How to build a 'win-win' relationship with your CNC machining shop

Machining Pulley Wheels from Stainless Steel at Challenge Engineering in Sydney

What is CNC Machining?

James Abbott - MD Challenge Engineering

CNC (Computer Numerically Controlled) machining is a manufacturing technique where the operator takes a CAD (Computer Aided Design) file and turns it into a list of commands for the machine; known as G-Code. The G-Code program is generated by CAM (Computer Aided Manufacturing) software. This file contains all the instructions for a specific machine to complete a part, the machine will then execute all the operations without the need for human supervision.

Taking the leap from a CAD model to a real part is often daunting for those not familiar with the field of CNC machining. To get the most out of your machinist you need to know what to submit. This article aims to give some guidance on the topic.

Machine Compatibility

First and foremost, make sure that your part is machinable. It’s not always possible to create one solid part. Your design might need to be split into sub-components, and then assembled post manufacturing. Your machinist will be able to enlighten you on the limitations of their machinery and will advise on the best way to achieve your desired result.

Organic shapes are difficult to machine since they require finer cuts, which leads to increased machine time. As a result, the cost will also increase. Furthermore, complex shapes may require the use of a more complex CNC machine.


It is always important to make sure that you indicate the dimensions and tolerances with proper GD&T (Geometric Dimensioning and Tolerancing) practices. If you are uncertain about the best practices, make sure the machinist understands the function of the part and how it is meant to interface with other components. They can then advise you on the tolerances that provide a good balance between cost and functionality. Also make sure that you indicate the surface finish you require on your part. A highly polished surface for example vs. a rougher finish.


If you are submitting your part in the form of a 2D manufacturing drawing, ensure that it is clear and includes all relevant GD&T information as mentioned previously. There should be no ambiguity. Some software packages allow you to include all the GD&T information in the 3D part file. This 3D file can then be viewed by the machine operator.

If you are unable to create detailed manufacturing drawings, then a hand drawn sketch with the correct dimensioning and tolerances can be used. The best machine shops will have drafting capabilities. Make sure that you review their 3D model before manufacturing begins.


You won't create savings by providing your own material. CNC machining shops purchase material in bulk, and as such get the best possible prices. These materials are supplied with a material certificate that guarantees the quality of the material. So if you supply your own, not only may it cost you more in the end but you also put yourself at risk of using subpar materials.

Not all materials behave the same way when machined. Softer metals like aluminium machine much faster than steel. Sometimes it may be cheaper to use aluminium even though it is more expensive per kilogram, due to the time saved on the machine.

It is also important to note that there are many different grades of a specific material and this can have a big effect on the final cost.

Plastic parts cannot maintain tight tolerances and tend to warp due to their sensitivity to temperature.

Planning Ahead

When it comes to CNC machining, the cost per part decreases with higher production quantities. This is due to set up, fixturing and machine time. More than one part can be manufactured at a time. Essentially, you’re manufacturing parts in parallel rather than in series, thus reducing machine time.

If you require parts on a periodic basis make sure that your machinist is aware of this so that you do not end up with a situation where you require parts urgently and your machinist has already scheduled other manufacturing runs on their machines.

If your CNC machining activities are based on sales and you are unable to confidently predict sales quantities, then it is best to opt for a lean production model where you monitor long lead manufacturing items and maintain a buffer of parts. Once you reach a set level, you will be notified and you can arrange for a production run with your machinist to ensure no down time.

In Conclusion

Machinists are highly skilled at what they do and if you supply them with accurate detailed requirements, you can rest assured that you will receive your parts as required.

At Challenge Engineering, in Sydney Australia, we machine custom metal and plastic components, and pride ourselves on great customer service. We can manufacture and deliver anywhere in Australia. Please contact us and find out how we can assist your business.

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