Thomson Bars, V-Rails, or Linear Rails

[Chuck_M]

I am in the process of designing my own CnC Plasma Cutting Table, starting with a 4x4 table that could be expanded to 4x8'. A small shop table aimed at general cutting and some art work.

I find 3 different options for linear transport ... V-Rails, Thomson Bars, Linear Rails, in roughly that order of cost.

If cost were not a factor, is one of these more appropriate than the others for the Y and X axis?

[Glacern]

Linear ball bushings with supported rails are our preference for shop built CNCs. That is why we'll be offering them on our web store later this month (around xmas time) with rail lengths from 13" to 120" at very affordable prices.

Linear rails are pricier. They can be very accurate and have very high load capacity, but they're only as accurate as the surface to which they're mounted. VMC castings are very flat and straight. Steel tubes and aluminum extrusions... not so much.



-Sol
Glacern Machine Tools

[Chuck_M]

Thanks for the info.

I think it was THK that I asked about linear ball bushings vs. linear rails, as they manufacture both. They said that the higher load rating of the linear rails made them cost competative with the linear ball bushings. They also said they were trying to move customer to linear rails for all new applications, and kept the linear ball bushings in their line for backward compatibility.

But I guess the real advise is to not use v-roller bearings.

[Chuck_M]

Glacern,

P.S. Can you say at this time which Manufacturer you are going to carry, Thomson, NB, Pacific Bearing???

[Glacern]

Chuck,

We'll be carrying our own brand. Our products are made to similar quality, tolerance, and load specs to those of the name brands, but will be priced much lower. Our ball bushings and hardened rails are made for us in Taiwan, while our aluminum pillow block housings and rail supports are extruded in the USA.

I was not aware that THK made linear ball bushings and round rails. Their linear rails are certainly top notch, as are the similar offerings from Hiwin, NSK, Bosch Rexroth, etc. For high precision mills and lathes, profile rails can't be beat.

But for wood routers, plasma cutters, and other machines with looser required tolerances, the inherent design features of round rails offer significant cost advantages. Round bars are centerless ground, a process that is much less expensive than profile grinding. Extruded aluminum pillow blocks are also much less expensive than steel blocks which require many more steps to complete. Also, since the blocks are free to pivot a few degrees about the centerline of the rail, they can prevent binding when a pair of rails are mounted to surfaces that aren't coplanar.

-Sol
Glacern Machine Tools

[Chuck_M]

Your comments on cost are interesting. I priced a pair of THK SSR15XTB1SS cars and SR15-1480LY rails for ~$700 on their Online store. I compaired this to the NB Ball Bushing TWD10WUU cars and WSS10x60" (5/8" dia./full support) for ~$800 on the VXB Bearing site. Now these are at the low end of the scale load wise. Haven't looked to see how this would compare at larger sizes.

I have found that the continuous supports for the round rail adds a significant percentage to the cost. At least for the low loads I calcuate for a 4' x 4' Plasma cutting table, it appears that the two are a wash.

Also, I have just seen seen some really nice looking used THK rails on EBay HSR35s x 1640mm. For little more than the new SSR15s. However, the Ebay store describes them as in excellent condition, but sold as is, with no returns. Ya Gotta Luv It.

[judleroy]

The 20mm (a little over 3/4") round linear rails on vxb are $159.00 for one fully supported 55" rail with 2 pillow blocks. That would be roughly $320.00 for a set before shipping. You can get a set of 2-60" 20mm medium duty linear rails with four bearing blocks made by hiwin from automation4less for under $600.00. judleroy

[Chuck_M]

Very interesting. judleroy you are correct. The metric line on VXB is less money. They also do not carry the NB label, which I hadn't noticed before. So they appear to be an unnamed brand. Also they state hardened rails but don't specify a Rockwell spec, where the NB line states case hardened to 60-64 Rc.

Sometimes you get what you pay for, and other times you only pay for the brand name. The trick is to know which is the case. Anyone have any insight on this?

[cyclestart]

What type of side loads are ok for supported round rail ? The Y axis is a Lintech table from 'here-and-there'. The attached picture shows what I have in mind. Ideally I would like aluminum milling capability. Who needs CAD when scrap lumber is handy ?

btw: You have the cleanest machine shop I've ever seen judging by the videos.

Edit// According to numbers published by Lintech, that little table tolerates surprising side loads. I'll do it Old School...... build it and see what bends.
Now back to the regularly scheduled discussion...............

[tobyaxis]

As a note, "Stay away from Thomson Linear at all costs"!!!
Made in Mexico and they do not have a good quality control system.