New Machine Build- Structural issues

[$hantan]

Hi guys, this is a drawing of a stripped down version of my machine. What concerns me at this point is :

1. The distance between the X rails.

Is it too large? Any 'cocking' issues?(I hope I'm using the term correctly; I mean one set of blocks getting ahead of the other.)

2. The length of the gantry.

Is it too large? The Y rail suppport tubes are 3"x3" and are 1/4"thick. The wt of the Z axis assembly with the spindle and stepper would be around 30 kgs(assuming I'm right about the wt of the 2 MIC6 Al plates being 6 and 7 kgs). Will there be intolerable or acceptable sag?

I have tried to compensate for the length by using a wider gantry. Also, should I screw on a 3'- 4' long, 3/4" thick Al sheet connecting the Y rail support tubes from behind to reduce the sag?

Help and feedback required. Thanks in adv

shantan

[jjacstcy]

Looks to me with that design you will be able to cut with the z and y axis only. Are you putting another frame or something in between the gantry?

[Phife]

where is your bed going to go? how will it not interfere with gantrys lower cross brace?

[Harpye]

Just a quick shot - no deeper look -
- the gantry ase itself looks rather stiff ( stabilizer frame below is rather wide in X


don't know about the rail system you want to go for ... with a very stiff and low friction bearing you might be able to go trough but since you ( as you alrady mentioned) have a very wide y axis you may come into issues regarding asymetrical load on the X engagement what leads to a pivot point at the thread nut and as a reactio the sliding parts of the x rail might stick and slip ... solution 2 toothbelt sychronized Threads on one or 2 motors which are placed close to the rails....


Another issue due to the same asymetrical load application may be a twist of the 2 X railsaround the X tread

Just a short comment -

Hope it helps you to find your way

[jsheerin]

The plates that your x screw is mounted to look really thin. Those need to support the force that pushes your gantry, so they need to be fairly substantial.

Your x rails probably need to be tied together a bit more than you , but really you need to figure out how you're going to mount your material first since that could change the entire structure of that part of the machine.

Racking is the term usually used to describe an axis twisting and binding.

[routercnc]

1. 'cocking' is usually called 'racking' or 'crabbing'. With the central leadscrew there will always be a small amount of racking.
If the racking is severe then the axis could bind, but with linear rail or profile bearings this is less likely.
The issue then is how much is acceptable for what you want to cut and make. My homebuild cnc machine uses a central ballscrew driving a gantry about 600mm wide. I consider this to be the widest practical single screw width.

1914mm is very wide and I think you will be dissappointed with the accuracy when cutting away from centre. You should consider a twin screw design unless you are cutting balsa wood, or light engraving. There a various methods available, cheapest is to drive only one leg and use a wire and pulley system to slave to the other side, like an old drawing board. Next cheapest is to drive one leadscrew and use a pulley and belt to slave to the other. Or you could put the motor in the centre and use a belt to each leadscrew. Most expensive, but simplest is a motor on each leadscrew.

Twin leadscrew designs also make the design of the cutting area much simpler because the gantry doesn't need to join underneath.

2. Gantry sag due to Z axis weight.
Taking your data of 2off 3"x3" x 1/4" box sections (assume steel) over a 1914mm span, and Z axis weight of 30kg gives a maximum sag of 0.074mm or 74 microns. You need to decide if this is acceptable.
If you join them together with a plate at the back as you suggest this will reduce this value significantly, since you will create something approaching an 'I' beam. This will also help in torsion when you cut in the X direction, which tries to twist the gantry about the Y axis.

I agree with the other posters that the design of the bed is not clear. Can you clarify how that part will work?