sliding gantry or sliding Base ?

[JohnJ]

I didn't mean to offend you Rick,
just wanted to clarify that I already know the disadvantage of the machine being larger for the same workspace, i just didnt mention it in my first post, as it seems kind of obvious to me, sorry for the misunderstand regarding machine size, i should have said i would use 600mm rails if the gantry moves.

Could you tell me what experience you have made with the two methods please, thats whats most interesting for me.
Is the advantage of building a fixed gantry big enough to justify a larger machine foot print ?

[Ricardoco]

As you have probably noticed on your travels moving Gantry machines are almost always used for machines such as plasma cutters no load at tool tip, routing machines low to mid load at tool tip. Aluminium seems to be the bridge between both types and i have certainly seen more Steel machines with a moving X rather than a moving gantry.

[JohnJ]

Thanks for your answer,
so, if space wouldn't be an issue for you, you would opt for the fixed gantry, right ?

Regarding my experiences,
i only know a few industrial Milling Machines that are built with a bridge design,
for example the Datron Machines, which are good worthy Mills, have a bridge design, but they have a moving gantry and fixed table, which is probably due to size, as size is a big sales argument for industrial machines.

You are right, all the bridge type machines I can think of, have either no load at the tool tip, or only light load.
I can just think of industrial machines, as thats where my experience is from.

For now i will complete a 3D Design of a fixed gantry type machine, lets see if I like it at the end.

Nevertheless, i would love some more people to join the discussion and share their experiences.

Regards,
Clemens

[Ricardoco]

Thanks for your answer,
so, if space wouldn't be an issue for you, you would opt for the fixed gantry, right ?

Now that you ask i suspect i would, assuming the size didnt bring its own considerations then yes i would.. I think you would gain accuracy and greater versatility of materials...

I suspect there will be some more input shortly...

Rick

[Jonathan]

With a fixed gantry you can make the gantry as strong as you want without worrying about its mass too much, since it's not moving. Clearly since the rails now need to be bigger it's likely to cost more, so you're using more space and money to get better rigidity which is nothing new. Also the motors now have to move the mass of the bed and whatever you cutting, which could add up to a considerable amount with an aluminium bed. Clearly this means you may have to spend more on the motors, although since the bearing blocks can be spaced out a lot, you should only need one ballscrew so that saves a bit compared to a moving gantry.

It will be significantly cheaper to use steel for the main frame, instead of extrusion. With a fixed gantry you can afford to get some big box section...

[JohnJ]

Thanks for your answer Jonathan,
i'd rather spend a little more money for a sturdy machine, than being dissapointed afterwards, so that wont be a problem for me to justify a little more money.

I am not sure about the steel box section, i could only bolt them together, because I dont think I can weld good enough so that it wont distort and warp in every direction :)

Whats your opinion on that ?


Regards
Clemens

[Ricardoco]

Thanks for your answer Jonathan,
i'd rather spend a little more money for a sturdy machine, than being dissapointed afterwards, so that wont be a problem for me to justify a little more money.

I am not sure about the steel box section, i could only bolt them together, because I dont think I can weld good enough so that it wont distort and warp in every direction :)

Whats your opinion on that ?


Regards
Clemens

If i could i would like to comment on the welding question.

If you look in my gallery you will see i am using box section in my latest build, Like you i was worried about warp, a friend of mine who has been welding for years (surgical instruments) helped me, i made up a Jig with angle iron and used a single tack on all joint faces and from there i built up the joints gradually on each side untill i was finished, there was no measurable warpage. as long as the two pieces to be welded are held clamped in the Jig and you are in no hurry i dont see it as a problem.. and it taught me how to get some great mig welds as well...

Rick

[D.C.]

Have you considered epoxy?

I can weld decently enough, I studied it at college and my cousin is a crazily skilled welder with 25 years experience but I'm still going to use a mix of bolts and epoxy when I build my machine because I think it is a better fit for the task. 3M's DP range of epoxies are used to hold airplane wings on and by BMW/Lotus for car chassis and a well done epoxy join will outperform a poorly done weld.

DP-420 3M EPOXY overlap shear test by Doctorbass PART 1 - YouTube

[Ricardoco]

Have you considered epoxy?

I can weld decently enough, I studied it at college and my cousin is a crazily skilled welder with 25 years experience but I'm still going to use a mix of bolts and epoxy when I build my machine because I think it is a better fit for the task. 3M's DP range of epoxies are used to hold airplane wings on and by BMW/Lotus for car chassis and a well done epoxy join will outperform a poorly done weld.

DP-420 3M EPOXY overlap shear test by Doctorbass PART 1 - YouTube

Ive no experience with epoxy being used in this manner, but we do live in modern times and welding does even seem a little primal when you think about it LOL


Rick

[JohnJ]

I am still not sure about the steel box section yet, because when making a rectangular frame out of box section, the areas of the 4 sections touching each other, are rather small, i am not sure if the epoxy would be strong enough when the contact area is so small.

Nevertheless, i got round making a first design of the gantry it self, the Z Axis is just thrown together as of now, i dont like it either :)


I used 40x40 Aluminum Profiles, the "Spindle Base" is a 20mm Aluminum Plate.


The other plate is also 20mm Aluminum.


All Components used are from zapp, Y and Z are SFU16 Ballscrews with 5mm pitch, the rails are TBR20.
The big Profiles are 120x40 both.

I had the design in mind, that was chosen as the most rigid one in the "heavy duty design" paper, regarding the orientation of profiles,
as well as the orientation of the Y axis Rails.
The Assembly, as pictured, weighs in at 38kg right now, without stepper motors, only the parts you see.

For the profiles i would use ITEM profiles, as I know them well, because we design dust covers for our products using them at work.
I would use the heaviest option of profiles, ITEM has to offer.

Further plan would be to space the gantry up, as far as necessary to reach the workpiece height i desire, by using another two 120x40 each side, reinforce it by 45 degree pieces inwards, little bit restricting the workspace, but obviously useful.

And then creating a base, either chunky 60x180 Alu Profiles, or as mentioned, steel box section,
just rectangular , reinforced by 45 degree pieces and cross beams, with two TBR20 Rails and one Ballscrew for the X Table.

What do you think ?

Regards,
Clemens