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Pipe Organs and Related Topics <[log in to unmask]>
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Sat, 27 Feb 1993 22:17:28 EST
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Hello, OrganNet!
 
Bobby, doing "unit" sliderchests really isn't too hard if you mind the
details *real* careful!
 
Larry Chace's diagram is about as clear as can be as to the general layout.
The thing to be sure of is: as you go "upwind" of the pipe toe, the
cross-sectional area of your wind supply must always have extra capacity
designed in.
 
For example, say you have 4'C of a gedeckt stop.  The toe hole will be
about 16mm diameter max.  (ALWAYS base calculations on full open toe, even
for stuff you know - or think you know - won't be.)  The area of this toe
hole is 201 sq mm.  We always add 30% to the toe hole area to get the
channel area, so we are up to 261 sq mm.  Then, our pallet slot must add
another 30%, so 339 sq mm.  Assuming a slot width of 15mm (5/8"), the
slot for this one pipe would have to be 23mm long.
 
When we are calculating for a whole division, we add up the areas of all the
toe holes, add 30% to get the channel area, add 30% to get the slot area.
This usually accounts for the fact that the pallet hinges at one end,
giving two triangular openings on each side, plus a rectangular opening
at the free end.  We assume that the pallet will open 8mm at the free end.
 
Our chest grids are made with rabbets in the front and back rails (usually
32mm poplar) to accommodate dividers of various widths.  Often, we will have
blank channels in the bass to keep the weight down, and use 6mm plywood
for the dividers here as well as in the treble.  The pallet board and the
table board are 9mm voidless birch plywood (usually Baltic birch).  The
outside frame is glued up, then the dividers are glued into the rabbets.
 
At this point, if we have "unit" stops, we just glue a piece of plywood
in the channel to form our isolation baffle.  No rabbetting is
necessary.  Then, we run the whole thing through a 48" wide stationary
belt sander to get the top & bottom of everything even.  The pallet board
and table board are glued to the grid in a glue press.  (All this is done
with good old Titebond.)  The pipe holes in the table board and the pallet
slots in the pallet board are drilled/routed before assembly.
 
Note, however, that if you are unifying like this, you should keep your
pallet openings, etc. fairly close to the same dimensions for both stops,
so that the airflow, volume, etc. is kept within reasonable limits.
 
Now, the fun begins.  You **must** glue-size all the channels.  Cover all
pallet slots with masking tape.  Spread out lots of plastic sheeting.  Get
a couple of wallpaper trays, and put the chest up on sawhorses table board
up.  Invite a few of your closest friends and buy a case of beer.  Mix
Titebond 1:1 with water.  Pour it into the toehole of C1 until the channel
is full of glue; let it sit for 5-10 minutes.  Now crawl under the chest
and pull the masking tape off.  If you are lucky, the glue won't pour
down your arm and into your sleeve, or get into your hair.  You probably
won't be lucky.
 
This wonderful task insures that there will not be runs across channels,
or across your isolation baffles.  It also insures that you and your friends
will be hermetically sealed for days.
 
Regards baffles in channels to prevent "undesired interactions", I think
these are often required because the channel is too small in area and
you get shock waves developing within the channel.  We have never had
such problems.  One thing we always do, however, is to drill a small hole
(about 1/8") at the end of the channel *opposite* the pallet and cover it
with a small felt muffler; this effectively damps out any potential
resonance problems.  On "unit" subdivisions, this probably won't be
of any consequence.
 
Yes, the reeds are often separated, and a lot of people think that French
reeds *have* to be separated to prevent the powerful pulses from getting
back into the mixtures.  Again, by carefully designing your winding system,
this does not present a problem.
 
Well.  Now we know a lot about both penguins *and* walruses.
 
****************************************************************************
Allan J. Ontko                                      [log in to unmask]
Ontko & Young Co. Inc.                                   Pipe Organ Builders
Charleston, South Carolina                                OrganCADD Software
            Tradition and Innovation in American Organbuilding
****************************************************************************

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