big budget cnc

Hi Peter,

If you cannot stretch to linear rails at the moment, you might like to have a look at mechmate design.
http://www.mechmate.com
http://www.mechmate.com/forums/index.php
I'm not necessarily saying use the whole mechmate design as its a little dated, but it has a lot of features you describe in your initial post.
Lots of info for making roller bearing rails, rack and pinion, and other stuff

Personally I would always keep the gantry as short as possible. 3m will offer huge challenges.

Cheers

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Originally Posted by mekanik

Hi pet30
Welcome to the forum
I can't see any logic in having your gantry that long the machine would normally have the gantry spanning the shortest width.
trying to construct a gantry that length you are on a hiding to nothing, have a look through some of the build logs.
Enjoy the forum.
Regards
Mike

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i need the gantry to be this way because my workshop( ist at my parents house) and if i go home then my mothers car is parked in the garage so i this way i can still have a cnc .

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Originally Posted by pet30

i need the gantry to be this way because my workshop( ist at my parents house) and if i go home then my mothers car is parked in the garage so i this way i can still have a cnc .

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3m's is just too long though and you will be creating yourself a world of trouble, there are plenty of machines out there with 3m gantries they are very very chunky and use serious slides the cost of the gantries are far beyond what most of us spend on our entire cnc machines.

If you build it like you suggest it will not work very well at all and you will spend a lot on a frame you can't use.

https://www.kickstarter.com/projects...foot-cnc-machi

This seems like it might be more useful to you although I would never consider this suitable for real production (I thought it looked crap but mate thought it was ideal for a small workshop for occasional jobs.) It might be something that ticks your boxes given the lack of space and tight budget.

There is another choice you could make the gantry 1.5m's and mount the CNC so it folds to the wall when not in use you can get more space and only use when the cars is not there.

You probably don't want to cover your mum's car in sawdust anyway if you still want the cnc machine their in a months time ;)

the beams are 160x80x5 mm steel
they will be perfectly flatten at a machine shop not far.
then i wanted to put a flat steel plate of 120x5 mm and on the 2 side beams so the bearings can ride on this.
on the bottem of the flat bar ther will be 2 bearing that are pushed against the flat bar with a spring so its really thight .

the steppermotors would be nema 23 directly to the rack ( the rack is bolted to the flat bar on its side) and behind the rack ther would be a bearing so the gear can't jump a way
what do you think or what should i change

i was also thinking of using the mechmate V groove rails etc instead of the ball bearings.

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Originally Posted by pet30

what do you think or what should i change

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Your issue is going to be that massive 3m gantry beam - even at 160x80x5 it will flex and twist like a bitch.

Then there is weight - that beam will weigh about 55kg before adding motors, spindle, guides, motion etc - two nema23's will not do unless you add some decent gear/belt reduction - this will be a slow machine, this size stuff is servo motor territory i feel.

To stop the beam sagging, you add triangulation - a support from end to end raised in the middle and joined with a strut, this will not stop twist though and all this will add weight, I would guess a working 3m beam gantry would weigh in at around 200kg or more - this is silly and would mean using Aluminium to build it.

I'm not bragging on you here - just trying to save your money, i've built some dodgy small machines in the past but nothing this big ;)

no problem thanks for the help

i know this is not easy to build but i think ist the only way i can have a cnc in the garage.

if i was to change the machine 90 °( making the machine 3 meters long and the gantry 1600 mm long) could it work then?

what do you mean with :
you add triangulation - a support from end to end raised in the middle and joined with a strut, this will not stop twist?

and how hard will it drop and twist?
if the cnc is accurate to about 1 mm on the 3 meter then its ok i can live with that
i am using it for shelfholes( 5 mm holes like ikea cabinets) and making some MDF doors , some holes for blum hinges etc.

Yes make the gantry 1600 and things get much much easier.

I think for a router you will still possibly need 2:1 reduction on the drives though if using rack/pinion, ball screws would be ideal but much more money.

Triangulation - draw a long thin box - this is your beam, now draw a triangle on top spanning end to end of the beam, from the apex draw a vertical line down to the beam. You now have a box with much more vertical rigidity, triangles are very strong when used right. The trouble is this will not stop twist - you will not stop twist with a single beam that long - it will need two parallel beams with the spindle/z axis mounted between them, this will be very sturdy but 2 x heavier - there is no free lunch here :)

I doubt you will achieve 1mm tolerance in the middle of a 3m beam, plus you have lift - when the cutter plunges into the wood, the beam will lift causing tapered cuts etc.

Big machines need lots of design i'm afraid, and lots of money to make them work.

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if i was to change the machine 90 °( making the machine 3 meters long and the gantry 1600 mm long) could it work then?

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There are a few examples of machines built vertically. Dean (Jazzcnc) has made a few and Neil (njhussey) has done one as well.
Deans are built with the long axis horizontal and Neil has done it with the long axis vertical.

Save some money and buy proper linear bearings, at least round supported. Which in fact are very cheap. What you want to do is like driving on the Autobahn 200km/h with this:

Attachment 21991

I have SBR25 and would recommend that as the minimum for the longest axis and it weighs a ton so I wouldn't want it on my gantry. I'm putting 1.5m 15mm Hiwin clones I got on ebay with 4 carriages for €120 delivered on my gantry to keep the weight down.

I like my supported rail it's lovely and smooth but it's no Hiwin.