Thumbnail Shinking Dies for a homebuilt P-Hammer

[tdoty]

Carey,

Thanks for the response! Yeah, my lower die has a big ding in it from the upper die rotating

Yeah, the dies are stretching a bit, but I think a lot of that is setup. I just wanted to try it out. Some of the shrinks actually shrunk a bit better, but were quite ugly :shock: . The ones in the pictures were just the first pass. They shrink, if you move the metal in and out really fast. Too slow and it starts to stretch. Maybe lower air pressure? I'm using 50 psi for aluminum and 80 psi on the steel - might try 30 and 60, or any combo I can think of .

Another thing I've noticed, for those who say an air hammer won't do this job, is that it's actually thinning the metal where the dies hit. If an air hammer won't hit hard enough to move 18 gauge steel, how's it thinning it out ?

I'm going to keep playing with the dies to get the setup and technique right. I modified my hammer last night to allow a set screw to seat on a flat ground on the upper die shank. The barrel of the gun wasn't that hard :roll: . I tried to make my first set of dies from the hydraulic cylinder shaft material I've been using for regular lower dies, but couldn't even make a mark in the shaft with a center drill or center punch. I used grade 8 bolts for the dies because they drilled a lot easier. the air hammer barrel was just a bit tougher than a grade 8 bolt, but no match for a center drill .

I'm also thinking about making another P-hammer with about an 8" throat just for shrinking - mostly to avoid having to change my dies .

Tim D.

[Jay Hayes]

You can keep the upper and lower thumb nail shrinking dies from rotating by incorporating a technique that blacksmiths frequently use to make spring fullers. A piece of rod or flat bar is bent in a hair pin shape and the ends are attached to the upper and lower dies. This allows the dies to open and close but they can't rotate out of alignment. A piece of rod a couple feet long will make a spring that gives you about a ten inch throat

Jay

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[timf]

would machining the bottom die in the form of a shrinking dolly not work.



so that the metal is gathered.

[Richard K]

We at Shopdog have been making thumbnail shrinking dies for 2 years. It is not a simple task. Making a set that shrinks is one thing, making a set that shrinks without marking the metal is a whole different ballgame.

Here are my theories on how power tuck shrink dies work. We will go through the steps as the metal is put into and then retracted from the dies.

Important Point... Think of the shrink die set as two sets of dies in one.
The rear half (away from the operator) "gathers the metal to form a tuck as the metal is push into the dies.
The front half flattens the tuck as the metal is pulled out of the dies.
1. As the metal is fed into the dies it must be "gathered"; not "stretched" over the thumb. The die set must allow the metal to slide into the die from the sides as the thumb forces the metal into a tuck. When using the die you should feel the front corners of the panel being forced downward a bit and pulled into the die from the sides. If the die is pinching the metal as the thumb deforms it ; the tuck is being formed by stretching not gathering.
To test for "gather"
1. Mark the metal with a pair of lines perpendicular to the edge at 3" apart.
2. Feed your metal into the dies about 2". and stop the hammer.
3. Remove the metal and measure the distance between the marks.
4. The measurement must be less than 3" IF the die is "gathering".
If the die is not "gathering" on the way in it cannot shrink.

We will add to this post later tonite for the second part of the operation, flattening the tuck, and then some observations on why the die set marks the metal.

[anders nørgaard]

Richard,

EXITING tutorial

:shock: TO BE CONTINUED...

Can't hardly wait

Anders DK

[Gene_Olson]

You two have been watching too many season finale cliff hangers on tv.

My Guild of Metalsmiths monthly notice says that "Shopdog" is also on the demonstrators list for the Guild Feb meeting in Plymouth.
What with Richard and Don Hammer demonstrating is sounds like we should have a good metalshaping 101 lecture going.
Feb, 9th in Plymouth MN, stop by if you are in the neighborhood.

more on the meeting, time, and a map at:
http://www.metalmeet.com/forum/viewtopic.php?t=2555

G.

[Doug98105]

Regarding shrinking dies for Pullmax, etc.....

I've noticed with my home made dies there's a fine line, adjustment-wise, between having them shrink or stretch.

If the die clearance adjustment is too tight I get a stretch rather than a shrink. So it appears it's a balance between a wide enough clearance for forming the tuck with as little stretch as possible and having the a small enough clearance to cause shrinkage on pull back of the panel.

My thought is to add an attachment to the Pullmax lower adjusting screw. This would be a pneumatic cylinder actuating a short lever attached to the screw operated by a foot pedal that allowed a slight amount of clearance change on the dies between infeed and outfeed. The adjustment screw pitch is relatively coarse so a 30 degree or so rotation will make quite a difference in die clearance.

The theory is the dies would have a wide clearance on infeed forming the tuck. The wide clearance would assure very little stretch. As the panel is pulled out the operator would hit the foot pedal to close the die clearance so it was right for shrinking.

Any thoughts? Anyone tried this?

Doug

[Richard K]

I will continue on with my shrink die theory. Many who are successfully making dies have been down this path. Maybe a different path than I have found. These are my observations and I feel I have been making progress in developing a die giving good shrink results with minimal marking.

As I mentioned previously, the rear half of the die "gathers" The metal on the way in. How does this happen and what characteristics does the die need?

1. The die needs to have clearance in this part that will allow the metal to be pulled into the die from the sides as the thumbnail forces it upward. From the base of the thumb (intersection with the die face near center of die) the top face of the die needs to relieved towards the rear of the die. As the thumb gets taller the clearance to its left and right needs to increase. I try to give this area a 4-5 degree slope from the center to rear and toward right and left. Do not get carried away as the rear corners of the die need to support the metal as the thumb provides an opposing force.
2. The metal seems to rise above the actual top of the thumb as if it is being forced by the intersection of the top die groove and the sides of the thumb. therefore a scuff type mark will be made on the underside of the metal at the base of the thumb. Three things I have found help to alleviate the marking.
A. A polished radius between the thumb and the top surface of the die. To do this when a thumb is created by burying a rod in the base is difficult. I have TIG welded filler in the area and ground it smooth. Alternative is to mill the die and thumb one piece.
B. The shape of the top of the thumb. A slight arch shape seems to help flow here. Start with a 17 degree angle and almost flatten the thumb top at the rear of the die. Round off the rear end of the thumb.
C. The shape of the inside of the groove. As I mentioned in "A" the metal rises up in the groove. Make it match the thumb plus metal thickness plus clearance in front and rear. In the mid portion of the groove open it up like the inside of a teaspoon.

All surfaces of the die need to meet in VERY Smooth transitions.. Polish, Polish, Polish. The die surfaces need to have a mirror finish to work well and not mark.

Next we will discuss the front half of the die that crushes the tuck

As doug9815 has found stretch dies are easy to make.

[anders nørgaard]

Hi Richard,

This is getting better and better

My dies marks the work piece on the top side makes 2 scratches in the area right next to the thumb. The dies are made from bolts and maybe it's the narrow area (A and B) that causes the metal to be stretched/marked.

Do you think it would get better if I added surface in these areas

Thanks for all your good information Looking forward to " The Next Episode"



Anders DK

[jlrussell4]

Hi (insert name),

would machining the bottom die in the form of a shrinking dolly not work.

so that the metal is gathered.

All I have ever been able to do with one of those dollys or the serrated hammer is to mark the daylights out of a panel. If you are using bondo they give a good "tooth" for the bondo to adhere to .

Jim