Figuring out angle of a cut using "rise over run"?

[brian]

Hi all,

I'm really new to woodworking. In fact, I haven't really done any yet. However, I am laying my own wood floor (5"x3/4" plank cherry, nailed), and I've bought just about every tool under the sun by now, so I'll be doing more after this project ;-)

The key, though, is finishing this project, which requires that I get around obstruction #1, which is a brick half-wall. The wall is roughly

48" wide, and the flooring runs parallel to that wide side of the wall. My problem is basically that I'm not wonderful at math, so I'm hoping someone here can help me figure this out:

According to all the measurements I've taken, my flooring is parallel with both an exterior wall, and the interior wall which runs parallel to it. If the brick half-wall was also straight, I'd need to rip a board to be about 2" in thickness. However, it isn't perfectly straight. The variance is roughly 1/4" over the 48". So I need to rip my board with that in mind. Does anyone know the calculation for figuring out the angle to set my board as it goes over the table saw blade?

Thanks. brian.

[Australopithecus scobis]

tan(theta) = opposite/adjacent sin(theta) = opposite/hypoteneuse cos(theta) = adjacent/hypoteneuse

theta = arctan(opposite/adjacent) = 0.25/48 = 0.3 degrees ~= 18 arcminutes.

That's the math. One or more of the tablesaur guys will tell you how to set up the saw without the math.

[Charles Spitzer]

measure how wide at one end of the wall where the board needs to be. measure at the other end of the wall. transfer measurements to board. draw line. cut on line.

[Paul Franklin]

I *think* this is right...

tan (X) = .25/48

Solving that X=about 0.3 degrees.

If it were me and I had one to do, I'd cut it freehand with a circ saw or jigsaw and clean it up with a sander.

HTH,

Paul

[patriarch]

Australopithecus scobis wrote in news: [e-mail]:

Rip it straight, and get out yer trusty hand plane. This won't take an #8, willit? ;)

Patriarch

[Doug Miller]

In article , [e-mail] (brian) wrote: [snip]

Here's the math: angle = atan (rise / run) where atan is the "inverse tangent" function (shown as Tan -1 on some handheld calculators, or accomplished by using the Inv and Tan buttons on others; in the Windows calculator, check the Inv box, and click the "tan" button).

In this case: angle = atan ( 0.25 / 48 ) = atan ( 0.0052 ) = approx 0.3 degrees

You may have a tough time setting up a taper jig to cut a taper that shallow on a table saw.

Here's how I'd do it:

1) mark the cut line on the board 2) trim the board close to the line; preferred tool is a bandsaw, but a saber saw will work 3) finish up with a hand plane

-- Regards, Doug Miller (alphageek-at-milmac-dot-com)

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[Larry Kraus]

[Mike Marlow]

If your brick wall runs true, so that the runout is fairly straight from 0 to 1/4in, then I'd just strike a chalk line or a pencil line from 0 to 1/4in on the floor board and freehand it on the table saw.

[Doug Miller]

Either (a) you misspelled "band saw" or (b) you're an idiot. Freehanding

*anything* on a table saw is *very* dangerous, and advising other people to do it is both dangerous and stupid.

-- Regards, Doug Miller (alphageek-at-milmac-dot-com)

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[U-CDK_CHARLES\Charles]

You probably spent too much on tools . . .

You don't want to calculate anything.

What you want to do is take a compass and use it to scribe the profile of the wall onto your plank. You probably want to leave a 1/4" gap at the edge, which you'll cover with your baseboard.

Once your plank is scribed, cut it with a sabre saw or a band saw.

You might want to get one of the many books on the uses of a steel square. There are very few things that you need to do in the shop that can't be done with a square.

An adage from higher math comes to mind: Numbers are the enemy. Avoid using them. In math, that means you simplify first, then plug in the numbers. In the shop, I'd turn that around: Mark your measurements on a story stick and never use numbers until the next project.

[Mike Marlow]

Well, I did mean table saw but that may or may not make me an idiot. I have free handed a great number of things on a table saw over the years without a problem and more importantly, without the suggestion of a problem in the making. If you can rip a board with a fence in place then why would it be dangerous to guide a like board without that same fence in place? Slow and easy is the key to a good cut. I disagree that freehanding "anything" on a tablesaw is *very* dangerous.

[Doug Miller]

And so you conclude that it's safe. I conclude that you've been lucky.

Because without the fence, you have nothing to guide it with, and the board can easily become skewed. It takes only a very slight degree of skew to cause a kickback.

You might be the only person to hold that opinion.

"Never cut freehand under any circumstances!" -- Kelly Mehler, "The Table Saw Book"

"Never freehand on a table saw! ... I've watched some old-timers do this and get away with it -- at least while I was watching. But I've also noticed that many of them can't count to ten on their fingers..." -- Jim Tolpin, "Table Saw Magic"

"Never crosscut freehand on a table saw! If you inadvertently twist the stock even a tiny bit, the blade will jam in the kerf and throw the piece off the table with tremendous force." -- Tolpin

Suture self... it's your fingers. You've been lucky so far. But don't advise other people to adopt practices that are universally recognized as unsafe just because you haven't gotten hurt -- yet.

-- Regards, Doug Miller (alphageek-at-milmac-dot-com)

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[JAW]

Find a short marking piece for each side of the cut out, mark it and transfer to the real piece. Draw a straight line from each cut out side and cut.

Tools needed - pencil.

brian wrote:

[Doug Miller]

And a bandsaw. :-)

-- Regards, Doug Miller (alphageek-at-milmac-dot-com)

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[Swingman]

Although I see freehand table saw use done all the time on job sites by modern day "carpenters", it is usually done with an underpowered saw that would easily bog down before it bit back at you.

While you might get away with it over time, it is definitely not the safest practice on any table saw ... do it on a cabinet saw and you're just asking for trouble.

Just my tuppence ....

[Mike Marlow]

I conclude that there is no problem when proper care is taken in feeding the stock.

This is not true at all. It takes a reasonably high degree of skew to potentially cause kickback. It is also quite easy to guide the piece so that you never reach that point.

Obviously not in light of the comments you saw fit to place below. I have seen it done by more experienced woodworkers than I can count and I have done it myself. That makes for a pretty good base of experience. Much more so than what I may have read in a book.

As the author states - they got away with it - yet, he attempts to convince the reader that the practice is so dangerous. The author loses credibility with comments like his closing comment.

If that were absolutely true then all of the misaligned table saws in the world would be killing and maiming their unfortunate owners since they would be experiencing the skew he talks about - even a tiny bit.

You presume that I and others who have guided stock through a saw freehand have been lucky. You have no basis for that but you're entitled to believe it. The practice of carefully guiding a piece through is not universally recognized as unsafe. It has been done and still is done by woodworkers today. It requires care but so does every aspect of operating a table saw. Any cut on a table saw can be made to be dangerous, and this one, no more so than any other.

I respect your position not to do this and my only point is in response to your original reply to me. Chose to follow whatever degree of safety practice makes you most comfortable, but also recognize that despite what a book says, or what has become something of a mantra in a group like this, every day, common experiences prove the absoluteness of your claim to be a bit much. Many woodworkers chose not to freehand cuts. That's fine. Many chose to do it. Equally fine. The practice has not generated the amount of injuries that some warn are inevitable.

[Mike Marlow]

My saw as well as plenty of others that I know of are well capable of going past the bogging down point. Clearly, the more power, the greater the risk of any kind of problem, but my original reply as quoted above was really taking exception to the very dangerous statement - that's why I included Doug's use of the astericks surrounding the word very.

I've seen it done on a lot of cabinet saws and frankly beyond the fact that I respect more powerful tools even more than I respect lesser powered tools, I never gave another moments thought to one saw versus another. In fact I might be more concerned for a woefully underpowered saw - I'm not fond of saws that will stop and start on their own bases on whether a piece of wood is on them or not.

[Phil Crow]

Snip

I'll second that emotion. I just bought off e*ay a book that's 450 pages of small print on the uses of the steel square. You can figure out the friggin meaning of life given a steel square and a pair of dividers.

I know it's a little off the topic of the OP, but now every time I pick up my framing square, I do so with a small sense of awe.

-Phil Crow

[Doug Miller]

A conclusion entirely unsupported by the evidence.

Absolutely false. To cause a kickback, you need skew the board only enough to bring the wood in contact with the teeth at the rear of the blade. This is a tiny fraction of an inch.

Sure is -- if you use a fence.

"The practice of carefully guiding a piece through" is synonymous with using a fence, sled, miter gauge, etc. Freehand is not guiding, it's pushing. And to state that it is no more dangerous to freehand than to make any other sort of cut is just plain absurd.

Like I said -- it's your fingers. But to advise others to adopt practices that are known to be dangerous, especially without warning of the attendant hazards, is irresponsible.

-- Regards, Doug Miller (alphageek-at-milmac-dot-com)

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[Mike Marlow]

As is exaggerating the dangers of a common practice.