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Mr.Chips
12-17-2004, 03:51 PM
I have seen machines made in all configurations of having the linear rails mounted on the Top, Side, and Bottom of the base.

Could I get some Pros and Cons for each orientation.

I’m favoring the top mount because the force seems like it would be handled better by the bearings.

Hager

buscht
12-17-2004, 05:23 PM
Hager, I like the idea on the right the best because you table will help to keep the bearings clean.

The bearings and rails are so lightly loaded per their specifications that I doubt if the load is a factor. It more like what works best for your design.

Chagrin
12-17-2004, 05:30 PM
The pros are all in the method illustrated by the first example and is the method that the bearings are really designed for. The proper documentation on the specs of linear bearings should be available; reading them you'll see that the bearings have different maximum load ratings depending on orientation.

Mr.Chips
12-17-2004, 07:42 PM
Here are the specs for these bearings, It's greek to me but I'm told they are OK for a all alum 24 x 36 moving gantry machine.
I like the first one (top) because I can fully support the base.
Hager

ger21
12-18-2004, 08:49 AM
I'll agree with Trent. Try to design it to keep them as dust free as possible. I think most 25mm rails are rated at about 2000 lbs. Our router at work has a cantilevered gantry with the rails on their side, like your second example. The gantry weighs over 1000 lbs, and hangs about 5 ft in front of the bearings, although probably half the weight is on the opposite side for balance. And I'm pretty sure they're 25mm rails.

Mr.Chips
12-18-2004, 01:21 PM
Thanks for the input. In light of the comments I have redrawn the two most promising configurations. Please review these and let my know if I’m headed in the wrong direction.

Requirements for X Axis:
1. Have a solid mount, free from sagging.
2. Have it orientated to optimize the rail/bearing design/life.
3. Locate the bearing to minimize dust on the rails/bearings.

Thoughts:
1. For the best solid mount I have narrowed it down to Fig 1 and 2. Because the “Base” made of ¼” extruded steel tubing will be as long as the bearing “Rail”, and have full contact with the “Table” again as long as the rail, thus eliminating the sag potential. In the bottom mount design the “Base/Rail” would have to be supported by the ends only, leaving a sagging condition possibility.
2. It’s my understanding that Fig 1 will optimize the bearing orientation. It does add a little extra height but I can live with that.
3. The sides of the “Cutting Table” will be extended as close as possible to the sides of the Gantry, helping to prevent dust migration. The extra width will be out of the cutting area, but improves dust handling. I’m thinking that Fig 1 will be the best dust proof location because.
a. In Fig 2 the bearing groove in the rail would be facing upward, and dust would settle in it.
b. In Fig 1 the bearing grooves are on the side and I’m thinking less likely to collect dust.

Summary:
Fig 1 looks to be the best orientation, for all three requirements. Again, If I’m missing something I would appreciate corrections.

Bloy2004
12-18-2004, 01:47 PM
Hi Hagar,
Your cutting table will need to be better supported (making it thicker and more rigid), unless it will be granite :), otherwise the cutting forces and even simple weight of the object will cause unwanted deviations. unless maybe you experimented with a THIRD rail under the center of the cutting table with the truck mounted to the gantry's under-side cross support. But then you would have more space under the cutting table as a result and would be able to facilitate a stronger cutting table without any third rail....that's another mess of counter force thought.

The second drawing you post is the better as it is from the point that the truck is mounted directly to the gantry upright..whereas the first drawing allows the gantry weight to "spring" the under cross support of which the truck is situated. Although weak, use of gussets and other angulations can remedy that to a point.

I'd suggest you draw up several plans exploring various options for design

Bloy

ViperTX
12-18-2004, 03:25 PM
How much do you think the gantry will weigh?

I like figure #1...just move the mounting location of the truck closer to the end and the end of the gantry should be attached to the top of the cross-member that goes to the other rail's truck, not to the side as in your drawing. Your base table could actually be the same 3 x4 x1/4 inch tubing....I suspect you're using steel because you have access to a welder....if not then you might consider the extruded 80/20 aluminum.

Mr.Chips
12-18-2004, 03:48 PM
Bloy and Viper
Thanks for the comments. More gladly accepted.

I'm thinking the cutting table would be the rectangular 8020 extruded aluminum in the vertical position and topped with not what I’m sure yet. I’ll post questions/concept on the table construction in the future, when I have some ideas on it.

I’ve redrawn the configuration in Fig 3, moving the Base/Linear bearing as far as possible to the side. Hoping this will minimize the "spring" of the cross support you mentioned. The cross support will be approximately 9” in the X axis and trucks mounted at the extremes to get the most support.

Haven't seen anyone use a third rail configuration. And it would get in the way of the ball screw which will be in the center of the table

Hager