Solved! Measurement accuracy - Touchdown!!

[Bill Huber]

I am trying to figure this out, my scale is not accurate at all and I can't get it to be.

Here are 2 images, the first is the fence is away from the blade but is parallel to the fence, in the second one the fence is close to the blade but is still parallel to the blade.

This is were I don't understand it, if the scale is set how does it change when you move the fence?

Saw-1.jpg Saw-2.jpg

[John Piwaron]

I am trying to figure this out, my scale is not accurate at all and I can't get it to be.

Here are 2 images, the first is the fence is away from the blade but is parallel to the fence, in the second one the fence is close to the blade but is still parallel to the blade.

This is were I don't understand it, if the scale is set how does it change when you move the fence?

Saw-1.jpg Saw-2.jpg

Nothing about the stick on scale on the guide tube changes. 6" is still 6". I'm using 6 just for discussion.

Your drawing *is* what I was thinking was going on. The distance the guide tube scale is measuring is the hypotenuse of a right triangle. The distance from the blade you actually move it is the long leg of that triangle. A distance that will be less than what you intended. For me, that was 1/64" + at 6". For someone else that error will be different depending on how out of parallel the guide tube is.

[Bill Huber]

Got it... thanks.

The first one is 4.27 and the second one is 4.14

saw-3.jpg saw-4.jpg

[johnny means]

Not exactly what you're saying. The error was a LOT more significant than that. When I'd slide the fence over to 6",k it down, the measure fence to blade with the Starrett rule, the distance was really 1/64 to 1/32 short. Move the fence more and get MORE error. So at 6" the error is 0.02, the the error at 36" would be 0.12" And all the while the fence is parallel to the saw blade.

I'll make a drawing if confusion persists. All I know is that now whatever distance the scale on the guide tube says the Starrett scale now agrees with.

If you had that much of discrepancy than there was obviously some issue. A complete tear down and setup may have fixed it, but the geometrics dont work out for the rail angle to cause that large an error.

[Brian Kincaid]

When I had this problem I realized my tape measures were inaccurate. Bought starrett tape measures and all my measuring tools (Incra, etc) now match up
-Brian

[Jerry Miner]

When I had this problem I realized my tape measures were inaccurate. Bought starrett tape measures and all my measuring tools (Incra, etc) now match up
-Brian

I think this is a much more likely cause of the issue. If it were strictly an out-of-parallel fence issue, with these numbers:

So at 6" the error is 0.02, the the error at 36" would be 0.12"

then the fence would have to be out of parallel by almost 3 inches! (if the hypoteneuse is 36.12 and the long leg is 36, then the short leg is the square root of (36.12)^2 - (36)^2 , or 2.94.)

[Jerry Miner]

I don't want to beat this one to death, but let's look at the math from another angle, so to speak:

Suppose you have a 50" Biesemeyer fence. You're installing it on your saw, and the mounting holes give you 1/8" adjustability for parallel to the front of the saw. Let's say you install it as far off as you can (1/8").

Your fence would then travel along the hypoteneuse of a right triangle whose long leg is 50" and whose short leg is .125" (1/8").

You zero-out the scale at 0.0" and move the fence to the 50" mark. Instead of travelling 50", your fence will have travelled 50.000156", a little more than one ten-thousandth of an inch too far! That's why the manual doesn't address this issue. It's a level of precision that is beyond reasonable for a tablesaw.

Even if the rail is 36" long, and the installation is 1/4" out of parallel, the error at 36" is less than a thou!

[Mark Kornell]

If you had that much of discrepancy than there was obviously some issue. A complete tear down and setup may have fixed it, but the geometrics dont work out for the rail angle to cause that large an error.

Johnny is correct. Assuming that the vence was parallel with the blade, and given A^2 + B^2 = C^2, using A = 5.98 and C = 6, results in B = 0.49. In other words, the tube would need to be out of parallel with the front of the table (in actuality, since you are referencing off the blade, you mean off perpendicular to the blade as Dave Richards said) by about 1/2" over a 6" span. About 4" over a 48" span, and I'm certain you would have noticed it being that far out. Your tube rail may have needed adjustment, true, but there was also something else going on.

My saw rail can be adjusted by about 3/4". So if the rail was out of perpendicular by 3/4" over 48", and the fence had been adjusted to be parallel to the blade, the measurement error would be .006" - about 1/170 of an inch. Over 6", the error would be less than .001". So fence rail perpendicular-ness is pretty much a non-issue, which could be why the manual doesn't mention it. My saw's manual doesn't mention the issue, either. It has about 20 pages on aligning the blade, mitre slots and fence, but basically says to eyeball the rails. Having done the math, I now know why.

I did make sure my rails were within 1/16" side to side when I set it up, which puts my measurement error at something less than the day-to-day movement of wood. :-)

[Mark Kornell]

Oops, I see Jerry beat me to the punch while I was working on the math. Must've had better marks than I in high school...

[Richard Wagner]

I'm one who is very particular about alignments; especially the TS blade to fence and DP bit to table top. But when we start talking about errors in the fourth decimal position, I become somewhat uninterested.

Disinterested unless I am working at the Bridgeport. Nonetheless, this was a very interesting thread. Examples like this should have been used by my math teachers. If they had shown such "real world applications" I might have retained more of what was being taught (presented). Teaching does not occur unless there is some learning.

[Barry Wixey]

I thought maybe I would kick this up a notch since I am questioned daily about accuracy issues on table saws that use our digital readout. Most of the time the discrepancy is because the original fence scale is not accurate, however, we have tracked down large discrepancies on a few occasions to fence rails that were actually slightly bowed. If you do the math on a rail bowed on a 60 foot radius you can be way off the farther you get from the blade. This issue can be easily tracked down because the fence will be parallel to the blade at 0 inches but will swing its way out of parallel as you move away from the blade.

[John Piwaron]

Math aside, which given the numbers I posted indicate a vanishingly small error. An amount of error that's most likely not observable by the human eye.

Yet, my direct observations were not wrong and what I did to fix that after combing through how it was setup, has, in the end, apparently yielded the result I was looking for. The distance between the saw blade and the fence, as measured with a Starrett scale, now matches the number the crosshair in the head of the fence over the scale on the guide tube. I'm good. At the moment. I only checked with 2 cuts - one cut at 1" and another at 9". So I might be making an assumption that won't hold true for me at longer distances. I'll do more test cuts. FWIW - when I say the two distances match, I mean that not only do the two measurements match but also the resulting cut is what both scales said it would be. THAT is the proof in my pudding.

Is something else at play? Maybe. Probably. But I'm no longer going to worry about it. For anyone else, this remains something to consider if after everything else someone is still not getting what they want. Make no assumptions. Check everything. I spent north of 3 grand for this saw. My expectations went up with the dollar figure. And in this regard, brand is unimportant.

This is also about trust and confidence. Trusting that the scale on the tube is where it should be and when the fence is positioned at some distance, that the width being looked for is the width that results. The confidence comes in that such a thing remains so going into the future. I think for a while I'll be double checking the setting until after a while I know that doing so is redundant.

The last thing - I agree with Mr. Wagner above. This is the sort of real world example math teachers should include with course material. The practical application of the match being learned. It'd help with the "what am I ever going to use this for?" question.

[John Piwaron]

I thought maybe I would kick this up a notch since I am questioned daily about accuracy issues on table saws that use our digital readout. Most of the time the discrepancy is because the original fence scale is not accurate, however, we have tracked down large discrepancies on a few occasions to fence rails that were actually slightly bowed. If you do the math on a rail bowed on a 60 foot radius you can be way off the farther you get from the blade. This issue can be easily tracked down because the fence will be parallel to the blade at 0 inches but will swing its way out of parallel as you move away from the blade.

Wow. *More* to check. This is reminding me more and more of what happens when a plastic injection mold isn't giving the results it should. We now leave the world of plane geometry and enter the Twilight Zone. I mean 3D geometry.

[Bob Lang]

I believe that practical experience in the shop trumps sitting at a desk and running trigonometry equations. If I'm understanding the issue, the rail is actually differing in height from the saw table. I bought a Biesemeyer fence in the early 1980s, before they were a part of Delta. At the time this issue was addressed in the manual, and an L-shaped jig cut from 1/4" Masonite was included with the fence. You placed the long section of the jig on the saw table and mounted the angle iron that supported the rail so the it touched the short leg of the L. Following those instructions, as well as the advice to calibrate the cursor with dial calipers (I had to go out and buy) gave me a fence that I could trust the tape on from kissing the blade to the end of the fence.

Bob Lang

[Mike Goetzke]

I agree to disagree with many calculations here, but, maybe look at the assembly instructions for your fence. I had a Uni with a Biese fence and there was a template used to align the fence tube.

Mike