[pianotech] Measuring Crown Radius

[Joe Goss]

Q Is there a way to measure crown from the bridge side of the board?
Just Checked the crown on a SnS and had a wonderful time laying on the
spider supporting the piano. NOT
Joe Goss BSMusEd MMusEd RPT
imatunr at srvinet.com
www.mothergoosetools.com
  ----- Original Message ----- 
  From: David Love 
  To: pianotech at ptg.org 
  Sent: Tuesday, July 12, 2011 1:06 AM
  Subject: [pianotech] Measuring Crown Radius


  Speaking of special gauges to measure the crown radius, you don't need anything that special.  Attached are a couple of pictures of how to use a simple 15" straight edge and a simple formula to figure out the crown radius on a soundboard.  A bit easier than stretching strings but beware of uneven crown or undulations along the width of the board.  

   

  Using a set of calipers (the one pictured is ordinary but you can use a digital too) measure the width of your straight edge and then tare the scale on the calipers so that when you set the straight edge against the panel and measure the distance from the top of the straight edge to the panel itself you will get a reading of only the difference.  That's how much crown there is in that segment along the length of the straight edge.  Using simple circle math you can then figure out the crown radius given the length of your straight edge and the measured displacement.  

   

  The formula is:

   

  r = ((a^2 - d^2)/2d) + d

   

  Where r is your radius, a is equal to ½ of the length of your straight edge and d is the measured displacement or crown along the length of the straight edge.  You'll have to be consistent with units of course.  If you want radius in feet then you'll have to convert your d measurement to feet.  If you want the measurement in meters then you'll have to convert your d measure in mm's (probably) to meters.  The table below gives you an idea though.  On my straight edge measuring 15" long or 380 mm (381 actually but who's counting).  The right hand columns are the measurements taken and the crown radius is then calculated.  A displacement of .01 inches means a radius of 233 feet or 71 meters.  The board I was measuring in the picture had about this much.  I'd say it had flattened out a bit.  

   

  BTW there's a good article on this particular subject in the February 1984 PTJ written by Bob Hohf.  

   

  On an unloaded board, what would be the minimum amount of crown you'd be willing to work with and why? 

   

   

   

   

   

   

       To measure crown radius
        
            
        Ruler Length  mm (15")
       Length x .5  mm
       Radius (m)
       Radius (ft)
       Measured Crown (mm)
       Measured Crown

        (in)
       
            
        380
       190
       71
       233
       0.25
       0.01
       
        380
       190
       36
       117
       0.51
       0.02
       
        380
       190
       24
       78
       0.76
       0.03
       
        380
       190
       18
       58
       1.02
       0.04
       
        380
       190
       14
       47
       1.27
       0.05
       
        380
       190
       12
       39
       1.52
       0.06
       
        380
       190
       10
       33
       1.78
       0.07
       
        380
       190
       9
       29
       2.03
       0.08
       
        380
       190
       8
       26
       2.29
       0.09
       
        380
       190
       7
       23
       2.54
       0.10
       

   

   

  David Love

  www.davidlovepianos.com

   

   
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