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Ross77
02-06-2009, 07:00 PM
Hi
Just planning ahead, about 6 mouths or so, but Is it a good idea to have the gantry rail (y axis) move up and down (on LM rails)to incrase the bed capacity or will it just introduce more play in the system?

I want the machine to be as versatile as possible but the main aim is ridigity. the 175mm on the z axis should be enough for most purposes, but you never know :smile:

HiltonSteve
02-06-2009, 07:11 PM
I would say its not impossible but you would have to put some thought into your design.

I used to work on an old Webster and Bennet machine for a while at British Rail (those were the days!) which had a similar configuration to what you are talking about. The only thing was it was a lot bigger and i mean a lot bigger! You could fit 2 railway bogies on the table easily.

That machine had a moving table and the gantry sides were bolted to the floor, the whole gantry moved up and down on the 2 side columns and then you had a moving vertical Z for the spindle, in fact 2 spindles, one at either end of the gantry. the spindles then travelled across the gantry as normal to give you your Y axis. Machine was rock solid but it everything was pretty heavy which worked in its favour.

Hope that makes sense and may give you some food for thought.

Ross77
02-06-2009, 07:29 PM
Cheers, did the gantry slide on dovetails?

That was the same design as I had in mind, but then I bought some THK HSR 35 rails and thought it would be a good idea to make it move as well.

Probably looking at a square braced frame for the moving gantry and let the Y axis beam move up and down within it, I am also worried about keeping it aligned.

Although this is primarily to make guitars, Im in to Rc cars and offroad buggies, so scope for larger work would be good.

BillTodd
02-06-2009, 07:31 PM
Basically it's just more sliding joints to cause a problem.

If the gantry doesn't need to move during a job then it doesn't need precision rails, it just needs to be adjustable with clamps to secure it in place.

You could have an extended Z axis (requiring long strong precision slides) or make the whole gantry as the Z axis. The mass will be similar to a Y gantry,so will require a similar motor/drive. It would require spring/air/mass to balance gravity and you'd have to find a way to stabilise the Y axis (i.e. dual lead-screws). Complex maybe, but do-able.

HiltonSteve
02-06-2009, 08:37 PM
Just a quick thought, instead of moving the gantry up to give you more room why not move the bed down??

By this i mean make a false bed which sits on top of your existing bed, with the false bed you would get say 0-150mm and without it 150-300mm. You have to remove it everytime you put a large workpiece in and it wouldn't give you the flexibilty to do the whole 0-300mm on one job.

It also would mean making your gantry sides a lot higher which would reduce the rigidity.

Personally i wouldn't bother though....!

Ross77
02-06-2009, 09:43 PM
Thanks guys, I have considered most of those ideas so its good to know im thinking along the right lines.

Kip
did you mean 1mm or 1 m? What are rod less cylinders(probably regret asking that...) are they air driven?



Basically it's just more sliding joints to cause a problem.

If the gantry doesn't need to move during a job then it doesn't need precision rails, it just needs to be adjustable with clamps to secure it in place.


this sounds the most favorable, Think I saw someone on here suggest multiple holes on the Z to gain more bed height, perhaps allow fixings on the end at 50mm increments and then move it when neccessary.


Just a quick thought, instead of moving the gantry up to give you more room why not move the bed down??

Not keen as there will definatly be a rotary table going in for 4th axis

Any one used dual X axis to extend travel?, ie. Moving gantry and moving table.

rinderpest
03-06-2009, 01:37 PM
Snap! Mine are 550mm stroke tho. Very nice things; just not yet sure what to do with them. The ones I have are made by SMC.
Shame air is too springy to allow positional accuracy.
What do you plan on using yours for?

Back to the original posters question I would go for a movable bed too.
I saw a very handy post and/or image on cnczone(maybe?) that had a side view of a moving gantry machine with lots of forces drawn on it ie given 50N force on cutter then with n length of Z axis the twisting forces on the gantry will be (x)N.
It was well worked out and made the point you wanted the centre of gravity of the Z axis to be between the two gantry bearing trucks etc.
Saw it once, was impressed, and now I actually need it to help me design a machine I cant find it anywhere.
Anyone know what Im talking about?
Jim

PS Kip, do you live on a boat? That photo looks like it was taken looking to the side of a narrow boat or a broads cruiser.

Ross77
03-06-2009, 07:51 PM
Any closer to finnishing your machine Kip? Forgotten how nice that y axis rail looks'

Given my obssesion with buying things "that may come in useful oneday", how much do you want for them. Are they capable of taking high loads in all directions?

Rinderpest
I found lots of info from here, Really good explanation of forces.
http://www.cncroutersource.com/homemade-cnc-router.html

Ah Just looked and the best pages have disapeared :evil: Sure ive got a paper copy here some where......

Ross77
03-06-2009, 08:02 PM
My connection must be crap, Found it now

http://www.cncroutersource.com/do-it-yourself-CNC-router.html

Huumm no way to direct link Url stays the same.go to builders guide, x axis frame designs


Copied another link but looks like the same address
http://www.cncroutersource.com/build-your-own-cnc-router.html

Ross77
03-06-2009, 09:34 PM
You may be able to buy them from my widow on ebay :evil:

Ok, looked for them but cant find any in the uk.
Any idea what the item no. is?