KeepItSimpleStupid made a comment in another thread about having to look these things up in a table. I wrote up a response describing how I do it; then I thought it would be of more general interest in its own thread rather than buried in another thread.
Example: you want the decimal equivalent of 27/64". That's 1/64th below 28/64 = 14/32 = 7/16. That's 1/16 below 1/2. Thus, doing the problem in mils (0.001"), the answer is 500 - 62 - 16 = 500 - 78 = 422. Thus, you get 0.422".
After a while you come to know the 16ths too; thus, since this is 1/64th below 7/16, you just say "438", "428", "422" in your head. This method will usually get you within a thousandth or so of the true value.
Businesses like Cleveland Twist Drill used to sell handy little circular "slide rules" that would do fraction and number drill conversions along with cutting speed calculations (see attached photo) -- see if anything similar is sold at a local industrial supplier (I've had mine since the 60's). This is handy to keep at a machine. If you can program in e.g. Postscript, such things are not hard to make. Or, find one of the old Gilson circular slide rules, as they had nice fraction to decimal conversions on them. Or print out scale.pdf from **broken link removed** (has other useful stuff on it too).
Memorize that 1/16" = 0.062, 1/32" = 0.031, 1/64" = 0.016. Memorize the decimal equivalents of all the eighths. Then start from a number you know and make corrections.I didn't mind mills and thousandths when machining things, but I kept having to look at the pesky table for fractions to decimal equivalents.
Example: you want the decimal equivalent of 27/64". That's 1/64th below 28/64 = 14/32 = 7/16. That's 1/16 below 1/2. Thus, doing the problem in mils (0.001"), the answer is 500 - 62 - 16 = 500 - 78 = 422. Thus, you get 0.422".
After a while you come to know the 16ths too; thus, since this is 1/64th below 7/16, you just say "438", "428", "422" in your head. This method will usually get you within a thousandth or so of the true value.
Businesses like Cleveland Twist Drill used to sell handy little circular "slide rules" that would do fraction and number drill conversions along with cutting speed calculations (see attached photo) -- see if anything similar is sold at a local industrial supplier (I've had mine since the 60's). This is handy to keep at a machine. If you can program in e.g. Postscript, such things are not hard to make. Or, find one of the old Gilson circular slide rules, as they had nice fraction to decimal conversions on them. Or print out scale.pdf from **broken link removed** (has other useful stuff on it too).