Manufacturing is dominated by CNC machines
due to their high speed, precision and versatility. When designing parts, it is critical that you understand the finer details of the process so that you can design your parts to be manufactured efficiently and cheaply, without reducing part quality.
This article will discuss how to design parts for CNC machining by focusing on specific design practices that will reduce machining time, eliminate problems and reduce cost.
Types of CNC machines
CNC is a very general term. It that can describe any type of machine that uses servos, stepper motors or linear motors along with hardware and software systems, to move a tool around the work space and perform a specific manufacturing operation on a billet of material. However, when most people talk about CNC machining, they are basically referring to either a CNC lathe or a CNC mill.
A CNC lathe works by adding a billet of material into a chuck and spinning it to a predetermined speed and then moving a tool into this spinning material to remove material. A CNC mill typically works by attaching a billet of material to a work bed that can move from left to right as well as forwards and backwards. The tool is then spun to a predetermined speed and the material is moved into the tool, resulting in material being removed. The tool bit can move up and down.
Design for Manufacturability (DFM)
DFM refers to the engineering practice of designing parts that can be manufactured easily. The DFM principles are intricately tied to the manufacturing technology of choice. For example, DFM guidelines for plastic injection moulded parts are completely different from DFM guidelines of CNC parts.
The list below gives 5 areas of DFM that need to be focused on to create high quality, low cost CNC parts.
When designing parts, care needs to be taken to choose the correct features for the task.
Use standard drill sizes.
Do not use flat bottomed holes.
Avoid partial holes i.e., the axis must not be near the edge of the part as the tool may drift.
Avoid deep holes as this can result in tool breakage and hinder chip removal.
Drills must enter the part only on surfaces that are perpendicular to the drill axis. Angled surfaces will result in the tool drifting.
When machining a pocket, use the largest possible radius on the corners of the pockets. The larger the tool, the stiffer it is and the faster it can feed through the material before deforming.
If a sharp corner is required for assembly purposes, a hole can be drilled before the pocket is machined.
When a part requires a high degree of flatness, bosses can be used so that the flatness tolerance can be maintained on the bosses only, reducing machining time and cost.
On outside corners, do not use a radius as this requires special tooling. A chamfer can be cut relatively easily with a common tool.
2. Material Selection
Some materials are cheaper to machine than others. For example, aluminium can be machined more easily than titanium. This results in a significant cost saving, as machining tougher materials requires more machine time as well as specialised tooling. If possible, design parts to use easy to machine materials. Only use tougher materials when you specifically need their superior mechanical properties.
Use tight tolerances sparingly, as highly accurate features can result in significant costs due to machine time, fixturing, scrap rate and load times. Only apply stringent tolerances when a highly accurate fit is required. High overall tolerances don’t automatically mean a more functional part. If possible take the time to learn proper GD&T techniques.
4. Material Shape
Design parts to fit into standard stock sizes with some clearance to allow for cleaning up and facing of the part. Designing parts with standard material sizing in mind will allow for significant cost savings due to machine time and material cost. If you are unsure of the best stock material to use, contact your machinist for advice.
5. Machine operations
Design the part to reduce the number of operations required to reach the desired result. If the part needs to be re-set up in the machine more than twice, then consider removing or modifying some of the redundant features.
CNC machining is extremely versatile and if the correct factors are taken into consideration during the design phase, significant cost savings can be achieved seeing as 80% of the product cost is determined during this initial design process. The ideal solution would be to consult with your machinist during this phase as they can provide invaluable advice that will save multiple headaches down the line.
Challenge Engineering is an Australian ISO 9001:2015 certified CNC machining company based in Sydney. To find out more go to www.challengecnc.com.au.