Monday, February 25, 2013

Stamping Dies and Cutting Clearances

Stamping Dies and Cutting Clearances

I have been a tool and die maker for over three decades now. It still surprises me that a large number of toolmakers, designers and manufacturing companies for stamping tools do not fully understand the issue. Companies spend tens of thousands of dollars on all kinds of improvements, yet they ignore the basics, and the foundation of their business.

Metal Stamping and Manufacturing the Tools for Metal Stamping

The investment can be put toward training tool and die designers and toolmakers to improve the bottom line. Making tools that last longer, and work longer before maintenance is required. I am talking about "Old School" professionals. They should know better and not make mistakes about the foundation of metal stamping.

The general rule is to make the cutting clearance 10% per side. That means when the stock thickness is e.g. 0.100 of an inch, the cutting clearance should be 0.010 of an inch per side. This 10% value is more like a guide only. If you don't know any better, use it. Just don't be surprised when the result is not as expected.

There are different materials that you can pierce or blank.
Put them in at least 3 different categories, such as:
• Low (soft), like copper, Aluminum, and plastics
• Medium, like CRS and Milled Steel, galvanized steel etc.
• High (hard), like spring steel,tool steel or other alloys.

The more carbon the steel has, the harder it is.

LOW - Often called mild steels, low-carbon steels have ~ 0.10 percent carbon and are the most commonly used grades.

MEDIUM - Medium-carbon steels have from ~ 0.25 percent carbon. Increased carbon means increased hardness and tensile strength, decreased ductility, and more difficult machining.

HIGH - With ~0.50 percent carbon, these steels can be challenging. There are even harder steels with 0.75 to 1.0 or more carbon content.

Because there are different metals to be pierced or blanked, with different molecular properties, the cutting clearances must be different as well. Do not ignore the facts. Get to know them to improve both the blanked parts and their tooling.

When the stock thickness changes within the same material, the cutting clearance needs to be different. If you punch two different sized holes in the same sheet, the cutting clearance should not be the same. Blanking and shearing have different cutting clearances as well.

There are at least 6 different variables that can tell you what cutting clearance should be used.

Improper cutting clearances have different results.
• The 1st and obvious is the excess bur on the blanked part. It caused by excess or too little clearance.
• Reduced punch or die life or premature wear.
• Punches and or die edges are chipping
• Die plate cracking
• Difficult stripping
• Deformed or rounded parts (not flat enough)

Improper cutting clearance will not only negatively effect both the finished product and tooling but the whole manufacturing process as well. Poor tooling must be taken off the press, components to be sharpened that create downtime, unwanted change in the process and loosing valuable time and money.

You may want to take a minute and calculate your loss when a tool is taken off the line. Is this happening often? There are tools that could be used for about ~15,000 parts between sharpening. Some tools will go for 500,000. I've seen two tools in my career that run millions of parts over the span of 20 years without the need of sharpening. I know this is extreme and the tool had other features that supported the long life.

My point is, that a tool  producing a large number of parts is not needed to be more expensive than the ones that perform poorly. They are simply made better to last longer and add more value to the end user.

I know it is not easy for everyone to know the details, or the variables that effect the cutting clearance. Each tool is unique with its own challenges, yet the same principles can be applied for best result.

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