operation cnc

[kingcreaky]

evening all.
first off, thanks to all that have posted on this forum. It really comes across as a friendly inviting community of like minded individuals.
As I mentioned yesterday in my 'hello' post. The time has come for me to commence (or at least thinking a bit more seriously about) my CNC machine. Well, in actual fact CNC router I guess.

Ive sold an my pride and joy '68 mini van' which has freed up my workshop area. The nights are drawing in, the caravan is packed away and its now workshop season.

The intention is, to manufacture a flat bed miller. In order to fundamentally cut / carve wood. However, while I am there I wish to make it well enough to cut aluminum which I think will be ultra handy for some other projects I have in mind.

Id like to make it about 3m by 3m however I dont think my bank balance will afford it. I dont want to compromise too much on size just because of the cost so if I can achieve a working area of around 1m x 1m I wont be too disappointed.
My original idea was to copy some of these angled ali / bearing linear ideas I see so frequently on youtube with a wooden frame etc. However after reading some of your thoughts and comments on here Ive decided to do it properly, so if at any point I seem to be cutting corners please feel free to point it out.
Today I have learnt alot. The differences for example between Trapezoidal Spindles and Ballscrews for example. Vibration, jitter etc

On request of fellow members of this forum, I think its prudent I make some drawings discuss all my ideas etc before I go balls deep and order the gear. So today has been a crash course in 'sketchup' which incidently ROCKS. Ive attached pictures of my gantry design

It is, to be fair a bit similar to motoxys. But as there was so much discussion around this subject I hope he doesnt mind me harnessing the wealth of ideas around this very subject.

So assuming the gantry is 1300mm does this look about ok?

(If there is anybody out there, not used sketchup before, or just want to copy this design, ive uploaded my sketchup file for the below for free download to anybody who is interested http://www.pensys.co.uk/cnc1.skp

[kingcreaky]

spent most of last night laying awake. I get a bit like this when im 'on one' with something like this.

The more I read, the more ive established that the cutting ali is all about getting the strength in the right places. You dont want things flexing. I fear this may be copying another idea ive seen somewhere, but instead of having big lanky gantry support uprights with the linears at the bottom... how about having the linear rail at the top. this way, I dont have to decide now what the maximum height I want to work with is, plus I think its likely it will be very rare that im dealing with anything 'deep' as such. therefore I can cater for bigger objects by lowering the cutting deck and from an amount of movement I have available in my z axis?

I have an abundance of wood available to me, so Im thinking the uprights and cross members that support the Y linears etc (the table as it where) is going to be made of wood. then the gantry out of ali... does anybody think this is a really bad idea?

anyway, ive made some more pictures available...


I think im going to have two ballscrews for the gantry (one at either side) with two steppers.

[Jonathan]

The design you've just posted with the tool operating below the X-rails is far stronger, especially with an adjustable height bed. See mine here (or come and see it in real life if you want), don't pay much attention to the Y-axis though as that's weak.

I have an abundance of wood available to me, so Im thinking the uprights and cross members that support the Y linears etc (the table as it where) is going to be made of wood. then the gantry out of ali... does anybody think this is a really bad idea?

I do...it is far easier to make a machine to cut aluminium efficiently if you only use metals in the design. You'll need to use coolant if you're cutting aluminium regularly and clearly the wood wont last long or stay put if it's getting wet. You'll be much better off with making a steel frame. I get steel box section from a place called Adey Steel in Loughborough, which is not too far from you. 60mm box section is a good size for the frame and if you stick to 3mm wall thickness it's much easier to weld with a cheap welder.

Not sure what thickness it is in the drawing, but the Z-axis plate looks a bit thin. 20mm thick is a good size.

Put a few supports for the long rails on the frame to help stop them bending towards the middle.

(Edit, forget this bit - written before previous post.

With the one piece of box section the gantry torsional stiffness is quite poor - imagine what happens when you apply a force to the cutter when the Z-axis is at it's lowest point, the gantry will twist. If you just put a second piece of box section below the top one, aligned to the bottom face of the aluminium plate then the torsional stiffness will be much improved.

If you're after 1m travel then a 1300mm wide gantry is a bit generous since 200mm is plenty for a Z-axis.

Instead of putting the rails for the Z-axis on the Y-axis carriage, put them on the plate which holds the spindle and moves up/down and have the bearings attached to the Y-axis. This is generally a stronger configuration, especially with an adjustable height bed, since it minimises the vertical distance between the cutter and the support bearings (the overhang). This also can save some aluminium, depending on where you put the stepper motor.)

[Jonathan]

As irving has said, you need to use 10mm pitch (RM1610) ballscrews on the X and Y axis to get respectable feedrates without using a larger diameter ballscrew.

Clearly it depends on what the minimum feedrate you can accept is, but I wouldn't go above a 1500mm long RM1610 ballscrew without using a larger diameter screw or rotating the nut. If you're interested I can make the rotating ballnut mounts for you, let me know if you wanted the details. I wont be able to make them any time soon, but it looks like it will be a while until you're ready anyway.

The 3Nm stepper motor shafts are 8mm, so you would need 8mm to 10mm couplers. However I would strongly advise using timing belts and pulleys since this enables adjusting the drive ratio to get better speed or resolution with the added bonus of adding damping to the system to suppress resonance.

[irving2008]

Another thought/idea: if you're going for that layout is to consider putting the x-axis motors/rotating ballnuts on the gantry rather than rotate the ballscrews. This way you could easily upgrade from 1m to 3m by changing out the ballscrews & x-rails and retain the same motors/etc. and you dont need to invest in expensive bearings for the ballscrews. The downside is marginally more weight on the gantry and constructing rotating ballnut mounts is more tricky.

[kingcreaky]

Like that Idea, I noticed on Jonathons this idea was used, but disregarded its main advantage. This way I could buy, say 2m x 1m setup now. make the gantry 2m and the x axis x axis 1m. If its as successful as I hope I can then re-make the x axis longer. Need to think of a cunning way to make the rotating housings... I can worry about that at a later date. Going to start from scratch in sketchup again. this time with my newly quoted 60mm steel box from Adey Steel, and with the new design wide gantry and static screw.

[blackburn mark]

spent most of last night laying awake. I get a bit like this when im 'on one' with something like this.

that's a good sign :)
its interesting and quite easy at the outset of any new design... in my experience though it gets harder the further you get into the nuts and bolts of the job... one small amendment can have a bitch of a knock on effect :(
its frustrating when you want to get building but try to design the knickers out of it before you commit because those amendments and knock on effects can be heart breaking if your half way through a build

having a Y axis that is longer than your X is pretty unconventional for obvious reasons... i can see your logic in expandability, if you will definitely expand if all works well i would keep the faith and go for glory at the outset on the proviso that the consensus was that your design would do what you required

iv not built a machine of that size but there are a few people on here that will give you a good idea if its doable 3m x 3m is quite a large machine... i'm not sure anyone has built one of that size on this forum ?

good luck getting to sleep lol

[Jonathan]

3m x 3m is quite a large machine... i'm not sure anyone has built one of that size on this forum ?

I don't recall any that size here. You can get 3m long rails rails easily, but if you decide to use profile rails then it will be hard to align them over that distance. You're getting into mounting the rails on self levelling epoxy territory. The SBR series rails tolerate a lot more misalignment than the profile rails, so that option would be a lot easier and I'm confident you could make a strong enough machine that size to cut aluminium with them. The gantry clearly needs to be stronger than the current design if it is extended to 3m. You may want to consider placing the Z-axis between two beams with the rails on their inner faces.

With 3x3m you would also need rotating ballnuts on both X and Y. Just in case you've not already found it, there's some ideas about how to mount a rotating nut here. It sounds like you might have a milling machine so as long as you've got a boring head or lathe you can make them.

[WandrinAndy]

... in my experience though it gets harder the further you get into the nuts and bolts of the job... one small amendment can have a bitch of a knock on effect :( Its frustrating when you want to get building but try to design the knickers out of it before you commit because .....

Very true! That is precisely the design phase I have been wading through for the past many weeks, and it's very frustating but, with what I have discovered and had to correct in my little design, I'm very thankful to have found these problems before the build proper!

[kingcreaky]

thanks for the comments gents.

however, although id love to build a machine 3x3. I cant afford too!. Im simply make the biggest I can whilst maintaining my marriage :D

anyway, done a bit more on the design. but those bearings look mighty small for such a large spanning gantry....

might have to have a re-think...

what do you guys think?