[PotatoMill]

I am currently planning to build an aluminum-framed router that will eventually have capability to cut different ferrous alloys. In addition, I hope I can do some high-speed tool-paths in aluminum. In the early stages of the design, I planned to use aluminum profiles for the build, without any need for welding, but profiles are expensive and did not have the stiffness I wanted. Then I went on to steel, but that got to heavy. So now I am going to use 60x60mm t=4mm aluminum square tubes welded together. I have to practice my aluminum welding some more before I start with the build. I have done FEA on most of the build, where I started with requirements of a frame stiffness of 10-20 N/μm (10'000 - 20'000 N/mm)(57'000 - 114'000 pound/inch). Deflection is proportional with force so I have checked the stiffness with lower forces and then multiplied up to 10'000 N and 20'000 N. I have done many iterations on the frame and the FEA images under is only the most relevant for the current design. Obviously the FEA is based on a lot of simplifications and assumptions, but it gives a good overview of how the machine deflects and a good basis for a design.

Planned working area is 1000x500x250mm

Complete assembly

The plan is to use 20mm profile rails and 16mm ball screws on all the axes. With dual 1000mm drive one the X-axis. I am going to use nema23 stepper motors but I am still uncertain on the torque requirements I have to do some more calculations there. Though it seems like most of the steppers have a very high corner speed and to utilize all the torque and speed of the motors they should be geared down about 3:1. Any thoughts on this?


Base assembly (500N load)



Stiffness; 223 N/um. I tried some different setups for vertical loads on the base. Five vertical supports on the sides gave 4.8 times more stiffens on the worst case compared to three vertical supports


Stiffness; 53 N/um. No need for cross bracing on load in the X-axis it is more than stiff enough.

The weak link where the forces in the Y-axis, this was fixed with some side bracing

Stiffness; 51 N/um. The side bracing gave 4.7 times higher stiffness. Success! (I tried many different potential setups before deciding on this one)



Gantry (100N load)

The whole weight of the gantry including the Z-axis came to 32kg (70lbs), I’m going to see if I eventually can cut the weight down. I had hoped for something in the range of 20kg (44lbs).

Single aluminum tube thick gantry. Were within 10 N/μm but not 20.

Stiffness; 32 N/um. Double thickness improved the stiffness 2.7 times, and the weight gain was small compared to the gain in stiffness.



Z - Axis (100N load)


The Z -axis ended up on 12kg (27lbs). I have looked for ways to reduce the weight but often the stiffness suffers.

I am going to use a 2.2kW water-cooled spindle. However if it is going to be use for ferrous alloys I have to gear it down to reasonable speeds. The plan is to use a belt reduction, but it is probably going to be a later update.

Under is some pictures of the results on the Z-axis. This is clearly the weakest link on the whole machine. It is no point making the the rest of the machine stiff if this part has 1/10 of the rigidity of the rest of the machine. The two vertical bracings on the front improved the stiffness but it is still not inside my requirements. It would be great if someone could come with input and suggestions on the design before I start ordering stuff or it could become very expensive.

Stiffness of the Z-axis in the X direction (worst load direction) 3.9 N/um.

Stiffness; 0.85 N/um


Last one was a failure

[DonKes]

Why aluminum frame for something that big.

Is it free?

If so, sell it and buy steel.

Don

[PotatoMill]

As I wrote in the original post. Weight is the main reason for aluminium.

The cost savings are relatively small compared to other components.

[wizard]

As I wrote in the original post. Weight is the main reason for aluminium.

The cost savings are relatively small compared to other components.

The problem there is that weight isn't a bad thing if your intention is to machine steel. In fact for most machinery, that isn't intended to be portable, weight is usually seen as an advantage.

As for cost savings I have to disagree, steel is and has been very cheap. Further a design like yours, with lots of welded components will lead to machining even in aluminum so There is no real advantage there either.

I'm not a FEA expert but I'm puzzled by your images of the various runs. It appears that you are assuming a solid surface for the frame to sit upon. I would think your gipussets of braces would need more support underneath the horizontal members.

As for machining steels, I guess it depends upon what you mean by machining but in my mind this machine is way too light weight to accomplish anything useful. I'm not that certain that it would be useful for aluminum. This will take a bit of study really as I'm not sure elevating the X axis rails makes sense for such a machine. It strikes me as adding a lot of complications especially if this is a welded together machine.

In a nut shell if your long term goal is to machine ferrous metals aluminum isn't the answer for the machine structure and further you need to reconsider the focus on weight.

[Christian_S]

My suggestions, only considering the z-axis:
1. It looks like you can increase the spacing of the z-axis rail-blocks considerably, they should ideally be mounted on top of the y-axis rail-blocks, else the y-axis rail spacing and gantry height is not utilized properly. Increase the z-axis plate and rail lengths as necessary.
2. The distance from the lowermost z-axis rail-blocks to the point of cut in the lowered position can be more significant than the stiffness of the z-axis itself, the lowermost rail-blocks should therefore in all cases be flush with the bottom of the z-axis plate at the top position.
3. Do you really need the bottom of the z-axis plate to lift higher than the bottom of the gantry or the collet to extend below it? If you do, I think you will be better off placing the rail-blocks on the z-axis and the rails on the y-axis plate. Alternatively consider if you could move the spindle lower only when needed.
3. The dual y-axis plates should not be necessary and only adds weight and moment arm for the load on the y-axis rails. It should be possible to design them out using the possibilities of wide rail-blocks that can be bolted from either side, bolt heads countersunk in the plate etc.
4. It looks like the y-axis rail blocks are not flush with the top and bottom of the gantry, consider moving the rails as needed.

[PotatoMill]

A lot of good suggestions.

My suggestions, only considering the z-axis:
1. It looks like you can increase the spacing of the z-axis rail-blocks considerably, they should ideally be mounted on top of the y-axis rail-blocks, else the y-axis rail spacing and gantry height is not utilized properly. Increase the z-axis plate and rail lengths as necessary.

The reason I did not space them further was because the z-axis became quite tall. However it is potential stiffness to gain, so I guess ill check it out.

2. The distance from the lowermost z-axis rail-blocks to the point of cut in the lowered position can be more significant than the stiffness of the z-axis itself, the lowermost rail-blocks should therefore in all cases be flush with the bottom of the z-axis plate at the top position.

Agreed, there will be a increased momentum on the blocks with a increased distance. And if I remember my statics right the deflection increases with L^3 from the "fixturing". This could help my Z axis problem. I should have seen this myself earlier, I guess I get a bitt blind to the design after staring at it for a while.

3. Do you really need the bottom of the z-axis plate to lift higher than the bottom of the gantry or the collet to extend below it? If you do, I think you will be better off placing the rail-blocks on the z-axis and the rails on the y-axis plate. Alternatively consider if you could move the spindle lower only when needed.

The rails are very rigid and helps minimizing deflection on the Z-axis. I also save a lot of weight with this solution.

3. The dual y-axis plates should not be necessary and only adds weight and moment arm for the load on the y-axis rails. It should be possible to design them out using the possibilities of wide rail-blocks that can be bolted from either side, bolt heads countersunk in the plate etc.

I'm not sure what you mean but if you mean the plates that elevates the blocks for the Z-axis it is to make space for the ball-screw. I am not completely satisfied with the solution but I currently don't have a better one.

Very observant

4. It looks like the y-axis rail blocks are not flush with the top and bottom of the gantry, consider moving the rails as needed.

There is only gains with this change. So ill probably fix that.


And to the weighty steel lover's. I need to be able to move it in tight spaces without an army of body builders. I can add weight when I have placed it. But it needs to be removable.
Steel is good to make rigid machines but because of my limitations I can't. Instead I compensate by having an exact understanding of the deflections in the machine to gain the best possible results.

[G59]

Can you handle steel just for the Z?? A C Chanel?? Wouldn't add that much more weight. A machine this size, I don't think you'll want to move it everyday either. It will be plenty heavy on it's own.
You shouldn't be counting on the rails ridgidity for any of your calculations. They bend and thats the end of your ridgidity and precision.
Cutting steel with it? Possibly, albeit slowly. Wrong spindle for steel.
BTW, FEA although very good, doesn't calculate or account for errors very well. And most if not all of the FEA softwares out there, don't allow or have provision for you to change the amplitude of the frequency. That makes FEA a little less real then in the real world. Interpretation is now dependent on experience and knowledge.

Ooops. just realized your not doing modal tests. LOL

All in all your design looks better then most I've seen here.
Good luck.

[pippin88]

That gantry is not heavy from a stepper / driving point of view.

My gantry / Z weighs well over 100kg and I can rapid at over 20metres/min with dual drive 15mm pitch ball screws and 381oz.in steppers.

Good work on the design.

With the Z: A box section would be best. Easier is a C channel with bracing I posted a possible one in the Router for Hardwoods sticky (on phone so hard for me to search sorry).

[PotatoMill]

Can you handle steel just for the Z?? A C Chanel?? Wouldn't add that much more weight. A machine this size, I don't think you'll want to move it everyday either. It will be plenty heavy on it's own.

Good point. Steel on the gantry and Z-axis is less of a problem when it comes to movability so this is a possible option. I'm going to explore some options with steel on the Z-axis. Since it have 1/10 of the stiffness of the gantry, the rest of the machine is meaningless til I fix it. I will at least be inside 1/5.

Cutting steel with it? Possibly, albeit slowly. Wrong spindle for steel.

I know it's not ideal for steel, that's why i'm thinking of having belt reduction on the spindle. Then I can keep the spindle in it's optimum rpm range and at the same time cut with a speed suitable for steel. However this is a long term goal, first i'm just going to use it for foam, wood, CF, maybe some aluminium.

That gantry is not heavy from a stepper / driving point of view.

My gantry / Z weighs well over 100kg and I can rapid at over 20metres/min with dual drive 15mm pitch ball screws and 381oz.in steppers.

With the Z: A box section would be best. Easier is a C channel with bracing I posted a possible one in the Router for Hardwoods sticky

Nice to know with the weight, seems like a lot of people get way to powerful steppers on their machines.

I will probably test out how a box section works out soon.

[wizard]

[QUOTE=PotatoMill;1874076
And to the weighty steel lover's.
[/quote]
I hope that isn't me. I just find aluminum construction to be very expensive and the only benefit you gain is a light machine (relatively).

I need to be able to move it in tight spaces without an army of body builders.

Not soon after caveman left the cave he discovered the wheel.

I can add weight when I have placed it. But it needs to be removable.

Actually dry sand is good for this.

Steel is good to make rigid machines but because of my limitations I can't. Instead I compensate by having an exact understanding of the deflections in the machine to gain the best possible results.

You will still have a machine that weighs a lot. I can't imagine one man picking it up an walking off with it. Beyond that the form of these sorts of machines makes them awkward even for two guys to handle. Moving these machines around becomes a personal safety issue.

Given that have you considered putting effort into the design to make it easy to break down into lighter components? For example putting a 150 pound limit or whatever feels right to you, on components size. I've actually put some thought into the portability issue sometime ago. Since my shop is currently in a cellar eventually the machines would be moved out of that cellar thus it would pay to make the machines in such a way to easily do that. Part of the solution is a weight limit on machine component size. The other is to design in such a way that parts can easily be broken down from the assembled machine and transported with rudimentary tools (hand trucks).

Interestingly I never really considered aluminum for this, mainly for issues of cost. It isn't the cost of the aluminum either though that is an issue it is the cost of processing that aluminum. For example Welding aluminum requires welders I don't have access to but steel can be done on simple welders one of which I own. There are other add ons not seen with steel, for example thread inserts.

In any event don't get me completely wrong here, portability is one good reason to go with aluminum for a machine design. However the awkwardness of these machines means aluminum isn't the big advantage you seem to think it is. No matter what material you ultimately choose you need to put a lot of consideration into how the machine will be moved around.

[wizard]

It is interesting your focus on portability but I fear this machine is too big for that focus to really matter.

Given that I have considered, in the past, making a machine portable like a contractors table saw. Something you can wheel around or even pick up if you really wanted too. I just don't see the point in making a machine this size similarly portable. You can make something easy to move around on wheels but it won't be contractor saw easy. At a certain point akwardness and weight simply beat one man portability and even two man safe handling.

Good point. Steel on the gantry and Z-axis is less of a problem when it comes to movability so this is a possible option. I'm going to explore some options with steel on the Z-axis. Since it have 1/10 of the stiffness of the gantry, the rest of the machine is meaningless til I fix it. I will at least be inside 1/5.

I'd serously consider a design alluded to in one of my other posts where the gantry is easily dismounted no matter what material it is made of. I'm trying to understand how important this portability is to you and how to achieve it in this large of a machine. From my perspective what has been implied so far indicates to me that an easily knocked down machine is going to be a requirement.

I know it's not ideal for steel, that's why i'm thinking of having belt reduction on the spindle. Then I can keep the spindle in it's optimum rpm range and at the same time cut with a speed suitable for steel. However this is a long term goal, first i'm just going to use it for foam, wood, CF, maybe some aluminium.

It depends upon your expectations. If you expect to do engraving and very small tool killing it might not be that bad. More traditional milling will be an issue. Even an old Bridgeport mill will vibrate significantly if you are aggressively running a large roughing end mill.

Given that the design will certainly be good enough for most of the materials you list. Milling aluminum will give you a good idea about the feasibility of doing steel. When it comes right down to it my big concern is about the machines feasibility to do anything beyond aluminum.

Nice to know with the weight, seems like a lot of people get way to powerful steppers on their machines.

I will probably test out how a box section works out soon.

When I think machining steel I generally see a Bodgeport in my mind. Obviously small mills can be had and they can handle lighter jobs in steel, but what you can accomplish in a given time drops significantly. Sometimes the small mills are good enough, in the case of your machine I'm simply not that sure it will handle steel in the way you want it too. Build this machine though and you might be able to use it to build parts for a machine more suitable for machining steel.

[dmalicky]

It's great you are using FEA to analyze the component stiffnesses, and improve the design based on that.

As others have mentioned, you have some aims that will strongly compete with each other: machining steel, lightweight, and portable. I'd agree that it's probably not realistic to achieve all these, at least for a machine of this work envelope.

That's great you have looked up the stiffness needed to cut steel. Yes, 10 N/um (57k lb/in) is a good target. I don't think any DIY machine has achieved it in reality -- it is very difficult, since any one flexy component ruins stiffness. It will take very thorough and sophisticated (contact) FE modeling and analysis, extensive understanding of good joint design, some other analysis beyond FEA, and an expert build and assembly process.

Keep in mind the 10 N/um target is for stiffness-at-the-cutter-tip (relative to the workpiece), due to the deflections of all the components. Some important points:
- There are about 10 components from the workpiece to the cutter tip. Stiffnesses add by the inverse law (1/k = 1/k1 + 1/k2 + 1/k3...). This means each component needs to have a stiffness of 100 N/um, for the total stiffness to be 10 N/um. 100N/um gets tough.
- Since 'adding' stiffnesses is confusing, I find it easier to simply assume a standard cutter load of 100 lb (445 N) (along a suggested vector of 2i + 2j + 1k), then calc max acceptable deflection-at-the-cutter, then look at how each component contributes to that deflection. Deflections add normally, and it's easier to see where our losses really are, and what doesn't really matter.
- The frame stiffness cannot really be analyzed without a gantry on it. We need to actually apply the load at the cutter tip, then let FEA calculate its distribution across the gantry, then propogate into the frame, then calc how the frame deflects, and how those deflections propagate back to the cutter tip.

With FEA, it is easy to make assumptions that are much different than reality, and these assumptions have a huge effect on the results. For example, I noticed there are constraints all along the bottom surface of the frame. This simulates the frame as epoxy bonded to an infinitely rigid block below it. Given the aim for portability, I don't think you plan to do this. A safe approach is known as "3-2-1 constraints": https://www.google.com/search?q=3-2-1+fea+constraints That is the least constraining, which is most realistic for portability.

I liked reading your statement "It is no point making the the rest of the machine stiff if this part has 1/10 of the rigidity of the rest of the machine." I found the same thing when designing other machines, especially for the Z axis. This is part of why FEA is great for learning and design.

Again, it's great you are approaching this quantitatively with a good computer tool. My post is some tips to help it be accurate.

[PotatoMill]

Update on the design. I have finally managed to get sufficient stiffness in the Z cart. The solution was no surprise, maximizing second moment of area, and... steel. I also moved the Z blocks down as far as I could and placed all the Z blocks over the Y blocks. I am now planning to use a 100x200mm t=4mm square steel tube. The Z cart became twice as heavy, now 20 kg (44 lbs). However, about 20 times stiffer. The large cut on the backside is so I can get more space for the ball-screw and eventually remove the spacers for the Z blocks. I did test if the cut would compromise the design but it held up well to loads in all directions and torsional load.


Now that the Z cart is good, torsional stiffness in the gantry is the problem. I am also doing the analysis on the whole gantry with Z and Y carts now as suggested by dmalicky. The connections between the blocks and rails is not correct. They are just bonded. I do not know of any good ways in SolidWorks to make them in an efficient way, without messing around with spring connections. Therefore, it is a gross simplification, but it should give an approximate overview of the deformation.

The image is with both gravity and a 100 N (22.4 lbs) load in the X direction, a potential worst-case load. It gives a deflection of 0.014 mm (0.55 thou). With some vibrations, joints, and the base, the deflection will be bigger. At this point, the structural frame is not the weak link any more, but ball-screws, rails, steppers, drivers, and so on. The stiffness is at 13 N/um (72000 lbs/inch) now. As I am a newbie on this, I wondered if this is a good point to settle?

The gantry, Z and Y carts weight about 40 kg (37.8 lbs) with a aluminium gantry. And 65 kg (143 lbs) with a steel gantry. So I guess steel gantry is not out of the question.


I also looked into changing the design on the gantry, by using two parallel beams and having the Y and Z cart mounted in the middle. This had many advantages on the rigidity and weight distribution.

However, this would require rails on both sides of the Z cart and I cannot find any good way to prep the surface and mount them accurately. With the classic gantry design, I can use epoxy, but with the parallel beam design, the Y and Z cart becomes a problem. The design would also be a lot more complex with parallel ball-screws on Y. In addition, four rails on Z or parallel ball-screws here too. And I have enough problems to worry about already.

[G59]

The image is with both gravity and a 100 N (22.4 lbs) load in the X direction, a potential worst-case load.

Please, how did you arrive to that number(22.4)lbs? How far away from the spindle flange are you applying this load?

Now I know this is DIY but enlighten me bit as I'm having trouble understanding.

Just a suggestion here, your gantry is too narrow for its height. It should also be boxed(plate welded) at the ends and if you insist on using tubing then add a top and a bottom plate welded to them. Make it out of steel and problem solved.

[PotatoMill]

A usual estimation for steel milling is 75N then I just added 25N to be on the good side. I usually use 100N because it is fast to calculate stiffness in N/um. I am applying loads 40 mm from the spindle flange.

I can do a test with a wider gantry. Currently most of the build is based on 60x60mm aluminum tubes for practicality when ordering and assembling the machine. I get some good prices on aluminum from a local supplier, but only on larger quantities.

Steel is an option. However, I am somewhat worried how the weight will affect acceleration on the gantry. In addition, the increase in price on the steppers and drivers.

[wizard]

A usual estimation for steel milling is 75N then I just added 25N to be on the good side. I usually use 100N because it is fast to calculate stiffness in N/um. I am applying loads 40 mm from the spindle flange.

I can do a test with a wider gantry. Currently most of the build is based on 60x60mm aluminum tubes for practicality when ordering and assembling the machine. I get some good prices on aluminum from a local supplier, but only on larger quantities.

Steel is an option. However, I am somewhat worried how the weight will affect acceleration on the gantry. In addition, the increase in price on the steppers and drivers.

Weight obviously impacts acceleration but the question is how much of a differential in weights will there be to get equal structures mechanically. This especially in the face of your many tubes approach which adds a bit Of dead weight to the structure. If acceleration is this important I'd look into a large aluminum extrusion if you can find one big enough. Your other option would be to cast your own beam.

[PotatoMill]

Little update on the Z and Y cart design, after a lot of iterations i'm now converging on a better solution for the design. However there are still changes to make and improve upon, and I hope on some feedback on my design.

A image of the whole thing. With the 100x200mm t=6 steel square tube in the front.

New setup on the stepper motor for the Z axis with a belt and pulley system. The Stepper is also mounted so it's easy to tighten the belt. It's also easy to pluck out of the machine.
The end of the ball screw it fixed with a custom double ball bearing fixture. Using 10mm thick SKF 6201 ball bearings.

Mounting of the Z and Y blocks over each other.

Here is the square tube see-trough so it's possible to see the mounting of the spindle and the ball screw mechanism. I had to make a cut in the square tube from the top to make space for the ball screw. This did not however weaken the rigidity of the Z cart significantly.

Weight obviously impacts acceleration but the question is how much of a differential in weights will there be to get equal structures mechanically. This especially in the face of your many tubes approach which adds a bit Of dead weight to the structure. If acceleration is this important I'd look into a large aluminum extrusion if you can find one big enough. Your other option would be to cast your own beam.

Good point, a large extrusion could also save me a lot of time and problems when i'm building it. I'm going to look for a supplier, ill also se what it does to the stiffness of the frame, I guess it will not work without closed ends.

[CaptainVee]

You mention you are looking for light weight, but I did not read why?
As mentioned earlier, weight is a factor not to be overlooked if you want good milling.

But still, you may want low weight for the right reasons.
But building a precision machine out of welded aluminium?

I think you are mad.
Especially if you are not a master aluminium welder.

I think that for constructing a good router in aluminium a bonded torsion box is the only way to go.

I too am designing a "light weight" router, primarily to be able to carry it out the door myself. It is still going to be over 400kg for 1x1.25 meter working area.

Oh, when I look at the image for the connection between the y axis and the Z axis, it seems that there is no way to access all the bolts to be able to set the angle of the z-axis to the Y axis after assembly.

[PotatoMill]

You mention you are looking for light weight, but I did not read why?
As mentioned earlier, weight is a factor not to be overlooked if you want good milling.

But still, you may want low weight for the right reasons.
But building a precision machine out of welded aluminium?

I think you are mad.
Especially if you are not a master aluminium welder.

I think that for constructing a good router in aluminium a bonded torsion box is the only way to go.

I too am designing a "light weight" router, primarily to be able to carry it out the door myself. It is still going to be over 400kg for 1x1.25 meter working area.

Oh, when I look at the image for the connection between the y axis and the Z axis, it seems that there is no way to access all the bolts to be able to set the angle of the z-axis to the Y axis after assembly.

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?

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.

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.

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.

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?

[CaptainVee]

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.

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.

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.