How do you measure "flatness"
 

I got ready to measure flatness of my bench, and realized that I don't
know exactly what is meant by the various measures I have seen.

I've seen statements about a table's "flatness", expressed in
thousandths of an inch or in thousandths of an inch per foot, e.g flat
to .002" or flat to .002"/ft. Are people using those interchangeably,
or does the first mean literally over the entire surface, deviation
from one point to another is no more than .002? And in the second
measure, does that mean that using a 1-ft straight-edge, any dip in
the middle is less than .001" and any rise in the middle will cause
one end of the straightedge to be high by .002" when the other edge is
held down?

In a wooden bench, I am much more worried about eliminating twist than
absolute flatness, but I wonder what is a reasonable goal? I figure
that if I can't fit a .005 feeler gauge anywhere under my 4-ft
straight-edge no matter how it is oriented, that I have attained more
flatness than I can reasonably expect to keep for any length of time
due to normal wood movement. Is that right, or should I strive for
better?

This is my first experience flattening with hand planes, and I am
pleasantly surprised at how easy it is to get pretty darned good
flatness with just a home-made scrub, a flea-market #5c, and my prized
"worker" 100-year-old #8.
--
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

On Sat, 15 Jan 2005 12:32:30 -0500, alexy wrote:

I got ready to measure flatness of my bench, and realized that I don't
know exactly what is meant by the various measures I have seen.

Doesn't move when you sit on it, flat and level enough not to spill
your coffee mug.

I care much more about stability than I do about flatness. If I was to
start obsessing about flatness, I'd go for the assembly table, not the
workbench.

--
Smert' spamionam

On Sat, 15 Jan 2005 12:32:30 -0500, alexy wrote:

I got ready to measure flatness of my bench, and realized that I don't
know exactly what is meant by the various measures I have seen.

I've seen statements about a table's "flatness", expressed in
thousandths of an inch or in thousandths of an inch per foot, e.g flat
to .002" or flat to .002"/ft. Are people using those interchangeably,

not properly...

or does the first mean literally over the entire surface, deviation
from one point to another is no more than .002?

yes.


And in the second
measure, does that mean that using a 1-ft straight-edge, any dip in
the middle is less than .001" and any rise in the middle will cause
one end of the straightedge to be high by .002" when the other edge is
held down?

if the high point is in the center, yes.



In a wooden bench, I am much more worried about eliminating twist than
absolute flatness, but I wonder what is a reasonable goal? I figure
that if I can't fit a .005 feeler gauge anywhere under my 4-ft
straight-edge no matter how it is oriented, that I have attained more
flatness than I can reasonably expect to keep for any length of time
due to normal wood movement. Is that right, or should I strive for
better?

that would be a nice flat wood bench.





This is my first experience flattening with hand planes, and I am
pleasantly surprised at how easy it is to get pretty darned good
flatness with just a home-made scrub, a flea-market #5c, and my prized
"worker" 100-year-old #8.


fun, eh?

"alexy" wrote in message
...
I got ready to measure flatness of my bench, and realized that I don't
know exactly what is meant by the various measures I have seen.

I've seen statements about a table's "flatness", expressed in
thousandths of an inch or in thousandths of an inch per foot, e.g flat
to .002" or flat to .002"/ft. Are people using those interchangeably,
or does the first mean literally over the entire surface, deviation
from one point to another is no more than .002? And in the second
measure, does that mean that using a 1-ft straight-edge, any dip in
the middle is less than .001" and any rise in the middle will cause
one end of the straightedge to be high by .002" when the other edge is
held down?

It's a matter of relativity.

Flat is good enough for what's it's doing. If pieces don't rock on your
bench, either both are flat enough or both OTL.

Andy Dingley wrote:

Doesn't move when you sit on it, flat and level enough not to spill
your coffee mug.

I care much more about stability than I do about flatness. If I was to
start obsessing about flatness, I'd go for the assembly table, not the
workbench.
Thanks, Andy. I suspected that the right answer for flatness of the
bench was "flat enough that flat workpieces don't rock when you are
working on them". On the other hand, I am concerned about avoiding
twist and that the front of the bench (which will be the rear jaw of
my shoulder vice) be perpendicular to the top. Better to plane pieces
square and cut square joints than to force them square during
assembly! g

And yes, I was obsessing about flatness, just for the fun of seeing
what is possible with how much work. I suspect that I've got it a lot
flatter than I need it, and that I will not maintain it to this degree
of flatness.
--
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

George wrote:

Flat is good enough for what's it's doing. If pieces don't rock on your
bench, either both are flat enough or both OTL.

Nonsense. If the bench isn't flat to within 0.000001" your projects will
explode due to PVC dust in your 18 HP shop vac.

--
Michael McIntyre ---- Silvan
Linux fanatic, and certified Geek; registered Linux user #243621
http://www.geocities.com/Paris/Rue/5407/
http://rosegarden.sourceforge.net/tutorial/

alexy wrote:

And yes, I was obsessing about flatness, just for the fun of seeing
what is possible with how much work. I suspect that I've got it a lot
flatter than I need it, and that I will not maintain it to this degree
of flatness.

Oh, for my real answer... I have a hunk of Scarey Sharp(tm) granite. I
used that to test the top for flatness. I can slide that thing around
pretty much everywhere without it rocking, so that's probably flat enough.

I'm worried about flatness because I also use my bench for gluing, say,
chess boards together. That granite thing seemed to do it though. No
feeler gauges or anything, just making sure a piece of granite
approximately the size of 1/3 sheet of sandpaper doesn't rock anywhere on
the bench. I'm sure it's not up to the standard I just posted about, but
I'm too lazy to drive somewhere and find a set of feeler gauges anyway.

--
Michael McIntyre ---- Silvan
Linux fanatic, and certified Geek; registered Linux user #243621
http://www.geocities.com/Paris/Rue/5407/
http://rosegarden.sourceforge.net/tutorial/

Silvan responds:

I'm worried about flatness because I also use my bench for gluing, say,
chess boards together. That granite thing seemed to do it though. No
feeler gauges or anything, just making sure a piece of granite
approximately the size of 1/3 sheet of sandpaper doesn't rock anywhere on
the bench.

Grizzly has a no-ledge 24"x24" chunk of granite accurate to within .00015".
That ought to do it for you. By the time it arrives, you'll have laid out
something close to $230, and you'd best have braced your bench to take its 234
pounds. You'll also need a couple helpers to get it ON the bench.


Charlie Self
"One of the common denominators I have found is that expectations rise above
that which is expected." George W. Bush

Charlie Self wrote:

Grizzly has a no-ledge 24"x24" chunk of granite accurate to within
.00015". That ought to do it for you. By the time it arrives, you'll have
laid out something close to $230, and you'd best have braced your bench to
take its 234 pounds. You'll also need a couple helpers to get it ON the
bench.

Nah, free is good. I think this was a piece of counter backsplash. I used
to deliver to a place next door to a stone counter maker. I picked up a
whole gob of this stuff. Unfortunately, I'm down to two pieces. It hasn't
stood up very well. The top, smooth layer eventually wants to flake off
after a few times in the saddle as a Scary Sharp(tm) stone.

I might have to see about getting some pieces of plate glass or something.

--
Michael McIntyre ---- Silvan
Linux fanatic, and certified Geek; registered Linux user #243621
http://www.geocities.com/Paris/Rue/5407/
http://rosegarden.sourceforge.net/tutorial/

"Silvan" wrote in message
...
Charlie Self wrote:

Grizzly has a no-ledge 24"x24" chunk of granite accurate to within
.00015". That ought to do it for you. By the time it arrives, you'll have
laid out something close to $230, and you'd best have braced your bench
to
take its 234 pounds. You'll also need a couple helpers to get it ON the
bench.

Nah, free is good. I think this was a piece of counter backsplash. I
used
to deliver to a place next door to a stone counter maker. I picked up a
whole gob of this stuff. Unfortunately, I'm down to two pieces. It
hasn't
stood up very well. The top, smooth layer eventually wants to flake off
after a few times in the saddle as a Scary Sharp(tm) stone.

I might have to see about getting some pieces of plate glass or something.

go to a place that makes windows and does shower door installs. they won't
give you any scrap usually, because of liability, but the one i went to
said, that dumpster over there gets filled at the end of the day, and we
lock up and go home at 6pm.

that said, i just cut up a 3/4" glass 4'x4' dining table top i got for a few
dollars at a yard sale.

--
Michael McIntyre ---- Silvan
Linux fanatic, and certified Geek; registered Linux user #243621
http://www.geocities.com/Paris/Rue/5407/
http://rosegarden.sourceforge.net/tutorial/

Charles Spitzer wrote:

I might have to see about getting some pieces of plate glass or
something.

go to a place that makes windows and does shower door installs. they won't
give you any scrap usually, because of liability, but the one i went to

Good suggestion, but actually I know a guy who works in a glass shop, who
owes me a favor. :)

(Too bad he doesn't work for a lumber dealer. I would have cashed in the
favor much sooner.)

--
Michael McIntyre ---- Silvan
Linux fanatic, and certified Geek; registered Linux user #243621
http://www.geocities.com/Paris/Rue/5407/
http://rosegarden.sourceforge.net/tutorial/

Depends on what kind of "flatness" you are talking about. In the highway
installation sector, the flateness of the road is a part of the installation
contract. If there are complaints of the "flatness" of a particular section
of road, the DOT has a vehicle that is sent along the offending stretch of
road with meters and insturments (obstensibly set to some desired limits) to
measure the flatness of that stretch. If the measurements are exceeded, an
adjustment is required. I'm not an expert in this, but you can be assured
they do require and make adjustments. "Flatness" is a measurable component.
Wolf-==-


"Charles Spitzer" wrote in message
...

"Silvan" wrote in message
...
Charlie Self wrote:

Grizzly has a no-ledge 24"x24" chunk of granite accurate to within
.00015". That ought to do it for you. By the time it arrives, you'll
have
laid out something close to $230, and you'd best have braced your bench
to
take its 234 pounds. You'll also need a couple helpers to get it ON the
bench.

Nah, free is good. I think this was a piece of counter backsplash. I
used
to deliver to a place next door to a stone counter maker. I picked up a
whole gob of this stuff. Unfortunately, I'm down to two pieces. It
hasn't
stood up very well. The top, smooth layer eventually wants to flake off
after a few times in the saddle as a Scary Sharp(tm) stone.

I might have to see about getting some pieces of plate glass or
something.

go to a place that makes windows and does shower door installs. they won't
give you any scrap usually, because of liability, but the one i went to
said, that dumpster over there gets filled at the end of the day, and we
lock up and go home at 6pm.

that said, i just cut up a 3/4" glass 4'x4' dining table top i got for a
few dollars at a yard sale.

--
Michael McIntyre ---- Silvan
Linux fanatic, and certified Geek; registered Linux user #243621
http://www.geocities.com/Paris/Rue/5407/
http://rosegarden.sourceforge.net/tutorial/