Humankind has been drilling objects for millennia, but precision was always sort of elusive. When modern computer-based control of machines was developed in by James Parsons at the Massachusetts Institute of Technology, the aim was to make durable and consistent aircraft parts. While the first generation of CNC machining was still somewhat imprecise, it was a vast improvement. Parsons used the technology of the day'punched paper tape'to send instructions to the machine that told it where to move and where to mill.
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We've come a long way since the days of punched paper tape spools. On some of today's mills, the machining process can take place on five different axes without removing and restaging the part in a fixture, making for faster, more accurate output. CNC turning, which uses your 3D CAD models to create cylindrical parts, makes parts on a high-speed CNC-controlled lathe. With turning, it's basically the same idea as cutting away slivers of wood from a maple log (or white ash, if you prefer) as it spins on its long axis, ultimately making a baseball bat. It takes time to do all this right, but we mill and turn parts 24/7.
Unlike 3D printing, in which you add material to form a part, with CNC machining, it's all about subtraction. You start with a solid block or cylinder of metal or plastic and cut away from it to achieve the desired geometries. It's kind of a high-speed version of sculpture, but with drill-like cutters called end mills, instead of bits and chisels. For milling, the cutters'called end mills'spin at incredible speeds measured in the tens of thousands of revolutions per minute. Speed can be adjusted to avoid cutting errors or potential damage to more sensitive material. Sometimes slow and steady wins the race. CNC machining's goal is precision, and feature tolerances can be as small as ±0.001 in. (±0.025mm).
Our entire process, from quoting to physical cutting in a mill'more commonly known as a machining center'or on a lathe is part of a digital thread, which ensures data integrity and helps maintain consistency. Essentially, we take your uploaded 3D CAD models and perform a design for manufacturability (DFM) analysis. Once any DFM issues are fixed in the CAD model and the quote is approved, the digital file is translated into G-code, a language that tells the machine how to cut away material to define the desired shape of the model as an actual 3D object. The mill or lathe then does the work as instructed until the part is complete, or so it can move to any desired finishing options'more about those later.
The thing to notice here is how this has all happened digitally. This simplifies communication and speeds production. So, the initial iteration can be done on the digital twin that our systems create, rather than with the traditional method that involves machining a part, followed by laborious back-and-forth discussions about how to adjust it, and more iterations until complete. With digital manufacturing, the goal is speed, efficiency, and accuracy once the physical part is produced.
But how does the machine know which kind of cutter to use? While you can choose any size end mill, we will direct you to the fastest solution to get your part finished. Each machining center (we have more than 500) comes with a set of standard end mills in various sizes. The mill set is chosen to offer a range of possibilities and is based on how often they are requested. Avoiding tool changes, and especially avoiding a change to a special, non-standard end mill, saves production time.
The technology behind machining has also come a long way. For many years, the standard lineup in any machine shop consisted of two-axis CNC lathes and three-axis machining centers. Some were horizontal and others vertical, but for the most part, work bounced between the two until all machining steps were completed. Lately, thanks to some clever machine tool builders, the line between lathe and mill has grown quite fuzzy. So called multitasking machines combine a milling spindle and tool-changer with a lathe-style head and turret (the part that holds the tools). Similarly, mill-turn lathes combine both rotating and stationary cutting tools, while machining centers with turning capability have become increasingly common. For example, we use lathes with live tooling to accommodate features such as axial and radial holes, flats, grooves, and slots.
Machining centers may also have more than three axes. A five-axis mill, for example, can move along all its axes simultaneously, an attribute that's useful for producing parts like impellers and many other complex geometries. And a traditional three-axis machining center might be equipped with a head that tilts and/or rotates. This 3+2 capability is perfect for machining multiple sides of a workpiece in a single handling, avoiding re-fixturing, which always adds production time. Whatever the exact configuration'and there are many'each style of machine tool is designed to reduce machining operations and increase production flexibility.
CNC machining has become a manufacturing staple for product designers and engineers who need low volumes of prototype or production parts, fast. This guide shares tips on designing efficient and manufacturable CNC-machined parts as well as how it can streamline your product development processes.
Get GuideWhether prototyping or creating end-use parts, our CNC machining provides multiple benefits, including:
Compared to injection molding, CNC machining provides parts more quickly. Also, it offers greater dimensional accuracy than 3D printing (although this may change with further innovations) and produces parts with better mechanical properties in all three dimensions than 3D printing. However, CNC machining usually comes with a higher price tag, especially with smaller volumes of parts, and has less flexibility with complex geometries, compared to 3D printing.
As mentioned previously, CNC machining is a go-to manufacturing process to get precision parts, whether for prototyping or end-use production. It is widely used throughout the aerospace, medical, automotive industries for its ability to rapidly manufacture precise parts in production-grade materials, such as:
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If you're someone who designs parts for a living, you might be wondering: Who cares? If I get my parts on time and at a reasonable price, it doesn't matter to me how they're made, right? Well, not necessarily. Just as you need to have a basic understanding of how a motor vehicle works to arrive safely at your destination, a reasonable grasp of machine tool technology is important knowledge for any parts designer. It is especially important when working with an automated digital parts supplier such as Protolabs, which can drastically accelerate the machining process'and often reduce costs'if certain design guidelines are followed.
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The primer just offered is a good starting point, but the best advice short of programming and operating a machine tool for a few years is to work closely with your machined parts supplier. Ask questions, in particular ones such as: 'How can I make my parts easier to machine?' With that in mind, here are eight considerations when exploring ways to optimize part design for machining:
One of the most common mistakes that even experienced design engineers and product developers make is to overly complicate their part designs. Consider breaking up single-piece, multi-faceted 'super parts' into simpler components that can be bolted, glued, or screwed together. Unless required for functionality, avoid swept surfaces, as these generally require lengthy, more costly machining with a ball-nose end mill. Design parts with features that can be cut from one side wherever possible. This avoids multiple operations and possibly special fixturing, as well as having to use a more expensive five-axis machining center, or one with tilt-rotary (3+2) capabilities.
Cutting tools are made of hard, rigid materials such as tungsten carbide and high-speed steel (HSS). Despite this, they do deflect ever so slightly when subjected to machining forces, a phenomenon that becomes increasingly troublesome the farther out the tool protrudes from the toolholder'depending on the operation, carbide cutters are good to a 'stick-out' of roughly 4x the tool diameter, maybe a bit more, on soft materials, while HSS tools become problematic at about half that distance. This leads to chatter (an ugly rippled surface), difficulty meeting part tolerances, and poor tool life. The lesson for a designer? Watch out for deep, narrow pockets, or part features situated alongside tall walls, or cutter deflection will create problems for those doing the machining.
Similarly, thin-walled parts are subject to deflection, but because most workpiece materials are nowhere near as rigid as the cutting tools used to machine them, the rules are a bit more stringent. Again, it depends on the part feature and material, but a good rule of thumb is to design walls that are no more than 2x thickness deep and understand that any wall thinner than around 1/32 in. will likely cause problems. As always, check with your machining expert for advice.
Hole-making is the most performed of all machining operations. It's often done with drill bits that are a little different than the ones you find in any hardware store. As holes get deeper (say five to six times the drill diameter) evacuating the metal chips'called swarf'formed during any machining operation becomes increasingly challenging. If your product design requires deep holes, so be it, just know that the deeper they are in relation to their diameter, the more expensive the part or parts will be.
In machining, corners can be tricky, too. Say you're designing an electronics housing for a product. On one side of the part you need a pocket, inside of which will sit a 2 in. square circuit board. Someone unfamiliar with machining practices might design a square-cornered pocket, one just slightly larger than the board itself to provide clearance. Not a good idea. Those square corners will cost you dearly, as the only way to generate them is to burn them out with EDM, a machining process common in the injection molding and tool and die industries. Space permitting, consider oversizing the pocket so that an end mill can be used instead'in this example, a 1/2-inch tool might be appropriate, which means adding half its diameter (1/4-inch) on all sides of the circuit board, plus whatever additional clearance is needed to fit the circuit board. Another option is to cut reliefs or 'dog ears' at all four corners. This might give the pocket a cloverleaf or T-shaped appearance but will make machining much easier.
There's plenty more to think about. Just as deep pockets are a no-no with milled parts, overly deep grooves can be challenging to turn, and long, slender shafts can be tricky, too. Breaking the edges on turned parts with a radius or chamfer is no big deal but requires an extra machining step on ones that are milled. And since we're on the subject, be sure to ask your machining provider about its preferred method for deburring parts'some use abrasive wheels, while others 'tumble' parts in small rocks, or blast them with tiny glass beads or bits of walnut shell. Each has its own advantages, and may affect the appearance of the finished product, as well as its cost.
Internal threads can be created with a tap, a cutting tool that closely resembles the bolt or fastener that will at some point be screwed into the workpiece, or a special cutter known as a thread mill. Either way, deep threads are difficult for the same reason that deep holes are difficult, namely chip evacuation, and in the case of thread milling, radial cutting pressures. In most cases, a thread depth of twice the diameter provides plenty of strength, and costs less to produce. If this is insufficient, consider using inserts such as Heli-Coils or key inserts to bolster thread integrity, especially on plastic CNC parts. Finally, thread tolerances are specified with an 'H' limit, the most common being H2 or H3. The latter has a tighter tolerance, however, and is therefore a bit costlier to produce, so the best bet is to call out an H2 for all but critical applications. Scroll down for more about threading.
Our automated CNC machining service provides the ability to easily add threaded features to milled and turned parts. Threads are specified within our automated interactive quotes. When a quote is returned, the 3D display will show which thread types are possible for each feature. Different threads (where possible) or no threads can be selected on a feature-by-feature basis. View a sample quote to explore a 3D model with threads or check out our guidelines page.
For those having trouble with the interactive display, a 3D PDF is included in the demo quote that shows the thread options available. After an order is placed, an order confirmation will be emailed with the thread sizes selected. Please review the order confirmation for accuracy.
We offer a limited selection of UNF, UNC, and metric threads for machining along with coil and key inserts (but do not supply or install the inserts). On both milled and turned parts, threaded holes must be modeled at the proper diameter, however, threading options differ for milled and turned parts. Currently, automatic external threads are not available for milling. It is possible to machine approximate external threads using ball and flat end mills if threads are modeled and are at least 1/2-13 (M12) or larger. Threaded holes on turned parts are also available and external threads are offered on the axial diameters. Simply model the nominal diameter; do not model the threads.
Is it possible to get perfection plus fast and cost-efficient prototypes and production parts from a CNC machining supplier?
In truth there is a balance to be made when it comes to manufacturing parts using on demand CNC machining. After all you wouldn't expect a Bentley for the price of a Mini, yet both do the same basic job of getting you from A to B.
Just as there are a number of important points that would lead you to a decision on your next car purchase, there are a number of considerations that you can make when designing parts for milling and turning processes. These decisions can accelerate production and reduce your costs.
In this post we will look at some decisions that will help you make the best CNC design. We will cover hole depths and diameters, sizes and types of thread, text on parts, wall heights and feature widths, live tool lathes and multi-axis milling.
There is a lot of debate between using a drill bit vs an end mill.
For rapid production and to help lower cost, end mills provide greater flexibility in the hole sizes possible with a given tool and they offer a better surface finish. They also allow us to use the same tool for machining slots and pockets and thus cut down on the cycle time and cost of your part.
As a designer you do need to be aware that holes that are more than six diameters deep become a challenge due to an end mill's limited length. Any holes longer than this may need machining from both sides.
Instead of using a tap to cut internal threads, check that your supplier uses a thread mill to create the thread profile. It creates an accurate thread and a single milling tool can be used to cut any size that shares that pitch to save production and set up time.
To illustrate the point UNC and UNF threads from #2 up to 1/2in and metric threads from M2 to M12 are all possible from a single toolset. Find out more about threading here.
Less really is more if you want to save time and money as far as text is concerned. Ask yourself do you really need a part number, description or logo milled onto your parts?
If the answer is still yes, then the spacing between characters and the stroke used to 'write' them should be at least 0.5mm. Also make the text recessed, instead of raised, use 20 point or larger fonts such as Arial, Verdana or other sans serif options.
Wall heights and feature sizes are very dependent on the individual part geometry as well as the toolset being used.
Remember even the strongest tools deflect, as do the metals and plastics being machined. So, avoid very small features and tall thin walls. At Protolabs for example the minimum feature thickness we can achieve is 0.5mm and the maximum feature depth is 51mm, but that doesn't mean you can design a ribbed heatsink using those dimensions.
It pays to do your research when outsourcing a supplier as capabilities do vary considerably and this may affect what it is possible.
For instance use similar toolsets to the ones on other machining centres. It means that off centre holes, slots, flats and other features can be machined parallel or perpendicular to the long axis of the turned workpiece. It will typically follow the same CNC design rules as those applied to the parts made on machining centres.
Contact us to discuss your requirements of Rapid Prototyping Services. Our experienced sales team can help you identify the options that best suit your needs.