A number of customers have indicated they like the idea of using quality German chisels but are turned off by the idea of having to work with metric sizes. We put this little tutorial and chart together to help those folks. You may not only find this information useful when working with these fine cutting tools, but you might find it handy with general woodworking that involves metric measurements.
~For starters, remember that, when working with Imperial measurements, you break an inch in half, then the half in half, and so on until you get down to 1/32 inch. Moving on...
~There are 26 millimeters in an inch. In other words, a millimeter is 1/26th of an inch. Unfortunately, you can only halve 26 millimeters one time without dividing them. Trust me...dividing a millimeter is literally like splitting hairs! HOWEVER, we now have two conversions. One inch = 26 millimeters and one-half inch = 13 millimeters.
~So why not round off fractions of a millimeter and assign numbers to standard Imperial fractions? After all, the differences here will be miniscule. So, let's build a chart:
IMPERIAL FRACTION
METRIC EQUIVILANT
1/8 in. 1/4 in. 3/8 in. 1/2 in. 5/8 in. 3/4 in. 1 in.
3 mm. 6 mm. 10 mm. 13 mm.* 16 mm. 19 mm.* 26 mm.*
*Two Cherries uses 12 mm for 1/2 in and 20 mm for 3/4 inch. Remember, we are working with measurements literally less than a hair's width!
~SMALLER THAN 1/8 INCH: Two Cherries also offers a 2 mm chisel. These are equivalent to 1/10 in and are handy for clearing the mouth of shop made side escapement planes.
~LARGER THAN AN INCH: This is relatively simple. Simply add whatever number applies on the chart above to 26 mm. Here again, Two Cherries uses a slightly different number on occasion. For instance, a two-inch chisel is 50 mm.
~Also keep in mind the fact that the metric system is divided by ten. This means you can split the difference between two metric equivalences and come up with a pretty close Imperial equivalent. For example: There is 4 mm difference between a 6 mm (1/4 in) and 10 mm (3/8 in) chisel. This means that a chisel half way between these two, say an 8 or 9 mm would be 5/16 in.
~Still squeamish about converting from metric to Imperial? Keep in mind that when you get to tolerances of around 1/26 in, the wood you are working with probably moves far more than that seasonally!
It's surprising how quickly you will learn to convert these numbers on the fly once you get comfortable with them!
