Aligning Table Saw Sled Fence - Five Cut Method

[Jon Rista]

Here’s the correct math for the five-cut method:

A=((T-B)/4)*(R/L)

Tim

That's the same math. Just organized with different grouping. I actually should have at least grouped the T-B , but the rest is the same.

A = (T-B)/4/L*R
  = (T-B)/4 * 1/L * R
  = (T-B)/4 * R/L

You are right, the T-B needed parens (order of operations/precedence), but the rest should just follow standard rules. If you divide by L first, or multiply by R then divide by L, or divide R by L first then multiply, the result is the same.

 

[Jon Rista]

Yep. I find I actually need to build a new sled now but going to do it up the same way - Evidently while it was stored in the back room where I stashed it to put it out of the way, a rough cut board fell over and cracked it, I'll see if I can remember to pull it out and grab a picture, but yeah - I drill a through hole in the pivot end of the fence just far enough inside of the saw kerf to fit through a bolt for it to pivot on, and another at the other end of fence at some convenient point where I decide to put the knob, (in a drill press to drill holes square to the fence) then drill through the sled itself where fence is to be located, flip sled over to the side runner will be on and stick trammel with router and 1/4 inch bit using the pivot side as trammel pivot line up with the swing hole and route a perfect arc to get my range of adjustment, then a 1/2" straight bit in same router setup to cut a recess just a touch deeper than the carriage bolt head height following the same arc - then just a carriage bolt up through sled and fence and a lock knob on top. that way I can adjust my fence to pretty much any angle I need to cut, and I can dial it in pretty precisely.

I'll see if I can remind myself to get some photos tomorrow when I go out to the shop... I've already got some plans in mind to improve on it for the new sled I'm making (Just waiting for the sled runner to come from amazon ( https://www.amazon.com/Aluminium-Slider-Woodworking-Fixtures-Router/dp/B091Q78CK8 ) which may help vs the jury rigged one I have now.

Yeah, if you manage to dig it out, share some photos. What do you do about the blade cut into the fence...if the fence swings around, would the blade eventually chew that central part up? (This is actually a small concern of mine, that over time, the fence will end up getting chewed up at the point where the blade slices it... However, I was also thinking, I would ONLY use it in 90 degree orientation, and never angle the blade. I'd probably build a more advanced sled with replacable clearance plates and maybe fence plates, so that I could angle the place if I wanted to, and not have to rebuild the sled every year or something like that.)

Regarding those runners...wow, $11!? I think I spent over $30 on the HDPE, and it can't be adjusted for optimal fit in the miter slots. I may have to pick a couple of those up myself...

 

[Tim Hunter]

That's the same math. Just organized with different grouping. I actually should have at least grouped the T-B , but the rest is the same.

A = (T-B)/4/L*R
  = (T-B)/4 * 1/L * R
  = (T-B)/4 * R/L

You are right, the T-B needed parens (order of operations/precedence), but the rest should just follow standard rules. If you divide by L first, or multiply by R then divide by L, or divide R by L first then multiply, the result is the same.

You’re trying to do the same thing, but the order of operations will foul your proposals. The parenthesis's are needed to be correct.

Tim

 

[Kevin Jenness]

That's the same math. Just organized with different grouping. I actually should have at least grouped the T-B , but the rest is the same.

A = (T-B)/4/L*R
  = (T-B)/4 * 1/L * R
  = (T-B)/4 * R/L

You are right, the T-B needed parens (order of operations/precedence), but the rest should just follow standard rules. If you divide by L first, or multiply by R then divide by L, or divide R by L first then multiply, the result is the same.

This is what I meant about the 5 cut method being fiddly. I kept getting confused and adjusting the fence in the wrong direction. The 3 cut method provides an intuitively obvious visual of the magnitude and direction of the fence correction required even to a foggy brain like mine.

 

[Jon Rista]

You’re trying to do the same thing, but the order of operations will foul your proposals. The parenthesis's are needed to be correct.

Tim

For the subtraction, you are correct, I missed that in my OP. However all the rest of the parentheses are not necessary. This can easily enough be proven:

R = 22.75
L = 5.25
T = 0.445
B = 0.423

My Original:

A = (T-B)/4/L*R
= (0.445-0.423)/4/5.25*22.75
= 0.022/4/5.25*22.75
= 0.0055/5.25*22.75
= 0.00105*22.75
= 0.0238

Yours:

A = ((T-B)/4)*(R/L))
= ((0.445-0.423)/4)*(22.75/5.25))
= ((0.022/4)*4.333)
= 0.0055*4.33
= 0.238

The excess of parentheses kind of gets in the way, IMO. Basic order of operations for simple terms means multiplication/division have higher precedence than addition/subtraction, otherwise you run left to right. It doesn't matter if you move the /4 as a 1/4 * (R/L) even. The results are still the same:

A = ((T-B)*((R/L)/4))
= ((0.445-0.423)*((22.75/5.25)/4))
= (0.022*(4.333/4))
= 0.022*1.08333
= 0.238

Or we could even do this, running the M/D left to right:

A = ((T-B) * (R/L) / 4)
= ((0.445-0.423) * (22.75/5.25) / 4)
= (0.022*4.333 / 4)
= 0.095333/4
= 0.238

I forgot the parens for T-B originally, but once that's added, you can use simple algebra to rearrange the formula between my version and you version simply enough. The (T-B)/4/L is really (T-B)/4 * 1/L. Once you have the 1/L term, multiplying by R gives you R/L. You can easily enough seprate that into R/1 * 1/L, and combine the L back with the original (T-B)/4 term... I think it keeps things simpler, if you limit the number of parens (as, technically, "parenthesized/bracketed" groups take the single highest precedence, meaning you have to expend more cognitive effort to parse through the formula with all the parens. Keeping the parens around just the one term that actually needs a bump in precedence, (T-B), it keeps the cognitive effort lower.

PEMDAS, is the acronym, right? Parentheses, Exponents, Multiplication/Division (equal precedence, run left to right), Addition/Subtraction (equal precedence, run left to right). I think there is another acronym, BODMAS, which is basically the same thing. Technically they can both be reduced to PEMA/BOMA, since division is multiplication by a fraction, and subtraction is addition of a negative number. In any case...thanks for pointing out I missed the parens on the subtraction, that DID need to be done first.

This is what I meant about the 5 cut method being fiddly. I kept getting confused and adjusting the fence in the wrong direction. The 3 cut method provides an intuitively obvious visual of the magnitude and direction of the fence correction required even to a foggy brain like mine.

If you give yourself a simple rule, its easier:

Negative -> Move Inward (swing end of fence moves into sled)
Positive -> Move Outward (swing end of fence moves out of sled)

Then it becomes less fiddly, at least math and concept wise.

For me, the frustration has been, it seems like even a little flex in the system screws the pooch.

I'm all for simpler, though. I started watching videos on the 3-cut method... I'll have to run through them again. Effort wise, I don't know if I'm convinced its easier? Mathematically, there is less math, but...I'm good with math usually, so the 5-cut method works for me. This guy had a pretty good video...is this the same method you are referring to?

View: https://www.youtube.com/watch?v=2l5DFlCRGKo

 

[Kevin Jenness]

is this the same method you are referring to?

View: https://www.youtube.com/watch?v=2l5DFlCRGKo

No, here's how I do it:

1. Straigten the long edge of a workpiece as wide as your sled will cut and at least twice as long.
2. Make a crosscut in the middle of the piece.
3.Flip one of the halves over end for end, so that it is upside down with the same reference edge against the fence and crosscut it again.
4. Flip it back over and butt the the crosscut ends together in their original orientation, with their reference edges against the fence. Any gap is twice the divergence from square. Assuming the fence is truly straight and your runners are a good fit in the slots you should be able to dial it in to +/- .005" over 24".

 

[Jon Rista]

No, here's how I do it:

1. Straigten the long edge of a workpiece as wide as your sled will cut and at least twice as long.
2. Make a crosscut in the middle of the piece.
3.Flip one of the halves over end for end, so that it is upside down with the same reference edge against the fence and crosscut it again.
4. Flip it back over and butt the the crosscut ends together in their original orientation, with their reference edges against the fence. Any gap is twice the divergence from square. Assuming the fence is truly straight and your runners are a good fit in the slots you should be able to dial it in to +/- .005" over 24".

Gocha. I think I did that yesterday on a test piece, although, it was not as large as you recommend. I'll give it another try. I assume that on this large piece, you would want to make 1 initial reference cut, then rotate so that side is along the fence...THEN follow your steps 2-4 above?

 

[Brian Gustin]

Yeah, if you manage to dig it out, share some photos.

Here they are...
Pic 1 shows the fence swung out, pic 2 shows bottom side including the dual runners I made from an HDPE cutting board from dollar general


I don't worry about the fence overhang of the kerf - It is easy enough to stick in a piece of hardboard scrap along the fence if I need the zero-clearance cut (minimizing end grain tear out)

 

[Jon Rista]

Here they are...
Pic 1 shows the fence swung out, pic 2 shows bottom side including the dual runners I made from an HDPE cutting board from dollar general
View attachment 74995View attachment 74994

I don't worry about the fence overhang of the kerf - It is easy enough to stick in a piece of hardboard scrap along the fence if I need the zero-clearance cut (minimizing end grain tear out)

Ah! I see...yeah, that makes sense. So the blade isn't really cutting through your fence...the blade basically passes by the end of it. Interesting. I had something similar on one of my early bandsaw sleds. I made that out of MDF, it was I think my second sled, and it had many problems. I used it mostly for trying to make segmented pen blanks. I need to revisit it and make another one, but yeah, it had a swinging fence like that. I was thinking of something totally different when you first mentioned this, but now it makes a lot of sense.

 

[Mike Selser]

McLaughlin sure adds a lot of steps to a very simple procedure. Do it the way Kevin Jenness does and save several steps. The 5 cut method is technically more precise but I doubt it makes any difference. This is woodworking not a machine shop. Also adding the R/L factor makes it more precise than marking the measuring point with the test piece especially if the fence is significantly longer than your test piece.

 

[Jon Rista]

McLaughlin sure adds a lot of steps to a very simple procedure. Do it the way Kevin Jenness does and save several steps. The 5 cut method is technically more precise but I doubt it makes any difference. This is woodworking not a machine shop. Also adding the R/L factor makes it more precise than marking the measuring point with the test piece especially if the fence is significantly longer than your test piece.

I can understand that.

I don't know what accuracy is required, to say cut segments for segmented turning pieces (probably mostly vases, to start.) I know that you have a lot of cuts with segmented turning... With all of those cuts, would any error compound and cause problems? That's been the main thing in my mind, I guess. Second to that, I want to make some small boxes. Pen boxes, maybe small trinket/jewelry boxes. Eventually I'd make larger boxes, but I only have the capacity to start small right now. With boxes, making four cuts along each side...at what error level, does a compounded error on the fourth cut, actually matter? I know that 0.020 is too much, I could see the discrepancies with my own eyes. I think, right now, I am probably around 0.006-0.008 I think. I'm concerned that might still be too much, but I don't know...I lack the actual experience cutting and making things with a table saw to actually know.

 

[Brian Gustin]

I can understand that.

I don't know what accuracy is required, to say cut segments for segmented turning pieces (probably mostly vases, to start.) I know that you have a lot of cuts with segmented turning... With all of those cuts, would any error compound and cause problems? That's been the main thing in my mind, I guess. Second to that, I want to make some small boxes. Pen boxes, maybe small trinket/jewelry boxes. Eventually I'd make larger boxes, but I only have the capacity to start small right now. With boxes, making four cuts along each side...at what error level, does a compounded error on the fourth cut, actually matter? I know that 0.020 is too much, I could see the discrepancies with my own eyes. I think, right now, I am probably around 0.006-0.008 I think. I'm concerned that might still be too much, but I don't know...I lack the actual experience cutting and making things with a table saw to actually know.

For Segmenting, you outta have a wedgie sled (someone here will be sure to be able to link to segmented woodturners, I can't think of the URL offhand, its bookmarked on shop computer) Segmenting is indeed compounded errors so the more precision you can get there, the better. For boxes and squaring up boards for boxes, dovetails, and other flatwork, you really ought to have and use a shooting board (See Rob Cosman on YouTube for example, or Wood by Wright for another, or Rex Krueger - many excellent shooting board build videos) because even slightly out of square results in compound errors - so as I mentioned, I may use a sled to cross cut boards "close enough for government work" as they say, but if I am doing a box or other similar fine woodworking flatwork (with cabinetry it's easy to hide little errors , with fine woodwork, it ain't) then I'm going to square and true up my parts with a shooting board and hand plane , marking knives, etc (besides, nothing like that silky smooth finish left by a properly sharpened hand plane - not even sandpaper can beat it) as for Segmenting, I built my wedgie sled based on plans from Segmented Woodturners (Look up @Jerry Bennett on here.) which is an entirely different animal from a typical crosscut sled.

 

[Jon Rista]

For Segmenting, you outta have a wedgie sled (someone here will be sure to be able to link to segmented woodturners, I can't think of the URL offhand, its bookmarked on shop computer) Segmenting is indeed compounded errors so the more precision you can get there, the better. For boxes and squaring up boards for boxes, dovetails, and other flatwork, you really ought to have and use a shooting board (See Rob Cosman on YouTube for example, or Wood by Wright for another, or Rex Krueger - many excellent shooting board build videos) because even slightly out of square results in compound errors - so as I mentioned, I may use a sled to cross cut boards "close enough for government work" as they say, but if I am doing a box or other similar fine woodworking flatwork (with cabinetry it's easy to hide little errors , with fine woodwork, it ain't) then I'm going to square and true up my parts with a shooting board and hand plane , marking knives, etc (besides, nothing like that silky smooth finish left by a properly sharpened hand plane - not even sandpaper can beat it) as for Segmenting, I built my wedgie sled based on plans from Segmented Woodturners (Look up @Jerry Bennett on here.) which is an entirely different animal from a typical crosscut sled.

Thanks, Brian. I actually have looked into shooting boards a few times. I have a couple of hand planes now, and that makes sense, to hand plane to perfection rather than rely on the sled to cut perfectly. I'll plan on making a shooting board and do that once I get into the boxes more. I've watched a bunch of Rex's videos and shorts. I LOVE that guy! I think he is one of the most practical woodworkers around right now. He keeps things simple, practical and on point, all the time. Great guy.

I did pick up a nice marking gauge recently too. It was actually a gift. I've also watched a bunch of Rex's stuff on using marking gauges as well, and for the most part its pretty strait forward. Lot of useful little techniques for getting your marks exactly right.

I'll check out Jerry Bennet here, and see what he's got. Not sure if I have enough wood left for another sled or anything...but I'm also not quite starting the segmenting just yet, and I'm hoping that I'll be out of my financially tight spot here fairly soon. Once I am it should be easy enough to fund another sled.