I had a discussion with another 'Zone member this morning and he was telling me about the problem he had with sag when his router was in the middle of his Y axis. I'm saving parts for a large table myself and have been thinking about this too.

The solution I'm thinking of trying is to attach a parabolic arch across the axis-something like a 1/8" thick by 1" wide steel strip with a 1" cube screwed to each end and drilled through.

A bracket on either upright would be slotted to accept the cube and a retaining pin would pass through the bracket, the cube, and the other side of the bracket. The arch would be a little under-curved, so that it would have to be bent into shape like a bow is when stringing it. This would ensure initial tension.

A length of small angle would bolt across the top of the Y-axis. The vertical portion would be drilled though every few inches. Bicycle spokes would be fed through these holes and up through corresponding holes in the arch. The same end pieces that are used on bike wheels would cap the spokes and allow tensioning.

This would be best suited for use on a very wide axis where the build material for the axis is extruded aluminum, like the 80-20 stuff. It should allow for a lightweight yet sag-free axis.

Thoughts / comments? :idea:

The downward force (gravity) exerted on a gantry by the weight of the router whilst in the middle Y position would be almost negligable compared to: (a) the upward force caused during a pierce in aluminium or (b) the frontwards/ backwards forces and/or twisting forces exerted upon the gantry during cutting of say MDF where sometimes you have big diameter tools + fast travel speed + thick material. If the gantry sags when the router motor is at middle Y point then I'd hate to imagine how unaccurate the cut parts would be due to the flex in other directions. The only exception to the above would be if one had used a profile (for the gantry) where the width was much wider than the height. (5" wide x 0.5" high for example) Yes, in case there would be minimal front/back flex but an unacceptable amount of up/down and twist movement. (i.e. poor choice of material size). A rectangular section aluminium profile that is correctly sized in the first place is the cheapest and strongest method rather than thinking about beefing up other methods which aren't strong enough in the first place.
Skippy

Make the Y axis beam across the gantry taller, and build a lightweight plywood torsion box in between your 80/20. This is similar to the approach I'm taking. I'm using rollerblade bearings and tried to find a way to eliminate the sag commonly found in rollerblade designs. This is what I came up with. The table top I made is 32 x 59 and needs well over 100 lbs to see any deflection, when only supported at the ends.
http://www.cnczone.com/forums/showthread.php?t=1356

Skippy-I hadn't even thought about the reaction forces to plunging. You are absolutely right! Thanks for setting me straight.

Ger, can you clarify? I was envisioning a single piece of 80-20 with 3"x 6" cross section. Span would be over 80". I can understand going to two, an upper and a lower with each carrying one linear rail. The torque box would bridge the gap between them? If that's the case then wouldn't a pair of plates (front and back) that sandwiched the two 80-20 beams do the same, or am I missing your point?

Give the span I'm crossing I just don't want something that weighs too much.

Thanks for your input-I appreciate it!

I didn't realise it was 80". Yes, I was talking about putting a torsion box between them. If you sandwiched the 2 pieces with plates, I think you would need to bond them with epoxy to stiffen it up. If you just bolt the plates on, it would still flex more than if bonded on. I think you could build it lighter with maybe 3/8" baltic birch plywood skins, and about a 4" x 4" gridwork inside the 2 skins. You could get away with smaller 80/20 at the top and bottom, too. But I think you'd have to go at least 12" tall on the wood framing. Taller is better. My Y axis I show in the link I posted is 40", and way overbuilt, but there is no way it could sag without several hundred pounds on it. It ways maybe 15-20lbs. I could have built it half as light and just as strong. If it works out well, my next router will have a 60" Y using similar construction. I already have the IKO rails for it.

Ger,

I see what you mean-that is a piece of work! Very nice. For some reason I've got this mental image of a metal table-I hadn't really considered wood. But a lattice or grid like yours topped with a skin (and I agree-bonded) should do fine. And for less of the green stuff. You've made the little wheels in my thickish skull start to turn! Thanks again.

Lance

Lance: I hadn't realised you were talking about an 80" span either. Getting back to your first drawing, if you modified the layout of the spoke idea it would work (against sag that is). You'd just need to place angle pieces of both the front and back (top) edges of the Y axis profile and connect the spokes alternately to front and back angle pieces. This is to triangulate the load the same as a bicycle wheel has spokes going alternately to each side of the hub. Otherwise the arch would simply fall over. Secondly the arch is under a compression load (not tension) so I'd use a formed tube. Even a 1" light wall tube (or square section) would do then get it rolled (very cheap) into an arc, drill the spoke holes on the inside and the bigger access holes on the outside and away you go. (Sydney harbour bridge! he he) I tend to cut a lot of aluminium using coolant so the wood route is not for me (painted or not) but it all depends on what you are going to cut, what you have best access to and what you are most comfortable with.
Gerry: Not the first time I've looked at your torsion box and not the first time I've come to the conclusion it's a work of art! Did you ever post pics of your kitchen?
Skippy

Skippy,

I suppose I ought to have mentioned the span-that was my concern. I agree totally with your idea for the triangulation-a much better approach! I got my idea for this from some Discovery channel show on the history of the arch-and you again are right-they only work in compression. I said tension due to the thought of bending the piece into its final position (I could try to save face and say that I meant the spokes would be in tension-but I won't-you got me).

Of course your original point concerning the upward reaction forces to the milling head still apply-and I'm wondering if Gerry's stressed skin beam isn't the best way to go-if the strength is there it'll handle loads in either direction.

Or maybe a combination. This is the problem with inexperience! Indecision.

Thanks again for the helpful input.

Lance

Skippy mentioned coolant, but you did say router in your first post, so I was assuming you'll be cutting wood with this?

Ger,

I've been aquiring parts for some time now and have the basic stuff-some nice THK rails, a few different slide assemblies (slides, ground ball screws, limits, and a couple with motors), and other stuff to build three tables and maybe retrofit a lathe.

My first table is is going to be small, at most 2' (X) by 2' (Y) by 5" (Z). I'm really doing nothing more than bolting together three complete slide units, slapping on some motors, a 2-1/4 HP, variable speed (8000 RPM -27,000 RPM) Craftsman router, and some Geckodrives. I've got both servos and steppers: I'm not sure which way I'll go.

This unit is a temporary, disposable unit I plan to use solely to help make the parts and pieces for the bigger units. Once I'm done with it I'll canibalize it for the parts.

Why?

Two of the slides came off of an X-Y positioning video camera inspection rig called a SpotSeeker II made by a company called MicroVision. They are highly accurate (9 mils/step), but somewhat wimpy in the leadscrew area. Without some serious work they are not going to last forever. All they were doing was moveing a small CCD camera, so they didn't need to be beefy.

Machine #2 will utilize 80" long THK HSR series 20mm rails on the X axis, 48" rails on the Y axis, Z as of yet undecided. I've got an 1-1/4" dia. ground ballscrew, 5 turns per inch, with 60" of threads for the X axis and a 1" dia. ground ballscrew, 5 turns per inch, with 50" of threads for the Y axis. I have a pair of 1,100 oz-in NEMA 34 steppers, but also some 50 lb-in, 5000 RPM Baldor servos. Haven't yet decided which I'll use. This will be a metalworking machine. Don't know where I'm going wih a spindle/drive/tooling yet-that's my weak area, as I have virtually no experience. This will be a sturdy, workhorse machine, so I don't want to skrimp, but I'm also somewhat "financially challenged" and that places limits on what I can do.

Machine #3 will use 3 pairs of the 80" THK HSR20 rails: two end-to-end for an X axis with 10' of travel even with an 18" wide gantry base, and the long Y axis we've been discussing. I've got another pair of the 60" and 50" ground ballscrews available for use. I've also got a very stout 30" slide unit for the Z-axis.

I can't for the life of me imagine needing to mill anything that big: this machine will be a dual purpose PLASMA torch and woodworking machine. I hope to add an indexer to allow carving of wind turbine blades about 10' long.

I'm doing all of this primarily to learn something new and see if a money generating making venture can spring from it.