Exactly Measuring for, cutting, and fitting a Sound Post...
...Fitting the sound post accurately is made more difficult because both the top and bottom plates of the violin are curved at the contact with the ends of the sound post; besides this, the angle of the end is usually different at the two ends.
...The sound post is approximately 1/4" in diameter and is made of spruce wood. The grain of the sound post is oriented at 90 degrees to the grain of the wood for the top plate... that is, the grain of the one goes across the grain of the other. Normally there should be between five and seven growth rings across the 1/4" width of the sound post. Less grains for a warmer tone and more for a colder tone, is the general rule.
...In this article I will outline my method for carefully measuring the exact length of sound post needed for a particular instrument and how to measure the end angles and accurately cut them, all at the correct overall length (length of the centerline of the sound post).
First, I will be helpful if we have some basic sound post tools:
Sound post retreival tool (for when the post is loose inside instrument.
Sound post pliers, to manipulate the top end of the sound post.
Sound post setter, adjustment tool; useful to adjust the bottom postion of 'post.
Hand made sound post installing tool; or just use the sound post setter's pointed end.
Twin-layer thin plastic gage for measuring position of sound post through the "ff" hole
...This is the tool that will allow you to tell if the sound post is exactly vertical when inside the violin. The bottom has a shape that 'cups' around the sound post and is parallel to the vertical shaft of the tool.
...The soldered-on brass rod is to allow the tool to be easily turned but is not absolutely necessary.
...On top (out of the picture) is a 'bulls-eye' level mounted on a wood block that slides over the vertical shaft. (I remove for storage convience when not in use).
...This tool is commercially available and the inner metal shaft slides up inside the brass tube so that it can measure the centerline length of the sound post that is needed. The thumb screw locks it into the correct length; the tool is then removed from the violin "ff" hole opening and measured. You will notice in this photo that I have added two wooden ends which are tipped to match the inside plate contours...more on this later.
...The plastic post location tool. The two pieces are identical and the end is cut as shown so it can hook around the sound post when it is in position inside the instrument. The end are stapled together or that the two pieces stay in 'registar'.
...Thus by looking at the outside piece you can tell where the inside piece is compared to the bridge.
...A vernier caliper measures the center-line length of the sound post to the nearest 0.001". be sure to measure in the center of the tipped ends of the wood tips (90 degrees to the tool...not shown)
...This tool measures the end angles of the sound post tool so we know what angles to 'cut' on the sound post ends to accurately fit the insturment. Usually the end angles are different so it is a good idea to put a mark on the sound post as to which way is 'up'...I draw an arrow on the side facing the "ff" hole so it is clearly visible throught the sound hole.
...(this is the bottom of the holder fixture) Notice that the top section lifts off the four support feet. This allows you to move the instrument so that you can recapture the sound post after it falls over and is rolling around inside; without having to unclamp the instrument from this holder (saves time and frustration).
...This is the final tool you need to make which allows for the accurate hand sanding of the end angles and which also allows you to exactly control the length of the sound post.
...It is bolted to the mitre gage of my table saw and slides in the slot. The end angle of the post is set on the mitre gage. The sanding disc is flat and the cut is made with by sliding the mitre gage in and out by hand while feeding the motionless sanding disc out (my 'Shop Smith' table saw allows the saw spindle to slide in and out with a lever...perfect for this application.
...These three photos show more details of the sound post tool that allows you to measure how vertical the sound post is once it is installed in the instrument. It fits in through the right hand "ff" hole by turning it sideways.
...Now I will show you the whole process of fitting a new sound post for and instrument from the beginning...
...The first step is to mount two cut off pieces of sound post stock with oversize holes drilled in their center about 70% of the length deep (just larger enough to allow the wood ends to pivot and conform to the shape of the inside of the insturment, when put inside and raised into position). Then some 5 minute epoxy is mixed up and put on the steel shaft ends of the sound post length finding tool. The ends are covered with the two wood 'post ends and then they are gently held in position while they are passed through the right hand "ff" hole. Once inside, the tool is manuvered so that it is in the position of the sound post and then the tool is lengthened to firmly contact the top and bottom plates and then the thumb screw is firmly locked down. Do this with the tool vertical as shown by the bubble balance. The wood ends tip and conform to the inside plate shape, then the glue sets up.
...It is best to fitt the sound post with the instrument lightly strung up with the bridge in proper position and just enough tension in the strings to make them straight. Once the sound post is in position, then the string tension can be brought up to proper tension and that string pressure will hold the sound post firmly in position.
...Here the end angles of the sound post are being measured with the protractor tool (and recorded (of course) for future reference....
...The needed length of the sound post may change over time but the needed end angles should remain constant.