Q: Y Axis - Gantry vs. "bridge" designs

[Zoid]

Hi all!

I've been lurking here for weeks, well, actually months. I have completed a design for a 24" x 30" for routing foam and soft woods. I've started purchasing frame and bearing materials for the X Axis.

But I have a question about the Y Axis. My design, like many others here, has a "vertical bearing" moving gantry - that is, the Y bearings are in a vertical plane.

I have seen a few designs where the Y bearings are in a horizontal plane, I guess I would call it a "bridge" Y axis - it is like an industrial bridge crane. My design could be easily revised to use this for the Y axis.

What are the pros and cons of a vertical gantry vs. a bridge style Y axis? Does the gantry have some special benefit that I'm overlooking? To me, a bridge would be easier to build but I don't see it used very often.

Thanks

[samualt]

I chose to use the bridge type. That is, both Y-axis rails are horizontal. And the z-axis is mounted inbetween.

=========Y
Z
=========Y
Top View

This way the weight is more evenly distributed as well as other forces. See Finagling the Gantry in a previous post.
Actually, I don't think it matters that much if you have a stiff design. But seeing as how this is my first router and it's pretty big I decided to give myself the best chance of it working ;-)

[Zoid]

Thanks for the "Finagling the Gantry" reference, somehow I missed that thread. That "validates" my thinking. The bridge has no drawbacks that are readily apparent. I'm going to fire up AutoCAD tonight and redesign my gantry. My concept is pretty much like you show yours - Z Axis between the 2 Y rails. It just seems more stable with respect to torque around the Y axis.

Thanks. When I finalize my design I will post some CAD pics and photos. I'm using Bosch Structural Aluminum (like 80-20) and Bosch linear bearing assemblies. I haven't seen any posts here that have used Bosch bearings. I'll post the good, bad and ugly.

[High Seas]

This spot seems "bout right top post my Bridge details for the Y-Z axis. This is lookingg along the Y Axis at the Driven side of the Z Axis. The Steper Motor is on top of the Z Axis (I'd move to the bottom and offset the Router/spindle -- in a future mod).
Note the rails move up and down with the Z axis - and the cars are fixed.
Here Goes:

[High Seas]

Here is a side view - tha may better show the relationship of motor and Z axis:
NOTE - the Z axis cars are centered (vertical) between the dual cars on the Y axis.

[High Seas]

Looking from the opposite side of the earlier photo - along the Y axis: (router side)
NOTE the Z Axis is a complete "car" or channel set up. I'll be building a set of mounting plates to let me "swap" out the Porter Cable Trimmer with a Dremmel, or the homebuilt digitizer probe I'm starting on soon. The channel will help in alignment - but is a bit heavy and could save some weight by using some "rational" drilling of holes!

[jstuedle]

Your design looks very sought, indeed. Instead of drilling holes to reduce mass, you might be better off milling pockets to reduce the thickness in non-critical areas to about 1/8". Use a moderately large end mill with about 1/8" corner rad. to reduce stress in the corners. You will be able to reduce the weight a greater amount over a larger area using this method without a noticeable reduction in strength. OTH, the extra mass will lend to less vibration and harmonics, with more apparent rigidity. Just a thought.

[High Seas]

John - I like the "drilling of pockets" idea you passed along. And the details of endmill and radius must be from some experience. Thanks for that.
Now I'll just have to get that other machine built first - so I can take this one apart!
Can never be easy - eh? Cheers - Jim

[jstuedle]

My experience is in aircraft engine test and design. My only home cnc machine is a pc board driller. The "thinning" concept is used heavily in aircraft airframe and engine design. It leaves gussets or ribbed areas for strength and leaves the "meat" in areas where extreme strength is needed and where other parts are mounted.

[High Seas]

Righto Mate - That's where we keep the cookies and snacks (the cockpit cubbies) for inflight lunch on transatlantic delivery flights! Surprising how much junk you can stuff into a fighter's cockpit - but when you unload (neg gs) watchout!
cheers - Jim