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Reader Question: We’ve recently received repeat orders from customers, not because of volume, but because of problems found downstream at plating or assembly. We made the parts to print, but further processes revealed issues that started with machining. The customer wasn’t upset, but how can we identify these issues ahead of time to be a better partner for our customers?

Every manufacturer depends on a network of suppliers to get a product to the finish line. The faster things move, the more every link in that chain is under pressure. When parts make it through your CMM but fail at plating, anodizing or assembly, it might not be because you did anything “wrong,” but it still started with machining. Understanding that connection between surfaces, shapes and tolerances, and then designing your process with these nuances in mind, is what separates a vendor from a partner.

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In today’s pace of manufacturing, being a reliable partner isn’t just about hitting a dimension, it’s about understanding what happens after the part leaves your shop. The shops that thrive are the ones that combine technical excellence with foresight. They don’t just machine to spec; they strive to machine for downstream success.

To breakdown where to start looking for these nuances, let’s examine some common post processes: removal processes, chemical processes and mechanical processes.

Removal Processes

Often, our parts will move on to further metal removal operations than just machining. While many shops will have the more common ones in house already, such as surface or ID/OD grinding, some are less common and may go to specialty processes. Processes like abrasive polishing, honing and lapping are common for production parts and require special consideration for incoming form tolerances. Honing requires a very straight bore to start with, but this spec may not be obvious on a typical print as it’s considered an “in-process” spec. Polishing with a wheel is a “less is more” process. Too much polishing can begin to introduce inconsistencies or wear in small areas of a surface. 

Chemical Processes

Chemical processes can present some odd issues for machined parts. A combination of part shape, finish and flow of the chemicals in and around the part can lead to plenty of unique chances for a good part to go wrong.

The first step is to recognize if the process is more additive or subtractive in nature and how this may affect sizing. For example, a plating process is a process that goes on top of surfaces, so internal features get tighter while external features grow. A chemical polish will do the opposite as it’s eroding the metal to achieve its end. Anodizing also grows on the surface, but depending on the flow of electricity through the part and if the chemicals are also flowing evenly throughout. Knowing how to pre-size your features appropriately, larger or smaller, so that the final part is in spec will set you apart in this area.

Second is to recognize how the part design will interact with these processes. If a feature needs to retain a sharp edge, some of these processes may round that off some. Or if the feature is too sharp, it may lead to premature flaking or chipping of that finish. Offering your customer a service like tumbling to soften edges or advising them to mask certain areas before coating may give the final product that performance edge.

Mechanical Processes

Once the part has been finished and coated, it still needs to be assembled, and that’s where many issues finally show up. The more we understand about how a part is used, the better we can machine it.

Start with assembly holes and fasteners. Many drawings show drill depth equal to thread depth, which we know isn’t possible, due to lead in of a tap. This is a common design mistake, but it’s easy to make when it looks like the bolt has clearance in the 3D world. Always check that there’s proper relief beyond the thread minor diameter. A simple tweak at the machining stage can save a seized fastener or a loose mating part later.

Next, consider sealing surfaces. Seals rely on three things: form, texture and direction of the tool marks. A gasket land that looks perfect to the eye can still leak. A spot face interpolated with an end mill behaves differently than one plunged with a boring tool. Think about the texture and direction and verify it matches the sealing requirement. When possible, measure surface finish along and across the cut to confirm.

Even part geometry affects sealing and assembly success. A bore that measures perfect at your shop could have changed during processes like heat treat. Take note of where tolerances stack, and if you suspect a part will distort downstream, share your concern.

The machinist’s job doesn’t end when the chips stop flying, it ends when the part works. Downstream processes are where that truth is often tested. The good news is that the fixes are rarely complicated; they just require awareness.

Just as we read a 2D drawing or do a spin in 3D to anticipate machining issues, we need to look at the whole life of the part to anticipate manufacturing issues. A simple chamfer, a drain hole or a change in cutter direction can mean the difference between a smooth anodizing run and a rejected batch. Those choices show foresight, and foresight builds trust.

Repeat business is good, but repeat business caused by scrap isn’t. When customers see that your parts perform cleanly through the whole chain, they stop seeing you as just another supplier and they start seeing you as the one shop that makes their product work.

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.