Author Topic: Draw Dies  (Read 9035 times)

Offline Dave G

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Draw Dies
« on: August 05, 2012, 03:30:30 PM »
Here is a project that I did  a few years go. I built Bob Shore's Silver Bullet which is liquid cooled and needed a radiator. The plans called for the tanks of the radiator to be fabricated from multiple pieces of brass and soldered together which is how I built it. After finishing I got to thinking how nice it would be to have radiator tanks formed from brass sheet. For this I needed a draw die. I will attempt to describe how I designed and built the draw dies to make these tanks.



Here is a pic of the die components for the first operation. On the right is the blank pattern. This was developed after the first die was complete. In the first op I wanted to make a draw of the material to form the major tank body. A second op is needed to form the second lip of the tank.

The material is .02 " thick brass that came from a roll of brass stock. I haven't tried brass shim stock but I think it should form well also.

Offline Dave G

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Re: Draw Dies
« Reply #1 on: August 05, 2012, 04:01:02 PM »
Heres another pic of the die components.



The die is at the bottom. The stripper is above it and the punch is at the top. The bars on the left are for support of the stripper. The cavity in the die determines the size of the tank body and was arbitrary, the stock size I had was the limiting figure in the design.

The die cavity was milled to size and then the draw radii were milled with a corner cutter. To provide for tangencies at the junctures of the surfaces I measured the small dia of the corner cutter which dictated what size endmill to use to cut the cavity leaving the correct radius that would match the radius of the corner cutter. The corner cutter radius is determined by the stock thickness. The draw radius should be 4 - 10 times the stock thickness. I used .02" thick stock so I went with a .125" corner cutter which will give me a radius to stock ratio of a little over 6:1.

Once the radii were milled they were polished smooth with sand paper. The die was made from 4140 RC 28-30. The stock will need to flow around these radii so they must be very smooth with no imperfections. Normally the die would be made from A-2 or another tool steel and be hardened to RC 62 but for a few pieces this worked just fine.

The stripper was dowel pinned to the die for alignment and then the slot for the punch was milled to size. A clearance of .001" was used all around. The stripper and die should be milled in the same setup to make sure the slots line up perfectly.

The punch size was determined by the die minus stock thickness minus .001" clearance all around. A corner cutter was used to make the radii just as the die was. The radii were now polished.

The first pic shows the brass stock blank placed on the die. It was positioned by hand and then then stripper is placed over the die.
I found that the stripper was lifting up when in use so a couple of support bars were placed over the stripper for added strength.

Offline Dave G

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Re: Draw Dies
« Reply #2 on: August 05, 2012, 04:17:30 PM »
This pic shows the assembly in the press and the punch has been forced to the bottom. The brass blank and the draw surfaces were lubed with draw lube before forming. Draw lube is a very heavy and sticky lube.


Offline Dave G

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Re: Draw Dies
« Reply #3 on: August 05, 2012, 04:21:31 PM »
Heres a pic of the die flipped over with the blank still in the die. The blank can be pushed out of the die with ease now. The die is open bottomed and the depth of the draw is controlled by the punch bottoming the blank out on a flat plate that the die sat on in the press.


Offline Dave G

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Re: Draw Dies
« Reply #4 on: August 05, 2012, 04:26:48 PM »
A pic of the blank after the first operation.


Offline Dave G

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Re: Draw Dies
« Reply #5 on: August 05, 2012, 04:29:19 PM »
A side view of the blank. The blank is on top and a finished part was used to prop it up.


Offline Dave G

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Re: Draw Dies
« Reply #6 on: August 05, 2012, 04:38:33 PM »
Since the edges of the blank are irregular they will need to be trimmed before the second op. A trim fixture was made. The blank fits onto the fixture and then the edges are filed to size using the fixture as a guide.



From here our blank needs to have another lip formed on it, this will be the second draw operation.

The second pic of the blank seems to be after the second operation and it shouldn'd have this lip on it at this point.

Offline Dave G

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Re: Draw Dies
« Reply #7 on: August 05, 2012, 05:34:10 PM »
The second operation draw die components. The die is on the bottom, the support is above the die and the punch is on the left.



The brass blank has been trimmed to size and is ready for the second draw. This draw will form a lip for strength and will form a pocket for the mating piece to fit into easily to provide a good solder joint.

Offline Dave G

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Re: Draw Dies
« Reply #8 on: August 05, 2012, 05:38:39 PM »
The brass blank has been inserted into the support. The support will be inserted into the die and it will float in the die. When the punch is pressed down the support will float with the blank until it reaches bottom and then it becomes a coining plate.


Offline Dave G

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Re: Draw Dies
« Reply #9 on: August 05, 2012, 05:48:34 PM »
The blank inserted into the die and the punch positioned on top. Everything has been lubed well and placed into the press. This pic shows the blank before forming the second op in the press.



This pic shows the punch at the bottom depth of travel and the lipped formed.




Offline Dave G

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Re: Draw Dies
« Reply #10 on: August 05, 2012, 06:22:56 PM »
A pic of the tanks after both drawing operations. A quarter for size comparison is included. There is now a pocket in the bottom for the mating piece to be soldered to.



I worked as a Tool & Die maker for almost 30 yrs with the last 10 years spent in the prototype dept of a Bearing manufacturer. We were responcible for all the prototype construction for product development engineering. I also got to spend the last year of my apprenticeship in the same dept in the early 80's, we were doing alot more developing back then and many small jobs would come to us.

We had only a few guys who would get the draw die work because of their experience, time was always a limiting factor and management knew who to go to. I got to apprentice with these gentlemen and built up a good working relationship with them. When ever they got an interesting die job I always picked their brains for info, they were normally very generous and would share what they learned with me. Later when working in the same dept as a journeyman myself I got a few of these jobs but they were few and far between at that time. I still kept close to every die job as I could without getting in the way, the guys I worked with were good at sharing info.

This project came about because I wanted to see if it could be done at home with common materials and machinery.

I thought I would share some of what I learned with you. Be very careful when using dies because of the tonnage needed to do certain operations. I didn't need to figure tonnage in this job because of the small size but is a very important step in designing dies.  I used materials that weren't so overly hard. Hardened steel will explode if over pressured so proper guarding is required.

Dave

Offline Dave G

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Re: Draw Dies
« Reply #11 on: August 05, 2012, 06:26:35 PM »
One thing I failed to mention and I should have is that I cut one of these in the corner to check for thinning of material. Because this will be pressurized I didnt want the corners to be thinned in the drawing operations. Something that should be done before using in it's application. Safety First, Dave

Offline ref1ection

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Re: Draw Dies
« Reply #12 on: August 05, 2012, 07:11:30 PM »
Thanks Dave,

For those of us that wouldn't have the opportunity to see this done there was a ton of info and a great explanation. Very interesting indeed.

Ray
Indecision... the key to flexibility!

Offline Dan Rowe

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Re: Draw Dies
« Reply #13 on: August 05, 2012, 08:55:35 PM »
Dave, many thanks for sharing this work. You mentioned the bars were to keep the stripper plate in place. Is the blank free for the first draw or do the 4 screws control the tension on the blank?

Dan
ShaylocoDan

Offline vcutajar

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Re: Draw Dies
« Reply #14 on: August 05, 2012, 09:26:42 PM »
Very, very interesting.   :whoohoo:

Thanks Dave for the detailed notes.

Vince

 

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