Gantry design and FEA analysis

[eci22]

Hi all,
So a thread of mine asking about advice on different X axis rail configurations (http://www.mycncuk.com/threads/13497-Choosing-between-alternative-X-axis-bearings-designs) kind of deviated into a discussion about the suitability of the material chosen which is 50x50x3mm mild box steel, I didn't mind as all advice is welcome, but I wanted to see some data to backup what I was being told, which is the main point of this thread so I did some FEA analysis:

Goal for the machine
mill 6082T6 Aluminium at 1 to 3mm DOC with an accuracy of between 0.1 to 0.3mm

Dimensions:
27944

600x350mm not a huge gantry by any means

Fixed Ganry Designs- 50x50x3mm mild box steel

A: No bracing used more as a baseline for comparison against other designs
27945

B: Vertical bracing
27946

C: Diagonal bracing
27947

All FEA analysis' uses 980 Newtons force = 100kg

FEA Analysis Type 1, no Z axis plate

27948
Design A: Max displacement of 0.01 mm under a force of 100Kg

27949
Design B: Max displacement of 0.009 mm under a force of 100Kg

27950
Design C: Max displacement of 0.009 mm under a force of 100Kg

FEA Analysis Type 2, with Z axis plate (200x300x20mm Cast Tool Plate Aluminium)

27951
Design A: Max displacement of 0.09 mm under a force of 100Kg

27952
Design B: Max displacement of 0.07 mm under a force of 100Kg

27953
Design C: Max displacement of 0.08 mm under a force of 100Kg

Questions

1.Which is a more realistic FEA setup, Type 1 or Type 2 or something different (and why) ?

2. Is 100kg force a good ballpark figure to use given the cutting parameters of my machine ? This number was a pure guestimate on my part, what is the correct way to calculate?

3.The displacement figures seem to suggest the material (50x50x3) is suited to the task- 0.09 to 0.07mm displacement under 100kg seems perfectly acceptable to me unless I've missed something ?

4. What else have I missed ?

All help appreciated

EDIT:
Voicecoil pointed out incorrect decimal places in second set of results, thank you!

[Voicecoil]

Setup 2 is much more realistic since you want to design for decent performance when the cutter is hanging down a fair way below the gantry, 'coz that's where it will often be used. Having an equal horizontal force on each beam really isn't going to happen as that would mean the workpiece being considerably higher than the bottom of the gantry. 1000N cutting force is maybe a bit much unless you're using big cutters, I've heard figures of 200N tops bandied about before for typical machines such as people on this forum build though I haven't seen the sums behind that. I did find this reference a little while back though:

https://www.ctemag.com/news/articles/understanding-tangential-cutting-force-when-milling

Putting some numbers into the formula for a 1.5mm DOC on a 3 flute cutter at 0.1mm feed per tooth I get a tangential force around 90N for the best 6000 series ali.

It would also be interesting to do an analysis with a plate fixed on the back of your 2 bits of box section as that will make kind of a super box with the Z plate.

PS looking at your pics again, there might be a decimal point adrift in the text for the second set of results..... :whistle:

[routercnc]

Agree with Voicecoil.
Analysis 2 is closer to worst case as this puts a twisting moment on the gantry as the force is applied at some distance away from the structure and will usually cause the biggest deflection.

I wrote a spreadsheet a long time ago just to play with the numbers and I looked at 3 load conditions.
Vertical load at the centre of the gantry due to the mass of the Y and Z axis and estimate the sag.
Lateral load applied at the tool tip as this will want to turn the gantry into a parallelogram- one reason why raised bed sides is stiffer than tall gantries.
Foreaft load at the tool tip to twist the gantry. This usually causes the most deflection and you need to make the gantry have a shape with material around the outside not 2 isolated beams. So yes plate them together is an option.

I used 80N load for cutting aluminium.

Remember this is all to see relative performance and get a feel for what aspects are important as it will assume perfect joints and you would need to model the whole machine and the cutter to get close to the actual deflection and even this is the steady cutting force not the deflection caused by vibration which will add some more.

The stiffest gantries in terms of numbers are raised bed sides with a large box section as this gives good stiffness for all load cases. But practical considerations mean other options are good including the famous L shape gantry with 2 good sized rectangles joined in an L shape.
In the end you want a single shape with as much material as possible away from the central axis it is trying to twist or bend about (the neutral axis). Once you have the largest outline you then up the wall thickness for many reasons including getting the load into the section from discretely mounted components. Depending on the design the very large sections may need baffles or subdivisions.

It’s fun to look at the numbers and get a feel for it but also look at successful machines to see what has worked before and bare in mind the practicalities of building it and having adjustment capability.

[Kitwn]

I know you are designing on the basis of 50 x 50 material but rather than the complexity of the braces in the designs above have you considered two pieces of 100 x 50 welded together to give a flat face 200 high? Your overall height looks to be about that anyway. I'm really asking because that's what I've built and I'd love to see the deflection calculations.:nevreness:

[eci22]

I know you are designing on the basis of 50 x 50 material but rather than the complexity of the braces in the designs above have you considered two pieces of 100 x 50 welded together to give a flat face 200 high? Your overall height looks to be about that anyway. I'm really asking because that's what I've built and I'd love to see the deflection calculations.:nevreness:

Hi Kitwin, do you mean like this ?
27955

This seems like a very stout design and is better than the bracing I used. I have used it to generate some numbers below- hope they are of interest. But the analysis suggests the biggest component for deflection is the material of the Z plate, it seems to contribute the most. If you are interested let me know your exact materials ,including Z plate and I would be happy to run the numbers for you

I've changed all model calculations to use 100 Newtons of force. Also the Z plate is now 200x300x12mm Aluminium

27963

My main question is where did you perform the welds for the two pieces of 50x100?, A seam weld covering the entire length would lead to significant warp id imagine and tack welds wouldn't hold up, bolts would work I suppose. How did you make yours ?



Agree with Voicecoil.
Analysis 2 is closer to worst case as this puts a twisting moment on the gantry as the force is applied at some distance away from the structure and will usually cause the biggest deflection.


Hi routercnc, can I just say the videos on your CNC build is really inspiring. I learnt so many subtle things the info is worth its weight in gold to me. It's something I come back to time and time again. Thank you for putting it out there !




I wrote a spreadsheet a long time ago just to play with the numbers and I looked at 3 load conditions.
Vertical load at the centre of the gantry due to the mass of the Y and Z axis and estimate the sag.


(All simulation diagrams use 100 newton force)

Vertical Load like this ?
27956

Presumably the only force is gravity and the weight of your cutting tool, which I haven't included.



Lateral load applied at the tool tip as this will want to turn the gantry into a parallelogram- one reason why raised bed sides is stiffer than tall gantries.


lateral load
27957

correct ?



Foreaft load at the tool tip to twist the gantry. This usually causes the most deflection and you need to make the gantry have a shape with material around the outside not 2 isolated beams. So yes plate them together is an option.


Is that the same as moment force around the Y axis of the Z plate ?

27961



Remember this is all to see relative performance and get a feel for what aspects are important as it will assume perfect joints and you would need to model the whole machine and the cutter to get close to the actual deflection and even this is the steady cutting force not the deflection caused by vibration which will add some more.

Yes definitely, I just find this really interesting to do. It's a good way of comparing different designs. I'll use this for guidance only and for learning FEA which is something I have not done before. I also appreciate not having the Y axis, the feet, basically the rest of the machine modeled will skew the results somewhat



The stiffest gantries in terms of numbers are raised bed sides with a large box section as this gives good stiffness for all load cases. But practical considerations mean other options are good including the famous L shape gantry with 2 good sized rectangles joined in an L shape.

Would you mind directing me to a picture of this gantry design ?



Setup 2 is much more realistic since you want to design for decent performance when the cutter is hanging down a fair way below the gantry, 'coz that's where it will often be used. Having an equal horizontal force on each beam really isn't going to happen as that would mean the workpiece being considerably higher than the bottom of the gantry. 1000N cutting force is maybe a bit much unless you're using big cutters, I've heard figures of 200N tops bandied about before for typical machines such as people on this forum build though I haven't seen the sums behind that. I did find this reference a little while back though:

https://www.ctemag.com/news/articles...e-when-milling

Putting some numbers into the formula for a 1.5mm DOC on a 3 flute cutter at 0.1mm feed per tooth I get a tangential force around 90N for the best 6000 series ali.

It would also be interesting to do an analysis with a plate fixed on the back of your 2 bits of box section as that will make kind of a super box with the Z plate.

PS looking at your pics again, there might be a decimal point adrift in the text for the second set of results.....


Thanks for the link Voicecoil, much appreciated. I updated the the force to 100 newtons in my above simulations, I also made my Z plate thinner as I realised 20mm thick tool plate is out of my budget. If I have done the simulations correctly then the numbers still look promising. I will look to add the fixed plate at the back tomorrow. Thank you very much for your input

[Kitwn]

Hi Kitwin, do you mean like this ?
27955

My main question is where did you perform the welds for the two pieces of 50x100?, A seam weld covering the entire length would lead to significant warp id imagine and tack welds wouldn't hold up, bolts would work I suppose. How did you make yours ?


That's about it. just two pieces of 50 x 100 welded together to give 200 x 50. The whole is then welded to the gantry uprights.

Bear in mind that this gantry was the first time I ever tried welding, I bought a cheap stick welder specifically for this job. To minimise the warping I clamped the pieces together and used a series of short welds alternating between the front and back seams. I use a welding technique I've called "Bird Poo" since that's what the result most resembles. Copious use of an angle grinder, gobs of car body filler and a layer of paint make it look much better than it is. The front surface was flattened (not very well) using epoxy.

[Voicecoil]

I also made my Z plate thinner as I realised 20mm thick tool plate is out of my budget. If I have done the simulations correctly then the numbers still look promising. I will look to add the fixed plate at the back tomorrow. Thank you very much for your input

Tip: if you can add some bits of bar to stiffen your Z-plate (i.e. turn it into a channel section) you will do better cost for cost than a flat plate. For example a 10mm plate 180mm wide with some 30 x 15 bar fixed down the edges is about 2.5x as stiff front to back as a piece of 20mm plate 180mm wide - go simulate it if you want to. I mention this because you generally end up with some space between the front and back z plates due to having to accommodate the carriages, ballscrew etc., so why not put something useful in there. The front (moving part) of the z-axis maybe isn't such an issue as the z rails will stiffen that.

PS If you want 0.1mm accuracy, you will likely need to design for static deflection figures rather better than that - remember that cutting metal produces a lot of vibration which can mess things up.

PPS what FEA package are you using? looks good.

[routercnc]

Glad you found the CNC videos useful. It was a good way to pass on the ideas I had seen, and add some of my own thoughts.

In the analysis:
Yes, vertical direction forces are mostly due to gravity but they will be much higher than 100 N as this only represents a Y and Z assembly of about 10 kg. More like 25-50 kg depending on the design, possibly more. This is to see how much the gantry will sag if you are surfacing a plate as in the extreme you would cut a dished shape. There is also a pull down force when using spiral fluted cutters, and there will be forces when drilling or plunging.

The lateral direction analysis is correct for the load direction but because of the grounded sides in your simple analysis it won't mean much on the results you have there.

The fore/aft analysis is in the wrong direction - it would be in the 'Z' direction using the coordinate system you have in the top right corner and would cause the part of the Z axis plate which is hanging down to all bend forward or rearward, and cause the gantry to twist about the 'X' axis as per your coordinates in the picture. This is usually the worst case of all the load conditions.

There are lots of square and rectangular gantries out there but the L shapes gantries were 'invented' by JazzCNC:

27981

[Boyan Silyavski]

Combine both and that's what you will have if you do not design properly the machine.

If you design your Z properly , use proper bearings and mount your spindle correctly, variant 2 will be non existent, so variant 1 is the real deal.
As if you clamp your spindl properly its body will strengthen the Z and only a couple of braces will make the Z equivalent to like a solid chiung of 100x100mm metal


Here is some design wisdom to you: it does not matter what profile you use. A well designed machine will have such design as to simulate at least 10cm wide x 3cm thick steel plate against all cutting directions. Against the 3cm no the 10cm. Or the equivalent. in whatever material/s you do it.

So go and make it 50x50mm but remember what i said if you want your machine to cut vibration free aluminum or even steel.

Check my build at page 7 (http://www.mycncuk.com/threads/6619-Quite-an-Unusual-one/page7) for the gantry and page 16 (http://www.mycncuk.com/threads/6619-Quite-an-Unusual-one/page16) and 17 for the Z, to grasp the idea of how serios a gantry and a Z have to be to machine fully extended at 200mm.

Its not only my machine i am bragging about. There at least a couple machines on forum that are seriously heavy duty and one way or another they are build like that.

[eci22]

Tip: if you can add some bits of bar to stiffen your Z-plate (i.e. turn it into a channel section) you will do better cost for cost than a flat plate. For example a 10mm plate 180mm wide with some 30 x 15 bar fixed down the edges is about 2.5x as stiff front to back as a piece of 20mm plate 180mm wide - go simulate it if you want to. I mention this because you generally end up with some space between the front and back z plates due to having to accommodate the carriages, ballscrew etc., so why not put something useful in there. The front (moving part) of the z-axis maybe isn't such an issue as the z rails will stiffen that.



Do you mean like this ?
28048




PS If you want 0.1mm accuracy, you will likely need to design for static deflection figures rather better than that - remember that cutting metal produces a lot of vibration which can mess things up.
PPS what FEA package are you using? looks good.

The FEA software is Fusion 360, it is really nice once you get used to it. What kind of ballpark figures should I be looking at for in the deflection analysis ? At the moment I'm not actually taking figures as real world numbers, but more to compare the deltas between different designs.



Yes, vertical direction forces are mostly due to gravity but they will be much higher than 100 N as this only represents a Y and Z assembly of about 10 kg. More like 25-50 kg depending on the design, possibly more. This is to see how much the gantry will sag if you are surfacing a plate as in the extreme you would cut a dished shape. There is also a pull down force when using spiral fluted cutters, and there will be forces when drilling or plunging.

Great I will, add the additional forces for future analysis



The fore/aft analysis is in the wrong direction - it would be in the 'Z' direction using the coordinate system you have in the top right corner and would cause the part of the Z axis plate which is hanging down to all bend forward or rearward, and cause the gantry to twist about the 'X' axis as per your coordinates in the picture. This is usually the worst case of all the load conditions.


So rotating around the X axis would would be the following force, correct ?
28049



Bear in mind that this gantry was the first time I ever tried welding, I bought a cheap stick welder specifically for this job. To minimise the warping I clamped the pieces together and used a series of short welds alternating between the front and back seams. I use a welding technique I've called "Bird Poo" since that's what the result most resembles. Copious use of an angle grinder, gobs of car body filler and a layer of paint make it look much better than it is. The front surface was flattened (not very well) using epoxy.


Yes I'm quite familiar with the bird poo welding technique : ). When you were welding, did you let the weld cool before removing the clamps and turn the piece around to clamp the other side or did you weld one side un-clamped, turn it over clamp it down and then weld the other side ? It you clamped down the first side then welded and turned over to weld the second side the clamps wouldn't have much effect on the first side at least ?- I'd be really interested to hear your workflow as I've alredy welded my Y axis and I am trying to improve my amount of warp in the rest of the build ( I'm using gaseless MIG, a very very basic machine), I've seen a bunch of you tube videos but haven't dialled in my sequencing yet.



Combine both and that's what you will have if you do not design properly the machine.

If you design your Z properly , use proper bearings and mount your spindle correctly, variant 2 will be non existent, so variant 1 is the real deal.
As if you clamp your spindl properly its body will strengthen the Z and only a couple of braces will make the Z equivalent to like a solid chiung of 100x100mm metal


Here is some design wisdom to you: it does not matter what profile you use. A well designed machine will have such design as to simulate at least 10cm wide x 3cm thick steel plate against all cutting directions. Against the 3cm no the 10cm. Or the equivalent. in whatever material/s you do it.

So go and make it 50x50mm but remember what i said if you want your machine to cut vibration free aluminum or even steel.

Check my build at page 7 for the gantry and page 16 and 17 for the Z, to grasp the idea of how serios a gantry and a Z have to be to machine fully extended at 200mm.

Its not only my machine i am bragging about. There at least a couple machines on forum that are seriously heavy duty and one way or another they are build like that.



Hi Boyan, thanks for your input- do you mean your build log from project 1 in your signature ? If so I just want to point out one thing from the first page of that build log:



2. Money is not an issue for the frame or the length of the supported rails, so i am not going to go cheap to save 10cm of rail or steel profile.


Unfortunately this is not the situation I am in, so I am trying to generate as much data and perform as much analysis on my design so I have enough information to make informed decisions about my build. Do you have images of your final build, it would be great to see an example of the Z axis you are describing. Thanks

[Kitwn]

Yes I'm quite familiar with the bird poo welding technique : ). When you were welding, did you let the weld cool before removing the clamps and turn the piece around to clamp the other side or did you weld one side un-clamped, turn it over clamp it down and then weld the other side ? It you clamped down the first side then welded and turned over to weld the second side the clamps wouldn't have much effect on the first side at least ?- I'd be really interested to hear your workflow as I've alredy welded my Y axis and I am trying to improve my amount of warp in the rest of the build ( I'm using gaseless MIG, a very very basic machine), I've seen a bunch of you tube videos but haven't dialled in my sequencing yet.


You are overestimating my skill and ability! I used a cheap stick welder I bought online which is the only welder I have ever actually used. The pieces were clamped together with C-clamps and vice-grips and if I remember correctly I used a couple of pieces of wood as braces to keep the pieces as flat as possible. I don't have a welding table so worked on the gravel path outside my shed. I clamped everything as tightly as I could before I began and didn't undo any of the clamps until it was finished. Having spent the last couple of days properly aligning this machine for the first time I can say how pleasantly surprised I am with the result of my first attempt at welding. It's needed some shims to get everything level and parallel but has turned out very well I think. The important thing is to think about how you will make your design adjustable, where you will need to have joints that can take shims for alignment and how you will access those joints when the machine is complete without taking half of it to bits again.

[Voicecoil]

Do you mean like this ?
28048

Not really, I'm pretty sure it would be best to have the Hiwin rails fix directly onto the plate (which will add some rigidity already, especially with the extra stiffness of steel, so would be worth adding into you simulation) I was meaning maybe either side of the ballscrew (assuming there's enough clearance) or even on the front either side of the spindle mount if there's space. Just trying to help you add some cheap rigidity :-)


The FEA software is Fusion 360, it is really nice once you get used to it. What kind of ballpark figures should I be looking at for in the deflection analysis ? At the moment I'm not actually taking figures as real world numbers, but more to compare the deltas between different designs.
I'm no expert on dynamic loads whilst cutting, but as it's not exactly a smooth process (hence the noise!) I would expect peak loads to be may 1.5...2x the static???

[eci22]

You are overestimating my skill and ability! I used a cheap stick welder I bought online which is the only welder I have ever actually used. The pieces were clamped together with C-clamps and vice-grips and if I remember correctly I used a couple of pieces of wood as braces to keep the pieces as flat as possible. I don't have a welding table so worked on the gravel path outside my shed. I clamped everything as tightly as I could before I began and didn't undo any of the clamps until it was finished. Having spent the last couple of days properly aligning this machine for the first time I can say how pleasantly surprised I am with the result of my first attempt at welding. It's needed some shims to get everything level and parallel but has turned out very well I think. The important thing is to think about how you will make your design adjustable, where you will need to have joints that can take shims for alignment and how you will access those joints when the machine is complete without taking half of it to bits again.

Thanks for the advice, yes I think the joints and adjustability becomes really important- so many choices!



Not really, I'm pretty sure it would be best to have the Hiwin rails fix directly onto the plate (which will add some rigidity already, especially with the extra stiffness of steel, so would be worth adding into you simulation) I was meaning maybe either side of the ballscrew (assuming there's enough clearance) or even on the front either side of the spindle mount if there's space. Just trying to help you add some cheap rigidity :-)


Like this ?

28052

That does seem like a great idea to add rigidity

I think I've read a few times that it's better to have the guide blocks fixed and the linear rails moving down on the Z axis- I've also seen this quite a few times on builds, apologies if this is not what you mean by saying ('best to have the Hiwin rails fix directly onto the plate')

[Voicecoil]

Well sort of.... The direct fixing of the rails thing was more aimed at how you're spacing the front and back Z plates apart. Currently you have the carriages fixed hard onto the back plate, and then some bits of square bar lifting the rails off the front plate presumably to clear the ballscrew bits. This will make the front plate wonderfully stiff front to back, but it would be worth doing your FEA to see what lateral/twisting rigidity is like. The other option would be to have the rails direct onto the front plate, and lift the carriages on a couple of bits of rectangular plate, if this leaves the front plate assy. too bendy, stick the stiffening bars onto the front plate instead of the back. My gut feeling tells me that getting the front plate stiff is more important as it's that that will be dangling down at maximum Z extension. It will be interesting to see what the FEA predicts for the different options.

PS do think about the locations of the fixings for both X and Z carriages - get them overlapping and you might end up with a wonderfully rigid assembly that it's impossible to assemble :disillusionment: And also think about making sure there's a way of tramming the Z-axis (adjusting it so it's perpendicular to the bed)

[JAZZCNC]

Your concentrating too much on deflection and not thinking about resonant vibrations which are the real killers of tools and finish. That Z-axis shown will resonate because the plate is too thin with a large gap between the rails acting like a trampoline.

Going back to the 50mm box section then I can tell you from experience building machines that it's not the best choice unless you use at least 6mm wall thickness and good bracing. The rectangular tube is much better when used in the correct direction and braced correctly, however you still need a thick wall to help with vibrations.

The simple FEA you are doing won't tell you anything about how the machine will perform in real life. You would be better advised to drop the FEA and put the time into reading builds and asking questions about any designs you like. By that, I mean directly asking the builder and not posting a thread asking the questions about a particular design as you'll get lots of conflicting advice and often from people with little experience or worse armchair experience.

Can't beat experience and to get the experience you need to get on with it so put the PC down and get the welder out.!!

[Kitwn]

Thanks for the advice, yes I think the joints and adjustability becomes really important- so many choices!

You then have to choose what measurements you need to make and instruments to buy in order to set those adjustments correctly. There are a lot of worms in the can but you learn a lot too.

[Boyan Silyavski]

You don't like to hear it and i would not say it again, but one last time for the benefit of all people who will read that in the future:

-50x50 is not right for a machine that will mill aluminum, 80x80 or 100x100 3mm is the right profile to form the gantry and the sides or the whole frame

-saving on box section size=5 times more cutting and welding diagonal braces, where if 80x80 or 100x100 there is no need for diagonals and simpler construction

-there is no such thing as 0.1- 0.3mm accuracy, especially cutting aluminum. If the machine is made right it will be repeatable 0.05mm and accuracy will be ~0.01- 0.02mm
0.3mm accuracy is for MDF machines or crappy cheap machines, in other words trash that will not cut aluminum at all.

-Modeling wil get you to nowhere, only wasting your time. Check what people build on forum, choose a build, see the results and imitate it. Then you will be successful. Make a build log, ask questions and listen to them answers..

-use common sense, if you lack it, there is no software to help that. Go to a local welder and find a 2m piece of that 50x50 profile, 80x80, 100x100, touch it , try to bend it then for 1 min everything will become clear to you. 3m 100x100 is flexible and 50x50 is like a macaroni

-every crappy machine can cut aluminum, scratching or slow, but the better the machine the most perfect the finish will be.

-if on a build of 60x90x20 for example machine you think that 10cm more or less is a great thing, better not start it. 10cm of rail and steel more or less in abuild are no more than 100-150e, and if you dont have 3k for parts and 500-800 for tooling at least, you are in the wrong place.

[Neale]

Personal view here, but based on using my own machine for the last 2-3 years. It's described somewhere on this forum - search for "AVOR" and my username if you want to look it up. It's built mainly of 50x50x3 with some 100x50x3. Steel, all welded, various bits and pieces from 20mm Ecocast aluminium. I suspect that if you did a static FEA job on it, it would come out as being pretty stiff (for some definition of "stiff", anyway). It's not "Boyan bullet-proof" but it isn't bad. That 3mm box is strong enough - if well-braced. If I were doing it again, would I go the same route? No - it's strong enough but resonance is always a problem, and that's not something you can easily fix after the event. I should have gone for 5mm, I reckon, although there's not much else I would change. That would have meant that I could drill and tap directly into it for fixings instead of needing reinforcing strips or other workarounds.

So, what can t cut? Can it cut aluminium? Well, yes, probably, but it's horrible stuff to cut and with a plywood bed, I don't want to do "wet" cutting so I try to avoid it. Of course, it cuts wood/ply/mdf etc with no problem. But it also cuts - regularly - brass and steel. I use small (typically 2-4mm) carbide cutters which are happy to run at high speed and cut dry with no problem. No, I'm not carving engine blocks from large lumps of steel but I cut lots of fiddly little bits which would be difficult to do on the manual mill because of the detail needed - like a brass lost wax master for a lapel badge. One limiting factor is not machine strength or spindle capability but just work-holding - one area that I would revisit another time around is how to build a bed with clamping capability for the kinds of things I now do, as opposed to what I thought I would do when I first built it. Difficulty in just holding things firmly limits cutting forces you can apply.

I'm also in the "just put something plausible together, listen to comments based on experience, then fire up the angle grinder/welder" camp. The time you spend analysing you could cut and weld a couple more braces...

[eci22]

Well sort of.... The direct fixing of the rails thing was more aimed at how you're spacing the front and back Z plates apart. Currently you have the carriages fixed hard onto the back plate, and then some bits of square bar lifting the rails off the front plate presumably to clear the ballscrew bits. This will make the front plate wonderfully stiff front to back, but it would be worth doing your FEA to see what lateral/twisting rigidity is like. The other option would be to have the rails direct onto the front plate, and lift the carriages on a couple of bits of rectangular plate, if this leaves the front plate assy. too bendy, stick the stiffening bars onto the front plate instead of the back. My gut feeling tells me that getting the front plate stiff is more important as it's that that will be dangling down at maximum Z extension. It will be interesting to see what the FEA predicts for the different options.

PS do think about the locations of the fixings for both X and Z carriages - get them overlapping and you might end up with a wonderfully rigid assembly that it's impossible to assemble :disillusionment: And also think about making sure there's a way of tramming the Z-axis (adjusting it so it's perpendicular to the bed)

Yes thank you for pointing the overlapping XZ carriages, this is something I tend to forget, I will try the FEA on your suggestion and post an update- thank you very much Voicecoil !



You then have to choose what measurements you need to make and instruments to buy in order to set those adjustments correctly. There are a lot of worms in the can but you learn a lot too.

Yes, just waiting for some of those instruments to arrive, got a surface plate, square, dial indicator/ test indicator and some 123 blocks I think that should have me covered




Can't beat experience and to get the experience you need to get on with it so put the PC down and get the welder out.!!


I completely understand where you are coming from, I just like doing it- it adds to the enjoyment of the build process to me personally. I'm not trying to get others to do it or recommend it as a technique to anyone else, it's just my personal preference.



Your concentrating too much on deflection and not thinking about resonant vibrations which are the real killers of tools and finish. That Z-axis shown will resonate because the plate is too thin with a large gap between the rails acting like a trampoline.

Going back to the 50mm box section then I can tell you from experience building machines that it's not the best choice unless you use at least 6mm wall thickness and good bracing. The rectangular tube is much better when used in the correct direction and braced correctly, however you still need a thick wall to help with vibrations.

The simple FEA you are doing won't tell you anything about how the machine will perform in real life. You would be better advised to drop the FEA and put the time into reading builds and asking questions about any designs you like. By that, I mean directly asking the builder and not posting a thread asking the questions about a particular design as you'll get lots of conflicting advice and often from people with little experience or worse armchair experience.


It's not a one stop solution to everything and I'm not going to treat it as such, but I do think comparing the deltas between different design choices is useful and interesting to do. It is also useful in pointing out the best candidates for bracing, this may be something that is second nature to someone like yourself and many others but for me, seeing this visualised is very helpful.

I know vibrations will something that needs to be compensated for, I appreciate 6mm would be better than 3mm and 8 mm would be than 6mm etc- but I have what I have and need to make the best use of it. I enjoy the flexibiity that CAD gives you in terms of experimentation and will try to combine it with the vast array of insight in the forum.



when used in the correct direction

Would you mind expanding on this please ? Thank you




Personal view here, but based on using my own machine for the last 2-3 years. It's described somewhere on this forum - search for "AVOR" and my username if you want to look it up. It's built mainly of 50x50x3 with some 100x50x3. Steel, all welded, various bits and pieces from 20mm Ecocast aluminium. I suspect that if you did a static FEA job on it, it would come out as being pretty stiff (for some definition of "stiff", anyway). It's not "Boyan bullet-proof" but it isn't bad. That 3mm box is strong enough - if well-braced. If I were doing it again, would I go the same route? No - it's strong enough but resonance is always a problem, and that's not something you can easily fix after the event. I should have gone for 5mm, I reckon, although there's not much else I would change. That would have meant that I could drill and tap directly into it for fixings instead of needing reinforcing strips or other workarounds.

So, what can t cut? Can it cut aluminium? Well, yes, probably, but it's horrible stuff to cut and with a plywood bed, I don't want to do "wet" cutting so I try to avoid it. Of course, it cuts wood/ply/mdf etc with no problem. But it also cuts - regularly - brass and steel. I use small (typically 2-4mm) carbide cutters which are happy to run at high speed and cut dry with no problem. No, I'm not carving engine blocks from large lumps of steel but I cut lots of fiddly little bits which would be difficult to do on the manual mill because of the detail needed - like a brass lost wax master for a lapel badge. One limiting factor is not machine strength or spindle capability but just work-holding - one area that I would revisit another time around is how to build a bed with clamping capability for the kinds of things I now do, as opposed to what I thought I would do when I first built it. Difficulty in just holding things firmly limits cutting forces you can apply.


Thank you Neal, I also believe your CNC machine is larger than mine ? my cutting area is 3040 with a 600x600 frame, so I'm thinking the relatively smaller size works in my favour. I believe the thicker material gives you a bigger operating window and the thinner stuff means your pocket is smaller and probably slower and I am OK with that.

[JAZZCNC]

-use common sense, if you lack it, there is no software to help that.

Boyan that's a Class statement and 100% spot on.!! . . . . . That's going to be on my Tag line..:encouragement:

[Boyan Silyavski]

Steel box i buy for 1e per kg, so 30-40 euro more on a machine when i value my hour 25-100e should not be a big deal, right?

[JAZZCNC]

Would you mind expanding on this, please? Thank you

Well, it's strongest when used on its edge, so for instance if fastening rails onto it you wouldn't use it flat unless it's supported from below. When properly used and supported it's very strong. Hence why my "L" shape gantry is works so well because together 2 pieces of rectangle form a very strong gantry in both directions but still allow reasonably lightweight Gantry that is stiff and dampens vibrations.

Now don't get me wrong I'm not saying don't play with FEA or design in CAD.!!... ALL... my machines are designed in Solid works down to the very last detail and done FEA on some of them in some key areas just for my own interests to see if it matches the real thing. And even with the detail, I've model them to they rarely match with anything I can see or measure.

Just pick a design that suits your needs and get on with it.!! . . . . If needs to be stronger get the welder back out.! . . . Or start Mk2 because I can tell you know no matter how well you model the design and try to think things through there will always be things you'd do differently the next time around.!

[Neale]

Or start Mk2 because I can tell you know no matter how well you model the design and try to think things through there will always be things you'd do differently the next time around.!

Now, ain't that just the truth!