ok, first attempt using sketchup 8

[luke11cnc]

I did write a reply but I think the mod's must have it ??
James

[JAZZCNC]

I did write a reply but I think the mod's must have it ??
James

So what did it say.?????

[deannos]

Ok, still struggling a little with sketchup. This is my latest attempt and i have put the ballscrew on top of the i-beam. Would this work, i'm struggling with ideas on this.




Ignore this

[deannos]

Ok, i think i'm finally getting there. All plate is 20mm thick ali and also using I-beams. I'm not sure if the screw mounts are ok doing it this way. Would the bearing blocks need spreading out more. All the rails are 20mm with 1610 screws. Some pictures to show my progress and please tell me where i have gone wrong and where improvement can be made

I will have the ballscrew end machining increased from 15 to 30mm as i want to use belts and pulleys.
The RSJ's are 178x102x19mm, would it be better to use a smaller size on the gantry, 127x76x13mm.
Also just realised i haven't taken into account the thickness of the beams

[deannos]

Ok have redone the RSJ's and added steppers.

[Jonathan]

The RSJ's are 178x102x19mm, would it be better to use a smaller size on the gantry, 127x76x13mm.

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Although it's convenient to use RSJ on X and Y, you'd be better off using a different cross section. The torsional stiffness of RSJ is pathetic, hardly much better than a sheet, so especially on the gantry where the cutting forces will tend to twist the beam you would get a lot of deflection. Similarly RSJ is poor under shear stress, which is the case on the X-axis. To an extent you have compensated for that by using a large thickness, but with less material you could use box section and get much greater stiffness.
One reason not to use smaller RSJ is it reduces the vertical spacing of the Y-axis bearings, which is something you want to maximise.

What you've drawn is still fairly strong, so it would work - but that is purely because of the large thickness.

Try folding an RSJ cross section and a box section out of card then try and twist them and you will see how big the difference is. This spreadsheet lets you compare them.

[deannos]

Although it's convenient to use RSJ on X and Y, you'd be better off using a different cross section. The torsional stiffness of RSJ is pathetic, hardly much better than a sheet, so especially on the gantry where the cutting forces will tend to twist the beam you would get a lot of deflection. Similarly RSJ is poor under shear stress, which is the case on the X-axis. To an extent you have compensated for that by using a large thickness, but with less material you could use box section and get much greater stiffness.

Ok, would i be better using Ali extrusion, or are you talking about steel box section. What size would you suggest

One reason not to use smaller RSJ is it reduces the vertical spacing of the Y-axis bearings, which is something you want to maximise.

I'm still confused on the axis direction, by "Y" are you referring to the bearings the longer gantry is riding on

I want to get it right the first time, or as best as can be


How do you use the multi quote option, i clicked it and nothing happens

[Jonathan]

Ok, would i be better using Ali extrusion, or are you talking about steel box section. What size would you suggest

I was thinking steel box section, but aluminium extrusion is pretty good too. If you're happy to do some welding then steel box section is the cheapest and strongest option for making a frame. The standard length is 7.5m, so it would be prudent to design the frame to use an integer multiple of 7.5m! I used 60x60mm, 3mm wall thickness box section for most of the frame and 100x60mm, 3.6mm wall thickness for the two lengths the X-axis rails mount on.

How long is the piece of RSJ you've drawn for the gantry?

I'm still confused on the axis direction, by "Y" are you referring to the bearings the longer gantry is riding on

I always call the axes on the gantry Y and Z and the other axis X. So by that definition in your case X is shorter than Y.

How do you use the multi quote option, i clicked it and nothing happens

The what.. never seen that! I just copy and paste the [QUOTE] tags in the relevant locations. I've put them in on your post for you, so if you click 'Edit Post' you can see how...

[deannos]

[/COLOR]

I was thinking steel box section, but aluminium extrusion is pretty good too. If you're happy to do some welding then steel box section is the cheapest and strongest option for making a frame. The standard length is 7.5m, so it would be prudent to design the frame to use an integer multiple of 7.5m! I used 60x60mm, 3mm wall thickness box section for most of the frame and 100x60mm, 3.6mm wall thickness for the two lengths the X-axis rails mount on.

I can weld , it wouldn't win any contests though, i would like to keep the RSJ for the X axis

How long is the piece of RSJ you've drawn for the gantry?

It measures out at 1715mm, but could be slightly longer.

[deannos]

Instead of using a rsj for the gantry, could i use 100mm x 60mm x 3mm steel box section. I can get my hands on a piece long enough for a tenner, and its very local.