October 27, 2009
Tool design shortcuts, oversights, and errors can be costly. Following some fundamental design laws can help you anticipate and prevent possible problems, simplify operation and maintenance, and improve production and quality.
Designing tools, like most tasks, goes more smoothly when you follow guidelines to achieve the desired result. Implementing these 10 design laws can help you produce the best tools for your stamping application.
1. Have no other goal except your personal best. The results are a reflection of the design. Think beyond the traditional "design the tool"; think "design the process."
Design is not about creating 120 AutoCAD® drawings that all fit together to produce a die that will stamp a part to printed specifications. It is more about creating a die that maximizes tool life, yields minimal variation relative to the part tolerances, and reduces the skill level and time required to service the tool quickly and cost-effectively.
2.Minimize strip lift and die stroke. The shorter the lift, the faster the production and the more dimensionally stable the parts are. The less you need to move the strip, the more stable it will be. A shorter stroke reduces vibration and increases tool life. Vibration to tooling is like sand in your transmission—poison!
3. Design lifters so that they keep the strip level during progression and as the die closes. Strips must be level at all times. Do not have large gaps between the lifter that will allow the strip to sag, and always set the lifter to keep the strip level throughout the entire progression.
If you are using spring-loaded punches, make sure the springs are sufficient to prevent any punch from backing up during the bending.Waviness in the strip progression will cause dimensional variation.
4. Balance all work being done. Balance any action on one side of the strip with spring preload pressure on the opposing side. This applies to both front to back and left to right.
Many times I have seen a tool with several tons of nitrogen cylinders loaded at the exit end, because the previous operation was used for heavy forming. The press shut height has been adjusted to ensure the die closes on the bumpers in the new operation. This imbalance causes an overhit on the entry side of the tool where relatively light work is being done.
The imbalance also causes the die to close unevenly relative to any ram slope that occurs. Imbalances and uneven closure lead to excessive uneven wear in the machine and tooling.
5. Design for simple, repeatable maintenance. Design the tool to ensure that service can be performed in the same way and achieve the same results every time it's necessary. Minimize the difficulty of service.Use throwaway inserts on high-wear forms.
Build the tool so that service involves flat grinding and shimming. Eliminate the need for regrinding sweeps, forms, clearance slots, and heels when servicing cutting components. Spending a little more effort upfront with inserts and quills and splitting items into multiple components as necessary save a lot of maintenance headaches.
6. Design out people skills; design in machine capabilities. Machines make parts; people maintain components. Do not design components that require maintenance by highly skilled technicians.
With today's wire electrical discharge machines (WEDM), there is very little need for people to grind forms and shapes. Today's WEDM can produce a surface finish to even the tightest requirements, with the exception of a very few types of tooling that still requires vision grinders. When people skills are needed, clearly define the requirements on the printed specifications. See law 9.
7. If the print is changed, update the revision (every time, no exceptions). We all have been caught in the trap of ordering or making a component only to find out requirements had changed on the print, the print revision was not updated, and we wasted time and money making something that was wrong.
Follow the golden rule of design: Always update the revision, even if you think the change does not matter.It actually is best to make the revision a part of the detail number.
8. Design to achieve perfect execution. Execution is either perfect or not. It needs to be fail-safe and duplicated.
During the die design phase always review the progression. At each pitch of the progression ask what is expected in that station, what can go wrong, and what can be done to mitigate possible problems.
When a tool is in development, how many times must you go back and re-engineer to get it right? If you have slug pulls, do you go back and add a bazooka, or did you anticipate this upfront and build it in?
If a form is off location, do you have go back and recut tooling, or did you anticipate this possibility upfront and insert the tool?
If you examine your development process, record every issue, and then, when done, review all the re-engineering, you will find that most of the time, you could have anticipated these issues. Time spent planning for and mitigating risk upfront and adding duplication-like qualifiers, inserts, slug darts, and bazookas to ensure that the tooling operates effectively in production is time well-spent.
9. If it needs to be done, it needs to be on the print. If it is on the print, it needs to be done—every detail. This is very important. If corners need to be broken, then we need to state by how much and with what tolerance. Clearance slots that have no functional purpose require specified dimensions and tolerances on the print.There should be no room for interpretation errors, or leaving any items undefined based on the old adage, "Everyone should know that."
10.Nothing is the only thing that is insignificant. By nature, if you can identify something, it is subject to change and, therefore, significant and required to be defined in the design. Never fall into the trap of thinking something is insignificant. I can tell you horror stories related to dowel pin lengths, springs, screws, carbide grades, and punch lengths that can have a greater range allowed in service than on the design print—the list goes on. All of these items were thought to be insignificant.
Remember from the ten tooling laws, if nothing changes, then nothing will change.
Also remember the three R's of design:
Repeatability—Do something the same every time and you will get the same results.
Reliability—Make sure the process is robust and not subject to variation caused by minute outside influences.
Replication—Design a process or tool that is easy to duplicate and not subject to human factors or skill.