OMG, 30 y/o go-to ruler incorrect

[David Utterback]

I know many members have been correct in recommendations to ensure comparability of measuring tools. But, I am just not careful enough.

But, wow, what an awakening. Using my go-to quick measurement tool, I discovered recently that it is inaccurate. By about 1/16" over some of it's length.
I purchased the 24" cork-backed ss ruler from a drafting supply retailer over 30 years ago. It is etched engraving and I incorrectly trusted it without question all this time.

It has been used on every stick of furniture I have made. In some ways, it has been responsible for my lack of confidence due to limited success. I only hope that the new ruler will help somewhat with future pieces. But after all these years, my vision is declining, tremors increasing, and memory evaporating:eek:. Will give it our best shot, though.

The morale - listen and follow all instruction provided on this web site. It is a treasure trove.

[Mike Cutler]

David

As long as you used only that ruler as a reference, It "accurate enough". It's not the absolute accuracy of the measurement, it's the repeatability of that same measurement.
That being said, it's always very, very, nice to know that you have "accurate standards" in the shop. Be it a ruler, square, triangle, or level.
How do you know the ruler is 1/16th off over it's length?

[glenn bradley]

Hoping I don't start a storm here . . . Gathering a matching set of reasonably accurate metal rules can elevate your work and your confidence when things go together as expected. Be careful of the reiteration effect of the often repeated "as long as you use the same measuring thing for the whole project, you're fine". A moments thought on this will reveal the many ways in which this can cause problems. It might be OK if all measurements from the offending device were used to make parts that didn't have to go together.

If I am off 1/16" over 24" and I use that device to measure four 8" pieces in the first 8" of that sliding error and then expect them to fit between something that I measured as 24" on the same device . . . nope. Even if the measuring device were off on a perfect graduating scale, each piece would be 1/64" off. For those who think 1/64" doesn't matter, make some dovetails, box joints or lay in a dutchman :D. A dozen erroneous 64ths over a project could have dire effects. It could certainly make me ask things of myself like "why can't I get this right?". :mad:

[Darcy Warner]

Maybe it was a pattern makers ruler.

[michael dilday]

I had several 6 foot folding rulers that were off 1/16 over 6 foot. I went to Home Depot looking for an accurate folding rule and there were none. Now I use a tape for just about everything. If it isn't accurate I throw it away.

[Jacob Reverb]

Just don't use it to measure the length of your Plancks!

[John Goodin]

But now you have an excuse for 30 years of cutting boards too short.

[Mike Cutler]

Hoping I don't start a storm here . . . Gathering a matching set of reasonably accurate metal rules can elevate your work and your confidence when things go together as expected. Be careful of the reiteration effect of the often repeated "as long as you use the same measuring thing for the whole project, you're fine". A moments thought on this will reveal the many ways in which this can cause problems. It might be OK if all measurements from the offending device were used to make parts that didn't have to go together.

If I am off 1/16" over 24" and I use that device to measure four 8" pieces in the first 8" of that sliding error and then expect them to fit between something that I measured as 24" on the same device . . . nope. Even if the measuring device were off on a perfect graduating scale, each piece would be 1/64" off. For those who think 1/64" doesn't matter, make some dovetails, box joints or lay in a dutchman :D. A dozen erroneous 64ths over a project could have dire effects. It could certainly make me ask things of myself like "why can't I get this right?". :mad:

Glenn
There shouldn't be a storm. The example you have is a little outside of what I was referring to. You're laying out cuts that have to mirror each other, so any error is multiplied by 2.
My position goes back to the term "accuracy". There is no accuracy without a NIST CERT. There are no "accurate" rulers, tapes, or squares, in a Big Box Store. Not one, Not in any of their stores.
If you're buying online,or in a quality supply store, Incra, Pinnacle, Veritas, and Starrett, to name a few, all come with manufacturers certifications of specifications, that should refer back to a NIST traceable standard, and that's why they cost the premium that they do. Sometimes folks drive themselves to distraction over "accuracy", when it is repeatability that they are really after.
On the flip side of that, It really bothers me when someone is trying to achieve a high level of accuracy, makes the investment in quality tooling, and then is derided for wasting their money because, "it's just wood". The process of being accurate, is never a waste of time, or money.
A quick check for any machine rule is to mark out a measurement, flip the rule and measure from the other end.You're using the rule as it's own "standard".

Believe it or not, I used to calibrate machine rules to NIST Standards early in my career. I worked in a mechanical standards lab calibrating every tool a machinist would use. Even the old school DRO's on a Bridgeport. I used to love messing about with the Mills.

[Peter Kuhlman]

I probably have 30 or more measuring things. Rulers, tapes, Mylar and metal strips, squares, etc. I use my Starrett 24” and 18” rules as my baseline for accuracy as they cost me the most but really have nothing else to use for a baseline. It can be difficult to figure out what is the most accurate device to use for comparison. For years I used my squares made by Bridge City Tools but kept wondering why my other squares were always off a little bit in comparison. Turns out the BCT squares were all off! Price does not reflect accuracy it turns out.

[David Utterback]

The problem is that 5" in the 10 - 15" range is unequal 5" in the 0 - 5" range. The former is off by about 1/16". What this means is that if I measure an opening in the 10 - 15" range and then set my power saw to cut this length to fill the opening, the resulting piece, e.g. drawer front, does not fit.

I routinely used a variety of starting points on this ruler to make measurements even using it backwards from high to low at times. As a result the error is sometimes in the range and other times it is not.

This type of manufacturing error for the ruler is incomprehensible. For how long has a inch been an inch?

[scott vroom]

If I am off 1/16" over 24" and I use that device to measure four 8" pieces in the first 8" of that sliding error and then expect them to fit between something that I measured as 24" on the same device . . . nope. Even if the measuring device were off on a perfect graduating scale, each piece would be 1/64" off.

I'm not following your math. How would one expect four ~8" pieces to fit a ~24" span?

If your 24" rule is really 24 1/16, and assuming the error is evenly distributed over the length of the rule, using the rule to make a cut at 8" would yield a "true" 8.020833333 length, 3 of which would fit perfectly into a 24.0625 span, no?

I'm in the repeatability camp.

[glenn bradley]

I'm not following your math. How would one expect four ~8" pieces to fit a ~24" span?

:D:D:D Wait . . . isn't that the new math!?! Sorry about that. My imperial engine got tangled with my long-division engine but, you get the drift. The point being that where along the length you measure with a variable reference tool can effect where the error is, how large it is, etc.

I spent a short period of time many years ago and got a set of rules that matched each other and my chosen reference. . Once I get to 128th's, my eyes, steadiness of hand and choice of marking device start to vary more than the measurement :). We all have our point of 'close enough'. It's when multiple parts have to fit together well that a poor measuring tool can bite you.

So much of the impact of tools and methods vary with how we work and our skill sets. For me a poor measuring stick is a negative variable I don't need. I have other skill areas where I can be pretty casual and get it right most of the time. Measuring for parts isn't one of them probably due to my eyeballs. Pr-retinal detachment I could eyeball pretty close. Now I can't trust that anymore :o.

[scott vroom]

If the error is evenly distributed along the entire length of the measuring device, then any portion of that length will be precisely proportionate to the total error. Therefore, as long as the same measuring device is used throughout the project, everything will match perfectly...i.e., repeatability trumps the absolute accuracy of a measuring device.

[Steve Rozmiarek]

I had several 6 foot folding rulers that were off 1/16 over 6 foot. I went to Home Depot looking for an accurate folding rule and there were none. Now I use a tape for just about everything. If it isn't accurate I throw it away.

Careful with the tapes, mine invariably land hook first when I drop them on the floor, which can easily add or subtract a 16th when the hook bends. I try to keep a pampered reliable one in the shop for use there only, but often the daily carpenter tape gets packed in the shop and used, which has definitely caused problems.

[Ron Selzer]

"If the error is evenly distributed along the entire length of the measuring device, then any portion of that length will be precisely proportionate to the total error. "
I agree with the above statement, HOWEVER I never have found it to be true in real life.
Have found that somewhere the tapes, rules, etc. that are off (don't match as I don't have NIST traceable to check with) are off in different areas, some even were true to others at longer distances and didn't match somewhere in the middle.
Best I have found when wanting matching rules/ tapes, etc is to lay them out in the store and match them up ALL the way out to the end. Sure get some funny looks when stretching 30' tapes out 6-10 wide

[Mike Cutler]

I probably have 30 or more measuring things. Rulers, tapes, Mylar and metal strips, squares, etc. I use my Starrett 24” and 18” rules as my baseline for accuracy as they cost me the most but really have nothing else to use for a baseline. It can be difficult to figure out what is the most accurate device to use for comparison. For years I used my squares made by Bridge City Tools but kept wondering why my other squares were always off a little bit in comparison. Turns out the BCT squares were all off! Price does not reflect accuracy it turns out.

Peter
You're probably pretty good with the Starretts. They have always made nice tools. I have a 6' Starrett machine rule that lost it's NIST CERT, thus making it 6' straight edge and not worth anything.
It was heaed for scrap metal dumpster at work and instead found a way to my shop. That's my "go to" reference.

It's odd, and unfortunate, that your BCT square is not square. John was really anal about machining accuracies?????

[Max Neu]

This reminds of a saying I always think about;
"A man with 1 watch will always know what time it is,a man with 2 watches will never know what time it is. " :)

[David Utterback]

"If the error is evenly distributed along the entire length of the measuring device, then any portion of that length will be precisely proportionate to the total error. "
I agree with the above statement, HOWEVER I never have found it to be true in real life.
Have found that somewhere the tapes, rules, etc. that are off (don't match as I don't have NIST traceable to check with) are off in different areas, some even were true to others at longer distances and didn't match somewhere in the middle.
Best I have found when wanting matching rules/ tapes, etc is to lay them out in the store and match them up ALL the way out to the end. Sure get some funny looks when stretching 30' tapes out 6-10 wide

Your observation about variance over the length of the ruler is exactly what I have. Undersized at 1 - 3", variably over between 3 and 10", etc. etc. What I do not understand is why this type of error occurs in the manufacturing of linear measuring devices. I know they cannot all be NIST traceable, but how hard can it be to set up machinery to reproduce rulers that are systematically close to accurate. My ruler has engraved markings. I do know what kind of machinery is used to manufacture them. Apparently, there was some slop between the engraving head and the substrate holding mechanism.

[Brian Holcombe]

I check calipers against Jo blocks then check rulers against calipers. I compare machine fences with test cuts against long calipers (24”). Throw away anything that can’t be calibrated and is out of spec enough to cause error.

Doing this, in addition to squaring fences to a standard in my shop, became and incredible time saver. So nice to never second guess, and just go around and check machine fences and wear items every few months. I can’t remember the last time I needed to interrupt my work to check things or second guess squareness or accuracy.

I still cut to a knife mark when fitting things but that’s a different animal and I may be able to eliminate most of that by adding a DRO to more tools. Similar to scribing for machine work, it’s useful but the DRO is actually more accurate.

These precaution are cheap by comparison to screwing up something expensive.