Cantilever Tube strength - Best Option Question

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I'm going to build a simple project that will use (2) lengths of tubing 18" long each on a horizontal plane (Total extension 36", but rarely extended past 30").  It will be mounted to a vertical 2" square 16ga tube approx. 30" tall. Something along the lines of this:

I want the cantilever portion to be the best combination of strength and light weight.  The maximum weight at the end of the structure will be no more than 15lbs (most likely 7lbs).I'm looking at 2" square tube, or 1-7/8" round tube - 16ga each, as the possibilities.  In your opinion, which of these three options would be the stiffest bet:*The round tube*The square tube with the flats parallel to the ground*The square tube with the flats at 45deg to the ground (diamond orientation?)The end of the tubes will be welded to vertical pieces of 1-1/4" solid steel shafting, which will be held with pillow blocks top and bottom of each shaft.Last edited by Shootr; 8 Hours Ago at 07:21 PM.Yeswelder MIG-205DS(3) Angle Grinders at the ReadyJust a hobbyist trying to improve
Reply:BeamBoy V2.2
Reply:All tubing, whether square, or round, resists torque.  Some of your thing will have to resist torque, some will have to resist simple beam stress.Square tubing is best at resisting simple beam stresses, but still has strength in the torque loading scenario.  Judging by the size of your mockup, I don't see a lot of load on that thing.  Either way, considering all the joints, and the various angles involved, I wouldn't expect a lot of rigidity.............but that depends on the load.
Reply:Run the lengths out to what you envision, mockup aside, and material selection aside, you're still looking at a loss of rigidity.  I don't believe the hinge strap orientation will have much effect on anything.At full extension, 15lbs is quite a load, looking at it again.  No problem holding that load, but I wouldn't expect it to resist any kind of movement to a great degree............if you're looking at a precision deal, or have forces acting on the load.  Forces increase loads.  On a lathe, that's a disaster.
Reply:If you're not working to thousands, which is unlikely when profiling a wood workpiece, it ought to be fine I guess.  You'll find out what deflection you have, and work accordingly.Tool overhang is dangerous if taken to extremes.  Be careful, and take shallow cuts.  Personally, I wouldn't do anything like this........but I'm an anal bastd.
Reply:I was trying to keep the project secret in case it monumentally fails, but...It is to be a plasma cutter pattern copier.  Sort of like this but a bit simpler:

If I can keep tolerances to 1/16" I'd be happy, 1/32" - ecstatic.  The torch weight is miniscule - 1lb, with the hose I'd figure 7-10lbs would provide plenty of conservative margin.Yeswelder MIG-205DS(3) Angle Grinders at the ReadyJust a hobbyist trying to improve
Reply:The tube used is "not your enemy",,The enemy is the joints,, with all the freedom of movement you are designing in,, the tolerances IN the joints will result in more than the 1/16" of movement.You could build the arms out of 3" titanium tubing,, the joints will be the issue.Look at the blue pantograph,, the reason for the height is to make the joints incredibly rigid.
Reply:Back the 1980's I had a Pantagraph torch.It used seal ball bearings. I would use wood for making master patterns or one off part. The best magic is a DC solenoid easy to come by.Dave

Originally Posted by Shootr

I'm going to build a simple project that will use (2) lengths of tubing 18" long each on a horizontal plane (Total extension 36", but rarely extended past 30").  It will be mounted to a vertical 2" square 16ga tube approx. 30" tall. Something along the lines of this:

I want the cantilever portion to be the best combination of strength and light weight.  The maximum weight at the end of the structure will be no more than 15lbs (most likely 7lbs).I'm looking at 2" square tube, or 1-7/8" round tube - 16ga each, as the possibilities.  In your opinion, which of these three options would be the stiffest bet:*The round tube*The square tube with the flats parallel to the ground*The square tube with the flats at 45deg to the ground (diamond orientation?)The end of the tubes will be welded to vertical pieces of 1-1/4" solid steel shafting, which will be held with pillow blocks top and bottom of each shaft.
Reply:If you do build the pantograph, the follower has to be motor rotated, to give precise speed control of motion.A knurled 1/8" follower comes to mind, unless you are willing to have a larger offset, then a rubber follower would be nice.If you try to use a point as a follower, you will not be much better off than freehand.I built a CNC plasma (also 0xy-fuel) and exact speed control was the secret to a great surface finish of the cut.I could see a difference in the surface of the cut with a 10% speed change.
Reply:1/32 inch at one end will translate to larger error at the other end.  Bushings would provide tight joints and still be relatively easy to build.Your tubing selection would handle the load.
Reply:I had absolutely no idea that the thing was going to be a sort of pantograph.  I still think my rigidity forecast is right, and I think SweetMK added a very important consideration as to the depth of the web.