Stiffening 50x100 extrusions

[electrotwelve]

I'm still designing my fixed gantry build using Misumi's 50x100 High Rigidity extrusions. I was wondering if it would be a good idea to fill the interiors of the extrusions with some dense substance to give it more heft. At first I thought about using Concrete but after reading a few more threads here, I realised that the shrinkage coefficient of concrete and Aluminium is incompatible and might cause the extrusion to crack or warp. Many recommend granite epoxy but finding granite particles is kinda difficult in my part of the world not to mention expensive. I was wondering if filling the extrusions with epoxy and hardener combination is a valid idea? Would that give it the additional heft I'm looking for?

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[bhurts]

They don't have sand and rocks where your at?

Ben

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[peteeng]

Hi Ben - Aluminium is 70GPa stiffness and epoxy is 3.4GPa stiffness so filling it with the stuff does not give you any more stiffness. Plus the cost will be much more than the extrusion. You need to bolt steel or aluminium plates to the outside of the section to make a gain. Do you want to double or triple the stiffness? If your still in the design stage then your a lucky fella. You need to double up the extrusion or select a bigger one. Or go one better and do not use construction extrusions they are inefficient sections Use a plain square or rectangular extrusion will be cheaper and stiffer. See attached...Peter

[fretman_2]

That's interesting! One of the advantages of the construction extrusion is having places to attach t-nuts.

Hi Ben - Aluminium is 70GPa stiffness and epoxy is 3.4GPa stiffness so filling it with the stuff does not give you any more stiffness. Plus the cost will be much more than the extrusion. You need to bolt steel or aluminium plates to the outside of the section to make a gain. Do you want to double or triple the stiffness? If your still in the design stage then your a lucky fella. You need to double up the extrusion or select a bigger one. Or go one better and do not use construction extrusions they are inefficient sections Use a plain square or rectangular extrusion will be cheaper and stiffer. See attached...Peter

[peteeng]

Hi Fretman - I'd call it a convenience rather than an advantage. T nuts pull up on thin edges which is not very rigid. I've just expanded the gantry info. If your still designing consider solid plywood or MDF for this style of machine. Timber is also very damp...see attached. Peter

Fretman - I've made a couple of necks for guitar builders interesting shapes. Difficult to register when turned over. But the router makes a neck in a

[rodw]

Very interesting thread thanks peteeng. When I built my plasma cutter, I thought the cost of extrusions were excessive despite seeing them in use on a number of commercial tables. My local aluminum supplier had 80 x 40 x 4mm box section which I went with. Yes it was a bit of stuffing around to drill and tap holes on a 1600mm long beam but its worked very well. A M6 thread still has 4 threads to hold which is more than enough for any engineering. Also don't forget there is additional stiffening added by the time you bolt on linear rails and gear racks. My next door neighbour spent $100k or so on a table that used extrusions then complains of lack of accuracy becasue the surfaces are not machined.... So thanks for confirming my decision was sound.

[peteeng]

Hi RodW - In that sort of application I use the thinnest extrusion possible yet the stiffest section possible (biggest). I then get a laser cut Al or SS backing plate cut and thread all the holes (or you can use flat bar if you have a drill press etc). This locally stiffens the section and is easy to tap. Cheers Peter

[rodw]

Hi RodW - In that sort of application I use the thinnest extrusion possible yet the stiffest section possible (biggest). I then get a laser cut Al or SS backing plate cut and thread all the holes (or you can use flat bar if you have a drill press etc). This locally stiffens the section and is easy to tap. Cheers Peter

Yes, laser cutting is the way to go. I laser cut the mounting rails for the table from 12mm steel and they were able to cut 5mm dia holes perfectly to mount the gear racks and HGR25 rails. These were tap ready for M6 threads so I spent a day hand tapping them with a magnetic tapping jig.

There is no cutting forces in plasma so I'm happy the way it turned out. The Gear rack had mounting holes in the rear so I drilled a clearance hole thru the reverse side which is similar to what you suggest. If I did it again, I'd only use one larger HGR rail instead of 2.



The other stiffening member is the drag chain support at the back






This started as a 90mm x 40mm piece of angled profile mounted to tapped holes on the lower side but more recently I had to upgrade the drag chain becasue I added a bigger 120 amp torch so I folded up a replacement that was wider. If you needed more stiffness, the edge on the underside could be folded down to create a stiffening spine. You can see the clearance holes for the gear rack mounting bolts. Just hold them on an allen key with a dob of grease and it is easy to fit them.

[wizard]

I actually see the T-Slots on most extrusions as being the worse reason to buy T-Soits. First off there isn’t a lot of mechanical strength in those thin T slot rails. I’ve literally have seen multiple cases where the Aluminum gets busted out, if not busted distorted enough end up useless. The best examples of suitable extrusions I’ve seen have been purposed design to mount linear rails and other components. These extrusions have web thicknesses of a 1/4” or more. This stuff is hard to find, one machine builder we have worked with went to steel for gantry beams some time ago. Steel is just too cheap to ignore.

That's interesting! One of the advantages of the construction extrusion is having places to attach t-nuts.

[wizard]

I'm still designing my fixed gantry build using Misumi's 50x100 High Rigidity extrusions. I was wondering if it would be a good idea to fill the interiors of the extrusions with some dense substance to give it more heft.

Mass might give you more damping, how much stiffer the beam might be is hard to say.

At first I thought about using Concrete but after reading a few more threads here, I realised that the shrinkage coefficient of concrete and Aluminium is incompatible and might cause the extrusion to crack or warp.

That isn’t really the biggest issue, concrete is corrosive in contact with aluminum from what I understand.

Many recommend granite epoxy but finding granite particles is kinda difficult in my part of the world not to mention expensive. I was wondering if filling the extrusions with epoxy and hardener combination is a valid idea?

It seems to work well for many when filling steel or concrete beams. Use ally the goal isn’t so much stiffness but damping

Would that give it the additional heft I'm looking for?

Honestly if you want heft in a fixed gantry beam why not consider square steel beams? Far cheaper. If you go with aluminum extrusions you still need to consider machining. Since the steel beam will also require machining to achieve best accuracy cost becomes a wash. For a little extra you can have the beam drilled at the same time.

[peteeng]

Hi Electro - If the gantry is fixed then there is no concern about weight or space. If you can't do the engineering math to calculate the stiffness required then whatever you think you need x3 or x4. Make it in steel to maximise stiffness and minimise cost. If it's super stiff it won't vibrate or its vibration modes will be very high so won't matter. 50x100 is not a particularly stiff or large section for a fixed gantry unless its a tiny machine. Peter