[Graphicman]

Hello guys
this is my first post here
I am trying to build up my cnc machine, attached is the design please let me know the pros/cons of it
machine specification:
max travel XYZ: 73x59.6x7cm
steppers: nema 23 287oz/in
guides: thk/rexros
ballscrews: Chinese BS XY: SFU 2010 , Z: SFU1204
spindle : at least 1.5kw
target accuracy : not worse than 0.05mm
I intend to make the frame mostly from hard wood + supporting metal parts
I care most about the accuracy/rigidity of it not the speed

hope to hear from you and thanks

[Eldon_Joh]

accuracy is mostly a limit of reliable step resolution divided by the ballscrew pitch.. cheap rolled ballscrews are of course dubious.. however, times have changed since 5 years ago...

regarding making the machine from wood... it can be done but the big problem is changes in humidity will lock up your machine.

but if you do want to go with a wooden table.. make it a stack of plywood of about 10 layers of 18mm plywood epoxied together, it just might work. the X axis slides will be sucked together and pushed apart with humidity changes but if the Gantry is high enough, the cross beam will flex to take up the stress.

[Eldon_Joh]

ballscrews: Chinese BS

can't help but laugh.. for a router table you really don't need ballscrews... spend the money on software and broken milling bits while you learn how to use the machine....

[aarggh]

I'm not a fan of using wood for cnc machines for anything other than the spoilboard. As a suggestion why don't you pick up some 50mm x 50mm steel square and bolt it together. The Chinese rails and ballscrews will have greater tolerance and accuracy than anything you'll make out of wood without major effort, and with using steel, you can shim where you need to to get it perfectly aligned.

Be aware though, if whatever your machine is sitting on has any degree of movement or flex, this will manifest itself in your machine. So planning the machine also means planning how to reduce the effects of table movement or variations in the floor surface beneath the machine's frame.

cheers, Ian

[Graphicman]

-Thanks Eldon, i know about the accuracy thing but i bet this applies when the machine is rigid enough, and that's what i care most here from this design as i did not make any cnc machine before so i am trying to avoid as much mistakes as possible before i start it
-very true about the wood and humidity , i was thinking sealing the wood should take care of this problem, if not then i might consider replacing some important parts with 80/20 for example.
-i know how to operate a cnc very well as well as designing on artcam ect.. i just need to have my own cnc now
i dont understand what do you mean by i dont need ballscews for a router table?

[Graphicman]

Thanks Ian
I agree with you, of course i will make sure the machine is fixed properly to something that is stable enough underneath.
I can see that the main concern here is for the frame material
the idea is that i will be able to deal with wood better than anything else
but if this is not enough then you better send me some links or photos for how to use this 50mmx50mm steel squares
the steel is much harder to deal with though and i am afraid the machining/finishing of it might not be as accurate as dealing with hard wood
but basically what do you think about the design itself?
do i need to change anything for better performance or easier to do?
i am still confused about the way people install the linear guides, some install it on top and some on side, what is the right thing?

[Eldon_Joh]

i am still confused about the way people install the linear guides, some install it on top and some on side, what is the right thing?

there are two kinds of linear rails, take a look at the company datasheets for diagrams of the bearings.
when you work out how the cutting forces are applied to the bearings then it will make sense. but some of it is just preferance.
mounting the rails on the sides also keeps them cleaner.

[Graphicman]

I have seen a video where it says on top is better (in term of rigidity)
does it make any difference if the linear guide is 4 or 2 races?
i am going to use this SR15W2UU 750mm Used Rexroth Bosch LM Guide 2RAIL 4Block CNC Route Linear Bearing | eBay

by the way what do you think of a good condition used linear guides on ebay, do you think it will be good enough or it will most probably have backlash, I am afraid they are sold out because they start to wear out!

also I asked the seller about this specific linear guide and he assure it has four races every block , however i see only 2!
please let me know