Subject: Need for extreme accuracy?

From: [email-masked] (Michael Rainville)
Date: Sat, 08 Feb 1997 14:20:10 GMT

Is it me or does it seem that people in this group are running around
with calipers much more than is necessary? I've known many woodworkers
who have done fine work without ever uttering how many thousandths
this or that is over, under, whatever. Seems that most have been able
to read to 1/128" (7.8 thousandths but usually known as half a whisker
in these parts) off a tape measure or folding rule. Anything in wood
needing to be much more accurate will be different by morning anyway.
I did break down and buy calipers a few months ago for our production
shop but mainly because we are having some equipment built by an
engineer who needed information to his level of accuracy and
understanding (he is having difficulty relating to the degree to which
wood can change).

Everyone goes to align their TS blade to their miter slot to .00x but
but do they do it when its running and hot (guaranteed it will be
different, even on a PM66) and do they set their miter gauge to the
same level of angular accuracy?
 
I am not lobbying against accuracy but there are limits, especially
when working with wood. Seems that things should be judged on the
results you are getting not by the reading on a dial. Again, is it
invasion of the machinists and engineers or am I out of touch?   

Mike Rainville

---

From: [email-masked] (remove the _ before and after the @ to De-spam) (Keith)
Date: Sat, 08 Feb 1997 19:32:14 GMT

On Sat, 08 Feb 1997 14:20:10 GMT, [email-masked] (Michael Rainville) wrote:

>in these parts) 

<<Snip>>
Actually, The variation for average deciduous species as well as composite
materials that have been dried will show variance of .02mm or .00078" (78- Ten-
thousandths) from dawn to dusk, day in and day out with varied humidity and
temperature changes causing the largest effect on movement..  In our production
facility here, we try to set up machine tolerances to 100% more than the largest
material variance (or .01mm) for all standard panel saw setups as well as boring
machine settings.  This insures that we stay within the +/- .02mm variance of
the material movement / fluctuation itself.  .02mm is about the thickness of a
sheet of toilet paper, for example.

>invasion of the machinists and engineers or am I out of touch?   

<<Snip>>

If you are making widgets for a living, and your widgets have to match other
widgets made in different parts of the world, then there has to be some +/-
Variance standard set.  If you are making window boxes in your garage, that's a
different story.

Sincerely,

Keith

Keith Hill, Project Manager, Forbes & Company
1002 Investment Blvd., Apex, NC. 27502-1954
(919)362-4277/387-7997(Fax)/303-0240(alt-Fax)
 
keithhill @ mindspring.com
www.mindspring.com/~keithhill/

---

From: Colin Montoya-Lewis <[email-masked]>
Date: Sat, 08 Feb 1997 18:45:47 -0700

Michael Rainville wrote:
<snip>
> invasion of the machinists and engineers or am I out of touch?

Hi Mike,
I have to agree with what you're saying here. 
When I was in school my instructors frowned every time 
I pulled out a measuring device.  
They quickly proved to me that with
every tool there is a way to test accuracy with the tool running, by
using sample pieces.  My instructor's biggest pet peeve was people
using a try square to set the miter gauge at 90 degrees.  "That,"
he'd say, "ain't the way it's done."
-Colin

---

From: David Mackenzie <[email-masked]>
Date: Sat, 08 Feb 1997 19:15:57 -0800

I find that when I am working on my small novice projects, I have to
keep telling myself "This ain't rocket science".

David M.
Making some bird feeders to sell.

---

From: [email-masked] (Lonnie Smith)
Date: 10 Feb 1997 16:22:48 GMT

> Agreed,but thank god I read the article on adjusting my Delta 10" TS 
transum here in rec.ww. I swapped blades untill I was blue in the face and 
still couldn't stop my chipping problems. I happened to have an old dial 
indicator from my auto mechanic days and found that my blade was .036" from 
parrallel with my miter slot. I set it parrallel within .00x and now no more 
chipping. Bottom line is that new tools require setup and having an accurate 
means to align them is essential to their proper operation. Just try test 
cutting your transum back in line.

In article <[email-masked]>, [email-masked] says...

---

From: [email-masked] (Michael Rainville)
Date: Mon, 10 Feb 1997 20:20:07 GMT

>Variance standard set.  If you are making window boxes in your garage, that's a

I would agree that a production environment like yours requires a
higher concern for these matters than the average rec.www'er I was
addressing. 

The movement figures you quote offer no scale: per inch? per foot? per
meter? per panel? thus they don't enlighten much.

My figures are (partly as interpreted from an article on WOODWEB):

Dried lumber (<6% moisture content) requires relative humidity under
30% to remain stable. Temperature plays a relatively minor roll. A 7%
change in RH will, over time, change MC by about 1%. That time is
about 4 weeks for it to get 2/3 of the way to being equalized,
depending on how it is stacked. The rate of adjustment is not linear
and will be the fastest in the most immediate period of time (I do not
have a curve for this so I will assume linear and simply suggest the
overnight variance to be higher.)

For maple, expect a size change of .35% for each 1% change in MC
across the width of a flatsawn board.

So, say you have a panel of maple 12" wide and RH changes 7% from one
day to the next morning, your panel is now off by more than .001". A
week at the same RH differential and your panel is now off by more
than .007". 

Although RH currently is pretty well fixed at 40% around my shop, it
easily shifts 20-30% day to day in the warmer months inside and out.
So much for worrying about extreme accuracy where such accuracy isn't
warranted anyway.

I now apologise to myself for getting so technical.

IMO, the greater problem, than size change issues, is twisting and
warping after cutting due to stress relieving from imperfect drying or
MC changes after the fact. These problems are nearly non-existent in
composite materials.

Mike Rainville