VenterMech leadscrew decoupling mechanism

  • On order, email julie@fastbikegear for ETA
  • Manufactured by: FBG

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As this is a free solution please do not attempt to add it to the shopping cart. This is an information page only. Please email Liam@FastBikeGear.co.nz for free instructions, BOM, STEP and STL files.


VenterMech © 2021 by Liam Venter is licensed under Attribution-NonCommercial-NoDerivatives 4.0 International 
 

Please note additional intellectual property protections for this invention are pending by the third party who own the commercial rights to this invention, but they have agreed to free non commercial use.

Our solution is completely free for you to use under the terms of the above registered Creative Commons license.

If you decide to contribute to Liams' coffee fund to keep him up late at night doing more of this stuff, he would appreciate this, but there is absolutely no pressure or obligation to do this. His PayPal account is Liam@FastBikeGear.co.nz
 

What do Ventermechs do?

For many years designers have been trying to isolate or decouple the wobbling motion from lead screws which commonly causes artefacts in printed or machined parts....and inexpensive, good, simple solutions have eluded them.

Solutions such as lead screw guides, flexible motor to lead screw couplings, wobble rings and wobble wings all attempt to do this with mixed degrees of success and complication. The VenterMech works as well as these other solutions but is very simple and over hundreds of hours of testing on commercial print runs has proven completely maintenance free.

FBG design ethos has two broad guiding principles.

1. Parts left out don't cost anything and don't fail
2. Design the simplest solution possible and only add complexity if required.

The VenterMech embodies these principles as well as any other of our industrial designs, such as our Scalpel suspension valving system used by many of New Zealand's top motorcycle racers.

The sizes and tolerances of each component are important to the operation of the VenterMech. The toleranceds can be achieved with any good consumer level printer. 

The image below depicts the VenterMech in it's simplest form. Both STL and STEP files for this are available by emailing liam@FastBikeGear.co.nz

The VenterMech consists of both a receptacle and leadscrew star drive and three other cheap, readily available components that you can source from Aliexpress, Amazon, etc.
VenterMech leadscrew decoupling mechanism - Click Image to Close
Please note only the VenterMech consisting of the receptacle, unique bearing system and lead screw star drive are covered by our licensing and any other intellectual property protection.

We provide you with the STL and STEP devices of the mechanism and it is up to you what device you embed it in. 

The image below is a generic depiction of how the VenterMech can be integrated in to a bed support arm for a 3D printer. 

Critical to the design is achieving a low friction interface between the leadscrew star drive and the receptacle. We did a lot (A LOT!) of experimentation to find the perfect solution and this is achieved by way of a simple component costing about $5 US for a pack of three from Aliexpress, Amazon, etc.

FAQ


Should I apply any lube to any of the printed plastic parts?
You should apply a small amount of your normal lube to the lead screw thread and the bearings. No lubrication is required on the printed parts.

Why bearings?
When developing the VenterMechs, I and the beta testers tested a variety of mechanisms and material. We could get good and worthwhile (but sometimes inconsistent) improvements using traditional Oldham couplers. At each step of the way we discovered, anything we did to reduce the friction of the movement resulted in noticeable improvements to print quality (After several months of staring at test prints under critical single point light sources, we got very critical of results). One idea I had, that was quickly discarded for practical packaging reasons, was to add bearings to Oldham couplers to reduce their friction. We also experimented with teflon sliders in the mechanisms.
It was not until I tried some mechanisms with bearings that I achieved a breakthrough in getting rid of lead screw induced artifacts...everything else up until then was merely a  good improvement. At this point we simplified everything by devising the VenterMech mechanism to specifically enable the use of low cost off the shelf lateral bearings (created by flipping the bearing races on radial bearings).
Low resistance (friction) to lateral movement of the leadscrew is the key to eliminating lead screw induced artifacts.
 
Are there any downsides to using VenterMechs?
You will initially notice more noise on Z hops. My printer sits on my design desk next to my CAD station where I spend a lot of time designing stuff.
You can get some noise from the slight force of the faces of the star drive striking the corresponding faces of its receptacle. If the threads of the star drives are cleanly printed and properly bedded in, the lead screw will commence rotating within the star drive as soon as the faces of the star drive lightly meet the faces of the receptacle and the striking force becomes negligible and there should be little to no additional noise.

Is there any backlash in the system?
Contrary to popular myth, no. Gravity keeps the parts in contact with each other. Only if vertical accelerations are greater than that of gravity (9,800 mm/s) would it be possible to get back lash. If you see people suggesting otherwise, please make your own evaluation of their engineering skills.

Can I use Z hop?
Yes absolutely and I do with a lot of prints. With Z hop enabled the bed lowers during non printing moves so the nozzle does not drag across the top of the print and then rises again at the start of the next printing move. With VenterMechs after dipping the bed will return to EXACTLY the same height every single time just as before in you installed the VenterMechs.

This has been confirmed by moving the bed up and down with both small (0.5 mm) and larger (0.4 mm) values for Z hop and measuring the bed movement with a dial gauge. While it would seem obvious that this would be the case, I am a firm believer that all theories need to be tested and measured. Theoretical modeling on its own has a history of overlooking factors that a room full of clever designers overlooked.

Is there a quick way to do back to back testing to compare print quality with VenterMechs fitted to not having them fitted?
Yes, just put some tape on the sides of some of the faces of the star drives, this will stop the star drives from being able to move in the receptacle. This can be useful if you also want to compare noise levels with and without VenterMechs.

Should I use metal bearings rather than the printable TPU Wiggle Woggles in the recommended lead screw guides.
While originally sized to also accept bearings, we found the bearings over constrained the lead screws and impaired print quality and made the printer louder. I highly recommend the TPU version of the Wiggle-Woggles

Is it critical that the flat sides of the bearing races face the bearing assembly?
Yes, if you place the grooved side against the bearing assembly this will constrict the movement of the bearing to radial movement only and will stop the VenterMechs doing their job.

Why did you change the angle of the pins in the bed arms
To better meet the Maxwell Criteria. I have also discussed this issue with Rat Rig devs and explained the importance of it. While originally disagreeing, they now understand the value of this and I am guessing they will correct the angle of the pins in their arms at some date.

Why did you move the linear rails
It helped achieve the geometry necessary to improve the Maxwell Criteria and it also reduced the rotational torque moment imparted by the lead screws on the bed arms. Having a POM lead screw nut bolted to the sides of the bed arms as Rat Rig did makes it all but impossible to achieve desirable geometry or centering of the rear leadscrew.

Why do you recommend that the star drives are printed from PETG rather than machined from POM (Acetal)?
It is a popular fallacy that POM (Acetal) has a low coefficient of friction. In the scope of plastics POM has a relatively high coefficient of friction. POM has a dry running coefficient of friction of 0.3 ~ 0.4, compared to just 0.05 - 0.13 for the range of industrial plastics normally selected for low friction applications.... but POM has handy advantage of being cheap and more importantly easy to machine. We have machined a lot of POM in the FastBikeGear workshop over the years for these reasons. As mentioned previously Low resistance (friction) to lateral movement of the leadscrew is the key to eliminating lead screw induced artifacts.

Pom also has a self lubricating process which is why it is usually described as a self lubricating material. The self lubricating process occurs when very small particles wear off the parent material and act as miniature bearings between mating surfaces. This works when a shaft rotates in a POM block and these 'bearings' can stay in place between mating surfaces. In applications where these 'bearing particles' get wiped away, by a sliding action between mating surfaces, you are better to regularly apply a lubricant ....and then the coefficient of POM is irrelevant, and the coefficient of the lubricant is then of more interest. For dry low running applications there are much better plastics to use...or better yet use bearings (especially if the mated surfaces are under load.)

Are VenterMechs the end game to eliminating leadscrew induced artifacts?   
Definitely not, While  Ventermechs work extremely well and are a cost effective de-coupling mechanism for use on a low cost printer, they are still a band aid for a compromised Z system. Many printers like the new Bambu X1 appear at first glance to have a similar Z arrangement, but a closer examination reveals that there are fundamental and important differences to their Z mechanisms, that greatly negate the need for de-couplers in the bed arms.

I am currently working on and testing a new Z kinematic architecture. This solution if it meets all goals will feature first on the 1P printer. A retrofit version for a V-Core 3 may be possible, but the mechanism would need to be redesigned to fit and there is at least six months of testing and measurements to discover what I might not know yet, before releasing the design.


Alternative Rear Arm
An alternative centralised option for the rear leadscrew and rear arm is also included in the attached files. This is primarily a cosmetic upgrade.It requires the following STLs:

  1. Centralised STL for the rear bed arm
  2. Centralised rear motor mount
  3. Extra cross frame extrusion to support the motor mount above
  4. Centralised rear lead screw guide 

Feedback

I love feedback so please let me know how you get on.
Remember to post up your before and after photos of prints on the forum where you first heard about the VenterMechs.



A discussion on leadscrew induced Z banding.

This is by no means a comprehensive discussion, but I thought it might be valuable as an intro to the topic.
Please excuse my poor grammar and typos!


Leadscrew induced Z banding.

Leadscrew induced Z banding is primarily caused by lateral displacement of the bed arms by the radial movements of the lead screws. In a perfect situation the Z linear rails and the carriages fixed to the bed arms would be able to resist this lateral movement.

Many 3D printers describe their bed mounting system as being a Maxwell kinematic coupling. According to the Wikipedia definition, “Kinematic coupling describes fixtures designed to EXACTLY (my emphasis) constrain the part in question, providing precision and certainty of location”

At first glance these bed mounting systems look like perfect Maxwell Couplings…but they are not, because the fixtures (balls sitting in pins and arms bolted to linear rail carriages, mated to the linear rails) do not solidly and EXACTLY constrain the bed, simply because there is a very small amount of movement between components in this restraint chain.

Under good circumstances the weakest point in this restraint chain is the clearance tolerances between the carriages and the linear rail. The clearances could be reduced by using higher grade, but more expensive linear rails/carriages. The effects of these clearances could be reduced by using wider rails and/or longer carriages, again at some increased expense. Prusa for example uses two linear bearings stacked one above the other to increase their effective length as part of their solution to Z banding.

Money can help solve a lot of problems, but so can clever design with cheaper components, and the aim with a hobby/prosumer printer is to get the biggest bang for buck.

In practice, because the large cross section of these is able to withstand bending from the applied forces it is very unlikely that any flexibility in the printed bed arms contributes to Z banding. However, a small amount of movement between the bed arms and carriages is possible and does occur if one or more of the screws that hold them on to the carriages work loose due to plastic creep or vibrations backing off the screws. Checking the tightness of these cap crews should be part of periodic maintenance.

Other issues such as loose grub screws on both X and Y motors and the Z lead screw flexible couplings can also cause artefacts that look like lead screw Z banding. In addition, the beds are not fully constrained against the pins by more than the weight of the bed assembly and sometime with the added assistance of quite weak magnetic force.

When one arm moves laterally, the balls in the other two arms can either slide on their respective pins which allows the bed to rotate fractionally or climb up on one pin in the pair to accommodate this, which if/when achieved would/will fractionally lift the bed. Both these options cause imperfect layer stacking, which can be viewed as Z banding in the prints.

Contributing to the issue is that the length of the bed arms amplifies any lateral displacement caused by the lead screws.

Before trying to engineer a new solution it is bet to ensure that your printer is as mechanically fine-tuned as bet as possible. It may be after doing this tuning you deem you do not have a problem that needs solving.


 

What causes the lateral movement of the lead screws?

1. Radial misalignment of the stepper motor shaft and the lead screw. Radial misalignment is when the centre of these two rotating ‘axels’ is not perfectly aligned. 


2. Angular misalignment. This is when an extension of a line drawn along the two axial centres of two shafts intersect each other. 


3. Angular misalignment of the lead screw with the linear rail. I have published sturdy lead screw alignment tools on Thingiverse to assist with reducing this misalignment.


4. Bent lead screws. Lead screws may not be straight to start with, but are also likely to become bent over time.


All of the above cause lateral (sideways) forces to be present at your lead screw nuts which are attached rigidly to your bed arms.

The only thing constraining the lateral movement of the bed arms is the chain of restraint from the from the linear rails to the carriages to bed arms through to the lead screw nuts

In attempting to move the arms sideways the printers lead screws are a competing with this chain of restraint…and you definitely want the chain of restraint to win this argument.


So how can you help the chain of restraint win?

For the chain of restraint to win this argument you need to try and do one or more of the following:

1. Reduce the lateral forces applied by the lead screws by making the lead screws more flexible. You could do this by making them thinner or by making them from more flexible material so that the lateral forces they can apply to the bed arms are less.

2. You could reduce the lateral forces applied by the lead screws by making the lead screw couplers softer, so the couplers can ‘accommodate’ misalignment. This is not a bad idea. I replaced the flexible cushions in my printers supplied lead screw couplers with ones I printed from Ninjaflex. (STL on Thingiverse)

3. You could buy perfectly straight lead screws, non flexible Z screw couplings (or lead screws integrated with your steppers, like Prusa does) and gimbal mount your motors to ensure that no angular misalignment is introduced by the lead screw coupler - but this is complicated and you don't have room to gimbal mount your motors.

&nbs

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