Requesting Guidance on Designing a CNC Machine for Machining Wood and Polystyrene

[POORIAPK]

Hello dear friends and engineers. For about two months, I have been working on designing a CNC machine that can machine wood and polystyrene without any issues in dimensions of 2500x1300x500mm. Additionally, this machine has a fourth axis and a rotary, which are separately placed on the machine's table.

Due to my lack of sufficient experience in CNC design, I request you to take a look at my machine and point out any flaws so that I can fix them.

Furthermore, some parts of the machine have been removed to clearly display its structure.
Also, this is a preliminary design!


Materiels : 1 - 80x80mm Aluminium Extrusion T-Slot Profile (8 slot), slot width 10mm
2 - Closed Loop Stepper Motor NEMA 34 - (85Kg XY , 120Kg Z)
3 - Linear Guideway Rail (HG20 XZ , HG25 Y)
4 - Ball Screw Diameter 20mm Pitch 5mm
5 - 2.2kw Spindle motor water cooled
......

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[arturod]

We can help you with the controller for this. Please contact us: https://cnc4pc.com/contact

Arturo Duncan
https://cnc4pc.com

[joeavaerage]

Hi,
just a couple of quick comments:

Aluminum extrusions are nice to use but as about as rigid as soggy spaghetti. Rigidity is absolutely the key to ANY machine for accuracy and freedom from vibrations.
Thick walled steel sections are vastly better than aluminum extrusions.

You intend on using a 20mm ballscrew over 2.5m long? They will whip at anything above slow speeds. For the long axis you might be better off with rack and pinion.

Craig

[peteeng]

Hi PRPK - 1) The Z axis plate is too thin, it needs to be much stiffer. 2) If you use ballscrews 5mm pitch will be too fine for XorY but good for Z. I see the Y axis is R&P so use R&P for the X axis (gantry). I think you have done that? 3) Otherwise looking good. I don't totally recommend using construction extrusion its expensive not very stiff, but it is convenient. Good luck with the build and rethink the Z axis plates... Peter

[POORIAPK]

Thank you very much for taking the time to review and comment, my friend joeavaerage. The reason I chose aluminum extrusions is because high-quality welding is quite expensive. Additionally, mounting Linear Guideway Rails requires a boring machine, which also incurs high costs. Moreover, other steps required to build a quality metal structure, such as stress relief, also have their own costs, and my budget is limited . However, we plan to fill the inside of the aluminum extrusions with a mixture of cement to increase rigidity and stiffness. Do you think this will meet our needs, or do you have a better suggestion? Should I bear the costs and build a metal structure?

Yes, you are absolutely right. A 20mm ballscrew at a length of 2.5 meters will whip at high speeds. According to the plans, we will use the ballscrew only for the Z-axis, and for the X and Y axes, we will use rack and pinion.

And finally, I am very glad and thankful that you are guiding me.

[POORIAPK]

Hi PRPK - 1) The Z axis plate is too thin, it needs to be much stiffer. 2) If you use ballscrews 5mm pitch will be too fine for XorY but good for Z. I see the Y axis is R&P so use R&P for the X axis (gantry). I think you have done that? 3) Otherwise looking good. I don't totally recommend using construction extrusion its expensive not very stiff, but it is convenient. Good luck with the build and rethink the Z axis plates... Peter

Thank you, my friend peteeng, for your helpful advice. Actually, I was debating between using a 15mm thick aluminum sheet and a 180x45mm aluminum extrusion (For Example) for the Z axis design. So I think this axis needs to be revised, right? Do you have a better suggestion?

2 - You are absolutely right. I used rack and pinion for the X and Y axes, and only a ballscrew for the Z axis.

3 - The use of aluminum extrusions is mainly for cost reasons, and I plan to fill them with a mixture of cement to increase rigidity. Do you think this will improve the situation?

And finally, I am very glad and thankful that you are guiding me.

[POORIAPK]

We can help you with the controller for this. Please contact us: https://cnc4pc.com/contact

Arturo Duncan
https://cnc4pc.com

I reviewed your website and will definitely consult with you on the electronics stage. Thank you!

[joeavaerage]

Hi,

3 - The use of aluminum extrusions is mainly for cost reasons, and I plan to fill them with a mixture of cement to increase rigidity. Do you think this will improve the situation?

No, I do not think it will improve the situation. Even aluminum has a modulus of 70GPa, whereas concrete is in the region of 20Gpa. The stiffer material (aluminium) being less compliant, especially as it
is at the circumference will transmit 99% of the load, while the more compliant material (concrete) especially as it is interior and therefore less geometrically favoured will transmit 1% of the load.
The compliance of the entire structure it almost entirely that of the aluminum alone.

Don't get me wrong, I understand the appeal of aluminum extrusions, but that does not change the fact that they are not stiff.

There are two ways to maximize the rigidity (minimize the compliance) of a structure:
1) Increase the dimensions so that the first moment of area is maximized but use the same material of the same modulus.
2) Use the same size geometry but utilise a material of higher modulus.

I would suggest that if you want to produce the most rigid machine you can of the dimensions you want you employ both strategies, ie increase the geometry AND use a stiffer material.

Here in New Zealand for instance 100 x 100 x 9 steel SHS is quite widely used and stocked. Regrettably 200 x 200 x 16, in fact any of the 16mm thick RHS or SHS sections are only available on indent order.
Even 100 x100 x 9 steel is going to be many times stiffer than your 80 x 80 aluminum extrusion. What do you want 'cheap and floppy' or 'difficult/expensive but stiff'?

Craig

[peteeng]

Hello PRPK - Concrete and aluminium are not friends. You run the risk of the aluminium corroding rapidly if you are in humid conditions. Normal concrete shrinks (and does not bond to the AL) so will not form a good connection to the tubing when it shrinks away from the internal surfaces. So the short answer is do not fill the tubes with stuff. Get the tubes size correct from the beginning and you don't need to fill them with anything. Aluminium construction extrusions and their bracketry are not cheap, they are convenient and that creates limitations. Having said that many successful machines are made from the stuff. Have you actually costed it yet? Everyone has a concept, a purpose and a budget. The cheapest material to build your first machine with is good quality plywood, actually MDF is half the price of ply and my work horse machine is half MDF and its 4 years old. MDF is easy to work with using std timber tools. Plywood is stiff and damp and makes machines that can cut aluminium easily. Next step up is std steel sections bolted together. Steel is cheaper then aluminium and stiffer. Then up from that is laser cut steel parts bolted together. I have done a few like this and they work very well. My machines are all over the forum do a little search to see them. Then if it was me I'd go aluminium welded or steel welded. Then I'd have a go using Carbon fibre. Comes down to your resources.

The image is of Scoot my MDF base machine. Its still square and true and I built it in 2020 and I give it quite a hard time. Since then I have built plywood gantries that are stiff and damp and cut aluminum easily, see Frankie image. I'm currently building a 8x4' machine mainly from ply and it will have a 3m long gantry. I choose ply because its easy to work in my workshop vs metals. I'm not saying this is the way you have to go, I am saying its possible and cheaper than extrusions.

I've built 7 machines and what I have learnt is that the Z axis assembly needs to be really stiff and thought out to be successful ie cut without wobbling. Your current Z axis will disappoint I feel. You have started designing at the machine base and worked towards the Z axis. The Z axis is the most important part of the machine so start there. Keep at it. Peter

[routalot]

Whenever I see plans for a machine with a large Z axis travel I have to ask myself whether the designer/builder has fully considered the limitations that are inherent.I have a feeling that the plan is to carry out quite large three dimensional work and it needs to be considered that there are limitations to the depth at which cutting can occur.The angle between the tip of the tool and the corners of the Z axis backplate being perhaps the most obvious and then there is the angle between the tool tip and the spindle mounting brackets.I hope the file I am attempting to attach shows some of this and that your projects may be of a nature that will still work as it can be a major disappointment to discover the reality after a lot of time,effort and money has been expended.

[joeavaerage]

Hi,
any given beam or section has a certain stiffness. It is inversly proportional to the cube of its length.

Lets say you have an 80mm x 80mm aluminum section of 2.5m length. It may prove that if you fix one end and apply a 10N force at the other end it will bend by 5mm say.
If you shortened that section to 1.25 m, ie half then it would deflect by 5 x (1 / 2)³ = 0.625mm. That is a remarkable improvement.

To OP: I understand that budget and simplicity is driving you towards aluminum extrusions, but that will result in a very 'floppy' structure. One way to reduce that is to make a smaller
machine even if you still use the same aluminum extrusions.

This is the downside of saying 'I want a machine of X by Y meters' because that will imply that you have to use large section steel or some other stiff material to achieve it.......only to be thwarted
by the cost and complexity of that material.

The other way of looking at it may be to say 'Cost and complexity constraints limit me to aluminum extrusions.....so what is the largest machine I can construct using that technique and STILL
have a rigid enough machine?'.

What is also true is that a smaller machine costs a lot less than a large machine. My guess, or at least my experience, is that to double the size of the machine will cost somewhere between five and
ten times as much. It is very dramatic.

My suggestion would be to build a smaller and much cheaper machine....and see how it goes. You will very soon learn what is important and what is not, and most particularly what is 'rigid enough' and what
is not. Use that experince to build a second (or third or whatever) machine.

Craig

[POORIAPK]

Hi,



No, I do not think it will improve the situation. Even aluminum has a modulus of 70GPa, whereas concrete is in the region of 20Gpa. The stiffer material (aluminium) being less compliant, especially as it
is at the circumference will transmit 99% of the load, while the more compliant material (concrete) especially as it is interior and therefore less geometrically favoured will transmit 1% of the load.
The compliance of the entire structure it almost entirely that of the aluminum alone.

Don't get me wrong, I understand the appeal of aluminum extrusions, but that does not change the fact that they are not stiff.

There are two ways to maximize the rigidity (minimize the compliance) of a structure:
1) Increase the dimensions so that the first moment of area is maximized but use the same material of the same modulus.
2) Use the same size geometry but utilise a material of higher modulus.

I would suggest that if you want to produce the most rigid machine you can of the dimensions you want you employ both strategies, ie increase the geometry AND use a stiffer material.

Here in New Zealand for instance 100 x 100 x 9 steel SHS is quite widely used and stocked. Regrettably 200 x 200 x 16, in fact any of the 16mm thick RHS or SHS sections are only available on indent order.
Even 100 x100 x 9 steel is going to be many times stiffer than your 80 x 80 aluminum extrusion. What do you want 'cheap and floppy' or 'difficult/expensive but stiff'?

Craig

Adding weight with materials that do not transmit vibrations well can help increase rigidity. I have previously filled aluminum profiles with materials like cement, resin, and stone powder, similar to what we use for fiberglass, and this improved the situation, but for a smaller machine! Your point is completely valid that aluminum and concrete are two different phases that do not combine well, but adding a bit of resin can improve the situation (though not perfectly).

Regarding increasing dimensions, could you explain a bit? Should I increase the dimensions of the table (with increased length we generate more torque, and I will have to use larger rails, wagons, and ball screws) or should I increase the number of profiles to distribute the incoming vibrations among them and thus reduce overall vibrations?

I think given the current situation, your best suggestion for my work would be to set aside aluminum profiles and redesign using steel profiles, accepting the costs of welding, machining, boring, etc., to ensure that the time and money spent yields the highest efficiency and best results, correct?

[POORIAPK]

Hello PRPK - Concrete and aluminium are not friends. You run the risk of the aluminium corroding rapidly if you are in humid conditions. Normal concrete shrinks (and does not bond to the AL) so will not form a good connection to the tubing when it shrinks away from the internal surfaces. So the short answer is do not fill the tubes with stuff. Get the tubes size correct from the beginning and you don't need to fill them with anything. Aluminium construction extrusions and their bracketry are not cheap, they are convenient and that creates limitations. Having said that many successful machines are made from the stuff. Have you actually costed it yet? Everyone has a concept, a purpose and a budget. The cheapest material to build your first machine with is good quality plywood, actually MDF is half the price of ply and my work horse machine is half MDF and its 4 years old. MDF is easy to work with using std timber tools. Plywood is stiff and damp and makes machines that can cut aluminium easily. Next step up is std steel sections bolted together. Steel is cheaper then aluminium and stiffer. Then up from that is laser cut steel parts bolted together. I have done a few like this and they work very well. My machines are all over the forum do a little search to see them. Then if it was me I'd go aluminium welded or steel welded. Then I'd have a go using Carbon fibre. Comes down to your resources.

The image is of Scoot my MDF base machine. Its still square and true and I built it in 2020 and I give it quite a hard time. Since then I have built plywood gantries that are stiff and damp and cut aluminum easily, see Frankie image. I'm currently building a 8x4' machine mainly from ply and it will have a 3m long gantry. I choose ply because its easy to work in my workshop vs metals. I'm not saying this is the way you have to go, I am saying its possible and cheaper than extrusions.

I've built 7 machines and what I have learnt is that the Z axis assembly needs to be really stiff and thought out to be successful ie cut without wobbling. Your current Z axis will disappoint I feel. You have started designing at the machine base and worked towards the Z axis. The Z axis is the most important part of the machine so start there. Keep at it. Peter

You are absolutely right; plain concrete causes shrinkage after the water evaporates. Initially, I considered using MDF in my design, but due to its lack of sufficient strength and the risk of breaking or damaging the machine during operation or transportation, as well as the low rigidity during machining of hard materials, I decided to use stronger materials and invest a bit more.

The machines shown in the images look amazing and seem to offer the highest efficiency on a low budget. However, I believe I can afford to spend more and build a larger machine because I also plan to use it for machining polystyrene foam, which requires relatively large dimensions.

Regarding the Z-axis, you are absolutely correct. I used two aluminum plates with a thickness of 15 mm, and I couldn’t think of an alternative solution other than using aluminum profiles. To reduce vibrations during machining, especially when the spindle is at its lowest position, I tried to use stronger rails and carriages to handle more torque. I think I need guidance on these aspects if I decide to build the machine with steel instead of aluminum

[POORIAPK]

Whenever I see plans for a machine with a large Z axis travel I have to ask myself whether the designer/builder has fully considered the limitations that are inherent.I have a feeling that the plan is to carry out quite large three dimensional work and it needs to be considered that there are limitations to the depth at which cutting can occur.The angle between the tip of the tool and the corners of the Z axis backplate being perhaps the most obvious and then there is the angle between the tool tip and the spindle mounting brackets.I hope the file I am attempting to attach shows some of this and that your projects may be of a nature that will still work as it can be a major disappointment to discover the reality after a lot of time,effort and money has been expended.

Thank you for pointing out this important issue. To solve this problem, longer bits are typically used for machining, and in the cases where I intend to use this machine, an accuracy of a tenth of a millimeter is not required, as shown in the picture below

[POORIAPK]

Hi,
any given beam or section has a certain stiffness. It is inversly proportional to the cube of its length.

Lets say you have an 80mm x 80mm aluminum section of 2.5m length. It may prove that if you fix one end and apply a 10N force at the other end it will bend by 5mm say.
If you shortened that section to 1.25 m, ie half then it would deflect by 5 x (1 / 2)³ = 0.625mm. That is a remarkable improvement.

To OP: I understand that budget and simplicity is driving you towards aluminum extrusions, but that will result in a very 'floppy' structure. One way to reduce that is to make a smaller
machine even if you still use the same aluminum extrusions.

This is the downside of saying 'I want a machine of X by Y meters' because that will imply that you have to use large section steel or some other stiff material to achieve it.......only to be thwarted
by the cost and complexity of that material.

The other way of looking at it may be to say 'Cost and complexity constraints limit me to aluminum extrusions.....so what is the largest machine I can construct using that technique and STILL
have a rigid enough machine?'.

What is also true is that a smaller machine costs a lot less than a large machine. My guess, or at least my experience, is that to double the size of the machine will cost somewhere between five and
ten times as much. It is very dramatic.

My suggestion would be to build a smaller and much cheaper machine....and see how it goes. You will very soon learn what is important and what is not, and most particularly what is 'rigid enough' and what
is not. Use that experince to build a second (or third or whatever) machine.

Craig

It is absolutely correct that increasing the length results in increased torque, and vice versa. I think I misunderstood your previous comments.

Given your guidance, and the advice from others, it seems that a complete redesign of the machine is necessary, and I need to reevaluate everything.

Thank you very much for your advice and for the time you’ve spent helping.

[peteeng]

Hi PRPK - Whichever machine material you choose is fine. You need to understand its pros and cons and what you are trying to do. Your first entry says wood and plastic. An MDF machine will be fine for those and be lower cost. If you want to mill aluminium and steel then MDF won't work. If you choose metal you can bolt the machine together or you can weld it together. Both approaches work and use a different philosophy & resources. Make the machine the size required and make the Z a bit taller then you think you need. The Z height is the one that gets me into trouble with long tooling. To solve the Z height issue consider an apron on the machine or a bed well. This allows larger objects to sit lower so you can work on them. I don't know what you intend machining so the "best" machine configuration I can't help with at the moment. There is a company in Germany that make very large routers for cabinet makers and their machines are plywood. Gantries are 3m long and machines can be 16m long or longer. So ply as a machine material is fine if you take the time to understand it.

What Craig is trying to say is make every component as big as possible. Take up every available space for that component. Geometry is your friend and a big object is significantly stiffer then a small object even considering the material modulus. With construction extrusions there is a lot of material on the inside of the section that does not contribute to the global stiffness of the section. They also have a lot of thin free edges which vibrate, Speaking of which the dynamic stiffness of a machine is a very big topic but if its static stiffness (initial rigidity) is sufficient you have sorted most of the issues. Keep at it - Peter

[awerby]

If you're adding a 4th axis, consider installing it so that the center of the rotary axis is at or around table height. This will minimize the length of the Z axis, compared to simply perching the assembly on top of the table. Since cutting is almost never done any lower than the center point, you don't lose any useful area that way. It will require either cutting out a section of the table, or mounting the 4th axis and tailstock assembly outboard, but neither are too difficult to implement.

[joeavaerage]

Hi,
an aluminum SHS of 80x80x6 has a moment of inertia of 3,413,225 mm⁴
An aluminum SHS of 100x100x4. has a moment of 8,333,289 mm⁴


If you had two sections, both 1000mm long, the 80mm section will deflect 2.44 times as much as the 100mm section. Both of these section weigh the same, that is to say they have the same amount of material
in each, but the bigger 'geometry' is nearly 2.5 times stiffer.

The idea is to use the biggest geometrical section you can to maximize the stiffness.

Aluminum has a modulus of 70GPa , while steel has a modulus of 207GPa. If you had two sections 100x100x4.8, both 1000mm long, then the aluminum section will deflect 2.95 times as much as the steel section.

The idea is to use the material of the highest possible modulus. For practical CNC purposes that is steel.

If you have two sections of 100 x100x4.8, one 1m long and the other 2m long the long one deflects 8 times as much!!!! That is huge.

I suggest you avail yourself of one of the online beam deflection calculators, they are very instructive. As an example:

https://skyciv.com/free-beam-calculator/

Just to put some real numbers on these assertions. From the calculator a 1m long cantilevered 80 x 80 x 6 aluminum SHS loaded with a 1kg (10N) force at the very end deflects by 14mm.
A 1m long cantilevered 100x100x4.8 SHS loaded with the same 1kg (10N) force deflects by only 6mm. The same weight of material in the two sections but the larger geometric section deflects by less than
one half.

A 1m long cantilevered 100x100x4.8 SHS but of steel, loaded with the same 1 kg at the very end deflects by only 2mm, about one third of the same section in aluminum.

A 2m long cantilevered 100x100x4.8 SHS steel loaded again with 1kg at the very end deflects by 16mm....so fully eight times the 1m steel section.

The takeaway is that if you made your machine half the size (1.25m x 0.65m) you would in general expect it to be eight times stiffer than the full size (2.5m x 1.3m) machine made from the same
materials.

Big geometric sections are your friend for stiffness, as is steel by comparison to aluminum, and a smaller machine tends to be a LOT stiffer than a large machine.

Craig

[routalot]

Thank you for pointing out this important issue. To solve this problem, longer bits are typically used for machining, and in the cases where I intend to use this machine, an accuracy of a tenth of a millimeter is not required, as shown in the picture below

They work very well in foam but in harder materials they will increase the load on the spindle bearings quite a lot.They are also quite expensive and hard to find once you look for diameter/length ratios above six.

[peteeng]

Morning Pooria and others - In prior posts you have said that vibrations "transmit" through the machine. Other people also think that stress "flows" through structures. These are incorrect ways to think about the process (I appreciate its the conventional way to think about it but there are better ways to conceptualise it) . Everything vibrates even people. Structures and their parts have a "natural vibration frequency" and if there is a freq or a harmonic near that the structure or part it will vibrate at its natural freq. You can also get things to vibrate at non natural freqs (forced vibration). A vibration implies a deflection which can be tiny or large. The issue with machines is that if the driving freq is persistent (like the teeth of a tool impacting the cut surface at constant timing) then the deflection (vibration) will reinforce itself and become bigger (resonance). If you wipe your finger around a champagne glass the sound starts small but if you continue exciting it the sound becomes louder as the deflection becomes bigger (moving more air). Now materials have internal friction. Materials are not continuous but consist of "grains" or chains that are kept together with friction. When the material deflects these grains rub together and convert the movement to heat. Just like when you rub your hands together and they get warm. Some materials have large slippery grains like cast iron some have tiny grains that are locked together quite tight like steel. That's one reason steel is strong as its grains are highly locked up. This is called material hysterisis. So every part in the machine is vibrating and at any point in time its also damping itself. We need to avoid resonance or a forced vibration ie the vibration input gets bigger then the internal damping can slow it down. So to get back to the point.

An operating machine is vibrating everywhere. When cutting material the main issue is to not get chatter. This is when the teeth of the tool excite the machine at a freq that is detrimental to the cut and particularly if the machine head has a resonant freq near the cutting speed. Look up tap testing, input shaping, active damping. These are advanced methods of vibration control. Material damping is not enough to produce an overall "damp" machine. Currently we more or less operate machines in zones that avoid resonance. The 3D printing market has had to address light machines with low rigidity so implement input shaping for instance and jerk control... These produce much smoother paths then maker lever cnc machines. Peter

see page 10 - materials are not damp enough

Two good controllers that have jerk control are Dynomotion and Buildbotics...