CSC SYS 50 as a replacement for a cabinet saw?

[tjbnwi]

Please explain how read from both ends of a shared ruler works using left edges. And why there’s no mark at the very ends.

My Starrett rules are marked all the way to both ends.

I don’t understand the first question.

Tom

 

[smorgasbord]

Take a close look at the Starrett C635-E metal ruler:

It's marked from both ends. Note the tick marks for 20mm from the left and 130mm from the right. If what you claim is the correct way to read rulers, then the left edge of the 20mm tick would line up with the right edge of the 130mm mark (as viewed in this orientation). But, that's not the case - the 20mm and 130mm tick marks have their centers lined up, as do all the tick marks on opposite sides of the ruler.

Now take two metal rules and use one to measure the other's tick marks, but offset them by at least one tick mark. Line up the tick marks and see where the edge of the ruler intersects the tick mark on the other. It'll be at the center.

That the NIST engineer used the center of the lines, not the left edges, is also indicative that the correct way to read rulers is to use the center.

Try measuring your 4" block with a metal hook ruler as well.

 

[mino]

Take a close look at the Starrett C635-E metal ruler:

It's marked from both ends. Note the tick marks for 20mm from the left and 130mm from the right. If what you claim is the correct way to read rulers, then the left edge of the 20mm tick would line up with the right edge of the 130mm mark (as viewed in this orientation). But, that's not the case - the 20mm and 130mm tick marks have their centers lined up, as do all the tick marks on opposite sides of the ruler.

Now take two metal rules and use one to measure the other's tick marks, but offset them by at least one tick mark. Line up the tick marks and see where the edge of the ruler intersects the tick mark on the other. It'll be at the center.

That the NIST engineer used the center of the lines, not the left edges, is also indicative that the correct way to read rulers is to use the center.

Try measuring your 4" block with a metal hook ruler as well.

Hate to merge in, but you are unintentionally conflating two things here.

When you check/calibrate a tape measure for precision, you check for TWO things:

A) Is the tape exact and uniform along its length - I am not sure of NIST procedure here, but both DIN(German) and ČSN (Czechoslovak) standards would require that calibrating a tape measure checks every 200 mm of its length for deviations. This is because you can have a tape that is exact end-end by accident but have big variations through the scale with "short" and "long" sections.

The standard calls for maximum accuracy here, so the most precise checking method is used - reference of a mark edge as is the rule in all metrology. If you ever did metrology, or even analytical chemistry, when you take visual readings of a scale, and you are after maximum precision, you never go to the centers of the mark. The way a human eye works is it is reference-based. Meaning you will be more accurate if you take two "left" readings or two "right" readings of a scale and subtract.

When proper science classes are*) done, this is hammered into kids as the first thing as it is useful in daily life. Pretty sure it was at our 6th year labs (K7 US terms).

B) Is the end stop zeroed? To do this, you check the end position vis-a-vis a given mark close to it, say at 200 mm. You do it using magnification kit ideally.

When you did both these readings, you then combine them into a single calibration value.

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In short, there are two rules to read a scale for best precision:
1) Whenever possible, calibrate (and read) either the left or right sides of the marks. Do not read by the center, excepting point 2).
2) Always keep note of how a given instrument was calibrated if you you cannot follow rule 1).

Scales starting at zero cannot use the left/right approach thus are calibrated to the center of the mark. While this is the less accurate method, a scale starting at zero cannot be made other way.*)

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What this all means for practice:
1) When you can, always take a relative (subtractive) reading using the left or right edges of the marks on both ends of your measurement.
/With tape measures this is even more important as the end stops are the things the most abused/damaged. A tape measure will usually keep its relative accuracy even after its end stop gets bent inevitably./

2) Be aware that the ends of zero-starting measuring instruments are calibrated to the center of the mark, so when using the zero start/stop, you must use that.

3) When calibrating scales on machines, for maximum precision calibrate to the left or right edges of the marks and make sure every user of the equipment is aware which side it is calibrated to on that specific machine.
If you cannot ensure all users are aware which side a scale a machine was/is calibrated to, a common scenario, then it is preferable to eat the precision loss and calibrate to the center than having people using the "wrong" side. In machine shops it was common to have shop-wide policies of "left of mark" or "right-of-mark" readings when manually controlled machines were in use by multiple people. In such settings, many workers had no clue why and just followed the policy, so lots of anecdotal "this is the correct way" stuff is out there. These days of CNCs, it is mostly irrelevant as anything needing to be precise is not made by hand/eye.


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Now to put this all in context.
We are talking about precision differences in the 0.1 mm (0.005") range and smaller. Talking about this stuff in the context of studs positioning would be just ridiculous. But above is how it goes if you need the best precision from a given scale reading.

If you do not need it, or multiple users are involved, just stick to using mark centers which are unambiguous. No point wasting time/energy on precision that is not required.

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*) were done, notice the "proper" term too. To align us to US/Western "quality education", Physics labs at middle school level were abolished 20 yrs ago and the Chemistry labs were castrated to a teacher show by EU about 10 yrs ago to "protect the kids from chemicals knowledge". These days one will probably hit this only at a trades school or a natural sciences higher edu course. It still is the fundament of visual instrument reading, nonetheless.

 

[smorgasbord]

A) Is the tape exact and uniform

B) Is the end stop zeroed?

I guess you didn't watch the NIST video.

First, are you suggesting that tape measures are designed to be read differently than metal rules? Sure, you can modify the hook end of your tape measure such that the reading is at the left edge of the tick marks, but that's not how they're designed to be read. And, on a metal rule, you can't add material to the zero end to make it read that way.

Here's the Starrett metal rule and a modified version when I thickened two tick marks, keeping the left edge intact, to emphasize what is continually being ignored here:

As you can see, the thickened ticks don't line up because Starrett didn't design them to read from the left edge, but from the middle of the tick. The middles of the ticks on opposing sides does line up.

Go grab yourself a metal hook rule, such as the nice ones Woodpeckers makes (here and here, for instance) and see for yourself where your accurate 123 blocks line up.

Now, sure, you can burn-an-inch for outside measurements to use the scale any way you want, BUT, for measuring from the end of the tape (most common), or using a hook rule or taking an inside measurement, you're pretty much stuck with how the ruler was designed. And that is to use the center of the tick marks.

 

[mino]

Um, this is indeed tricky to convey. Not being a native speaker is not helping either.

When you use edges of marks, you always compare two marks on the same scale and their respective edges. I.e when you need to measure 2", you look at the left edge of the 1" mark and the left edge of the 3" mark. Alternatively (which side is more suitable is chosen) you look at the right edge of the 2" mark and the right edge of the 4" mark, etc.

As there is no "edge" to the 0" mark on that particular rule, the start at zero cannot be read that way, so all marks are calibrated as "centered" with respect to the 0-starting edge of the measuring instrument. That is the "special case" I refer to when zero-starting commodity measuring instruments are at play. It is that way because that is the ONLY way how to make them practical to use. That is a limitation of sorts, forced by them starting at zero, not a "feature". There is no reason to abide by that when one is not using the zero-start capability, outside the additional precision is not being needed and approximate measurement being more than adequate for the use in question, which does happen to be most woodworking scenarios.

This is the exact same as when marking stock. One always marks such that the cut is to be located on the edge of the mark. Not the center.
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With a ruler that is not starting at zero, thus has a proper 0 mark, you can use left, right (more precise) or center (less precise reading). This holds as long as the marks are same-width. Which they are on any serious measuring instrument. For exactly this reason so edge-reading is possible.


Add:
When you compare two scales, you then leverage both edges, center is ignored as that is how a human eye works when comparing the relative placement of two objects of a similar width. You cannot see a center or a different-color line, you /your brain/ can only approximate it. But you do see its edge(s).

 

[smorgasbord]

all marks are calibrated as "centered" with respect to the 0-starting edge of the measuring instrument. That is the "special case"...

Glad you agree with me on how rulers are designed, but using tick centers is not a "special case," it's the standard.

What you and @tjbnwi are talking about is a special way to use a ruler that can only be applied, as I said, when you do something like "burn an inch." But for tape measures, which is how this tangent got started, I'll argue that tick mark bleed variation makes trying to use the edges problematic, if not even more inaccurate. that the NIST engineer chose to use the center for his tick mark to tick mark accuracy measurement was not by accident or lack of knowledge.