[Harp-L] Re: Embossing and Chamfering

["Vern Smith" <jevern@xxxxxxx>]

Thank you, Rick!

There is no substitute for quantitative information from an expert.  

Lord Kelvin quotations:

"To measure is to know."

"If you can not measure it, you can not improve it."

"In physical science, the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be." [PLA, vol. 1, "Electrical Units of Measurement", 1883-05-03

 

----- Original Message ----- 
  From: Rick Epping 
  To: harp-l@xxxxxxxxxx 
  Sent: Saturday, December 27, 2008 12:42 AM
  Subject: Embossing and Chamfering


  A few notes on embossing and chamfering:



  Embossing:


  ...What clearance do you aim for when you emboss?



  When embossing for maximum performance I aim for a clearance of about 0.01-0.02mm along most of the length of the reed. Approaching the rivet end however, this clearance might be reduced up to a zero clearance without causing difficulty, at least in the higher-pitched reeds.  This is possible because there is little reed movement near the rivet end to interfere with the slot, particularly in the higher reeds, which are relatively thick there.  Some accordion reed specifications have included a zero clearance near the rivet end.



  I emboss the free end of the slot much the same as the sides, but if there's excessive initial clearance I don't emboss to the degree that the level of the burr is more than a couple of tenths of a millimeter lower than that of the sides.  After embossing, it's important to lower the reed at its rivet end into the slot to compensate for the embossing having lowered the effective edge of the reed slot.



  One result of embossing is that it creates a very short moment in which the swinging reed almost completely shuts off the airflow through the slot, similar to the effect produced by a thin reedplate.  This amplifies the upper partials of the note produced by the reed, producing a brighter tone - not necessarily a desirable characteristic. An unembossed harp with decent factory clearances, though requiring somewhat more effort to play than a closely embossed harp, can produce a tone more suitable for some musical applications.  I play Marine Bands with DymondWood or sealed pearwood combs and closely embossed reeds when playing loud, hard-driving music and prefer Special 20s with more moderately embossed reeds for softer music.


  ...How do you measure the clearance?



  During the course of developing new reed/slot clearance specifications with Hohner, I evaluated a number of vintage, production and test sample reedplates, the clearances of which were measured by Hohner's CNC video measuring microscope.  Using this machine while at the factory I was also able to measure the clearances of reeds I embossed.  



  I had no such means of making absolute measurements back in my office in Richmond but I was able to make relative comparisons of different reed/slot clearances using a microfiche reader I salvaged when Hohner switched its record keeping to a CD-based system.  By placing reedplates on the fiche carrier I was able to get good silhouette images of clearances, which were then easily photographed.  



  From this work I developed a reasonable eye at estimating reed/slot clearances, so, while I don't measure the clearances of the harps I now work on, I know fairly well what to look for through a lens.  





  I have two tools I use for embossing, both of which are shown at the following link:



  http://docs.google.com/Doc?id=dckkn2d6_22m6mx4gf8





  One tool is for quick and easy embossing, where both sides of the slot are embossed at the same time but where the slot can only be embossed about 2/3 of the way back to the rivet without affecting the offset of the reed.  The other tool is for embossing the entire slot, one side at a time.  Also shown on the above link is a tool I use for clearing the slot if too much of a burr is raised.  It's simply a strip of .001" feeler gauge set into an X-acto blade handle.  The haft cut off of a blade is set into the handle to keep the strip secure.





  Chamfering:



  The link below has photos of a reed before and after chamfering.  Included are photos of the chisel I use to chamfer along with a description of how to make and sharpen the chisel:



   http://docs.google.com/Doc?id=dckkn2d6_32vjf4qr





  Best regards,

  Rick