CNC Machining Guide

A Guide to CNC Machining

__________

 

At Penta Precision, we’re experts in CNC machining. We’ve been manufacturing CNC machined parts for customers across the UK since 1998.


Here we’ve put our years of knowledge into this comprehensive CNC machining guide.

If you need parts manufactured but aren’t sure whether CNC machining is the most appropriate method, read on and things will become clearer.

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

3D Printing Machine

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.

CNC Mill Drawing

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

CNC turning of shaft

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

 

Aluminium 7075 T6 bracket

 

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

Blue acetal circular component

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

Aluminium L168 side member

2014A / L93 / L157 / L168     7075 / L95      5083       6082 / L111 / L113 / L114 / L115        

6061          6063          1100        2024      Cast Tooling Plate          5251          M82       

 

Stainless Steel

Stainless steel component

316 / 316L      304 / 304L      303      420      15-5 PH      17-4 PH      440C    

 

Yellow Metals

Brass component

Brass – CZ121      Copper – C101

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.

Engineering drawing

Here at Penta Precision, our machining parameters are:

Milling

X travel = 1000mm