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Discussion Starter #1
My never ending quest for flat square wood is progressing. very slowly I have to admit, but progressing.
anyway, now i have to ask if i'm on the same page as the rest of you.

i have a sliding table on the front of my router table, and have made a wooden jig that allows me to quickly clamp flat pieces to run them across the cutter to make flat sides.

So, i start with a 9mm thick flat piece of hardwood about 6" (150mm) roughly square,
i run the piece across the cutter to get a flat edge. By turning that wood once and repeating the cut, i get two sides at 90 degrees. but if i do all 4 sides, one from the next, i dont get 90 degrees at the end of it.

After a lot of trial, i realised I had to do 5 sides to get rid of any tolerances from the first cut.
I'm close, the final angle is ONE degree adrift, which means I'm a quarter degree out on each cut.
Without a complete redesign and lots of time and effort, my wooden jig is as good as I can get it.

Remembering my complete ignorance of this kind of thing, am I there yet?
Or is it normal to get all four completely square?
 

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Obviously the closer the better. The main thing is if there are no gaps in any joints when you put your boxes together. If you can't see a problem then for all practical purposes there is no problem. If the 1/4* is causing a problem then instead of starting from square one and risking even worse results is it possible to shim some part of your procedure? The shim might only need to be a couple of thicknesses of paper.
 

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What happens if after you square the end if you use the same side to square the opposite end ,then run the other side? Like Chuck said a piece of paper or tape might correct the "problem", if it is a problem. Wouldn't be a problem for me. Maybe the "dial indicator guys" might have a solution.

Herb
 

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Well now you have opened a can of worms! I remember a recent discussion about whether dial indicators have a place in a woodworking shop. Some of us are hardly more than carpenters. If a cat can't crawl through the crack, it's close enough. Others are much more demanding, requiring joints you can't even feel. The way we get there varies from the artist who carefully planes a joint until it fits just right, to the technician who carefully sets up machinery to make "perfect" cuts every time. "Close Enough" depends on who you are and what you are building. It's not a destination, it's a journey. Your own definition of "Close Enough" will change as your skills progress. Meanwhile, just keep finding ways to get closer.
 

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Discussion Starter #5
The jig is like all my stuff, as basic as basic can be. No way to shim the leading edge without starting all over with a different design.

I thought a little while ago (while I was in the barbers chair) about reversing the wood after the second cut, I'm wondering if that will reverse the angle? if it did i would be dead on. I shall try that as soon as I can.

I have an engineering background which is why I'm trying for such close tolerances, but I've already learnt the hard way that wood moves about a lot.

The effect I really want to achieve is almost marquetry, where the box is made of different woods, and the floor has panels of different colour woods. this is why I am trying so hard for accuracy. Yes, I aim high (lol)
 

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Sunnybob,

As soon as you said 5 turns it made me think of William Ng and his method for squaring TS sled fences. If I understand it right the process requires one to compound the error X number of times then measured with calipers. The error is measured over the length of the cut not at the vertex. If your calc of .25° at vertex is correct then at 6" sides, you are out of square opposite the vertex approx 13/512" + or -.
 

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ok, youve blinded me with science, where do i get a tape measure in 512ths? (G)
I reckon do two sides rotating the wood. The flip the wood and do the other two sides. i think that equals plus minus minus equals zero.
How is the sliding table indexed to the main table? As Ghidrah states, the 5-cut method is typically used for checking squareness of a sled, and it checked by measuring the "taper" of the strip left after the 5th cut. For your router table, where you're just truing up the edges of a piece of wood, the adjustment has to be made in the mechanism which locate the sub-table which slides past the cutter. Look at how this table is referenced relative to the cutter, and see if there is a way that you can make this adjustable, adding/subtracting paper shims is a good (low tech) way to accomplish this. I have an old Delta shaper where the wooden fences are shimmed to the table top with strips of playing card - I always meant to take it to work and have them skim it on the mill but never got around to it, the strips of card have worked for over 20 years with no problem.
 

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Wouldn't doing the 'do 1 side then flip and do the opposite side' simply give you two pairs of parallel sides? Basically a parallelogram with the corners not necessarily at 90deg?

This is reminding me of my SiL working with me doing the new 2x2 spindles on his back porch. I wanted to do the center spindle then use a 2" spacer to set the adjacent ones consecutively going in both directions away from the center.
Nope. Apparently that's not accurate enough; he had to do each one by measuring top and bottom to a 32nd" ....zzzzzzzzzzzzzz ;)
 

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Discussion Starter #10
Tom, you might be on to something there. Its a very basic system with wooden slats sliding between two ally rails. i always push the unit towards the cutter to take out the very minor slop in the slider. I could just about get a thickness of paper on the front edge of the rail which might tip the whole sled enough.

Dan, TWO sides, not one.
Cut one side, then use that as reference to cut side two. This now has a quarter degree angle. use side two to cut side three. This now has a half degree angle. Flip the wood top to bottom and do side four, this has removed a quarter. do side three again, and your back to 90 degrees.

Thats my theory, based on nothing more than hope. i shall try to prove or disprove it tomorrow morning, but we are going away for 3 days, so it depends what time the packing is finished.

Ordinarily, I would agree that this is very small potatoes, but i want to match five different shapes into one box base, so I'm testing myself.
 

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Tom, you might be on to something there. Its a very basic system with wooden slats sliding between two ally rails. i always push the unit towards the cutter to take out the very minor slop in the slider. I could just about get a thickness of paper on the front edge of the rail which might tip the whole sled enough.

Dan, TWO sides, not one.
Cut one side, then use that as reference to cut side two. This now has a quarter degree angle. use side two to cut side three. This now has a half degree angle. Flip the wood top to bottom and do side four, this has removed a quarter. do side three again, and your back to 90 degrees.

Thats my theory, based on nothing more than hope. i shall try to prove or disprove it tomorrow morning, but we are going away for 3 days, so it depends what time the packing is finished.

Ordinarily, I would agree that this is very small potatoes, but i want to match five different shapes into one box base, so I'm testing myself.
Firstly, you need all the slop out of your guide system and then you need a second fence at 90° to the initial guide. In essence what you're trying to do is mimic a jointer/planer. You start off by truing up the first edge, but just rotating the part and using the 3rd edge as reference will only give you a trued edge that's parallel to edge #4, you haven't cut it square to edge #1. As Dan said, if you start with a parallelogram with no square corner, your set-up is not going to correct the starting condition. If you have the second fence and put side #1 against it, then side #2 will be at 90° and so on. Think of squaring a block on a jointer - flatten side #1 and then, with side #1 against the fence, machine side #2 at 90° to side #1. At that point, you normally switch to the planer and make two (or more, depending on material removal) passes with face #1 and then face #2 against the table - this makes #3 parallel to #1 and #4 parallel to #2, and everything is square because you started with faces #1 and 2 at 90°.
 

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Wouldn't doing the 'do 1 side then flip and do the opposite side' simply give you two pairs of parallel sides? Basically a parallelogram with the corners not necessarily at 90deg?

This is reminding me of my SiL working with me doing the new 2x2 spindles on his back porch. I wanted to do the center spindle then use a 2" spacer to set the adjacent ones consecutively going in both directions away from the center.
Nope. Apparently that's not accurate enough; he had to do each one by measuring top and bottom to a 32nd" ....zzzzzzzzzzzzzz ;)
Get him a dial indicator for Christmas,Dan.

Herb
 

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Sunnybob,

I don't think there is a usable tape at that range outside of a microscope, but if you want to drive yourself mental, calipers is the closest you're likely to get.

Degrees = (A/R) • (180/PI)

This can be worked both ways to find "A" or "degrees" the only constant is (180/PI) which is rounded out to = 57.2957795 = 1 radian. To me, at best this a mind exercise, to actually measure "A" or Degrees accurately is worse than watching paint dry or a golf tournament or my oldest playing an RPG.

If you were able to physically and accurately mark out .25 degrees, (I can't, I think even my Exacto tip is too thick)
 

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Bob,

I understand that you're making smallish boxes, and trying to square up the individual pieces and cut them to size on the router table. I think that there's a way to accomplish this without any elaborate sliding fixture, assuming that you have a fence and enough room on your router table to the FRONT of the bit to accommodate the size part that you need. Starting with pieces cut roughly to size and not square, you need a way to get two edges 90° to each other, and I think that this will work. Starting with edge #3 against the fence, take a skim cut on edge #1 opposite to it. Now you need a fixture - just a piece of plywood, a little narrower than your finished size and with a fence at 90° to the length screwed to it. Put the workpiece on top of the plywood fixture, and with edge #1 against the fixture fence, run it past the table fence taking a skim on edge #2 - edges #1 and #2 are now perpendicular to each other. Take the fixture off, adjust the table fence to give you the final size and take two passes, one with edge #1 against the fence and the final one with edge #2 - the part is now square and to finished size. If you attach the fence to the fixture with screws, you can adjust the fence while making test pieces until you have as accurate a 90° angle as you need - you should have an accurate square in your shop that should let you get pretty close for a first try. The explanation is a little involved, but it's really pretty simple - the way it works is because the router table will always give you parallel surfaces, the trick is just to get two of the surfaces perpendicular first before cutting to the finished size.
 

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Sunnybob,

I don't think there is a usable tape at that range outside of a microscope, but if you want to drive yourself mental, calipers is the closest you're likely to get.

Degrees = (A/R) • (180/PI)

This can be worked both ways to find "A" or "degrees" the only constant is (180/PI) which is rounded out to = 57.2957795 = 1 radian. To me, at best this a mind exercise, to actually measure "A" or Degrees accurately is worse than watching paint dry or a golf tournament or my oldest playing an RPG.

If you were able to physically and accurately mark out .25 degrees, (I can't, I think even my Exacto tip is too thick)
I agree with Ghidrah.

Herb
 

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Sunnybob,

I don't think there is a usable tape at that range outside of a microscope, but if you want to drive yourself mental, calipers is the closest you're likely to get.

Degrees = (A/R) • (180/PI)

This can be worked both ways to find "A" or "degrees" the only constant is (180/PI) which is rounded out to = 57.2957795 = 1 radian. To me, at best this a mind exercise, to actually measure "A" or Degrees accurately is worse than watching paint dry or a golf tournament or my oldest playing an RPG.

If you were able to physically and accurately mark out .25 degrees, (I can't, I think even my Exacto tip is too thick)
Not sure what arc length (A in your drawing) has to do with this problem. It's simply a matter of scale. A 0.25* error from 90* will result in less than 1/32" across 6", in most cases acceptable. If you extend that error to 6' the error is now greater than 5/16" which is unacceptable.
 

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Think of it this way, how perfect were the joints used by craftsman in the 17th century? They were using hand tools yet everything looked perfect. I can't picture what a 1 degree error would look like but I'm sure that once you change anything at all on your saw (such as a new blade) this one degree is going to change somewhat. Even raising or lowering the blade will change things.Or for that matter a piece of wood that is surfaced differently will ride through the saw differently. I put a square on the cut piece and if I don't see light I call it square.
 

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Checking squareness with squares is folly.
Squares are indicators not measuring tools.
Precisie parallelism and squareness measurements require sophisticated measuring tools
and conditions. If rough meaurements in a foot or so of material show errors
on the order of the meaurement error (i.e. using a rule to 1/32" allows meaurement only to the nearest 1/32") it might be more prudent to get on with the project.

Just 10' of angle error (1/6 of a degree) will produce a 1/32" error in parallelism/foot.
Cancelling that error, x rotating the work against the saw fence 5 x, is a rediculous practice.
Chasing that kind of error with measuring tools to 1/32" is a waste of time. Moreover, without a lens, you cannot read a rule to 1/64"ths, a common scale interval.

Jigs and fixtures, on the other hand, do require precision.
If you must chase a mil (.001") do it with your fixturing.
If your jigs are screwed up, all the cuttings from those jigs will be a guess and x-golly.
Controlling squareness, straightness and parallelism demands more than ordinary
equipment. Took me > 4 months to design and create a rectangle maker.
With its sleds I can hold .001"/24" of parallelism in wood, plastic and aluminum.
Squareness requires a special calibrated x-cut sled. You don't want to go through
that much hell for a dining room table top.
Just cut, mill, assemble, don't worry , be happy.
 
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