Pieces are formed by removing unit cubes from rectilinear solid pieces.
A burr is notchable if it can be made with just straight cuts.
Some burrs have a "key" piece that slides out.
More complex ones have a number of internal voids (called holes),
where removing the first piece may require sliding several pieces.
An assembly of a burr is a solved shape.
An assembly is a solution if it can be achieved by starting with the pieces apart and making legal moves.
The level of a solution is the minimum number of moves required to remove the first piece
(or separate the puzzle into two parts).
The level of a burr is the lowest level of its solutions.
Note that to compute level, we use
where the movement of several pieces together,
or the consecutive movement of pieces in the same direction,
counts as a single "move".
Burr level can be expressed with more than one number;
e.g., 3.7.2 means 3 moves to remove the first piece,
7 moves to remove the second piece,
and 2 moves to remove the third piece.
Standard Six Piece Burrs
The most well known burr is the standard 6 piece burr,
with 2 x 2 x 6 unit pieces
(or sometimes 2 x 2 x 8).
For example, the figure above shows
Coffin's Improved Burr,
which requires 3 moves to remove the first piece
(letters show how pieces fit, numbers indicate an order in which they can be disassembled).
The number of holes in a standard 6-piece burr:
Standard 6-piece burr records, from the computer work of Bill Cutler:
- Volume of six solid pieces = 6 x 24 = 144 (or 192 for 2x2x8 pieces).
- Volume of a solid burr = 24+24+16+16+12+12 = 104 (or 152 for 2x2x8 pieces).
- Volume difference = 40.
- Holes = (total number of unit cubes removed from the six pieces) - 40.
- Highest level for unique solution with 3 holes = 7.
- Highest level for unique solution with 4 holes = 8.
- Highest level for unique solution with 5 holes = 9.
- Highest level with a unique solution (uses 7 holes) = 10.
- There are no standard 6-piece burrs of level 11.
- Highest possible level (its the only one, but has non-unique solution) = 12.
- Highest level for unique notchable (has 7 holes) = 5.
- Highest level for notchable with non-unique solution = 10.
Example Variations of 6-Piece Burrs
• Fractional or rotation moves.
• Non-rectangular cuts.
• Solutions with exposed holes.
• Ball bearing(s) inside that may have to move during solving.
• Additional moves to remove the second piece require more moves than the first.
Other Types of Burrs:
Non-standard 6-piece burrs have six pieces but don't adhere to standard construction rules.
Burrs in the theme of the standard 6-piece burrs but with more pieces can be difficult,
especially when combined with non-standard constructions.
Burrs with as few as 3 pieces can also be quite difficult
(e.g., the Cuter Level 8
Three piece "knots" fit together in a simple but non-obvious way.
Some three piece burrs require unusual twists or diagonal motions.
The basic idea of a burr is quite old.
presents a wood knot as "Cross Keys" and a 6-piece burr as "The Nut".
Slocum and Botermans New Book of Puzzles,
on page 52 discusses the
Spears Puzzle knots manufactured in Bavaria in 1910 and marketed in England;
it is also mentioned that six piece burrs appeared in Bestelmeier's 1803 Toy Catalog.
Johnson and Smith Catalog,
on pages 254-255,
shows a 6-piece burr,
a two burr stick,
and related wood puzzles.
Puzzlers' Tribute book,
on page 260 cites a 6-piece burr called the
and a 24-piece burr
Large Devil's Hoof
in a Catel's catalogue of 1785,
and credits David Singmaster's example of a 6-piece burr in a 1733 Spanish book by Pablo Minguet E. Irol;
also, on page 262 it credits the Mikado Puzzle as shown in the 1915 C. J. Felsman Catalogue:
The 1942 Filipiak book has a substantial discussion of burr puzzles;
here are figures it shows of a 3-piece wooden knot,
a 6-piece burr,
and a 6-piece burr set:
There have been many burr patents; for example,
here are the figures from the 1890 Altekruse and 1917 Brown patents:
Examples of Burr Patents
from: www.uspto.gov - patent no. 393,816
from: www.uspto.gov - patent no. 430,502
from: www.uspto.gov - patent no. 524,212
from: www.uspto.gov - patent no. 588,705
from: www.uspto.gov - patent no. 779,121
from: www.uspto.gov - patent no. 781,050
from: www.uspto.gov - patent no. 985,253
from: www.uspto.gov - patent no. 1,099,159
from: www.uspto.gov - patent no. 1,225,760
from: www.uspto.gov - patent no. 1,261,242
from: www.uspto.gov - patent no. 1,350,039
from: www.uspto.gov - patent no. 1,455,009
from: www.uspto.gov - patent no. 1,542,148
from: www.uspto.gov - patent no. 2,836,421
from: www.uspto.gov - patent no. 4,148,489
from: www.uspto.gov - patent no. 4,880,238
from: www.uspto.gov - patent no. 5,040,797
Rob's Puzzle Page,
Cutler's Holey 6PB Booklet,
Cutler's Computer Analysis,
IBM Burr Page
Math Games Page,
Wikipedia Burr Page,
Mathematische Basteleien Page,
Mr. Puzzle Page,
Source Forge Page,