Crosscut sled accuracy using the 5 cut method

[bgreenb]

 

Insert a precision-ground bar in the kerf at/in the rear fence : http://www.mcmaster.com/#8954k146/=107ynwi

 

This is awesome.  So simple.  I just placed an order.

(Also love MMC - fast shipping and a ridiculous catalog of stuff).

[ponderingturtle]

Got to .0021" over 20". I'm happy with that but in real terms it made no difference. Before when it was over .006" I did a long test cut and it measured exactly 90 deg using a digital protractor. Did the same and it was still 90 deg. So for practical reasons a few thou makes little difference. For what I do this setting is fine. I am not doing precise engineering metal work.

I now understand Marc's comment about "in the real world .005" is good enough".

And it is for wood, an unstable material that moves on its own and will warp, bend and swell.   It makes sense for the material to be the limiting factor but you only need to go so much better than the fundamental changeability of wood before it is pointless to go for being more accurate.

[Andy Wright]

I guess I am not eligible for this poll because I don't even have a way to measure the difference between .005" and .006" Master craftsman have existed for a lot longer than digital measuring tools - just go build something useful and stop worrying about 1/1000th of an inch in your tool setup

[Trip]

==>And it is for wood, an unstable material that moves on its own and will warp, bend and swell.

Obviously, you're right... But sometimes stacking tolerances come into play, so if you can get it under 2thou for less effort than the 5-cut method, why not? At least that's my excuse...

 

==>I don't even have a way to measure the difference between .005" and .006"

That's the entire point of the 5-cut method... You don't have to...

Of course, you could extend it to the 9-cut method if you like...

[Chris H]

I am not an expert, but if your blade isn't squared to your t-slot, the squareness of the fence is irrelevant.  You are going to have north/south and east/west travel as you advance any jig past the blade, if your blade is out of square with your slot.  May not matter on a very narrow board (depending on the variance), but once you start getting bigger, it's not just inaccurate, but unsafe (bad kickback, if it is pinching toward the fence). 

 

If your blade isn't squared to your slots.  Put the sled down, and fix the more pressing issue with your saw.  

Edited December 16, 2015 by Chris H

[Dan S]

My sled was square to 0.0005" when it was built, but that's mainly because machining is another hobby of mine, and thus my tool set is a little more diverse.

[Coop]

If you want to tweak something, work on the National budget but leave .0067 the heck alone! Cut it!

[collinb]

Just some passing thoughts ...

I guess the real test would be to make 6 cuts in board, perhaps every 10 inches in a 6 ft board.  Lay out the pieces in order. Then flip every other board.  Tighten them together and see how true either edge sits.  If your sled is off by 0.006, then a difference of 0.036 over 6ft may or may not be important. That's about 1/30 of an inch. It's a visible error.  My suspicion is that its importance will depend on the application.

As to measuring with a rule or the 5-cut: The five-cut reminds me of an old rule I learned in grade school for cutting a piece paper in half.  Sure, you can get a precise measurement with a ruler.  But why not just fold the paper in half and cut at the fold.  There are times when the purest, simplest analog system surpasses anything measured.  Faster and more precise.  The 5-cut is an analog approach, simply multiplied.

[bgrella]

On ‎12‎/‎12‎/‎2015 at 4:05 PM, barri said:

Marc quoted in his video that .005" error over 24" in the real world was good enough. I get .0067 over 24" and the large rectangle left over from the 5 cuts appears to have four 90 degree angles as well as a test cut, checked with a set square. I guess that if my sled can only cut to a bit more than 24" and current cuts of that length appear square then i should stop here and start enjoying my sled but I am curious as to where others have stopped and said that's good enough. By the way, I have almost no places to put more more screws as that was my 4th attempt.

Would you stop here or should I try to get under .005" over 24"? What is considered an acceptable figure to aim for? .008"? .004"? Where did other people stop?

If you are getting 0.0067 (not sure how you are measuring out to 0.0001, most cheapo calipers only are reliable to neasest thou +- 0.001) over 24" then that means your sled is aligned to 0.016 degrees from 90 or 89.984 degrees.  And actually, if you include the error from the caliper you are potentially a little further away from 90 degrees.  Not bad.  But, for the amount of work you put into your alignment (you said it took you 4 tries) you are not getting as much bang for your buck as could with a different method. 

Consider this: with an 8" square and a dial indicator, you can get to within 0.002" (0.001" error for the DI and ~0.001" error for the square) over 8".  If you read a difference of 0.001" over 8" of a square (with at least accuracy class2 - most decent squares are class2) your fence will be aligned to 0.0186 degrees of 90 or 89.981 degrees on the first attempt.  No re-do, no test cuts, no headache.

In my opinion the 5-cut method just isn't worth the extra time and hassle.

Another problem with the 5-cut method is that there is a large probability of something going wrong in between the 5-cuts, like a sawdust getting in between the fence and the wood on one of the cuts.  Try doing the 5-cut method and repeating it.  See how close you get to your original error. 

 

 

 

[Dan S]

18 minutes ago, bgrella said:

In my opinion the 5-cut method just isn't worth the extra time and hassle.

Another problem with the 5-cut method is that there is a large probability of something going wrong in between the 5-cuts, like a sawdust getting in between the fence and the wood on one of the cuts.  Try doing the 5-cut method and repeating it.  See how close you get to your original error. 

 

There isn't anything wrong with the 5 cut method! Stuff like saw dust can throw off any method, 5 cut, square, anything else someone can think of. Care must be taken when performing the test, just like it should when you are doing any other finicky task where accuracy matters.

You don't even need special tools to use the 5 cut method, a $10 set of feeler gauges from the local hardware store will get you within 0.002"/ft.

If someone is familiar with the procedure, it will most likely take 2 tries. The first pass is so a measurement can be taken to see how far out the sled is, and the second is to verify the results after a correction is made.

 

[bgrella]

36 minutes ago, Dan S said:

 

There isn't anything wrong with the 5 cut method! Stuff like saw dust can throw off any method, 5 cut, square, anything else ...

 

There are negatives to any method, especially in woodworking.  No method is flawless.  However., when it comes to alignment - something that can be quantified in absolute terms, there are factors one should consider: speed, repeatability (precision) and potential accuracy.  The DI method is tough to beat when considering these factors.

Special tools required?  There are too many uses for a DI not to already have one. 

Use what ever method makes you feel good and squishy on the inside. 

[woodsmoke]

Notice the brass 1/8" bar has a tolerance of +\- .oo8"?

That nay not matter if the error is constant the whole length.

 

 

[gee-dub]

If your saw is well aligned and your fence on your sled is reliably adjustable, .001" is reachable and .002" is very reachable.

[BobInAustin]

Mine was "good enough".  I don't remember the number but I cannot tell that anything is not square.

[barri]

I'd like to resurrect this old thread as I tried Brian's dial indicator method and its absolutely brilliant. I used a good quality square which has a 10" edge and I got an error of .001". I then checked with the 5 cut method and got exactly the same error over 10". All within a few minutes without one cut. I am a former math teacher and It makes sense that the fence is aligned with the slots not the blade as that's the path the sled takes.  If the blade isn't perfectly aligned with the slots then you'll still get a straight square cut but with a wider kerf. See video previous page. If you own a dial indicator then I'd love you to try this and report back before you criticise this. I am a total convert. Thanks Brian.