router design trade-offs

[Drools]

Am I correct in thinking that a moving gantry style router is great for maximizing your usable space but it comes at the expense of what?
A fixed gantry benefits from what but you do not get full travel on the axis.
Then there is the mill (XY table) type setup, obviously your work area is smaller but what are the benefits?

Thanks for the help.

[Drools]

I didn't think this would be such a tough question. From watching the builds on here I have my assumptions but I was hoping for some clarification.

[Bubba]

Drools,
My take on it the fixed gantry is more rigid at the expense of foot print.
Whereas the moving gantry has a smaller foot print at the expense of rigidity.

Think about all the threads where the moving gantry folks talk about "racking" of the gantry and the need for two lead screws or some other method of keeping things square.
It all boils down to how much realestate you can afford to put into the machine you want!

[Drools]

Thanks Art, that is pretty much my take on it as well but you never see (well I have not seen any) larger fixed gantry routers. All the fixed gantry builds are smaller envelope builds 20"x20" around there.


Thanks

[Bubba]

Basically, as the the travel of the table goes up, the footprint increases drastically. I am working on one right now that will have a 14x14" travel and the table movement axis is right at 30" long. If I had a moving gantry, it might be closer to 18" long. But for this size, I feel the rigidity is more important than the foot print. Now if I was looking at say 36" movement, I would need upwards of at least 6' of length to get that kind of travel and that is a lot of shop space!

[Drools]

Have you seen any hybrids where the original machine is a moving gantry but if needed for small fine work a moving table frame could be mounted and the gantry locked into a fixed position. Yes you would effectively loose half of the gantry travel but you could now do that PCB that requires the rigid gantry.
I have done PCBs with my moving gantry machine but I have to run it at 20ipm or less to keep the pads round...
Think&Tinker wants their bits to be moving at 45ipm or over for best results.

Thanks

[louieatienza]

Originally Posted by Drools Thanks Art, that is pretty much my take on it as well but you never see (well I have not seen any) larger fixed gantry routers. All the fixed gantry builds are smaller envelope builds 20"x20" around there. Thanks

Some large indudtrial machines are made with a fixed gantry... Check out CR Onsrud's Extreme Series for an example...

[Bubba]

No, haven't seen such a hybrid. It would seem that you would loose and awful lot of Z movement to do that. I designed my machine primarily for doing PCB work as I have a mill drill that is for general purpose work. At present, the spindle will do 30,000 rpm and if plans go as predicted, it will be good for 70,000 with mod. (read full ceramic bearings)
My personal opinion is that a PCB etcher/drilling machine is a specialized unit that has unique requirements for its purpose. For instance, I read recently that one had measured the power requirements for doing PCB work and it was 10's of watts not 100's or multiple HP! I agree with you that we need accuracy/repeatability more than absolute speed. One source quoted a feed rate of 1IPM per 1000 rpm of the spindle as a good guideline. So this is what I am shooting for with my setup.

[Drools]

I asked that question as sort of a primer to this one. My new machine is a moving gantry 48"x38", I have an extra set of 48" THK rails that I could make a frame for (existing parts on hand) and mount this sub frame and basically have a moving table. I do have the Z travel to this. It is either that or take the left over parts and make a dedicated small machine for doing PCBs, which would require more electronics, more steppers etc. I have 2 sets of THK 24" rails which I was planning on using turning into a dedicated machine until I got this hair brain idea.

[JohnZ]

Originally Posted by Drools Thanks Art, that is pretty much my take on it as well but you never see (well I have not seen any) larger fixed gantry routers. All the fixed gantry builds are smaller envelope builds 20"x20" around there. Thanks

The build I am working on right now has 48" x 32" travel with a fixed gantry. The footprint is 55" x 39", but the table cantilevers over the edge of the base frame when in use by a foot or so out into the walk space in front of the machine. I could not put it against the wall due to the concrete foundation sticking out from the wall, so I only lose about 4 or 5 inches back there for the tables motion. When not in use the table is centered so it does take up any extra space. The table is mounted similar to a mill drill table with the two bearings mounted in the center of the table on a 36" supported rail resulting in 32" of usable travel.

[Drools]

Do you have a build log John?

[JohnZ]

Originally Posted by Drools Do you have a build log John?

Sure do, but it has been several weeks since an update. I'l have to head on over there and post an update soon?

http://www.cnczone.com/forums/cnc_wo...e_rebuild.html