What guage and size steel to use?

[yukonho]

Hi guys, it has been a while since I posted, but I sold the old machine and am about to embark on a steel machine.
This machine will have a 4 foot gantry (x) and 3 foot Y. It will be driven by leadscrews on each side by 280 oz in steppers driven on 68 volts through geckos.
I am thinking that 2" X 4" tube will do the trick for the gantry, but what guage do I use? Or perhaps should I go to a larger size? I am not familiar with the standard sizes.
Any suggestions?
Also looking for suggestions for the endplates for the gantry that will go down to the linear slides beneath the table.
No drawings yet, you get to help design this one with me!
Thanks
Colin

[mxtras]

2X4X.125" is easy to get, relatively easy to cut and weld and it's dimensions make it a convenient choice. 1/8" wall may not be substantial enough to hold a thread larger than #10, though. I have tapped 1/4-20 into 1/8" wall for holding a pneumatic regulator, etc and it is fine for that kind of application, but it may be too thin for attaching something like a motor mount or a linear bearing.

Scott

[yukonho]

Thanks for that. Will that be stout enough for my 4+ foot wide gantry? My Z axis will be fairly light, but I will be using an 11 amp router (heavier than the roto zips or trim routers)
I would plan on using nuts and access holes to bolt anything on. I plan on welding the gantry endplates on.
Colin

[mxtras]

What are you planning on cutting?

2x4x.125 is going to deflect if you load it heavily (anything deflects - it's a matter of how much your process can tolerate). It also depends on how the tubing will be applied - if there are large cantilevered forces, the tubing might not be stout enough to span 4' and perform as you might want and you might look at making some alterations to the design, or include additional members to stiffen the beam - too many variables to say right now.

Scott

[yukonho]

Cuts will be MDF and plywood with 1/2" bit at the most. I want a very rigid machine, and I expect to use feeds around 60 ipm with rapids in the mid 200's. Gantry will be supported and driven at each end, thus the worst possible place for deflection will be in the middle of the gantry.
What is the next larger size rectangular tubing?
Colin

[mxtras]

Going with larger tubing may not be the answer. Larger tubing is going be heavier and you will have to accelerate the added mass.

You might want to investigate making a stiff structure for your gantry - maybe even using smaller tubing, but using more than just one beam. Something like three arranged in a triangular manner, tied together with spars could be made lightweight and stiff. I don't know if this makes sense... There are lots of ways to get a stiff structure without adding a lot of mass.

Tubing is available in pretty much every inch dimension - 2X2, 2X3, 3X3, 3X4, 4X4, 4X6, 2X6 etc. Common wall thicknesses are 1/8", 1/4", 3/16" and many sizes of tubing are available with gauge thickness walls - check google for 'steel tubing' and you will find online suppliers and their stock lists, and quite possibly pricing, but you will likely get a better price from a local supplier.

Scott

[yukonho]

Thanks for the info.
Being driven by two steppers, I am less worried about the added mass, however I do agree that overkill in mass is not the way to go. I am not keen on using more than one tube for the gantry, too many alignment issues and less simple to build.
I will simply have to get my hands on some steel and make the decision then on size.
Again, thank you for your help.
Colin

[Geof]

To give you some idea about weights 4" x 6" x 1/4" wall tube is around 17lbs per foot. My gues is that this would be plenty stiff enough, probably more than stiff enough so you could maybe go down to 3/16 wall which is around 13lbs per foot for a total weight of just over 50lbs just for the tube.

[mxtras]

Ok. Here are some available sizes and weights per foot of tubing. I also included a quick review of the calculated deflection of each using 500 pound load in the center of the tube with the ends simply supported. I used the tubing in the most favorable condition for the deflection calculation.

4X2 tubing:
.120 wall = 4.7 lbs per foot .027" deflection
.135 wall = 5.2 lbs per foot .025" deflection
.148 wall = 5.7 lbs per foot .023" deflection
.188 wall = 6.9 lbs per foot .018" deflection
.250 wall = 8.8 lbs per foot .015" deflection

4X6 tubing:
.188 wall = 12.0 lbs per foot .0043" deflection
.250 wall = 15.6 lbs per foot .0034" deflection
.313 wall = 19.0 lbs per foot .0028" deflection
.375 wall = 22.3 lbs per foot .0024" deflection


Scott

[jcc3inc]

Mxtras,

Thanks for the good listing of deflection vs load! It's good to see some technical info here instead of "I think that will be OK" approach.

The original poster can see that his deflection for a 50# load using the 2 x 4 x .120 wall will be .0027 inches!! He will have a heck of a lot more deflection/lost motion in the rest of his machine.

The engineering approach is very welcome.

Regards,
Jack C.

[Geof]

....The engineering approach is very welcome.

Regards,
Jack C.

Yes but the old fogies guess was pretty close.

mxtras how about a torsional load of about 100 lb ft applied at the center, the cutter forces do not act directly on the tube. I guess this will prove to be okay also.

[mxtras]

Yes but the old fogies guess was pretty close.

mxtras how about a torsional load of about 100 lb ft applied at the center, the cutter forces do not act directly on the tube. I guess this will prove to be okay also.

At about what distance would the forces be acting? Are you asking for 100 pounds of torsion about the center of the tube? I would guess that the torsional forces would be produced by the cutting forces at the tool which might be a few inches from the center of the tube.

With a 4"X2"X.120 tube standing vertically (4" in the Z axis) and a 100 pound force acting about 4" from the center of this simply supported tube, it would deflect about .0002". If the tube was layed on it's side and the same conditions apply, it would deflect about .0006". A 4"X4"X.120" tube would deflect .0001" - theoretically, of course.

So Geoff's guess is proven to be OK, I guess. It's luck, I am sure....



Scott