Short CNC machine with wide gantry or...

[Mr.Ed]

Hi everybody...

I noticed that Balsaman made a second CNC machine wich is a bit longer with a narrower Gantry, compared to his first one, wich was a bit shorter with a much wider Gantry.

What would be the best way to aproach this?

I have aquired THK rails with a length of 640mm and 820mm. I have to make a choice wich will be the x, and wich will be the Y-axis.

Any suggestions? Much appreciated.

Ed.

[bio_eng]

Just the beginning of the thought process...

The main considerations are:
1. moving mass (lower is better)
2. vibration damping (higher is better)
3. stiffness (higher is better)

shorter gantry:
1. lower
2. ?
3. higher

longer gantry:
1. higher
2. ?
3. lower

[High Seas]

If you turn the box sideways - You might fall out! Then all of your thoughts will be ....

How about:
1. a wide Fixed Gantry, and
2. a short moving table ?
Seem to recall thats what Balsaman did on the first? Or did the final rbuild of number 1 end up with a moving gantry and not the moving table?
Or, Mr Ed, did I missinterpert the question? You were considering a Moving Gantry in your thinking/proposal - right?
Jim

[balsaman]

Wide gantry on my first machine 'cuz it was fixed with moving table. Machine needs to be twice the size of the table in this config.

Narrow gantry on moving gantry design because it's hard to make the gantry track correctly if it's real wide. (It will skew).

[Mr.Ed]

Ok, good points here.

Actually, the narrower gantry is what i have now on my first machine. The Gantry (Y-axis) "stands" on a longer x-axis wich might bend downwards if it's made too long or too thin.

I strongly believe that there is some sort of ideal meeting point in between, where lenght of the X-axis and width of the Gantry is equally balanced, with things like skew taken into account.

My biggest concern would be the skewing when machining along the edges of the CNC machine. Disadvantage with a narrow Gantry is that you can only handle narrower wooden board.

Anyway... I accuired THK HSR25 slides with a length of 640mm and 820mm, hoping that i stay within critical dimensions so that flexing and skewing would be minimal / acceptable.

Thinking about making the structure with angle iron, but a bit worried that weight might get critical here. Any hints on the skewing part?

Thanks for your replies folks, i'm a big fan of your project Eric.

Ed.

[balsaman]

A solution to the skewing issue is two lead screws. Other than that it is almost imposible to eliminate. It takes some effort to make it skew, however so if you avoid very heavy cuts it's not an issue.

Eric

[Mr.Ed]

"If you turn the box sideways - You might fall out! Then all of your thoughts will be ...." ???

Are you perhaps referring to the phrase "thinking outside the box"? Otherwise, i dont know what you're talking about. I feel a bit offended here. Tell me i'm wrong...

Anyway...Eric...A second lead screw is one thing i will consider on my second machine. Right now i do have a double lead screw with a spring in between. More or less what you mentioned.

Thanks,

Ed.

[High Seas]

Mr. Ed -
Sorry Mate - yep - outta the box! And no offence intended! I sincerely apologize if I did!

Seems yer idea of the sideways (long) Y axis - got me thinking a bit "outta the box" - and again sorry for being all too cryptic!

There are a lot of non/atraditional ideas floating around in this site. Just thinking that if I start to explore all the different ways to put this rig together - I might not ever get done! (Wait til I post some pictrures.. uggh too many choices).

But, back to my reference to turning the box sideways - one of the first and easiest steps to moving "outta -the-box - is to change the frame of reference -- and my intent was to applaud - not offend your thinking. A refreshing change of view!

But again too long a breeze (wind) here too - eh? - or to steal and twist Shakespere - '...do I protest to much?'
Jim

[Mr.Ed]

I was considering the wide Gantry, because if i place the machine on a rectangular table (placed parallel to the wall) and sit in front of it, one could access the working area much better, then when one uses the narrow Gantry. This way one of the legs of the gantry would be "in the way".

When the Gantry would be the longest, it would have to be able to carry it's own weight including the weight of the router. On the other hand, if i have the longer x-axis, it could bend down by the porce applied to it by the gantry, Y-axis and router.

Since my x-axis and y-axis are not that different in length, it doesn't really matter. I just found the first design of Balsaman "more convenient" when you sit in front of it. Downside is of course the moving table wich is much smaller compared to his second design. That was a small detail that i forgot about.

Apologies apreciated, though i was a bit "trigger happy". Sorry for that... I think this is solved now...

Maybe i should publish some of my pics just to let you guys in on my project. Then you would see that i do think inside AND outside the box.

Ed.

[yukonho]

I thought about the same idea for the router I am building now. I want to be able to cut balsa sheet, which comes either 3" or 4" X 36", so I wanted my travel to be 12" X 36". Like you mention, it is much more convenient when you can sit on the long axis of the machine to load and unload your materials more easily.
I dismissed the idea because I figured I would need two lead screws to avoid binding. I didn't want to increase the complexity too much on this, my first, machine. And, adding a belt drive or another stepper along with another leadscrew and nut would cost more too.
I ended up making my machine so that it can be placed very close to the wall. The gantry on that side is very flush fitting to the table, the stepper is on the other side, (where you sit) and the gantry moves far enough out of the way to the left that I can easily unload and load the machine.
As a note, I decided to put my stepper for the long axis on the front of the machine, so I can tuck the back end into a corner, and the machine will take up very little space. Not including the steppers sticking out, the machine's footprint is only 18.5" X 45.5" and I ended up with 13" X 37" total travel.
co

[limbo]

Originally posted by Mr.Ed When the Gantry would be the longest, it would have to be able to carry it's own weight including the weight of the router. On the other hand, if i have the longer x-axis, it could bend down by the porce applied to it by the gantry, Y-axis and router. Ed.

This is a good point. I'm running into that now with my second router. The original frame (see "second router in progress" thread) had wide Y-gantry and short X-rails. This would need two leadscrews. I have done that on my first router with two steppers. Works OK but the problem is that you are never sure its still square (missed steps or different startup conditions in the drivers). I ended up re-squaring the thing once in a while. (So if you go for two leadscrews I'd use a belt). Anyway, in my second router I changed the thing to long X and short Y to have just one leadscrew. The YZ assembly is VERY heavy so now I have a bit sagging in the middle of X, and understand why the original frame was the other way around....
But if I understand correctly you have THK rails for x and y, so in that case X can be supported over the whole length?

greetings,
John

[Klox]

Guys, sorry for barging into your interesting discussion! I know this sounds a lot "outta the box". If space is a serious problem why not swop the z axis & x axis. The table would be upright & the gantry would move up & down......
Gravity might be of help on certain aspects. I have seen a BIG mill that works this way.....

Klox