Very Large Gantry Router Questions

[IAD]

Hullo...

I'm a rank newbie here, and I've got a few questions, regarding the design of a 66"x 40" x 10" gantry router... I'm looking to build a design that's accurate (inaccurate?) within ~0.015" (obviously, [much] more accurate would be better) for milling foam, and possibly some balsa. (Very light duty, sideloads measured in ounces, around 10 oz. max..) A friend of mine is building the driving hardware, so I'm all set in terms of interface/driving board.

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Attached are a couple sketches of my current design... (Dimensions in inches.)

I'd be using 25mm/1" open linear ball-bearing units at all bearing points (http://www.vxb.com/page/bearings/PRO...ystems/Kit7445 or similar) and would place supports under the X-axis (66") rails, every 12" or so. (Alternatively, could I use closed bearings, and allow the gantry support columns to rest on the router bed, taking the weight loading off the rails?)

The spindle would be driven by a Dremel, with a flexible pen-tool attachment, though I'm definitely open to other (more powerful) low-dollar solutions. (As currently designed, the Dremel would actually hang on a slide above the router, with the pen-tool clamped to the Z-axis, to minimize weight loading on the gantry assembly.)

For axis driving, I'd be using four stepper motors; one driving each gantry column (to prevent skewing) and one each on the Y and Z axii, with Acme leadscrew and dumpsterCNC.com anti-backlash leadnuts on all axii.

As for questions:
Where does one purchase shaft/rail material in 6' lengths, at "reasonable" cost? I know these things are expensive, but I would like to do this as cheaply as possible. (Obviously, it needs to have precise OD, if I'm going to use the ball-bearing units. I am tempted to use one of the adjustable rollerblade-bearing designs, but the amount of machining required puts me off a bit.)

All structures shown would be built from MDF, with a few pieces of plywood here and there. Again, I'm open to alternatives. (I have access to a manual milling machine, lathe, etc.) I'm aware that I need a 'box' type structure to better support the Z-axis. Would MDF be sufficient, or should I redesign the gantry columns etc. to be fabricated from aluminum plate?

If I were to drill both of the supports for the X-axis at the same time (clamping both blanks in a milling machine, and drilling away) would it be fair to say that I could count on the rails being parallel? If not, what sort of rail adjustment mechanism would be recommended? Also, how wide should the supports be, in order to achieve some approximation of cantilevered shaft/rail mounting? (Right now, I'd be using several layers of MDF, for a total width of ~2".)

I'm sure I've made some gross oversight somewhere, so any input would be much appreciated.

~Luke

[Coogrrr]

I am using the 20mm linear setup from VXB I love that stuff BUT!!!! the to Y rails you have will flex even at the 25mm size so what you need to do is support them vertically some way. I love the design but I am thinking that 2 flat bars sitting vertially and joined to the points at each end just below the y rails will give you more support and stop the flex you will get the beaings need to be the "c" style ones or open rather than full circle ones. this will stop the issue cold I think.

I can whip out a sketch for you and would direct toward my thread and the pics I post mostly the last one dealing with this flex issue.

http://www.cnczone.com/forums/showthread.php?t=29877

Let me know on the sketch

Coog

[IAD]

Ok. Oh, golly, so the post went through, huh?

That means there could (must?) be about 5 copies of this thread...! (If so, VERY SORRY! Mod, can you clean that up?)

It kept telling me "the thread ID was invalid, please contact the admin"....

~Luke

[IAD]

So, you're saying I should tie the two columns together with something flat and rigid, and then place supports between that and the Y-axis rails, right?

Again, instead of using the C-bearings, can I allow the Y-axis carriage to slide on the flat supports? (I'm just tempted by the VXB 20mm "4-closed-bearings-for-$25" set.)

I'm thinking I could put some round-headed bolts on the bottom of the carriage, so they would be adjustable height, and could slide along some steel square stock, or similar?

(See attached.)

~Luke

[Al_The_Man]

Check out the Pacific Bearing products Supported rail and lube free bearings.
http://www.pacific-bearing.com/produ...oduct.cfm?ID=1
There is an application mentioning a Router Manuf.
Also Misumi have similar products.
Alos ITEM Extrusion Products sell a profile that stainless bearing rod clips into the profile.
Al.

[Coogrrr]

in your last drawing you show the support but the gantry is wrapped around the rails insted remove the bottom portion where it sits between the flat bar and the round bar and go to a open bearing like in my thread.

VXB link
http://www.vxb.com/page/bearings/PRO...Systems/kit989

then you can use 1" of flat standing vertical directly underneath.

pics attached.

Coog

[IAD]

That's what I was asking, though... The closed 20mm bearings are 50% less expensive than the open 20mm/25mm bearings. [cheapskate grin]

I'm guessing that I'd be playing with fire (friction) by allowing the carriage to be directly supported by the anti-flex member, the way I sketched it?

~Luke

[Coogrrr]

you want as smooth as possible movement so friction is bad BAD IAD BAD!!! dont cheapskate when it comes to the rails and motors you'll only pay twice insted of 1.25 to begin with.

the open bearings are your ticket and that design is nice with a double bar gantry and some verticle supports it will be solid and run reall good plus your z-car sits on 2 sides of each of the bearings.

I am a long way into mine and I would (if money permitted) change to this design of yours in a heart beat! In fact I might have to as I am not liking my dual rail vertical solution I get horizontal flex where you would get vertical.

Anyway AWESOME design and yes I am contemplating a change....

Coog

[Coogrrr]

why do you have 2 rails per x axis? if they are going to be supported (I suggest the same as the Y axis chat) then one would be fine and it would be easier to build and align.

Thoughts?

Coog

[Oce]

Looks very nice IAD, I'm planning for the same thing with my design.
But:
Which way does the gravity work?
I would suggest to "flip" the C-bearing blocks so that the opening is facing downwards. Then you could add some kind of supportblock if needed, and the weight distribution around the bearing should get a lot better... Just simple thougths and physics thou, I still haven't recieved mine...

[IAD]

The paired X-axis rails were used to helped reduce the sagging at the center, by halving the weight load that each side had to support.
Changing the design to use the C-bearings, the only reason I can think of for leaving them is to make each gantry column a rotationally rigid structure, which would support those massively overbuilt gantry columns nicely. But you're right, there's really no need, and the ~6' rails are quite expensive.

I would definitely orient the opening of the C-blocks downwards. The design shown was originally designed based on 'tube-running-on-tube' bearings, but common sense gained the upper hand.

I learned about the going-cheap-costs-more-in-the-end thingy... Both by observation and experience! Before I start buying, I just want to make sure I've explored all options.

~Luke

[IAD]

I just had a rather crazy idea, but I guess it's worth asking about.

What if I added a 2-3 rollerblade bearings as shown, to support the weight? Added friction should be negligible, and the columns don't have to be made any taller, to allow the 10" clearance I want underneath. Of course, how straight is ~0.25" x 1.25" steel/aluminum flat stock?

Anyway, the long axis will use the C-bearings no matter what, for simplicity's sake.

By the way, the CAD drawings I've been posting are really just sketches I've done, to try and get a feel for how all this will work, so many things are not shown in any sort of build-worthy form. (Just my disclaimer, for component-size errors in the pictures, and what-not.)

~Luke