ok, first attempt using sketchup 8

[Swarfing]

Looks like the KISS approach is completely being over looked here? there looks like a complete ton of needless work for what will amount to very little gain in a home built machine. Welding all that extra in will no doubt make what will not be perfect in straightness even worse. Deanos if you want to make precision NASA approved telmetry parts go for. If you want to build parts that any motoring manufacture would be pleased with don't bother. The sort of numbers we have been looking at will not make a bit of difference to you i truly believe that. By not adding all this extra rubbish you will be keeping down the weight and expense, you can always make the parts with the machine once it is running.
KISS KISS KISS KISS

[Swarfing]

Please for you

[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

[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]

Yes i'm going with the rsj as well. Its simple in its design and doesn't require any fancy tools. With its mass i believe this could just be bolted down on to a very sturdy table and save me messing about with a frame. I also think this would be easier to get square and level as well. I'm only interested in cutting wood not ali so the design suits me down to the ground. Jazz that design you have done in post 38 is excellent, looks like a million dollars but is cheap as chips:tup:. I'm going to base my design on that.

Cheers Dean

[JAZZCNC]

With its mass i believe this could just be bolted down on to a very sturdy table and save me messing about with a frame. I also think this would be easier to get square and level as well. I'm only interested in cutting wood not ali so the design suits me down to the ground.

Here's a bit of food for thought for ya.!!! . . . . Why not build 2 small concrete block walls with a plate securely bolted down onto it. Then with adjustment jack nut n bolts thru both to level the RSJ's. Then you could just push a strong bench under neath fasten down then surface it flat and away you go with virtually no material height restriction.
Obviosly you will have Z axis travel restriction but not material height.!!. . . The machine use's and flexibilty regards material sizing become far greater.!!. . . . Imagine some one wanting an old table surfacing or V carving. . . . how easy does that become.? . Ye I know not probable but you get the idea.
The same plate/jack system would work good on a bench thou.

Some time towards the end of the year I'll be building just such a machine that use's R&P and will be used for cutting granite slabs 3x2mtr into work surfaces. This will use the same principle but have a free standing concrete table with a hydrolic tilting frame for loading slabs from near vertical to horizontal.

[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.

[luke11cnc]

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