A simple method for finding center on multiple objects of different sizes w/o math

[liam c murphy]

I'm posting this in hand tools because I think it suits the methods of handwork over power tools. This is probably common knowledge for the experienced folks. But, I figured it might help out the folks who are just getting started. I'll show this method in the context of building a table apron. I'm sure it is useful in other contexts as well. The purpose of the post is to show how to find center on multiple objects of various lengths, quickly, and without arithmetic. The underlying idea is that it is easier to find center over a shorter distance, than a longer one. In context, the benefit is that, you can find center without knowing the exact length of the block. When I build my aprons, I don't worry about making all the "diagonal blocks" the exact same size. As long as their lengths are within 1/4", they are close enough for my purposes. However, I still need to find center. In this case, I have many blocks to make, and I don't want to do the math to figure out where center is on each block.

I start by setting my combo square to slightly less than where I think center will be. I make a mark.

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Then, I do the same thing on the other side.
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Next, I mark where I think center is. At this point it is pretty easy because the distance is so small.
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Finally I draw a line through approximate center.
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If the line is the same distance from both edges, then you've marked accurately. Usually, I don't check. I've found this to be a reasonably effective way of finding center on objects that I don't want to measure.

Finally, here is a shot of a finished table and bench. Many hand tools were used to make these.
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[Frederick Skelly]

Nice furniture! Welcome to SMC Liam. Are you Stateside or overseas?
Fred

[Brent Cutshall]

Thank you for asking this question. I needed an answer for it to. Sorry about on being able to help you. Oh, and welcome to the forum. Beat you to it Jim!:)

[Patrick Chase]

The sides of the object have to be parallel for that to work (or else the combo square will be tilted differently when coming in from each side, and that kills accuracy), and if they're parallel then there's a faster technique that delivers similar accuracy:

1. Take a ruler, and pick the next major division (inch, half-inch, whatever) up from the size of the surface you want to divide
2. Lay the ruler diagonally across the surface so that the interval you chose in (1) is filled.
3. Mark center.

As a concrete example, if you had a surface that was about 4.5" wide then you might choose 5" in step (1). You them lay the ruler across the surface such that 0 is at one edge and 5" at the other, and you mark center at 2.5. Done.

Note that the "diagonal trick" works equally well for divisions other than half. If I wanted to divide by thirds in the example above then I might choose 5.25" in step (1), and mark out at 1.75" and 3.5" in step (2). I guess that's technically math, but it's the sort that takes a fraction of a second to do mentally.

This is a very, very, very old trick BTW.

[Mike Henderson]

There's also center finding rules, which have two scales: One that is normal inches and one that is one half the normal side. You measure using the normal side then find the measurement on the second side. So if you measured something and it was 7 inches, you just find 7 on the second scale and you're in the middle.

The other approach is to use a set of dividers and just step over the space, adjusting until you make the distance in two steps. You can hit the exact middle with very few adjustments.

Mike

[Derek Cohen]

Thanks Liam ... and welcome to the forum.

What you demonstrate can also be done with a cutting/marking gauge on the side of a stretcher to approximate the mid point. Similarly, as both Patrick and Mike describe, there are other ways to do the deed. It is relevant to note that, depending on the task, these centre points may not need to be perfectly accurate ... as long as one is working consistently from a reference side.

A few years ago I posted a little gauge I make for centering a pencil or awl point (via the centre hole), or for centering a chisel to mark the position of a mortice. I have copied-and-posted the article fro my website (http://www.inthewoodshop.com/ShopMadeTools/MorticeMarkingGauge.html) for the interest of others ...

The other day I received an email inviting me to purchase the latest one-time tool from Woodpeckers, a mortice and tenon centre gauge ..

http://www.inthewoodshop.com/ShopMadeTools/MorticeMarkingGauge_html_4ad19d58.jpg

My first thought was that this was brilliant - what a great, quick way to mark a mortice (or tenon) in the centre of a stretcher. Then I looked at the price ($150), and that one had to pre-order now for 5 months time. I put away the credit card.

I still think that it is a terrific idea, and I must admit that the concept haunted me for some days. Today - Sunday - I decided to build my own version. So I looked around the workshop for something to cannibalise.

I found an old dovetail marker that had never worked and had been tossed into a drawer ...

http://www.inthewoodshop.com/ShopMadeTools/MorticeMarkingGauge_html_m7041ad15.jpg

This is the centre gauge I built ...

http://www.inthewoodshop.com/ShopMadeTools/MorticeMarkingGauge_html_668de7ae.jpg

It slips over the stretcher this way ...

http://www.inthewoodshop.com/ShopMadeTools/MorticeMarkingGauge_html_m2bf4fdfc.jpg

Marking out for a 3/4" wide stretcher calls for a 1/4" wide mortice. This one will be 2" long.

Step #1 is to mark off the 2" length (with room to avoid blowing out the end) ...

http://www.inthewoodshop.com/ShopMadeTools/MorticeMarkingGauge_html_55a5011e.jpg

Now place the centre gauge between the two end lines (anywhere, it is not critical). Set the 1/4" mortice chisel against the centre, eyeballing it square ...

http://www.inthewoodshop.com/ShopMadeTools/MorticeMarkingGauge_html_m7ffbf4d0.jpg

Press done hard so that you leave a clear imprint of the chisel ...

http://www.inthewoodshop.com/ShopMadeTools/MorticeMarkingGauge_html_f09b54e.jpg

Now you have a choice - either use two single blade cutting gauges to mark each line individually (you will need to set these aside for the tenon) ...

http://www.inthewoodshop.com/ShopMadeTools/MorticeMarkingGauge_html_1357d380.jpg

... or use a mortice gauge, aligning the blades with the outer edge of the imprint ...

http://www.inthewoodshop.com/ShopMadeTools/MorticeMarkingGauge_html_m64acb938.jpg

It has never been critical to centre the mortice, but this method does speed up the process (even though this is all a bit tongue-in-the-cheek). Is it accurately centred? I did three or four. None were absolutely perfectly centred. A little error creeps in. It does not matter. As long as you use the same gauge(s) (from the initial set up) and score from the reference side, each mortice/tenon will be marked at the same distance from the sides.

Regards from Perth

Derek

(written in November 2013)

[Pat Barry]

The sides of the object have to be parallel for that to work (or else the combo square will be tilted differently when coming in from each side, and that kills accuracy), and if they're parallel then there's a faster technique that delivers similar accuracy:

1. Take a ruler, and pick the next major division (inch, half-inch, whatever) up from the size of the surface you want to divide
2. Lay the ruler diagonally across the surface so that the interval you chose in (1) is filled.
3. Mark center.

As a concrete example, if you had a surface that was about 4.5" wide then you might choose 5" in step (1). You them lay the ruler across the surface such that 0 is at one edge and 5" at the other, and you mark center at 2.5. Done.

Note that the "diagonal trick" works equally well for divisions other than half. If I wanted to divide by thirds in the example above then I might choose 5.25" in step (1), and mark out at 1.75" and 3.5" in step (2). I guess that's technically math, but it's the sort that takes a fraction of a second to do mentally.

This is a very, very, very old trick BTW.
One worth remembering too!

[Jim Koepke]

Welcome Liam, my only excuse for lateness is being asleep.

This is an old method but good to bring up for anyone who hasn't seen it before.

I have used this method many times with my Odd Jobs. It also works with a marking gauge.

For bigger surfaces a four fold ruler is good for finding centers.

I also have a ruler that was made in a print shop where I used to work. It is 2" long with a zero in the center and the measurements going in both directions from there. It was handy in the print shop, but I do not use it much these days.

jtk

[Bill White]

I've used this method many times, and Derek makes stuff that simplifies.
Derek, do you just sit around all day figuring stuff? :)
Bill

[Derek Cohen]

I've used this method many times, and Derek makes stuff that simplifies.
Derek, do you just sit around all day figuring stuff? :)
Bill

Bill, my patients think that my furrowed brow indicates that I am concentrating on what they are saying. :)

Regards from Perth

Derek

[Jim Koepke]

Bill, my patients think that my furrowed brow indicates that I am concentrating on what they are saying. :)

Regards from Perth

Derek

Do they ever look at your note pad aghast at the Rube Goldberg like drawings? :eek:

(just kidding of course)

jtk