big budget cnc

[pet30]

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.

[Davek0974]

what do you think or what should i change

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 ;)

[pet30]

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.

[Davek0974]

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.

[Clive S]

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

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.

[Boyan Silyavski]

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:

[Desertboy]

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.

[Boyan Silyavski]

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.

Thats what i am talking. Its cheap, its not Hiwin but hey, its supported bearing. Thats what i call the real "budget solution". It will work much better than any V and skate bearings. And i have such small machine /V with skate bearing/ from the time when there were no cheap Chinese bearings or machines. And i am telling you this is one of the things in life that have frustrated me most. I dont wish that to anybody. It will go in the museum of Bad engineering

[Davek0974]

Looking at the dimensions more, i can only repeat earlier comments - this is not a machine for what is called "plates and skates" motion - ball bearings running on steel plates. It works reasonably for small stuff - my first plasma cutter was plate & skate but it jammed a lot due to crud on the plates and it was only 650x650mm, over a length of 3000mm the motion will not track square or level.

This would be a big machine, heavy and expensive I feel, it will need to be if it is to do any sort of work at all. If it were me building big, I would be looking at ball screws, 25mm linear rails, 750W servo motors at least, and a heavy steel frame. I would not even go with the round supported rail at this size. Just my thoughts of course. :)