6 ways to cut machining costs

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By Tony Holtz, technical specialist at Proto Labs

Cutting machining costs starts with

Machining gets a bit more complex every year, which means it can be challenging to keep pace with the do’s and don’ts of part design. But cutting the cost of machined parts while improving functionality can still be achieved by a few relatively simple adjustments to your part design or material selection.

Automation software is often used to quote parts and highlight features that need design attention. The software will catch features that are not manufacturable up front, but it will also highlight areas that do not necessarily require change, but can improve the design’s overall machinability—thin walls, engraved text, deep holes and complex geometries, to name a few.

To design more cost-effective parts, we’ve identified some tips that can be applied as follows:

1. Provide relief to corner pockets

Consider the corners of a machined pocket—the inside of an electronics housing, perhaps, or a bracket used to capture the body of a rectangular component. One common design oversight is leaving the intersection of the vertical walls on those part features perfectly sharp. To illustrate, think about machining a stainless steel box to hold a collection of baseball trading cards. The only way to get the perfectly square corners needed to fit those Babe Ruths and Hank Aarons is with EDM, a slow and expensive process.

Instead, equip your (or your supplier’s) machining center with the smallest end mill available to clean out the corners. In 304 stainless steel, that means a 0.031 in. (0.8mm) end mill, which leaves a corner radius of 0.016 in. (0.4mm). That’s pretty sharp, but the depth is limited—the length of most steel-cutting end mills in this size range maxes out at five times the cutter diameter, barely deep enough to fit your favorite center fielders. Machining with small end mills such as this is also slow and delicate work, driving up the cost of your project.

A more budget-friendly approach is machining a relief in each corner of the pocket. This removes that pesky radius, leaving a U- or C-shaped clearance instead (see illustration). It also allows for far deeper pockets — by cutting a 0.25 in. (6.35mm) wide relief in each corner, functionally sharp corners to around 1-1/4 in. (32mm) in depth are possible. And by switching to aluminum or even plastic, pocket depths twice that of steel are possible. Best of all, designing pockets in this manner reduces part cost, since larger end mills can be used and material removal rates increased accordingly.

2. Consider deburring or part finishing

How is your part going to be finished, what is the function of your part, or what is the finish requirements? You have options on how edges may be broken or deburred and they each have pros and cons for them. Automated deburring may add small chamfers or radii on your part and is done in the CNC machine while other options might be more labor intensive such as hand removal of these burrs and even grit blasting. The process for removing burrs may depend on cost and functionality of your part. Do you require sharp corners or do you want to reduce machining times — and ultimately production costs — and opt for manual removal of the sharp corners? Will your parts need smooth finishes or are textured finishes okay to disguise toolmarks and remove sharp burrs.

3. Avoid text until molding

Similarly, text engraving is an aesthetically pleasing but time-intensive operation, one that might be best to avoid if possible. Here again, a ball end mill is used to trace whatever letters, numbers and symbols are called for on the CAD model. It looks cool, and might be a valid requirement on your machined part, but is probably more appropriate on injection-molded parts, where additional machining time is amortized over higher part volumes.

Small tool diameters add machining time so consider removing text or logos from machined prototypes.

4. Be cautious of thin walls and features

For most products, part tolerance is +/- 0.005 in. (0.127mm). If you have a feature that is 0.020 in. (0.5mm) or smaller, it may be categorized as thin wall geometry. That doesn’t mean it cannot be machined, but the machined part may differ slightly from your original design. Further, any thin walls that are 0.020 in. or less are not only subject to breakage during the machining operation, but may flex or warp afterwards. Beef them up as much as your part design allows.

5. Keep it simple

Very deep pockets are a no-no, even if the corners are relieved. Not only does it take a lot of machining time to remove all that material, but any residual stress in the raw material tends to show its ugly face as pockets get deeper and walls taller. Gussets or support structures might be used to support these workpieces and prevent movement due to stress, but these tend to increase machining costs. Keep it simple is the best advice for any manufacturer or product designer of machined parts.

The same principle applies to overall part geometry as well. Don’t attempt to make parts do more than they should. Maximizing material usage may create workholding or machining problems, in turn increasing costs. If the design gets too complex, consider breaking it into multiple components and using fasteners to assemble them. No one likes assembly costs, or the complexity that goes with multiple pieces, but it might be the best approach for difficult-to-machine parts. Sculptured surfaces, cavernous slots (think heatsinks), super deep holes and threads — these are some of the common machining cost drivers that can chip away at your project budget.

6. Explore alternative materials

One of the simplest ways to stay within budget—assuming it meets your requirements—is by switching to a more machine-friendly or less expensive material. Proto Labs’ material selection includes a range of metals and plastics, each with its own engineering attributes, aesthetics, machining considerations and material cost. 17-4 PH stainless steel is a bear to cut. If high strength and corrosion resistance isn’t critical, try 316L or 304 instead.

Copper is a great electrical conductor, but it’s far more expensive than aluminum, which is nearly as conductive and easier to machine besides. If hardness is a concern, 4140 might be your first choice, but 1018 is very low cost and takes an admirable case hardening.

And one sure way to make a machinist smile is to give him or her an order for some brass or magnesium parts — both are free machining, and might just have the mechanical and chemical properties needed for your application.

On the plastics side, all of them are relatively easy to cut, which often equates to lower cost parts. Some plastics offer superior wear, corrosion, or chemical resistance, others perform well under heat or flame, while still others offer excellent strength, impact, or electrical properties.

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