Gold Series Rat Rig V-Core 3 VenterMech arms.

thingiverse

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. The solution is completely free for personal 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 To get the full files sent to you please email Liam@FastBikeGear.co.nz with 'VenterMechs' in the subject line. Update 18/03/22: I have now added an alternative option centralised rear bed arm/leadscrew option to the files I send out. <b>What do Ventermechs do?</b> 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. My design ethos is always guided by the follow two design 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 sizes and tolerances of each component are important to the operation of the VenterMech. The tolerances can be achieved with any good consumer level printer. 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. The files we sent to you have these already modelled into my own original design bed support arms to fit a Rat Rig V-Core-3 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. 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 bearing costing about $5 US for a pack of three from Aliexpress, Amazon, etc. <b>FAQ</b> Q: Will the mechanism be long lasting and reliable? I have probably done about 600 hours with the printed mechanism in my new printer. I also used printed lead screw nuts on one of my Prusa's for a couple of years. We do low volume production printing for customers. No indication of any wear or other issues. No traces of any material wearing off. Haven't needed to print any replacements yet. I have tested them in PLA, PETG and Nylon. I have not tested any in ABS and I would not recommend printing them from an abrasive filament. Q: Looking at the pictures on your webpage, my initial thoughts scream BACKLASH at me but if that does happen, I guess it would just be a case of increasing the Z-Hop distance to compensate. Yes it would seem at first that there would be some slack when the Z stepper motors reverses direction when you use Z hop, and they do. The amount of free play translates to just over 0.1 mm of vertical free play when reversing lead screw direction with a TR8 x 4 pitch lead screw. If you want to account for this you can just add 0.1 mm to your Z hop settings in your slicer. If you are using 0.4 mm of Z hop you might elect to increas that to 0.5 mm. So far I haven't had any issues with Z hop except for some slight extra noise of the lead screws on the lead screw limiters at the top of the lead screws that I chose to use in conjunction with the VenterMech. Q: I can visualise a bent/misaligned screw moving in XY causing the drive gear to move as the triangles slide which will cause a slight rotation in the 'nut' and change Z. A: They are designed to deal with bent lead screws. They can handle up to 1.5 mm of bend through the entire length of the lead screw. Yes in theory a bent lead screw must cause some very small rotation of the star drive in the receptacle, but in reality this is so small (much less than you might think) that I have not observed effects from it in any test prints. I had a solution in mind to address this if needed, but in practice there was no need to add complexity to the design to deal with this. Q: How many people are using VenterMech on Rat Rig printers A: I have sent out over 200 sets of files, BOMs and Intructions so far. probably about half the people I have sent these to are already up and running with VenterMecsh on their Rat Rigs. <b>A discussion on leadscrew induced Z banding.</b> This is by no means a comprehensive discussion, but I thought it might be valuable as an intro to the topic. 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. <b>What causes the lateral movement of the lead screws?</b> 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. <b>So how can you help the chain of restraint win?</b> 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 …and It won’t help if your lead screws become bent. 4. You could attempt to perfectly align perfectly straight lead screws with the linear rails so there is no argument between them and perfectly align the lead screw with the stepper motor shafts. This requires very stiff and expensive lead screws/ball screws, couplers and motor mounts that perfectly align the motors with the lead screws with neither angular or axial misalignment (or integrated lead screws) and a very stiff motor mounting. And you need these conditions to be maintained over the life of the printer. You are venturing into the realm of expensive pieces of precision machined metal to achieve this. My lathe is built like this, it weighs over 1 ton and uses widely spaced parallel massive linear rails for the carriage’ (called a saddle on a lathe) and it still uses a relatively flexible, by comparison lead screw – so that the chain of restraint comprised of linear rails, carriage mating and big rigid bits of precision machined steel win any mechanical arguments. It is worth noting that there are also adjustment screws on the carriages that can be used to reduce the clearance between the carriage and linear rails as components wear. 5. You could make your linear rails beefier (go up to 12 mm rails for example). But you don't have room to fit them on them on many printers using smaller extrusions. Or use two Z linear rails per bed arm on different faces of the vertical extrusions….an expensive option that needs perfect alignment. Longer carriages or stacked carriages could also be used but this would add to cost and reduced Z printing capacity. You could also use a combination of the above, but some are at best poor pairings and some pair like using lead screw nut de-couplers and softer cushions in the lead screw/motor couplings may work well. <b>Flexible motor shaft to lead screw couplers</b> Flexible couplers uncouple the lateral forces applied to the lead screw nuts so that these forces are not transmitted to the bed arms. In doing so they solve some problems and introduce other potential issues (cost, complexity, maintenance and mechanical). One issue when you use flexible lead screw to motor shaft couplers is that the lead screws are now much less restrained in their lateral movements. If this movement is too extensive it can overwhelm the de-couplers capacity to accommodate their movement which means that lateral forces will be transmitted to the arms – at this point they stop being de-couplers. Centripetal force can come into play with fast Z screw rotations that you might encounter with Z hop for example. If there is angular misalignment of the lead screw it’s centre of mass is no longer axial with the stepper motor shaft. When this happens it is like swinging a bucket of water around you on a longer rope rather than a shorter rope. At slow speeds it is not too much of an issue but at faster speeds with the larger arc the centripetal force increases and the bucket needs more constraint to stop it overcoming the restraint and travelling in a wider arc around your body (or heading off at a tangent). One solution to this problem might be a very loose restraint at the top of the lead screws to restrain their wildest movements. The other solution is to apply a countering self centering force within the de-coupler mechanism which it is possible to do with a suitable compact mechanism. Or you could bypass all the hassle and complication and try the VenterMechs