# Crosscut Sled Quandry



## MarkwithaK (Sep 12, 2009)

The other day I built a new crosscut sled. I watched the video by William Ng and used his equation to square up the fence.

A-B/4/ Length of cut X distance of fence.

After plugging in my numbers I get:

1.093 - 1.089 = .004/4 =.001

At this point I'm thrilled.

My issue stems from when i plug that number into the rest of the equation it tells me that I need to move the fence up .0026.

Being at .001 is great…I wouldn't even gripe too much at .002 but having to move almost .003 starts to concern me.

Am I correct in that the .001 is how far I am off square as it sits and .0026 the amount I would have to move based on a the full length of my fence?


----------



## Jeff82780 (Mar 15, 2010)

its been a while since I saw William ng's video, but I don't ever remember plugging a number back in, I believe you are off .001 and you need to use a .001 feeler gauge and move it that distance. Anything up to .0003 is extremely good and I would leave alone. You may end up making things worse if you keep fiddling with it. Good Luck!

-Jeff


----------



## RPhillips (May 16, 2013)

I don't recall it saying how far to move the fence, as this would pend on the distance between the axis that the fence would move on, which would have to be calculated into the equation as well.

I'll be building one soon following William Ng video as well.


----------



## MarkwithaK (Sep 12, 2009)

Sorry, the equation is to calculate the error ratio which according to Ng is the distance to move the fence. In my case that would be 26" from the pivot point (far right side of the fence).

About 7 or 8 minutes into the video.

5 Cuts ti a Perfect Cross-Cut Sled


----------



## BJODay (Jan 29, 2013)

Mark,

A piece of paper is typically 0.004" thick. Don't beat yourself up over such a small discrepancy.

BJ


----------



## MrRon (Jul 9, 2009)

The fence distance can be anything you want. If you make the fence length a distance measured from the pivot point to say 24" to the left of the pivot, that is where you measure your deviation at. If 30", then that is where you measure the deviation. Just make a mark any distance you want from the pivot and that is your reference point where you measure your deviation. The further away you get from the pivot, the greater will be the deviation.


----------



## MrRon (Jul 9, 2009)

What is your length of cut? You have to divide .001" by length of cut and then multiply that by 26".


----------



## rg33 (Dec 1, 2012)

the setup for this as I recall is you cut four sides (rotating 90 degreees each time) then you cut a strip of the fourth end (the side you first cut). Measure the difference in widths and that is how far out of square you are on the ENTIRE PERIMETER OF BOARD you just cut. So if you had a 20" by 20" piece this would be the error in 80". I dont know how big the piece you were cutting was but if your total error was .004" then forget about any more adjustments. to give you an example I was cutting a piece roughly 20X20 and I ended up with .018" difference in width on the strip I cut that means that I am only out of square .0045" per 20" or less than .003" per foot which is more than fine for 99.999% of any woodworking anyone will ever do. The woodwhisperer has a good point on accuracy here:
http://www.thewoodwhisperer.com/articles/how-much-error-is-too-much/


----------



## waho6o9 (May 6, 2011)

Maybe measure your diagonals.

If they're the same you'll be fine. Leave the .001 stuff
for the machinists.


----------



## bowedcurly (Aug 31, 2013)

.0003 hell that's to bearing fit, there is nothing on a table saw that's even near that, just sayin


----------



## wormil (Nov 19, 2011)

Been a long time since I've watched that video but if memory serves it was the most convoluted way to square a fence I've ever seen.


----------



## jmartel (Jul 6, 2012)

bowedcurly, wouldn't you say that the arbor bearing in the table saw has a bearing fit?


----------



## oldnovice (Mar 7, 2009)

I use 1/4" tempered Pegboard mounted to the runners and mount the fence using the holes in the Pegboard. I have used this technique on a number of sleds and found less than .002" error.

I have two steel rails that have both 1/4" through holes (at the far ends) and threaded holes between them. I drive dowels through the Pegboard into the rails (typically a very tight fit) and then counter sink the holes that align with my rails threaded holes. The fence must be carefully constructed since that determines true 90°. I drill 1/4" dowel holes and 1/4" threaded holes (with inserts) to align the fence to the Pegboard and use the same methodology when mounting the fence as I used for the rails.

The reasoning for using peg board is that the holes are very accurate and I can also make a 45° sled by using the appropriate holes in the peg board.

My PC is in for motherboard repair otherwise I could post the entire process!


----------



## wormil (Nov 19, 2011)

The pegboard idea is very interesting. I look forward to your motherboard returning so you can post pics.


----------



## exelectrician (Oct 3, 2011)

Buy a good quality sliding chopsaw and ditch the sled, forever.


----------



## comboprof (Aug 26, 2013)

It seems you have not told us what the length of the cut was, i.e the distance between where you measured A and B. But your calculations suggest this was 10". Also you give 26 as the distance from the pivot to where on the fence you plan to make the adjustment. If so you are correct and need to adjust by .0026. By why are you concerned. It is what it is. The error could have simply been introduce by you when you set the original sled rear fence. Because it is so easy to make the adjustment I would do it. On the other hand as they say here it may not make any noticeable difference in your projects anyway.

In my opinion William Ng method is brilliant, accurate and easy to use. It is not convoluted at all.
The crux of his method is that it finds a way to use the digital caliper to accurately measure the error.


----------



## Shan (Aug 17, 2009)

You are making this too hard, and too complex. I made a video showing to to make your crosscut sled dead on accurate. NO MATH INVOLVED. 



 Hope this helps


----------



## BigMig (Mar 31, 2011)

When I built a crosscut sled, I used the 5 cut method that the Wood Whisperer uses. It's pretty easy to do and to calculate your error. But beware of seeking perfection. It's elisuve and not likely worth all the time it'll take to dial out 0.001. Plus - you're assuming your measurement tool has no error - not likely, right?


----------



## HorizontalMike (Jun 3, 2010)

In addition to what @BigMig says regarding "seeking perfection", I would add that you will lose some of that perfection as you use your crosscut sled, over time. The runners do wear and over time you will find that you will develop ever-so-slight side to side slop with regards to 90-degrees crosscuts.

I see that error build up even on my own Super Sled . The error is not enough to really worry about and "technique" can correct that most of the time. Just be careful how you are moving the sled across the blade.

My 2-cents…


----------



## usnadad (Aug 26, 2011)

William Ng's method is fine, precise and not that hard to do.

The basic issue is accuracy.

There are a couple analog steps in the process that introduce variation.

If you repeat the same process with the same fence position 5 times, I bet there would be variance in the result to a greater or lesser degree.

When you measure thickness of wood with the calipers, fer instance, do you apply the exact same amount of pressure when squeezing the calipers to the wood's thickness? Wood does compress, as do you finger tips. Do you place the calipers in the same place? The surface of wood itself varies in texture introducing a thousanth or so error, potentially (I have no idea how much, but some). Wood varies (in the same piece of lumber) in compressibility, due to orientation of grain, knots, etc. When you squeeze the calipers, you compress the wood a little.

Do you think you can repeat the measuring process with less than a few thousands of an inch accuracy?

I can't.

The upshot is that Ng's method is fine, but the analog elements (the meat device…us) introduce error.

When you start talking about thousandths of an inch and wood, I think you are in the area of wood quantum mechanics.


----------



## wormil (Nov 19, 2011)

Much faster to use a dial indicator.


----------



## oldnovice (Mar 7, 2009)

*usnadad* you "hit the nail on the head" with your assessment of William Ng's process. Each of the steps and measurment technique can/will introduce errors by themselves. So how many iterations are required before all errors are removed … if they can be at all!

*In theory William Ng's process is perfect for setting the sled true … the catch is that it is performed by humans.*


----------



## Woodbum (Jan 3, 2010)

I found the Ng method fairly simple and easy to use. You MUST be patient however, cause it takes time to get it right on the mark. BTW, I think that Marc's 5 cut method is Williams Ngs method, in shorthand. No method is perfect due to the factors that usnadad stated, but this gets me closer than all other methods that I have tried. Thanks William. For me, there is no such thing as perfection. While we may strive for perfection, this elusive quality is just varying degrees of "close enough for what it is and who it's for".


----------



## NiteWalker (May 7, 2011)

You can buy metal runners (incra's SE versions are really nice) and not have to worry about wearing those out.


----------

