sliding gantry or sliding Base ?

Hello Folks,

sorry if this question has been discussed before, if so, i couldnt find it :)

I am currently thinking out my Design and I am not sure how to make the X Axis.

Either the whole Gantry slides, or the Gantry is fixed and the "Baseplate" slides (lack of the right english word, sry) ?

I will probably use 600mm TBR20 supported round rails for x and y, so thats what the size will be.

Anybody has empiric experience comparing the two methods ?

I want to design the gantry it self as per the paper posted in the faqs, problems, solutions forum regarding "heavy duty design".
I mean just the structural design, not the materials used.

Built from 40x40 Alu Profiles triangular reinforced, i already drew it that way but of course I forgot to make a screenshot :)

I personally prefer the "fixed gantry and sliding baseplate" design, that way I can make a really sturdy gantry.

Another slightly off topic question, that just popped in my head, would be how to fix the TBR20 Rails to the profiles, I dont know if I like the idea of just bolting them down with T Nuts, otherwise I would have to make adapter plates, which in turn would add some of the much desired mass to the system.

THanks in advance,

Regards,

Clemens

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Originally Posted by JohnJ

Hello Folks,

sorry if this question has been discussed before, if so, i couldnt find it :)

I am currently thinking out my Design and I am not sure how to make the X Axis.

Either the whole Gantry slides, or the Gantry is fixed and the "Baseplate" slides (lack of the right english word, sry) ?

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Ok sliding Gantry will give you almost double the working X travel for a given length of rail so how much workable area do you need

Quote:

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Originally Posted by JohnJ

I will probably use 600mm TBR20 supported round rails for x and y, so thats what the size will be.

Anybody has empiric experience comparing the two methods ??

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With Moving Gantry you will get 600mm travel and 600mm working X axis minus the thickness of the gantry and the spindle offset, i can do pics if you need them, with the moving X worksurface you will get 300mm workable area/length if you want 600mm workable area then you will need 1200mm rails. and possibly a bigger area to put the machine in than you have..

Regards Rick

I am aware that the machine footprint will be almost double the length if i am not using a sliding gantry.

My question is, is the advantage in structural rigidity worth it , or are there arguments against it, except the larger machine footprint.
I am mostly interested in

Quote:

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Anybody has empiric experience comparing the two methods ?

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If i go with sliding baseplate, i obviously will use longer rails on the X axis.

*edit*
Regarding the mounting question of the supported Rails,
i probably will just use 40x80 Profile where the rails are, and just mount the rails with t nuts, as the spacing of the TBR20 is 40mm.
Any arguments against that ?
I want to avoid machining that large adapter plates.

Thanks,

Regards,
Clemens

Quote:

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Originally Posted by JohnJ

I will probably use 600mm TBR20 supported round rails for x and y, so thats what the size will be.

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This is why i replied as i did, because the size will be different......

Quote:

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Originally Posted by JohnJ

Anybody has empiric experience comparing the two methods ?

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I have owned and built both types.. is that empiric enough for you??

Rick

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 ?

Im Just a mirror my friend, i was just trying to clarify what you knew as you have few posts and as we get machinists from expert to never touched a spanner, the answer to your question is this, i decided to keep with the moving gantry design, even though there may be a requirement for larger motors, and more complex design, the effective working area and space needed sold it for me(7'X4'). But then with a design as small as yours i may have been tempted to go for the moving work area as The mass being moved would mean faster machining speeds, the chap to ask is jonathan as he has experience of both small and large machines alike although im not sure about the moving X in his case..


Rick

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.

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

Quote:

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Originally Posted by JohnJ

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

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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

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...

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

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Originally Posted by 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

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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

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

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Originally Posted by 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

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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

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 :)

Attachment 7315
I used 40x40 Aluminum Profiles, the "Spindle Base" is a 20mm Aluminum Plate.
Attachment 7317

The other plate is also 20mm Aluminum.

Attachment 7318Attachment 7319
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

Back to the fixed/moving gantry question for a mo'...

If you have something solid to bolt the machine down to I would suggest fix the gantry.

If the machine simply rests on something solid I don't think there is much advantage either way.