[fretman_2]

Whew...I forsee lots of flex in that design. I agree with routalot that there's going to be lots of leverage on that gantry. You could still build your machine economically using other materials like MDF using torsion box design. That would be much more rigid than your current iteration.

[imbaine13]

Whew...I forsee lots of flex in that design. I agree with routalot that there's going to be lots of leverage on that gantry. You could still build your machine economically using other materials like MDF using torsion box design. That would be much more rigid than your current iteration.

Thanks for the recommendation to switch to MDF, fretman_2. Unfortunately, I don't access to power saws, or sanding machines to guide me with cuts. Most DIY machines I've seen other hobbyists do using MDF are usually cut by either another (sometimes bigger) CNC router, or a couple of wood working power tools, which I don't have either. If I can cut MDF straight and true using hand tools, I'd gladly consider using MDF. It's interesting that I thought the first concern most CNC'ers would have would be the linear slide design for the Z-axis with the ball bearings running a long the rim edge (because of the small contact area and lateral load on the bearing), but that doesn't seem to bother you guys that much.

Cheers
Isaac.

[fretman_2]

Well...the linear slide could be a concern, but likely less than the flex. A long moment arm is the same as using a long stick to move a big rock...takes less pressure on the other end to do the work. You might ask yourself if you really need all that clearance under the gantry. You didn't really go into what you intend to cut with your machine. I designed mine to cut out guitar necks and bodies. So I limited the Z cutting travel to about 3 or so inches to keep the forces down when I cut. I lacked the engineering skill to project those forces, but I did have an understanding that they could be higher with an excessive amount of Z travel.

Nothing particularly wrong with using skate bearings the way you have them I suppose. It looks as if you have a cross member under the machine that keeps the two bearing assemblies from spreading. You might have some issues later with using steel on aluminum...depends on how often you use it.

Thanks for the recommendation to switch to MDF, fretman_2. Unfortunately, I don't access to power saws, or sanding machines to guide me with cuts. Most DIY machines I've seen other hobbyists do using MDF are usually cut by either another (sometimes bigger) CNC router, or a couple of wood working power tools, which I don't have either. If I can cut MDF straight and true using hand tools, I'd gladly consider using MDF. It's interesting that I thought the first concern most CNC'ers would have would be the linear slide design for the Z-axis with the ball bearings running a long the rim edge (because of the small contact area and lateral load on the bearing), but that doesn't seem to bother you guys that much.

Cheers
Isaac.

[imbaine13]

Well...the linear slide could be a concern, but likely less than the flex. A long moment arm is the same as using a long stick to move a big rock...takes less pressure on the other end to do the work. You might ask yourself if you really need all that clearance under the gantry. You didn't really go into what you intend to cut with your machine. I designed mine to cut out guitar necks and bodies. So I limited the Z cutting travel to about 3 or so inches to keep the forces down when I cut. I lacked the engineering skill to project those forces, but I did have an understanding that they could be higher with an excessive amount of Z travel.

Nothing particularly wrong with using skate bearings the way you have them I suppose. It looks as if you have a cross member under the machine that keeps the two bearing assemblies from spreading. You might have some issues later with using steel on aluminum...depends on how often you use it.

Right,... thanks to you guys, I realised many things of vital importance my gantry lacked that I had completely missed!!! Only after I imagining the machine cut along each axis at a time did I notice how much flex is actually inherent in this particular design, so i came up with a major revision! I intend to use it primarily for engraving lettering in wood and plastic (mostly acrylic for signs although i do sometimes need custom cut parts for small speaker enclosures out of MDF. As i mentioned earlier, I live in Africa and the nearest CNC in 350kms away, and only good for about 150mm x 200mm x 20mm cutting volume, so that's useless! I know for sure I'll get clients for the machine, but I don't want to get ahead of myself just yet. So, in rev 2, solved most of the flexing problems with the gantry as you can see.

[imbaine13]

Rev 2 modifications:

1) Added triangulating bars on both sides of the gantry. that should take care of flexing when cutting along the X-axis alone.

2) Also added a piece of timber snugly between the each pair of the gantry uprights. That should prevent flexing when cutting along the Y axis alone and finally;

3) Added another piece of timber whose face runs parallel to that of the cutting table, underneath the table. This should resolve racking (or at least minimize it) of the gantry from side to side probably by keeping the X-axis bearings nice and parallel.

I can't think of anything else I could possibly add to the gantry to make it any more rigid that it already appears. As an extra precaution, to prevent the X axis rails from sagging under that weight of the gantry, I think I'll add some 2mm L-section steel running along the edges of the table (Not yet added in the pictures). This will serve the purpose of both stiffening the X-axis rails and keeping saw dust from jamming the X-axis linear bearings during operation.