By Marshall Taylor
At Marvel Manufacturing, we primarily build to print. The part could be a coaster for your boss’ coffee mug or a precision component that is crucial to launching a multibillion-dollar project into space. Usually, I don’t really know what the part does.
In either case my goal is to meet or exceed the customers’ expectations by delivering parts that are within the tolerances set forth in the provided drawings. I have always appreciated opportunities to collaborate directly with the engineers who designed the parts. It not only gives me a sense of satisfaction that I am part of their team, but I also gain critical insight as to their intent regarding the functionality of their products. There are always tradeoffs when designing parts for manufacturing. A designer who gets carried away with over-tolerancing and adding unnecessary features is going to pay way more for their components, while failing to specify design requirements could lead to an unusable part. I have seen both ends of the spectrum.
When designing for manufacturing there are three main factors that should be considered: functionality, manufacturability, and cost. Let’s dive into each of these to find out how to design parts that can be manufactured at the best price with the best results.
Remember, the best part is no part. Although I am in the business of making parts, I would be remiss if I didn’t point this out right off the bat. Excluding parts eliminates the cost and maintenance associated with it.
Obviously, the design needs to be functional. Missing a tolerance on a critical feature like a bore could mean that the part won’t fit when assembled. In worst case scenarios such as products that are high risk and or extremely expensive, the consequences can be substantial or even catastrophic. In these cases, I usually see engineering erroring on the side of caution by adding extra zeros to the tolerance blocks. This tactic might avoid scrutiny on their end if something does go wrong but it should not be a shotgun approach to boiler plate drawing templates.
When it comes to manufacturing there are tons of tips that I can give you from experience that will certainly affect the price. Not all engineers or designers understand how their parts are made. If this is you, I encourage you to tour various factories or even spend some time on your own production floor to grasp different manufacturing processes. I always learn new things about manufacturing when I go on site tours to places like #Harley Davidson and #Schweitzer Engineering Laboratories. At the very least, try to visualize how the part is going to be made and what types of machines will be used. This will go a long way to designing a part that is suitable for manufacturing. Here is a list of things to consider when designing your parts.
Tolerance parts appropriately - Adding decimal places to your drawing will add zeros to your quote… enough said.
Minimum Corner Radius - Whether the part is turned on a lathe or machined in a mill, consider the tool that will be used for the feature. Milling cutters are typically cylindrical so internal corners usually need to have a radius equal to or greater than the tool radius (except for the intersection of floor and wall features). The same is true for carbide turning inserts because they usually have a corner radius as well.
Consider Tool Diameter - Design internal radii to be slightly larger than the nominal tool size. A tool needs to have consistent tool pressure to maintain a specific chip load. When the toolpath has to make a sharp corner, it is forced to push the leading edge of the tool into the adjacent wall, and it has to change directions in a fraction of a second. Making your corner radius slightly larger than the nominal size allows for constant chip load and a smooth toolpath. This helps reduce chatter and allows for increased feed rates.
Tool Length/Diameter Ratio - Be aware of the L/D ratio. This goes hand in hand with deciding what size your internal corner radius should be. A 4:1 ratio or less is comfortable, larger L/D ratios are achievable with bigger tools while smaller tools will require an even lower ratio. This doesn’t mean those features are out of the question, but they may require additional processes such as EDM or broaching. The same is true when considering the length of a turned component or a bore. The further the part or the tool must hang out the less rigid it will be.
Avoid Chamfers and Radii - Despite what I just mentioned about internal corner radii, I suggest eliminating external chamfers and radii on parts unless they are necessary. These may be more aesthetically appealing, but they require more tools, programming, and setup. If you must choose one, then a chamfer is usually easier and faster to produce. Edge breaks can be either modeled or simply called out in the notes.
Avoid Undercuts and Narrow Slots - These types of features usually require special tooling and in some cases are extremely difficult to achieve. I have machined a water jacket into an aluminum head that was a culmination of an undercut, excessive L/D ratio, and too small of a corner radius which ended up being a success, but it is not something that I would wish on anyone.
Material Choice - This should be evaluated first for functionality but then analyzed for the best option for manufacturing. Some materials are a breeze to machine while others can cause a person to consider changing careers. Mild steel machines are ok but can be difficult to achieve a decent finish. Anything cold rolled is going to have more residual stress than hot rolled due to the process used to shape the raw stock. This is a huge factor when flatness and straightness are important. Not all plastics are created equal. Delrin or acetal are my first choice when machining plastic. It is stable and machines well. On the other hand, UHMW is challenging because of its low melting point. It requires very sharp tools to get an acceptable finish and is difficult to maintain tight tolerances. While composites are machinable (even glass filled can be machined with off the shelf tooling) it wreaks havoc on machine tools. Unless a dedicated machine for composites is used, a detailed cleaning will be necessary to remove the fine abrasive material.
Material Size - There are many ways to approach how a part will be machined. I usually use the carrier method of holding stock, which means I purchase material that is larger than the finished part so I can clamp it in a vise to machine all sides. The part is then flipped over for the second op and the extra material that was use for clamping in the first op is machined away. In these scenarios designing a part slightly under the nominal size makes sense, otherwise the manufacturer has to use the next size larger material. On small parts this is not a big deal but on larger pieces of stock, there could be a significant difference in the price of material. On the contrary, when working on thinner sheets and plates it is usually easier if the part thickness matches the nominal thickness of the material. That is because it can be challenging to hold thin pieces to machine the top surfaces without bowing and avoiding clamps. Keep in mind raw material tolerances vary so be generous with the thickness tolerance if you decide to go this route. Raw stock finishes often have defects, primarily scratches and dings from processing and transportation. To mitigate flaws in the surface when choosing to go with a stock finish, we request the material be free of defects from our vendor. We also apply finish techniques such as sanding or bead blasting to improve the appearance. If finish or flatness is critical, we evaluate the best method to accomplish the requirements. Sometimes this involves special fixturing or sending it to an outside vendor for grinding.
Visualize the Workholding- In subtractive manufacturing, there needs to be a method for holding the part. There are countless options for workholding including vises, fixtures, chucks, and collets. In any case study the part to determine how it will be held for manufacturing. A milled part without any parallel sides will be a challenge to hold onto. Sometimes, adding extra stock or a feature specifically for fixturing, such as through holes in plates to use for locating and clamping, are good options that can save money.
Both functionality and manufacturability will have an impact on the cost of producing parts. When price is not an issue, even the sky is no limit. However, there is usually a realistic balance of manufacturing products that work and adding details that may add value in other ways. In short, the harder the part is to make, the longer it will take and the more it will cost.
Bringing your supplier into the design phase can help avoid sticker shock on quotes or having RFQ’s come back with no-bids. I am glad to discuss DFM solutions with customers, which sometimes means that I suggest manufacturing options other than CNC machining. Marvel Manufacturing is truly focused on “People first, parts included” because our business is serving people not just making parts. When you succeed, we succeed.
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