when it comes to the average contractor tablesaw, how flat is flat? How is it measured, 1/000th inches per 12 inch of surface? ETc.
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Replies
The flatness of granite surface plates is specified by the distance between two parallel planes, one passing through the highest point(s) on the surface and one passing through the lowest point(s). The $30 9"x12" plates are typically specified at 0.0001".
By a machinist's standards, the tables on most tablesaws are nowhere close to flat but for most woodworkering the tables are adequately flat. If you keep the insert level with the table and adjust the blade tilt stops carefully, the fact that the table isn't perfectly flat shouldn't cause you any problems.
If you want to check the table for flatness you need a machinist's straightedge and a set of feeler gauges to measure the gap between the straightedge and the table's surface. Most saws will have an overall dip or a crown of a few thousandths of an inch in the top, but occasionally a top will be wavy. Machinists straighedges are quite expensive, it isn't worth buying one to check a tablesaw for flatness.
John W.
In my experiences dealing with wood machine manufacturers as a repair technician .015" for tablesaw top was the common allowance from Delta, Powermatic, etc. So anything less than .015" is considered acceptable. That is measuring with a straight edge the table length, width or crosswise.
Edited 3/5/2004 10:52:51 AM ET by rick3ddd
Really? 15 thousandths? I was looking at a Tablesaw with a really nice finish. I used a 4 foot level (not laid flat but on it's corner). I noticed a dip. Using my car point gap feeler gauge I measured 6 thousandths. That seem high. Then again if it is 6/1000ths over 12 inches you could only be off square by .5/1000ths when cutting a 1 inch board. And that would be the worst case.
On a tablesaw .015" isn't a problem. I had a 16" Oliver jointer that was dipped .018 on the outfeed table that was giving a less than acceptable edge joint. I had it reground to within .007" and it made a big difference. We are talking a 18" x 48" table. Two tables actually as the jointer was 8' long over all. Plus I use machined straight edges and feeler gauges to measure with. I wouldn't put much emphasis on thousandths using a level as a measuring tool. Don't confuse quality of finish with flatness of the table. Improper grinding can make a flat surface even worse. When grinding a jointer table it must be supported properly or the magnetic chuck will distort the metal and it may appear flat after grinding, then when you release the magnet it goes distorted again. You can only grind it so many times before it's junk.
By the way...Oliver considered .010" to be an acceptable tolerance on the 16" jointer tables.
Edited 3/5/2004 2:18:18 PM ET by rick3ddd
No, I am not confusing finish with flatness. That was my exact point. I noticed the finish FIRST and then wanted to see how flat it was.
As for the level, your point is well taken. However, I used a couple of other "straight" edges as well and all read the same spot as low. I am sure that my measurement with my feeler gauge could have been off a couple of thousands one way or the other.
I had never looked at a saw for flatness before, so I did not know what a good measurement would be. Hence the question.
Your question reminds me of a question I pose to "MENSA" members any time I bump into one:
Carefully place a perfectly smooth and spherical ball on a perfectly flat and level plane. Which direction is the ball likely to move?
The answer won't help you much with your "flat" question. But when it might when you start wondering about being level.
4DThinker
Which hemisphere?
DaLE
Hemisphere? Of the Earth or the sphere? Either way you can pick. It doesn't matter.
4DThinker
If you take away all outside influences, and start with no external perturbations, it will initially oscillate about the CG.
"If you take away all outside influences, and start with no external perturbations, it will initially oscillate about the CG."
You win, drduvall, if you'll specify what the CG is and whether it is of the sphere or the plane. Now explain why to everyone else. Then what happens after the oscillation and why?
And thanks to all for playing. These have been far more and more interesting responses than the question has ever gotten in a MENSA forum. And if drduvall finishes his answer, then he'll be the only one I've EVER encountered who knew the answer and why it was the answer.
4DThinker
You are going to argue that the flat plate is a gravitational shaped bowl and the sphere rolls around until it finds a local min?
You might as well say the ball spirals into the center.
There is a great deal of difference between an answer you like and the "right" answer.
If you take a way "all" influences, the atoms in the ball will continue moving apart as will the atoms in the plate.
"You are going to argue that the flat plate is a gravitational shaped bowl and the sphere rolls around until it finds a local min? "
No. I'm not going to argue. Or wait, did I just do that?
"You might as well say the ball spirals into the center. "
Why would it spiral? That would take some vectored force to start it on a path not toward it's final destination. Limit yourself to just gravity if that makes it easier to contemplate.
"There is a great deal of difference between an answer you like and the "right" answer."
OK.
"If you take a way "all" influences, the atoms in the ball will continue moving apart as will the atoms in the plate."
Only if they have started moving apart can they continue moving apart. So I'm declaring since they hadn't moved apart (for whatever reason) when the sphere was set on the plane then those atoms will not be moving apart until at least long enough for one to observe which direction the sphere might have started to roll.
4DThinker ;-)
My final(?) comments, as I don't think we've really answered the question about table saw flatness. [In fact, I'm going to measure mine today!!]
Limiting the argument to Newtonian (Classical) Mechanics:
The ball will fall to the lowest point of the plate (i.e., the point closest to the earth's CG). With no friction, etc., it will oscillate about this point indefinitely (The sum of the potential and kinetic energy will remain constant, since this is a conservative system) ... but ... unless you're on the equator, it will appear to precess (it's path will appear to rotate as the earth spins beneath it) as well. You've essentially constructed Foucault's Pendulum (not Eco's book!!). So hemisphere and latitude actually do matter.
Dale
Edited 3/8/2004 12:41:33 PM ET by drduvall
Edited 3/8/2004 12:42:51 PM ET by drduvall
How is your flat plane supported, and how strong is it? Is the sphere perfectly evenly weighted? What about wind, magnetic fields, and other moving stuff like cars and bullets? "Carefully" does not always mean without motion, as is demonstrated pretty often by those who carefully place fairly smooth spheres on fairly flat surfaces, hoping to watch them travel sideways a good distance and knock down a lot of pins.
"How is your flat plane supported?"
Sufficiently enough to keep it flat and level, with neither the sphere's weight or the plane's weight causing any deflection of the plane.
"and how strong is it?"
Strong enough to keep it flat and level. See above.
"Is the sphere perfectly evenly weighted?"
Yes.
"What about wind, magnetic fields, and other moving stuff like cars and bullets?"
No wind, or any other object coming into play including magnets. In fact consider there to be a vacuum as in outer space if you want.
"Carefully" does not always mean without motion"
OK, so I'll amend the wording from "carefully" to "without directional or rotational (spinning) motion".
Hope that helps. ;-)
4DThinker
Edited 3/5/2004 11:52:18 PM ET by 4DTHINKER
OK, you intellectuals humor me here. If you place a sphere onto a motionless perfectly flat surface, and all forces that can act on it are nonexistant, including gravity, then it should sit still, right? But, don't electrons even have mass, so that their impact would eventually move the ball, assuming no friction? Seems to me that if it did move, it could only be because of some force acting on it.
Steve
Edited 3/7/2004 1:21:27 AM ET by Dirt Stirrer
Yes, but the ball will roll toward the centre of the plate regardless because of gravitational attraction if the whole thing was in a vacuum and somehow or other managed to avoid things like friction and inertia.
Two objects will be attracted to each other - if the ball is off-centre on a flat plate, then there is a lot of mass on one side of the ball and a minimal amount on the other, then the ball will be 'attracted' towards the centre of the plate. I can't remember the scientific term, but the force of attraction is proportional to the mass differences.
Enough of this but - I'm now grounded firmly in practicalities.
Cheers,
eddie
Eddie,
Makes sense, however, like you said, being firmly grounded in practicallity, that situation could never be achieved could it? How would you get rid of all forces acting on the ball in reality, there is always something to act on it that would mess this result up. Fun mind exercise, nothing more I guess.Steve
Without any forces if the ball had any motion initially it will never stop.
In the absence of all forces thermal motion makes the atoms move apart.
Without the sub atomic particle forces, the sub atomic particles will move apart.
Flat has no meaning without the method of measuring it. Gravitationally flat using a perpendicular to a plumb bob is different than flat using light.Different "flats" lead to different answers.
Those Mensa people are a lot smarter than you are.
The plane is level, as stated in the first problem statement. All techniques I know of to measure and declare a plane level involve gravity, so gravity is one force we must let be.
Gravity seems to be a parallel downward force to us on earth, but it is really a radial force toward the center of the earth when you stand back far enough (or far enough away from earth) to see it's effect on things.
A flat plane must also be balanced to remain level on average. So at it's center of gravity (the center of the plane) the force of gravity will be the strongest. Any direction on the plane away from that center is actually "uphill" relative to the earth's gravity and the plane's center. This is easier to visualize if you imagine the plane to be very large and the earth to be small. If the sphere is placed exactly on the center, gravity will hold it there. But if it is put anywhere "uphill" and away from that center, gravity will pull it downhill toward that center. It may gain enough momentum to roll past the center, but then it will slow and eventually reverse direction back toward the center.
As for those MENSA folks being smart, I agree. For every one question I can create that will stump most of them they can usually find thousands that stump me. I like my question though, because an answer CAN be derived from what is given in the question and "common knowledge" about what is given. But it takes a good problem solver/observer to not try and complicate the question.
My conclusion? Some Woodworkers are better problem solvers than some MENSA members.
How's that for diplomacy?
4DThinker ;-)
All other things being equal, I'd guess "west", due to coriolis forces...
CharlieI tell you, we are here to fart around, and don't let anybody tell you different. --K Vonnegut
Huh?
Coriolis force is a pseudoforce; it doesn't act on anything.
Really? Didn't know that; I was under the impression that along with temperature changes, coriolis forces were responsible for wind..? Never pretended to be a physicist...just a simple country woodworker...
CharlieI tell you, we are here to fart around, and don't let anybody tell you different. --K Vonnegut
"All other things being equal, I'd guess "west"."
"West" will be right only occasionally if the experiment is tried many times.
4DThinker. ;-)
Pearls are graded using an approximation of this procedure. A perfectly spherical pearl is described as an eight way roller.
Who really cares anyhow! By the way I know the answer but you did not use the coifficient of friction quantity. Two "perfectly flat" and frictionless planes put together can not be taken apart!
"By the way I know the answer"
OK, Dick. But please don't tell anyone here. And PLEASE don't tell any MENSA members. Thanks.
4DThinker ;-)
The answer to your question depends on where you are on the earth as well as where the moon is and the level of seismic activity.
If you have an answer that does not take those variables into consideration, your answer is wrong.
I think the ball will move towards the center, as a perfectly flat plate will not be following the curvature of the earth - the edges of the plate are minutely further from the earth's center.
Bill
You win too, Bill. Good work!
4DThinker
4D:
I went back to your original question:
"Carefully place a perfectly smooth and spherical ball on a perfectly flat and level plane. Which direction is the ball likely to move?"
Maybe one of the reasons your MENSA contacts have difficulty with the question is that it is poorly expressed. You assume gravity is the only external force on the two bodies, but you don't tell them that. You assume that "plane" is a physical plate, composed of massy stuff and having finite dimensions, rather than the normal definition of a plane (two dimensional, massless, infinite in extent).
Reminds me of many of the Sherlock Holmes stories which involve clues known only to Holmes but not to the reader--"While you were away, Watson, the killer dropped by and explained everything."
But you seem to enjoy stumping MENSA folk, so more power to you.
my $.02-
assuming the ball is initially placed at the exact "gravitational center" of this perfectly "flat" (as a theoretical absolute- not relative to any other reference), perfectly "level" (relative to a theoretically perfectly spherical earth), infinitely large but massless, plate/plane (or as you and others described, at the point on the surface closest to the center of gravitational pull), and is done so infinitely gently without inducing any vibrations, etc, it should just sit there without moving anywhere at all. if it is placed 'off-center' it will seek the center. if we're allowed to consider the rotation of the earth in the equation, it should have a slight tendency to roll in a generally westward direction but only until it reaches equilibrium with the gravitational force pulling it to the aforementioned center.
(and yeah, i couldn't help but smirk at the typo in the title of this "deep thinkers" thread)
m
"You assume that "plane" is a physical plate, composed of massy stuff and having finite dimensions, rather than the normal definition of a plane (two dimensional, massless, infinite in extent). "
True, Donald. Since I always am writing down the question from memory, some times I don't get it as perfect as others. From now on I'll try and remember to use "plate" instead of "plane".
Thanks for the pointer.
4DThinker ;-)
Mfg's spec on my cabinet saw is .012" over the top, edge to edge, F-B- Corner to corner.
I finally got around to measuring my PM 66. The primary table in flat to within 0.015". Most of the table, measured front to back, side to side, and diagonally is better than 0.006/running foot. One of the corners on the metal extensions was slightly high, but I think that I can correct some of that error. I agree with the other guys, this is good enough for woodworking.
Dale
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