Aluminium framed FEA optimized CNC router

[PotatoMill]

I expect it to be a bit off. I'm going to epoxy level the surface for the profile rails. And then mount one side of the rails and use it as a datum for mounting and drilling holes for the other side.

I still can't find anything good for a setup to adjusting the angle/squareness of the Z and Y cart, after the machine is assembled.

[CaptainVee]

I have read that some people are happy with their leveling using epoxy, but from the spills I have had, I would not rely on it. The spills did not have a mirror flat surface.
Still, no harm done in trying.

"I still can't find anything good for a setup to adjusting the angle/squareness of the Z and Y cart, after the machine is assembled"

You mean, "how to go about measuring and correcting?

[PotatoMill]

90% of reading about stuff in the CNC world is finding the right word and phrases. Found the right word now "tramming" found also a lot of good info when I had the right search word

Edit: I'm probably going to set it up so the spindle can be adjusted within the square tube. Hopefully it's going to be a quick way to tram it.

[PotatoMill]

Update: Pictures is not good for viewing a design so I have placed it on GrabCAD for everyone to view in 3D. https://grabcad.com/library/z-cart-cnc-1 it's currently only the Z and Y cart since i'm working on changing the gantry and the base.

[wizard]

My machine will consist of parts that are easy enough to handle for two persons.
I started with trying to design for one person but I found that the resulting gantry would be too light.

This is one reason I was suggesting a machine that can be easily knocked down for transport. It is the only way I know of to deal with the awkwardness of handling these machines. My thinking keeps going back to contractor saws which are supposedly man portable but man two people can make a huge difference in handling. It doesn't escape my notice that one of the most popular accessories for a contractors saw is a set of wheels.

Wheels can make moving any machine a thousand time easier. If the use of wheels is integrated into the machine the ultimate weight isn't such a big deal. The use of wheels has solved a lot of problem with awkward machines from the old pen plotters once common, to saws, to transmission dollies. While managing weight helps some I think designing in facilities to add in moving the machine will help more.

It is now about 60 kilos.

The rest of the parts should be less, aiming at most to be no more then 30 kilos.

As for welding, I'm sure it is not that hard to weld aluminium.
I mean that it is impossible to weld it and end up with a straight machine, especially with the amount of welds your design requires.

Depending on the alloy you can loose considerable strength in a welded aluminum structure. I think your first thought here was right you really need to know what your are doing with aluminum weldments.

That is why I said that I feel that a bonded aluminium torsion box design is probably the only way to go, but forgot to mention I meant laser cut aluminium, or maybe water jet cut.

Frankly if weight is that important I'd seriously consider an aluminum casting if a suitable extrusion can't be found. However for the gantry I'm still leaning towards steel as the best option. The thinking is that it is better to to suffer a bit of weight in the gantry instead of compromising the structure and find a way to make it easy to handle the weight.

This is actually a very interesting discussion for me as a portable CNC has haunted my imagination from time to time. I've considered designs that amount to a wheel barrel to a quick assembly machine. The big problem is that once you get past a certain size the awkwardness of handing the machine is as big a danger as its weight. Having handled more than a few machine over the years it is the ones that are awkward, top heavy or other wise not well balanced that causes me concern.

The common drill press is a good example. Put one on a set of rollers and you need to be extremely cautious about a roll over. Put an injection mold machine on a set of rollers and it won't likely roll over on its own. I could go on but I think you get the idea.

[wizard]

Thanks for the insight.

I'm looking for lightweight so it's easy to move, and manoeuvre without any safety hazards. It must also be possible to move it though several doors. Edit: How do you think of moving yours?

The problem I got is with the size of your machine. Two guys won't be able to lift it up and carry it through most single doors. In english terms your cutting area isn about 20" wide which isn't too bad. But then you have the extra width of the vertical supports which will likely mean the total width of the machine will be wider than the average single door way. I don't believe you have noted the overall width of the machine but this is a reasonable issue unless you have double wide doors where ever you expect to move the machine.

If not the only reasonable option in my mind is to design in the capability of mounting a set of wheels to allow movement of the machine on edge. This would give you a machine that is fairly easy for two guys to handle. The key here is to handle safely.

Weight is a easy way to get a sturdy machine but is not the only way. The Youngs modulus for steel is higher than aluminium, but it's possible to compensate with a high second moment of area, polar moment, and the length of parts. When the machine is stiff enough it is a matter of fixing the base.

I don't see weight that way. While I agree that you can design a stiff and light structure that structure can still suffer from issues. It really comes down to how well the machine is designed and what the users expectations are. by the way one can design a very heavy machine and not have the sturdiness required to do the job, it is a two way street.

However weight has other positives beyond sturdiness. One is the mass keeps a very active machine from walking all over the place.

Mutch of the z and y cart, and also some of the gantry will probably be steel. The weight of these parts will not be too heavy anyways, and I needed the higher modulus of elasticity. The base will be aluminium if I don't find a way to split it into two or more parts.

Id rather see a one piece base myself. One piece int he sense of a welded structure that doesn't disassemble.

I have welded some aluminium before, it's not as hard as many people want it to be. Just the right settings, a steady hand, and prepping the surfaces well. A foot pedal is also helpful.

Please do your research as Welded aluminum looses significant strength. I'm not expert here so don't take my word for it. My limited understanding though is that it does depend upon the alloy and its tempering.

I haven't found much useful info on adjustment of the z straightness, but I would like to know more, so do you have some tips or links?

It depends upon the design of the machine. Some machines are built such that the final alignment of the parts is what you get which implies making the parts of the machine, the gantry and Y axis saddles to a high level of precision. If you want adjustability you have to design that in. For example Z orientation in one plane could be bad adjustable by mounting the Z axis assembly on a subpart on the Y axis saddle that can be rotated for alignment. In the other plane you would likely need a way to rotate the entire Y axis gantry. On a lot of old machinery, gantries like these would have had the plain bearings scraped until the required alignment was achieved. This could be done with the Y axis saddle with some effort.

[PotatoMill]

Now the whole thing begins to come together again. I have done some more FEA on the whole frame. I will just add that they are approximations and gives a rough overview. Joints and mounting points will add to the deflections. But it gives a good view on the relative stiffness of the parts and what parts is necessary to focus on.



Base:
I did some test with and without the side supports. The side supports make the base a lot more rigid. However, without the side support the deflection on the base is still insignificant compared to the gantry and the y and z carts. I also looked at how steel would increase the rigidity, but since the deflection on the base already was insignificant on the total frame, the increased stiffness from the steel was very small on the whole frame. The base is about 57 kg (125.4 lbs) with aluminum and 105 kg (231 lbs) with steel. Without the side supports it is possible to get it trough doors wider than 96 cm (37,8") without tipping it on the side. In addition it is also easy to add wheels on the side without the side supports. Drag off gantry -> Tipp base on side -> Roll out.




Gantry:
The gantry have a lot to say on the stiffness of the frame especially torsion that add to deflection on loads in the x axis. The gantry is changed from a frame with a lot of 60x60mm square tube to a single 300x200mm t=6 rectangular tube (as suggested by wisard). The stiffness of the change is about the same, but it makes the assembly a lot easier. Which means there is less areas to make mistakes the rigidity of the machine will suffer from. I had to go for steel on the gantry to get it rigid enough, an aluminum rectangular tube was just not enough. Casting aluminum could be possible, but that will again make it more complex and prone to errors. The gantry is still movable on its own but now at 50kg. The Y and Z cart is at 20kg so in total the gantry and Y and Z cart will be 70kg. I think this also need wheels :P


The design on GrabCad is now updated https://grabcad.com/library/z-and-y-cart-gantry-cnc-1. However, the ball screws for moving the gantry is not finished yet. And I see that mounting of the rails for the Y cart makes it impossible to attach the Y cart so I have to find a solution for that. Moreover, it would be great if someone have a good suggestion.

[CaptainVee]

You now have two panels that carry the carts for both the Y and the Z axis. If you make the panels slightly bigger but leave the holes for mounting the carts where they are, can't you then add bolts to mount the panels to each other?
Add a pin in the center and you can use it for fine tuning too.

[PotatoMill]

That's a very good idea. It could potentially help with both assembly, disassembly and tramming.

[DonKes]

2 Points

1 Aluminum welding is easy. ---- Distortion control is the hard part. --- Think Distortion all the time !!!!!

2 Not all Epoxy is created equal. Talk to the vendors. Listen to the vendors !!!

Been there done that on both counts.

Don

[dbsharp]

You have put a lot of time and work into this. I'm sure you will have a nice machine when your done, but your expectations of stiffness are not realistic. I made an aluminum tube frame cnc router that can cut steel. My machine is about 800 lbs all said and done. I got about 15k lbs/ in deflection in reality on the finished machine. This is enough to cut aluminum nicely with a small endmill (1/2" or less), but not enough to do any serious work on steel. I have a build thread somewhere in this forum.

I would not build another mill out of AL, I would choose steel weldment, and I would have it stress relieved.



If you need a portable cnc that can machine steel, do a conversion on something like this Mill / Drill | Grizzly Industrial

[PotatoMill]

I have been away fore a while from the forum, exam reading. I still read for the exams but i'm going to try to make some progress on this project the next two weeks. I have spoke a bit with a steel and aluminium supplier, and workshop who can help me out with some of the cutting of the tubes. So I plan to order it in the upcoming week. I'm still not 100% decided on the aluminium. It could be that the "aluminium famed router" becomes more and more a steel framed router. In addition I dont have access to the same welding equipment in the summer, so steal would be considerably easier.

You have put a lot of time and work into this. I'm sure you will have a nice machine when your done, but your expectations of stiffness are not realistic. I made an aluminum tube frame cnc router that can cut steel. My machine is about 800 lbs all said and done. I got about 15k lbs/ in deflection in reality on the finished machine. This is enough to cut aluminum nicely with a small endmill (1/2" or less), but not enough to do any serious work on steel. I have a build thread somewhere in this forum.

I would not build another mill out of AL, I would choose steel weldment, and I would have it stress relieved.



If you need a portable cnc that can machine steel, do a conversion on something like this Mill / Drill | Grizzly Industrial

Your machine is one of my inspirations. On the video it seems like it works fine, but it's probably more to it. Could you elaborate on why it couldn't do steel well? What was the weakest link?
Also when it comes to steppers, according to the calculations and reading I have done a nema 23 3nm (425oz-in) seems like more than enough, what is your experience on this? The gantry weight is at about 60-80kg.

[PotatoMill]

I'm currently considering making a stationary gantry design. Since the weight of the gantry pluss the z and y cart comes inn at 80kg (mostly in steel).
The cons will be;
-Longer machine
-Longer rails
-Potensial for sag in the bed

Pros
-Less weight to move for the steppers
-Simpler design
-Less twist on the z axis, the largest factor for displacement

The reason i'm posting this is because it probably a lot more pros and cons than I can come up with, that I hope someone here will add.

[PotatoMill]

I have been cutting a lot of steel today. Luckely I had good help from a friend and the right cutting equipment. I redesigned the whole thing for a fixed gantry design yesterday, since the steel delivery was today and I'm moving everything to another part of the country tomorrow. Yes, it ended up with steel. The weighty steel lovers were right. Going to start welding in a few days.

[PotatoMill]

I have started looking at stepper and servo motors for the machine. I have decided that I will be using closed loop motors. But i'm still not sure if I shall go for closed loop steppers or servos. I have manly been looking at the products from BST automation, JMC, Leadshine and Clearpath. I like the quality feel and good documentation from Clearpath, but the price is still high compared to closed loop steppers, and they don't ship to Norway. However I cant find a lot of good reasons to choose servos over steppers.

[Roadsidejobshop]

Hey Potato,

i'm really interested in seeing the new design. Keep in mind that steel likes to distort and distortions are kept in the frame if you don't stress relieve it.

Also, I'm building a cnc router - thread here: http://www.cnczone.com/forums/diy-cn...-aluminum.html - and my frame without any addional parts was 155 kilograms. If I had enough money and time I would have made it 300.

On the steppers - why go closed loop? Unless you are doing some crazy precise parts, I would rather go for NEMA23 3Nm or NEMA32 4.3Nm motors and step them down 2:1. The mill I am currently using is an optimum BF46, 2.2kw motor, and has 6Nm steppers which are stepped down about 1.33:1. They can take pretty chunky cuts in steel with a 20mm endmill and rapids for 1000 mm/min is easy (even 2000 is easy, but thats too fast for my skills level). Never have I ever stalled a stepper but sure have chipped some endmills.

Also, consider that you are using HIWIN HGR15 rails - they are super nice, have very low friction (the optimum is dovetails) and super stiff. If I remember correctly, the max load that each carriage can take is 1.1 tons (or 1100kg). So make the frame heavy, make the gantry heavy, fill it later with epoxy granite and create a great machine

[PotatoMill]

I have been a bit absent from the forum lately. With the steppers you are correct, after a long discussion over at mycncuk I decided to use open loop 3Nm nema 23 motors with low inductance and 5mm lead ballscrews. Direct drive.
I'm going to use a 2,2kw spindle with HIWIN HGR20 rails on all axis. The machine will weigh about 150-200kg. I have already ordered a lot of the parts so I'll start welding and assembling the machine soon.