[Patuc]

Hello,
I need help, I hope someone can help to me. I thinking which y axis designs to choose, I have 3 options:
1. Ordinary: both rails in one row, example: https://www.workshopaddict.com/forum...d-dsc_0763.jpg
2. Rails in different places, example: https://idea2reallife.files.wordpres...axis-sbr16.jpg
3. Like this: http://i36.photobucket.com/albums/e1...CNC-Y-Axis.png and video how it looks like in real: https://www.youtube.com/watch?v=gg2UvbRsLP0&t=588s

How you think: which design is best (most stable and most accurate). I will use SBR20 support rails.

[Patuc]

106 views and no answer. Is there someone who can answer to my question: which (1-3) design is most stable and most accurate?

[adamdyson]

I'm also in the planning stages of building a CNC and the Y axis design was something that took me a while to decide on.
Firstly, if accuracy and strength is your goal, I would consider using square support rails instead of round.
As for the 3 design options, I think if the right materials are chosen and you take your time with aligning everything, making sure everything is square, neither design is better than the other as far as strength and accuracy is concerned. In my opinion the 3rd design is the easiest and cheapest option when building your own CNC, but I'll be building my CNC using design 1 because unlike designs 2 & 3, the rails are not facing upwards which is bound to catch allot of dust while cutting. I also prefer design 1 over design 3 because you don't have reach over or through the Y axis supports when loading and positioning material. With design 1, it's possible to design your X axis so the spindle can travel past your gantry risers without clashing, allowing you to utilise the full width of your bed.

[Edit] I forgot to mention (probably because I'll be driving my Y axis using rack & pinon) but design 1 is the only design that allows you drive your Y axis with a single motor/ballscrew.

[Patuc]

Yes, I know that square support rails would be better, but HIWIN is too expensive to me right now Ok, thanks, so in first place is design number 1 X axis will be 1900mm, Y axis 1300mm and z axis 500mm

By the way, thanks for your answer

[Patuc]

More info: cnc frame design will be like in that video ( https://www.youtube.com/watch?v=gg2UvbRsLP0 ), so now I'm thinking about Y axis design

[pirate323i]

I too am doing design 1, however i like design 2, i just cant articulate why...
I realise this is of no help, sorry!

[wizard]

I like design # 1 myself. In any event you need to think about manufacturability of your machine. If the design you choose keeps the rails in one plane then you greatly reduce the complexity of your build.

The complexity comes from developing a flat surface to mount the rails on that keeps them in the same plane. On design one this can be done via machining or some of the other methods often discussed here like Epoxy leveling. Design #2 is especially bad because you need to mount the top rail on a plane that parallels the other rails mounting surface precisely. Oh #2 arrangement also requires that the top surface that the rial is mounted to is also parallel to the lower rail. As for design #3 the big problems there are two things in my mind. The rials up orientation creates a channel to collect junk and it adds unneeded complexity to the Y axis saddle which will support the X axis.

In a nut shell when looking at machine designs you need to ask yourself the question "how will I build this". This includes what you can DIY and what you might have to farm out. Also due to the size of these machines farming out work to a machine shop isn't always cheap, especially if you have work that needs to happen on a bigger mill. BridgePort type milling machines are a dime a dozen around here but they really aren't that useful when it comes to machining large gantry beams so all your friends with Bridgeport mills aren't much help.

In any event #1 is the only viable design approach in my mind. The more you think about it the more the others have design considerations that suggest they aren't the best approach.

The other thing that bothers me about the selected pictures is the lack of attention to the beam cross sectional size. A fairly large beam , in cross section, is generally your best bet. Stiffness is so important when it comes to getting good machining results and should be the primary driver of your design process. In a couple of those pics there really doesn't appear to be much of a beam at all.

[1Jumper10]

Design 1 is better I think. The machine in the video I don't think will be very rigid. It would help a lot if the square tubes on the gantry had braces welded between them. Then they would act like one single structure and be more rigid, like Design 1.

[Patuc]

Thanks for all answers! I don't want to make new threat, so I will write in this topic. So: Y axis will be like design #1; cnc size: X axis 1900mm (Ballscrew 2005 and SBR20 from both sides), Y axis 1300mm (Ballscrew 2005 and SBR20), Z axis 500mm (Ballscrew 1605 and SBR16), stepper motors Nema34 1600oz.in. Frame design similar to frame like in this video: https://www.youtube.com/watch?v=gg2UvbRsLP0

*What forum members think about this design?
*What do you think: which steel profile size to use: a.) 70x50mm wall thickness 2mm; b.) 80x60mm wall thickness 2mm; c.) 80x60mm wall thickness 3mm. Or maybe all profile sizes is too small?

[ger21]

1600oz steppers are a very bad choice. Using them with 5mm ballscrews is even worse choice. The machine will be very slow.

Use 2010 or 2025 ballscrews, and 600-900oz motors.

[Patuc]

Thanks for answer 1600oz is about 12 Nm Holding torque, so if speed increase the real torque decrease; I want to mill hard wood, aluminum and sometimes steel, so if motors would be weaker (600-900oz) can they do the job with harder materials (hard wood, aluminum)? Of course speed is important, but if speed is increasing does precision decrease?

[awerby]

You won't be cutting steel with that machine - you don't have the right spindle or enough rigidity - aluminum would be questionable. Speed is important for cutting wood; if you go too slowly, the tool starts scorching the wood, leaving dwell marks. High-torque motors tend to have high inductance, which requires more voltage to overcome. But the drivers won't tolerate too much voltage. With any given motor you can increase torque by using finer screws; that gives you more precision at the expense of speed.

[ger21]

10mm pitch screws give a good balance of speed, and still plenty of precision.

[Patuc]

Thanks for answers. Motor inductance is 5mH, I will use 60V 350W power supply for one motor. Spindle 3kw air cooled. But I have the motors and rails ans screws, so I can't change it now.
So:
*What forum members think about this design? (like in video)
*What do you think: which steel profile size to use: a.) 70x50mm wall thickness 2mm; b.) 80x60mm wall thickness 2mm; c.) 80x60mm wall thickness 3mm. Or maybe all this profile sizes is too small?

[jannos]

Wall thickness 6mm is minimum. I have used 250x100x6 steel tubes for my router.

[Patuc]

Wall thickness 6mm is minimum. I have used 250x100x6 steel tubes for my router.

If steel minimum 6mm, and you use 250x100, this is steel, so what to talk about people who made routers from aluminum profiles, if 6mm steel minimum, so aluminum have to be 300x300mm minimum? by what calculations you calculate 6mm minimum? In this forum and youtube you can see that a lot of frames is thinner than 6mm and frames is solid, router work an everything is ok. 250x100x6 is aprox 32kg/m. So I think it have no sense, because a lot of people build cnc from aluminum profile, and some people build cnc frame from steel, because they wand less vibrations and more solid frame; 6mm wall thickness is good, but I don't thinks it is necessary

And thanks for answer

[ger21]

Everyone has different expectations for what a cnc machine should be. What works fine for one person, may not work at all for another.
So, there is no "minimum" or standard requirement.
One reason for using 6mm wall thickness, is that if you'll be drilling and tapping for screws, you may only have 2-3 threads if using 3mm wall.

[Patuc]

Everyone has different expectations for what a cnc machine should be. What works fine for one person, may not work at all for another.
So, there is no "minimum" or standard requirement.
One reason for using 6mm wall thickness, is that if you'll be drilling and tapping for screws, you may only have 2-3 threads if using 3mm wall.

True You are community moderator, i believe you have a lot of experience, you see a lot of different cnc machines. What do you think, I will build cnc as in that video from youtube,just y axis will be like in design #1. I think to use 80x60x3mm steel profiles, nema 34 (5mH) 60V, ball screw 2005, sbr20 and air cooled 3kw spindle, "awerby' say: "You won't be cutting steel with that machine - you don't have the right spindle or enough rigidity - aluminum would be questionable" So if I want mill aluminum my parts for cnc is bad? or cnc frame design is bad? Why people can mill aluminum with aluminum frame cnc, but with my parts and steel frame aluminum would be questionable?

[ger21]

You can mill aluminum with just about any machine. I've milled aluminum on my MDF machine.
The issue is whether or not you can do it efficiently, and the quality of the results that you'll achieve.

If it were me, I'd go with the 6mm wall thickness.

[Patuc]

Thanks, but 6mm thickness profiles is heavy, for example Y axis will be 1.5m long, so it would be about 45kg+ z axis would be about 30kg, so y axis would be 75kg. I use supported linear rails 20mm (SBR20) for X axis, does 75kg is not to heavy for them?