[peteeng]

Hi guys.... we intend to create a dual fixed gantry build (X and Z axis sandwiched between two fixed gantries). do you think that this, coupled with otherwise proper design might make for a setup that is notably more rigid than its single gantry counterparts?....

Hi 308 - In response to this question:
1) shopbot cnc started making routers a long time ago in this configuration. They soon changed to a conventional config. So there are evolutionary reasons why this config is not any better
2) If you consider that the Z axis is a vertical structure and then you have to attach another rail structure to the back of it, it makes sense to have those rails vertical too. It is simpler and simple is best in any machine
3) A machine design is about stiffness especially for cutting steel as you mention. So every part of the machine has to be justified on a stiffness basis. If you have two parallel members not connected with a shear structure they behave independently. There is no global gain in stiffness, which I think is the perceived gain in this config.

The carriage in the middle can only transfer load across the gap via the lead side deflecting. But you have to make the lead side very stiff for your application so it does not deflect much. This makes the lagging beam redundant in the limit case. This also goes for the transfer of moments if you use rails. Rails are rigid in the sense that they resist transverse moments. So the lead beam will resist most of the applied moment and only transfer a small moment to the lagging beam. A way around this is to use round shafting bearings so that they cannot resist moments and allow the two beams to moment share. But shafting has less capacity and probably these days you would not use them. If you do an FE of the gantry this will show up the load share issue. It will show up better if you do a non linear geometric analysis as a couple of these things can be non linear if the beams are not very stiff and you don't model the bearings correctly.
4) In a fixed gantry design the beauty of it is to have a hugely stiff gantry that does not move ie no inertial loads to cope with. So you may as well have a very big single beam or even a cantilever like a bridgeport machine. Which depending on the size of this machine you will need a bigger beam than you expect.

So: how big is this machine? benchtop or 4x4ft? and are you aiming at 0.1mm, 0.01mm or 0.001mm accuracy?

In terms of machine stiffness this is measured as the "static and dynamic stiffness" The dynamic stiffness is hard to measure but the static stiffness is easy. You are going to need a stiffness at the tool in the order of 10-100N/0.001mm Small hobby mills are around 1-5N/um and the owners of these try to make them stiffer. So I expect 10N/um is minimum.... Peter

[Daedalus308]

Hi 308 - In response to this question:
1) shopbot cnc started making routers a long time ago in this configuration. They soon changed to a conventional config. So there are evolutionary reasons why this config is not any better
2) If you consider that the Z axis is a vertical structure and then you have to attach another rail structure to the back of it, it makes sense to have those rails vertical too. It is simpler and simple is best in any machine
3) A machine design is about stiffness especially for cutting steel as you mention. So every part of the machine has to be justified on a stiffness basis. If you have two parallel members not connected with a shear structure they behave independently. There is no global gain in stiffness, which I think is the perceived gain in this config.

The carriage in the middle can only transfer load across the gap via the lead side deflecting. But you have to make the lead side very stiff for your application so it does not deflect much. This makes the lagging beam redundant in the limit case. This also goes for the transfer of moments if you use rails. Rails are rigid in the sense that they resist transverse moments. So the lead beam will resist most of the applied moment and only transfer a small moment to the lagging beam. A way around this is to use round shafting bearings so that they cannot resist moments and allow the two beams to moment share. But shafting has less capacity and probably these days you would not use them. If you do an FE of the gantry this will show up the load share issue. It will show up better if you do a non linear geometric analysis as a couple of these things can be non linear if the beams are not very stiff and you don't model the bearings correctly.
4) In a fixed gantry design the beauty of it is to have a hugely stiff gantry that does not move ie no inertial loads to cope with. So you may as well have a very big single beam or even a cantilever like a bridgeport machine. Which depending on the size of this machine you will need a bigger beam than you expect.

So: how big is this machine? benchtop or 4x4ft? and are you aiming at 0.1mm, 0.01mm or 0.001mm accuracy?

In terms of machine stiffness this is measured as the "static and dynamic stiffness" The dynamic stiffness is hard to measure but the static stiffness is easy. You are going to need a stiffness at the tool in the order of 10-100N/0.001mm Small hobby mills are around 1-5N/um and the owners of these try to make them stiffer. So I expect 10N/um is minimum.... Peter

Just like my last reply, I'll try to respond to your points one at a time

1) Do you have any idea what models or years that was produced? I would love to be able to see some of those and research how the end users liked them.
2)I'm not really sure what you mean here, I do intend to have the Z axis traveling on rails, one pair per side attached to a gantry. Perhaps we could further discuss this so I can better understand what you mean?
3) could you expand on this? i'm not familiar with the concept of a shear structure
4) This is intended to be a standalone machine with a 2ftx2ft working area (subject to change), I would like to achieve a positional repeatability of at least as accurate as 0.001 inches, so about 0.025mm repeatability for a machined part (given otherwise correct machine setup, proper tooling, speeds, feeds, etc.). In the end I would like to achieve stiffness equal to or greater than that of a Haas minimill (I don't know the numbers, but i do have personal experience to use this as a reference).

Thanks again for the thoughtful responses! The information you are giving me is enlightening and it is greatly appreciated!

[peteeng]

Just like my last reply, I'll try to respond to your points one at a time

1) Do you have any idea what models or years that was produced? I would love to be able to see some of those and research how the end users liked them.
2)I'm not really sure what you mean here, I do intend to have the Z axis traveling on rails, one pair per side attached to a gantry. Perhaps we could further discuss this so I can better understand what you mean?
3) could you expand on this? i'm not familiar with the concept of a shear structure
4) This is intended to be a standalone machine with a 2ftx2ft working area (subject to change), I would like to achieve a positional repeatability of at least as accurate as 0.001 inches, so about 0.025mm repeatability for a machined part (given otherwise correct machine setup, proper tooling, speeds, feeds, etc.). In the end I would like to achieve stiffness equal to or greater than that of a Haas minimill (I don't know the numbers, but i do have personal experience to use this as a reference).

Thanks again for the thoughtful responses! The information you are giving me is enlightening and it is greatly appreciated!

Answers:
1) That was 20 years ago and everyone would have moved on by now. Call Ted Hall I'm sure he has time for students or someone there can help. He has a successful business so the users must like them
2)Please publish some images to ensure we are on the same page
3) An "I" beam consists of flanges and a web. The web transfers shear between the flanges, the web is the shear structure. We make I beams to improve the structural efficiency of the material. In a twin gantry design as I think you are designing you are using two tubes as the flanges but you do not have a web. I think you expect that two widely spaced tubes are stiffer then one large central tube. This is incorrect thinking. If you do the correct math this will become apparent

Mind experiment : You have two parallel beams well supported at the ends (your twin gantry). There is a stick in the middle that slips in between. You push on the lead beam, how much load gets transferred to the lagging beam via the stick? If the lead beam is very stiff not much, so the lagging beam is redundant. So in the twin beam approach you eventually have to have two beams the size of the correct beam. The twin gantry is actually evaluated in Bambergs thesis and rejected for being less stiff. I rest this issue here.

4) Speak to HAAS and ask them the stiffness of their minimill. HAAS is a philanthropic organisation with great student interest. The stiffness number will scare you I think... Plus look up accuracy and repeatability they are different.

I think you need to define what this machines real reason to exist is. A HAAS is a venerable machine to emulate but so far I can't see you getting there. A machine has to have a strong reason to exist otherwise it will rust in a corner, if in fact it gets built. You need to expand the vision. Companies and people are happy to help and money is not the issue, there is more money in the world now then there ever has been. For instance why do you want to cut tool steel? Make the machine out of carbon fibre? do you realise that by the time you are a good engineer, mills maybe dinosaurs? taken over by 3D printers? Companies like HAAS are looking for the next stuff not the stuff that was done 20 years ago. If you where tasked to take a mill to the moon to cut some moonium would you make it out of 2T of steel? What does your sponsor want? Think beyond your thinking... that opens doors

I'm not invalidating your current trajectory I think there's an opportunity here bigger than you think...
There are a million reasons to halt this project, you only need one really good reason to have it happen Peter

[Daedalus308]

Answers:
1) That was 20 years ago and everyone would have moved on by now. Call Ted Hall I'm sure he has time for students or someone there can help. He has a successful business so the users must like them
2)Please publish some images to ensure we are on the same page
3) An "I" beam consists of flanges and a web. The web transfers shear between the flanges, the web is the shear structure. We make I beams to improve the structural efficiency of the material. In a twin gantry design as I think you are designing you are using two tubes as the flanges but you do not have a web. I think you expect that two widely spaced tubes are stiffer then one large central tube. This is incorrect thinking. If you do the correct math this will become apparent

Mind experiment : You have two parallel beams well supported at the ends (your twin gantry). There is a stick in the middle that slips in between. You push on the lead beam, how much load gets transferred to the lagging beam via the stick? If the lead beam is very stiff not much, so the lagging beam is redundant. So in the twin beam approach you eventually have to have two beams the size of the correct beam. The twin gantry is actually evaluated in Bambergs thesis and rejected for being less stiff. I rest this issue here.

4) Speak to HAAS and ask them the stiffness of their minimill. HAAS is a philanthropic organisation with great student interest. The stiffness number will scare you I think... Plus look up accuracy and repeatability they are different.

I think you need to define what this machines real reason to exist is. A HAAS is a venerable machine to emulate but so far I can't see you getting there. A machine has to have a strong reason to exist otherwise it will rust in a corner, if in fact it gets built. You need to expand the vision. Companies and people are happy to help and money is not the issue, there is more money in the world now then there ever has been. For instance why do you want to cut tool steel? Make the machine out of carbon fibre? do you realise that by the time you are a good engineer, mills maybe dinosaurs? taken over by 3D printers? Companies like HAAS are looking for the next stuff not the stuff that was done 20 years ago. If you where tasked to take a mill to the moon to cut some moonium would you make it out of 2T of steel? What does your sponsor want? Think beyond your thinking... that opens doors

I'm not invalidating your current trajectory I think there's an opportunity here bigger than you think...
There are a million reasons to halt this project, you only need one really good reason to have it happen Peter

1) Is Ted Hall an employee at shop bot CNC? if so I will make an effort to contact him!
2) our modeling is at a very early stage right now but I will post an image of what I am referring to by dual gantry. (please disregard any other concepts in the design, it was a very early design point) https://imgur.com/a/JhBX06k
3) Thank you for the definition! I think i can see in your own design, maximus, where you utilized a shear damper as shown in bamberg's work. Bamberg's Thesis is lengthy, however,and may take me a week or two to fully read through (I promise i fully intend on doing so), do you know where he denounces the dual gantry within the thesis (general page range or section)? please don't regard this as laziness on my part, I am simply trying to forward our conversation without delaying until I have fully read the material.
4) you're right! I will try to do that this upcoming week! I've been trying to contact organizations for sponsorship and knowledge but so few actually respond via email... I do see what you're saying about defining purpose and forward thinking. I will make a point to do a write-up on purpose. Thankfully, our current sponsor doesn't have any requirements for us other than our use of their software.... perhaps other potential sponsors may have stricter requirements. I will do what I can to be forward thinking in my design, and application

Also, I am a mechanical engineering student with a manufacturing background. We do have a couple of EEs on our team however!

As always thank you for the thoughtful response, I really appreciate the amount of effort you are putting in to help me out!