[Project_Hopeless]

So the gantry beam is supported by 4 bearings (2 each side) for table travel. Ideally I assume you want the cuting tool dead center of the 4 bearing. But That is rarely the case for home builds.

Can the cuter be outside the bearing blocks? Are there any do's and don'ts?

[wmgeorge]

Best way to get your answer is to look at some ones already up and running, including commercial ones.

[wizard]

So the gantry beam is supported by 4 bearings (2 each side) for table travel. Ideally I assume you want the cuting tool dead center of the 4 bearing.

Many arrangements are possible. Which one to choose depends upon what you want to get out of the machine.

For example a wood worker that wants a machine that can do end work on a vertically mounted board might be happy with a router that overhangs the bearings by a bit. This would allow working vertically mounted boards at one end of the machine. There are other design issues to consider such as the gantry axis (Y) being able to move past the vertical gantry supports. It really comes down to what you want to get out of the machine and your ability to build it in a solid manner.

But That is rarely the case for home builds.

Unless you have a well equipped home shop, access to one and plenty of building experience, it is best to choose the simpler designs. As long as that simple design is mechanically robust you will have far fewer problems building and running the machine. I see a lot of really complex gantry beams in this forum as an example of making more work that is really required.

Consider the arena of tool room or manual lathes. You can buy a machine today that hardly deviates from what was built in the 1960's. Why? Because they have found the right balance between complexity and functionality.

Can the cuter be outside the bearing blocks? Are there any do's and don'ts?

Sure the cutter can be any place you want it to be. In the ideal world you minimize overhang, have infinitely stiff structures and and materials that never transmit vibration. The ideal world doesn't exist so you have to go with a design that best addresses your needs.

As for Do's and Don'ts, this often comes down to opinion but I can add my thoughts which I'm sure deviates from others. So:

1. Do keep a design simple. Complexity costs in many ways.
2. Don't use extruded aluminum for critical parts of the machine. This can be controversial as many do use extrusions to good ends. Also be aware that extrusions vary widely in suitability for machine tool usage.
3. Do use profile bearings if at all possible. Frankly I see this as a critical point if you are looking for maximum performance.
4. Don't be buying controls or even linear motion parts until you have the basic design down. About the only qualifier here is if you find extremely discounted used parts and design around them.
5. Do try to nail down size requirements, that is the largest pieces you intend to work on. With these dimensions in mind, add a few inches to support fixture machining and such.
6. Don't waste money on oversized parts or likewise undersized.
7. Do understand how twisting moments will impact the machine, especially when it comes to the load on bearings.
8. Don't forget that your design needs to accommodate ancillary structures. Here I'm thinking drag chains, switch cams, air hoses, vacuum hoses and the like. Things like this can be significant.
9. Do have fun!

I'm off to bed so I hope that this is coherent.

[ger21]

Ideally I assume you want the cutting tool dead center of the 4 bearing.

Not necessarily.
For cutting forces, yes.
But consider the weight of the gantry, and where it is in relation to the bearings.
Most likely the other forces will be higher than any cutting forces, and should take precedence in the design.

[Project_Hopeless]

I'm off to bed so I hope that this is coherent.

Yes very much so, the feedback is definitely appreciated.

I feel I have enough smarts, just not the equipment or tools on hand to build one the more advanced router builds. So I've settled on a simple but functional wood/plastic CNC router/pattern cuter, this is my immediate need. I'm trying to keep cost reasonable, hence the SBR bearings for X and Y travel. Everything is a compromise.

[cyclestart]

My router has sbr rails mounted to the top/bottom of the gantry beam instead of the front. This has the advantage of bringing the spindle slightly closer to the beam. It has the disadvantage of being a bit fussier to assemble.
This guide posted here many times, it gives a good idea of the forces involved:
Link --> Do it yourself CNC router: Design Considerations, the Gantry