Author: Gordon (NZ)
Date: 2014-02-03 03:48
It may be of use to somebody, so here is what I once wrote: (Perhaps thicker, longer pins for a bass clarinet, so that there is more surface area for the glue around the pins.)
PINNING A BROKEN PLASTIC BODY
Some technicians have promoted simply gluing broken plastic bodies (and tenons). I am reluctant to trust an adhesive in a rather small surface area in a butt joint. In careful use a clarinet body is subject only to sheer (twisting in this case) stress and mild compressive stress during assembly, mild tensile stress during disassembly, and some insignificant bending stress during some fingering. However I suspect that such a mend will fail as soon as it is subject to significant bending (tensile) stress, perhaps in a mild accident or in the hands of a rough user.
The traditional method of boring the body at the break, and grafting a plastic sleeve is fine, but I offer an alternative of pinning, which may be different in some important respects, from the pinning jobs we have all seen that fail. Both grafting and pinning are similar methods in that they both use a butted glue joint, but reinforce this joint by introducing new material parallel to the bore, i.e. across the face of the break. In one case the introduced reinforcing material is a plastic cylinder, and in the other, a series of metal pins. Both methods capitalize on the high SHEER strength of adhesives.
As I see it, the advantages of the pinning method as I use it, are:
1. With a graft there is potentially a significantly weakened area of the wall of the instrument at each end of the graft, where part of the wall has been removed and replaced with adhesive. This is not the case with pinning.
2. Pinning does not need a lathe or similar. Grafts do.
3. Pinning does not require obtaining stock body material for a graft.
4. Pinning does not interfere with any tone holes or post holes.
5. Pinning is probably stronger, providing the method is good.
6. In my experience of using both methods, pinning is considerably quicker.
7. The pins themselves are stronger than the graft would be.
A disadvantage is that it needs the broken faces to be reasonably clean, and free from the adhesive of failed previous attempts, to ensure good alignment of the two body parts.
I use epoxy glue, but reinforce the mend by fitting seven 1.2 mm diameter stainless steel reinforcing pins across the break, i.e. parallel to the bore of the body. Each pin is about 50 mm (2") long, but more if extra strength is required. The trick is to get adhesive all along the pins so that the SHEER strength of the adhesive is utilised. (Kraus's straight stainless steel springs may be suitable as pins, but I buy straight stainless steel in long lengths. It is probably available from http://www.smallparts.com/products/descriptions/swgx.cfm))
1. Mark out the 6 or 7 pin locations, at the break line, on the outside of both parts of the instrument body using a white pencil. (A greasy one used for black photo album paper is excellent). Choose positions carefully to avoid lining up with tone holes and posts.
2. I use a hand-held dental micromotor to drill 1.4 mm holes from the break surface starting half way across the wall thickness of the break itself, to a depth of about 25 mm on both sections of body. The holes are parallel to the bore. A sharp drill, slow speed, and plenty of withdrawing to clear swarf are required so as not to melt the plastic while it is drilled. I BEGIN the holes with a short drill with 2.3 mm shank because it is more rigid for starting accurately half way across the body wall. A small dental burr would do instead.
3. An air vent hole is required at the 'blind' end of each of the holes. This is necessary in order to get adhesive along the entire length of each pin-hole, as described later. Put a straight piece of brass wire (e.g. a straightened paper clip) down a hole to gauge the depth and mark (with the pencil) the depth, i.e. the location of the bottom of the hole, on the outside of the body. Leave the wire there in the hole for visual alignment and drill from the outside, straight into the body wall towards the bore, near the bottom of the pin's hole. I use a 0.5 mm drill. I know when I have broken through to the pin's hole because the paper clip wiggles. Repeat this for each pin's hole. Blow air through these vent holes to ensure that they link to the pin-holes.
4. Cut the pin wire to length, i.e. almost as long as the total length of each matching pair of pin-holes. These pins, approximately 50 mm long, may be of slightly different lengths. Fit them one at a time, checking that the two body parts are not prevented from meeting freely when each single pin is installed. If corresponding holes of the two body parts do not line up quite right, this will be obvious because it will prevent accurate alignment of the two body parts. If so, then the first few mm of the hole can be 'widened' in an appropriate direction to stop the pin from distorting body alignment. Make sure that you know which hole each pin belongs in.
5. I rough up these pins using the edge of a course file, for better glue adhesion.
6. Epoxy glue wrecks the surface of ABS plastic so it must not get on visible surfaces. I use Sellotape (other types such as plastic masking tape may be better still) to protect the outside of the body, everywhere that glue may ooze. Apply it around both parts of the body right up to the very edge of the break. Sellotape has enough stretch to accommodate most irregularities. If necessary, trim the tape exactly at the break and roughly around nearby posts with a sharp knife.
7. Also wrap Sellotape around the body where the air vent holes are, and then prick through to the holes with a pin so that they will still vent air and glue.
8. Protect the bore of the instrument (and any tone hole surfaces involved) from the epoxy sticking, by smearing thinly with cork grease over the susceptible areas.
9. I mix my epoxy with a little black paint pigment powder and put it into a disposable 3 ml syringe. I actually use the metallic base of NON-disposable syringe with the actual needle tube cut off. This makes a better nozzle than a disposable needle's plastic base. However the latter will do. (The glue will not travel through a long thin needle)
10. Squirt glue RIGHT THROUGH each 1.4 mm hole, from the break end, until the glue begins to ooze out the air vent (through the pricked holes in the Sellotape). Apply some more glue to the surfaces of the break itself.
11. Smear each pin with glue. I simply roll it in a small 'puddle' of glue on the mixing pallet). Insert each pin into its appropriate hole in ONE section of the body. Take care to wipe off excess glue exiting the vent holes before it drips or moves off the Sellotape. Then slide the other part of the body over all the pins at once, until both body parts meet.
12. Generally clean up excess exterior glue, ensuring that there is slight excess remaining at the break itself and at the vent holes.
13. I use cotton string to pull the two pieces of body firmly together, by winding it from post to post across the break.
14. Lay the body horizontal so that any glue oozing inside the body does not flow off the area protected by cork grease.
15. I sometimes use a 100 watt incandescent bulb neaby to warm the body, hence improving adhesion and shortening setting time.
16. Leave until the glue is firm, no longer sticky to the fingers, but not fully cured. Remove the string. Remove the Sellotape, being careful not to pull the soft glue out of where it belongs. This is the best time to remove bulky excess glue, especially inside the body. I use a hard plastic (acrylic) rod sharpened at an angle at the end. This does not scratch the surface of the instrument.
17. When the glue is harder, cut away any excess at the vent holes and the break. I use a chisel-shaped piece of acrylic sheet for this at the break because it is soft enough to be unlikely to damage the surface of the body.
18. I drill the glue out of the needle base (with a 1.0 mm or smaller drill) so I can use it again next time. The syringe itself can be thrown away or the remaining glue pushed out with a wire. Compressed air helps too.
This process may seem very complicated but it has never failed. Another local repairer has tried to copy it but had failures because the rods were not adequately glued, on account of lack of any vent holes.
I work fast and each job takes me 60 to 90 minutes, excluding glue setting time. I have not been brave enough to use fast-setting epoxy!
For broken tenons I simplify the method somewhat, and often do not even need to change the original tenon cork. The main differences are:
• I drill the pin holes first through the tenon from the break side, then hold the tenon to body as a guide, and partly drill into the body via the tenon. Then complete the drilling of the body without the tenon in place.
• Cut the length of the pins so they finish about 1 mm below the surface of the end of the tenon. (Each pin tailored to each hole)
• Fill those spaces at the ends of the holes with black epoxy, and trim neatly.
Post Edited (2014-02-03 08:34)
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The Clarinet BBoard
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