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TL;DR:

On my particular printer design, it definitely appears the company switched from flex couplings to solid couplings on purpose to improve the printer, and to prevent well-meaning users from adjusting the previous flex couplings. Whether or not this is a good design overall, is left for the reader to decide. It's certainly simple and cheap to use the motor itself as one of the leadscrew thrust bearings, but in the future I might add a micro-adjustable bearing block in place of the motor, and then couple the motor to the leadscrew with a flex coupling. That would be more typical of the way it's done on CNC machines, where the stepper bearings are isolated from any machine loads.

Original post: I bought a Artillery X1, supposedly the latest V4 version. It's my first printer, but I have experience with CNC machines. Although the pictures and YouTube videos all show the Z axis steppers connected to the leadscrews with flexible (spiral type) couplings, the printer I got has solid couplings.

I thought flex couplings were important to prevent binding, so I was going to buy some flex couplings. But as I look at the printer, it appears the leadscrews actually are supported by the stepper motor itself at the bottom...there are no thrust bearings, so the weight of the printhead is supported by the stepper motor directly through the coupling. Since the spiral flex couplings can compress slightly, is the use of solid couplings deliberate in order to make a more precise machine? If I install flex couplings, will it make my printer worse by introducing "spring" into the system? Is this a typical design, or do other printers have thrust bearings or "hang" the leadscrews from the top?

There are a lot of reviews and videos about this printer where people noticed the leadscrews were internally touching the end of the motor shaft inside the coupling, and people thought that was wrong, so they spaced them back apart. Now, I wonder if the shafts touching was a deliberate design decision, and maybe the company even switched to solid couplers to prevent people from adjusting the couplers back. Please help me solve this mystery of the solid motor couplings.

EDIT: I found a related application where the axial rigidity of the stepper couplings matter. In the following YouTube video the author deliberately jams together the lead screw and stepper shaft in order to eliminate spring from the flex coupling. This tends to solidify my theory that this printer uses solid couplings deliberately, and other printers with similar Z axes could potentially benefit as well. The X1 uses floating support bearings for the X stage to prevent binding, which probably helps.

Also, in this YouTube video the author describes how he deliberately decided to use rigid couplings on this printer to improve his Z axis and references other sources which come to the same conclusion.

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Yes and No

Let's start with the obvious: Flex couplings or rigid couplings are a deliberate choice in designing the printer. Each has its benefits and drawbacks. So let's look into the 4 types of drive-train setups that I know to be possible:

  • Spiral couplings make it possible to operate the machine with a slightly misaligned motor to the shaft (<1°) but can add a measurable slop, which can be seen as a systemic error.
  • Rigid couplings have minimal slop but need to be much better aligned by the user in setup.
  • No coupling at all - direct drive to the shaft - has no slop at all, but is both more expensive and needs the most diligent work on setup.
  • Geared. You could couple the motor with the Z-Axis via gears, allowing to trade speed for detail and vice versa. It also can allow mounting the motor 90° to the printer, but it also adds some backlash and slop. Because good gears are expensive, this is really rare.

Using spiral couplers right

A spiral coupler can actually be used in two ways:

Keeping the leadscrew tensioned into a position.

This butts both parts together, it reduces the slop induced by the coupler to the minimum, but you need to forcefully make sure that the parts push together. You lose some of the misalignment-correction.

Decouple vibrations from one shaft to another.

This keeps a little gap between the two. The slop can be larger, but you get the benefit of the shafts on both sides not transmitting all the vibrations 1:1 to the other shaft and you get more self-realignment if the motor and shaft are not perfectly coaxial.

What is better?

Well, it depends. If you want the perfect drive, you'd go direct drive. If you want the finest motor control (by virtue of getting the steps to a much smaller rotation), you might use gears. But the spring-coupler allows you to do the setup easier for a consumer that might not be the most reliable in setting it up himself. Making the spring-coupler butted can increase repeatability and reduce slop to almost the same as a stiff coupler but not lose the ability to have a slightly crooked setup that still works.

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