What do machine builders need to consider when outsourcing CNC machining?

Quotation

The process starts with a request for quotation (RFQ). It is certainly easier for machinists to quote against 3D CAD models rather than 2D drawings, though the latter remains an option. Clearly, if the parts are required quickly, then a quote will also be needed as fast as possible. For smaller machine shops, this can be difficult if they are busy, whereas larger businesses are more likely to employ experienced people whose job it is to turn around quotes in the shortest possible time – and raise queries should any arise when viewing the CAD files. 

Online portals and systems for uploading files can streamline the quotation process and save time for the customer as well as the supplier. Some organisations use fully automated quoting systems but these run the risk of potential issues not being picked up until the order has been accepted. This is why it can be valuable to have an expert pair of eyes viewing CAD files during the quotation stage. 

Speed and delivery

When requesting the quotation, the machine builder will know how soon the part is required. Here at Prototype Projects, we offer three production speeds: Express 3 Day, Standard 7 Days and Economy 12 Days. With the Express CNC Machining service, parts are machined within three working days, while Standard parts are machined in seven working days and Economy parts in 12 working days. All parts, regardless of the production speed, are then sent using Next Day Delivery (or are delivered next day in our van to customers within a 15-mile radius). Note that there are limitations with the Express service, relating mainly to size, materials and finishes; to aid our customers we have clear criteria that need to be met for our Express service.

However, it is most important that promised delivery dates are met. Some machine shops make promises that they cannot keep but Prototype Projects manages scheduling and material stocks very carefully, as well as having robot-tended 5-axis VMCs that can run overnight, so we are always confident of meeting the delivery dates that we quote. 

Materials

One of the key pieces of information the design engineer specifies on the drawing is the material. Although we can procure and machine almost any solid material, our Express CNC Machining service is only available for the materials we hold in stock. These are aluminium alloy (grade 6082), stainless steel (AISI grades 304 and 316), acetal (black and natural) and clear acrylic. If a different material is specified, we might be granted a concession to use a stock material if the part is needed urgently. Otherwise, we can buy-in the material or, sometimes, the customer free-issues the material so we can supply the part through our Standard or Economy service. 

Other materials we typically machine include copper, brass, phosphor bronze, mild steel, tool steels and a broad spectrum of engineering plastics. In addition, we are occasionally asked to machine tooling board, Styrofoam, composites (eg Tufnol), investment casting wax and MDF. 

Quantities

Machine builders can easily get quotes for one-off parts, and plenty of machine shops are eager to quote for thousands, but small batches can be problematic. Since we invested in our five-axis VMCs and robots, we have become very competitive for small batches of, say, five to 20. With the machine tool and robot programmed, and the pallet loaded with blanks of the correct blanks, we can step back and let the technology do the work. If need be, batches can be produced overnight, with the machines left running unattended.

We can CNC machine your low volume production work on our 5 axis VMCs. Whether you need 50 or 500 parts (or anywhere in between!), and your parts will fit into one of our billets, we can help. We can machine in Aluminium (black/natural), Acetal and Acrylic. Production speed will vary, depending on the quantity needed.

Complex geometry

Manually milling compound curves is difficult and expensive. On the other hand, five-axis CNC machining such surfaces is straightforward. Five-axis CNC machining also simplifies the production of other complex features. Machine builders might prefer to use their in-house facilities to machine simpler components, while outsourcing the more complex parts to companies such as Prototype Projects.

Tolerances

We quote a general tolerance of ±0.1 mm and a hole tolerance of H7 on CNC machined parts but bear in mind that this covers all materials and all sizes. In reality, we normally achieve tighter tolerances and, if required, we can work to specific tolerances provided these have been indicated on PDF copies of 2D drawings. 

Our standard finish of vapour honing does not affect tolerances but finishes such as painting, powder coating and electroplating might.

Finishing, assembly and marking

All of our CNC machined parts are deburred, vapour honed, cleaned and greased. Often, therefore, parts are ready to be installed as soon as the customer receives them. However, alternative finishes are available on request. For example, we can apply EMI/RFI shielding to plastic parts, paint mild steel parts, and polish parts made from materials such as clear acrylic, brass or stainless steel.  In addition, we can subcontract anodising, powder coating, electroplating, electropolishing, heat treatment of tool steels, and other specialist processes. When we supply parts that are finished and ready to install, it saves time and money for the machine builder. 

Similarly, if plastic or aluminium alloy parts need threaded inserts to be inserted, we can do this as well. 

Machine builders – or their customers – might want certain parts to be identified using part numbers or unique serial numbers. Rather than reach for a hammer and the number/letter stamps, we can utilise our laser cutting/marking machine. Not only can this indelibly mark or engrave metal and plastic parts with alphanumeric text, but it can also mark machine-readable codes such as bar codes, 2D Data Matrix codes and QR codes.

Quality assurance

CNC machining is an inherently accurate manufacturing technology. Nevertheless, a machine builder’s own ISO 9001 quality assurance scheme might necessitate the use of ISO 9001 certified suppliers, such as Prototype Projects, and not all CNC machine shops are certified. 

Furthermore, critical parts might need to be accompanied by inspection reports. Prototype Projects has a co-ordinate measuring machine (CMM) that can be readily programmed to inspect key features and generate a report. If we are machining a batch of parts, we will agree an inspection regime with the customer. This might include a comprehensive inspection report for the first-off part, followed by reports on key dimensions at agreed intervals thereafter. 

Technical advice

There will be times when a design engineer is not 100 per cent sure about something, whether that is the choice of material, a part’s geometry, the best finish, or simply whether or not something can be machined. In such circumstances, it is vital to work with a supplier that has the depth of knowledge and breadth of experience to provide unbiased technical advice.  

Prototype Projects is unusual in that alongside CNC machining we also have complementary manufacturing technologies; as well as five different 3D printing technologies (SLA, SLS, DLP, PµSL and PolyJet), we also have vacuum casting, traditional model making, and we can 3D print patterns for investment casting. Our experts are always happy to spend time talking to customers about their projects to help ensure the best possible outcome in terms of function, aesthetics, lead time and cost.

Talk to us

If you are a machine builder in need of outsourcing CNC machining, please contact us on 01763 249760 or request a quote.