Newbie question on Gantry design

[Tartan5]

Hi Everyone,

I have a quick question on gantry design. I noticed that all of the designs I've come across (so far) have a large Z offset between the X and Y rails. My concern is that this would apply large sideways forces to the X bearings when cutting in the Y axis.

My thought is to extend the base legs up to the Y axis plane, giving a rigid structure up to the Y axis plane. Then the x axis rails and the Y axis rails would be mounted within the same plane (above the work surface).

The additional benefit is that the x axis rails would be subjected to less debris from the milling process.

Can anyone think of other pros / cons? Are there examples of this type of gantry design around?

Best regards,
Howard

[jsheerin]

Yes, there are many examples of this construction. One disadvantage is that your y axis needs to be longer between bearing supports for a given work envelope.

[Tartan5]

Hi Jsheerin,

Thanks for the reply. I'm not exactly clear on what you mean....Do you have an example in mind that I could examine? Does this type of gantry have a specific name that I could search the forums for?

I love this site, but it is so freakin' HUGE I feel like I wander around lost half the time

Thanks again!
Howard

[jsheerin]

I'd read peoples build threads in this forum to find some examples. Here's a commercial one although there aren't many good pictures:http://torchmate.com/products/routermate/

On the disadvantage, picture a 4x8 sheet of wood - say that is the size part you want to cut. Now picture the x beams under the edge of this sheet of wood sticking out just enough for the x bearings to clear the material. Then you have vertical pieces up to the y axis. The y axis can then be longer than the material to allow the spindle to move past the edge of the material so that it can cut all the way to the edge with whatever the largest diameter cutting tool will be. However the unsupported length of the y axis between the vertical pieces will be roughly the width of the material (4' in my example). In contrast, if you put the x beams above the material those beams need to allow space for the material between them plus the width of the z axis plus the diameter of the largest cutter you'll use.

[ger21]

I don't like that design, because it's much harder to load material. I think ShopBot's use that design, and the Mechmate does too. www.mechmate.com

[florins]

Hi Tartan5,

See this thread http://www.cnczone.com/forums/showthread.php?t=107251. This is a design close to what you described above, in my opinion. The router is not finished, but from the first tests is showing no signs of vibration or instability. More, loading the material is actually easier from the sides of this model (and, with proper support, you can load oversized material as well).

Hope this helps,

Florin

[Aryantes]

I wanted to create a machine with similar design, with raised X axis rails.

I think lowering the height of the gantry seems good to do, or, at least, keeps your linear motion point closer to the middle of your gantry instead of it's legs.

The reason why I didn't make a machine like this, is because I wanted to use 80/20 1530 series extrusion and it would increase the cost of my 3'x5' machine by a few hundred dollars.

There's a build thread around here somewhere using a FineLineAutomation kit that illustrates exactly what you are envisioning.

[Tartan5]

Hi Guys,

Thanks for the info and links. It definitely gives me something to think about.....

My 8020 raw stock finally came in last night (1545, 1530, and 1515) that I won off ebay. It's getting time to choose my path.....

Florin, I hadn't considered having my Y-rails straddle the gantry......How is it working out for you? Also, I have been considering using the supported linear bearing system like you....How well do these stand up to chip debris?

Thanks guys!
Howard

[louieatienza]

Howard, both styles of machines have advantages and disadvantages.

Raised x frame advantage:

- can make the gantry attachment stiffer (or reduce load on bearings)
- linear and drive components above the work area
- allows under-table components, storage
- machine could theoretically be placed on TOP of workpiece if necessary

Raised x frame disadvantage:

- legs or tall frame needed, must be strong (though simple to implement)
- PITA to place work over rails into machine
- Usually necessitates dual leadscrew, or R&P, timing belts, dual stepper/servo

Tall gantry advantage:

- ease of access to table area
- less material
- smaller footprint
- leadscrew can be under table, protecting drive nut

Tall gantry disadvantage:

- longer lever, puts more strain on bearings; weight and CG more of an issue
- more potential flex in gantry legs
- if leadscrew under table, limits under-table options
- machine usually cannot be placed over a work surface

I don't see why some of the better aspects of both designs could be incorporated, for example raising the x rail height, and lowering the gantry height, to have more of a mid-rail design.

[florins]

Originally Posted by Tartan5 Hi Guys, ...I hadn't considered having my Y-rails straddle the gantry......How is it working out for you? Also, I have been considering using the supported linear bearing system like you....How well do these stand up to chip debris?

Hi Tartan5,

The two sides supported rails axis is looking OK after the first tests, and the movement is smooth and with very low drag (push the Z carousel at one end, rolls fine 1 meter away). I still have some small fears on the medium term. The way that these rails are designed to work is a bit different than the way I am using them, so I took some chances ... and precautions - I locked the rail support screws (the ones that are holding the round bar on the aluminium support) with thread locker before installing the rails. I'll let you know how it is working on long term.

I intend to install bellows in order to protect the rails and the screws, so the chips should be no problem.