[zambrano]

Hi, i complete my second cnc router, this time i use square linear rail. Working area 1200 mm x 2700 mm. For x axis i use 2005 chinesse ballscrew and for Y axis 3610 THK ballscrew. However the bridge twist under external forces.

Any help with this issue?

Miguel

[alan_3301]

What is the gantry / bridge material and size?
Can we see pictures? Wow I've never seen a 36mm ballscrew.

[zambrano]

Hi Allan. Here You can find a photo before i install 3610 ballscrew.

Message - My Photo Gallery

Lateral base are a-36 steel 3/4" thick. X axis base is "U" canal of 10".

I am reading a post where use cable to avoid the twist of the gantry.

Miguel

[alan_3301]

what is the cross section of the bridge plate?
If it is flat plate, I think that is your problem. tube is much better.
Is the ballscrew only on one side? That is a problem also, unless the gantry is incredibly stiff.
Cable can work to keep sides even with each other, but the gantry will bend in the middle where it is unsupported.

[ger21]

Your talking about the gantry racking? Space the bearings on the gantry uprights further apart.
I doubt that a cable system will work on a machine that size.

Driving the gantry from both sides is probably the best option.

[louieatienza]

Hi Allan. Here You can find a photo before i install 3610 ballscrew.

Message - My Photo Gallery

Lateral base are a-36 steel 3/4" thick. X axis base is "U" canal of 10".

I am reading a post where use cable to avoid the twist of the gantry.

Miguel

I think you can remove a lot of the rack by simply making a transverse plate on the bottom, the whole width of the gantry legs, spanning the entire length, and securely fixed (or welded.) It would help greatly with each side moving away in relation to each other.

[Eldon_Joh]

you need two ballscrews for a machine of that size and aspect ratio, it is that simple.
alternatively you can drive the gantry from the bottom, with the ball screw centered under the table, that will cut the racking in half, but won't get rid of it completely.

[John Hill]

I am still building my first cnc router table, 900mm x 2100mm, and I really hope the cable method will work as I am driving one side with cables to keep the gantry 'square' to the table. I hope it works!

There are two styles, at least, of cable configuration that would assist with controlling wracking.

One style uses two cables that are fixed and do not move. One cable is fixed at a corner of the table and passes via pulleys across the gantry then continues and is fastened to the corner diagonally from the start point. The second cable runs in similar fashion between the other two corners of the table.

The second style uses a single moving loop of cable. There are idler pulleys at all four corners of the table, two of them are at one end of the table (we will call this the 'head' of the table) and are mounted with a horizontal axis, the two idler pulleys at the far end of the table (the 'foot' end) are mounted with vertical axis. The cable is fixed to the gantry slide then passes around the vertical axis pulleys across the foot of the table then up to the far side to the pulley at the head on that side. The cable then comes back and around the foot again, up to the head end and back to a fixing point on the gantry close to were the start end in anchored. The far end of the gantry is clamped to the cable that is moving in the appropriate direction.

'Aircraft' cable is what you need for this job.