# Thread: Definately a scrapyard challenge

1. I wonder what would happen to lock in Jazz and Jonathan in a big brother house. Make them to make ONE cnc machine.
That would be most interesting BB ever. For a first time I would watch it.

2. Originally Posted by wiatroda
I wonder what would happen to lock in Jazz and Jonathan in a big brother house. Make them to make ONE cnc machine.
That would be most interesting BB ever. For a first time I would watch it.
Easy question to answer.!!. . . My bezzy mate and I would agree to disagree then have it built in record time both hopefully learning something new in the process:tup:

3. For me and I bet for many others both of you are great source of knowledge and experience. You just have totally different approach hence you are great contributions to this forum complementing one another.
Both of you developing one project would create perfect WHOLE :tup:

4. LMFAO. . . I've got to tell you my son's response to seeing me smiling to my self while writing that.!

" No you wouldn't, You'd test the Moment of inertia of his head with the ballscrews to see if they produced enough Kenetic energy to clear the Big Wall"

Personally I don't know why he'd think that.?? . . . . I didn't know there was Big wall:naughty:

5. Originally Posted by JAZZCNC
LMFAO. . .

" No you wouldn't, You'd test the Moment of inertia of his head with the ballscrews to see if they produced enough Kenetic energy to clear the Big Wall"

Personally I don't know why he'd think that.?? . . . . I didn't know there was Big wall:naughty:
:rofl::rofl::rofl: Did he mean Big Wall around Big Brother' house?:rofl:
So you would do empiric aspect of the job , leaving formulas and calculation for Jonathan ??:naughty:
Last edited by wiatroda; 06-01-2012 at 10:31 PM.

6. Jonathan - i am having that quote as my signature. Thanks.

As for all the spat Your both absolutely right. Without calculations we would have no idea what could be achieved. But suck it and see can be so much fun. Somewhere in the middle would be where you find success.

Now if its alright with you guys I would like to have my thread back. :wave:

I have thought about my gantry today. Repairing fences can be boring. How about 80 x 40 Al profile as the cross rail with the 10mm thick ally side fixed to cut down version of the 10mm thick angle? I will draw over the weekend.

Bruce
Value frame are cheaper than marchant and dice.

7. Just been given an old computer for the project. Checked it out....chucked it out. Boy some comps are really old.

Got the balls back in the screw but i will have to make up a guide tomorrow as they push out when you try to screw it together.
I am going to check out my supplier to see what ally box he as available and will then decide what to do about the gantry.

Bruce

8. Bruce roll up some cereal packet cardboard for now and find something to pack the middle out once inside.

9. 2e0poz you are yet another genius on this site....I have a functioning ball screw. Thanks everyone for your help. Nice to realise I am not the only curious one on this site that justhad to look inside

Bruce

10. The main problem with the 150x10mm plate is that that cross section has very poor torsional stiffness. The deflection neglecting twisting is not too bad, though it can easily be improved. Torsional stiffness is very important since the Z-axis acts as a lever applying a large moment to the gantry, so poor torsional stiffness leads to high tool deflection = chatter, poor finish, lower loot life and not being able to cut aluminium well.

The best cross section for torsional stiffness is one where as much material as possible if far from the centroid, that's clearly a tube, and since the stiffness constant is proportional to the radius to the power 4, a small increase in wall thickness or diameter has a big effect. Unfortunately it's rather difficult to attach rails to a curved surface! So the next best thing is to use box section. As a reference, the torsional stiffness of 80x80mm aluminium box section, with just 3mm wall thickness, which is what I use on my machine is almost 40 times greater than 150x10 plate. For forces parallel to X (i.e. orthogonal to the gantry) the stiffness is also significantly greater for that size box section.

If you add two pieces of 50x10mm aluminium to the plate, attaching 10mm surface to the top and bottom of the 150mm surface, the deflection parallel to X is massively reduced (to the point where it's about equal to that for the box section) however the torsional stiffness is still 16 times less than my box section. Add a thin plate on the back (3mm or whatever) and it's a whole lot better. But then you've effectively just make a box section! Adding aluminium profile could work to, but bear in mind the same rules as in this little example apply (look at the cross section of the aluminium profile).

Perhaps you could instead use the 150x10 for the Z-axis (as torsional stiffness there is irrelevant) and add the plates as above to the Z-axis as 10mm is probably not enough on its own.

Hope that helps / gives you some ideas.

Another important factor is the rigidity of the joints between the piece(s) between the gantry, so bolt it well. Doesn't matter how rigid the beam is, if the ends are not connected firmly it will bend and the joints. At the moment you want some more bolts nearer the edge of the aluminium angle, but I guess you've just not got round to putting those in yet.

(Note in the calculations above I didn't take into account the rails themselves. These will add to the strength, but I think relying on the strength of the rails to make the gantry sufficiently rigid is going about it the wrong way. You should get it rigid enough to start with.)

What's the width of the gantry between the supports?

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