When it comes to machining, “Time is Money.” And there is no cost driver as significant as this. More often than not, machining costs will outweigh the cumulative material costs, set-up costs, and finishing costs (anodizing, powder coating, etc). So, the best way to reduce the machining costs in a product is to emphasize reducing the machining time as early as the design phase of product development. Here are a few thumb rules you can internalize when designing a product if you need to reduce the costs.
The variables that drive the “Time”
- Geometry
- Set-up difficulty
- Materials
General Thumb rules
1) Limit using tight tolerances
In a typical component, only a few surfaces perform critical tasks like assembly fit or precision mounts. It is unavoidable to machine with tight tolerances at those locations. But to achieve high tolerances, the work has to be precision machined, and that process is much slower than machining with open-tolerances. If a part has a lot of features that require tight tolerances, then the machining time will go up, and it will take the machining costs with it.
So, it is best to mention tolerances, where it is absolutely necessary. This is one of the biggest drivers of costs when machining a part. For a regular-sized (150 mm x 150 mm), open tolerance is 50 microns. Anything less than that will cost time. If not necessary, it will be like money spent in vain.
2) Use Softer materials and post-process them to achieve desired properties
Using softer materials can help us reduce overall costs in three different ways. One, Softer materials are easier to machine. For example, Aluminum 6061 can be cut easier than 7075, so the machining time is lower, and so is the cost. Two, reducing material hardness reduces costs by increasing the tool life. Harder materials wear off tools faster than softer materials. Three, there is a general positive correlation trend between material cost and hardness. Higher the hardness, higher the costs. So, using softer materials will inherently be cheaper. Heat treatment or anodizing can be done to increase the hardness of the material after machining.
3) Avoid monolith parts for complex shapes
If a part’s shape is intricate and does not demand extreme precision, it is better to avoid machining monolith structures. Because you can achieve a similar shape by machining a few better machinable parts and join them with welding, gluing, or brazing. The Structural strength will also be comparable, albeit with a little wider tolerance, which can be controlled with jigs. But this can reduce the machining costs greatly since a single complex part will always take more machining time and setting time, than a few better machinable parts.
4) Order high quantities to reduce costs
If an order is for a prototype part, the time for setting up, G code generation, and quality control will have a higher proportion of the overall time. Along with that, the profits for a machining operator for machining a prototype is marginal, so they don’t do prototype parts without higher margins. These two parameters make the costs to balloon by reducing the number of parts manufactured. Most companies quote a cost per part for slabs (For example, 50-500 parts x $/ part).
DFM Thumb rules
1) High internal fillet diameters
Internal fillets in a pocket are not something that is deliberately machined. It is created by the diameter of the cutting tool. Bigger the milling tool used to machine the pocket, the bigger the fillet radius. But what drives the cost here is the relative feed rate difference between the smaller and larger tool. The bigger the tool diameter, the faster one can machine because they are structurally stronger. So, if the inner fillets are bigger, the machining time will be lesser, and vice-versa is also true.
2) Avoid deep pockets
Milling tools are subjected to tremendous torsional and bending forces. And the tools are extremely hard, so fragile. The better structurally mounted a tool is, the faster we can machine the work. And a milling tool can be cantilevered only so much. The standard ratio for the depth pocket and the diameter of the tool is 4; if a tool is of 10 mm diameter, the pocket depth is supposed to be no larger than 40 mm. If the design requires deep pockets, the machining speed will be substantially reduced, increasing the cost severely. After a certain threshold, we cannot machine that with conventional methods, and we have to resort to EDM machining.
3) Avoid external fillets and chamfers
Unlike internal fillets, external fillets are not automatically formed. They have to be deliberately machined. The cutting tool has to take a circular path near the edge of the work to cut an external fillet, and it is time-consuming. The same with chamfers; it has to be deliberately cut. Any design that slows the tool movement is going to increase the machining costs.
4) Avoid thin walls
Thin walls create complications on two fronts. One, the deep thin walls are difficult to machine as mentioned above. Two, thin walls are usually not stiff. So, the forces from machining operations induce the walls to vibrations. It can force us to reduce our machining speed, thereby increasing the costs. More often than not, it is better to use other manufacturing methods, including fabrication, rather than machining because of the costs. If that’s not feasible, keep the thin walls at least over 1 mm (for Aluminium).
5) Avoid giving features in different planes
In a 3-axis milling machine – which is the most common one around – the machine tool can only work on one plane at a time. So, if you have designed with features on different planes, then you have to remove the work from the fixture and set it up again which costs time. So, limiting the features to a single plane can reduce the costs beyond anything else. With smart engineering, you can almost always achieve what you want in a single plane itself. You can machine such parts without changing set up in a 4-axis or a 5-axis machine, but they are quite rare and so expensive. If cost-cutting is your primary priority, limiting features to a single plane can do wonders.
6) Maintain consistency of dimensions
For example, there are two slots in a part. One is 10 mm in width and the other is 15 mm in width. We either have to use two different tools to machine them or we have to do multiple passes to machine the 15 mm slot with the 10 mm tool itself. Both of them are going to take more time. Whereas, if it is designed with two slots of each 15 mm, we can use a single tool to machine them quickly. So, keeping consistent dimensions for features in a part can save us huge time either by tool changes time or by machining time.
7) Standard drill hole sizes
Non-standard drill holes are made using endmill cutters by revolving the tool in a circular path to create the hole of the given dimensions. Or by using boring tool. And machining a hole with an endmill cutter or boring is always going to be several times slower than taking a standard drilled hole. So, it is better to design holes with standard diameters so that they can be drilled by standard drills.
8) Limit aesthetic surface finishing to where it is absolutely necessary
VMC machines can produce parts with an almost mirror finish, but that does not mean we have to do it all the time. For some places, it is necessary. Whereas in others, it is just redundant. So, limiting the surface finishing to where it is necessary can help you cut costs greatly.
9) Try to limit features
The word feature, in engineering, includes holes, slots, pockets, tapped holes, threads, keyholes, or any machined shape on the work. The more features a part has, the more time it takes to machine it. But we cannot remove features just like that because the part has to fulfill its functional requirement and features usually play a big part in it. But with smart engineering, you can find ways to limit the number of features in a part.
If there is anything you can take away from this article, let it be “When it comes to machining, Time is Money”. The more time it requires to machine apart, the more expensive it is going to be. If you still struggle to find DFM issues in your design, we, at Custiv, have a proprietary free DFM checker that can help you out to reduce your costs in machining.