X Axis Rail Question

[Dman65]

I am totally new to CNC and I am wanting to build a starter machine in the 2'x4' range since I think this will make it possible for me to then cut out the parts I would need to build a better machine. I am not good with jig and band saws. That is one of the reasons I want to build a table to begin with.

I had been planning on something patterned after the JGRO design, and I have ordered some skate bearings from VXB. However, I have been reading this evening about all kinds of other types of rails.

There were some who say that the skate bearings on a cylinder is a bad idea because they are not built for radial stress and they should be used on a square surface. I have also read about linear bearings, both round and square, as well as as using V bearings.

I am wondering what the best way to go in a 2x4 project would be.

1. The JGRO skate bearings on round pipe

2. Skate bearings on steel square tube (how do you setup the bearings for this?)

3. Round linear bearings on round tube

4. Round linear bearings on round cold rolled bar

Also, if you run the round bearings on round bar, what bar diameter should you use? I saw some stainless in 1/2" on e-bay that wasn't too bad a price, but I don't know how the strength measures up to the gas pipe.

I have ruled out the flat bar linear bearings because they push the cost up more than I am interested in.

There are a lot of things to get your head around in this endeavor.

[benjamint76]

I am not familiar with your skate bearing design but they are very similar to the V-bearings which my gantry runs on, my Y-axis(100 inches)rack and pinion. I love the v-bearings because I can reach speeds of at least 1000ipm, although I am really afraid to run it that fast and my 1 3/4 hp router would never keep up. I have never had a problem and I would guess that my gantry weights in at about 120 lbs.
I use the 20mm linear rods for my x-axis(50 inches) ball screw and its ok but I can only get up to 150 ipm before my steppers stall. If I modify my x-axis, I will replace the ball screw with more rack and pinion to up my speeds...I must say that your choice will depend on your application. I primarily do wood working so +/- 0.005inches is great for me, a sander takes more than that i the blink of an eye. I have started doing some metal working and wow... I am better that 0.005 but I don't have anything to measure that small. Hope that helped.

[Dman65]

For my first machine, I want to be able to cut wood and HDPE, UHMW, and delrin plastics.

I forgot to put v-bearings in my list. Was way too late when I was typing that up last night.

Do you have a picture of how your v-bearings work?

[jsheerin]

Most types of steel are similar in their modulus of elasticity. This plus the cross sectional shape and area effect how strong a piece will be, or for our interests, how much it will deflect under load. With round bars, I'd strongly suggest support the bar. Ideally you would fully support it, but several supports spaced along its length would still be much better than just supporting the ends. You can calculate all this stuff if you want to.

Personally I use Ahren's skate bearing parts on 1018 cold rolled steel attached to pieces of 80/20 aluminum extrusions. This works great. The only downside I've seen is that you need to come up with a way to get debris off the rails or keep it off to begin with, but that is true of all designs. Check them out at cncrouterparts.com.

[benjamint76]

ok so my y-axis has the v bearings. I am happy with that, but if I were to do it again I would shoot to have another set on the bottom so that it is basically pulling the gantry down as well. Right now all that holds the gantry down are the stepper motors with some big springs and the weight of the gantry.

The rods for the x-axis are fully supported but I was only able to find then in lengths up to 54 inches. I would have preferred 60" so that my z-axis could have a larger base/platform. Hope this helps some. As far as dust goes some people put brushes around the pillow block which pushes larger debris off. I just use the air compressor from time to time. Also if you use a dust collection system while cutting, that will eat at least 80% of the dust if not more...

[judleroy]

Those v rails and bearings should be rotated 90 degrees from what they are now. That way the right and left rails are keeping the axis from moving right to left by pushing or pulling against each other. The v is keeping it from moving up and down.

[ger21]

Originally Posted by judleroy Those v rails and bearings should be rotated 90 degrees from what they are now.

Doing that would reduce their load capacity by about 80%. Their radial load rating is about 5 times more than their axial load rating. The correct thing is to have another set below, which would both prevent lifting. By preventing lifting, you eliminate any side to side movement that may occur.

[judleroy]

Although i agree with Gerry that another set of rails below would work and give you more strengh i dont think its needed. The figures i've seen dont show 5 times more radial load its actually about 3 times more. That being said unless your moving more than 2-300 pounds rotating 90 degrees is your easiest solution.

[ger21]

Originally Posted by judleroy The figures i've seen dont show 5 times more radial load its actually about 3 times more.

I got the info from the Bishop WiseCarver catalog. #2 wheels are rated for 141 lbs axial load, 596lbs radial. Although going back and looking at the #3 size, it looks like those are about 3.5:1.

[benjamint76]

And in fact, I did get these from Bishop Wisecarver

I do not have any problem with side to side movement at all, that's what the "v" is for and the gantry never lifts off,( unless I leave the work space cluttered and it ramps over a drill...lol)

[Dman65]

Thanks Benjamin. I appreciate the pictures. Definitely helps me understand.

How would you mount a second set of rails here?

[Trucks]

Hey Dman65,

I am in the process of building my first machine, so far it is 3' x 5' and i am planning on using the skate bearing idea, but i am building the guide or rail support out of steel, as i feel it will be stronger and easier for me to weld together. I am putting a piece of angle on the bottom and the top of a piece of tube, and then will make the bearing blocks out of angle as well. I looked at all kinds of linear motion rails, but i couldn't make up my mind and my wallet did not want to cooperate either. I also ordered the skate bearings from VXB, i got 32 of them for 20 bucks with free shipping, way better than 16 for 44.99 at the sporting goods store. I should be able to post some pictures this week as i build it, maybe if you figure out what you are doing you could post some pictures as well?

I am interested to see what others come up with.

Andy