Thread: Triangular Gantry Beam

The optimal gantry shape may be a triangle. peteeng has posted some good information and simulations on triangular shaped gantry beams.

The real test is can a good triangular gantry be practically built (and beat the alternatives) by / for the hobbyist.

Basics:
Steel appears to be the best material. Steel being very stiff, easily laser cut, and relatively cheap.
Laser cutting (or water jet, or plasma cutting) is likely optimal, being reasonably affordable and precise

Fabrication
How do we stick the components together to form the gantry?

Welding
Pros:
Readily available
Low entry cost
Can fill loose joints.

Cons:
Distortion, distortion, distortion
Increases residual stress -> stress relief optimal / may be required -> added $$$

Brazing
Sounds great in theory - low heat, so does not cause an increase in material stress
Can joint dissimilar metals

BUT requires tight fit-up and good cleaning.
Most sources quote an optimal joint space ("fit up") of 0.003-0.006" (0.0762 - 0.1524mm) for maximum strength (looser tolerance is possible, but strength falls off and chance of a bad joint increases)
Cleaning flat exposed surfaces is not hard, but cleaning the inside of laser cut slots (with width = material thickness) may be difficult (and may affect fit-up if using abrasive).

The need for tight fit-up is a major limitation in this application where 45º joints are involved. Almost all laser cutting / waterjet / plasma will be on 3 axis machines - thus a 45º chamfer edge will not be produced by these machines. Adding this chamfer whilst retaining tight tolerance appears unlikely for the hobbyist

Glue / epoxy
Low stiffness

Optimizing the shape
A 90 degree right angle triangle with 45º angles seems optimal and easiest for most situations.
Linear bearings on the vertical face - both on the same face makes machining and aligning much easier

Vertical ribs - spacing of ribs etc will depend a lot of method of fabrication - need to access each joint in turn.

Hi Pippin - Cleaning steel is easy if you use vinegar. It will remove mill scale laser scale etc. Just need a bath big enough. About 6 hr soak will do it.... Then there is cast concrete or engineering grout. That would be my go to at present. To produce 90deg joints a flat can be bent into the back plate so the connection is square... Peter

see attached images -
gantry tri is the ideal way to make the tri gantry. The rail should be mounted on a web. This means the load transfer is directly to the structure. If SHS is used and the rail is away from the corner because of the radius this means the metal has to move slightly to take up load. This is called membrane action. This action maybe small but it all ads up to a large deflection. A machine has primary deflections ie deflections caused by direst loading. These are flexural and shear deflections typical calculated via equations., Then there is secondary deflections. These are deflections caused by primary deflections. If you use construction extrusions all those little edges deflect oddly due to the gross deflection of the section. Then there are membrane deflections as discussed. Thin members with loads on them have to change shape to be able to take up load. This is the same as a hammock. At rest its floppy and free but when you lie in it it strain stiffens into a "strong" shape. This happens with thin sections and local loads. Thin is a relative description, seemingly thick structures still have membrane actions in them... and membrane actions contribute to vibration easily.

The entry timed out in the middle of me writing it.. Peter

if you cast a square concrete solid gantry then being solid it does not lozenge so can be square to take advantage of the extra inertia.

ganttry tri can be aluminium or steel

Re Brazing - either torch or tig brazing- I think if you used this process it would be braze welding not flow brazing so the fit up is not critical. But with all welding and brazing methods the better the fit the better the result.

Hi All - To illustrate this discussion I built two steel gantries, A) 1200mm wide 200x200x10mm SHS. B) same with the back cut off to make a triangle. In this case the "bearings" are mounted on the same spot as the square gantry. The bearing land is 200mm wide. Ideally they would be mounted at the edge or web. I made the z axes "rigid" so only the gantry deflection is measured. 100kgf is placed at the bottom of the z axes. So this is very apples to apples. The SHS deflects 68um the Tri deflects 54um. So the tri is 20% stiffer. So its up to the Maker to take advantage of this info. The tri weighs 45kg and the SHS weighs 51kg so that's a plus as well. happy to answer any questions. Cheers Peter

edit - for completeness I rang a transverse and plunge case. The tri gantry is 15% less stiff in transverse stiffness but 20% stiffer in plunge. The 15% trans can be made up via changing the plate thicknesses since this is likely to be a fabrication. Since we have 6kg to play with as the SHS is the benchmark...

trans Tri 6.7um shs 5.8um are we worried about 1um delta trans stiffness? I suppose so as we're a but OC around here....
plunge tri 24.8um shs 31.2um 100kgf applied

Peter, thanks for your input.

I lost access to any simulation / FEA a while ago which has slowed me down. Hopefully have some access again soon.

Not a huge difference on those numbers. A square tube with appropriate bracing may win on ease of manufacture up to standard sizes like 200x200mm. I'm looking at 300x300 which is a bit harder (I do happen to have a 200x200x10mm tube 1m long taking up space. A 25mm thick, 300mm high front plate on the 200x200, with triangular bracing from top of 200x200 tube up to top edge of front plate may be a good solution

Hi Pippin - 20% not enough ? Peter error 403 again?

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Hi All - Because I'm into concrete at the moment I added a 200x200 E=40GPa gantry to the mix. In Oz via nearly any hardware store that's sells Sika products you can get E=56GPa material. Its really easy to make a square mould. I made it hollow until it was the same weight as the steel SHS 51kg so that's apples to apples and ran the solvers. Remember there's 56/40=40% extra stiffness available using the windmill grout...

Push concrete 50um tri 53um shs 62um

Trans conc 8.4um tri 6.5um and shs 5.7um

Plunge conc 26um tri 24um shs 28um

So it seems to me that I can vary the geometry to improve the trans result (make it a little deeper) and get a better gantry from the grout. So I suggest that the concrete path maybe better for the Hobbyist and professional. Damper, stiffer get the shape you want/need in your garage, no heat treat or welding. Bond on the steel bits for the rail lands, or embed bits up to you. I think the steel era should end... Peter

also this can follow thru to z axis assembly, machine bases etc. Any part can nearly be converted to engineering grout if it can be made thick>

can't load images at the moment and its doing odd things

Pippin sketch what you want to do and I'll run it. Would you consider grout? Peter

Originally Posted by peteeng

I think the steel era should end

hmm, for hobby use? I can see that happening. How is sika compared to uhpc? stuff like microcracking and longevity? is sika csa? durcrete's engineer, the one from the webinars, says once microcracking starts, the part becomes useless as a machine tool. He responds to emails so you can talk to him in more detail about it. Send me a pm for his email.

I've seen some concrete builds, some even claiming to be uhpc, that had not just microcracking but had big chunks of concrete break off. If cracking is resolved or at least becomes reliable to some degree, lets say good for 5-10 years, this would allow for some more interesting builds from the diy community. I'm really sick and tired of everyone doing the same old thing, its either a router or a C frame, boooring...

Hi Ard - Bluey and Sika guarantee their product for 40 years. Some projects 100 years... If a ground anchor micro cracks and parts company it's a big issue for the contractor. Or a runway cracks...Grouts are not Portland cement different chemistry. Hard to pick if some products are CSA as CSA is classed as cement so in the SDS it just says cement. It will say CSA in the SDS or literature if they want to differentiate the product. As you say Ard its time to move on from the iron age. Peter

What is Grout? - Bluey Technologies

Re -UHPC - UHPC is portland cement with additives that minimise the required water addition. Some use a small % of CSA in it others don't. There are many published recipes for UHPC. But I'd just buy a premade mix. You can add fibres to the mix to help with cracking. But machine stresses are really low compared to commercial use so service stress cracking is very unlikely. Onward to my first cement machine....

Re - concrete - I've seen videos say stacking the Langmuir machine with very dry concrete mix pushing it into corners etc. Thats how not to do it. Grout is pourable, plastic and conforming. Just pour it into the mould , no need for vibration in fact don't vibrate it at all its not recommended, and its done. Slightly underfill to the side you want "flat" then use self levelling grout to bring to flat. Sounds easy?? Peter

Originally Posted by peteeng

service stress cracking is very unlikely

I suspect so, the cracking I've seen in builds 100% from incorrect recipe/material, I've noticed a lot of people underestimate the importance of shrinkage reducers, they used just the cement part(tegno) without any additives. The %wt of the additives is also important, it varies even with durcretes E45 vs E80 recipes. A one stop solution that only requires water would be very much welcomed.

this guy had his concrete saddle crack even before any use, visible at 1:05

Hi Ard - doing your own grout is like trying to formulate your own epoxy. Why do it? the cement companies have spent the $$$ and done their homework for us. I can see the surface finish in the video says it all. Its too dry and its probably portland. The cracks are classic drying cracks from corners. Thats why the grouts are plastic in the first drying stage so they have give in them. Anywho move along. Use Class C grout from your local supplier with the highest modulus you can get... Peter

Plus you can colour it with ochres. Bunnings have polka dot yellow!

I'm certainly not against grout / concrete.

What is the shrinkage of the Sika stuff like compared to the Bluey HE80?

Hi Pippin - I would use Bluey HE80AG as it has added aggregate and a E=40GPa or thr Sika 3350.

Sika 3350 shrinkage is 0.025% HE80AG is 0.017% ie 0.17mm per metre...These are less then epoxies typical shrinkages...Peter

It all sounds good and all but even the availability of said products might be an issue, how's the minimum tonnage per order Pete? Might need a middle man company like mcmaster to order big and sell small to private customers.

Hi Ard - I can get the sikagrout 3350 E=56GPa at the local hardware for $80AUD for a 20kg bag. I'm waiting on a quote for the HE80AG at moment. I can get Lanko 701 class C for $37AUD for 20kg at the local hardware.. So a few choices there. So availability here no problem... Peter

Peter - I think you have corresponded with Bluey? Is the HE80 their best product for this? They list other non-shrink grouts (e.g. BluCem HS60 - Expansion 2.5%. BluCem HS120 Expansion 0.6%) but don't list Drying Shrinkage for all of them. HE80 major point seems to be rapid set / early stength gain).

Hi Pippin - I spoke to the MD about the application (wow time flies over 2 years ago in my records) and he recommended the HE80 (E=32GPa) or HE80AG for more modulus less shrink. The AG has 3mm blue metal added which raises the E>40GPa. At the time it was $32AUD per 20kg bag. Waiting on new quote. Available arcacia ridge Qld... or other Bluey depots. My plan is to cast the main structures in grout, haven't decided yet but Bluey sent very good info so feel they helped alot, Sika are a bit scarce on MOE data. The guy I deal with has to go to superiors for info I ask for... Then use Tetrium-steel or carbon for the rail mounts etc. This will allow me to route the surfaces on my router. Next machine is going to be long and thin so I can make gantries and drive systems... Got to make a bigger machine so I can make small machines...

The Sikagrout 3350 is premium product, premium price. I'll see how the FE goes to see if its justified. Then theres lanko 701 cheap and at Bunnings. Sika own Lanko...

So tell me what your thinking for your 300x300mm steel gantry (how long how thick?) or publish a step file or send to me and I'll run it. Then we can convert it to eng grout. happy to collaborate/help. In 5 weeks Frankie leaves the building and a space doesn't stay a space long around here. Frankie moved today WOOHOO Peter

Thanks.

I got access back to FEA/Simulation. I am definitely an amateur here.

An early play - steel skin tube appears twice as stiff (at 2.88um/1000N) as grout without skin (at 5.91um/1000N). This is not surprising given the material modulus.

Take the grout fill out of the steel skin tube and there is a huge drop in stiffness: from 2.88micron displacement (347N/um) to 51micron displacement (19.6N/um). (Tube twists and becomes a parallelogram)

Does that sound right???

Edit: Unfortunately CNCZone heavily compressing the images