Well, today I finally broke down and finished a cross-cut sled for the tablesaw. This was motivated by needing more square cuts than I could get with the miter gauge; no matter how hard I tried to adjust the gauge. This problem was really evident when cutting material for the Leigh jig.
Now I'm wondering why I waited so long (OK, one reason was because I didn't want to take the time). The test cuts have come out to less than
0.0015" over 5", which equates to less than 0.003 out of square over 12". Much better than anything I could have gotten with the miter gauge.
I had built one of these several years ago using oak and plywood -- the mistake I made with the original was that I made it too large -- it was unwieldy and a pain to try to put on the saw and use. In addition, the plywood I used from Payless Cashway was not stable enough to remain flat. The new one is smaller, easily placed on the saw, and made using an MDF base for stability.
The only thing I am uncomfortable with is that I see no way to emply a splitter with a sliding table; I suspect this won't be a problem, but I'm not used to using the saw without one for through cuts.
Being a newbie, with a sled on the next to-do list, was wondering if you used both miter slots or if you used just one? I am wondering how to get two rails aligned with the sled's base, with the slots, so it wont stick when you push the sled through the board?
I'd venture that most of my crosscuts on a sled are less than the width of the blade and wouldn't even engage a splitter. There are exceptions of course, like when crosscutting panels, but generally those are done on stable materials that are not likely to close up on the blade, and anything laying on the table of the sled is basically stationary from the surface to surface friction since it is the sled that's moving, not the material.
IME with a sled, much of the hazard to precision can to come from pulling the cut piece back though the blade after the cut is made. On critical pieces, I don't even attempt to pull the sled back to the starting position until the blade stops on each individual cut ... too many times just the slight kiss of the blade on the drawback (either from the inevitable slop in the runners, or slight movement of the part) is enough to lose the precision of the sled.
...and, if you're like me, don't forget to put the splitter back on when you take the sled off.
That is the easy part. You make the rails and put them into the miter slots. No place the platform of the sled on the saw. put two screw in each and they will be aligned. Once in place, you can mark the location, remove them, and glue and screw as you desire. Ed snipped-for-privacy@snet.net
Make them a little less thick than the slot is deep, and shim them up when you do the initial assembly. This gives you a little clearance between the bottom of the rail and the bottom of the slot for sawdust and gunk to fall into and not interfere with the motion.
There are many little tricks, but the basic philosophy is to lay the runners in the miter slots _before_ you attach them, then position the base of the sled on top of the runners, fastening them temporarily to the base with small screws from the top (or alternately, with pre-positioned double sided tape on the runners).
TIP: use your fence as a guide for the right edge of the sled base during this above operation.
Now carefully remove the entire assembly without upsetting the position of the runners, and permanently mount them to the bottom of the base with the remaining screws.
The next trick is to get the back fence of the sled perpendicular to the saw blade once the runners are in place.
The philosophy behind this is to fasten one _end_ of the fence with a screw to the base, and the opposite end of the fence to the base through a slightly oversize screw hole, just big enough to give you a bit of wiggle room for adjustment on ONE end.
Screw both ends down, make a cut, flip just one of the pieces edge for edge and see if the cut edges have a gap. If so, loosen the single screw (on the oversized hole end of the fence) and move the fence slightly in the appropriate direction, then re-tighten and repeat the steps until there is no gaps between the cut edges when one is flipped.
Your fence is now aligned perpendicular with the blade. Remove the sled and add more screws, through the base, to the fence to hold it securely in position.
TIP: Do not glue the back fence to the base as you may have to remove all but the original two screws at some point in the future if the parts move due to the dimensional instability of wood.
Good point ... the easiest way to do this, and to get the runners up against the base, is with two dimes in each miter slot underneath the runners. Dimes are just the right thickness for this "clearance".
Of course, since this is an "international forum", you may not be able to do this ... but you will at least have one more reason to hate us here in the US.
I built a sled 30 years ago and still use the same sled today. I recently replaced my table saw and was surprised to find that the sled fit my new saw. The two slots and the blade align perfectly.
I don't quite understand your logic here. If you put two pieces together the error is additive. An error of 0.0015" across 5" will cause a joint to be 0.003" out of square across 5".
I think you're misunderstanding what he is saying. If he cut a board 12" wide, the error would be that much larger than it was at 5". And if he cut a 24" board, it would be even larger. See?
That it did NOT, Bob ... it had nothing to do with your remark, which I must of overlooked in any event, sorry.
It just so happens I was listening to NPR on the shop radio this morning (all the other stations are either in their bi-annual menstrual cycles, bleeding for money, or broadcasting paid-advertisement-live-forever-modern-snake-oil-health-potions from Doctor's who graduated last in their class).
NPR was taking great delight, nay gloating, for the umpteenth f*cking time, about just how much "the whole world hates Americans".
Perhaps if NPR, and the rest of the media, could comprehend just how much I really give a shit, they'd maybe lay off, eh?
The error in "squareness" is based upon a defect in the angle of the cut relative to the side of the board that is supposed to be perpendicular to the cut in order for it to be "square" (as in a right triangle). As I am getting this, when we take the side of the board parallel to the rail of the table and the one that the blade engages is taken as the reference line from which the kerf is supposed to be absolutely perpendicular in order for the cut to be "square," then the error on the other side of the board (the one on the rail side) is such that the deviation from perpendicularity is only
0.0015" over a board 5" wide?
Side A of board ____________________________Vertex A | | | | ____________________________| Vertex B Side B of board
When sawn on a perfectly aligned machine (one that can exist only in the imagination) then Vertex A = Vertex B = 90 degrees iff Side A is parallel to Side B (not demonstrated by the original poster) and the right side of the board is "square".
But if the alignment is not "perfect", then the angles of Vertices A and B would not be 90 degrees and the end would not be square. So, is the gentleman saying that the deviation from perfect square across a 5" distance between *lines* A and B is 0.0015"? If that is the case, then we have:
Side A of Board ____________________________ Vertex A /| / | / | / | _________________________ /__| *Vertex B Side B of Board C D
Highly exaggerated, of course but *Vertex B (also Point D) would be the perfectly aligned position. However, due to the error, C becomes the Vertex B and the distance from C to D is 0.0015". Is that what you understand this to be?
Since the distance A to D is 5" and the distance C to D is 0.0015", we can find the angle CAD from:
Tan CAD = 0.0015/5 = 0.0003 and angle CAD = Around 2 seconds of arc!
WOW!!!! I doubt that this degree of error would ever be worried about by any woodworker - if that is what the original poster meant by error over 5". Hell, I'd bet a damn good roofing square is off by at least that much. You've got to be a good distance away from Vertex A before such a small error would be noticed. Consider D to be 5 miles away from Vertex A. Then C would be out of place by 0.0015 miles and that comes out to be a misplacement of only 7.92 feet!
So, I ask again, how was this error measured? It seems to me that just tightening a tape measure could compress wood by that much at the application point... then you've got the error of the tape measure and the error of your eye and the error of the changing water content of the board and some other errors thrown in. I'd say it's close enough for government work ... IN THE OLD SENSE!!!
Oh, yeah. Note that in the second ASCII Art attempt, the misalignment could have been rotated the other way but the 2 seconds of arc would be the same since the ideal vertex would form the midpoint of the base of an isosceles triangle.
OK up to your last number. The tan of 0.0015/5 =0.0003 = 62 arc seconds. Still a very small number. It is fairly reasonable to measure this . Take a 4" long board and cut it in the middle. Now flip one piece over and put the two cut edges together. The angle error is doubled. If you push one piece against a straight edge the other piece will deviate from the straight edge by 2 X 0.0015/5 X 24" or 0.014". This is still a small number but it is within reach to measure that amount.
I probably don't, the two potential issues I see are if the piece being cut tries to close on itself and to provide some additional protection to keep the off-cut piece from contacting the back of the blade.
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