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Ask the Stamping Expert: The 10 laws of die design
- By Thomas Vacca
- October 10, 2016
- Article
- Bending and Forming
Q: Can you provide me with some guidelines for die design?
A: When it comes to die design, I find there are two best practices to live by:
- Follow Tom Vacca’s 10 Die Design Laws.
- Use a die design continuous improvement check list.
Following are my 10 Die Design Laws. Look to the November/December 2016 issue for the continuous improvement check list.
1. Have No Other Goal Except Perfection
Start with a clear objective. Balance the overall tool cost with stability, quality, hits per service, and maintenance. Think beyond the task of just designing a die, and look for pitfalls that can crop up in maintaining and setting up the tool. The design needs to ensure that setup and maintenance are as poka-yoke (mistake-proof) as possible.
Be thorough in the documentation process. Remove the “tribal knowledge” effect by including clear service instructions on the prints for consumables.
2. Minimize Strip Lift and Die Stroke
Vibration is the poison of tooling. Stagger your cutting punches and minimize strip lift. This helps minimize the die stroke and imparts better dimensional stability to the strip because of less movement and tool component travel and wear.
Reduced die stroke yields reduced ram speed relative to strokes per minute. Less movement results in less shock and vibration. Carbide is like concrete; it will disintegrate before it wears.
3. Design Lifters as Needed to Keep the Strip Level in Progression and Level as the Die Closes
The strip needs to be parallel through the entire press cycle. Make sure the space between the lifters is not so big that it allows the strip to sag.
If by design you are doing work as the stripper closes, the lifters must sufficiently clamp the die strip under the pilots and against the stripper face plate to maintain a consistent part. If you are forming with spring-loaded punches, calculate the force required and double it to prevent punch backup as the die closes.
4. Balance All Work Being Done in the Tool and in the Press
The goal is, by design, to have the tool naturally want to close evenly on all four corner die stops. Balance the tooling forces under the press ram.
Remember, every tool and press has some lateral play, which in turn affects stability and tooling life. The tool guide pins and bushings need to be robust enough to overcome these lateral forces. If heavy forming is done on the right side of the tool, consider adding balancing springs on the left side.
5. Design for Repeatability
As a tooling designer, you must design in the maintenance process. If you don’t, how will you know it can be done accurately and repeatably?
Design the die so that all cutting and forming sections cannot be put in reversed, backwards, or upside down. If a punch can be shimmed to the point at which it interferes with other tooling, then you need to put maximum shimming instructions on the component print. Don’t assume the service technician will figure it out.
All purchased components, such as springs, screws, dowels, keepers, and shims, must be designed or specced out. The die builder and toolroom should not have to guess at your intentions.
6. Design out People Skills, Design in Machine Capabilities
Machines make components, and people maintain them. Skill levels vary. Do not design components that require a highly skilled maintenance worker to ensure repeatability.
Today’s wire electrical discharge machining (EDM) can deliver the accuracy and surface finish required. Put those requirements on the print. Spell out surface finish if polishing is needed and corner breaks where required. No exceptions. Design in repeatability, and remember the three R’s: repeatable, reliable, replicable.
7. Design for Zero Development
Success is based on understanding what to expect from each station in the tool. In die development, if something is done twice, you didn’t achieve the objective the first time.
Design to meet capability requirements. Get input from the people who run the tools. This is one of the single largest factors that affect the success of the design process. Look at similar existing stamping tooling and its output relative to the design and product dimensional stability.
Plan on facilitating development by anticipating what can go wrong, and design in risk mitigation. For instance, insert trims where they might need to change. Add extra pilots if there is a concern that the strip might be pulled during heavy coining or forming. Have an idle station in the tool in case it is needed later.
8. If It Needs to Be Done, It Needs to Be on the Print
Every single detail on the print needs to be completed as designed. Everyone should know that, right? Well, they don’t! Every detail has to be very clear, with no room for interpretation, to ensure consistency and quality. Absolutely no changes can be made to clearance slots, springs, duplicate component locations, and screw size and length. Too many times people change things with the belief it is meaningless. Don’t let this happen.
9. Nothing Is the Only Thing That Is Insignificant
If you can identify something, it is subject to change. Law No. 4 of my 10 Tooling Laws states, “If nothing changes, nothing will change.”
A good design minimizes the subtle changes that happen over time, usually caused by wear and maintenance. For example, you can design out rocker cam and form wear problems by using a cam-bypass design rather than angle-on-angle to drive tooling, which is subject to change from wear.
10. Grow and Improve
Every process should include continuous improvement. Changes are required! If you don’t innovate, the competition will pass you by.
Review the results of every tool build, particularly the development of the tool and the part capability data. A well-designed tool will have zero development. A robust design will yield parts with great capability. Incorporate the lessons you learn into your next design.
About the Author
Thomas Vacca
Micro Co.
Has a shop floor stamping or tool and die question stumped you? If so, send your questions to kateb@thefabricator.com to be answered by Thomas Vacca, director of engineering at Micro Co.
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