[OP]
Contributor
So I thought I'd check my sled using the 5 cut method before starting a new project. Measured the cut off after the 5th cut it's 20 inches long and I have a difference of .020 from one end to the other. Is .001 per inch an acceptable amount to be off?
Confidence: The feeling you experience before you fully understand the situation
Guest
Your calculation is off. Your difference of .020" is divided by 4 to give you an error of .005" over a distance of 20". You can get it closer but that is up to you. Divide by 20 to get an error per inch of .00025". That may sound good but that's per inch error.
Getting something around .001" to .002" per foot (easy way of thinking about it) is quite good and very doable. I set my sled after a few tries and got half a thousandth error over a one foot cut. I then cut multiple acrylic triangles, 8" and 12" to aid in tool setups. These match or exceed the accuracy of a Woodpeckers triangle. That's why a well set cross cut sled can be so valuable.
Watch the William Ng youtube video for correcting the sled and also for the calculations.
Also here's a PDF I found explaining it all: http://valleywoodworkers.org/wp-cont...thodSquare.pdf
Last edited by John Schweikert; 09-11-2016 at 9:28 AM.
In Memoriam
I wouldn't get too crazy over this folly.
The work's surface roughness is such that a measurement of +/-.005" is about
all you can expect. And that is independent of parallelism. Moreover, an edge has
to be indicated (a method of surveying parallelism and surface roughness), to assess its squareness.
Member
The 5 cut method is most assuredly not folly and will produce results more accurate than any other method you are likely to have at your disposal at home. The method has been recommended by thousands of professionals and competent hobbiests and it produces accurate and repeatable measurements if you do it correctly. Parallelism accuracy is almost irrelevant unless you have gross errors and those should be corrected when you first set up the saw. The angle and straightness of the crosscut fence is what determines accuracy. You could question the straightness of the miter slot but I have never seen a table saw with that problem. After the final cut, if you measure and calculate the difference in widths of the strip at both ends 5 times in a row, you will not see anything like 0.005" difference in your 5 numbers. The idea that surface roughness will make a lot of difference is just not right. I know because I have done it. I have spent an entire career doing accurate measurements on various things and I know how to determine accuracy and repeatability of measurements.
[OP]
Contributor
Originally Posted by pat warner
I don't think Pat meant the 5 cut method is a folly, I think he meant don't chase your tail around trying to get it perfect. That's not something I want to do, hence the question if I'm close enough.
Confidence: The feeling you experience before you fully understand the situation
Guest
Jerry,
One path forward is just go ahead and begin building things. If at some point you see gaps or errors being introduced into the project that were from the crosscut sled, then by all means do the five cut again after a correction. Each person has their own level of exactness. I prefer tools being as close to "my perfect" as possible. Projects will have fewer chances of compounded errors, gaps, etc.
Your current setup gives you .003" per cut foot. Not bad at all and will probably provide you very good results.
Contributor
Jerry,
The accuracy of your sled is important. The whole Idea of building one is to get accurate cuts without fuss.
I think you are as close as you need to be. My calculation is .02/5=.004 inches of error over 20". That's the ratio of rise over run and corresponds to .002" per foot or an angle of .01 degrees. (I'll happily split the difference with J Schweikert. Either way you are very accurate.)
Doug
Moderator
If you have a known good square, then hold it up to the corner of the piece. If you can't see light through it then your eye will think any square cuts it produces are perfect.
Member
If your saw is not square, good luck. If your saw cuts square and if you make a sled and square it, your cuts will be square.
My Incra V-27 is adjusted to cut square. The saw slots on my year 2000 Delta 10" table saw are square to the blade. Of course, the trunnions are adjustable and IIRC, I did adjust them when the saw was first unloaded off of the truck. The adjustment was making the blade absolutely parallel to the rip slots in the saw table.
The blade is 10" Forrest carbide tooth blade.
Pat Warner pretty much nailed it.
Last edited by lowell holmes; 09-12-2016 at 10:57 AM.
Member
.0002 is the thickness of the mark left by a sharpie....
A Sharpie...
Contributor
I think most of us would have trouble gauging the thickness of a sharpie mark. On the other hand, the width of a sharpie mark is quite a lot larger than 2 ten thousands of an inch!
On the question of your 5 cut result though the math seems to be lacking around here. 0.020" difference is the result of several cuts. The math I have seen indicates that you divide by 4 (4 corners all of which contribute to the error) which would give you .005" error over the 20 inch length or 0.00025 per inch. I'd say thats pretty dam good.
Guest
"The Ultra-Fine Point Sharpie has an extra-fine, hard nylon point for a 0.3 mm line width" from an online supplier
0.3mm = 0.0118" which is 59 times thicker than 0.0002"
As I said earlier, nothing wrong with the OP's 0.003" error per foot for his sled, but if he so desired it could be reduced. It's simply a matter of people's own desired level of accuracy in their tools.
Member
No no... I must have mis-stated my point... I'm saying... .0002 is the thickness of the Ink left by a sharpie... not the width of the mark.
What i'm saying is... it is irrelevant in what we do.
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