[jbro2]

So i have been looking around for about a year on any info on building a cnc router, at this point i have been collecting as much as possible in the way of parts from scrap stuff that i get thru work. Currently i have a 1''x2'' steel tube frame that is a rough size of about 80'' long and the 60'' wide and the gantry is going to be about 64''1/2'' wide 33'' tall. i would like to make it as economical as possible but we all have to make compromises someplace. So im not apposed to resizing anything. With an 80'' span on the x axis the gantry will move on, what is the best LM setup for such a long distance. Im assuming that the gantry with all the peices constructed will be an est. weight around 150lbs +- 20lbs. with that said will a 7/8 linear bearing shaft with the right bearings stay true with the movment along the x axis. I am more worried about the bearing shaft flexing downward in the center and with lateral gs fwd and back. Also i was thinking one center lead screw to move the entire gantry. Any advice is appreciated as this is the planning phase so i want this to be spot on before i buy anything. Any other info would be appreciated if im missing something, this will be my first build of this type.

[Bear5k]

I'm not sure I follow the issue. The LM rail must be attached to a frame structure, so "flexing downward in the center" will be more a function of the frame than the rail (deflection). The rail will provide some incremental support (and load), but the frame is the real issue for deflection.

That being said, you do want to be sure that you are within tolerance for the load the trucks will be able to bear along that axis. Here, it becomes important to look at specific mounting strategies and rail/truck types. For example, the THK rails I am using (SR30) have a significantly higher load capacity with radial loads (think orthogonal to the plane of motion) than with axial loads (think "sideways"). With my design, and by using dual trucks (or carriages), I have a large safety factor even with an axial load. Other types of THK linear rails (e.g., SHS series) have "equal load" capability. Unfortunately, specificity matters here.

That being said, with as much weight as you have and a 33" (or more?) tall gantry, you may run into issues with the force from accelerating or decelerating (i.e., moving) the gantry. Here, you want to make sure your dimensions take into account the relevant load arm (e.g., from the attachment point to the center of gravity). Your dimension of 33" sounds a little on the tall side for a moving gantry for a typical DIY build, especially using steel. I'd also worry about your weight estimate since I'm around that value, all-in, with my own aluminum frame (plus steel ballscrew and rail) gantry that's a foot shorter. For your weight estimate in figuring load, be sure you are accounting for all of the pieces (frame, running gear, motors, spindles, etc.).

Hope this helps.

[Jawno]

I think if you are talking about using a 7/8" round shaft 80" long to support your gantry then that is gonna be way too light for a 150 pound gantry. Droop in the center will be a problem. I would suggest looking at something that is fully supported along its length. You might look at other peoples designs and build something similar. these things can be very expensive and I'm sure you don't want to buy parts that you'll be unable to use or want to replace right off. I don't think any machines that large use the round shaft and linear bearings for movement. That is more of a desktop design.
On the other hand maybe I misunderstood your question as well. If so please ignore this advice. also of course I have no idea what you'll be cutting. I would still worry about droop though.

[hub]

Agree that the shaft must be fully supported. That distance would need a huge shaft, if supported in the ends only.

[Bear5k]

I think if you are talking about using a 7/8" round shaft 80" long to support your gantry then that is gonna be way too light for a 150 pound gantry. Droop in the center will be a problem. I would suggest looking at something that is fully supported along its length. You might look at other peoples designs and build something similar. these things can be very expensive and I'm sure you don't want to buy parts that you'll be unable to use or want to replace right off. I don't think any machines that large use the round shaft and linear bearings for movement. That is more of a desktop design.
On the other hand maybe I misunderstood your question as well. If so please ignore this advice. also of course I have no idea what you'll be cutting. I would still worry about droop though.

A 7/8" shaft over 80" unsupported is going to have a pretty good droop even with no other load than the shaft itself (measurable). It certainly won't be sub-millimeter accuracy for the total machine. There definitely needs to be a substantial frame if one wants to realize decent accuracy.