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5 cut method - how good is good enough?

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25K views 14 replies 12 participants last post by  Pezking7p  
#1 · (Edited by Moderator)
After watching the William Ng and reading a few comments over on his site, it seems 0.001" is the ideal tolerance. Now that I've given it a few tries and growing tired of measuring and re-adjusting my crosscut fence I'm asking myself, how good is good enough?

My latest measurement is 0.019" across a length of 18.4375" which to use the formula is:

Top of 5th cut, A = 0.625"
Bottom of 5th cut, B = 0.656"
Length of off-cut = 18.4375"
Distance from pivot point to point of adjustment = 44"

(0.625 - 0.656) / 4 / 18.4375×44" = 0.019"

Seems good enough to me. In what conditions would I see this measurement of error cause problems? The maximum length my crosscut sled can handle is 24 inches, which means I would see a -0.014" deflection. ( My math could be way off, please be gentle. )

Thanks!
 
#3 ·
In a way it is a personal thing. I have spent a lot of time on my woodworking machines to try to have them dialed in as much as possible. This is a good and bad thing. I agree with asking the question "when is good enough good enough". I guess that is a tail I still catch myself chasing. Wood is alive and will move, right? For me it is about eliminating guessing where I went wrong. If I know my sleds, or machines are dead-nuts, I do not have to look at them for blame in my errors. You would probably never notice the errors you are talking about. With that being said, I only had 4 test cuts and 15 minutes when I did my sled I got mine to 0".
 
W
#4 ·
I think your math is wrong. I think the 0.019 is the amount you need to adjust the fence to bring it to zero. To be sure I would have to go thought the video again and then try it in my shop. I got my fence dialed in to around 0.004. I found that I had more inaccuracy in pushing the sled along the miter slots.
 
#5 ·
I think your math is wrong. I think the 0.019 is the amount you need to adjust the fence to bring it to zero. To be sure I would have to go thought the video again and then try it in my shop. I got my fence dialed in to around 0.004. I found that I had more inaccuracy in pushing the sled along the miter slots.

- WoodNSawdust
Click to expand...
I figured it was 0.019" to bring it to 0 in my case, but then how is the deflection across length calculated?
 
#6 ·
Plywood cases go together twisted a little anyway. There's
usually a little persuasion and compromise involved. I
emphasize making the front of the case square and let
the back opening be crooked if it needs to be. You
won't succeed in tuning this kind of everyday carcase
twist out of your work by making your cuts square to
within 1/64th tolerances. There are methods to make
cases come out more square of course, but they have
to do assembly methods and tools like pneumatic case
clamps and wood-welders to set the glue in a few minutes.
This is beyond the scope of what you need to do working
on a small volume of material in a small shop.

Where it gets interesting, if you want to go there, is
tuning and fitting up smallish cabinets and chests
of drawers so everything looks and works just so,
fitting drawers nicely and things like that. Handwork,
really.

1/64" squareness error over 12" is very acceptable
in general, imo. I've found it difficult to get my
panels squarerer than half that error, which comes
out to a 1/32" error when doing a 5-cut test on
a 4' square panel.
 
#7 · (Edited by Moderator)
Ok, so I think I've figured out that my deflection is at a rate of 0.0017" given my current alignment. Using the slope formula. Again, I suck at math so I could be wrong here.

That's more than acceptable to me. Given the maximum cut capacity in my sled is 24" I would see a difference of 0.042" between the bottom and top of the cut, or 5/119ths in other words.

Thanks for indulging my random drivel as I scurried down this rabbit hole.
 
#10 ·
4.2% of an inch is too much for me.

I got mine to .002-.004 over 24" using the same method. It was confusing at first but after 1 adjustment it was close enough.
 
#13 ·
I wrestled with mine for a while and got it to about .005". That is about the thickness of a sheet of paper.

I always allow a little wiggle room when cutting and gluing up cabinets. If I try to cut everything the exact size desired, then something as simple as a burr of a tiny wood chip can throw it off. I think you have to cut yourself some slack.

BJ
 
#14 ·
I wrestled with mine for a while and got it to about .005". That is about the thickness of a sheet of paper.

I always allow a little wiggle room when cutting and gluing up cabinets. If I try to cut everything the exact size desired, then something as simple as a burr of a tiny wood chip can throw it off. I think you have to cut yourself some slack.

BJ

- BJODay
Click to expand...
This is the exact number I ended up on today. I decided to go back and give it another go and now I'm at 0.005" and calling it done. For now. ;)

I'll make another sled another day and do this all over again with a bit more experience and knowledge, hopefully getting me closer to 0 next time.
 
#15 · (Edited by Moderator)
OK, you have 0.031" off on your off cut. That is an error over 4 cuts, so your error is 0.031/4 = about 0.008" off over 18". That's not great. I personally would shoot for less than 0.005" in 24". Very do-able with a set of feeler gages. I can't remember if I used william ng's video or the wood whisperer's video, but I know one of them lays out the procedure for making the correct adjustment. If you follow the guide, you should hit near perfection after 1-2 tries.

I know a lot of guys say "it's just wood, wood moves naturally, etc etc", but trust me, when you start fitting together multiple pieces and frames, a few thousandths adds up quickly. There is enough variability in wood anyway, 90 degrees is one variable I like to remove. Things start to fall together quickly and without fuss when everything is flat, straight, and SQUARE.

EDIT: Dang, you responded while I was typing. Nice job and cheers.
 
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