Biscuit Joiner From a Grinder




Introduction: Biscuit Joiner From a Grinder

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

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.*


  • 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)


  • 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.

Step 1: Determine the Thread Size of the Angle Grinder's Handle

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.

Step 2: Head Attachment

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.

Step 3: Attach Framework Pieces

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.

Step 4: Ready for Welding

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.

Step 5: Cut a Base Plate

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.

Step 6: Cut and Weld Threaded Studs

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.)

Step 7: Align the Base Plate

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.

Step 8: Aligning Front to Back

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.

Step 9: Add a Fence

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.

Step 10: Final Frame Member

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.)

Step 11: Cutter Wheel Guard

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.

Step 12: What Is the Difference?

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.



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43 Discussions


2 years ago

Undoubtedly better than my Ryobi biscuit joiner. For the life of me I cannot figure out how that tool has the ability to sporadically place biscuits at random heights in work. But yeah, not a tool that gets used every day here. I could count the times I've used biscuits in a project on one hand I think.

4 replies

Is that one hand with _all_ the fingers intact, pfred2, or...

So far, the slots are all consistently and properly placed. Your comment makes me wonder if your joiner has play in the bearings or the shaft, but I know you are quite experienced and have surely checked for that. My grinder is even a Ryobi, too.

I've tried to figure out how the tool manages to randomly place slots. So far how it performs its trick eludes me. Being intermittent it is a difficult fault to track down.

You will discover the reason one day. After an older man died his wife gave me his nice slide rule. It sticks rather badly in certain spots. I finally discovered a good part of the reason after checking the thickness of the parts in various places. The discovery was by accident and visual rather than by measuring and checking. One end of the slider has swollen over the years. It is very odd.

Phil B

2 years ago

An operator's fingers and hands are closer and more exposed to a fly wheel cutter on a router table than they are to the cutter wheel on my biscuit joiner. On my biscuit joiner the cutter wheel is on the other side of a 1/8" steel plate. On a router table, there is only air between your fingers and the cutter.

Phil B

2 years ago

I found some short articles from about five years ago indicating Festool was working on one, too.

Phil B

2 years ago

Someone now makes a competitor to SawStop that does not destroy the blade when it engages. I do not remember the name, but someone may.

Phil B

2 years ago

I spent 18 years on a farm before leaving for college. News of injuries travelled quickly through the area. Although equipment had some guards, there were still open spinning shafts, teeth, cutters, chains, and wheels. Injuries occurred when equipment operators abandoned known precautions due to impatience or engaging in silly play. For example, a neighbor eventually died of injuries when he decided he could pull a stuck stalk from a cornpicker without disengaging the drive and waiting for the machine to stop. He was pulled into the machine and badly injured before dying in a hospital a few days later. A hired man with years of experience decided he could reach down from a tractor seat to get mail from a mailbox without putting the gear shift into neutral or climbing down from the tractor. He reached too far and slipped off of the tractor seat, and fell under the wheels as the tractor moved over him. Two boys were chasing each other with tractors in a pasture when one of the tractors hit an unseen rock big enough to pitch him off of the tractor and he was run over and killed. A man old enough to know better playfully allowed his tractor to back too far into a ditch. When he began to drive out, the clutch grabbed too quickly and the front of the tractor pivoted over the top about the rear axle and came down on him. He survived after a long stay in the hospital. True accidents involved things like someone overcome by fumes in a confined area or someone slipping and almost falling into a hopper. Those were very, very infrequent.

i think it's a great idea and a wonderful ible. there has to be idiots that mess with everybody about their problems.It's their problem and they should keep it to them selves. But since the don't they are the reason we are able to find comedy in this world and find it great to laugh at them. Piss on these little people with no ideas and no clues. and thankx for the laugh dipshits. Once again Great job. Thankx for sharing your idea.

1 reply

I bought a BJ from HFT years ago- Either the kerf was too large, or the blade wobbles. Either way, the slots were larger than I would like. Of course, if you have a router and a slot cutter bit . . .

1 reply

I really considered a slot cutter router bit, but the cutter was expensive enough. Then I had to add the price of a mandrel or arbor to that. But, the radius of the router bit is not the same as the radius of the arc on the biscuits. The slot made by the router bit would not fit as well as that made by a biscuit joiner blade.

Harbor Freight discontinued their plate joiner (their name) made with the plastic base and replaced it with a metal base plate or biscuit joiner. The replacement is pretty good, but the plastic model has problems. The cutter makes slots that slope uphill, unless the fence is used as a guide and time is taken to be certain it is parallel to the blade.

The biscuit will fit loosely so some fit adjustment is possible. The biscuit is supposed to swell and tighten the fit when it absorbs moisture from the glue.

Sorry. Nice project. May actually try it1 Thanks for posting it.

How much is a biscuit joiner from Harbor Freight?

1 reply

About $60, but $48 with a 20% off coupon. Add sales tax to that.

I like the piece that BeachsideHank put in about the history.I KNOW ALL safety wennies have to go a safety meeting and pay for the course on how to operate a huge money tool to do 5 cent job.I like your idea on this joiner and also the SAFETY mentioned by you.It is one hell of a idea and also a SUPER GREAT INSTRUCTABLE.BUT LIKE ANYTHING ELSE ALWAYS WATCH OUT FOR ANY DANGERS AND always prepare for the worst.I to do things require SAFETY things such as drive my truck or flush the toilette or text on a cell phone lol .But I do operate a bandsaw mill but I do with CAUTION.
So as Paul Harvey use to say : Now you know the rest of story. ;-)

Ignore the safety weenies. This is a very cool instructable. Everything in life has risk. We are loosing our ability to produce because of the whinging babies who have to be protected from the world. Here is a tip for those who complain, if you do not like it, don't build it. This is the most positive way of making this point.

1 reply

Thanks. I thought I had addressed all necessary safety concerns in step 11. When we bought a new car in 2012 I was told to read the 400 page owner's manual. After 70 pages I was still reading lawyer talk about how I should not operate the car while sitting in a bathtub full of water for the sake of safety.