Linear Bearing Idea

[dirktheeng]

Check out http://www.pbclinear.com/Integral-V-Technology

It may save you some trouble.... basically the same idea you had, but already comercial

[bdring]

the simplest 1 is about 18 to 20 dollars per foot

[macona]

Thats about double the price of even the 45x90 (I think) heavy extrusion that I had quoted by Bosch.

Why not instead have an extrusion made that keys into the top of an existing extrusion and provides a groove where the base from a T-based round rail like here:

http://www.vxb.com/page/bearings/CTGY/1 ... ionSystems

Just drop it into the groove and screw it down.

Thermwood uses this type of rail in their routers. 12mm diameter would be more than adequate for a laser or rapid proto and even a small router. The prices at VXB are not all the bad even. $10 per pillow block and $100 for ~79" of rail. The stuff can be cut with an abrasive chop saw. I bet you could have it shipped directly from china for considerably less.

[bdring]

I use the 20mm VXBones on my wood router. They are great for that application.

They allow the block to rotate on the axis, so you need two rails per axis. The blocks do not have much stiffness to twist so you generally use two per blocks spaced a few inches apart. Mine are not smooth as silk either. There is a noticeable amount of "cogging". If you have a heavy load and a robust drive like ball screws that goes away, but I would not suggest belts.

See my X Axis. I have two rails and two blocks per rail. Total cost per axis about $320. For the 12mm system it would be about $200. The proposed system would be about $20-$25 (rail + 3 wheels). Also notice the v-rails for the Z. Next size up from the laser. An issue with the steel rails is corrosion. On my laser some items product some very corrosive gasses.

[bdring]

Die Prices...Price per foot.

I am getting die quotes below $2000. I broke the tappable holes into the upper voids to reduce the void count. That should reduce the die price a little. The mass per foot is around .70 lb/ft. I am getting quotes in the $1.60 per foot. Some places don't have a large minimum buy on the first run of the tool. I have attached a PDF, DXF and STEP in a ZIP file if anyone wants to play with the design or numbers. One reality check is that Misumi sells the 20x40mm extrusion at $3.00 for one foot, cut to length with probably a decent mark-up.

Feel free to have anyone quote it. Please don't post names of suppliers with their quotes. Send them to me and I will list them as supplier A, B etc. I don't think it is cool to do that in a public place without their permission.

[macona]

You could get them bidding against each other!

The only thing I know of that produces corrosive gasses when cut is PVC and that produces chlorine. If you have that rust on steel is the least of you issues as it will go after everything else including optics. That, ABS, which creates cyanide gas, and Polycarbonate, which flares up, were the three forbidden materials in our lasers.

[bdring]

The worst I ever cut was vinyl records. I saw some cool things cut from them and gave it a try. It gave off some very dense brown gas. Oops. Click for Blog Post on Subject

Sometimes you don't know exactly what you are cutting. I have cut sand paper, gaskets with adhesive backing. Some probably are corrosive. I don't worry about myself due to the huge exhaust blower, put I have a few corroded items.

[bdring]

So you really cant adjust the bearings to take up the slop.

There is a little built in wear compensation.

The wheels are pressed against the rail using various methods. Some people use eccentric spacers or studs. I like to use a set screw to push the mounting screw. Either way stores a small amount of spring tension. This compensates for rails not perfectly mounted, changes due to expansion,etc and some wear.

[dirktheeng]

I would be a bit concerned about the straightness tolerences that one can get with extruded parts. I looked into PCB Integral-V and talked with the engineers there about being able to hold tolerances to make a home made CNC router earlier this year. Basically, they make the extrusion and then mill the guide rail slot and fix a hardened steel V into the milled slot to give a high wear surface. That's why the stuff is so stinkin expensive. For short runs, extruded parts may be able to hold >.01" per foot without milling, which may be good enough for some small cnc projects, but not so good for something when you want to make a 4ftx8ft router.

The other thing to consider is deflection under load and how far you can span without support. I was doing the design calcs for a CNC router earlier this year and I wanted to be able to mainain a 0.001" deflection standard (really helps reduce chatter when milling hard aluminum). I was going to use 8020 extruded parts and soon found that I had to use the 2040 profile supported every 2 feet to take a load of 80lbs from the gantry, z axis, motor, etc (live and static loads). It could be done, but it gets expensive. It was cheaper to use an I-beam and mount round rails and pillow blocks.

Granted, for a laser these may work ok because the runs are shorter and the loads are very low. All that is to say, beware of the potential for low straitness tolerances and the higher complience of extruded aluminum parts.

What kind of tolerances are you expecting to be able to get out of this?

[dirktheeng]

Other possible solution for light-medium duty applications:

I came accross http://www.modernlinear.com/linear_guid ... track.html when I was looking for pre-drilled vgrove track. They make a double grooved track:

http://www.modernlinear.com/pdfs/2008%2 ... g%20p7.pdf

and I'm going to look into the cost of this stuff when I check for cost for pre-drilled track. Basically, this is a steel solution to your aluminum extrusion. Granted it would have to be supported, but it provides near perfect alignment and I am sure that a milled track like that is very strait. I will ask about tolerances. I could envision that kind of track being used on top of a simple 8020 extrusion with some sort of cheap standoff (bushings maybe or thick washers????). That way you would have room to get a bolt head through a vgroove bearing.

Given that it is steel, it may be a cheap alternative and it will certainly wear better than any aluminum surface.