Check yourself

[Curtis Niedermier]

I've been on a bit of a thread-posting tear lately, so I'm keeping it flowing here.

You ever check your measuring devices against one another for accuracy/consistency?

I learned to do this the hard way. I built a lift-out saw till for storing my back saws in my tool chest and designed it to fit pretty snug between the front of my tool chest and the two sliding tills that store most of my smaller tools. The idea was that the saw till would hold the sliding tills in place when it was installed, for a bit of extra security when I move the chest around.

While building, I cut some of the nicest dovetails that I'm capable of and left the saw till's carcass just wide enough that I would have to take a few shavings off to make it fit.

When I went to test fit it the first time, it was a strong quarter inch too wide. Hmm... A whole lot of ugly planing (more like scrubbing) later, I finally got it to slide in place. That was a lot of material to remove.

Come to find out, my old Stanley wooden rule - a 24-inch fixed model, not a folder - doesn't match up with my tape measure. The inches on the wooden rule are longer than the inches on the tape measure. I used the tape to measure the opening and the wooden rule to lay out my pieces, and thus the carcass became too wide.

How far off are the two? 1 foot on the wooden rule equals 1 foot, 1/4 inch on the tape measure.

Dadgum.

For the record, the tape measure matches the scale on my combination square...and I so love that old wooden rule. It's just like what we used in woodworking class back in high school (which might explain why my projects came out the way they did back then).

The lesson: Pull your dimensions straight from the work whenever you can. And if you can't, make sure your measuring devices measure up!

[Marty Schlosser]

Tape measures are notorious for this. Part of the problem lies with the tongue, which is so easy to have bent it isn't funny. I usually get them as close as possible, and reserve their use for only rough measurements.

As soon as I'm beyond rough cutting and milling on any project, I put the tapes away and resort to using only those measuring tools that have confirmed against my known-to-be-accurate ruler. Same goes for calipers and such other measuring tools.

[Curtis Niedermier]

Hey Marty, I'm aware of that issue with tape measures, and it's the first thing I checked. But that actually isn't my problem. If I compare any 12-inch segment on the tape to any 12-inch segment on the wooden rule, they're still 1/4 inch apart. The wooden rule's inches are simply bigger.

[Robert Hazelwood]

That level of inaccuracy is crazy...perhaps the wooden rule was actually intended to measure with an enlarged scale for some now-forgotten purpose?

On the subject of tape measures, when I need to get an reliable measurement from one I will usually ignore the tongue/hook and start the measurement on the 10" mark. So if I'm marking a distance of 17-1/2", I'll place the 10" mark on my start point and make the mark where the tape reads 27-1/2".

[Mel Fulks]

Sounds to me like the wooden rule is a 1/8 per foot shrink rule for making patterns for cast iron.

[Tom M King]

or a fisherman's ruler.

[Patrick Walsh]

This happens to me at least a half dozen times a year at work. One guy will be measuring and fitting and the other guy on the ground and throwing or handing the pieces up. Its kinda really anoying as it always takes more missed cuts than you would hope before you remember what it is exactly thats going on. Then you feel like a dummy for not catching it sooner.

At home building furniture vrs homes i tend to as another poster suggested measure only to rough my stock and get my first cuts.

[Bill Houghton]

Over the years, I've gotten rid of multiple rules that didn't match my reference rules. And I'd never use a tape measure for precise work.

[Mel Fulks]

Well, I agree tapes used to be a little off. I think they have improved in the last several years. Posted once about the one dollar yellow tapes from Harbor Freight matching Starrett machined "blades" exactly.

[Warren Mickley]

Sounds to me like the wooden rule is a 1/8 per foot shrink rule for making patterns for cast iron.

As you say it is a shrink rule. I can't control the font, but:

Molten metal will shrink as it cools. To allow for this, patterns are made slightly over sized. A pattern that is 12 1/8" long will produce a 12" long part when cast in iron. Shrink rules are elongated rulers that take shrinkage into account. (A one foot shrink rule with 1/8" shrink per foot looks like an ordinary ruler. It is actually 12 1/8" long.) By using a shrink rule to measure parts for a pattern, the pattern maker does not have to scale up the dimensions of the desired casting. Each metal has its own shrinkage rate:

Metal Shrinkage


Britannia*
1/32" per foot


Tin
1/12" per foot


Iron
1/8" per foot


Bismuth
5/32" per foot


Brass
3/16" per foot


Aluminum
3/16" per foot


Copper
3/16" per foot


Steel
1/4" per foot


Lead
5/16" per foot


Zinc
5/16" per foot


* Lead free form of pewter

[Frederick Skelly]

Sounds like Mel and Warren got to the bottom of things!

[Art Mann]

This conversation reminds me of an old fake Chinese proverb "Man with one watch always know what time it is; man with two watches never quite sure".

[Jim Koepke]

1 foot on the wooden rule equals 1 foot, 1/4 inch on the tape measure.

There used to be a difference between Danish inches and SAE inches. Someone here once said Danish inches were 26mm compared to SAE inches being 25.4mm.

One place where I worked many years ago had a rule to use a single measuring stick or rule on a job so as to not run into this problem.

jtk

[Jeffrey Martel]

I just don't use measurements period. Use a tape or a ruler early on to get the initial cuts somewhat around where I want them to be and then I reference everything off of the workpiece. Faster and more accurate.

[Lenore Epstein]

One place where I worked many years ago had a rule to use a single measuring stick or rule on a job so as to not run into this problem.
That's been my rule (pun intended) since long before I started playing around with woodworking. My one recent try at working from the project instead of measurements resulted in a silly looking fail, but I suspect that I simply lack the methods that work.

My tape rules are for hanging blinds or pictures, figuring how much room I need or have for something or what size thing will be right compared to something I already have. I do stick with either inside or outside measurements just from force of habit, though.

[Jim Koepke]

My one recent try at working from the project instead of measurements resulted in a silly looking fail, but I suspect that I simply lack the methods that work.


Have you looked into story sticks?

Here is one I made:

http://www.sawmillcreek.org/showthread.php?226134-Story-Stick-Gauge

jtk

[george wilson]

The government specs for tape measures allow for tapes to be off as much as 3/16" per 6 feet! I won't have a tape that far off! Back when I was having my shop building built,I bought a very cheap tape at the grocery store. For some reason they had a large box full of them for something like $5.00 each for a 10 foot tape. I checked my tape (No doubt Chinese) out when I got it home,and it is exactly correct,or I'd have returned it.(It is terribly hard to find a micrometer that is 10' long!) (O.K.,I made that up!) I measured where every stud in the 30 x 40' building was,and put the measurements on my original drawings I had spent 2 weeks making. When I put in 450' of shelving after the building was finished and painted inside,I missed hitting a stud under the plaster board with screws only one time. I did use that same tape measure to locate the studs. Also made video of where every wire inside the walls is located. All 400 amps worth! Good insurance!! In fact,the contractor tried to lie to me about where he had put re bar in the floor. He wanted to pull a fast one on me and put in a steel post to reinforce a foul up he had made. When my wife said "We have video" to him on the phone over this issue,he shut up. I fully recommend that if you are having a structure built,you take these same precautions. I hate dealing with contractors. And these were supposedly fundamentalist Christians.

Yes,the rule in question is very likely a shrink rule. When I made the 30 casting patterns for the bronze castings in the 18th. C. original fire engine we made in 1982,I should have bought a shrink rule,but did not. I just allowed for the shrinking of the castings with a regular old Lufkin steel rule,and all was well. Probably just as well not to have a shrink rule laying about in the shop to make trouble!!!:)

[bridger berdel]

A couple of years ago I bought a certified machinist's square. I only use it to check other squares. I have only two regrets about this; that it isn't t he largest square in my shop, and that I didn't get it years ago.

[James Waldron]

You don't need a certified square to check a square. Set the square to an edge and strike a "square" line. Flip the square over and set it against that same edge again and strike the same "square" line again. If the two struck lines are an overlay, one over the other, the "square" lines are actually square. If they diverge, they ain't square. They are off by one-half the divergence.

For steel squares, you should check this at least once every six months or immediately after you drop it on the floor. A wooden square should probably be checked weekly. Fortunately, it only takes a moment to check.

Checking a rule is another matter altogether. That's why story sticks are the better choice once a base dimension is chosen, which may or may not involve a rule.

[bridger berdel]

You don't need a certified square to check a square. Set the square to an edge and strike a "square" line. Flip the square over and set it against that same edge again and strike the same "square" line again. If the two struck lines are an overlay, one over the other, the "square" lines are actually square. If they diverge, they ain't square. They are off by one-half the divergence.

For steel squares, you should check this at least once every six months or immediately after you drop it on the floor. A wooden square should probably be checked weekly. Fortunately, it only takes a moment to check.

Checking a rule is another matter altogether. That's why story sticks are the better choice once a base dimension is chosen, which may or may not involve a rule.


Flipping a square on itself is a fine way to check square- within about the width of your line. Sighting two squares against each other with a bright back light is more accurate for me- as long as one of them is known to be good.

[Bill Houghton]

I agree that testing a square for outside corners requires only a piece of stock with a dead straight edge, but a good machinist-grade engineer's square, kept as a reference square, can be quite handy because it lets you test squares for inside as well as outside corners.