So, I've seen two styles of thumbnail shrink dies: one that has the thumbnail that is straight and one that is curved. What would be the advantage of each? The straight one would be easier to machine. I've heard some folks say that the curved version has no advantage, but I think that Yoder produced a curved thumnail and I would think there was a reason. Would the curved version shrink to some degree on the instroke? Which do I build? Thoughts? -Mark
I built a set of straight ones utilizing a removeable pin (thumbnail) - I have nothing to compare them too, but they work pretty good and were easy to make. I'll dig up a picture if I can find one.
I've noticed the same thing. Clay Cook's are curved,and not inexpensive. Probably the best way to find out is make a die with a removable pin,then make a couple pins. One straight,the other curved on top.
I would not think the metal would know whether the thumbnail was curved or straight. I believe the place that the tuck is crushed is whats important. I have 2 different sets of thumbnail dies, one made by Shopdog and the other by Loren Richards. Big difference in the two sets, and both are straight , not curved. The Loren Richards dies do not mark the panel like the others, and shrink way more aggressively. I also have a set of phenolic thumbnails that you can shrink pre-painted aluminum with, and they wont mar the finish either. I also purchased them from Richards. They were about half the cost of Clay Cooks. For what I paid for them, I could not have made them myself for the same money. Tim @ www.irrationalmetalworks.com
I noticed the post and asked my Dad Cal Davis to address the question. He has been building the Metalcraft Tools version of the Power Hammer for ten years. After many hours of design, cutting on CNC, and “Hammering”, I have come to the conclusion that the pocket is much more important than the tongue. The tongue should be of a radius large enough and shallow enough to form a tuck and large enough to get a shrink. The tuck has to be small enough so that you will be able to draw the panel out, too large a tuck will mark metal and in heavier metal (18 ga-16ga) you will not be able to withdraw the panel. Now to answer your question, I believe that the radius tongue is the best choice, it is more gentle to the metal and I am sure that Yoder learned something over the many years they manufactured the hammer. The depth of the pocket has to be deep enough to clear the metal thickness-but if it is deeper and has a lot of clearance this is O.K. Now the most important thing is the radius going into the pocket, large enough to allow the metal to “flow” into the pocket and not “stretch”. If I remember right, die markers use 3x (3/16-16 gauge) thickness of material. I can tell you right off this does not prove out on shrinking dies. We now use ¾” or 12 x thickness of material (16 gauge) to transition into pocket. Now to talk about the flat or crush area-This area needs to be large enough, so as not to stretch metal out and small enough to maximum clearance on the panel so as not to be stretching out or flattening out the radius in the panel. I do not believe there should be any “flat” right or left of the tongue. This should fall off so as to clear radius of part. I hope to make a video of the shrink die research we have done. It will be on our website for free. There will be more detail and explanation of what we have found. A shrinking die should shrink without beating up the metal and making hard to repair marks. Also, let me add I believe round dies make more sense than square dies. The corners of a square die are just in the way. Also, the finish on the dies are very important. They should be polished to a mirror finish and a light oil should be used on your panel. We are experimenting with a product that is used to polish draw rings and punches in industry, Where tolerances are held to as close to 2 to 3 thou. This material gives an extremely high finish and impregnates the material so that it maintains its slipperyness. Hope this helps some. Cal info@metalcrafttools.com Metalcraft Tools Shrinking Dies
I've been thinking (and gathering steel) to build a power hammer for a while. Shouldn't the shaping dies be made of tool steel..... or is mild steel up to the abuse?
Depends how much you are going to use it,and what you are forming. Mild steel will work surprisingly well,for a while. 4140,or Tool Steel is a better idea. O1 or A2 would be beautiful. Forget S7,and S5.
That is a lot of great information. I never thought about the overall shape of the die - round vs. square. I can see why round would be better. I also agree that the round vs curved shape of the die is not the most important issue, but I still don't have much of an idea of the "why" curved vs. straight. Someone told me that they thought the metal was actually shrunk on the way in - just a little. I'm not sure if I buy that or not, but I could see as you got to the curved part that it would be possible. I don't imagine that it would shrink significantly though (if it does), so that doesn't seem like it would be the reason. Anyway, I have one of Cal's kits adapted to an old shear frame and really like it, but I too am collecting steel to make a bigger hammer that can handle a little more horsepower and has more throat than I have now. Always good to bounce ideas off everyone else, so thanks.
The panel is bent on the way in,forming a tuck. The panel is shrunk when the tuck is progressively crushed,towards the edge.
The Jalopy Journal
