Some Newbie questions

[roma1]

Hello
I've been wanting to build my own CNC for a while so i started designing it

As for the design the main purpose of the CNC is routing plywood, MDF and melamine(without chiping it) with occasional light aluminium works.
The effective work area should be 1.3m X 1.3m X 0.1m
The CNC will be located in my garage so i quietness is mandatory.
And the main requirement as in most DIY project - is make the project on the smallest budget possible.


So i started doing my research and i got wondering about a few questions that i couldnt find answers to (so im sorry if some of my questions are dumb but i'm just beginning the project)

1. I know wood has some good dampening qualities - so would wooden table help reduce vibrations?
2. I was thinking of building the CNC from square steel profile - what minimal dimentions would you recomend for my dimentions in order to reduce vibrations?
3. what can be done to reduce inetria problems? (we are talking about heavy steel gantry)
4. for Z axis i was thinking using two unprocessed aluminium plates(i dont have tooling to work with aluminium yet), so i was thinking to mount HGR20 rails and cartridges with SFU2005 ballscrew though the hight of the linear rail and cartridges is smaller then the hight of the ballscrew what can i do ? do i have to go with smaller ballscrew?
5. how do i know what is a good distance between each 2 cartridges ? is there some guide line to simple calulations?
6. why most designs use side plates?
like this one:
https://forum.makerforums.info/t/som...c-design/19429

and not spacer between the gantry and linear bearing (cartridges)or designs where the gantry is lying straight on the bearings?

like this one:
https://i.ytimg.com/vi/Tay5zPYahm0/maxresdefault.jpg

[ger21]

1) Rigidity is the key. If you are building a welded steel machine, I'd have a welded ladder style table, with 1-2 layers of plywood screwed and bonded to the steel frame, with an MDF spoilboard on top of that.

2) I'd use at least 50mm x 50mm with at least 5mm wall thickness. The bigger, the better.

3) Inertia is only an issue if the machine is underpowered. Unfortunately, speed costs money.

4) Not sure I understand, but it sounds like you'd just make the nut bracket to accommodate the required height.

5) The farther apart, the better.

[awerby]

I'm not sure what you mean by "quietness", but that's not a quality I'd ascribe to CNC routers. While you can minimize noise by using a 3-phase spindle rather than a hand router, and cutting softer materials like foam rather than harder ones like wood or plastic, the materials used for the table don't matter much one way or the other. You get noise from the spindle, from the interaction between the tool and the material being cut, and from the dust collector, which is pretty essential. If quietness in your garage is important to you, I'd suggest you go listen, in person, to some CNC routers in action before spending any more time and money on this project.

[roma1]

Hey
thanks for your quick reply!

i have been watching lots of builds on youtube and lots of them especialy large tube designs creates lots of chatter when milling aluminium - louder then the milling operation it self

i think that if i would like to analyze those vibrations i would model the machine as a complex of springs and mass
so covering steel table with wood - i think 2 springs in paralel would result in higher resonant frequency while as far as i read i should try and get the resonant frequency as low as possible to reduce chatter

while not sure if im right but if wood has good dampening qualities then it should have lower spring constant as far as i understand
so connecting steel frame to hardwood table would result in springs in series lowering the resonant frequency.
again not sure if what im saying is right or wrong...


as far for the dimensions - i do understand that in terms of vibrations more mass will be better but it will add cost and complexity to assemble as i sayed in the first post - im trying to understand what are the minimum i can get away with to reach my goals and not wake my neighbours when i operate the cnc in my garage
same about the distance of cartridges the more distance i give the less effective work area i get so the machine will be more expencive and it is a budget build ...

so the main quiestion is how to balance everything?

[routalot]

I took a look at the machines you linked images of.It might be perverse,but I actually prefer the wooden framed version as the aluminium framed machine has short rails for some reason and if you keep the gantry travel within the ends,the working area is pretty small.The very low gantry of the MDF machine ought to be stable but on the other hand it does severely limit the height of any project.Good to see it has ample triangulation on the gantry too.

Its good that you are taking care not to produce too much noise but I will add a caution about wooden tables.Pianos have a wooden soundboard to help project the sound-the clue is in the "board" part of the name.The inexpensive spindles seem to be a good bit quieter than the adapted hand held routers but either is far from quiet when working and I would recommend that you give your workshop some carefully applied insulation and minimise gaps around doors.

[peteeng]

Hi Roma - Dynamic analysis of machines is not easy. I do it occasionally with specialist FE software. My first router had a heavy timber machine base and a Makita router on it. Had to wear earmuffs at all times and could not talk to someone comfortably in the same room. Noise is a big issue with routing materials. If you delete aluminium from the requirement your design and build will be a lot easier. If you require aluminium you need a machine thats 20x (or more) stiffer then one for the other materials. Usually you try to drive freqs up, not down. You need to remove low freq resonances and you need damp materials to absorb other freqs. So:

1) Yes and No
2) All steel tubes will vibrate. Your question is too general to answer.
3) That's obvious reduce inertia!! use aluminium not steel or use thin steel but this is a problem with vibrations and with local stiffness. Maybe fill with foam or epoxy granite (lots of info if you search for this)
4) 20mm is a big ballscrew and you want to reduce inertia so use 12 or 16mm. Do the math. There is always a stack height delta with ballscrews and linear rails. You either use rail spacers or machine a groove to match the heights
5) As far apart as possible, there is no guideline. I started a thread specifically on this topic if you do a search...
6) Columns are used to achieve the working height and clearance height required by your design and work. "High Rail" designs are stiffer but then you can't side load the machine. Often sideloading is the preferred direction in commercial machines so the rails are under the table height and a column is used so boards can be slide through. On your size machine and if you don't need to sideload then a high rail design is stiffer.

How to balance a design? Firstly you design everything in CAD and source and cost everything. At this point (say 400 hrs in and at least on design no 15) you will have a complete picture of your design in terms of cost and performance. Then you can rebalance the design around cost or performance or both. Its a hard task to do properly cheers Peter

Here's a machine base I'm building at the moment that will take a 1.25x1.25 sheet. plus the cad of the machine... Technically it will be able to cut aluminium but until its built and tested we shall see.

[roma1]

Hey
Thank you for your replays !
Routalot - what do you mean by ample triangulation of the gantry?

Peter i understand that dynamic analysis isnt easy though im trying to relay on other people experiance and some engineering sense..

1. On what it depends?
2. I guess any machine even huge DMGs has some vibrations the question is how much they vibrate? And will the vibration be as loud as the routing itself or say 10dB more quite?
(BTW i was thinking about chineese 2.2kw water cooled spindle)
Again i understand that it also depends on the other moving parts , type of design and mostly on the joints for each part but im looking for some initial assumption to start with

3. Think i didnt stage the question right - the real question is how to balance those factors ?
For the second part - I have seen people fill the tubes with epoxy , granit , sand etc but i didnt see anyone do it with foam - what foam would you recomend? And how to spread it evenly in the tube ?(cant use vaccum and i dont think 1.5m straw will work)

4. Can i use 30cm X 12mm ballscrew with aluminium? And also should i use sbr16 instead of hgh20?

[roma1]

5) As far apart as possible, there is no guideline. I started a thread specifically on this topic if you do a search...

i read your post - realing interesting
so as far as i understand - so basicly if the cars im buying as far as i understand would clamp when applied moment of 0.27KN/m , the gantry hight would be 0.3m max
also i know that cutting forces of aluminium are somthing between 200-300N and lets say at some condition the machine could apply all this force on the cars at the distance of 0.3 (on a single side)
even if i use 1 car on each linear rail i have almost 3 times the force needed to clamp it so for 2 car a rail configuration im covered no matter what i do ?

and a second thing to verify i understand correctly : if im going for 2 cars a rail configuraion - the best thing i can to for the cars in my design is puting the beam of the gantry on the dead center between the to cars so the lever of each one of them is the same - and not try to compencate for center of gravity ?

im sorry if i got things wrong but im learning

[peteeng]

Hi Roma - Rereading your stuff I seem to think that you are dividing "resonance" into two things a) machine noise 2) cutting noise. The machine will make some noise but in terms of it annoying neighbours it won't be a problem. machine noise is generally due to the stepper motor vibrations and anything rubbing like bearings. They have small clicks and whines as balls move around. Its the cutting noise and spindle motor noise that is significant.

Balancing anything is up to you. Your motives, $$$ in pocket and expectations are different to anyone else so your balance is different to my balance. You just have to keep working at it until it gels in a way that brings a smile to your face...

Foam use 300 or 400kg/m3 PU foam. A 1.5m straw does work or drill holes to fill. Be careful with foam it expands & its the stickiest substance known in the universe. Aim at underfilliing then do a top up....

People do try to compensate for CG but they are in fact trying to gain benchspace. The Z axis has a forward stack or stick out if you build straight up and out. This creates a dead space behind the gantry that the machine cannot use. So the columns are canted backwards to recover that space. The dead weight of the Z axis is tiny compared to the static strength of the bearings so the CG thing is not important. Stiffness is important and everything on a cutting machine has to be uber stiff. Much stiffer then you think... Peter

You have two pathways as I see it 1) get a CAD system and start designing a machine, expect to design and redesign about 20 times to get some balance 2) start building a machine, use MDF its cheap and easily changed and learn on the fly.... by the way the smallest budget possible means whatever is in your pocket go buy a second hand machine. Your first build will always be well over what you expect in terms of $$$. But if your a Maker then that's what its about.

PU foam

Bearing specs
Roma you say "clamp" so I'll explain bearing specs. They have a static and dynamic strength load. The static strength is the load at which the bearing will suffer some damage. Usually brinelling of the race by the balls. The dynamic strength involves the fatigue life of the bearing and there is an equation and process you can use to estimate its life if you know its service conditions. The bearing manufacturers publish design manuals if you get n the hiwin or other sites they are downloadable...

[peteeng]

heres a great foam video!!

[routalot]

To illustrate my point about triangulation,take a look at the network of cables that hold up this transmitter tower.The proportions of a router Z axis are very different but the principles remain the same.



I wouldn't suggest using cables but you do need to brace everything securely to minimise chattering.I suspect peteeng could tell you a lot about calculating resonant frequencies for structures.

[roma1]

Hey guys
Your participation realy makes me want to just get up and start building! though as peter mentioned it will inflate the budget (and my wife will kill me - im not as worried about my life but i will not be able to finish building the machine!) so first i have to iterate with designs and carefully plan in cad only then start and take action (and yet i understand that i will do mistakes and will have to iterate building the machine)

Peter about the resonance - i mean while cutting - some machines amplifie the cutting noise and it realy becomes disturbing
(now that think about that - i read alot about how to eliminate the vibrations and noise from the machine that may be i mixed some stuff about eliminating vibrations from the ball screws etc and the cutting noise issue? so now im confused LOL)

For the motives: i guess i want it to mill aluminium (although it wouldnt be the main purpose of it) , and it should have working area of at least 1.3m * 1.3m * 0.1m and for accuracy i want it to be more accurate then 0.1mm
and the last and hardest to explain motive - since the machine will be located basicly in my house - it sould generate the least amount of noise possible although i know milling and routing is noisy but the main issue i want to tackle is creating a frame that is cheap and will not amplify the milling noise (may be i will add a cover for the machine and think about some insulation like routalot recomended - but first i need to worry about the machine it self)

about the cars spread - i would be realy happy is you could help me calculate the minimum distance between the cars while the main factors i think are : the force over the spindle is 300N (worst case) accuracy im aiming at is 0.1mm, frame size 1.5*1.5m - welded frame made from 100mm * 50mm * 3.2mm walls(or 5mm wall) , linear rail is HGH20 bolted to the frame and gantry, the rails on gantry are 7cm spread apart


routalot: thanks i didnt even notice that they put that plates there - i think i might add them too , but after finishing the machine and having my first precision tool :P

[peteeng]

Hi Roma - from experience place the cars a minimum of one bolt spacing apart prefer two. 20mm square cars have a longitudinal bolt spacing of 36mm so make the cars bolt holes at least 36mm apart. I do suggest you make a MDF machine first. Like the second link in your first post. All your mistakes are then easily rectified. All the electronic and motion parts will move onto your next better machine in confidence. You will get through the learning curve faster. MDF has all the same issues to solve as a metal machine but is easier and cheaper to fix.

Welding is not necessary the way to go. Welding distorts tubes, weldments need stress relief. Welded structures vibrate badly (since your interested in vibration) bolted structures absorb vibration and can be adjusted. Major machine builders do not weld mills. They are cast for various reasons. Step ahead and build a bonded aluminium or cast CSA machine. Look for the future solutions not the past solutions... Otherwise just buy a machine you will be making swarf faster. Peter