7 Steps to get optimal machine design

The purpose of the article is to share the machine design basics from a real-world R&D perspective.

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1.  Design concept phase Reaching for the best concept?

The best concept, in my opinion, is one that meets the requirements unambiguously. In the most cost effective way, which can be realized in spite of all the system constraints in a reasonable time and budget.

From my experience, it is very important to take the time in this phase, and to postpone the detail design as much as possible.

This is because details can be changed or adjusted on the fly, but to change the selected concept during the detailed design is almost impossible. Usually you would have to start all over again.

In the concept phase the output should be: General dimensions, external interfaces (ICD), external shape/Industrial design – if you work with an industrial designer, it’s important to involve him at this stage.

Assembly feasibility must be taken into account; manufacturing technologies and materials as well as an estimated cost for the selected alternative. 

2.  Product requirements

In general, not all of the requirements are critical; sorting them to critical and “flexible” will be very helpful. In the end, the product design will be determined by the critical requirements.

In certain environments such as startups and small companies. System Requirement Specification (SRS), or other organized lists of requirements, is not always managed.

My advice for the responsible engineers in cases like that, is to manage the requirements doc by themselves.To collect all the info from the meeting summaries, design reviews and any other non-official conversations.

This written list will serve as the guideline for the detail design.

Think about the famous Abraham Lincoln quote: “Give me six hours to chop down a tree and I will spend the first four sharpening the axe”.

3.   Detailed design phase

“The devil is in the detail”

From my perspective, order is the key element in this important phase.

Give all parts short names (or a shortcut of the project name) and following number. For example: DA-01, DA-02… 

It will make easy for the manufacturer to follow, and will save you the hassle of inventing a description for every part.

Later on you can add a description in the file properties.

Build the parts effectively ,choose the right features, work parametric without patches, although it’s not the final release, in this way you have a better control and it will pay off whenever you have to adjust.

Keep your tree organized, rename your features, use hole wizard. 

 Try to use the same part in several places, but remember that machining multiple simple parts is less costly than machining one complex part.

Share your design with your manufacturer in the early stages. He can track technological problems for you, such as tool holder clearances, how to avoid end mill chatter and more.

Try to use off-the-shelf parts (COTS) as much as possible.

Don’t let your cad system lead you to your comfort zone, if you’re thinking of something and you have difficulties to realize it, ask for help, consult and complete what you wanted to do, and you will also have gained knowledge.

Split your work to sub-assemblies, insert all of the system components. When your design is stable.

Insert all of the fasteners although it’s a Sisyphean task.

I’m sure we all agree that it’s better to invest the time instead of get stuck, and chasing after the correct screw lengths.

4. More calculations – less Intuition

Calculate anything you can, the numbers will give you the feeling. Always look for the worst cases, do not do it by hand – use excel or matlab.

Simulate motions to avoid unexpected collisions. Use a collision detector – sometimes it’s hard to obtain by viewing, use interference detection to look for penetrations.

Simulate strength problems in order to determine the amount of material. Even if you don’t have the full package, express products are effective tool for basic analysis of parts.

If you don’t have the experience and it’s critical,go to FEA consulting but stay focused on the required output, don’t let it become a new project.

Understand well the difference between stiffness and hardness when choosing materials.​

All the above will help you to stay close to your working point.

And will help you to analytically prove your design in front your team. The more your design will be based on calculations, the less others will intervene.

5.  Design freeze

How do we know when to freeze the design?

When the team is confident that the proposed product meets the requirements.

After all the design reviews,calculations,simulations,experiments, assembly animations were made and verified by the team – probably after quite a few iterations, this is the time to move to the next phase. 

6.  Drawings phase

Many engineers underestimate the importance of detailed drawings; my opinion is that this stage can make the difference between a successful prototype and a moderate/poor prototype. Usually the only communication between the engineer and the manufacturer is by the drawings and the 3D model.

Ensure that the drawings are clear,the text is easy to read,and that line font are dark enough to see clearly.

Avoid using any colors such as light grey, blue and especially yellow. Use the title block on all drawings, including revisions, requested material and required finishes. Make sure you have 100% compatibility between the 3D part and the drawing.

Make the drawing in accordance to the assembly in order to recall the relations between the parts, and then define the necessary accuracy.

Use geometrical tolerances in a way that the manufacturer will understand what are your important dimensions and datums.

Stick to ASME Y14.5, don’t ask for unreasonable tolerances values.

fully define simple parts, and in a complex parts give only important dimensions, if you have more than 3-4 different threads in one view, use hole table – the “TYP” definition is not clear.

7.  Files

Make sure to attach the corresponding native 3D format for every part. If your design is not confidential it’s recommended to share the relevant sub assembly, even a capture will help the manufacturer to understand how you intend to use the part. When you finish all the drawings, it’s recommended to get a complete BOM for the main assembly in order not to forget any parts.

Summary

There is much more to add to all that is written on this important topic.

I hope I was able to shed some light on the process in a nutshell, for those who deal with various types of mechanical engineering from scratch.

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