[fockghost]

Hi

I am in the process of my build but just have a question about Y gantry. I have planned the machine using 120mm x 80mm Aluiminum Profiles. but due to the cost i will install them as a "upgrade" at a later stage. so i have designed MDF Parts to the same dimentions as the Aluiminum Profiles, but have a concern about the Y Beam. it will be 120mm x 80mm x 2000mm my concern is that the 2000mm span is to long and will Flex/bend to much.

Attachment 210250

i know the MDF beam is temp but would still like it to be as strong and as Flex free as possiable.

Regards Andrew

[Trotline]

Much too thin in the cross section for that span. I'm no engineer, but I'd want at least 3X the cross section dimensions for a 2 meter span, and even that may be too little.

Luke

[fockghost]

So your saying my beam should be 360mm, 240mm x 2000mm?

if so that wont really work since the beam is ment to replaced by a aluiminum 120mm by 80mm profile.

anyother ideas on how to strenghen or mod then existing design to make it possiable to span that distance?

[ger21]

I don't think you're aluminum beam will be as stiff as you'd like at that length.

I'm building a 1700mm MDF gantry beam for my machine. I don't have the drawings here with me, but I believe it's about 125mm wide and 200mm tall. To hopefully increase stiffness, it's made from laminated 1/4" MDF. 4 layers on the vertical faces, and 3 layers on the top and bottom.
It's going to carry two spindles, so I want it to be very rigid. I did some deflection calculations, and don't anticipate having more than .05mm deflection. However, my beam will extend past the gantry uprights, so only about 1300mm will be unsupported. Here are a few links.
http://www.cnczone.com/forums/cnc_wo...ml#post1162526

http://www.cnczone.com/forums/cnc_wo...ml#post1168468

[fockghost]

My beam will also extend past the gantry walls, My beam is planned with 16mm MDF 1 layer per side. i was told i can put threaded rods through the lenght of the beam on all four courners to give it abit more strenght.

could i perhaps get a look at your plan at some chance?

[ger21]

My plans are constantly changing, and are not complete. I don't even have plans for the whole machine yet.

[Trotline]

Fockghost, what is the actual free span of your beam? That makes a difference in any discussion. In any case, replacing an aluminum extrusion with an MDF box beam of the same dimensions isn't likely to yield the same strength or stiffness. (Well, actually, I imagine there's a beam dimension where the two materials would intersect, but it's likely to be at a much lower aspect ratio.) If you need the same dimensions for it to work, then going from Al to MDF is going to be problematic. I'm not sure if using threaded rod to put the beam into compression will help much, but I'm not really qualified to say, without testing it.

Luke

[dmalicky]

It's great your wood beam is a box section and has bulkheads! Good advice above -- I'd agree 120x80mm alum would be flexy for a long span. I'd suggest 200mm tall or so. What do you plan to cut with the router?

The Young's modulus (stiffness) of MDF is about 500 kpsi, which is 1/20 that of aluminum. Plywoods range from 1000 to 1500 kpsi, so would be better for a long beam. If very well sealed from humidity changes, they should be stable.

Very roughly, for equivalent bending stiffness, replacing a 6mm wall thickness alum profile with a 15mm thick MDF profile would require a larger moment of inertia by a factor of 6/(15/20) = 8. The 4th root of 8 is 1.7 -- the beam dimensions would need grow by that amount, minimum. A MDF beam would probably be heavier, so I'd shoot for more. If using 15mm plywood, the growth factor is about 1.4.

Instead of trying to match beam sizes from alum to wood and build 2 gantries, why not build one good plywood gantry of appropriate size (say, 300mm x 150mm) and never replace it? The tall height keeps the bearing loads low -- something a 120mm alum beam could never do. Check out the Jerry Burks' Big Bamboo machine for inspiration.

[fockghost]

Thx for all the Replys,

I plan on cutting Wood mainly, some of the wood will be Sleeper wood.

The Beam will extend 75mm past the gantry walls on each side, the Y will have a travel of 1700mm.

I could make the MDF Beam perminante thats not a problem. You would recommend Ply over MDF? i cut a sheet of Ply to the sizes i need for my original design but found that 90% of the Ply bowed. what ply would you suggest also?

So im thinking somthing like 300h x 150w x 2000L Sound Better?

Gerry, i Noticed on your gantry design you had alot of ribs close together on the ends of the beam and fewer in the middle, what is your reason for that?

Regards Andrew

[ger21]

Gerry, i Noticed on your gantry design you had a lot of ribs close together on the ends of the beam and fewer in the middle, what is your reason for that?

There are threaded studs epoxied into the ribs at those location. At every location of a mounting stud, they actually go into the ribs rather than just the skins.
Threaded studs embedded into the ribs should be much stronger than screwing into the wood, or using insert nuts.

[dmalicky]

The best plywood is Baltic Birch or equivalent, but I don't know if you can get 2m sheets. In the US we also have a domestic grade which is pretty stable; not sure what you have in S.A.

If the warping is minor and the cut line stays straight, assembling the box section should pull everything straight. But MDF can work, too, just heavier and needs a bigger section for the same stiffness. MDF has more damping, so a plus.

I was curious how a long gantry behaves so I revised one of my FEA models. My brain works in inches, but I'll try to show both inch and mm:
- 300x150x1900mm gantry (actual dims 12"x6"x74", excluding your overhang on each end), made of 15mm (0.6") MDF
- Bulkheads of 15mm MDF, spaced every 180mm (7")
- 100mm wide legs (4"), also 15mm MDF
- Z clearance of 180mm (7")
- Gantry car is 300mm wide (12")
- Rigid Y-car/Z-car/bearings/spindle etc -- this model is just to size the gantry and legs
- Deformation plots are scaled to 10% of the model size, so it's easy to see what's moving. Legend scale is displacement in inches.

Here is a longitudinal load of 1000 lb at the tool, giving 0.14" max displacement, for a stiffness of 7k lb/in. With this 74" gantry, the longitudinal bending mode becomes as important as the usual torsion mode. (For the engineers, that makes sense given that bending deflection goes by L^3 while torsion is just TL/JG).
Attachment 210522

Here is a lateral load of 1000 lb at the tool, giving 0.038" max displacement, for a stiffness of 26k lb/in:
Attachment 210524

For cutting wood, stiffness at the tool should probably be in the ballpark of 2000+ lb/in, but that is affected by about 10 parts total. That means the gantry+legs should be about 5x that stiffness... 10k+ lb/in.

So the above MDF gantry is close, but we can do better with more section width. Going to 300x250mm gives a deflection of 0.067" and K of 15k lb/in:
Attachment 210526

That gantry tube+bulkheads weighs about 30 kg (66 lb). The whole gantry would probably weigh about 2x that--whether that's ok depends on the drive system. Plywood could get the same stiffness with thinner walls or a smaller section.

I also tried 10mm MDF in a 300x300mm section (12"x12"): longitudinal deflection is 0.069", K = 14k lb/in. Tube+bulkhead weight is around 23 kg (50 lb).
Attachment 210528

Spacing the bulkheads every 330mm (11") only increased deflection by about 5%. But no bulkheads increased deflection to 0.30" -- 430%!
Attachment 210530

Take-home messages:
Big tubes rock
MDF is fine if the section is big enough
Gotta stabilize the cross-section (bulkheads or a diagonal sheet)
Long gantries need ~square sections

[fockghost]

wow, thanks verymuch for the time everyone has put in to help me. looks like its going to be one hell of a beam. 300x300 is huge.

In SA MDF comes is (16mm) 2750mm x 1830mm and Ply come in sheets of (16mm/18mm/21mm) 2440mm x 1220mm so the size is not a prob.

[Trotline]

lol, the gantry beam on my machine spans only 130cm. Because I'm no engineer, though, I massively over-engineered it. it measures roughly 300x400mm, but it has both vertical and horizontal longitudinal members, "and" 16 bulkheads. (4 each in all 4 cavities). I'd hate to think what would happen to anyone who ran a truck through my garage door. That beam would probably knock the engine block right back through the cab...

Luke

[Bl@ckrat]

imo save your pennies and then go for a decent beam , even if you use square mild steel tube , thats hardly going to break the bank. 4 meters os 76x76 tube isnt 'that' much

[dmalicky]

Glad it's helpful. You could go with a smaller MDF section if you wanted -- the target of 2000 lb/in should give good stiffness for heavy cutting, but many DIY machines cut wood with much less stiffness (at lighter and slower passes). And good gantry stiffness only helps if the rails/bearings/cars are also in the target stiffness.

In plywood, assuming a modulus of 1300 kpsi (Baltic Birch), a 175x175x16mm section (with bulkheads) deflects 0.058", for a K of 17k lb/in.

With a modulus 60x greater than MDF, a small steel tube is indeed stiff. A 76x76x5mm tube (with endcaps but no bulkheads in between) deflects 0.062", for a K of 16k lb/in. Just right. Weight is about 25 kg (55 lb). Its major disadvantage is the short vertical span it gives for the gantry-car bearings. That would be ok for a ~75mm Z clearance. But a 175mm Z clearance would put high loads on the lower rail bearings, causing high deflection at the tool. In the US we can get 4x4x1/8" (100x100x3mm) -- that has 15k lb/in stiffness (with endcaps, no bulkheads) and is lighter/less $.

[Dea4arrison]

imo save your pennies and then go for a decent beam , even if you use square mild steel tube.

[fockghost]

Im going to go with 16mm MDF or Ply, will be 300h x 200w x 2000L. will try post a new drawing later today of the new design. Z clearance will be 180mm. I would like to do most of the construction from Wood, i have a 1500mm x 1500mm steel table but aside from that the rest of the construction will be Wood.

I had planned to put a bulkhead every 100mm. i was working on a theory that more is better?
The entire gantry construction will use about 3/4s a sheet of Ply, with a weight of 34kg per sheet. so the gantry beam and walls will be +-25kgs without the rails, drive, motors ect mounted. but will have to recalculate after i do the new design.

what about Pine Ply? its quite cheap.

[Bl@ckrat]

david, i was thinking two beams ... so for the z axis it would be 152mm
and i would go with a 3mm section ...

the only reason i say is rather than spend all that time making bulkheads and fabbing up a mdf gantry only to replace it 6 months/1 year/x down the road, just go straight to something that is not going to need upgrading anytime soon ... imo

[Trotline]

The strength of the beam is all in the outer skins. The bulkheads are really only spacers. They maintain the strength of the skins by preventing them from warping or bowing. 100mm is a pretty close spacing, probably closer than is really necessary. To stretch it out a bit, and also to save weight where you can, I'd consider in the neighborhood of 200mm spacing.

Pine plywood is not made to the standards of good birch ply. It'll have a lot of voids, and in general is not suitable for the kind of application you're planning. How you join the parts is also a factor. I used 5/16", (8mm), dowel nuts and bolts roughly 65mm long. That makes for a really good joint. I've had no problems in over a year of use.

Luke

[dmalicky]

If you can get stable ply, I'd go with a smaller section of ply vs a larger one of MDF -- the weight savings will allow smaller motors/drives, which will more than makeup for the small extra cost of good ply.

Agreed with Luke on bulkheads. For most structures, bulkhead spacing is typically between 1x and 2x the section height or width (whichever is greater). So a 150x200mm section would have bulkheads between 200mm and 400mm apart. A 'thin' walled tube often needs the 1x spacing; a 'thick' wall tube with good corner joints can use 2x spacing or wider. Finer spacing than necessary adds weight that is better put in the walls. The only exception is if there are loads in the middle of the wall, like an airplane hull -- then many bulkheads are used (CNC rail loads should be very near the corners).

Bl@ckrat, thanks, that make sense. Yes, 2 of those bonded together or separated by ~5" would be good. Still, I think plywood gantries are also fine for long term, if sealed well.