How close is close enough...

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Greetings all...
I have been constructing a new crosscut sled, and the thought came to me, how close does this need to be to make nice joints...
Instead of using a square I cut the sides off an 8" square of birch plywood, I then ripped a strip off the first side and measured the difference with a dial caliper, and over 8" there is less than 1/128 difference, but that would be compounded over 4 cuts right?
So would you guys try to get it closer of just leave it as is?
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I might try to get it closer but some times measurements can deceive or may not be done accurately. Sawing technique can often render measurements useless.
Use some scrap wood, make a frame and ask yourself, is that good enough?
Keep in mind that opposite parallel sides must be exactly the same length also or even perfect 45's will not close properly. Use a stop to insure same length "opposite" sides.
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its kinda funny....but I was able to get it almost perfect, within a needles width on the dial caliper, I would think that would be close enough, but last night I had painstakingly straightend and squared the bit that would become the main fence and after all that I managed to install it with the wrong face to the saw....after switching things around my test cuts came out much better....
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So why not eliminate the 'sawing technique error from the equation when calibrating the fence?? Hmmmm.

This won't do it. 'Sawing technique error still there.
--
Brian
www.garagewoodworks.com
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RE: Subject
Remember the old saying, "A flying Red Horse can't spot the difference from 1,000 ft."
Still works.
Lew
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When calibrating tools 'eye-balling it' is NEVER good enough in my shop - wooden or otherwise.
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Brian
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Garage_Woodworks wrote:

In lots of situations, the precision of decent eyeballs and finger tips is actually very useful for checks.
Two examples:
Thickness planer parallelism - plane a board, cut it in half, put opposite edges together, rub the fingers over the meeting point.
Table saw blade or miter gauge/sled 90 degree accuracy - Cut a board in half, flip one board, place the cut edges together and check for gaps.
If they don't work out, the calibration tools make recalibration easier and faster than more test cuts, but the cuts are good enough for in-service spot checks.
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Perhaps one day you will learn to make eye-balling work for you. It certainly speeds up production.
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Perhaps one day you will learn that precision is better and faster when you learn the tricks.
--
Brian
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That is not what I was talking about, I'd be willing to bet that you would find nothing wrong with my precision and or joints. It's that with years of experience you learn how to achieve that with out having to use high precision measuring tools with every adjustment.
Case in point, you bought a new Powermatic 2000 TS. If your measuring and precision set ups on your old Delta saw were good why change saws? I'm just saying that a precision measurement is not always the answer to a problem.
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I wrote:

It sparked a spirited discussion; however, it clearly defines a few things.
In the data acquisition business, if you truly want an accurate measurement, you make a differential measurement, not a single ended one.
Single ended measurements introduce instrument as well as people errors into the measurement, something differential measurements eliminate because they cancel out.
Using ANY measuring instrument to make a measurement is a single ended measurement.
Ask 5 people to make a measurement with a micrometer of vernier caliper, and chances are pretty good you will get 5 different answers.
Set up a table saw, cut a piece, then break the set up.
Now reset the fence to the same dimension, cut a 2nd piece and compare it with the first.
They will be close, but they will be different.
I submit your fingers are the best instrument for this measurement.
Biggest reason I know for "sizing" all the material for a job using a single setting for the tool (Planer, table saw, etc) as the first step in processing the wood.
BTW, you also do it all at the same time.
Weather conditions tomorrow will be different than today.
Lessons learned the hard way AKA: Expensive.
Lew
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That would be my point. I am indicating that measuring will not always over come techique and when you are talking a visible or invisible joint line the technique problem may be so small that it could involve dozens of factors. Including but not restricted to, is your table flat, is your stock perfectly straight, are you working with soft or hard wood, is you blade "sharp", is the surface of your table smooth, and the list goes on.

It certainly does it for me. Because we are not machines there will always be some degree of imperfection with every cut. Measurements only get you so close and if you were able to obtain the perfect setting your technique will always add some degree of error from one cut to the next.
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Then eliminate the 'factors' you have control over. Like a precise calibration.

All variables that effect final result. Which of those do you have control over?

You are missing my point. See above.

See above.

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That is fine but if the precice calibration only corrects 10% of the problem you need to learn how to make compensations to remedy the problem.
AND going back to my original points to the OP, dial calipers are good for measuring and setting up equipment. The results of using that equipment is not guaranteed to give good results when using wood as the material to construct with.

All of them if you can compensate, but for instance a precicely calibrated saw will not eleminate tear out on the back side of a cut. There are other factors to consider to minimise the tear out. Excessive tear out can ruine a joint and technique can certainly minimize tear out. For instance when cutting a 45 degree miter on your TS if you have the miter gauge clocked to 45 in a particular direction it prioduces a better/cleaner cut than if you clock the gauge to 45 degreesin in the opposite direction. This all falls into technique and no amout of precision will make both setting equal in results on a consistant basis.
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Garage_Woodworks wrote:

Geez, even optical flats have tolerances. Figure out what your tolerances need to be and then set up your system to maintain them.
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I couldn't agree more with everything Leon said, not the least being to try it out to see for yourself.
You are talking about making a wooden device that you think will hold tolerances to within one 0.0078125th of an inch. Think about it; a wooden jig that will hold completely true through humidity changes, temperature changes, techniques differences, movement of the sled in response to different weight, density and size of material, etc., etc. Not happenin'.
Don't cheat yourself For dead bang spot on, buy a calibrated miter gauge.
Or... just use your miter saw. I for one have never understood the folks that have to use their tablesaw for everything from precision miter work for small pieces like building picture frames to making raised panel doors.
Maybe its just me. I could see it on really large pieces I guess, but even then... how often does one need a miter on a 12" board?
Robert
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wrote:

So why not eliminate the error you have control over (the original calibration)?? Why settle for a sloppy calibration because you might experience wood movement?
Error Total = Error in calibration + error from wood movement over time + ...
The 'error in calibration' can be eliminated completely w/o making a single test cut.
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Brian
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You don't even need to settle for the 'error from wood movement'. Just check the dam thing before you use it and fix it.

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LMAO.... pretty hot issue, eh? I like it when someone replies to their own posting.
You missed my point. You should always get your jigs, measuring devices, etc. as close to perfect as you can.
We were talking about a >>WOODEN<< jig. In my experience, I have never seen a usable wooden jig hold to perfect tolerance under all conditions. Close, but not perfect. Obviously you feel differently. To me, if I am to put a lot of time and effort into making something, it needs to a design that can perform to the standards I want. Wood can be a satisfactory fabrication medium for certain things, but not repeatability of tolerances within a couple of thousands over a period of use.
But since DCH didn't post what size board he was using, how do we know what the table saw is the best tool for the job? What if he is cutting 4" or 6" wide material? Should he be using a table saw and a homemade sled?
Certainly if that is the case, I would put any of my three miter saws against his table saw and shop built jig. And for compound cuts, a table saw is not even a consideration if I can get it under my miter saw.
The milled aluminum and steel bed and fence calibrated with my machinest's square are much more comfortable for me to rely on than plywood, mdf, white glue,. hardwood, etc.
Robert
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wrote:

I missed it because you never made this point. Where was it posted?
You stated that you don't understand folks that like to use their table saw for precision miter work.

Check it before you use it. I do. And w/o a single test cut.

I would take that challenge.

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