[Fluxion]

I am already in the process of building a new CNC router. I have a frame and gantry built of welded steel tubing that was designed around SBR20 fully supported round rails. My frame is built out of 4"x8"x0.25" wall rectangular tubing with 2" square legs and some other bracing, its stout and weighs about 620 pounds as it sits, I estimate it will be around 780 when complete. The Gantry without the Z axis is made of 8" square 0.25" wall tubing and weighs in at 180 pounds. I would like to go to 20 or 25mm profiled rails but the cost is prohibitive. I will use profiled rails on the z axis and I am building in such a way that I can upgrade the x and y axis rails in the future. I have found some used THK 15mm rails that are more expensive but I could afford them.

So, ignoring any frame or gantry deflections, would the 15mm THK rails offer much of an improvement over the SBR20 rails?

[Bl@ckrat]

on such a stout frame going with SBR is like fitting snow tires to a ferrari ... i would highly recommend using the THK

you dont mention the size of the machine , you have to either have the frame ground flat, or you must use self level epoxy it to get it flat or you will have a nightmare to get it to run smoothly !

got any pics ?

[UA_Iron]

What's your cutting area? What do you intend to cut with your mill? (I am assuming metal with such a beefy frame).

What kind of positioning accuracy and repeatability are you looking for? What are you driving your axes with (ball screw, rack and pinion) and with what motors etc?

[Fluxion]

I'll get some more pics up later today but here is some off of my phone. please try and ignore the messy garage.

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My cutting area is 48"x48" and I will mostly cutting wood but I do make a few parts out of 1/4" G-10 and also 1/2" phenolic, mostly CE grade but sometimes XX. I have cut aluminum on my other routers (this is my 3rd build) so that may happen but I am also planning on a RF 45 Conversion after this router is complete. I was planning on driving all axis with ball screws but haven't settled on which motors, probably steppers. I will be using epoxy to level the surface and as far as accuracy I would hope to be within 0.001 with a repeatability of 0.010, that's about what I was getting on my last machine. I just don't know it the 15mm THK rails would be big enough. I won't be able to swing larger THK rails right now and I do have a few paying jobs that I have been cutting at work since I sold my last CNC router but I may be leaving for a new job soon so this needs to be done soon. Right now I am leaning towards using the 15mm rails on the x and y and shopping ebay for a set of 20mm for the z.

[Bl@ckrat]

that gantry tube is massive it needs to be capped to utilize its full strength

but no ways id put sbr's on that frame

what spindle do you intend to use ?

[Fluxion]

I'm thinking I will give a Chinese 2.2kw water cooled a try for now. I may even go bigger, I haven't made a decision just yet, it all comes down to money. I need to get this up and running and I have a set budget to get it done. If I choose to spend more it will just delay things. I agree on the tube being massive but I'm used to seeing the 12" square 3/8" wall tube on the router at work. Now that I think of it, the router at work only has 20mm rails with a 28' X axis, 5' Y axis and 11" Z axis, so I wonder if the 15mm would be fine for mine? The gantry on the router at work weighs 3500 pounds and will rapid at over 1100 ipm, it's impressive to see.

My immediate problem is that I have the router frame formed up to pour the epoxy and want to form up the gantry tube to pour it. If I was to use the SBR rails I was planning on putting one rail on top of the tube and one on the bottom. If I were to go with the THK's I was thinking of putting both rails on the front face of the tube.

[Fluxion]

Ok, I need to make a decision very soon. Are 15mm rails big enough or should I try and get 20mm rails? I found some used of each size, the set of 15mm will cost about $850 and the 20mm will cost about $1150. Both will cut into my budget quite a bit but It will be easier to upgrade some other items in the future. Upgrading rails would be a pain for sure.

[Bl@ckrat]

the rails will normally have a load rating on them , do you have a part number ? you should be able to get a catalogue ... this will help make the choice with a bit of confidence and not just be a stab in the dark

by what ive been reading the 15mm rails will probably be fine, but i dunno , when i look at them i feel so much better going for the over engineered look :P 20mm for me

$300 on the grand scheme of things aint 'that' much ... but having to upgrade later , you gotta get new rails and fit them !

[Bl@ckrat]

https://tech.thk.com/en/products/pdf/en_b01_008.pdf#1

read page 2

[UA_Iron]

https://tech.thk.com/en/products/pdf/en_b01_008.pdf#1

read page 2

I've seen that tech document before, but wow, I must have missed that part before! 14.07kN = ~3305lbs.

[ger21]

Maybe a little short, but a fraction of the price you were going to pay for 15mm's.

automation products from Automation Overstock: LY201060ALC2B01P41, NSK 20mm linear bearing/rail, NSK Linear Rail/Bearing Combos

[Fluxion]

Thanks for the advice guys. I am sorry I haven't updated this thread, I have been pretty sick the last 10 days. I did however make a decision to go with new Hiwin 20mm blocks and rails. Turns out they were within $40 of the used THK rails I was looking at on Ebay. Rails are ordered and paid for and I was given the estimate of 12-17 days for delivery. So, I guess this thread has run its course and I will be starting a new thread in the CNC Wood Router Project Log.

Thanks again, Bryan

[Bl@ckrat]

i really think you made the wise choice, even if you will have to wait a little longer to finish the project it will be worth it

[Fluxion]

This thread has ran its course for me but I am continuing the Build of my router Here: http://www.cnczone.com/forums/cnc-wo...ml#post1655360