Rigidity Questions

[yydoctt]

Hi,
I'm considering my options whilst designing a CNC router, which when I imagine what I want to create, largely happens to be in Aluminium, and largely is large! I also plan on using other materials, MDF, acrylic, etc. (No steel or hard metals!)
I want a build area of 500x500mm, and have settled on a Momus/CNCRouterParts type design using CRS and bearings. I have two main questions which I hope someone will have the answer to.

1) Will I get away with bolted Aluminium extrusions (8020 style?), or bolted steel box sections, or must I weld it, if so which method of welding?
I don't weld myself and would therefore need to outsource this.
I would hope to be able to cut through 10mm Aluminium without issues, with a decent finish and a reasonable DOC and feedrate.

2) Bearings on CRS vs Supported Rail vs Linear Guides (Sliders).
In my mind Bearings on CRS seems to be able to take the biggest load in all axes, is this right or is it the linear slides (typically speaking of course).
The hiwin rail has four rows of contact, the bearing system has four contact points but you can add pretty big preloads?

Any help would be greatly appreciated! I plan on conducting FEA but appreciate it's inaccuracies and actually how tough it is to turn values into realistic conditions. So I guess I'm more aiming these questions to mathematicians and those with experience with any of the above.

Thanks,
Tom

Similar Threads:

• DIY CNC Rigidity/Flex
• Rigidity and random questions
• Tooling Rigidity
• My First CNC router, Servo + rigidity questions
• X3 column rigidity?

[ger21]

I would hope to be able to cut through 10mm Aluminium without issues, with a decent finish and a reasonable DOC and feedrate.

If this is the main intended purpose, then I'd say steel would be more appropriate, but much harder to work with. If using aluminum, I'd use the largest sizes possible. If bolting together, use heavy plates wherever possible to tie them together.

Reasonable depth of cut can mean different things to different people. I wouldn't expect more than 2-3mm/pass max, if the machine is very rigid.

Hiwin or THK type rails are far, far superior to either of the other types.

If all you'll be cutting is aluminum, you might want to seriously consider buying a mill, rather than building a router.

[yydoctt]

If this is the main intended purpose, then I'd say steel would be more appropriate, but much harder to work with. If using aluminum, I'd use the largest sizes possible. If bolting together, use heavy plates wherever possible to tie them together.

Reasonable depth of cut can mean different things to different people. I wouldn't expect more than 2-3mm/pass max, if the machine is very rigid.

Hiwin or THK type rails are far, far superior to either of the other types.

If all you'll be cutting is aluminum, you might want to seriously consider buying a mill, rather than building a router.

Hi, thanks for the reply.

Bolting together steel seems to be the best option for me so far. I have access to a mill so can do all the holes properly. I guess I wanted to know whether the joints will be an issue on both types of material.

Thank you for the suggestion, I think I will probably invest in Hiwin rails then. Is 2 per axis adequate? Would it help for the Y axis to have one on the side and one of the top to counter the forces in different planes? Same question for X and Z though is more complicated to produce.

2-3mm I would be pleased with.

I have done preliminary calcs on rigidity and can use box steel to minimise deflection to fractions of a mm, what besides rigidity (which can probably be designed into a router) makes it more suitable? The spindle?
Mills don't go as large as I want (for the price) and are too heavy and consume too much power for the DIY environment I want to install it in.

I'm happy to make compromises but am set in the form factor and weight of a router, hoping to achieve as much as possible with it.

[dmalicky]

Great advice from Gerry, as usual. Preloaded skate bearings are probably not terrible, though they can't compete with profile rail. For each skate roller, probably only ~2 of the balls are actually taking load. Also the typical 1/4" steel rail hangs out 1/2", which my FEA models show is enough to be a flexy spot in an otherwise stiff machine. With profile rail, there are many balls loaded in parallel, and they can also be preloaded.

The Momus design is ok, though the high placement of the x-rails is not best for stiffness. The best x-rail height is the same height as the cutter. Above or below that, cutter loads create a moment that increases load on and deflection of the x-bearings. A Momus height is probably optimal for high gantry acceleration (~same height as the gantry CG). Try doing some FBDs of each subsystem to see how to minimize moments--a lot can be optimized just from stick models. Slocum is a god of this kind of machine design and has a great pdf:
http://pergatory.mit.edu/kinematicco...Principles.pdf
For CNC, esp see his slides on Bearings, Abbe (sine errors), Structural Loops, Centers of Action, and Stick Figures.

Steel or aluminum are both fine. The key is getting a large cross-section and appropriate wall thickness. Typically the problem with larger tubes (e.g., 6"x6") is that the thinest steel walls available can be very heavy, so alum is often a better choice there. Steel is easier to weld, and welding makes a stiff joint, but welding creates distortion and slowly stress-relieves over time, so accuracy suffers. Alum is easy to power tap, so with lots of bolts and good joint design, it's not hard to produce a stiff structure with it, too. Bolting + epoxy should be equivalent to welding for stiffness.

Yes, 2 rails per axis, and 2 blocks per rail. Many machines use 1 long block per rail. These aren't very stiff; the blocks have to be spaced out. Profile rail has equal load rating in all directions, so they can face the same way.

For a stiffness target to cut aluminum efficiently, it looks like 20,000 lb/in is a minimum. Good links:
http://www.cnczone.com/forums/diy-cn...tml#post944411
http://www.cnczone.com/forums/diy-cn...tml#post911744
University of Utah - ME EN 7960 - Precision Machine Design"); if (currentModule == "index") document.write(" - Main"); if (currentModule == "courseInfo") document.write(" - Course Information"); else if (currentModule == "projects") document.write("
http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf

It's great you're planning an FEA model. I've a lot of background in structures, but have still been surprised at where the flexy parts are in a given gantry machine. And it's challenging to get everything ~equally flexible (so not overbuilt or have an Achilles heel). Lots of complex load loops and the usual mechanics of materials equations don't predict some major flex areas like "collapse" of the cross-section for the main gantry tube (bulkheads or a diagonal sheet fix that very well). Be sure to model the bearings with contact conditions (so they can slide); this will show the importance of leadscrew placement (also inferred with a stick model, center of stiffness). It's pretty cool stuff!

[yydoctt]

Great advice from Gerry, as usual. Preloaded skate bearings are probably not terrible, though they can't compete with profile rail. For each skate roller, probably only ~2 of the balls are actually taking load. Also the typical 1/4" steel rail hangs out 1/2", which my FEA models show is enough to be a flexy spot in an otherwise stiff machine. With profile rail, there are many balls loaded in parallel, and they can also be preloaded.

The Momus design is ok, though the high placement of the x-rails is not best for stiffness. The best x-rail height is the same height as the cutter. Above or below that, cutter loads create a moment that increases load on and deflection of the x-bearings. A Momus height is probably optimal for high gantry acceleration (~same height as the gantry CG). Try doing some FBDs of each subsystem to see how to minimize moments--a lot can be optimized just from stick models. Slocum is a god of this kind of machine design and has a great pdf:
http://pergatory.mit.edu/kinematicco...Principles.pdf
For CNC, esp see his slides on Bearings, Abbe (sine errors), Structural Loops, Centers of Action, and Stick Figures.

Steel or aluminum are both fine. The key is getting a large cross-section and appropriate wall thickness. Typically the problem with larger tubes (e.g., 6"x6") is that the thinest steel walls available can be very heavy, so alum is often a better choice there. Steel is easier to weld, and welding makes a stiff joint, but welding creates distortion and slowly stress-relieves over time, so accuracy suffers. Alum is easy to power tap, so with lots of bolts and good joint design, it's not hard to produce a stiff structure with it, too. Bolting + epoxy should be equivalent to welding for stiffness.

Yes, 2 rails per axis, and 2 blocks per rail. Many machines use 1 long block per rail. These aren't very stiff; the blocks have to be spaced out. Profile rail has equal load rating in all directions, so they can face the same way.

For a stiffness target to cut aluminum efficiently, it looks like 20,000 lb/in is a minimum. Good links:
http://www.cnczone.com/forums/diy-cn...tml#post944411
http://www.cnczone.com/forums/diy-cn...tml#post911744
University of Utah - ME EN 7960 - Precision Machine Design"); if (currentModule == "index") document.write(" - Main"); if (currentModule == "courseInfo") document.write(" - Course Information"); else if (currentModule == "projects") document.write("
http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf

It's great you're planning an FEA model. I've a lot of background in structures, but have still been surprised at where the flexy parts are in a given gantry machine. And it's challenging to get everything ~equally flexible (so not overbuilt or have an Achilles heel). Lots of complex load loops and the usual mechanics of materials equations don't predict some major flex areas like "collapse" of the cross-section for the main gantry tube (bulkheads or a diagonal sheet fix that very well). Be sure to model the bearings with contact conditions (so they can slide); this will show the importance of leadscrew placement (also inferred with a stick model, center of stiffness). It's pretty cool stuff!

This is an excellent answer, thank you for taking the time to address all my questions in such detail!

Somehow I thought the Momus high X rails would be good for stiffness, I will consider the typical raised gantry and square loop under the table, thank you for pointing this out. I will FEA both as you suggest and just have a play with structures.

I'm also considering and perhaps now leaning toward a fixed gantry, that I could probably build out of extrusion and aluminium endplates quite easily.

Thank you for the links, that last one from Utah Mech Eng will be a very interesting read!

[dmalicky]

Glad it's helpful! Yes, Bamberg's thesis is great (and Slocum was his advisor). To clarify on the x-rail height, there's another machine configuration with short gantry legs (in between the Momus and the loop-under-table) used by CNCRP and some others: http://www.cncrouterparts.com/images...ender-logo.png
With that x-rail height, the moments are minimal and the gantry legs are easy to make stiff. I avoid the loop under the table as it wipes out helpful reinforcements in the lower frame. Or almost as stiff, most commercial machines put the x-rails just lower than the table, for better access:
http://cfnewsads.thomasnet.com/image...605/605670.jpg
http://www.cronsrud.com/images/machines/97M12.jpg

But yes, a moving table / fixed gantry is easier to make stiff, if you have the space. If not, a moving gantry can be made stiff enough for alum.

[yydoctt]

Glad it's helpful! Yes, Bamberg's thesis is great (and Slocum was his advisor). To clarify on the x-rail height, there's another machine configuration with short gantry legs (in between the Momus and the loop-under-table) used by CNCRP and some others: http://www.cncrouterparts.com/images...ender-logo.png
With that x-rail height, the moments are minimal and the gantry legs are easy to make stiff. I avoid the loop under the table as it wipes out helpful reinforcements in the lower frame. Or almost as stiff, most commercial machines put the x-rails just lower than the table, for better access:
http://cfnewsads.thomasnet.com/image...605/605670.jpg
http://www.cronsrud.com/images/machines/97M12.jpg

But yes, a moving table / fixed gantry is easier to make stiff, if you have the space. If not, a moving gantry can be made stiff enough for alum.

Thanks for more info! I like the idea of the mid raised gantry without under table loop. I suppose the loop was introduced to use just one leadscrew, but perhaps not.

When you say a moving gantry can be made stiff enough for aluminium, what performance do you think is realisticly attainable, using 8020 style structural members? Additional decisions such as linear rails and ballscrews should go to improve the performance also. I hope.

[charlieslasher]

Weight is your friend. Heavy structures are damp structures and this will minimise chatter when machining aluminium.

Id recommend Hiwin rails or better and would use bolted steel frame with epoxied contact surfaces to stop movement. Chiness spindels work well with High speed tool paths. Creating level surfaces for the rails is the hard part! good luck.

A moving table 800x500 can fit inside a 1.1m box. I did it. Make the table as wide as you can support. Use a good table 1inch 7075 plate canterleaverd on bearing blocks.

[yydoctt]

Weight is your friend. Heavy structures are damp structures and this will minimise chatter when machining aluminium.

Id recommend Hiwin rails or better and would use bolted steel frame with epoxied contact surfaces to stop movement. Chiness spindels work well with High speed tool paths. Creating level surfaces for the rails is the hard part! good luck.

A moving table 800x500 can fit inside a 1.1m box. I did it. Make the table as wide as you can support. Use a good table 1inch 7075 plate canterleaverd on bearing blocks.

Wow, just checked out your build! How did you get the beams and surfaces level? It's the only thing that worries me about bolting together steel at the moment.

You say you did 5mm DOC, can you show some pictures? That's impressive!

Do you think you would get similar performance with moving gantry on Hiwin rails? I'm aiming for a 550x550x100 cutting area, and would quite like to keep it reasonably small.

[charlieslasher]

5mm doc was with 16% width of cut. slotting i only run 1mm doc all in 7075. I pored epoxie for the y- axis rails. It wasnt perfect and i intend to redo this at some point when i upgrade my rails to hiwin. X axis i used set screws behind the rails. i wouldn't recommend this.

I plan to get some self levelling epoxy and make a surface plate on the ground. Then lay the x axis beam down to create another flat epoxied surface that the rails will sit on.

-All connections are epoxied and bolted together using madvac technique ( search him there is alot of good information in his build)
-Use hot rolled steel sections if you can. The have less internal stress/less movement over time. Dont use an i beam like i did. Box section would be ideal.
-4 set screws at the connection between the gantry beam and the gantry column allow for final position of the linear rails so they lie in the correct plane. This worked really well and is standard practice when building very large CNC machine.

I would only build a gantry router if you intend on cutting mostly sheet metal parts and needed a big table area. Datron make HSM moving gantry mills for aluminium work. So it can be done. They are also heavy.

[yydoctt]

5mm doc was with 16% width of cut. slotting i only run 1mm doc all in 7075. I pored epoxie for the y- axis rails. It wasnt perfect and i intend to redo this at some point when i upgrade my rails to hiwin. X axis i used set screws behind the rails. i wouldn't recommend this.

I plan to get some self levelling epoxy and make a surface plate on the ground. Then lay the x axis beam down to create another flat epoxied surface that the rails will sit on.

-All connections are epoxied and bolted together using madvac technique ( search him there is alot of good information in his build)
-Use hot rolled steel sections if you can. The have less internal stress/less movement over time. Dont use an i beam like i did. Box section would be ideal.
-4 set screws at the connection between the gantry beam and the gantry column allow for final position of the linear rails so they lie in the correct plane. This worked really well and is standard practice when building very large CNC machine.

I would only build a gantry router if you intend on cutting mostly sheet metal parts and needed a big table area. Datron make HSM moving gantry mills for aluminium work. So it can be done. They are also heavy.

Great information, thank you. As suggested I looked up madvac as well as using epoxy to level surfaces.
This seems like the way to go! Would the recommendation be to pour epoxy on two perpendicular surfaces of each beam? Then use the madvac method to join two pieces?

How do you find the Chinese spindle, how slow can you go on it and how suitable is it for Alu?

[charlieslasher]

Great information, thank you. As suggested I looked up madvac as well as using epoxy to level surfaces.
This seems like the way to go! Would the recommendation be to pour epoxy on two perpendicular surfaces of each beam? Then use the madvac method to join two pieces?

How do you find the Chinese spindle, how slow can you go on it and how suitable is it for Alu?

Spindel has been good with limited use so far. The faster the better i have found for aluminium with carbide 24k and 5m/min feed. i have drilled at 6k but i dnt think there is much torque below 8k rpm. You will need a mister and air blast setup.

[dmalicky]

Thanks for more info! I like the idea of the mid raised gantry without under table loop. I suppose the loop was introduced to use just one leadscrew, but perhaps not.

When you say a moving gantry can be made stiff enough for aluminium, what performance do you think is realisticly attainable, using 8020 style structural members? Additional decisions such as linear rails and ballscrews should go to improve the performance also. I hope.

Yes, the purpose of the loop is just as you say. Ballscrews with stiff axial end support and profile rail do help a lot (though if the structure isn't well designed, that $ gives only partial ROI).

For 8020, I've not modeled any of it yet, so don't have numbers. Structurally, I think it has pros and cons compared to a std box section:

+ If the section has internal diagonal webbing, that should help prevent "collapse" of the cross-section for edge loads (parallelogram mode). E.g., these are good:
http://www.8020.net/Images/End-40-8016.gif
http://img2.fastenal.com/productimages/0961289.jpg
These, not so much:
http://www.8020.net/Images/End-45-9090.gif
http://www.misumiusa.com/CategoryIma...HFSP5-4080.jpg

+ Super easy to thread into the ends, enabling some joints that would be difficult otherwise.

+ Raised edges gives better clamping for no extra work.

But,
- The t-slotted perimeter puts a lot of material closer to the neutral axis, reducing the bending stiffness. bh^3 says a std box will handily beat it, by weight, size, and $.

- For the smaller sizes, the t-slots intrude so much that the closed-section dimensions are very small, so little torsional stiffness:
http://imageserver.grainger.com/is/i...er/16U248_AW01
Even for the larger sizes, the inscribed closed section is about 0.7 * the outside dimension:
http://www.8020.net/Images/3030-Profile.gif
Now take (0.7)^4 --> only 25% the torsional stiffness of a same-sized box section.

- Limited selection of larger sizes -- probably the biggest disad for the gantry tube. I'm designing a 4' x 8' machine now and shooting for around 40,000 lb/in for the stiffness at the tool (combined X and Y). To get that, the gantry tube needs to be 8 x 8 x 0.25" wall alum (with an internal diagonal sheet, or bulkheads)... there just isn't anything close to that size in 80/20. Even if there were, it would probably add ~30 lb -- I'd rather add 30 lb of sand, for more damping!

I don't mind drilling and tapping holes in alum, so the T-slots don't help me much, and it's those very T-slots that hurt the stiffness so much. If they made 8020 without the T-slots but with a diagonal web and end holes, I'd love it. But then they'd have to call it something else... Engineer's Ideal Extrusion?