New Machine Build- Fixed Gantry Router Bearing Position Q?

[Gliderask21]

Fairly new to this but reading a lot so please be gentle

I am looking to make a small moving table router about 12" x 15" cutting area with the 15" being the width of the fixed gantry.

Looking at various designs can anyone advide me why the bearing blocks shouldn't be placed on the base with the rail on the moving table. To my mind this would provide constant support under the cutter, the downside being the longer bed and larger unsupported overhang. Is ther a good reason for not doing it this way??

I have attached a quick drawing of the two methods.

Thanks in advance - always willing to learn

[Festus440]

Another newbie here (post #1).
I had been wondering the same thing.
Seems like it would work for a small machine. I would think it could help stiffen the table a little.

[mlabruyere]

The best practice is to mount the rails to the most rigid and stable platform possible. This is normally the support structure not the table. I wouldn’t use the rail to add rigidity to the table.

[JD_Mortal]

I don't even have to look at the drawing to know what you are talking about...

There is nothing "Wrong", mounting that way. There are just different considerations to take into account...

FYI, My setup uses one set of reverse rails, so that I gain both the stability of the fixed-location slide, and the addition of a wider and more stable footing, and greater mounting surface.

Floating rails will require a longer table surface, and provide the worst "true" surface. Both the table and the rails will flex. The table surface will need to be excessively thick to combat the length-flex.

Solution 1: Thick "I" or "T" under-supports, with the slides mounted within the sides of the "I" or "T". Like this, o = rail, x = screw. ][o][x][o][

Solution 2: Use mounted glides, round or slotted or plate. Using fixed-rail guides essentially merges the rails into the support structure, essentially eliminating excessive support structure and excessive flexing. Saves on weight, accuracy, and vibration.

The down-side to using reverse-mounting, is the fact that you are now pulling the weight of the entire track, as opposed to only pulling the weight of the slide-mounts. With a light track, this is not an issue. With a 300 lb solid stainless steel, mounted track design, this is redundantly excessive dead-weight. Much easier to slide just the pillow-blocks. (Remember, you are also pushing the weight of the slide-table, the milling object and the milling clamps. You are also removing that large mass from the weight of the stationary device, and placing it onto a moving device. That will make the mill shake from the momentum of the heavy moving table, and will place additional "breaking resistance" damage on the screw and motors. If it takes 200watts to move that mass, it takes 400w to stop it, and make it change direction.)

I am using aluminum rails, and MDF structure. I don't have to tackle many of the issues above. I am also only reverse mounting my plunger (z-axis), so I have the best possible precision while milling close, as opposed to having less accurate precision at all depths. (While my plunger is retracted, like for PCB work, the distance to my supporting bearing is about 2" from the drill-tip. At full extension, there is an 8" distance from the bearing. In most normal setups, the distance is always 8" while the tool is up or down, which yields low accuracy, in the whole range.)

[Gliderask21]

Thanks for the fast responses guys,

I don't think weight will be a major issue as its only a 12 inch long travel (not going to be 300lb, I hope :-)). Although there may be a risk of flex on the overhang it should be fully controlled within the cutting force "window" between the bearing blocks this obviously assumes the bearing blocks are suitably fixed on a stiff base.

With the bearing blocks mounted on the moving table and cutting at the extreme travel points there is no support under the cutter and I would have thought that it would be likely to flex even more than if it was only supporting the weight of the workpiece as in the moving rails method.

I'm glad to hear you are using this method for the Z to provide closer support, albeit variable distance, to the cutting tip. Some of the guys on MYCNC in the UK have done this and recommend it.

Still working this one out so any more info or experience will be gratefully received. Keeping an open mind to get it right.

[Festus440]

In case you haven't seen it, here is a fixed gantry design that has a cutting area similar to what you specified.

DIY 80/20 Aluminum Extrusion CNC Machine | 8020CNC.com

[restless1]

Take a look at this thread

http://www.cnczone.com/forums/diy-cn...tml#post849702


oops did not read enough hmmm so your flipping the conventional set up hmmmmm different..