[In Depth] - The Kossel linear rails

Terence's note:  This blog entry is a work in progress.  We are releasing it ahead of our usual editorial process as our backers are now receiving their linear rail sets and the linear rails contains an instruction sheet that links to this article.  Photos and video will be forthcoming.  Thanks. Hello Kickstarter backers!

By now, all the "rails only" Kickstarter rewards have shipped via Fedex to our backers. You should have received a Fedex shipping notification email with your tracking number and information.

One of the things that separate the Kossel Pro from the rest of the machines on the market and at this price point is the use of linear recirculating ball rails on the machine chassis. There are a few great Open Source hardware options when it comes to linear bearings: Bart Dring's MakerSlide, Mark and Trish Carew's OpenRail and V-Slot, and Steve Graber's W wheels are three of the systems that makers are most likely to be familiar with. When designing the Kossel Pro, we also took into consideration patents that are existing in the field, namely this patent held by the 80/20 corporation. After all, we are a small start up challenging the status quo of expensive T-nuts with cheap extrusions in the T-slot framing space and we cannot afford any legal entanglements with a much larger corporation.

The decision to use linear rails become pretty apparent when you consider the following factors.  Being able to adjust the play between the carriage and the rail is good; not having to worry about that adjustment is even better.  Given that I speak both Cantonese and Mandarin, and given our investment in our trade and logistics capabilities, we are able to source these linear rails at a very competitive price.  In fact, our linear rail and ball bearing carriage subassembly BOM cost is actually lower than the retail cost of a comparable length of MakerSlide and its carriage and just slightly above the retail cost of 3 W-wheels from Mr. Graber.

The biggest advantage with the ball rails over other ball bearing on aluminum extrusion systems, however, is in the material selection. The rails are made from induction hardened tool steel.  In fact, the steel used in the rails is so hard, the vendor that we purchase the rails from uses a Wire EDM process to cut the rails to length. These rails are also precision ground to incredible levels of flatness. This, combined with our new extrusion vendor's awesome aluminum profiles, meant that the linear rail assemblies on our machines will be straight and true, and likely will stay that way unless the machine is put through serious abuse. Remember, Aluminum Extrusion Council's standard tolerances is about 1.1 mm per meter length and a competent vendor generally can be expected to deliver parts within half of commercial tolerances. That is still 0.25 to 0.5mm of deviation across the Kossel's travel on the linear axis, and we would like to do better than this. Our rails, being a ground metal product, is flat to within 0.02mm per meter - order of magnitude flatter than normal aluminum extrusions. They also add an incredible amount of rigidity to the machine, and that is the secret to how we can build such a large, rigid machine chassis on such a thin aluminum profile.

The rails we source are Chinese made clones of Thompson / THK / HiWin linear recirculating linear ball rails. Contrary to popular belief, they are NOT from HiWin! Early on in the Kossel's development, there was a cloner who would shamelessly come into Metrix, observe what people are working on, and proceed to stock his webstore based on his observations. As the cloner himself does not do any meaningful R&D work and we rely on future sales to recuperate our R&D cost and had already lost money due to his cloning of early legacy Kossel extrusion kits, we could not afford to lose the opportunity to sell the rails if the cloner beat us to the market. Therefore, we masked the true origin of the rails by calling it a (much more expensive) HiWin MGN-12C equivalent. In reality, our rails are the same generic ones that are sometimes sold on AliExpress; we just found a reliable supplier, qualified them, and verified that the rails that we are sourcing will work with our design.

There is a big cost difference between our rails and a genuine HiWin / THK rail. Although our rails are ground from hardened tool steel, the surface finish on our rails is rougher than a HiWin / THK rail, and as a result our rails run nosier. (Surface finish is measured in micro inches or microns. The rougher surface does not affect accuracy, just performance noise). The tolerance range on the grinding is also looser than what would be on a genuine HiWin rails and carts, and some carriages may be stiffer than others to move on the rails. None of these issues affect the accuracy or machine chassis rigidity of the rails and is not considered a defect, and a little bit of care and preparation can help improve the rail's performance drastically.

The rails ships from the factory with a retained ball carriage. This means that there is a wire cage running down the length of the rail to make it harder for the ball bearings from falling out when you remove the carriage. Note that it is still possible for individual ball bearings to fall out, but it is also possible to push the ball bearings back in. There is a gap in the chain of balls in the recirculating ball race - in a higher end carriage, there would be a special plastic cage piece between individual balls that contains a reservoir of lubrication grease. For maximum life, it is recommended that you replace the protective oil that the rails shipped from the factory with a proper grease in the ball bearings.

I've found it easiest to slide the carriage off by laying it on its back and sliding the rail out from the top. This way any displaced balls hopefully won't fall and get lost. A towel over the work area to capture the occasional lose ball is a good idea.

We are shipping all kickstarter rewards (and all Amazon rails) with a piece of lint free wipe to wipe off the preservation oil, as well as a light bearing grease for greasing the rails. The seals do a pretty good job at keeping debris and dirt out of the rails. Unfortunately, this also means that any grease applied to the outside of the carriage on the rail will likely be wiped off and will never make it to the ball race. We therefore recommend our users to remove the ball carriage carefully, and apply the grease directly to the line of ball bearings using the provided cocktail stick, moving the balls around and applying the grease. We have chosen a non-toxic grease that is rated for intermittent food contact that complies with the new regulations on VOCs in lubrication fluid.

i've taken a short video showing the cleaning process here. With a little bit of care, these rails can be a very high performance upgrade option to the 3D printer (or any other mechanical build). We hope you find this information useful for your project. Happy building!

-=- Terence, Rachel and the furry monster puppy