Mechanisms: Lead Screws and Ball Screws

Translating rotary motion to linear motion is a basic part of mechatronic design. Take a look at the nearest 3D-printer or CNC router — at least the Cartesian variety — and you’ll see some mechanism that converts the rotation of the the motor shafts into the smooth linear motion needed for each axis.

Hobby-grade machines are as likely as not to use pulleys and timing belts to achieve this translation, and that generally meets the needs of the machine. But in some machines, the stretchiness of a belt won’t cut it, and the designer may turn to some variety of screw drive to do the job.

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Homebrew Linear Actuators Put The Moves On This Motion Simulator

Breaking into the world of auto racing is easy. Step 1: Buy an expensive car. Step 2: Learn how to drive it without crashing. If you’re stuck at step 1, and things aren’t looking great for step 2 either, you might want to consider going with a virtual Porsche or Ferrari and spending your evenings driving virtual laps rather than real ones.

The trouble is, that can get a bit boring after a while, which is what this DIY motion simulator platform is meant to address. In a long series of posts with a load of build details, [pmvcda] goes through what he’s come up with so far on this work in progress. He’s building a Stewart platform, of the type we’ve seen before but on a much grander scale. This one will be large enough to hold a race car cockpit mockup, which explains the welded aluminum frame. We were most interested in the six custom-made linear actuators, though. Aluminum extrusions form the frame holding BLDC motor, and guide the nut of a long ball screw. There are a bunch of 3D-printed parts in the actuators, each of which is anchored to the frame and to the platform by simple universal joints. The actuators are a little on the loud side, but they’re fast and powerful, and they’ve got a great industrial look.

If car racing is not your thing and you’d rather build a full-motion flight simulator, here’s one that also uses DIY actuators.

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[James Bruton] Is Making A Dog: OpenDog Project

There was a time when a two-legged walking robot was the thing to make. But after seeing years of Boston Dynamic’s amazing four-legged one’s, more DIYers are switching to quadrupeds. Now we can add master DIY robot builder [James Bruton] to the list with his openDog project. What’s exciting here is that with [James’] extensive robot-building background, this is more like starting the challenge from the middle rather than the beginning and we should see exciting results sooner rather than later.

James' motor and ball screws
James’ motor and ball screws

Thus far [James] has gone through the planning stage, having iterated through a few versions using Fusion 360, and he’s now purchased the parts. It’s going to be about the same size as Boston Robotic’s SpotMini and uses three motors for each leg. He considered going with planetary gearboxes on the motors but experienced a certain amount of play, or backlash, with them in his BB-9E project so this time he’s going with ball screws as he did with his exoskeleton. (Did we mention his extensive background?)

Each leg is actually made up of an upper and lower leg, which means his processing is going to have to include some inverse kinematics. That’s where the code decides where it wants the foot to go and then has to compute backwards from there how to angle the legs to achieve that. Again drawing from experience when he’s done it the hard way in the past, this time he’s designed the leg geometry to make those calculations easy. Having written up some code to do the calculations, he’s compared the computed angles with the measurements he gets from positioning the legs in Fusion 360 and found that his code is right on. We’re excited by what we’ve seen so far and bet it’ll be standing and walking in no time. Check out his progress in the video below.

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Adding Screws To A DIY CNC Machine

When it comes to CNC machines, your SureFine has screws on its axes, and the Bodgeport does too. A shopbot has an amazing rack gear system, but when you start to dig into the small CNC routers available for under $2,000, you’ll only find belts moving a router back and forth. This isn’t to say belts won’t work — you can create a fine CNC machine with bits of rubber. However, belts stretch, they wear out, and if you want more precision screws and racks are the way to go.

The WorkBee CNC machine is the first desktop CNC router we’ve seen that uses screws instead of belts. It’s a project on OpenBuilds, and a reasonably well-configured machine is now available from ooznest for about £1,700 ($2,200 USD), or just a bit more than other CNC routers that consist of a Dewalt router and some aluminum extrusion.

The WorkBee CNC is based on the OX CNC machine, another cartesian router machine built around the OpenBuilds aluminum extrusion. The OX, while a fine machine for DIY tinkerers, uses belts. The WorkBee trades them out for screws, and should gain better accuracy, much lower maintenance, and deeper cuts. Screws are slower, yes, but do you really need that much acceleration when routing a thick piece of wood?

Taig Mill Anointed with Ball Screws (at last!)

Yup, we can hear a crowd full of “not-a-hack” loading their cannons as we speak, but this machine has a special place in the community. For years, the Taig milling machine has remained the go-to micro mill for the light-duty home machine shop. These machines tend to be adorned and hacked to higher standards, possibly because the community that owns these tools tends to enjoy machining for machining’s sake–or possibly because every single component of the mill is available as a replacement part online. For many, this machine has been a starting point to making chips at home. (In fact, Other Machine Co’s CTO, Mike Estee, began his adventure into machining with a Taig.)

For years, Taig has sold their machines with a leadscrew and a brass nut that could be tensioned to cut down the backlash. Backlash still remains an issue for the pickiest machinists, though; so, at long last, Taig has released a backlash-free ball-screw variant in two incarnations: an all-in-one machine pre-fitted with ballscrews and an upgrade kit for customers that already decorated their garage with the lead-screw model.

In the clip below [John] takes us on a tour of the challenges involved in cramming 3, 12-mm ballscrews into the original topology. As we’d expect, a few glorious chunks of metal have been carved away to make space for the slightly-larger ballnut. Despite the cuts, the build is tidy enough to fool us all into thinking that ballscrews landed in the original design from the start.

Confused why ballscrews are such a giant leap from leadscrews? Lend your eyes and ears a few moment to take in [Al]’s overview on the subject.

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Mastering Ball Screws

Most inexpensive 3D printers use a type of lead screw to move some part of the printer in the vertical direction. A motor turns a threaded rod and that causes a nut to go up or down. The printer part rides on the nut. This works well, but it is slower than other drive mechanisms (which is why you don’t often see them on the horizontal parts of a printer). Some cheap printers use common threaded rod, which is convenient, but prone to bad behavior since the rods are not always straight, the threads are subject to backlash, and the tolerances are not always the best.

More sophisticated printers use ACME threaded rod or trapezoidal threaded rods. These are made for this type of service and have thread designs that minimize things like backlash. They typically are made to more exacting standards, too. Making the nut softer than the rod (for example, brass or Delrin) is another common optimization.

However, when lead screws aren’t good enough, mechanical designers turn to ball screws. In principle, these are very similar to lead screws but instead of a nut, there is a race containing ball bearings that moves up and down the screw. The ball bearings lead to less friction.

Misumi recently posted a few blog articles about ball screws. Some of the information is basic, but it also covers preloading and friction. Plus they are promising future articles to expand on the topic. If you prefer to watch a video, you might enjoy the one below.

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A Very Professional Homemade CNC Router

[Benne] has a small workshop at home so he decided to make a very versatile CNC router for his final project at school. It took him around 6 months to arrive at the result you can see in the image above and what is even more impressive is that he was only 17 years old at the time.

[Benne] used the free cad program Google Sketchup to draw the different parts he needed around the linear rails and ball screws he already had lying around. The CNC’s travel is 730x650x150mm, uses Nema 23 (3Nm) steppers, 15mm thick aluminum plates and 30x60mm aluminum extrusions. In his article, [Benne] gives great advice to those who would like to design their CNC like his, providing very useful links to manufacturers. He estimated the cost of his CNC to be around 1500 euros (about $2000). We’ll let you browse the many lines of his very detailed build log, which makes us wish to be as talented as him even at our age…