[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....

:wave:

Scott

[mxtras]

....The original poster can see that his deflection for a 50# load using........

Just to be clear - I used 500 pounds for the deflections. (there is a typo in your post stating 50 pounds).

Scott

[Geof]

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....Scott

My phrasing was not clear; I have attached a sketch. The tool tip distance from the center of the tube is going to be half the vertical tube dimension plus the clearance between the lowest point on the gantry and the work surface. This is probably going to be more than a few inches. I have made the picture match my figure of 100lb ft of torque. The 100 lb cutting load may be overkill but it suffices for calculation.

The tube is going to twist with this torque load and the twist will be maximum at the center of the gantry. I was looking for the twist expressed as an angle of rotation but it could also be expressed as a linear deflection at the tool tip; near enough to linear that is because the deflection will be small compared with the length of the lever arm. The orientation of the tube should not affect the amount of deflection.

[Normsthename]

Hi Colin
I built my Steel Router using Box Section (See CAD Drawing)
It has a working range of X36" x Y24" x Z5.5"
The main frame was built from 60mm x 60mm x 2mm Box Section, and the Gantry was built from 30mm x 30mm x 1.5mm Box Section.
I think the 60mm Box section was way too strong, but if i had used a smaller Box Section I think I would have ended up having to brace it in certain areas to stop it flexing.
So taking your 4" x 2" Box section, that measures 12" around the circumference, and the tube I used for my main frame measures 9.5" around the circumference so I think the 4" x 2" might be a little too strong.
I TIG welded my frame and it was a real pain! I would have done it twice as fast with a MIG welder.
Not sure if the 280 oz Motors you mentioned will be upto the job. I originally built my machine with 1nm (142 oz) motors and they were nowhere near stong enough, they kept losing steps even at slow speeds.
I had to upgrade my X & Y motors and electronics.
I fitted 640 oz motors, and now they are very strong!

Hope this helps

Andy

[mxtras]

I will hit the numbers again tomorrow. I am off the CNC zone clock now....

Scott

[yukonho]

Thanks for keeping this rolling guys.
Geof's thinking is the same as mine, I am wondering about the tube torqueing and what that would equate to in deflection at the tool tip. I imagine my machine will have 4" of clearance under the gantry. Add that to 1/2 the 4" tube gives us approximately 6" lever arm, lets add 1/2" to account for the spindle not being centred under the gantry.
.0002" sounds nice!
Thanks Andy for sharing your router and experience.
I am definetely going to balance weight with stiffness, the motors should be plenty strong for this router, I am driving them at around 70 volts, and will have 2 driving the gantry.
Colin

[Normsthename]

I am definetely going to balance weight with stiffness, the motors should be plenty strong for this router, I am driving them at around 70 volts, and will have 2 driving the gantry.

Whoops, did'nt see that you were using two motors.
Are you using two motors on the Y Axis?
One on its own could be a little borderline??
On my machine with the 1 nm motor on the Y Zxis running at 45V it was nowhere powerful enough.

Andy

[yukonho]

Hi Andy.
We will have to see, I guess. I was running this set-up on another, smaller, machine with good results. My Z axis is going to be quite light, ie. not over built but certainly somewhat heavier than on the previous router. I will be using high pitch leadscrews (ACME 1/2 - 10, 5 start) which will keep my stepper RPM's down and allow me to take advantage of the highest torque possible on the steppers.
The 5 start is actually more efficient at transfering power than the single start screws. (or so I have read, will try to locate that thread)
Colin
Colin

[Normsthename]

Just for reference, my X & Y axis use 5mm Ballscrews.....

Andy