How accurate is your tape?

[pixelated]

For a price, NIST can tell you.


And, is 22 thousandths good enough?

 

[Packard]

My opinion is that it really does not matter.

As I recall there are two grades of measuring tape.  (Three grades, if you count the tapes made to no recognized standard).

So you have the non referenced tapes (most tapes at the big box stores), then you have Class 2, and finally Class 1.

But in reality, I grab my tape and measure a door opening, and then use my tape mark the door and cut it down to the correct size.  More important is my technique in getting the measurements.

I probably uses measuring tools less often that most.  For the door, I would take a reading on a story stick and transfer it. 

Or if the part is small enough, just transfer the dimension.  So if I were installing a new threshold at that same door opening, then I would carry the threshold over to the opening and mark the cut line directly to the part. 

It really only becomes an issue if you are measuring with one tool for one dimension and another tool for the mating part. 

So if I measured with a Class 1 tape, then the tape on my table saw fence had better be class 1 too or there is no purpose in having the class 1 tape in the first place.

More important is that the various scales in your shop are all matched, and that your measuring technique is highly repeatable.

I am probably in the minority on this subject. 

 

[ChuckM]

The accuracy of any tape measure matters nothing to me as long as I stick to the same tape throughout for any particular task. I have half a dozen tapes in my shop, and at the last time I compared and checked them, only two of them were identical in their measurements while the rest were slightly off from one and another.

The same applies to my steel rules and between my steel rules and tape measures. The scale on my Sawstop as well as other tape scales (e.g. miter gauge) are also not identical to any of the tape measures or steel rules in my possession. I suspect I have the same "problem" with my various combination squares/precision squares.

 

[smorgasbord]

The scale on my Sawstop as well as other tape scales (e.g. miter gauge) are also not identical to any of the tape measures or steel rules in my possession. I suspect I have the same "problem" with my various combination squares/precision squares.

Yes.

But, I'm not going to do everything in my shop with story sticks - it just isn't practical. Even if I could figure out how to set my tablesaw rip fence based on a story stick, would it be any more accurate? Which maybe points to a difference between making a door versus fitting a door. How many of you use marking knives on your story sticks because pencil widths introduce inaccuracies?
Now, for building things, we all do things like plane all the stock to thickness at the same time without moving the planer bed/head, or cutting all the rails and stiles to width without moving the rip fence, etc. , but there are always times when we messed a piece up and have to match. And then the challenge is how well can we reproduce a setting days weeks later?

Luckily, the observation that the smaller a measurement is, the more accurate it needs to be holds true. No-one marks and cuts dovetail pins to match previously cut tails using measurements, but I'll certainly rip a new floating panel to width using my saw's scale.

With DROs on my tablesaw rip fence, planer thickness and drum sander thickness, plus Incra teeth on my tablesaw crosscut sled and miter saw fence all tweaked to be the same as each other at a couple shared values (eg 20.0mm, 125mm, and maybe 290mm), based on a dial caliper and a shared decent manufacturer for rules (I use Shinwa rules mostly), I'm comfortable for many things to measure then cut. When I know my tablesaw rip fence DRO is dial-caliper matched at 20mm and 125mm, then I'm comfortable doing any kind of rip using the DRO, especially if I'm trying to match something I cut a couple weeks ago using that same rip fence (even if re-calibrated since).

If it's critical, like planing new stock thickness to match something previously done, I'll measure at the next to last pass to be sure I'm getting what I should be getting. And, of course, "sneaking up" on a cut is a pretty standard procedure in most shops for precision matching, but even when DROs aren't zero-calibrated or just off on long distances, they are always dead nuts on for the small deltas one would have one the final pass of something like thickness planing.

I remember when Norm Abram got his Biesemeyer fence in season 2 or something and he talked about it on the show. He said something to the effect of "I'll check with the tape measure because we still can't trust the fence." By the next season he had learned to trust the Biesemeyer.

We have a thread on Starrett not keeping up with manufacturing progress. I think not only have CNCs revolutionized how accurate tooling can be made by manufacturers, I think DROs and similar have revolutionized how accurate projects can be made in home or custom shops without always using the more lengthy trial and error procedures of the past. It's really not beyond a small home shop's capabilities to have trusted measuring and cutting devices that match well enough these days.

But, I suspect I'm in the minority on this.

 

[Oldwood]

I find the end hook on tapes to be the biggest problem when it comes to accuracy. I place a good steel rule on a piece of wood with a square edge flush to the end, then measure it with my tape. If it is not the same as the rule, I adjust (bend) the hook until it is. I then set the rule on my saw to cut parts that measure the same as that tape.

I sometimes do use a digital caliper to measure pieces coming off the saw to confirm they are where they should to be.

Of course shortly after this you drop the tape, and it lands on the hook and you start again

 

[ChuckM]

And then there're many who allow their tapes to snap back violently into the cases.

 

[Crazyraceguy]

[member=7143]Oldwood[/member] that is a noble gesture, that beats doing nothing, but that is only in the first few inches.
Manufacturing techniques/tolerances can make a difference in the accuracy as it goes further and further.

Keeping so many different machines "in agreement" with each other is always a challenge. The big, computerized beam saw is the most accurate (very repeatable) so I try to keep everything matching to it.

Tape measure accuracy has always been a thing with us. New guys are taught to compare theirs to the guys they are working with, especially if one is the "cut man" for others.

 

[rst]

I bought a Class 1 tape years ago that I compare all new tapes with. 

 

[Oldwood]

[member=7143]Oldwood[/member] that is a noble gesture, that beats doing nothing, but that is only in the first few inches.
Manufacturing techniques/tolerances can make a difference in the accuracy as it goes further and further.

Keeping so many different machines "in agreement" with each other is always a challenge. The big, computerized beam saw is the most accurate (very repeatable) so I try to keep everything matching to it.

Tape measure accuracy has always been a thing with us. New guys are taught to compare theirs to the guys they are working with, especially if one is the "cut man" for others.

We built cabinets, so most of the measuring was out to maybe 3' and I found once we set all the tapes to all read the same off the hook to our 2' steel rule they were sufficient for something that you read by eye in a cabinet shop.

We moved for most of our cutting to a CNC router and that reduced the measuring by about 80% and as you say that becomes the reference.