CNC Machining Guide: Process & Materials
1. What is CNC Machining?
CNC machining, or Computer Numerical Control machining is a form of subtractive manufacturing to produce parts and components made from materials such as metals and engineering plastics.
A process of manufacturing parts or components through material extraction, CNC machining uses pre-programmed computer software to relay a toolpath which tells the machine how and where to extract the material from a raw billet (block of material).
The machine will take multiple step overs usually defined as a percentage of cutter diameter in order to produce the desired profile. The extraction is undertaken by motorised cutting tools.
CNC machining is suitable for producing both one-off/prototype quantities as well as high volume production.
2. The CNC Machining Process
CAD
If you require a CNC machined part, you will need to supply an electronic version of the drawing or part. The format/file types of drawings will usually be stipulated by the machining company in advance. For example here at Penta Precision, we need a DXF 2D CAD file as well as an STP 3D CAD file. Don’t worry if this isn’t something you’re able to produce. Some manufacturers like us also offer a machining drawing service at an additional cost. This will help you turn a 2D pdf or even a sketch into a fully fleshed out CAD design.
CNC Programming
CNC programming is the process of creating a set of instructions using computer software in order to tell a CNC machine how and where to extract material to create a part exactly matching the CAD model. Another element to be taken into account at this stage are ‘speeds and feeds’. The speed refers to the spindle speed of the machine that is holding the tool, the spindle speed will differ depending on the material being machined and the feature being produced.
The feed rate refers to the relative velocity at which the cutter advances along the workpiece. The feed rate will again speed up or slow down depending on the material being machined as well as the feature and cutter diameter being used.
Programming will be carried out by an engineer who has been specially trained in using a CNC programming language. Common programming languages include Fanuc, Heidenhain and G-code. The language used does not ultimately affect the finished product.
CNC Setting
An engineer will choose the appropriate tools for the job and load them into the CNC machine along with the material. The cutter used will depend on which material is being machined. For example, some engineering plastics can be prone to chipping during the machining process. To counter this, particularly sharp cutting tools would be selected. An experienced machinist will know the limitations of machining particular materials and will be well versed at choosing the appropriate tools for the job.
The setter will also need to set a datum point on the machine. This is to reference the machine so the program will know where its zero point is.
Press go!
Now that everything is set up, the operator can tell the software that the part is ready to be machined and the operation will be carried out.
3. What are the Benefits of CNC Machining?
CNC machining has many benefits as a manufacturing process which we outline here.
High Accuracy
The use of CAD means that CNC machining produces extremely high accuracy parts with tight tolerances. Penta has a general machine tolerance of +/-0.1mm geometric tolerance and a surface finish of 1.6µm (micrometer) but tolerances tighter than 0.01µm can be achieved where required. See PDF General Tolerances to DIN ISO 2768.
Cost Effective
A cost-effective manufacturing process of components, CNC machining can be used for prototypes, one offs and small to larger batch production. CNC machining is most cost effective for one off and small to medium batches. There are far fewer upfront costs associated with CNC machining than there are with many other manufacturing processes. In section 5 of this guide we compare CNC machining with 3D printing and injection moulding.
Efficient
CNC machining is associated with reasonably fast turnaround times. The set-up time is comparatively small to other manufacturing methods. This ties back to cost effectiveness as time is money! It is also easily repeatable. Once the design is created, it can be accurately repeated time and time again.
Range of Materials
A high number of materials can be CNC machined. Where other manufacturing processes can be quite limiting, CNC machining allows designers to select the most appropriate material, taking both the desired properties and cost into account. A good CNC machine shop will be able to help you with material selection if you require.
Superior Quality
CNC machined parts are associated with quality and excellent physical properties. Where high performance is required, CNC machining is an obvious choice.
4. What are the Limitations of CNC Machining?
More Complex Parts Come at a Cost
The cost of a CNC machined part will depend on the complexity of the part being machined. Multi axis machining is more likely to be used for more complex shapes. Thus parts with high geometric complexity will come at a cost.
Workholding Restrictions
A part must be securely held in place while it is being machined. This is easily done with more simple shapes. More complex parts may require custom jigs and fixtures along with troubleshooting time from the engineer so this would increase the cost to machine.
Tool Access Restrictions
For the part to be machined, the appropriate cutting tool needs to be able to access the necessary surfaces. This may rule out particularly complex parts although experienced engineers are often able to find workarounds so it’s always worth a discussion with a CNC machining company before making a final decision.
5. Comparing CNC Machining to Other Manufacturing Methods
Clearly CNC machining isn’t the only manufacturing option so here we will compare with the other likely alternatives you may be considering.
CNC Machining vs 3D Printing
CNC machining is a type of subtractive manufacturing, 3D printing is additive manufacturing. This means that CNC machining starts off with a block of material and then undergoes a process to extract material resulting in a finished part. 3D printing involves a part being manufactured by building it up layer by layer using specialist machinery such as a laser or heated extruder.
CNC machining is associated with a very high degree of accuracy – hence why it’s often called precision machining. 3D printing tends to be able to offer more complex shapes than CNC machining.
Whilst CNC machining is compatible with a broad range of materials, 3D printing traditionally has more of a focus on plastics such as plastic resin and filaments.
More options to 3D print metals (metal powders) are being introduced, however in order to 3D print metals, much more expensive and high cost machinery is required. This could make the cost of 3D printing metals prohibitive, especially for prototypes.
Another big factor is that strength can be compromised with 3D printed parts and therefore it would not be suitable for all applications and industries.
CNC Machining vs Injection Moulding
Injection moulding is pretty much what it sounds like. It’s the injection of a material (in liquid form) into a mould in order to make the part. The material is then solidified to create the finished component. There are two major factors that can be prohibitive for injection moulding when it comes to lower volumes: time and cost.
Before the actual injection moulding process can take place, an initial prototype must first be made. This would be either CNC machined or 3D printed. Once the design has been finalised, a mould can then be designed and produced using this prototype. Only once the mould has been created can the injection moulding process begin. There is likely to be lots of testing at every stage of this process and tweaks or redesigns having to take place before everything is just right.
You can see that this process has consequences on both time frames and the cost of a project. Thus injection moulding should really only be considered when a high volume of components are required. For mass production, once the initial outlay has been recuperated, injection moulding becomes a cost-effective manufacturing process.
Less material is wasted during the injection moulding process as, like 3D printing, it is a form of additive manufacturing as opposed to CNC machining where material is taken away. Again this has an impact on cost but for low to medium quantities, CNC machining is still more cost effective.
Similarly to CNC machining, injection moulding is very good for consistency. Once everything has been set up, the parts produced should be as close to identical as is possible.
CNC machining is compatible with a far wider range of materials than injection moulding. Due to the nature of injection moulding and the fact that the material needs to be melted into liquid form before being returned to a solid state, it does rule out some materials whose properties would be compromised as a result of the process.
CNC Machining vs Manual Machining
CNC machining is much more accurate than manual machining; it’s much more precise and exactly the same process can be repeated in order to produce consistency across multiple parts.
Manual machining is sometimes favoured over CNC machining for prototypes and one off components where the set up time for CNC would not be cost effective.
CNC machines are significantly more expensive to purchase than manual machines so many CNC machining companies will usually have a plant list that includes some of each. This helps them balance their costs and pass savings onto their customers.
6. What Types of CNC Machines are there?
The two main types of CNC machines you are likely to hear about are mills and lathes.
CNC Mills
A standard CNC milling machine uses a cylindrical cutting tool that can rotate in various directions in order to remove material and create the desired shape. The milling cutter can move along multiple axes to produce the finished part. A CNC mill will generally have between 3 and 5 axes.
What is the difference between 3-, 4- & 5-axis milling machines?
The number of axis a CNC mill has determines the range of movement for cutting. Thus it is likely that more complex parts would require a 5-axis milling machine whereas more simple parts could be manufactured using 3-axis.
3-axis CNC mills allow parts to be cut side to side (X), back to front (Y) and up and down (Z).
4-axis milling machines allow all of the above plus 180° rotation around the X axis (A). 4-axis milling is useful when holes and cuts need to be made in the side of a part.
5-axis (sometimes called 3+2) milling machines are considered to be the best machines available and usually used to machine the most complex parts. They have the same axes as the 4-axis plus an additional rotational axis. The 5th axis is the 180° rotation around the Y axis (B).
Whether a part is machined using a 3-, 4- or 5-axis CNC mill will usually be determined by the CNC machining company you use according to the nature of the part and will be factored into the price quoted. You can usually assume that your part will be machined using the most time and cost-efficient machinery for the part in question.
CNC Lathes
The lathe is one of the oldest manufacturing technologies in the world with the earliest versions dating back to the ancient Egyptians!
Today, CNC lathe machines remain a popular choice for manufacturing more symmetrical cylindrical parts. The part/material is clamped and rotated at high speed by the main spindle and the cutting tools work around this. The tools do not rotate but they do move lengthwise and radially. The process undertaken on CNC lathes is called CNC turning.
A standard lathe will have two axes: X and Z. This kind of lathe is used to manufacturer standard parts with cylindrical profiles. However there are other types of lathes and add-ons which can produce more complex parts.
Y Axis Lathes
Many companies will also have a Y axis on their lathes – this enables the lathe to move across the job. The benefit of a Y axis lathe is that it can machine holes and features not on the centre-line of the component.
Driven Tooling
Driven tools are tools that mount to a lathe to enable other operations such a milling or drilling on the face or inner/outer diameter of the part.
Couple the Y axis lathe with driven tooling (which enables cutters to rotate similar to the mills) and you are now able to machine features such as keyways, flats, hexagonals, square profiles and other milled profiles.
7. CNC Machined Part Applications
CNC machining can be used to manufacture parts for an endless number of applications. Components that can be CNC machined include: enclosures, heatsinks, connectors, brackets, housings, shafts, bushes, bearings, guides, manifolds, handles, casings, spacers… the list really does go on!
In terms of industries, CNC machining has been adopted as a popular machining process by many industries including:
- Aerospace
- Pharmaceutical & healthcare
- Computing & electronics
- Military & defence
- Marine
- Original equipment manufacturers (OEM).
8. Which materials can be CNC machined?
CNC Machining is suitable for a wide range of materials including aluminium, stainless steel, engineering plastics and yellow metals. Here are some of the most common materials. Click on the material to see a material guide including properties, applications and design considerations.
Engineering Plastics
ABS Acetal / POM (Polyoxymethylene) Delrin Nylon Acrylic / Perspex PAI PVDF PEEK PEI / Ultem
Polycarbonate PET / Polyester / Ertalyte Polypropylene Polyethylene / PE / LDPE / HDPE / UHMWPE PSU (Polysulfone)
Polyimide / Vespel / PI PTFE / Teflon (Polytetrafluoroethylene) PPS (Polyphenylene Sulphide) PVC (Polyvinyl Chloride)
Aluminium
2014A / L93 / L157 / L168 7075 / L95 5083 6082 / L111 / L113 / L114 / L115
6061 6063 1100 2024 Cast Tooling Plate 5251 M82
Stainless Steel
316 / 316L 304 / 304L 303 420 15-5 PH 17-4 PH 440C
Yellow Metals
Bronze – Aluminium Bronze Phosphor Bronze
9. Designing Parts for CNC Machining
There are many different things you need to consider when designing a part that is going to be CNC machined.
Tolerances
The tolerance defines the acceptable range for any given dimension. If for example the part you want machined is ultimately going to be fit into another part, you will need to make sure that you supply the required tolerance in order for the parts to fit together.
Maximum Part Sizes
CNC machines cannot manufacture parts of unlimited sizes. Many CNC machining companies will publish their machining parameters on their website but if not, a quick call should clarify this. Because there are only so many CNC machine types, the parameters are unlikely to differ hugely anyway.
Here at Penta Precision, our machining parameters are:
Milling
X travel = 1000mm, Y travel = 475mm, Z travel = 550mm
X travel = 760mm, Y travel = 600mm, Z travel = 550mm
Turning
Ø260mm Billet x 550mm length
Bar feed = Ø3” (Ø76.2mm) x 300mm
Matsuura MAM 72-35V fully automated cell
Milling max size: Ø350mmm x 300mm height
32 pallets, 240 cutting tools, 24/7 machining capability
File Formats of Drawings
It’s worth checking which file formats precision machining companies accept before submitting your request to quote. For an idea, this is what we say on our website:
PDF files work best for 2D drawings
2D DXF files required for production
3D STP (preferred) or IGS file required for production
Material Selection
A good CNC machining company will be on hand to help you with material selection for CNC machined parts. Things to consider when choosing a material are:
- Comparative cost of material to others available
- Availability and lead times – some materials are harder to source and need to be imported so this would need to be factored into your project time
- Weight – does your part need to be lightweight?
- Aesthetics – is it a customer facing part?
- Properties – the application of your part will determine the desirable properties of the material. Do you need a strong material, one that is conductive, or corrosion resistant? Think about what is required and then work with your CNC machinist to select the material that delivers the properties you need
- Machinability – some materials can be more difficult to machine and thus the time implications will need to be factored in. If you can choose an easy to machine material that also delivers on all other fronts, why wouldn’t you?
Surface Finish
If you desire a particular surface finish, this will need to be stipulated and allowed for in the design. An unfinished part (i.e. one that has not undergone any secondary process) is called a milled finish. Some tool marks are likely to be visible on the surface. This finish will give the tightest dimensional tolerance possible.
However it is not always suitable, particularly if aesthetics are an essential requirement of your part. You may therefore decide to choose an alternative finish. Please note: secondary processes need to be accounted for when stipulating tolerances.
Common Additional Surface Treatments
- Anodising – including sulphuric acid, hard and chromic acid anodising – an electrolytic process that produces a thick oxide coating on an aluminium surface
- Bead blasting – the part is bombarded with small glass beads using pressurised air to produce a uniform matte or satin finish
- Powder coating – a thin layer of dry powder paint is applied to the part’s surface to provide a strong and wear resistant finish to metals.
3 Tips for Reducing Costs When Designing CNC Machined Parts
Here at Penta Precision we are always working with our customers to help them keep costs down wherever possible. We are often asked by designers which ways they can address this with their design. Here are 3 quick wins when it comes to shaving costs off parts:
- Tolerances – don’t tie up tolerances that don’t need to be tied up. Of course, sometimes tight tolerances are essential for the application of the part but often they are not.
- Surface finish – if one or more of the faces of the part aren’t going to be seen, there is no need for an aesthetically pleasing surface finish for those faces. For example, a normal milled finish may have some visible tool marks but this may not always be an issue.
- Corner radii – bigger internal corner radii are more time efficient, so small internal radii should only be used where the design/application requires it – our experience is that it often isn’t required!
10. Choosing a CNC Machining Supplier
So you’ve read our guide and you’ve decided that CNC machining is the best way of getting your part(s) manufactured. Now you need to find and choose a supplier of CNC machined parts.
A quick online search will list lots of companies offering CNC machining services, so how do you decide who would be a good fit for you?
There are usually three main factors involved in any buying decision – price, service and time. If someone claims they can offer an outstanding service, provide the part(s) overnight AND at a bargain price then that should be an immediate red flag. Decide which two of these three factors are most important to you and look for a company that is offering that service.
Have a look at the websites of potential CNC machining partners, then if you can, get them on the phone too. It’s a great way of getting your questions answered and seeing if they are a good fit.
Comparing CNC Machining Services
Here’s some things to consider in your research:
- What accreditations do they have? An ISO9001 registered company will be following a system that ensures they deliver a consistent standard of product and service. They will also be regularly audited to make sure they continue to meet the standard
- Do they specialise in a particular industry or industries? You want to make sure you pick a company that has a lot of experience manufacturing similar parts
- Do they deal in the potential materials that you’re considering? Some companies only deal in plastics or only certain metals
- What is their account management process? How will your order be managed through the business? How will they be communicating with you?
- What quality procedures do they have in place?
- Are they able to supply you with or talk you through any case studies of how they have helped past customers?
- Do they offer any other complimentary services that you may require such as machine drawing or finishing processes?
Next Steps – Reliable CNC Machining Service
Penta Precision is a trusted and highly respected CNC machined parts supplier in the UK. We manufacture high quality precision machined components, from prototypes through to production quantities. We have extensive experience and exceptional customer satisfaction.
"Penta provides an excellent service from start to end. Friendly and knowledgeable consultant and advice, superb communication throughout, outstanding attention to detail, flawless engineering and machining, on time supply, transparent pricing, clear and accurate paperwork. A genuine pleasure to do business with." Simon – Head of Fleet Maintenance
What Makes Penta Different?
We are committed to delivering an exemplary service to our customers. One significant way we achieve this is through the application of our unique ‘3 Cs’:
We Communicate well with our clients
We take Care of the projects placed with us
We are Consistent in our service performance
We believe we're onto a winning formula for delivering what our customers want. See our ‘Proven Process’ page to find out more
We would love to talk to you so please call to speak to our team on 023 9266 8334.
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