Stell frame cnc 1500X1000. Adjustable table your opinion please

[JAZZCNC]

Something else to bear in mind is that increasing the size of the steel you use can sometimes make the structure strong enough, with a lot less work than adding lots of small supporting pieces and without necessarily increasing the cost. For example, if you have a piece of 60mm box section with 3mm wall thickness supported at both ends, then in general increasing it to a 80mm beam of the same wall thickness will reduce the deflection to less than 2.4 times the original.

Also, it's better to increase the size of the beam (within reason), than to increase the wall thickness. Continuing with the previous example, the 80mm beam will cost about 33% (assuming it's priced by mass) more than the 60mm beam, so suppose you instead invested that 33% extra in getting a 60mm box section with 3mm thickness to match the weight of the 80mm beam. Both sections will cost about the same, however the 60mm beam is still only just over half as strong as the 80mm, as you've only made it about 33% stronger (not 240% as above). The reason for this is that increasing the wall thickness only gets a linear gain (y=k*x) in strength, compared to a quartic (y=k*x^4) relationship from increasing the size. So in general, you only use a greater wall thickness when you haven't got space to fit a bigger section.

Well ounce again I'm going to have to disagree.!! . . . Not on the strength basis has that's true but on the fact it's better to use larger.? It's only better if the application warrants it and in this case it doesn't. The thicker wall thickness will be more beneficial because it will be less resonant, going Larger on the tube size just increases resonance. For this application then 60mm tube will be more than strong enough so strength isn't a problem so any extra efforts to improve would be better focused on reducing resonance and going thicker on the wall would help here.

With regards to resonance, MDF is actually a great material to use for the machine bed as it has such good damping properties. I had a lot less problems with resonance on my machine with the MDF bed than I do now with aluminium. If I'm machining an aluminium part for which the finish is more important than the accuracy, I'll sometimes put a piece of 18mm MDF between the part and the aluminium bed as the MDF damps the vibrations, making the 'sweet spot' to get a good finish a bigger spot.

Oh dear again I'm disagreeing. . Lol . . I don't have any problems with my aluminium bed and differing finish.? I get the same finish regardless of whether I'm working direct on bed or on any other sacrificial surface. If your bed is affecting finish then reckon you've under built it or your clamping methods are poor.!!
If anything I get more trouble when using MDF than I do when working direct on bed because material can slide if cutting deep, direct on the bed I get much better hold down.
Also Poor finish comes mostly from the tool chattering so if your struggling with finish then I'd be looking at the machine strength and resonance, hence why I put more importance on material Thickness than Size when it comes to steel. (obviously size has to be fit for application)

BA 99297 I'll say it again to be clear "Any of the designs will be strong enough" so if any more focus is needed then I'd direct it to reducing resonance rather than strength.

Oh when I said about unsupported edges I meant you don't have any members to support the Bed base material at the edges. On your designs the edges between table supports won't support the material or any sacrificial material. It may seem a small detail but it can make a difference and for the little extra material/work it's not worth not doing IMO.!

[Jonathan]

Well ounce again I'm going to have to disagree.!! . . . Not on the strength basis has that's true but on the fact it's better to use larger.? It's only better if the application warrants it and in this case it doesn't. The thicker wall thickness will be more beneficial because it will be less resonant, going Larger on the tube size just increases resonance. For this application then 60mm tube will be more than strong enough so strength isn't a problem so any extra efforts to improve would be better focused on reducing resonance and going thicker on the wall would help here.

I don't disagree with you. If you read what I said carefully you'll see that I acknowledged other factors come into it, and was making general comments, not specific assertions for this build.

Oh dear again I'm disagreeing. . Lol . . I don't have any problems with my aluminium bed and differing finish.? I get the same finish regardless of whether I'm working direct on bed or on any other sacrificial surface. If your bed is affecting finish then reckon you've under built it or your clamping methods are poor.!!
If anything I get more trouble when using MDF than I do when working direct on bed because material can slide if cutting deep, direct on the bed I get much better hold down.
Also Poor finish comes mostly from the tool chattering so if your struggling with finish then I'd be looking at the machine strength and resonance, hence why I put more importance on material Thickness than Size when it comes to steel. (obviously size has to be fit for application)

I agree that if my gantry was stronger it would help reduce chatter, but it's the same gantry with or without the MDF bed and the MDF sheet placed on top of the aluminium bed on my machine definitely makes it easier to get a good finish, so it must be having a damping effect. I'll reiterate for clarity, by 'good finish' I mean 'looks good' - the part itself is clearly not going to be as accurate if it's clamped on MDF.

Also, that's not to say the machine wont get a good finish using just the aluminium bed - it's just more difficult to get the right parameters. Here's a recent example:

[ba99297]

BA 99297 I'll say it again to be clear "Any of the designs will be strong enough" so if any more focus is needed then I'd direct it to reducing resonance rather than strength.

Dean or anybody else could you please suggest me a link ( or links) where i can read about resonance dampening basics ( i mean what i should avoid in order to reduce resonance )
So far ( tell me if i am wrong ) i realize that low mass and symmetry amplify resonance is that correct?

Oh when I said about unsupported edges I meant you don't have any members to support the Bed base material at the edges. On your designs the edges between table supports won't support the material or any sacrificial material. It may seem a small detail but it can make a difference and for the little extra material/work it's not worth not doing IMO.!

Now i understand what you mean but i didn’t say that vertically to the table beams i will put t slot aluminium profile ( a friend of mine who makes solar parks on the houses roofs has many meters of them)

[ba99297]

Hi everybody
These days i have time to go on with my cnc design
For 3d design i use solid works
After the table i will go on with the gantry etc
I would like to know if there is any online library with ready to use elements like bf/bk bearings. profiled rails, ballscrews ballnuts that can be imported in solidwork
Thanks