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View Full Version : Any sugestions how to build my gantry



masinecc
02-05-2015, 11:38 PM
Here is a picture of my design. Working area 2.2m x 1.2m , everything is maiden out of steel. How do u like it for now? I ll update it every time i design sth new. Hope together with ur help i ll make a good and accurate cnc machine.
Am i on good way with this design?http://www.mycncuk.com/attachment.php?attachmentid=15247&stc=1

Boyan Silyavski
04-05-2015, 09:02 PM
Hi,
square box profile is stronger in all directions than I profile so i suggest you make all from square or rectangular profile. Usually for a normal machine i would suggest around 80x80x3mm for all structure.


Find online the tech data of 60x60x3 , 80x80x3 and 100x100x3 and look at the moment of inertia. Compare with the respective I you have chosen and you will see what i am saying. So you will chose then what you need.

Apart from that, i believe me and some people here at the forum have come to the perfect dimensions of most popular various types of table, so better read some build logs and choose what table you like, instead of reinventing the wheel. That same is valid for the gantry.

But my direct advice for your exact drawing will be, use 80x80x3mm only and move the leg profiles inside at about 22% the length of the table, counted from edge. Cause there is where they will do most perfect job.

masinecc
04-05-2015, 09:58 PM
Thanks @silyavski for your answer.
I ll change me I-profiles with squared. Here i am sending an updated picture from my cnc router, you can see that i have designed the x-axis. I use RHS profile 200x80x4mm, which i thing it has stiffnes more than enough.How about to use two squared 80x80x3mm instead the RHS 200x80x4mm? Vertical distance between My linear rails SBR25x1400mm is 150mm(center to center) which is max distance i can get using RHS200x80x4mm. Is this vertical distance from 150mm is enough?( i know that increasing this distance is better coz it reduces force due to torgue caused by the cutting force in y-axis)
Outer face distance between my SBR25UU on x-axis is 220mm, is it enoug?!( i know that increasing this distance is better coz it reduces force due to torgue caused by the cutting force in x-axis)
I havent designet my z-axis yet, after i am done with it i ll upload the pictures. After i design the whole gantry i ll find the center of gravity and i ll decide where to instal the SBR25UU on my y-axis. I know that center of gravity should be in the middle between the liinear blocks SBR25UU.http://www.mycncuk.com/attachment.php?attachmentid=15264&stc=1

Boyan Silyavski
04-05-2015, 10:30 PM
In my opinion distance between rails should be at least 250mm for the gantry. So 150 is not enough. As you say another option is to make gantry from 2x 80x80 and some perpendicular pieces in between, to look like a stair.

masinecc
04-05-2015, 10:43 PM
If i still use the RHS 200x80x4 and mount the linear rails one on the top and one on the bottom the center distance will be 260mm, so should be fine.How about the outer face distance between the linear rails blocks? Is 220mm enough?

Boyan Silyavski
05-05-2015, 07:28 AM
If X axis is what moves Z left right -yes. On long Y axis - the gantry legs min 250mm, i would say at least 300mm step is right. I see you plan to use 3 bearings there which is even better. When 3 bearings are used they have to be precisely mounted otherwise it could get worse

Tom J
20-09-2015, 04:04 PM
In my opinion distance between rails should be at least 250mm for the gantry. So 150 is not enough. As you say another option is to make gantry from 2x 80x80 and some perpendicular pieces in between, to look like a stair.

That was a question I ask myself - how far apart should get the blocks on gantry
I use calculation



http://www.mycncuk.com/attachment.php?attachmentid=16117&stc=1http://www.mycncuk.com/attachment.php?attachmentid=16118&stc=1
http://www.mycncuk.com/attachment.php?attachmentid=16119&stc=1http://www.mycncuk.com/attachment.php?attachmentid=16120&stc=1http://www.mycncuk.com/attachment.php?attachmentid=16121&stc=1http://www.mycncuk.com/attachment.php?attachmentid=16122&stc=1



There are few rules of thumb:
Clearance between table and bottom rail of gantry should not be bigger than distance between bottom and top gantry rails - X axis. Something like that D2=D1 - (D3/2)


http://www.mycncuk.com/attachment.php?attachmentid=16126&stc=1


There is compromise between forces on blocks and torques on plates and rails - that is why I use Mathcad to use those distances as variables and see what are the results.

What the point to overbuild design by using heavy strong profile if we do not consider reactions on bearings caused by own weight and cutting force.

IMHO should be compromise between calculation and try and error build. At the end we will see if chatter, vibration and deflections occurs - real life:)

Boyan Silyavski
20-09-2015, 05:55 PM
So what is the your conclusion exactly? That 6Nm is not a big deal?

I believe that all very much depends on:
-the friction of the bearing blocks, in that case excellent. carrying capacity is much bigger than to have in mind in our case.
-the exact placement of the Z and gantry so the center of gravity is so that spreads the total weight of both equally on the bearing blocks.
-at what height the ball screw moves the gantry , ideally this should be the height of the cut.

if all 3 above are perfect in the design, having in mind the capacity of the bearing blocks and that we have 4x blocks, the force of the cut itself is irrelevant and even if no separation of bearing blocks all will be ok even cutting aluminum.
but as the designs are not perfect, the forces are not so straight forward, in real life the separation will help greatly especially with reducing vibrations in the machine.

So i will still separate them to form a 300mm from end to end. At the end its no more than 50-60euro worth of rails and steel for even a big machine.

As i see it, most of the designs in forums and even commercial ones, suffer from the ball screw not being where it should be, due to this and that. And it should near the level be where the cut is. That also is valid for the X / gantry / . Thats why the L gantry design and the ball screw in the middle of rails work very good.

Tom J
20-09-2015, 06:17 PM
So what is the your conclusion exactly? That 6Nm is not a big deal?

...yes, not big deal as I did proportion wright, few cm in wrong way and is double that


I believe that all very much depends on:
-the friction of the bearing blocks, in that case excellent. carrying capacity is much bigger than to have in mind in our case.
-the exact placement of the Z and gantry so the center of gravity is so that spreads the total weight of both equally on the bearing blocks.
-at what height the ball screw moves the gantry , ideally this should be the height of the cut.

friction of the bearing blocks is negligible for calculations as you do not have reaction in direction of movement - they slide

Ball screw location is important - means nut bracket have to ideally between blocks and in center axis of travel - no additional torques. I did mine like that, much more complicated but is worth it

http://www.mycncuk.com/attachment.php?attachmentid=16127&stc=1http://www.mycncuk.com/attachment.php?attachmentid=16128&stc=1


So i will still separate them to form a 300mm from end to end. At the end its no more than 50-60euro worth of rails and steel for even a big machine.

As i see it, most of the designs in forums and even commercial ones, suffer from the ball screw not being where it should be, due to this and that. And it should near the level be where the cut is. That also is valid for the X / gantry / . Thats why the L gantry design and the ball screw in the middle of rails work very good.

Agree, firstly I was trying to gain every mm of travel, than I start sacrifice that for more important aspects like - stiffness rigidity ect.