What Are The Physical Considerations??
Tool size plays a pivotal role when looking at the features you need to produce. While a CNC machine is constrained by the minimum tool diameter, the nozzle diameter of a 3D printer dictates the minimum feature that can be produced. Nozzle diameters typically range between 0.25mm to 0.8mm, with a 4 x minimum feature size equating to 1mm to 3.2mm.
You can read more about the minimum feature size when reinforcing your parts with Continuous Carbon Fibre parts here.
While the finish on 3D Printed parts has come a long way in a short space of time, the finish from a CNC machine will always be superior. This is due to the nature of the non-linear cutting paths vs the linear nature a 3D Printed part is laid up. However, strides are being made for 3D Printers to move the the X,Y & Z plane at the same time giving a better surface finish, especially with complex surfaces on the top of the finished parts.
Modern 3D printers can hold a dimensional tolerance of +/- 12mn or 0.0005″ and are accurate enough to be used for press fit applications. 3D printed parts can of course also be machined down if required. You will need to increase wall thickness to take this into account. Results will vary based on material, strength and geometry.
Traditionally, 3D Printing has been the domain of non-structure parts. However with the advent of Continuous Carbon Fibre printing, which can produce parts with equal or better strength than Aluminium 3D Printing is moving into use applications where significant structural loading is applied.
While a machined piece of metal has isotropic strength (equal strength in all directions) it is important to point out that 3D Printed parts are stronger in the XY plane and weaker in the Z. You can read more here. This is typically designed into the part when the decision is made to 3D print it and is taught on our 1 day Design For Additive Course.
What Environment Will My Parts Need To Operate In?
Both CNC machines, and now some 3D printers can produce metal parts. Typically the service temperature of metal parts is 50% of it’s melting temperature on an absolute temperature scale. Alloys are able to push this temperature higher, but 65% is the maximum achievable.
Typically polymer parts have a much lower service temperature and shouldn’t be used in environments above 150°C. Datasheet available here.
While the effects of exposing metals in well known, some 3D Printing filaments can also lose strength when exposed to prolonged moisture or submerged. Much like painting metal, a coating such as Liquitex can protect the parts and lengthen their lifetime.
Metals are mostly stable with many chemicals but it is always worth checking the compatibility with the chemical manufacturer. Nylon based materials, such as Markforged Onyx are chemically resistant and unaffected by most petrochemicals (check chemical compatibility here) however you should steer clear of strong acids.
Below are some typical examples from Markforged customers who have decided to add a Markforged printer to compliment their CNC operations.
When Do You Need The Part?
CNC machining is capable of removing material faster than 3D Printing can deposit it. Therefore if the part is needed urgently and a CNC machine is free, with an operator, work holding & materials available this the best solution. Size is much less of a time constraint for machining than 3D Printing as the bulk of the material (roughing) can be removed very quickly. However if the part volume/ stock volume ratio is is very low 3D Printing may become a more viable option.
Size significantly affects 3D Printing times. As a rule of thumb if a part fits into your hand you can have it the same day. With these smaller parts it can be much faster to 3D Print rather than machine. It is common for small parts to have printed before the CNC machine has been setup.
CAM programs usually require an operator or CAM Engineer to write the code for the machine. This can be lengthy time consuming process. With 3D Printing software the slicing is done for you while you just tweaks the required parameters equating to a large time saving up front.
We can mitigate some of the longer print times on large 3D prints by changing nozzle diameter, layer height and infill. The same part can vary in print time from 2 hours to 24 hours so suitable setup is critical to success with 3D Printing.
How Many Do You Need?
Breaking down low volume manufacturing, you will find that the majority of the cost comes from the initial programming & setup time compared to the time you’re actually cutting metal. This becomes scale able when you are making larger setups and cutting more parts without supervision.
As part complexity and the numbers of features and surfaces increases, programming & setup time increases with it, but the cost per part drops of dramatically. CNC scales well into thousands of parts as the program is re-used.
3D Printing setup is done in a matter of minutes and complexity has little to no effect on slicing or print times. The number of parts doesn’t affect the price in the same way CNC operations do as the initial cost is already low. Scaling is typically done by bringing more printers online.
What’s Your Budget For Equipment & Operators?
Once setup correctly CNC machines can run unattended but critical to success is the full time operator & programmer. CNC machines also come with expensive maintenance programs adding to the lifetime cost of the printer.
3D Printers easily run unattended, operators require minimal training (usually 1 day will suffice) the programming couldn’t be any easier as the software looks after most of the slicing. While Industrial 3D printers have optional support contracts these are usually much cheaper than the comparable CNC costs and the machines them elves have less to go wrong and don’t require the same level of servicing.
Both CNC machines & 3D Printers come in a wide range of prices and it’s important to find the right machine for you but like for like, 3D Printers are a much lower investment.
The upfront cost of 3D printing is extremely low making is ideal for low volume manufacture such as tooling or prototyping. If you’re looking to manufacture high volumes, CNC is typically the better solution.
If you’d like to know more, or compare your current low volume CNC parts to a 3D printed part please get in touch below.