CNC machines are a marvelous tool for producing a wide range of products, but they have their particular size limitations. The standard machine shop or prosumer will tend not to have too large a machine because such a device can be impractical for a lot of scenarios. The question arises: how can they machine large parts? In specific, how can they go about CNC machining parts too large to fit the device? Such an endeavor is still entirely possible but requires a bit more machining savvy and additional planning. This article will explore some of the dos and don’ts of machining large parts.
Considerations for Large Part Machining
When machining parts that are too large for your standard machine, there are some planning issues that one should consider. The first of these is that you need to properly think through how the part is going to be processed in multiple different setups and runs. Large parts need to be machined in multiple setups where each setup gives access to a portion of the part and processes a different segment. While lining up each setup, the machine tools and the part need to conform in such a way that the accuracy of features machined in different setups relative to one another match up in quality.
The other major limitation that comes with such a task is overall size. If it is too large for your enclosure, it needs to be done in sections. Similarly, many machines have a weight limit, which includes the part and all the fixturing and height can be a make or break factor. If the part is heavy or too tall to get under the machine, it may not be worth doing at all.
Another problem can often be that there are very limited machine tools dedicated to the development of large parts. As most machines are dedicated for smaller lot sizes, many manufacturers have played into this market at the expense of any tools that specialize in large parts. Even with large machines this can be the case, with a lot of machine shops requiring operator attendance to finish up the part manually (which can be slow and laborious).
There are also accuracy issues that can arise when doing things in multiple setups. This will require a lot of diligence and attention to detail. Large parts also create another problem where they produce intense amounts of heat. Users may have to compensate for this heat in their pre-calculations.
Sometimes machining a large part can be simplified immensely by just breaking it into smaller segments that can be assembled later. If your customer allows for this, it is highly recommended. Needless to say, this does away with a lot of the headaches being discussed in this article, and can even result in some novel parts.
It’s also wise to pick the right machine and the right type of model for your work. If you’ve only been considering a lathe, keep in mind that some parts are better off with a mill, for example. 4-axis mills can produce some surprisingly lathe-like work with a different angle and setup that offer different possibilities. This can be cumbersome to decide on, however certain CAD software can run the numbers and decide which would be best to use in what given scenario. It’s all about planning.
There are a few things you need to get out of the way beforehand (both literal and figurative ones. It may be best to open the windows on your enclosure sides so the parts can protrude or even remove the enclosure to machine large parts. In some cases, you might even have to remove the tool, changing it to give your machine a little bit more room for a marginal part to fit.
Collisions, as always, are going to be a major issue and they only get worse with larger parts. Check very carefully that no collisions will occur before letting the program do its job. It may also help having a full solid model of your machine and enclosure interior to facilitate checking collision possibilities via CAD.
Since bigger parts also usually bring a higher level of weight to the mix, it’s best to deal with this. This usually requires the need for weight supports. Make sure any of part weight that’s not on the table is well supported with outrigger stands of some kind. In certain cases, you may require some kind of rollers so the machine can travel freely too. Lastly, make sure you have proper equipment to help load and unload parts too heavy to carry manually.
You can also prep the part with a different machine. CNC Routers have large work envelopes, allowing them to handle much larger parts than the mill may be capable of. However, they aren’t ideal for processing the entire part as they have a variety of issues. Routers can also provide some novel solutions that improve the accuracy for your machines. Putting in counter-bores that can act as reference for your mill is one great example.
Consider the Part Geometries
While lining up the part in a straight manner is standard practice, machining large parts takes a bit more ingenuity. On any rectangle, the diagonal between two corners is longer than any edge, so it may be best to try putting the part at an angle that doesn’t line up the straight edges while getting the part’s features lined up right with your machine’s travel path. It can, thus, fit better in the available space and economize the work envelope.
Speaking of rectangular parts: it may be better to slide them in certain cases such as when holes need to made along their entire length. One way to streamline this process is to use a couple of round pins installed on your table’s T-slots as stops for the work to slide against. Put them in parallel to the machine’s X-axis, making sure the part has a true edge along its entire length. This will help to maintain the squareness.
Round parts, on the other hand, should be spun rather than slid. This allows the machine to access every feature into the machine’s work envelope. Each time the part is spun, there should be some overlap with the prior setup so features can be indicated to orient the machine to the part. You should mount some round stops on the table that will form a tangent to the circle. You can mount them on the inside or outside, whichever is more convenient. They’ll ensure that as the round part spins, the center stays put.
You can also extend your Y and X-axes by sliding and spinning the part to compensate. You can cut the various corners one at a time and then rotate the part to another side. This can be made much easier with the use of locating pins that go right into the T-slot.
Certain tools like right-angle heads can turn parts around to an orientation that saves the day. Similarly, big fly cutters can extend your mill’s reach, particularly to get those harder to reach edges. It may not extend it by a lot, but when you’re machining a large part, every little bit of reach helps.