OT Math Trick

[Dave Crabbs]

Hi All,
I was taught this math trick to divide odd whole number(not odd looking:p but 1,3,5 etc.) fractions in half and thought I would pass it on to you. Say you have a board and it is 15 3/8" long and you want to put a knob in the middle. Here is the trick take the whole number and divide it in half and drop the fraction then, add the numerator and the denominator together and multiply the denominator by 2. Like this 15 divided by 2= 7 1/2 drop the 1/2, 3+8=11, and 8*2=16 or 7 11/16".
Clear as mud...What other math tricks do you use to help you in your shop?
Dave

[Glenn Clabo]

Here's how I find the middle of things...
I take a piece of paper...
cut it to length...
fold it in half.
Saves what's left of the braincells.
Never measure unless you have to!

[Dennis McDonaugh]

Dave, I use a tape measure that tells me what half the measurement is, otherwise I'd never figure it out.

"Like this 15 divided by 2= 7 1/2 drop the 1/2, 3+8=11, and 8*2=16 or 7 11/16".


Where'd you get the 3+8 in the example?

[Dave Hammelef]

or 15 3/8 half of 15 is 7 1/2 and half of 3/8 is 3/16 (Just double the bottom number). So thats 7 1/2 + 3/16 = 7 11/16

So the 3 + 8 came from the 3/16 plus 8/16 (or 1/2)

[Steve Jenkins]

lay a ruler diagonally across the board so it measures something that is easily divided by the number of divisions you want. For instance if you want to divide a board into four equal divisions it doesn't mater how wide it is just lay your ruler at an angle so o is on one edge of the board and a nice even number is on the other edge ,lets say 12, then your four equal divisions will be at 3,6,9. Steve

[Ian Barley]

Even easier 'cos there is no math involved. Take a rule (or tape) And select a measurement which is wider than the board you want to divide and also easy to halve (10,12,20...). Lay the measure across the board so that the 0 end of the rule and the selected number lie directly on the edges of the board. Then just place your mark at the halfway point on the rule.

Example - You have a board which is 9 13/64ths. Take a 12 inch rule and lay it diagonally putting the 0 mark on the bottom edge of the board and the 12 on the opposite edge of the board. The 6 inch mark on that rule is the center of the board. If you wanna divide it into three equal portions use the 4 inch mark etc.

Trust me - its a lot easier to do than explain and its foolproof - this fool has thoroughly tested it!

Ian

[Phil Phelps]

If it involves numbers, count me out.:eek:

[Howard Barlow]

Just show me the picture, I don't wanna think. :D

[Dennis Peacock]

Math? I don't do math.........I can only do "cipherin' "......I can do Cipherin' and my times-es and my ga-zin-ta's..........you know......2 ga-zin-ta 4 two times?

Math is the eact reason why I have been working on compound ANGLES for these #$*(%&*#$& stools I'm trying to make for over a week now.............................I just <b>knew</b> I should have paid more attention in school............

[Todd Burch]

My Grandpa taught me this when I was 10.

You want to find the middle of a board?

Draw two diagonals from opposite corners through the center. Where the intersect, that's the middle.


Todd.


P.S. Right now, we need to start a repository of this type of information. Who gets the honors? Keith?

[Dave Crabbs]

Dennis,
"Like this 15 divided by 2= 7 1/2 drop the 1/2, 3+8=11, and 8*2=16 or 7 11/16".
The 3+8 is the fraction 3/8 [numerator+denominator]
15 3/8 15 divided by 2 = 7 1/2 forget the fration(1/2) and keep the whole number(7)
Now add the 3/8 fraction together 3+8=11 (11 is the answer for the top number 11/?)
Now double the bottom number of the 3/8 fration or 8x2=16 (16 is the answer for the bottom number or ?/16 or 11/16)

Like I said this only works for mix frations with a odd whole number. This is easier for me to do in what is left of a mine then trying to convert the fration to lowest terms. This way does it for you.
I guess it would be easier for me to show how to do it in person then explain it.
Dave

[Steve K]

Originally posted by Dave Hammelef
or 15 3/8 half of 15 is 7 1/2 and half of 3/8 is 3/16 (Just double the bottom number). So thats 7 1/2 + 3/16 = 7 11/16

So the 3 + 8 came from the 3/16 plus 8/16 (or 1/2)

Exactly, quick and easy

Steve (http://www2.fwi.com/~krumy)

[Kirk (KC) Constable]

Hmmm. I had to read that three times to figure it out. :D

For the example given, I'd divide 15-1/2 by 2 and then take another 1/16 away...or divide 15 by 2 then add in 3/16. Going from 15 to 14 to start with seems like adding another step to me.

But then again I've never had too much trouble with fractions. I can't remember how I was taught, but it's obvious there are several ways to get the right answer. I'd hate to be a teacher!

KC

[Ken Frantz]

When I had my shop a fella that was a good cabinetmaker often wondered how I could answer a question when he would ask me what's one-half, one-quarter, one eighth of this fraction.

So I told him one day just multiply the bottom number by what you what you wanted it to be divide by.

As an example:

1/2 of 1/2----multiply the bottom number by 2 which = 1/4

1/4 of 1/2----multiply the bottom number by 4 which = 1/8

1/8 of 1/2----multiply the bottom number by 8 which = 1/16

Then to double the fraction, multiply the top number by how many time you want to increase the size.

As an example:

1/2 doubled = 2/2 = 1

11/16 doubled = 22/16 = 11/8 = 1 3/8

11/16 X 5 = 55/16 = 4 7/16

The most confused I think I have gotten on fractions was a time I was using a cut sheet to cut out some face frames for cabinets that was written by another fella. I was going along and cutting---lets say styles or rails-- 22 1/2, 26 1/8, 36 1/4 and so on and came acrossed the measurement of 41 12/16!!!!!! Now in my mind I could not figure out that measurement. 12/16 was not there in my head anywhere and the brain couldn't relate to the 12/16. It took about a couple of seconds to figure out the fraction hasn't been reduced down to 3/4 which the brain could relate to.
Boy did that make myself look stupid to my self!!!!!!!!!!

So when you do fractions, reduce them down to the lowest fraction!!!!!!!!!!!

THAT'S MY STORY AND I AM STICKING WITH IT!!!!!!!!!!!!!!!

RUSTYNAIL

[Dennis Peacock]

Actually this is a very good thread.......I would like to see more about how folks handle math in the shop....ya know....their tricks and tips of math solutions in the shop? ;)

Com'on folks.....get to posting you savy ways of math solutions.

[Steve K]

Originally posted by Dennis Peacock
Actually this is a very good thread.......I would like to see more about how folks handle math in the shop....ya know....their tricks and tips of math solutions in the shop? ;)

Com'on folks.....get to posting you savy ways of math solutions.

Okay, but remember, you asked for it. Hre's one;
When you're figuring for miters, a 45° miter is as long, short point to long point as the stock is wide. So, if you're mitering for a piece 10" wide with 2" stock, you need at least a 14" long piece to do one 10" side. If you're doing a picture frame, don't include the rabbet in the equation.


http://www2.fwi.com/~krumy/wwhat/COMBO.GIF


Steve (http://www2.fwi.com/~krumy)

[Jason Roehl]

A trick for squaring a number that ends in 5.

For example, 25x25=

Take the 2, multiply it by the next number (3 in this case), which equals 6. Now stick 25 on the end, so 25x25=625. This even works for much larger numbers...

105x105=

(10x11=110) tack on the 25 and

105x105=11025

Aw, heck, I could go on for ages with quick math tricks 'cause I liked math and had a few good teachers over the years who encouraged my ability to do math in my head (but still no competition for RainMan).

My favorite teacher in 7th grade had us learn "the number line." He would give us a quiz on it occasionally. Basically, it was 22 points between 0 and 1, and we had to give the fraction, decimal and percentage.

.1, 10%, 1/10
.125, 12.5%, 1/8
.1667, 16.67%, 1/6

...and so on. If I had to, I probably could still remember what all 22 of the points were.

Jason

[Aaron Koehl]

<b>To center an object B on object A:</b>

Take the length of A minus length of B. Half of this measurement
is the distance the edge of B should be from the edge of A.

(A-B) / 2.

_Aaron_

[Ken Frantz]

Here is another trick I have used in the shop:

First---- guess about where the center of the board is and make a mark at this point.

Second-----measure to this mark and remember this measurement. Then measure from the other end of the board with the measurenent you were supose to remember from the first measurement and make a mark.

Now divide the space between the two marks and that will be the center.

If you have guest close enough and the marks are within an inch or two, it is easy to divide an inch+- measurement than a larger measurement like 45 1/2.

Clear as mud? I hope not

RUSTYNAIL

PS: It's sure easier to show someome this trick than put it into words!!!!!!!!!!!!!!!!!

[Ken Frantz]

Once again I have some tips that I used in the cabinet shop. This is more for a production shop, but the math gymnastic can still be applied. The good Lord put a head on our shoulders, so we
need to exercise it ever so often.

Better pull up a chair and prop up your feet—this is a long post!!!!!!!!!!!!!!!!!!!

So first I have to set the “stage”, so you will have an idea what I am trying to get acrossed.

All most all of our cabinets were built to be covered with VT (1/32) vertical surfacing mica which is half the thickness of STD ( 1/16) which we used for counter tops.

The material we used was ½ inch thick (plywood or particle board or what ever) for the boxes and the height of the standard upper cabinets were sometimes 12 or 15"" for refrigerators, 15" or
18" for hood cabinets and 30", 32", 42" or what ever for regular storage cabinets above the counter top. The ends were cut to the height of the cabinets and the top, bottom and sometimes
the shelves were also fixed.

First the basic makeup of the cabinets using ½ material!!!!:

1) Wall to wall cabinets 2) Wall to finished end cabinets 3) Cabinets finished on both ends.

On the wall end of a cabinet there was a 1 inch scribe on the face frame to the cabinet side so the cabinet could be fitted (scribed) to the wall if it was not plum or straight.

Now is where the math gymnastic comes in.

This one fella that I mention before ask me one time how to cut out a set of cabinets since he did almost all of the building of the boxes and really never thought of the cut out process on figuring the length of the different parts. So in about 20 seconds he knew how to cut out a set of cabinets.

I told him all he had to remember 1 2 3 Here is how it works:

Wall to wall cabinets: The floor, top, and fixed shelf (if used) were face frame length -1 inch for scribe on one end -1 inch for the two ½ inch ends and another -1 inch for the other end of scribe making a total of 3 inches total. The bottom, top and fixed shelf
( for lets say a 33 1/4 cabinet on the face) would be 33 1/4 less the 3 inches would equal 30 1/4 inches for these parts. The 1/8 or 1/4 back would be 1 inch more than the cut bottom etc since the back was applied to the length of the total box (31 1/4).

Wall to finished end cabinets: Using the same cabinet—33 1/4— the top, bottom etc would be 2 inches less than the face size or 31 1/4 and the back would be again 1 inch longer than the cut
bottom .(32 1/4)

Finished to finished cabinets: Again using the 33 1/4 inch cabinet, the top, bottom, etc would be 1 inch less than the face which would 32 1/4 for the bottom, top etc and 33 1/4 for the back.

I wrote this so you could see how to do things the easy way and think a project through without having to figure each piece of a project. In fact I started to build a jig to use on the RAS for each of the three basic cabinet so I could set a stepped stop and put all of the material on the saw and cut it at one time.

So think about some of the post above, because they wii help you out a lot!!!

Sorry about twisting your ear so much. It will feel better after while!! HI HI HI I hope this ditty was clearer than mud to you.

RUSTYNAIL

PS: e-mail me if you have any question.

Since some of you have quote at the bottom of your post, I have one I like from a southern comedian by the name of Dave Gardner fom the late 59and early 60's times. here it tiz:

"IF IT WASN'T FER THOMAS EDISON, WE WOULD ALL BE WATCHING TV BY CANDLE LITE"

[Todd Burch]

OK, I'll jump back in. Here are a couple quickies.

Is a number evenly divisible by 3? Just add the digits:
2580? 2+5+8+0=15, 1+5=6, answer is yes, as 3 goes into 6.

Same way with 9:
72? 7+2=9, yes.
2577? 2+5+7+7=21, 2+1=3, answer is no.

Todd.

[Jason Roehl]

Originally posted by Todd Burch
OK, I'll jump back in. Here are a couple quickies.

Is a number evenly divisible by 3? Just add the digits:
2580? 2+5+8+0=15, 1+5=6, answer is yes, as 3 goes into 6.

Same way with 9:
72? 7+2=9, yes.
2577? 2+5+7+7=21, 2+1=3, answer is no.

Todd.

Don't forget 6--if it's divisible by 3 (see above) and the original number is even, then it's divisible by 6. Thus, 2580 is also divisible by 6.

Now if I could just remember the trick to see if a number is divisible by 7...
(It does exist).

Jason