Imran's personal blog

December 4, 2011

Making a cheap linear motion system

Filed under: Uncategorized — ipeerbhai @ 10:45 pm

Hi All,

I’ve been designing my own “better than RepRap” printer. I’ve gone through a design, and not liked it. It sort of worked, but the X carriage linear motion was too rough, and the X Carriage was huge — almost 7 inches! I would end up with only 4″ of X travel on a 13″ beam. One of the biggest challenges I have in design is in a linear motion system that isn’t too expensive. I thought I’d write a little about this.

Linear motion systems are expensive. You look at McMaster, Grainger, even eBay, and things cost a lot. At least $25.00 per bearing, plus the cost of whatever track system you use ( steel rods, extruded aluminum, etc… ). In keeping my design cheap, I needed to use a cheap bearing system. In the US, there’s only a couple of cheap bearings systems available. Here’s what they are:

Skate Bearings

608/605 skate bearings, as well as some assorted 5mm wheel-type bearings for a good price. Good meaning $0.40/bearing. Very affordable! This is why so many designs use 608 or 605 type bearings — they’re cheap. But they’re not linear bearings — they’re just wheels. Little steel wheels. They have no linear control. You have to combine them in interesting ways to get a linear bearing. Some people use angle metal and mount bearings on them. This is one of the cheap systems that work. Other people enclose the bearings into little carriages, made either of plastic or metal. I’ve done this. I’ve designed a linear carriage that rolls on 1/8″ extruded aluminum. It works. It’s cheap. $9.00 a carriage ( 8 bearings, 2 HDPE inserts, 1 starboard block, 6 brass inserts ). The extruded Aluminum can be bought online for about $1.00/foot, or you can buy precision ground tool steel for just a little more. It has a 2.5″ footprint. So, what’s wrong with this idea?

The problem is when I need to do 2 dimensions of linear motion — say X and Z — on the same assembly. Things become expensive fast, and complex. Not to mention my moment arms become quite large, and I worry that the system will break under heavy loads — say under CNC stresses. I can’t machine the plastic blocks to the tolerances I need with a drill press. If only I had a CNC mill, then this carriage would work. As it is, I can’t warrant that X and Z are 90 degrees to each other. I impart error to the Z arm from this assembly. I need a design that doesn’t require having a CNC to begin with.

Sliders on rods

Although it takes a bit of sourcing, this is by and far the cheapest system you can get.  Cheaper than skate bearings, and more reliable too.  There really is no need to design linear motion systems any more — Sliders on Rods can be bought super-cheaply.  I’m talking about $5.00/carriage ( a carriage is more than 1 slider unit ) and a couple of cents a millimeter for the shafts.  The rods can be made from steel or Aluminum.  The sliders can be made with roller balls or plain bearings.  The Sliders are as cheap as $1.00 or two for very low-force systems.  You can source both rods and sliders in the US or from China.  Though — the Rod is best sourced in the US, as shipping rod from China is expensive, and cheap Chinese rod doesn’t have the same quality surface finish as what we get in the US.  Several suppliers, Like McMaster, Grainger, etc… have very good pricing on the rods.  I like to source the sliders from China — they’re fairly light, so you can get a good deal on shipping those.

 

MakerSlide

I like this concept. Cheap wheels/bearings kits ( $2.85/bearing, need 3-4 per axis. ), $8.00 for a mounting plate. $0.01/mm for the track. So, really, a hair above $20.00/bearing ( 4 wheels X 2.85 + $8.00/plate + eccentric spacers, m5 screws, etc… misc hardware ). Their system can handle 2 dimensions of motion for only adding 3-4 more wheels ( so $33 for a 2-dimension linear system. Awesome! ).

What’s the problem with this? It’s meant for light-force projects, and it is hard to get. I got some by chance, and I like it. It’s good stuff for light use projects — laser cutters, 3d printers, and “Dremel” sized routers are probably the most it can handle. The v-rails are thin aluminum, and Aluminum’s Young’s modulus is only 69 GPa — meaning that larger loads will bend the rails on the extrusion — so you wouldn’t put a 2 HP, 1/2″ shank router on this stuff. The project has been shipping to backers from Kickstarter, and is beginning to ship to normal folks. It’s a single guy in his garage — literally! So, you have to wait until he gets enough in to sell to you. I give the guy major props — he set out to do something really hard, and did it better than I could. But if you want some, good luck getting your hands on it. Re-producing the system using McMAster 7/8″ rails and VXB V-Groove wheels ( or Window Bearings can also work, VXB sells nylon v-groove window wheels cheap ) is expensive and labor intensive. If you have a project for which makerslide is the right material, wait and buy it from them, or use SC12UU on hollow 12mm rail that you bought in China when you went to visit.

HiWin/Bank of Taiwan carriages.

HiWin is also known as “Bank of Taiwan” carriages. This design is a copy from a german design using an inverse T-slot rail, which you bolt on to something like extruded aluminum. The carriage is like an SC carriage — lots of little roller balls, which can be sealed into the carriage. Sometimes, the carriage has a built-in steel scraper for use in high-dust environments, like a CNC router. This design is very nice, and I’ve seen this system on commercial CNC routers in real machine shops. It’s just very expensive. Maybe you can find a carriage for 20-30 bucks at an overstock place. The rail is another matter. 15″ of rail will run you a couple of hundred dollars normally, even in China. My HK supplier quoted me $1,300 HKD just for the rail, and $300 HKD for the carriage. This works out to $1600 HKD for 15″ of rail — about $200 USD. You might be able to find the rail at an overstock place — but if you need more than a few inches of the rail, your prices will quickly exceed the cost of a computer!

My Design

My design is based on the idea of an inverse T rail, complete with carriage. I use 608 bearings rather than individual ball bearings, and the bearings can easily be replaced. This makes the design cheap. Sealed 608 bearings means that the design is dust-proof. It should work in a CNC router environment, though you may want to add a scraper to my design. The basic idea is to take 608 bearings from VXB and 1/8″ thick, 2.5″ wide extruded Aluminum bar from OnlineMetals. 1/4″ sould work as well, but you’ll have to drill the bearing mounting holes further apart. Both of these are really cheap. Take a plate — say plywood, and drill 8mm holes into it to mount the bearings. Put 1/8″ HDPE liner on the plate, facing the track. The 608 bearings should be on the same side as the HDPE liner. The holes should be drilled into the 4 corners of a square, about 1″ square should do, +- a little. Make the bottom holes a slot, by drilling two holes right next to each other, vertically. Think of each bearing as a wheel. This allows you to move the bottom wheels until contact is made with the 1/8″ track. Attach the wheel with 8mm button-head screws. the button head must be on the “inside”, as the clearance between the screw and the track is around 1mm. Attach the track to 80/20 15 series linear rail with 1/4″ button-head screws, using the 80/20 “Economy” 1/4″ T-Nuts. Attach two plates together with 1/4″ long screws. If you only need 1 dimension of motion, use 4″ long screws and a bunch of nuts and washer — 4 washers/screw, 3 nuts. Attach whatever you want to either plate. I’ll post some pictures eventually. This design is cheap, lightweight, and strong. It will do 1 dimension of linear motion very well. It won’t do 2 dimensions well, which is its key drawback. It’s also hard to produce the carriage without a CNC machine or precision mill of some sort. A good machinist should be able to do it, but I am not good enough to do it without these tools.

There are other systems that are sometimes cheap — Inverse T-Rail like HiWin, THK, etc… is sometimes available off eBay for cheap. You might be able to find some overstock occasionally at other places. But that’s a lucky find — you can’t design around lucky finds, as you can’t repair the equipment or make more of it when you need/want. IF anyone here knows of a cheap system that already exists, can do 2 dimensions of motion cheaply and easily ( X, Z ), and is available now and forever, please let me know!

Another cheap Design

Get some V624 bearings.  They’re really cheap — like $1.00 a bearing or less, though a bit hard to find.  You can get them on Ebay pretty often.  Go to onlinemetals.com and get some ground steel bar — I like the 1/8″ thick, precision ground O2 2″ wide steel.  Take a die grinder, and grind a 90 degree V on both the long edges.  This is now a V-Edge race, that you can drill holes into and mount onto a support.  This will give you a steel-on-steel bearing/race combination, so it should wear nicely.  It’ll also be cheaper than pretty much any other solution — but you’ll have to put in your own labor.  The hard part is grinding the edge.  You have to be pretty good with a grinder to get this nice and straight.

–Imran

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1 Comment »

  1. Cool, informative, and accurate based on my knowledge as well. We had to go through this for 3D printers and found linear bearings and makerslide work best.

    Comment by Michael Armani — February 7, 2013 @ 12:05 am | Reply


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