder,

Weld the stud to the base plate. (Welding is likely to cause some distortion. I put the base plate's edge in a vise and used a hammer pounding against a nut on the threaded rod to return the threaded rod to its position before welding. An alternative would have been to tap threads in the base plate. Then turn the threaded stud into the base plate so its end is flush with the bottom surface. Add a retaining nut on the top.)

Plywood makes an easy gauge for aligning the base plate with the cutter wheel. In the first photo the cutter wheel slopes uphill on the left side. In the second photo all is in alignment and the glue lines are parallel to the cut for the biscuit. (These cuts were actually made on a discontinued Harbor Freight biscuit joiner I borrowed. The fence on that model was also set for 1/2" stock.)

On my adapter the base plate is aligned by loosening a nut above or below the appropriate framework member and tightening the other nut on the threaded rod. See the third photo. Loosening and tightening nuts can also be used to raise and lower the base plate for adjusting to the thickness of the stock. So, if I loosened both upper nuts and tightened both lower nuts, the base plate would be nearer to the cutter wheel for making slots in thinner stock.

If it were necessary to change the slot depth often (switching back and forth between 1/2" and 3/4" stock) shims could be prepared on a table saw and placed temporarily between the cutter wheel and the base plate. These would make alignment easier and faster.I could also screw 1/8" shims to the bottom of the base plate, but would need to develop a new fence for this situation. I expect I will use the joiner almost exclusively on 3/4" stock.

The attachment can pivot on the screw studs in the grinder head. Raise or lower the front of the base plate to make the base plate parallel to the cutter wheel.

There are three sizes of biscuits for joinery. From smallest to largest they are #0 (47mm x 16mm), #10 (52mm x 20mm), and #20 (54mm x 24mm). All have the same contour, but the cutter wheel is allowed to extend farther into the stock for the larger sizes.

My fence is only 1/8" thick, but that is enough to know I have reached the correct depth of cut. See the first photo. The fence is fastened with screws and nuts to the bottom of the base plate. Conversion from one size biscuit to another will require more time on my joiner than would be needed with a commercial biscuit joiner. But, I anticipate using #20 biscuits almost exclusively. (I did borrow a #0 and a #10 biscuit and drew their outline on a couple of #20 biscuits. I could cut a #20 biscuit down to make a #0 or #10 biscuit for the rare occasion when I need that size.

I drilled a hole at each end of the 1/8" x 1/2" bar that is my fence. Then I clamped the fence to the base plate with two Vise-grip pliers and made trial cuts on scrap wood. When I had the fence correctly positioned for a biscuit size, I used the holes in the fence to guide drilling holes through the base plate.

The holes in the base plate are quite near to each other, so I drilled a second set of holes in the fence inward from the ends of the fence so the holes in the base plate for the middle size of biscuit would not interfere with the holes for the #20 biscuit or the #0 biscuit. See the third photo.

Notice the hole in the base plate near its center. This is a sighting hole for aligning with a pencil line that marks the center of the biscuit slot. I try to keep the marking line in the center of the hole.

I find I can control this biscuit joiner with about the same ease of the commercial unit I borrowed, even though it does not have the same spring loaded blade cover. I place one hand firmly on the forward part of the base plate to keep the joiner cutting straight (and to keep the base plate against the work's surface. Feeding the cutter slowly into the wood makes for better slots, too.

As I mentioned in an earlier step, the attachment could pivot about the screw studs that are in the grinder's head, especially if the nuts should loosen in time. I bent a 5/16" rod into a "U" shape and laid it over the body of the grinder. Then I welded the ends to the angle iron frame members. This will block a possible tendency for the front of the base plate to rise. (Part of the blade guard will keep the front of the base plate from falling downward.)

In order to use this biscuit joiner, both hands must be on the tool with one on the grinder handle (and switch) and the other on the base plate to make certain it remains in contact with the surface of the work. I did add steel around the teeth on the cutter wheel. So, there is no opportunity for my fingers to come into contact with the spinning cutter during use. Also, I do not remove the tool from the work until the motor has stopped spinning. But, the guard I added does protect the teeth of the cutter wheel. I may add more steel to the bottom of the tool to cover more of the cutter wheel.

Since publishing this Instructable, some have suggested a router with a wheel cutter would be safer. In actual practice, it would be more difficult to bring your hand or fingers into contact with the cutting wheel on my biscuit joiner attachment than with a cutter wheel on a router bit. The operator's hands are actually farther away from the cutter on my joiner than on a router. And, a base of 1/8" steel separates the operator's hands from the cutter on my device, but not on a router mounted in a table. Yet, I have never heard of an injury from a router cutter.

Pictured is the Harbor Freight plate joiner I borrowed. With it one could remove the front fence and slide the plate joiner on its base plate, except customer reviews of this particular discontinued tool say the cutter wheel is higher on one side than on the other and the slots slope uphill. That creates problems when the error is compounded during gluing by turning one piece a half turn so the two work faces meet.

If I want to make biscuit slots for pieces in a polygon, I would need to make a wedge base of the proper angle to support the base plate of my biscuit cutter. That would not be impossible.

Commercial units have a dust collection bag. Customer reviews often complain that the neck into the bag easily clogs. My joiner does not have dust collection.

My biscuit joiner attachment utilizes a tool I already have. My cash outlay for this project was around $15 total and most of that was the cutter wheel (blade). Even with a discount coupon at Harbor Freight, a commercial biscuit cutter costs three times what I paid, minimum. Most cost far more.

The cutter wheel on a commercial unit is fully hidden inside a shield until the tool is pushed into the work with some deliberate force. On my joiner, some of the blade is exposed underneath, even if unreachable when I am using the tool. It is a little like those warnings about not using an electric toaster while in a bathtub: some real creativity and willful neglect would be required.

Otherwise, my biscuit joiner seems to work as well as a commercial unit. I had the enjoyment of planning and building it for very little money. And, it will be ready when I need it, but is not a separate power tool sitting around waiting and waiting for me to use it.