Need feedback on design.

[Mike F]

I had originally posted this at the end of my previous thread but it got only one response - thanks HuFlungDung. I really do need some feedback on this before I start making parts. The ballscrews and linear guides are ordered but I don't want to make a fundamental mistake if my thoughts are invalid.

Previous thread: www.cnczone.com/showthread.php?s=&threadid=2849 Last post.

After a little pondering and taking in suggestions from this forum, I have had a re-think and come up with the following which addresses some of the problems associated with my first idea.

As this machine is for a school workshop I want it to be as versatile as possible and not wanting to be limited by what could be placed under the machine I decided to hang the whole thing off the wall. This enables me to remove the table, if necessary, and maybe drive a car under for some decorative bonnet engraving Seriously, it does enable large items to be put under the machine if required and a vast array of sizes.

The sacrificial table top can simply be dressed with a flycutter when necessary.

Total travel in X (into the drawing) is 2000mm; travel in Y is approx 500mm; and Z 150mm, with the table adjustment being down to the floor.

Support for the Y axis cantilever is triangulated in both the X and Z direction.

Any thoughts please.

Mike

[buscht]

I think the CNC device mounted to a wall will work well. You may have some setup issues with getting the rails straight and in line. Most walls vary in and out.

I would add something to the table for positioning and locking to the wall. That way if you move the table out of the way, you can get it back in the same spot.

If your table isn't heavy enough, your router might move the part and the table since they are seperate. Locking this in will solve that issue.

It will be vital to use the machine to skim cut the top of the table for squareness.

If you are cutting parts all the way around, you should be OK. But if you want to position a precut blank against stops and only do some interior machining, you may have registration problems. Again this is caused by the table not being fastened to the CNC.

I'd say go for it!!!
T

[Mike F]

Buscht,
Thanks for that. It may well be that I will fabricate a substantial, steel subframe that will be mounted on the wall to hold the rails. That way, if for any reason it has to be dismantled, at least the rails will not be disturbed.

Point taken and well made concerning the indexing with a moveable table. Locks should not pose too much of a problem.

Thanks again, much appreciated.

Mike

[cbcnc]

Mike,

You don't show the depth of the machine in your drawing. Have you thought of the forces on your X axis trolly when the cutter is out at the end of the Y axis?
I wouldn't think it would be a problem if the trolly is long enough and the Y axis is braced firmly enough to it.
Looks like a good design. Good luck!

Chris

[High Seas]

Mike - you'll have to no doubt deal with all that cable and wire-junk too. Could use an overhead swing arm or something to keep the extra out of the way? However its done you'd want to not put any undue loads on the system.
Jim

[Mike F]

Chris,

If you're talking about the depth of machine as in the Y axis, it is intended to have 500mm of travel, giving somewhere in the region of 600m overall length of gantry. This is not too dissimilar to many of the gantries of other homebuilts found on this forum. OK, I am hanging mine off a wall, making the forces on the X rails rather different but there should be no less rigidity. The total travel on the X axis will be 2000mm and the trolley, carrying the Y axis, will be as long as possible to aid rigidity in Y.

I shall post some 3D pictures of the proposed gantry design as soon as I can for comment. One big advantage is that I can mount the Y axis motor (servo/stepper?) right next to the X rails therfore reducing the moment around the bottom X rail considerably.

[Mike F]

Jim,

THK claim their rails will take nearly ten times the proposed loads so I am hoping there will not be a problem there - should be able to sit on the Y axis and go for a ride

Not even thought about cabling yet but I can see it is an issue that needs addressing.

Thanks,

Mike

[cbcnc]

Hi Mike,

I was talking about the forces exerted in the X + - direction when you are cutting. The fact that your Y axis is attached on one end will turn it into a lever and your X axis will be the fulcrum. The longer the lever arm the higher the force.
All I meant to say was that the X axis trolly needs to be long enough in the X + - direction to resist those forces. That will transfer all that force to your linear bearings and ultimately to the attachments on the wall.

Chris

[Mike F]

I appreciate it's difficult to say without all dimensions, speeds, forces etc., but in your experience will spacing the X axis blocks 300mm apart be in the ball park?

I am almost ready to post some CAD drawing of proposed Y axis arm. Please pass judgement when I get them on.

Cheers,

Mike

[kdoney]

Could I see the motor positions and mounts? I think the distances required might be a problem.

[Mike F]

As promised, a CAD drawing of the proposed Y axis. I have not yet got round to putting in the X axis drive or the sub-frame that holds the whole thing on the wall.

You can just make out where the Y axis motor is situated. Being able to put it here takes a considerable load off the X axis rails.

Comments please.

Mike

[Mike F]

Another image.