building my 1st milling machine - first steps in action

[M-Malek]

HI
This is my first post on forum and i hope make it in proper place.


after about 2 years of study about CNC now i wanna build my first CNC mill and as i start design (and of course i take a look at most designs available on internet) i become a little unsure about some aspect of design. So long story go short. I need some help and advice on one of most traditional question : "Fixed gantry or moving one?"


i know what i wanna do (milling Al) needs maximum rigidity and stability of machine that will be achieved using fixed gantry design but in other hand same pros of moving gantry (like lower cost, need less space , ...) just floating over my head and spinning there ... , need help and advise PLZ.


Can i achieve desired accuracy using moving gantry (at the same cost for fixed gantry)?

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[Jim Dawson]

''Can i achieve desired accuracy using moving gantry (at the same cost for fixed gantry)?''

I don't think so, it requires more hardware to build a moving gantry. It really comes down to your willingness to sacrifice floor space. High performance industrial machines are all fixed gantry. Figure the floor space at 1.5 times the length of the table travel.

[M-Malek]

Thanks for your answer Jim

At the moment floor space is not critical problem but price and skill both are limiting factors.
I think there are two kind of fixed gantries:

1) The design in witch gantry attached to base (main body) using bolt and bolted to that.
2) The design in witch these two sticks together ether by welding or casting as one part.

And I also think that ether of these two will provide better accuracy compared to moving gantry but the question is how much? (Supposed cutting aluminum)

Is there noticeable difference in accuracy between moving gantry and type 1 fixed gantry (bolted one)? (enough to say it has superior accuracy)

I know it may seems like foolish question but at the time I have only one shut and I don’t want to miss. I don’t want to make TOY CNC MILL but at the same side I cannot go too expensive as I have limited budget.

I know type 2 fixed gantry provides ultimate accuracy but finding some shops that does it at reasonable price where I live is like jock (Not funny one of Corse) so main comparison in my case will be between type 1 fixed gantry (bolted) and moving gantry.

My biggest fear is about Gantry Racking (moving one) and limited work piece weight (load one table/bed in fixed design)

But in any case I don't want accuracy be sacrificed!

Here are specification of machine that I’m gonna build:

1) X&Y travel 500mm (Square table/work area) (Yes I know most standard is rectangular)
2) Z travel 250-300mm will be OK.
3) Whole machine will made from steel (No aluminum part)
4) For linear motion guide I love profile LMG from HIWIN (Model depends on overall design and load)
5) Power transition on X & Y will be ball screw and on Z lead screw.
6) Water cooled spindle is only option I have (I think 1.5 HP will be more than enough)

[Jim Dawson]

I guess one question is what do you mean by ''accuracy'' If you want to hold 0.25 mm that is one thing, if you want to hold 0.001 mm then that requires a different machine.

Bolting the machine together would be fine. The more mass the machine has, the more stable it will be. But this requires larger motors to move that mass. The accuracy can be increased by use of servos and put the encoders on the load (axis) rather than on the motor by using linear scales. I use 1um magnetic scales on my machines.

[M-Malek]

Accuracy for me means both repeatability and dimension/movement.

It’s clear about repeatability and by controlling dimension/movement I’ll be OK with 10 micron (0.01 mm).

At this time I do not care about electronics and sensors (encoders rotary, linear, magnetic, optical or …) because that can be add on any time after main structure made and easier to change or play with them but any mistake at making mechanical parts will be pain in --- and hard to redo and time consuming.

Bolting the machine together would be fine.

I hope this means that accuracy I need will be achievable by such a machine. please let me know if I’m correct.

The more mass the machine has, the more stable it will be

I 100% agree with that. I’m thinking about mounting machine on stone (travertine) base which I believe will be OK (I know granite is preferred but I don’t have) I also have in mind to make posts and Y axis from solid piece of steel (not tubing) as well as entire frame if I can (I’m afraid of machining price)

I’m going to make CAD model and do some stress and deflection analysis and share it here. I’ll be pleased by any advice during designing stage.

[Jim Dawson]

Yes, bolting the machine together will give the accuracy you want, this also allows you to make minor adjustments during assembly.. Once everything is aligned, then you can install dowel pins to maintain the position.

[M-Malek]

Well I got something interesting. I do some tests and for the bar that going to be gantry I decided to test both profile (steel tube) and solid piece of steel with same size and same amount of load and results was pretty amazing. Guess what, the difference is 0.001 mm between those two!

The profile deformation was only 0.001 more than solid steel bar.

So I should confess that I over react about steel tube and it was innocent.

And I should confess that designing it is harder that what I assumed and I don’t supposed it to be like this.

But I still want to make posts from solid piece. I’m going to add Y travel a little bit maybe (actually 2X)

Here are test results:

profile (tube)


solid bar

[M-Malek]

I spent all of my past day studding existing fixed gantry designs (and even moving gantry designs) and I’m focused on the gantry itself.

As you know most of existing designs used round rail/bars (and almost all unsupported) witch I don’t like at all so those are not my business. Looking at some designs that utilized profile rail (lm) I see something that I believe is done wrong.

Most of designs with profiled rail mount them in wrong way (I think) so instead of putting the rail in such position that it takes force perpendicular to it and transfer it to surface underneath it (which I think is the key benefit of using those rails over round one, if we do not take tolerance in account) they installed linear rails in position that those rails taking force caused by weight of Z axis and I’m pretty sure that those rails not designed to deal with that at all

Take a look at images below, am I correct? Please let me know what you think about this

Is it proper way to install LMG? I really need some help about this

[Jim Dawson]

I think your drawing is correct. That is the normal method to mount the LMG. That is a nice design, very robust. Very much like my router.

[M-Malek]

I think your drawing is correct. That is the normal method to mount the LMG. That is a nice design, very robust. Very much like my router.

Thanks for your reply Jim

That’s one of designs I got from internet

I know this is used on many actual builds and even on some industrial CNC mills I see this

It may work (which I’m sure it does) but it may not be perfect

But the question is: Is this correct from mechanical aspect?

[Jim Dawson]

Yes, it's correct. The only other way I can think of to build this would be to put the Z axis between gantry rails so that it is not cantilevered off of the gantry. That way all of the force would be vertical. I have never seen this done, it would increase the parts count (and cost) dramatically. Properly sized rails (18-25mm?) will be more than adequate to support the load. I think the rails will support the rated load in any direction.

[M-Malek]

The only other way I can think of to build this would be to put the Z axis between gantry rails so that it is not cantilevered off of the gantry

That is what I’m thinking about

It have some advantage and some drawbacks. Such design gonna suppress unwanted force caused by weight of Z column but as you said increase cost

But using that design it is possible to use two bars with smaller diameter instead of one bigger bar that easier to find here.

One other thing I should be worry about is vibration. I don’t know how it will be !

I think the rails will support the rated load in any direction.

well, that's something I should be sure about

[kawazuki]

I think you need to add some more detail to your design before signing off on it! At present i'd say the vertical legs are the weakest and potentially most flexible part suffering twist back and forth from gantry weight and machining forces. Also at present the legs appear to be about 175mm long from base to u/side of gantry - bare? Now add Linear rails & trucks (35mm?) X axis table (25mm?) a vise or other workholder (75mm) Workpiece (say 100mm max) + drop down of spindle nose (100mm) = 335mm? ie almost double what you've allowed and twice as flexible! Easily cured with triangular braces from legs to base - 2/3rd height or more ideal then there is potential racking issues in the Y axis - sufficient bolting surface to prevent? and hold tolerance.
10 microns is pretty tight - that is less than half a thousandth - very big ask for even a commercial machine with a similar work envelope IMO!

[A_Camera]

I spent all of my past day studding existing fixed gantry designs (and even moving gantry designs) and I’m focused on the gantry itself.

As you know most of existing designs used round rail/bars (and almost all unsupported) witch I don’t like at all so those are not my business. Looking at some designs that utilized profile rail (lm) I see something that I believe is done wrong.

Most of designs with profiled rail mount them in wrong way (I think) so instead of putting the rail in such position that it takes force perpendicular to it and transfer it to surface underneath it (which I think is the key benefit of using those rails over round one, if we do not take tolerance in account) they installed linear rails in position that those rails taking force caused by weight of Z axis and I’m pretty sure that those rails not designed to deal with that at all

Take a look at images below, am I correct? Please let me know what you think about this

Is it proper way to install LMG? I really need some help about this

Advantages of fixed gantry is greater rigidity, higher possible speeds and lower costs for the same rigidity. Another big advantage is that it is easier to square, which is very important if accuracy is important. So in my opinion, the fixed gantry design is better than moving gantry, but it takes up about 1/3 more floor space.

Your design is basically the same as my design, though I am using fully supported round rails. I think it is best if the table rails are under the table, just like you designed. However, in my opinion the Y beam legs must get some support, otherwise you will get some flexing.

Just a question... you call Y X and X Y why? i.e. your X is my Y and your Y is my X. Doesn't really matter, but normally the axis which is parallel with my body when I am facing the front of the machine is called X and the one moving towards and away from my body is the Y.

[M-Malek]

I think the rails will support the rated load in any direction.

well, that's something I should be sure about

And now I’m sure, I take my time and read HIWINs Linear Guideways Catalog and in page 1 mentioned that “Because of the resistant effect between the rails and the blocks, linear guide ways can take up loads in both the up/down and the right/left directions.” As well as somewhere in page 4 “equal loading capacity in all direction”

And I also should say thank you Jim, it helped a lot

At present i'd say the vertical legs are the weakest and potentially most flexible part suffering twist back and forth from gantry weight and machining forces

First things first, thanks for your reply kawazuki

I think that’s right, that design is simplest way to build fixed gantry mill but not most accurate one (I take it from DIY CNC Router Build; Fixed Gantry, Steel - Wade'O Design) that’s why I’m going to design by myself and do some calculation and more and more search’s to avoid ending up with something useless.

BTW , I design new posts (legs) which I hope can deal with forces and provide better overall rigidity (and I’ll put it here (in next post) to get some advice and fix any problem with it before I build it.

and hold tolerance.10 microns is pretty tight - that is less than half a thousandth - very big ask for even a commercial machine with a similar work envelope IMO!

Humm. Unfortunely it gonna be hard job getting that accuracy! (if not imposable ) due to so many items that should be cared about it (like temperature). Reading more and more about it and even riding HIWINs catalog opened my eyes
Now I aimed it and try to get as close as I can.

Advantages of fixed gantry is greater rigidity, higher possible speeds and lower costs for the same rigidity. Another big advantage is that it is easier to square, which is very important if accuracy is important. So in my opinion, the fixed gantry design is better than moving gantry, but it takes up about 1/3 more floor space.

Thanks for your reply A_Camera

I agree

By now I’m 100% sure that I should build fixed gantry machine, it take a little bit more space but that worth it.

Just a question... you call Y X and X Y why? i.e. your X is my Y and your Y is my X. Doesn't really matter, but normally the axis which is parallel with my body when I am facing the front of the machine is called X and the one moving towards and away from my body is the Y.

Well, I didn't knew that, in my world longer axis called Y and shorter one called X that’s simple isn’t it?, HAHAHA, but it always is good to learn some things new and thanks for that. To be honest some time I mess up with them especially when I type long text like this one!

In next post I put a design of what I have in my head (for posts/legs) and I hope you help me with that.

I really pleased with help of experienced peoples like all of you, thank you

[M-Malek]

Well, in this post I put an image of my design for posts/legs and a provide a brief explanation of what I’m look for:

- I think about using a thick plate (40mm) instead of tube for gantry bar it sits on the top of post and bolts to it (like using tooling plate.
- Base of post going to bolted to another plate which act as base of machine too and fable rails sit on that plate.
- As you can see max deflection from weight of gantry including Z axis on post will be 0.0003 mm.
- For post itself I think about casting and then machining it and it weights will be around 40Kg.

And I have some questions:

1) Is it good to use plate instead of tube? (it weight same, plate little more, and deflection of plate will be ok too but from vibration aspect I don’t know)
2) How can I calculate vibration?
(I’ll try to simulate force of cutting and force from motion on post and share it here)

[A_Camera]

Well, I didn't knew that, in my world longer axis called Y and shorter one called X that’s simple isn’t it?, HAHAHA, but it always is good to learn some things new and thanks for that. To be honest some time I mess up with them especially when I type long text like this one!

In next post I put a design of what I have in my head (for posts/legs) and I hope you help me with that.

I am an engineer and the way I learned is that the horizontal axis of a chart is X and the vertical is Y, regardless of length. Horizontal is the one parallel with my body, is the one I am facing. I am not a machinist, so perhaps I am wrong in terms of machining.

[M-Malek]

I am an engineer and the way I learned is that the horizontal axis of a chart is X and the vertical is Y, regardless of length. Horizontal is the one parallel with my body, is the one I am facing. I am not a machinist, so perhaps I am wrong in terms of machining.

That’s right sir, I’m ok with that

In your previous post I miss understood
i thought you are referring to a naming standard on cnc machines

I have no engineering degree and even academic education on it. My academic education was on completely different field. I am self-educated on mechanical subjects. But I can remember about Cartesian coordinate, they told me that (I think in school) but my poor memory ….

BTW, what do you think about my previous post?