STOP! Using Fractions

If you have one of those dial calipers that readout in fractions…..THROUGH IT OUT! These are a crutch that will keep you from learning how to use decimals in your woodworking.

Fractions are fine for 3rd grade arithmatic, but they don’t work for furniture building. The smallest common fraction that we use is 1/64″. That fractionĀ translates to 0.015625″. This is way too much for furniture joinery. If you try to fit a mortise and tenon or a dovetail joint to a tolerance of plus or minus 1/64″ you are going to end up with a piece of scrap. We must use decimals in our woodworking. A fraction is nothing more than an unfinished arithmatic example, 1/4 = 1 divided by 4 = .25, or 3/16 = 3 divided by 16 = .1875.

So let’s learn decimals. The standard increment in industry and engineering today is one thousandth of an inch. The thousandth is expressrd 0.001″. In industry we commonly work to three decimal places. The first decimal place to the right of the decimal point is hundres of thousandths. Example 0.100 = one hundred thousandths, 0.200 = two hundred thousandths, etc., etc., etc.. The second decimal place to the right of the decimal point is tens of thousandths. Example 0.010 = ten thousandths, 0.020 = twenty thousandths and so on. And of course we already know that the third decimal place to the right of the decimal point is thousandths. Therefore, 1/8 = 1 divided by 8 = 0.125 or one hundred twenty five thousandths, 3/16 = 3 divided by 16 = 0.1875 rounded to 0.188 or one hundred eighty eight thousandths. As you can see it is not difficult. We learned the alphabet, and the multiplication tables we surely can learn the decimal equivilents.

Keep your calculator as close as your measuring tools and use it to finish those unfinished arithmatic examples. Before you start a project go over the drawing and put the decimal equivilents beside the fractions and work with them, not fractions. If you do your own design write your dimensions as decimals. It may be awkward at first, but in time you will beĀ converting most of the fractions to decimals from memory. Lose the fractions and take up their decimal equivilents and you will be a more accurate woodworker.

About these ads


Professional furniture maker and restorer. Dealer and collector of vintage and antique woodworking tools.
This entry was posted in Measuring & Layout. Bookmark the permalink.

10 Responses to STOP! Using Fractions

  1. Lex Terrae says:

    Why not know and understand BOTH decimals and fractions? Ain’t too hard!

    I noticed that you criticized fitting a tenon in a mortise at a tolerance of1/64th, but you didn’t tell us what decimal value would be appropriate. Does it really matter? Isn’t it more a matter of “fit and feel” than measurements?

    It’s your blog and your opinion but I see your position as being only an opinion. Proselytizing if OK, but don’t expect everyone to accept your choices.

    • Once you learn and become comfortable with decimals your mind will automatically convert fractions to their decimal equivilant. Fractions are fine for framing and carpentry work, but they are too course for fine furniture.

      My purpose is not to teach how to fit a joint, but to point out that fractions are too course for fine joinery.

      Finally, you are right. The above article is my opinion. An opinion based on 47+ years in the machine trades.

  2. Eric says:

    Hi. I’m really enjoying reading your blog. You have a lot of good information. I agree with you regarding the decimals. It’s what I use in my woodshop. I have a question about your description. You say “Example 0.100 = one hundred thousandths”. Isn’t 0.1 = one tenth? Wouldn’t 0.001 = one hundred thousandth? Or am I backwards?

    • Eric, thank you for interest in my blog. In elementary school arithmatic the number to the right of the decimal is tenths, but in the machine trades it is hundreds of thousandths. Thus 1/8 0.125 would be one hundred twenty five thousandths. The second number to the right of the decimal is tens of thousandths. Thus 1/16 would be 0.0625 or sixty three thousandths rounded up. The third number to the right of the decimal is thousandths in either system, but in the machine trades it is the basis for the system. If I haven’t cleared this up please ask more questions. I’m trying to pass on the knowledge I have so others can use it and benefit from it as I have.

  3. Eric says:

    I read this on another machinist website ‘”The shop trades have dropped the 10th and 100th divisions of an inch and refer to everything in thousandths of an inch.” Now it’s all very clear. And in the example I cited above, one-hundred-thousandth is the same quantity as one-tenth, just expressed in different nomenclature. So it all makes prefect sense to me now. And now I can talk to machinists in their lingo!

    • Not only that but you now have a much clearer picture of fractions. A fraction is nothing more than an unfinished arithmatic example that when finished yields a decimal. Now you have learned “machinist language” for decimals you can easily work with decimals and talk to others and everyone will know what you are talking about. Decimals make working with fractions of an inch easy. Thanks for your interest. I hope you learned something useful.

  4. lman1000 says:

    Have you ever found a drill set labeled in thousandths? Every one I’ve ever seen has been labeled in fractions or millimeters. Even the mills in the machine shop are labeled in fractions. I’m all for thousandths, but I constantly need to switch my measurements in thousandths over to fractions to match up my tools. I like inches, but I don’t like fractions.

    • Most drill indexes label the bits by fractions and decimals. When we were in school we learned the multiplication tables. The decimal equivalent of fractions is pretty much the same. If you do the math (1/4=1 divided by 4 = 0.250) the conversion is easy and after a time you will have the more common ones memorized.

  5. You can definitely see your enthusiasm within the work you write.
    The world hopes forr even more passionate writers like you who aren’t afraid to say how they believe.
    Always follow your heart.

    • Thank you for you kind words. My main goal for this blog is to keep as much information on the finding, cleaning and restoration, and use of old tools as I can. This would help the next generation to know the value of these old tools.

      There is a lot to be said for modern technology, but there will always be a place for our heritage.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s