View Full Version : to counter sink or not?
09-18-2002, 04:05 PM
I am making some bronze spreader tangs from layers of 1/16" silicon bronze ala Hereshoff. It would seem, not that I am second guessing the master, that round head screws would provide a better attachment to my spruce mast than the counter sunk method he recommends in Sensible Cruising Designs. It may not be as attractive but I am a little nervous about trying to get a counter sink properly in 1/16" material. They will be similar to the tangs specified for several of the boats in the book ie: built up from 4 layers and it seems that squeezing the whole arrangement under some round head screws would be better. Waddaythink????
The countersunk head provides more, umm, "sideways" pressure to the assembly, preventing motions due to the stresses applied during sailing.
Let me try again. You want the tang to not move, not only not away from the mast, but also neither up & down or nor fore & aft. The cone shape of the countersunk head helps prevent these motions. (It also provides a means of adjusting the diameter of the shaft to the size of the hole -- turn it in more, until it's tight.)
Someone else can explain this better.
Alan D. Hyde
09-18-2002, 05:23 PM
It automatically provides an interference fit.
09-18-2002, 06:13 PM
Makes sense I guess. I was worried about taking out too much material from the top layer. It seems to me the countersink will extend into the second layer of bronze and not leave much of a shoulder for the underside of the screw head to rest on. Are their any specialized bits that would be specific to countersinking for a #12-14 screw?
09-18-2002, 06:53 PM
Yes, there are special bits for cutting the countersink at the correct angle for the screw being used. Any hardware store should have them. The more cutting edges the better. Bit speed and pressure are important to avoid chatter but danged if I have mastered them so do as I say and not as I do.
I think 82 degrees is the standard countersink angle.
I would try this...measure the depth of the entire head cone of the screw...if it is deeper than 1/16, and I'm sure it will be, then countersinking will take you through the first layer. I realize that the countersink hole grows gradually narrower, but I'd worry that it would make the top layer of bronze useless, if not loose.
09-18-2002, 08:13 PM
If you're worried about material removed from the top layer, you could cut the appropriate countersink in a lower level and "dimple" the outer layer. I think they do this with sheet metal restorations. It might also be an airplane thing. You still get all of the centering, interferrence, etc., without losing strength.
09-18-2002, 11:46 PM
Originally posted by Paul Scheuer:
If you're worried about material removed from the top layer, you could cut the appropriate countersink in a lower level and "dimple" the outer layer. I think they do this with sheet metal restorations. It might also be an airplane thing. You still get all of the centering, interferrence, etc., without losing strength.That is a very clever idea!
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