Measuring Surface Flatness (was "Cabinet Grade Particle Board")

[Randall Bearden]

On 11/19/04 7:57 PM, "John Pavlick" <jpavlick at idseng.com> wrote:

> Mark,
> A good way to measure flatness / trueness (is that a word?) is the "winding
> stick" method:
> 
> Take two perfectly straight bars or rods, each painted lightly on one edge -
> one white, one black. Lay them on your bench with the colored edges facing you
> and sight from a distance. You should see one color only at first, then the
> other color should appear evenly as you stand up slowly. If you see it more on
> one end, that is the "high" side. This will show you any twist. To check
> overall "flatness" you can use a long straight edge or the string method. My
> bench is 8' x 27" and this method works for me.
> 
> As far as 2x4's being "overkill" for the bench frame - well yes, maybe they
> are but it's kind of like having too much money or too much horsepower...
> 
> John Pavlick
> http://www.idseng.com <http://www.idseng.com/>
>   
> 
> 
>  
>>  
>> -----Original Message-----
>> From: discussion-request at nsrca.org  [mailto:discussion-request at nsrca.org]On
>> Behalf Of Mark  Grabowski
>> Sent: Friday, November 19, 2004 11:21 AM
>> To:  'discussion at nsrca.org'
>> Subject: Measuring Surface Flatness (was  "Cabinet Grade Particle Board")
>> 
>   
> 
> We've seen some great ideas from several folks. However, a  common element to
> this discussion is that you need to measure how flat the  surface winds up.
> What tricks are you guys using to actually measure how flat  the surface is of
> the bench you just built?
> 
> 
> 
>  
> There is only one way measure true surface flatness and that is with a surface
> place and a precision height meter.  This is a great method if you 1) have a
> large enough surface plate and the space to use. (This could double as a
> dining room table, tell the wife it is solid granite and will match the
> counter tops). 2) you have the money to buy a calibrated height (plus or minus
> 0.0001 inch accuracy).
> 
> Strings are foolish, how do you account for gravitational pull on the strings?
> There is an arc you have to account for.  Boards will warp with temperature
> and humidity changes.
> Maybe your best bet is to go find a billiards' supply house.  Buy the slate
> table is built from. ( I saw this at Scott Anderson¹s house) It is very flat
> and true.  This will be your reference plane, next buy a true strait edge and
> a good set of feeler gages.  You can measure the any deviations of the surface
> with the flat edge and the feeler gages. The slate can be used to verify the
> flatness of the strait edge.
> 
> Now you have had your first lesson in metrology 101, there will be a test
> tomorrow. (LOL)
> 
> Randall Bearden
>  
> 
>  
> 
>  
> 
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