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How to use dividers

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Forum topic by wombataholic posted 09-17-2012 01:18 AM 3804 views 3 times favorited 20 replies Add to Favorites Watch
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wombataholic

3 posts in 799 days


09-17-2012 01:18 AM

I was reading through the Anarchist’s Tool Chest this morning and I don’t understand the use of dividers. In the book, the example given is dividing a 43.238475758” long board into 7 sections with 1/2” in between each section. I understand starting half an inch in from the end, then marking out the 7 sections, then going to the other end and doing the same, but how do you determine how wide each section is when using dividers? The steps right now read to me:

1. Start with a mark 1/2” in from the edge.
2. ???
3. Mark off 7 equal sections.
4. Repeat from other edge.

What am I missing in step 2? Thanks!


20 replies so far

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Roadster280

23 posts in 804 days


#1 posted 09-17-2012 01:41 AM

Er..divide…

43.238475758 – (8×0.5) = 41.238475758

41.238475758 / 7 = 5.89121082257143 (or 5 57/64” for government work). Plus the half inch spacing.

Set your dividers to 6 25/64”. How you do that of course is another matter, especially when the dividers will not be perpendicular with the plane of the rule. Equally difficult is sticking the point of the divider in the far edges of the piece.

Personally, I’d go with 6” and cut the rest off, waste pieces at each end.

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JJohnston

1585 posts in 1976 days


#2 posted 09-17-2012 02:01 AM

If the distance to be divided is smal, eyeball 1/7 of the distance and set your dividers there; otherwise, do the rough division (only subtract the half inch once): a little less than 43” divided by 7 is a little more than 6”, so set the dividers to 6” or so. Then, step off 7 segments. Unless you’re really lucky, you won’t hit right at the end of the board; you’ll have some error. Adjust the dividers an amount equal to 1/number of segments x the error, or in this case, 1/7 of the error (eyeball this). If you’re short, open them up. If you’re long, close them. Then try again. Do this a couple of times until you can’t get any closer, and that’s it.

-- "Sometimes even now, when I'm feeling lonely and beat, I drift back in time, and I find my feet...Down on Main Street." - Bob Seger

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Sylvain

566 posts in 1184 days


#3 posted 09-18-2012 12:34 PM

I don’t have Schwarz ’s book.
Are you sure the goal is to have exactly 1/2” between each section?

Normally the goal is to have the spaces between the sections of equal width and regularly distributed in order to layout the pin’s width for a dovetail assembly.

The exact space width (being anything, let’s say, between 3/4” and 1/8”) is not important as long as it suits you (and you use the same on the four corners of the chest).

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

View Tedstor's profile

Tedstor

1369 posts in 1318 days


#4 posted 09-18-2012 04:51 PM

I bought a pair of dividers and attempted to use them. I quickly found myself thinking “whats easier/better about this over a tape measure”?
Pretty much on par with chopsticks. Isn’t a spoon or fork an easier way to get it done?
And I can’t use chopsticks either.

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MrRon

2858 posts in 1928 days


#5 posted 09-18-2012 05:52 PM

Why are you starting at 1/2” from the end? The board is 42.238475758 long. There are (6) 1/2” spaces. Subtract (6 x .5) from the total length =39.23847575 and divide by 7. Answer: 5.605796536. Round to the nearest 32nd = 5-19/32. Mark the first point from the edge. Add 1/2” = 6.105796536. = 6-3/32. Step off that dimension from the starting point. Return to start and mark off starting from the point previously marked (1/2” from the edge).

View CplSteel's profile

CplSteel

142 posts in 849 days


#6 posted 09-18-2012 05:55 PM

I don’t clearly understand what the book is asking you to do, but there is no step two. The point of dividers is that you guess at the right width, walk them out, and adjust the width, in your case 1/7th of the error. Then do it again. You may have to adjust one or two more times but in the end your dividers will be set to exactly 1/7th the board. The measurment in a number is irrelevant.

It is very fast and easy.

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Gshepherd

1502 posts in 886 days


#7 posted 09-18-2012 10:20 PM

Well the principal was right. Ditchin Math class would come back to haunt me…..

-- What we do in life will Echo through Eternity........

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404 - Not Found

2544 posts in 1654 days


#8 posted 09-18-2012 11:57 PM

A case for going metric if ever I saw one.

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Sylvain

566 posts in 1184 days


#9 posted 09-19-2012 01:12 PM

Usually the goal is to layout evenly spaced pins of equal width without even knowing the length of the board and without any calculation.
You eyeball 1/6 of the length between the end marks (whatever they are)

In the example you end up with 7 equal “spaces” :
between the left 1/2” mark and mark “l”
between mark “a” and mark “k”
between mark “b” and mark “j”
....
between mark “e” and mark “g”
between mark “f” and the right 1/2” mark

Being metric or not is irrelevant. (I am metric)

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

View xylophage's profile

xylophage

70 posts in 1219 days


#10 posted 09-19-2012 01:59 PM

I thought I was going to have to set everyone strait on this until I saw Sylvain response. He is absolutely correct on how to use the dividers correctly. They are a great thing to have around in the shop. I use mine all the time. Once you get the hang of them. Its by far faster than figuring out the math. Good luck

-- D.A Winograsky

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CplSteel

142 posts in 849 days


#11 posted 09-19-2012 05:14 PM

Sylvain is probably right, and I just do not understand the aim of the measurment. For a more general example of how to use dividers, see this Underhill video where, around the 7:20 mark, Roy lays out some even spaces on a random width board. In fact, I think he lays out the entire project without measuring (by number) anything. Everything is laid out in reference to the material.

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Tedstor

1369 posts in 1318 days


#12 posted 09-19-2012 10:02 PM

OK. This thread has been an eye opener for me. Dividers suddenly make perfect sense.
Definitely easier than a tape measure, but still light years more useful than chopsticks.

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Dallas

3030 posts in 1172 days


#13 posted 09-19-2012 10:18 PM

7 equidistant marks on a wide rubber band or piece of elastic work easier.

Stretch from point A to point B and mark where the marks on the band fall.

-- Improvise.... Adapt...... Overcome!

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CplSteel

142 posts in 849 days


#14 posted 09-20-2012 12:01 AM

@Dallas, that would work well if the rubber band stretched consistently across its length, which it might, at least while new. Of course, getting the marks equidistant is important, and doing so on the smaller scale is harder and the error rate will be magnified when you stretch it out, but yeah, it will save you 45 seconds or so.

@Tedstor, chopsticks have their uses too.

@wombataholic and Jonathan – as it appears that all the project is trying to accomplish is 5 or 6 evenly spaced gaps, with 1/2” offsets on the top and bottom, then as long as the size of the gaps is irrelevant, sylvain is right on point. Just don’t change the spacing on the dividers between the right direction run and the left direction run.

View JJohnston's profile

JJohnston

1585 posts in 1976 days


#15 posted 09-20-2012 12:09 AM

Sylvain, why would you set the dividers to 1/6 of the length if you wanted 7 segments? Everything else is right, and it’s what I said up higher. I did this in AutoCAD today, if anybody wants to see essentially what Sylvain drew, but with computer neatness.

-- "Sometimes even now, when I'm feeling lonely and beat, I drift back in time, and I find my feet...Down on Main Street." - Bob Seger

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