01763 249760  office@prototypeprojects.com

Request a Quote
Specialist in the manufacture of prototypes - Prototype Projects Specialist in the manufacture of prototypes - Prototype Projects
  • CNC Machining
    • CNC Turning
    • CNC Milling
    • CNC Materials
    • CNC Finishing
    • Service Levels
  • 3D Printing
    • SLS – Selective Laser Sintering
    • SLA – Stereolithography
    • Projection Micro Stereolithography (PµSL)
    • Polyjet 3D Printing
    • DLP – Digital Light Projection
    • SLS Dyeing
    • 3D Printing Materials
    • 3D Printing Finishing
    • Service Levels
    • Additive Manufacturing
    • Additive Prototyping
  • Vacuum Casting
  • Model Making
  • Expertise
    • Low Volume Manufacturing
    • Rapid Prototyping
    • Laser Cutting
    • Inspection
  • Downloads
    • Frequently Asked Questions
    • Guides and whitepapers
    • Policies, forms and certificates
    • Process datasheets
    • Prototype materials datasheets
  • About
    • Customers
    • Our Team
    • Our values
    • Payment methods
    • Work with us
    • News & Articles
    • Case Studies
    • Plant list
    • Privacy statement
  • Contact
  1. Home
  2. CNC Machining
  3. Does CNC machining mimic production parts?

Does CNC machining mimic production parts?

CNC machining: does it mimic production parts?

While CNC machining is commonly used for manufacturing production parts, both as one-offs and in high volumes, more often the reason customers come to us for CNC machining is for prototype parts.

In the early stages of a development project, CNC machined prototype parts might be required for proving a principle. At this point, therefore, a part is effectively a set of features with defined positions relative to each other. With such parts, mimicking production components is not necessary.

Later in the project, CNC machined parts will be representative of production parts. A plastic component might be CNC machined to mimic one that will, ultimately, be injection moulded, or a metal part might represent a die casting.

We are sometimes asked ‘How closely does CNC machining mimic production parts that will be injection moulded or die cast?’ The answer is not straightforward, so this article looks at some of the factors that can have an impact. For the most part the article relates to milled parts but many of the points are equally applicable to parts produced on CNC turning centres (lathes).

Draft angles

Injection moulded and die cast parts almost always require tooling with draft angles to enable the part to be ejected. With CNC machining this is not necessary and, in fact, it is easier (and quicker and cheaper) to machine a part with vertical sides. Of course, if the prototype part has been designed as a 3D solid model with draft, then our CAM software can import the model and generate toolpaths as required.

Often it makes no difference to a prototype whether or not there is draft. In some cases, however, whether for reasons of fit, function or appearance, it will be important to incorporate draft.

Material properties

When comparing a CNC machined prototype plastic part with an injection moulded production part, you have to bear in mind that plastic in a moulded part will have flowed in the mould tool, which partially orientates the polymer chains and can result in anisotropic properties. In contrast, parts machined from solid will have largely isotropic material properties or, if there is any anisotropy, it will be different from the anisotropy in a moulded part.

Also, the moulded part may contain weld lines (knit lines or meld lines) that have the potential to be weak points as well as detracting from the aesthetics. Prototyping with CNC machined parts can conceal this risk. However, with moulding analysis software, such issues should be predictable and the problem can be avoided or minimised.

Note that injection moulded and CNC machined parts are unlikely to be manufactured from the same grade of polymer, so the precise grade is likely to be different from one that would be specified for injection moulding, hence the material properties will not be identical.

For metal parts, the grade of alloy used for CNC machining will almost certainly not be the same as that used for die casting, so the material properties will be slightly different.

Warpage and distortion

Generally speaking, the material properties of solid plastics used for CNC machining are isotropic, hence the properties in the machined part will be isotropic. In fact, there could be some residual stresses in the solid material due to the production method (e.g., extrusion) and a degree of anisotropy. Because of this, we might check with the designer to ensure the machined part will perform optimally. We might also orientate a part to minimise the risk of any distortion caused by residual stresses in the material.

Some materials can be stress relieved by annealing prior and during machining but this adds time and cost. In most cases, we avoid warpage and distortion by selecting the optimum part orientation and taking care to ensure toolpaths will not contribute to distortion.

Warpage and distortion can occur in CNC machined metal parts but is less common and seldom a problem.

Setup

If two or more setups are required, we need to ensure the semi-machined part can be securely clamped. Depending on the part’s geometry, this may require one or more special fixtures to be made. These are normally CNC machined, which is why the job takes longer and costs more.

Pockets and slots

Consider an injection moulded plastic part with webs that support walls. For prototyping, a CNC machined part might be expected to include these webs. However, whereas a moulded part would have fillets between the wall and web with a relatively small radius to minimise sinkage opposite the web, this can be difficult to reproduce with a CNC machined part. The reason for this is that the fillet radius cannot be smaller than the radius on the cutting tool. A smaller radius requires a smaller cutting tool and there are limits to the diameter of a cutting tool for a given length (and depth of cut).

The issue of internal radii extends beyond web fillets to include any internal radii in pockets or other features.

Similarly, the maximum depth of a slot depends on the diameter of the cutting tool.

See our CNC machining page for more information about the relationship between cutting tool diameter and depth of cut.

Surface finishes

CNC machined parts have a characteristic pattern on the surface due to the motion of the cutting tool. If the part is for a proof-of-principle model or functional prototype, the as-machined finish is usually satisfactory. However, if aesthetics are important, the surface can be hand-polished or lightly bead blasted. There are numerous other finishing options as well, depending on the material and intended purpose of the part, so it is best to talk to us about your requirements.

We can normally match the surface finish of a CNC machined part to that of a production part but bear in mind that, for example, a painted finish on a CNC machined plastic part might not be as durable as a self-coloured injection moulded part.

Threaded holes and inserts

Our CNC machine tools benefit from automatic toolchangers, so threaded holes can be readily drilled and tapped in metals and plastics. If a plastic part has been designed to have other types of inserts, such as pins, studs and bushes, it might be possible to install the exact same type in the CNC machined part. If not, an alternative part with the same or similar performance can be specified. And, if that Is not an option, a special insert can be CNC machined as a separate component and fixed in position as necessary.

Undercuts

Injection moulded parts often incorporate undercuts for clips, snap-fits and other purposes. These add considerable cost to injection mould tooling so a designer might want to use CNC machined prototype parts to check the performance before committing to tooling. Fortunately, undercuts can be CNC machined though, as always, it depends on the geometry and whether a cutting tool can reach.

Conclusion

Taking all the above into account, what is the answer to the question ‘Does CNC machining mimic production parts?’ We can usually get extremely close and, even if the result is not 100 per cent perfect, it is almost always close enough to prove or disprove a design. As a result, the project team can move forwards with confidence.

Talk to us

If you would like to know more about CNC machining prototype parts, talk to us on 01763 249760.

Recent Popular
SLS Close-up

When is SLS your best option for ...

27 February 2023 Comments Off on When is SLS your best option for 3D printing?
Materials choices for SLA - Headlight

What are the material choices for SLA ...

23 February 2023 Comments Off on What are the material choices for SLA 3D printing? 
Star of the Month - Prototype Projects

Star of the Month

6 February 2023 Comments Off on Star of the Month
Projection Micro Stereolithography

What do we mean by ‘Precision’ ‘Micro’ ...

23 January 2023 Comments Off on What do we mean by ‘Precision’ ‘Micro’ Stereolithography?
3D Printing

The beginner’s guide to 3D printing

19 January 2023 Comments Off on The beginner’s guide to 3D printing
Prototyping medical devices for patient contact

Prototyping medical devices for patient contact

22 September 2021 0
silicone tooling

LUMA-iD develops Buddy with silicone tooling

21 July 2022 0
Applications and design tips for SLS: a guide

Applications and design tips for SLS: a ...

27 November 2019 0
From 3D Model to Award-Winning E-Bike in Record Time

From 3D Model to Award-Winning E-Bike in ...

16 September 2021 0
SLS Close-up

When is SLS your best option for ...

27 February 2023 Comments Off on When is SLS your best option for 3D printing?

Comments are closed

Next Day Dispatch

Delivering Overnight Printing

Email: office@prototypeprojects.com
Tel: 01763 249760

Prototype Projects Ltd
Units 1&2, Greenfield
Royston,
Herts SG8 5HN

If you have questions about any aspect of our service, please contact us.


    UKAS
    Made In Britain - Prototype Projects
    Royston Business Awards - Prototype Projects
    Specialist in the manufacture of prototypes - Prototype Projects

    Sign up to our Company and Industry newsletter

    Sign up here
    Specialist in the manufacture of prototypes - Prototype Projects

    Prototype Projects, established in 1980, is a specialist in the manufacture of prototypes. The company is experienced in 3D Printing, CNC Machining, Rapid Prototyping, Model Making and Additive Manufacturing. The company is located in Royston, Hertfordshire.

    Trustpilot - Prototype Projects

    About | Our team | Our values | Customers | Work with us

    CNC Machining

    CNC Turning
    CNC Milling
    CNC Materials
    CNC Finishing
    Service Levels

    3D Printing

    SLS
    SLA
    PµSL
    Polyjet
    Digital Light Projection
    3D Printing Materials
    3D Printing Finishing
    Additive Manufacturing
    Additive Prototyping
    Service Levels

    Expertise

    Low Volume Manufacturing
    Rapid Prototyping
    Laser Cutting
    Inspection

    Contact us

    Prototype Projects Ltd
    Units 1 &2, Greenfield
    Royston
    Herts SG8 5HN

    office@prototypeprojects.com
    T: 01763 249760

    © 2022 - Prototype Projects | Terms | Privacy | Web Design Indigo Ross

    We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. By clicking “Accept All”, you consent to the use of ALL the cookies.
    However, you may visit Cookie Settingsto provide a controlled consent. Reject All
    ACCEPT ALL
    Manage consent

    Privacy Overview

    This website uses cookies to improve your experience while you navigate through the website. Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. We also use third-party cookies that help us analyze and understand how you use this website. These cookies will be stored in your browser only with your consent. You also have the option to opt-out of these cookies. But opting out of some of these cookies may affect your browsing experience.
    Necessary
    Always Enabled
    Necessary cookies are absolutely essential for the website to function properly. These cookies ensure basic functionalities and security features of the website, anonymously.
    CookieDurationDescription
    cookielawinfo-checkbox-analytics11 monthsThis cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Analytics".
    cookielawinfo-checkbox-functional11 monthsThe cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional".
    cookielawinfo-checkbox-necessary11 monthsThis cookie is set by GDPR Cookie Consent plugin. The cookies is used to store the user consent for the cookies in the category "Necessary".
    cookielawinfo-checkbox-others11 monthsThis cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Other.
    cookielawinfo-checkbox-performance11 monthsThis cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Performance".
    viewed_cookie_policy11 monthsThe cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. It does not store any personal data.
    Functional
    Functional cookies help to perform certain functionalities like sharing the content of the website on social media platforms, collect feedbacks, and other third-party features.
    Performance
    Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors.
    Analytics
    Analytical cookies are used to understand how visitors interact with the website. These cookies help provide information on metrics the number of visitors, bounce rate, traffic source, etc.
    Advertisement
    Advertisement cookies are used to provide visitors with relevant ads and marketing campaigns. These cookies track visitors across websites and collect information to provide customized ads.
    Others
    Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet.
    SAVE & ACCEPT