by dirktheeng » Tue Sep 14, 2010 11:19 pm
I think you are confused about which part I mean. I was going to attach an image of the inventor files I am drawing up, but I can't for some reason... the options are not there.
Anyhow, look up the part RSRR25-480 and is $27. The car that goes with it is RSRS25, and is $12. This is a 480mm track with 18.25" of travel. It has center line holes to bolt to anything you want (I'm going to use t-slot extrusions). The track is aluminum alloy, the car body is 304 stainless, the wheels are a very very hard plastic (polyacetal) (the same stuff they make really hard scate board wheels from). There really isn't much room for any slop. This is not a roller slide like a drawr guide. Given that the loads are so light, a hard plastic does not concern me, and frankly it will wear better against an aluminum surface than hard steel. I can do the calculations for deflection, but from experince I can tell you that the expected loads are so low, that plastic will not deform more than 0.001" I would only expect a few ounses of live load for a laser cutter application.
This length would be used for the Y axis, so I need 2 of them. The total cost for the guide, car, and bearings (in this case included in the car), is $78. That doesn't include the first time buyers 30% discount (which brings it down to $54.60)
Now lets compare to the current design. To get simmilar travel, I have to order the 2ft v-groove track from mcmaster. If I get the cheapest 5/8" rail (which is what is specified in the drawings you posted), I can get them for $12.78 a piece. Mind you, these are unhardened steel without any chrome coatings or pre-drilled holes. I have yet to find a cheaper source for these rails and I don't particularly want to use an uncoated steel piece given that I know it will corrode. VXB sells the v-groove bearings for $9.95 each. I am assuming you got some sort of volume discount on the bearings to be able to offer them at the price you do in your bearing kit. Lets assume you got 30% off and can pass those savings on to us. That means the bearings are $6.97 each. The design calls for 3 each for each car, so the minimum price for the car is now $20.89. That doesn't include any hardware or materials to make the car (which is included in your hardware kit and the xy kit). Lets assume that there is somewhere around $5 of cost for the car plate and hardware for each car. That means that each car is about $26. I need 2 cars and 4 v-groove rails. That totals $103 and I still have to drill the v-groove rails and drill and tap the extrusion, and I still have to worry about protecting the rails from corrosion (which could be a problem if you ever cut any plastic with chlorine in it as it will make hydrochloric acid vapor).
Furthermore, I am still going to be heavily influenced in straitness tolerances to an extruded part even if I use the steel rails. I am really beginning to think that this missumi extrusion and car may be the most cost effective way to build a linear rail for laser cutters. They also offer steel rails and cars with steel wheels in a larger size rail system, but it is more expensive and (in my opinion) a large overkill for this kind of system. People get great results from cheap chinese lasers that have nothing more than square tubing for rails and plastic bushings for wheel/bearings. I want more than that, but I think this is a reasonable way to go that is going to be about half the cost of using v-groove rails with about 90% less headache and assembly complexity.