PDA

View Full Version : Double gantry idea, What do you guys think?



craigrobbo
23-12-2012, 10:57 PM
Hey guys.

I had a bit of a profound idea and I hope I don't get flamed for this.

I was thinking that on my CNC 3040, the biggest weak point is flex in the gantry and lower rails when plunging/drilling, causes the entire gantry to 'lean' backwards and flexes.

My proposed idea was to literally mirror image the entire gantry, Effectively the spindle is being held from front & back by 2 individual gantry's

Positives:
No 'push-back' leading to the gantry flexing/leaning backwards
increased rigidity in the entire machine
Straighter drilling and plunging operations
4 pillow blocks, spreading out the load again increasing rigidity
increased lifespan of pillow blocks, bearings and rails

Negatives:
Reduced travel
Extra weight will require a larger stepper(possibly)
reduction of ease of access

Weighing up the good and the Bad, if I can justify the negatives then would this work the way I hope it should?

Thanks

Craig

wilfy
23-12-2012, 11:22 PM
i dont exactly know what a 3040 is, but if something is allowing that much movement i'd rather replace the thing causing the issue.. instead of losing all the travel would it not be better to start the gantry all over again and design it right so it doesnt flex? from a quick google it looks like its on rails.. would the next point of upgrade here be to replace them rails for supported ones? i'd imagine its the rail that bends and even if you double up it may still do it

D.C.
24-12-2012, 12:30 PM
It probably won't work the way you hope.

If the unsupported rails are flexing because they can't take the loading then no amount of strengthening on the gantry will solve the problem. Think about a a one ton load on a tightrope causing the rope to flex downwards. It doesn't matter if the load is jelly, cast iron or a baby elephant, the tight rope will still flex because of the load on it.

craigrobbo
24-12-2012, 12:45 PM
I don't think I have explained myself very well.

when the machine flexes, its making the gantry 'lean' backwards Some of the flex is in the Y rails and some is in the X rails.

Doubling up the gantry would give me 4 X rails and the load would be spread out more evenly on the Y rails, and also the load wouldn't be in one direction but opposing so in theory will reduce flex.

Think of a balerena on her tip toes dancing on polysteyrene, she would likley just sink into it, but then think of a tank with huge tracks, it would likely glide over due to te increased surface area and balance over a larger area.

Hope I am making some sense?

On a side note, IF I decide to do this mod, I will be adding supported rails too in order to eliminate that possibility.

D.C.
24-12-2012, 12:58 PM
There are quite a few different designs of 'cnc 3040' out there, would it be possible for you to post a few pics of your specific machine and tells which way X and Y is on your machine?

If you are adding supported rail anyway it would be better to get longer rails for the gantry to ride on and double the distance between the bearing blocks by reinforcing the legs of the gantry. That will help a lot with gantry tipping backwards problem.

craigrobbo
24-12-2012, 01:44 PM
There are quite a few different designs of 'cnc 3040' out there, would it be possible for you to post a few pics of your specific machine and tells which way X and Y is on your machine?

If you are adding supported rail anyway it would be better to get longer rails for the gantry to ride on and double the distance between the bearing blocks by reinforcing the legs of the gantry. That will help a lot with gantry tipping backwards problem.

Sure here is mine

http://i200.photobucket.com/albums/aa274/dj__kraid/P1120790.jpg
http://i200.photobucket.com/albums/aa274/dj__kraid/P1120789.jpg
http://i200.photobucket.com/albums/aa274/dj__kraid/P1120788.jpg
http://i200.photobucket.com/albums/aa274/dj__kraid/P1120787.jpg

D.C.
24-12-2012, 02:17 PM
Is that 1mm sheet metal all that is forming the back of the gantry?

I don't think you will get that much of an improvement using supported rail attached to that stuff, you will need to beef up the back of the gantry so that the supported rails can transfer force into gantry otherwise the supported rails will flex almost as much as the unsupported ones do at the moment.

Going to supported rail on the gantry will also mean that you will need a Z-axis mount (I think) as from the pics it looks like the bearings for your current rails are all in one block.

craigrobbo
24-12-2012, 02:28 PM
Is that 1mm sheet metal all that is forming the back of the gantry?

I don't think you will get that much of an improvement using supported rail attached to that stuff, you will need to beef up the back of the gantry so that the supported rails can transfer force into gantry otherwise the supported rails will flex almost as much as the unsupported ones do at the moment.

Going to supported rail on the gantry will also mean that you will need a Z-axis mount (I think) as from the pics it looks like the bearings for your current rails are all in one block.

I was only going to support the Y axis, The X axis is too much hassel with it being an all-in-one block.

Thats why I was thinking of the double gantry and supported Y axis

D.C.
24-12-2012, 02:39 PM
If you make a double gantry aren't you going to need a new Z-block anyway, otherwise how is the z-axis assembly going to be attached to the second axis?

martin54
24-12-2012, 03:21 PM
Think DC is right & you would be far better beefing up the almost non existent gantry. You can't change the Y axis to supported rail without affecting the Z axis as the bearings at present are in the same block as a major part of the z axis. Not sure a double gantry is the way to go & probably at least as much work as sorting this gantry properly. Don't have the experience to comment on the double gantry but guessing the idea came from the machine that was discussed on here not so long ago.

JAZZCNC
24-12-2012, 03:34 PM
DC these 3040's call the long Axis Y axis and Z axis is attached to X axis.!!

Craig the X axis block could easily be modified to accept open bearings then beef up the gantry with a strong plate and then drill and support the rails from behind.
This along with supported rails on the Y axis should help greatly and not cost a fortune.

Other than that then I wouldn't bother and sell the whole thing and build a far stronger machine from scratch.

wilfy
24-12-2012, 10:20 PM
Other than that then I wouldn't bother and sell the whole thing and build a far stronger machine from scratch.

when you factor in that my machine will be all steel built, have supported rails, the right steppers/psu as advised via this forum a cutting area of around 500x550 with a 2.2kw spindle and it's going to cost under 1k i'd say it's very likely you would certainly do well to follow deans advice.

craigrobbo
24-12-2012, 11:55 PM
Thanks for all the advice, Would adding supported rails really be sufficient enough to stop the leaning back?

Craig

Have a merry Xmas all too!

Tenson
25-12-2012, 01:34 AM
Are you sure the problem is the rails bending and not the bearings being poor tolerance?

wilfy
25-12-2012, 09:01 PM
i think it bends as the machine looks like it's made from cheese :D