Wooden Burr Puzzles

Something's got to give when pieces of wood intersect at 90° angles. That's the reality behind the curious assemblies shown here. Known as burr puzzles, because they resemble seed burrs, these brain-teasers consist of three or more notched pieces that go together at right angles. Give one of these tricksters to an unsuspecting friend and watch the fun. Disassembling each puzzle is the easy part. Putting the pieces back together is the real challenge!

Precision is the key to making the puzzles work effectively. Each part must be accurately marked, milled and cut.

The first step for all three puzzles is to mill long 3/4” x 3/4” blanks. Use a caliper to measure the thicknesses precisely and make sure the blanks are square. Then cut the individual puzzle pieces to exact length from the blanks.

A shopmade jig makes it easy to notch the pieces for all three puzzles on the tablesaw, using a dado set (Fig. A, below). This jig consists of a sled with runners, a clamp and a fence. The runners fit the saw's miter slots, so the sled makes perpendicular cuts. For clean, tearout-free results, a different part of the jig should be dedicated to each notch size. If, as shown, the blade is offset between the runners, you can use both sides of the sled. For safety, the jig’s wide fence houses the dado set from both directions. Stop blocks and spacers precisely position the pieces, so the notches (dadoes, actually) are accurately cut. Like the puzzle pieces themselves, the spacers must be precisely cut. To set up the jig, clamp one stop block to the right of the slot (the exact distance—called the Jig Set-Up Dimension—depends on the puzzle you're creating). Use a puzzle piece and the spacers to locate and clamp the other stop. After you've installed each piece, secure it with the toggle clamp before you cut the dado (see Source, below).

The dadoes have to fit perfectly, so always make extra puzzle pieces, and start by making test cuts. Testing the dado widths and depths is pretty easy, because most of the pieces go together with lap joints. When the dados fit snugly, their widths are correct; when the joint surfaces are flush, the dado depths are correct. The pieces will go together more easily if you lightly sand their edges. That's it; you're ready to go.

Click any image to view a larger version.

The Three Piece Burr

The Six Piece Burr

The Twelve Piece Burr

The Three Piece Burr

In appearance, this puzzle is my favorite, because of its perfect, simple symmetry. It's the only puzzle of the three that requires cutting dadoes in two sizes (see Fig. B, opposite).

Make the pieces

1. Cut 2-1/4" blocks from square 3/4" stock, including extras for test cuts.

2. Set up the saw and the jig to cut 3/8" by 3/8" dadoes.

3. Clamp the jig's right stop block 1-1/8" from the edge of the 3/8" slot.

4. Snug a test piece and both 3/8" spacers against the right stop. Butt the left stop block against the spacers and clamp it.

5. Cut dadoes in a couple test pieces. Fit them together to check the dadoes’ width and depth; make necessary adjustments.

6. Install Piece A and cut the first dado (Photo 1).

7. Rotate Piece A, reposition the spacers and cut the second dado (Photo 2).

8. Turn the jig around and set it up to cut 3/8" by 3/4" dadoes.

9. Clamp the right stop block 1-1/8" from the edge of the 3/4" slot.

10. Repeat Steps 4 and 5.

11. Install Piece B with one spacer at each end. Cut the first dado (Photo 3).

12. Rotate Piece B one-quarter turn toward the dado set and cut the second dado.

13. Install Piece C and repeat Steps 11 and 12.

14. Ease the corners of the bridge on Piece C to create an octagon.

Assemble the puzzle

 1. Connect Pieces A and C.

2. Install Piece B from the top.

3. Rotate Piece C one-quarter turn.

Fig. A: Dadoing Jig

1. Cut a 3/8" by 3/8" dado after clamping the stop blocks in position and installing Piece A with both spacers to the left.

2. Cut a second dado in Piece A after rotating it one-quarter turn toward the dado set and re-installing it with one spacer at each end.

3. Use the opposite side of the jig to cut 3/4" wide dadoes in Pieces B and C. Cut the first dado, rotate each piece one-quarter turn toward the dado set, and then cut a second dado. Make these cuts with one spacer installed at each end.

Fig. B: Dimensions

The Six Piece Burr

By all accounts, this burr is the most well known, because the six pieces can be notched in so many different ways and still assemble to create the same form.

Make the pieces

1. Cut 2-1/2" long blocks from square 3/4" stock, including extras for test cuts (Fig. C, above).

2. Set up the saw and the jig to cut 3/8" by 3/4" dadoes.

3. Clamp the jig's right stop block 1-1/4" from the edge of the 3/4" slot.

4. Snug a 3/8" spacer against the right stop, followed by a test piece and the remaining 3/8" spacer. Butt and clamp the left stop block against the spacer.

5. Cut dadoes in a couple test pieces. Fit them together to check the dadoes’ width and depth, and make necessary adjustments.

6. Set aside Piece A; it's already done.

7. To complete Piece B, install it in the jig with a spacer at each end and cut a dado.

8. Repeat Step 7, using Pieces C, D, E and F. 9. To complete Piece C, rotate it one-quarter turn toward the dado set. Install it with both spacers to the left and cut a second dado.

9. Repeat Step 8, using Pieces D, E and F. Complete these pieces by moving both spacers to the right and cutting a third dado.

Assemble the puzzle

1.Connect Pieces C and D.

2. Drop in Piece E from the top.

3. Slide in Piece F from the front.

4. Slide in Piece B from the side.

5. Slide in Piece A from the front.

Fig. C: Dimensions

The Twelve Piece Burr

A good nickname for this puzzle is "The Intimidator," because all twelve pieces are identical, and taking the puzzle apart is as confounding as putting it together.

Make the pieces

1. Cut 4-1/2" long blocks from square 3/4" stock, including extras for test cuts (Fig D, above).

2. Make a pair of 3/4" x 3/4" x 7/8" spacers.

3. Set up the saw and the jig to cut 3/8" by 3/4" dadoes.

4. Clamp the jig's right stop block 1-7/8" from the edge of the 3/4" slot.

5. Snug a 3/4" spacer against the right stop, followed by a test piece and the other 3/4" spacer. Butt and clamp the left stop block against the spacer.

6. Cut dadoes in a couple test pieces. Fit them together to check the dadoes’ width and depth, and make necessary adjustments.

7. Cut this dado in all twelve pieces.

8. Flip the piece end for end and reinstall it between the two spacers. Cut a second dado in all twelve pieces. Both dadoes should be in the same face.

9. Rotate the piece one-quarter turn toward the dado set and reinstall it with both 3/4" spacers on the left. Cut this dado in all twelve pieces. This last cut creates a tab, which can be used to help assemble the puzzle.

Assemble the puzzle

1. Assemble four pieces to form a tic-tac-toe grid. Orient two vertical pieces with their tabs on the right and facing to the front. Then install the two horizontal pieces with their tabs on the inside and facing to the back. A rectangular space should appear in the center.

2. Install the next two pieces with their tabs on the left and facing up. Slide in one piece from the left side, until it locks around the vertical piece. Slide the second piece halfway through the rectangular space from the front. Then move it to the right, to lock around the other vertical piece.

3. Install the next two pieces with their tabs on the inside and facing down. Slide them down from above and lock them in place. You now have two intersecting tic-tac-toe grids.

4. Slide the horizontal tic-tac-toe grid to the left.

5. Install the last four pieces. Orient the two vertical pieces with their tabs on the outside and facing to the right. Slide in one from the front and one from the back. Lock them in place. Orient the two horizontal pieces with their tabs on the outside and facing to the left. Install one of these pieces from the top, and one from the bottom. You may have to hold onto the bottom piece during the next step. 6. Complete the puzzle by sliding the horizontal and vertical assemblies together.

Source

(Note: Product availability and costs are subject to change since original publication date.)

Woodcraft, woodcraft.com, 800-225-1153, Toggle Clamp, #143938.

This story originally appeared in American Woodworker June/July 2009, issue #142.

June/July 2009, issue #142

Purchase this back issue.

Fig. D: Dimensions