Why make the gantry so high?

[tltaylor]

I am in the middle of both researching 3 axis and 5 axis machines for work and designing a 3 axis machine for home. All the designs seem to raise the gantry off the rails to the height so the z-axis then travels down to meet the cutting table. Raising the gantry to gain this z travel would necesitate a heavier duty block and rail system because of the twisting of the high gantry. Why not keep the gantry low beaf up just the z axis mount, and lower the cutting table the distance of your z travel? On huge cnc machines I have seen the rail be 8' above the floor. Just thinking out load.

Trent Taylor

[CNCadmin]

You can but you will be limited to the length of tool you can use and the thickness of material you can cut. That's the reason they are so I so as not to limit yourself to thin material.

[buscht]

Trent, on commercial machines the Z axis clearance height is set based upon what 1000's of potential customers might need. If you are cutting 1/4" plywood that might not be much, but if you are modeling a full size car on a 5 axis machine, it would be quite high. They try and hit a happy medium. I suppose if you want to cut the edges of a molded spa on a 5 axis machine, you'd need about a 3 to 4' Z axis.

On your DIY machine you can set it as low as you want. But if you cut 1/4" thick parts and only make the Z 1/2" you won't have any room to get the router bit out of the chuck.

These are extreme examples, to try and illustrate my murky points.
T

[tltaylor]

I will try to post a drawing of what I am thinking about. Basically the gantry rides on rails that are 10" above the cutting table. The Z axis could still travel down the 10". You just wouldn't have the gantry rise so much higher than the rails. Thanks for the ideas.

Trent Taylor

[cbass]

Trent,

Some commercial machines use this approach. Shop bots are designed this way. Uptime.ca also makes router/mills where the rails are elevated above the table (see attached file).

Carlo

[sol]

My thinking was the same as yours, why not work below the rails...
This is a not so good image of what I did.

[tltaylor]

Sol,
That is pretty much what I was thinking. Great site showig how you did it. What size motors run everything? Feel free not to say, but how much did the project run you?

Trent

[JRoque]

That's how I have mine Please welcome... "Contraption 1"

I thought the same thing. The torsion/lever factor is affected by the total length between the Z point of contact on the material and the supporting rails. The shorter this is, the stronger it will be. I better not say much more as I really don't know what I'm talking about but this design makes a whole lot of sense to me.

Later,
JR

[Macaba]

I can see what you guys mean, it just seems logical to do this design. One could even say it feels right!

As was said, the limitation was the ease of use. If you want to use a big workpiece, you will have to slide it into the open end. So the machine would have to go along the wall of a long wall of a garage to give sufficent access.

Oh, well, would you look at that! My first post!

[ger21]

The downside of that design is that with large pieces, it can be difficult to load and unload the work into the machine. Maybe not so bad with small pieces, but especially so with large 4 x8 size. If you can load from the side, one person can manage a 4 x8 sheet, but loading a 4x8 sheet from the end by yourself is not something I'd want to do often.

[sol]

Yeah like Gerry says!
Notice that my table's end is right at the double door. Sheets slide right off the truck onto, or into, the table; it would be a really big pain otherwise... and according to my back and knees wood gets heavier every year.

[fyffe555]

CBass - do you have a url for uptime.ca ? www.uptime.ca doesn't work..

Sol - big machine! how are you driving the X (long) axis? belts? rack?

thanks..

Andrew