3 Considerations for Revising Design for Manufacturing Efforts

Reader Question: We run a fairly new shop and have made our way so far with simpler parts, but as we progress in difficulty, we’ve been burned a few times trying to make poor designs work for our customer. Do you have any advice on reviewing parts for potential problems, and what changes are reasonable to ask of our customer?

Miller’s Answer:

While we value our relationships with designers, they don’t live in our world day-to-day and understand all the complexities of making parts. Often, we receive a part through the quoting hotline and immediately start shaking our heads. We are given a part with poor design, excessive tolerancing and unclear requirements, and the part usually has combination of some (if not all) of these issues.

A designer may think they’ve done design for manufacturing (DFM), but as the ones responsible for producing the part, it falls on us to educate the designer and offer revisions for our sake (simplicity, chance of success) and theirs (cost and schedule).

To answer your question about the process for doing these revisions, there are three primary avenues I investigate: the 3D CAD, the 2D drawing and the part’s functional requirements, such as material, coatings and purpose.

However, before we dive into what I look for in each of these avenues, it’s important to understand there is no right one to start with, as they all are interrelated and that all of these avenues of review are happening at the same time. For example, the 2D drawing may have some tough true position requirements, which changes how we view the 3D model and our setup goals. Functional requirements like material and finish may inform you on 2D tolerances that require post processes or can be loosened to reduce cost. Difficult, thin shapes in the 3D review may require a change to a material grade that is stress-relieved versus the standard grade that was called for. With all this in mind, the best thing to do is just keep all the requirements handy as you work through the process. Below are what I look for with regards to each.

3D CAD

There are several things that you will want to study on the the 3D design of a part, including the shape, threaded holes and chamfers. Shape would apply to things like small slots which require tiny tools or pockets with nominal radii. These should be changed so you can properly feed the tool through a corner versus the corner matching the end mill size. It may also apply to things like angles that require a fourth axis or could be done with a chamfer tool instead, if minor accommodations can be made. These changes may also enable fewer setups, eliminating that extra step for the one obscure face on the part.

Another topic of focus should be threaded holes. Designs where the thread goes all the way to the bottom of a blind hole are not possible, and things like missing chamfers impact the designer’s intended number of threads, so be sure to clarify these.

Lastly, I like to have the chamfers designed into the part, not left to the machinist’s interpretation. The reason: If you don’t deburr enough, then the customer may complain; but if you overdo it, an edge that should have been left sharp is no longer functional. So be sure to tighten up these details as well. 

2D drawing

When reviewing the 2D drawing, you’re primarily looking at tolerances and text. The tolerances then breakdown into size, position and form. For example, a size tolerance may require DFM feedback if it is more difficult than a cutter compensation adjustment, or requires the addition of a reamer or boring tool. If you know a hole is primarily for clearance, it may have too tight of tolerances or have a purpose you’ll want clarification on. A positional tolerance may influence how you want to approach the part with regards to workholding or number of setups. If too many positional tolerances compete due to stack ups, then you may want to ask the customer to prioritize them further, or even open the least important one to promote better processing. Form tolerances may also be impractical given their purpose on the print, or too hard to hold without significant post processing.

The text of the part print is often overlooked, but it can cause problems. This is often a catch all for some internal requirements, but can also refer to specific part requirements that drive additional processes. It’s good to understand if the text is a driving requirement or a left over from a previous version of the title block.

Part characteristics

Part characteristics are a catch all for the intangible aspects of the part that a customer may not be thinking about or omitted in error. Things like materials that dictate processing requirements or assemblies, as well as coatings and more, can all impact the DFM process. Will they accept any aluminum or do they need 7075? When it says 304, does it mean annealed, half hard, 304i or some other variation? A finish requirement may say “to be ground,” but could it accept a carefully milled surface that still meets the print roughness requirement? Does the customer need the tolerances to be pre- or post plating? Proposing or clarifying any of these items can save a customer money or can prevent disasters like making a part in the wrong material.

What’s reasonable?

To answer the second part of the question about what’s reasonable to propose, my answer is anything you think is a risk or could save both parties a lot of money and/or time. For a prototype shop making just a few samples of a part, it may just be a small list of cleanups. For a production design, it may be dozens of proposals to scrape every possible second out of the process.

As an up-and-coming shop, it may feel hard to push back for fear of the customer going somewhere else. Rather, I think you’ll find they’re happy to accommodate if the request is reasonable, and more importantly, it comes with a well thought out and well-presented justification. I normally will send a quick slide deck with screenshots of the current design details and what I’d prefer. This professional approach reminds the customer that you’re a technical partner, not just another machinist whining about a sub optimal design. To be sure, the customer will have some firm stances with their design, so respect those while also showing them how they benefit. The worst they can say is no.

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Do you have a machining question? Ask the expert. John Miller leans on more than a decade of industry experience to answer machining questions from MMS readers. Submit your question online at mmsonline.com/MillersEdge.