Sunday, February 1, 2015

Stamp Mill Shoes & Dies

Stamp mill shoes & dies perform the crushing action of the shoes hitting on top of the dies. No one ever sees the dies since they are under inches of ore, but they supply an important function and that is to provide a wear surface between the stamps and the mortar boxes. If there were no dies you would have to change out the mortar box when the shoes wear down the bottom of the box. The picture below shows the shoes (K) and dies (B) in the bottom of the mortar box. You will notice the false dies (C) that we will talk about later in the article.

The shoes & dies are made with hardened steel, generally chromium or manganese is added to the metal to give its hardness and resistance to wear. Shoes & dies come in various sizes, but a common 1,000 lb. stamp mill would have dies 9” in diameter and about 6” to 10” tall. They weigh about 120 pounds each and not easily handled.

Both shoes & dies are considered consumable, in that, they will wear down as time goes on. I have been asked how long they last and I say that it depends on what material they were crushing and run time. Most host rock would be a quartz composition and has a hardness of 7 on the Mho scale. You will get much faster wear with quartz as compared to a granite material and if you are running 24/7 you will get less time.

The big question is where can you get replacements and how much will they cost? Back in the early 1900’s there were quite a few foundries where you could purchase dies and shoes. I have a bill from Joshua Hendy for a shoe and a die at $21.00 and $21.50.

You have some alternatives when it comes to replacing shoes and dies on your mill. You can purchase new dies and shoes, you can have them cast at a foundry, you can fabricate them from steel stock and you can use some alternative configurations in lieu of the classic components. The following are five alternatives to obtain shoes and dies:

Existing Shoes and Dies: This method is more of a hit and miss type process. You can generally find a shoe and die or two, but they generally are not the same size,  which makes using them an exercise in mixing and matching to make the sizes average out. You never find a number of shoes or dies the same size, so you are faced with making adjustments on the tappets to make them work. This is not recommended especially in 5 or 10 stamp mills where it is critical to have shoes and dies of the same size. The picture below is the exception to the rule, but you have to go to Australia to get them.

Cast the Shoes and Dies: This is what the stamp mill operators back in the later 1800’s would do. They would just contact their foundry and order new shoes or dies. They would be cast from a mold that the foundry had in their possession. They would only have to purchase the steel and would only incur one cost for fabricating the molds for the shoes or dies one time. Shoes and dies were changed out frequently and they were always purchasing additional dies as the old ones wore down. The first stamp mill that I restored at the Arizona Mining & Mineral Museum back in the 1980’s needed dies. I had them fabricated at a foundry that has since gone out of business. I think the dies were about $100 each plus the pattern that I think they threw in. I recently performed a survey of known foundries in Arizona and nearby States. The table below shows the cost of dies from various foundries. The cost is expensive with the cheapest being an Idaho Foundry at $2,000 plus the cost of the mold & shipping for a 10-stamp mill. This could run in excess of $2,000 by the time they are delivered. Remember, the dies weigh about 120 pounds each and shipping is not cheap.  

Fabricate Dies from Round and Flat Stock: We have been forced to use other methods for making shoes and dies due to cost considerations. The most common method is using round and flat stock steel. The problem with that is most of the steel you can buy does not have hardeners like chromium or manganese. I found a local machine shop that repairs cotton gins that use bailing machines to make large bails of cotton. These machines use large pistons in the balers and the machine shop has several of these pieces of steel.
We needed dies at the Superstition Mountain Museum on the Cossack 20-Stamp Mill. We only wanted to run 5 of the 20 stamps. There was a need to make some calculations to determine the thickness and types of steel materials to be used. The (5) dies came to around $500 for the Cossack Mill. See the picture of the location for the dies and then the drawing below.

Fabricate Shoes from Round Stock This process was used at the Cave Creek Museum. They had NO shoes for their 10-stamp mill. The foundry route was out of reach due to the cost. The decision was made to purchase (10) pieces of 9 ½” round stock, 12” long and have them machined on a lathe. We purchased the round stock from the machine shop that does the cotton gin repairs and took them to a machine shop that can turn down the cones on the top of the shoe. The cost for the (10) pieces steel was $900 and the machining brought the cost to $2,000. In this case the need for a pattern and shipping was not required. It would have gone to about $3,000. The other factor was time. The foundry’s were not that interested in servicing our needs since it was a small job for them.    

The picture below shows the round stock at the cotton gin repair facility.

Fabricate False Dies: I have used a cheaper method of fabricating dies from flat steel plate. In this process you use 1” steel plates for the stamp contact area and place other cheaper steel items such as rail and scrap stock under the wear plates. We were replacing the individual dies with a single plate of thinner material. This is a process that was used at the Golden Reef 10-Stamp Mill at Cave Creek Museum. They had no dies and we went with this method. The next slide shows a drawing of the plate configuration at the Cave Creek Museum. We determined the dimensions on the bottom of the mortar box and the thickness of the materials needed to make up the difference in the height of the dies. We have a similar situation with metal hardness and wear on the plates, but the concern was not great enough to effect the mill operations. This came to about $300 for the materials. The picture below shows the configuration:

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