Things to consider when making a longer LongMill

One of the most frequently asked questions is “Can you make a bigger version of the LongMill?” Well the short answer may be “yes”, but there are many other considerations that you should to consider.

Screw whip

Screw whip, or “whip” refers to when a rotating rod wobbles or flexes, which is exacerbated by the length of the rod, the speed of the rotation, or how off balance the rod is. The LongMill 30×30 uses a 8mm diameter steel leadscrew that is approximately 1030mm long. At the speeds that the NEMA 23 motors turn at and the length of the lead screw are such that the effect of a properly assembled machine are minimally affected by whip.

When you start to go longer in lead screw length, the effects of whip become more pronounced to the point where you will either need to increase the diameter of the screw or use a lower motor speed, both of which present different challenges and downsides.

There are different lead screw designs that can be used for this application, but may need modification with bearings, mounting feet, couplers, and motor mounts.

Motor speed

For practical purposes, it is best for the machine’s speed to increase proportional to the size of the machine. For context, the Mill One with a work area of around 258mm x 185mm has a maximum speed of 1800mm/min. The LongMill on the otherhand, comes has a maximum speed of 4000mm/min, more than double the speed of a Mill One but also significantly larger.

It is important for a machine to offer faster speeds to accommodate for a larger size machine as doing large jobs at a slow speed would take forever. Typically, you can run a LongMill 2.5x the speed of a Mill One, which means that a project that takes 30 minutes to do on a LongMill would take 1 hour 15 minutes to complete.

If you want to run the machine faster, you will likely need to provide more power to your motors as well. You can do this by increasing current and voltage supplied to your stepper motors, which may also need upgrading your power supply and motors to support the changes.

Luckily the LongBoard controller can support higher voltages and current with a larger power supply, but you will still need to consider upgrading the stepper motors (rated for up to 2.8A).

Rail rigidity

The longer your rails are, the more flex you’ll have if you don’t fully support or reinforce the rail. While the Y-axis is easy to do, as all you need are more feet to support the rail, you may want to consider adding additional reinforcement to their X-axis rail which is a free-floating part.

Without modifying the rails, you may experience more deflection, which will need to be combated by decreasing your cutting speeds.

The other consideration to make is how straight and parallel your rails are. Aluminum extrusion is relatively straight due to the process used to manufacture them. However, deviance in straightness increases the longer your rails are. This also applies when considering if your rails are skewed as well.

For some deep dive into rail design and FEA, make sure to check out this post on the forum: https://forum.sienci.com/t/making-a-stiffer-3×3-angle-gantry/693/5

Logistics

A larger machine cost more to ship, as there are restrictions on how large shipments by courier can be. This can vary region and country, which limits the ability to transport the parts for the machine, which is why we don’t offer longer rails for the LongMill. That being said, you can typically find 2×2″ and 3×3″ angle aluminum from most metal supermarkets as it is a standard material.

Conclusion

We believe that the sizes offered for the LongMill are optimal in terms of price, performance, and usability. However, we encourage folks to build their own machines if they choose to, which is why we provide all of our design files open and updated for free: https://sienci.com/dmx-longmill/open-source-and-modifications/

On the other hand, there are a lot of things that need to be considered in terms of building a longer version of the LongMill which can add to the cost and complexity of the machine. There’s a reason why costs can go up exponentially as size goes up as well. There is also a strong case to be made to avoid expanding this design without significant modification or purchasing a pre-made, larger machine as well.

For those who want to do larger pieces without modifying their machines, consider looking at putting your materials in diagonally, or using “tiling“.

We hope you enjoyed this read, or even inspired you to mess around with our LongMill design. In any case, we hope you share with the community what you learn and what you build.