Will this work alright for guide rails?

[phantomcow2]

I am looking to make a 24x12 router, and have many of the parts already. What i dont have is some sort of guide rail system. Linear bearing slides are not worth the expense for something this size in my opinion, so im not doing that.
BUt what i was thinking was buying these linear bearings:
http://www.vxb.com/page/bearings/PROD/LM/Kit244
With 12mm drill rod like this:
http://www.use-enco.com/CGI/INSRIT?P...&PMPXNO=944459

Using 2 of the bearings for the X, and 2 for the Y. I've already got Z figured out.
Will this work alright, or am i missing something of importance?

[jeffs555]

Most people grossly under-estimate the diameter needed for unsupported rods.You would be surprised at how much a 24" piece of 12mm rod will bend. Just ten pounds of force in the center will deflect it nearly .04 inches. You probably either need to increase the diameter of the rod, or use open bearings with continuously supported rod.

[ger21]

Also, you can't use those bearings on drill rod. They'll wear grooves in them very quickly. You need to use hardened, precision shafting.

You might be able to live with 20mm shafting for a 12" axis, but even that may flex too much for your needs at 24". Supported shaft of some sort will always be the best option.

[phantomcow2]

alright 16mm may do then.
What are some other ways people are making guide rails? I see drill rod used very often, and skate bearings mentioned.

[jeffs555]

Those bearings would wear grooves because of contact area. It is pressure that causes the damage to the rods. The balls in those bearings are orders of magnitude smaller than skate bearings, so the applied force is concentrated in a much smaller area. For the same applied force in lbs, the pressure in psi will be hundreds of times greater with the smaller balls. Even the skate bearings will wear slight flats on small diameter unhardened drill rod.

Like Gerry said, even 20mm unsupported rod may be too small. Deflection varies inversely with the 4th power of the diameter of the rod. Doubling the size of the rod will decrease deflection 16 times. Since 16mm rod is 1.33 times larger than 12mm, and 1.33 raised to the 4th power is about 3 the deflection of 16mm rod would be about 1/3 as much as 12mm. A 24" piece of 16mm would deflect .0125 for each 10lbs of load.

Jeff

[fyffe555]

Originally Posted by jeffs555 Those bearings would wear grooves because of contact area. It is pressure that causes the damage to the rods. The balls in those bearings are orders of magnitude smaller than skate bearings, so the applied force is concentrated in a much smaller area. For the same applied force in lbs, the pressure in psi will be hundreds of times greater with the smaller balls. Even the skate bearings will wear slight flats on small diameter unhardened drill rod. Like Gerry said, even 20mm unsupported rod may be too small. Deflection varies inversely with the 4th power of the diameter of the rod. Doubling the size of the rod will decrease deflection 16 times. Since 16mm rod is 1.33 times larger than 12mm, and 1.33 raised to the 4th power is about 3 the deflection of 16mm rod would be about 1/3 as much as 12mm. A 24" piece of 16mm would deflect .0125 for each 10lbs of load. Jeff

Gerry and Jeff have given you good advice. 20mm could be too small for the 24" travel, drill rod cannot be used with linear ball bearings. Skate bearings have a bigger contact patch but will still wear a flat over time. Do a search here 'co theres been loads of discussion on the Zone about exactly that.

Jeff, shouldn't 'Deflection varies inversely with the 4th power of the diameter of the rod' be 4th power of the radius?... Moment of Inertia = ( pi * r^4 ) / 4

[jeffs555]

Diameter is always 2 times the radius, so it is proportional to either, and rods usually are specified by their diameter. (pi*r^4)/4 = (pi*d^4)/64

[andy_ck87028]

Gerry, Jeff

I have done as shown below on mine and am using skate bearings rather than linear bearings. The tube is stainless 31.7mm with 1.6mm sidewalls.

Does this approach raise any alarm bells with respect to wear, distortion?

Andy

[jeffs555]

Andy,
I don't see any major problems with that approach. The tube looks well supported, and is large enough diameter to have a fair sized contact area with the skate bearings. Over time it will still tend to wear a flat into the tube, but it probably wont be much. It will tend to be self limiting, because when it flattens a little, the contact area will increase, and the pressure will decrease. The wall thickness on the tube seems a little thin, but I don't know if that will cause any problems.

Jeff