Leveling feet ?

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jcfx
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Leveling feet ?

Post by jcfx » Sat Jan 19, 2019 8:44 pm

I had a stand welded together for my new mill and not thinking it thru properly ( meaning I screwed up on the plans )
the uprights where the leveling feet are in are too close to the ground when the stand is on it's wheels, I have to cut back
the uprights to get ground clearance for the leveling feet so the stand can be rolled.
This leaves me with about 2 3/8" of the threaded stud sticking out.
Supporting 1000 lbs of static weight ( the mill and the stand ) I'm not sure if I should go up to a larger threaded stud
like a 5/8 or greater, with that amount of stick out. The stand btw is made of 2X2 3/16 wall square tube.
The leveling feet I have are 4000lb cap. ea. and the threaded stud is 1/2 -13.

Searching around I haven't found any answers, I know the shorter the stick out the better, so I was wondering
what an acceptable maximum length of stick out of the threaded stud on a leveling foot can be ?

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GlennW
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Re: Leveling feet ?

Post by GlennW » Sat Jan 19, 2019 9:26 pm

If you will have 2 3/8 of stud sticking out, why not just leave the legs a bit longer?
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

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liveaboard
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Re: Leveling feet ?

Post by liveaboard » Sun Jan 20, 2019 3:39 am

5/8" would be rock bottom minimum; for a machine like that I'd go bigger. I made 16mm [5/8"] feet for my workbench. It's a heavy bench, but featherweight compared to your mill. The rod only extends an inch or less.
As you can see from the pictures, I threaded 30mm square stock for a solid attachment to the frame.
1/2"? no, no, no.
3/4" ok
1" better
I was never sorry about making something too strong.
Many times sorry about making something not strong enough.
workbench feet.jpg
complete adjustable foot.jpg
long steel top workbench.jpg

Inspector
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Re: Leveling feet ?

Post by Inspector » Sun Jan 20, 2019 10:56 am

Can you turn a pad extension that is about 1 3/4' to 2" long and weld them to the pads you have. Not ideal but there would 3/8" to 5/8" of exposed thread.

Pete

earlgo
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Re: Leveling feet ?

Post by earlgo » Sun Jan 20, 2019 11:42 am

liveaboard has it right. When I had mechanical design interns I kept repeating, "No-one ever complains if it is too strong." (We were not in the aircraft industry for those of you prone to finding exceptions for everything.)

The minor diameter of a 1/2-13 stud is .405. so if we apply Euler's formula for the max compressive load on a 2.4" long column before failure we get:
P/A = (C*3.14^2*E)/((L/r)^2)
where P = failure load
A = area of column = (.314* .405^2)/4 = .129in^2
C = constraint coefficient = 2 for practical applications except hinged ends
E = material modulus of elasticity = 29,000,000 psi assumed for a steel stud
L = length of stud = 2.4 in
r = least radius of gyration = (I/A)^.5 = D/4 for a solid circular cross section = .405/4 = .101
therefore P = A*[(C*3.14^2*E)/((L/r)^2)] or
the critical load on the 1/2" stud 1.5" long is about 130800 lbs. compressive load only. A bending load will change this considerably.
If your leg load is 1000/4 = 250 lbs then your factor of safety is 523.

Perhaps the threads will shear before the stud fails in compressive bending.
1/2-13 threads have a pitch of .076 in and the pitch line is half way between the minor and major diameter so the thread shear dimension is .035. ie half of the pitch. Shear is calculated by P/A so the area needed to resist the 250# load is A = P/ S allow
and a conservative allowable stress for steel is 30000psi so the area needed is
A = 250/30000 = .008 in^2
The shear area of 1 thread wrapped around the stud is Circumference * shear width
Circumference = [(Dia Maj-Dia Min)/2}* 3.14 = (.5-.405)* 3.14 = .298 in
Area of shear for 1 thread is .298 *.035 = .0104 in^2
So if you have one thread on your stud you have a factor of safety of .0104/.008 or 1.3 FS

I am guessing that if you have a standard setup with 1/2 to 3/4" of thread engagement your table feet won't collapse. Moving it without bending the studs is going to be a problem for your maintenance department. :)
--earlgo
Before you do anything, you must do something else first. - Washington's principle.

Mr Ron
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Re: Leveling feet ?

Post by Mr Ron » Sun Jan 20, 2019 12:38 pm

earlgo wrote:
Sun Jan 20, 2019 11:42 am
liveaboard has it right. When I had mechanical design interns I kept repeating, "No-one ever complains if it is too strong." (We were not in the aircraft industry for those of you prone to finding exceptions for everything.)

The minor diameter of a 1/2-13 stud is .405. so if we apply Euler's formula for the max compressive load on a 2.4" long column before failure we get:
P/A = (C*3.14^2*E)/((L/r)^2)
where P = failure load
A = area of column = (.314* .405^2)/4 = .129in^2
C = constraint coefficient = 2 for practical applications except hinged ends
E = material modulus of elasticity = 29,000,000 psi assumed for a steel stud
L = length of stud = 2.4 in
r = least radius of gyration = (I/A)^.5 = D/4 for a solid circular cross section = .405/4 = .101
therefore P = A*[(C*3.14^2*E)/((L/r)^2)] or
the critical load on the 1/2" stud 1.5" long is about 130800 lbs. compressive load only. A bending load will change this considerably.
If your leg load is 1000/4 = 250 lbs then your factor of safety is 523.

Perhaps the threads will shear before the stud fails in compressive bending.
1/2-13 threads have a pitch of .076 in and the pitch line is half way between the minor and major diameter so the thread shear dimension is .035. ie half of the pitch. Shear is calculated by P/A so the area needed to resist the 250# load is A = P/ S allow
and a conservative allowable stress for steel is 30000psi so the area needed is
A = 250/30000 = .008 in^2
The shear area of 1 thread wrapped around the stud is Circumference * shear width
Circumference = [(Dia Maj-Dia Min)/2}* 3.14 = (.5-.405)* 3.14 = .298 in
Area of shear for 1 thread is .298 *.035 = .0104 in^2
So if you have one thread on your stud you have a factor of safety of .0104/.008 or 1.3 FS

I am guessing that if you have a standard setup with 1/2 to 3/4" of thread engagement your table feet won't collapse. Moving it without bending the studs is going to be a problem for your maintenance department. :)
--earlgo
You know your stuff. Thanks for the engineering explanation. As usual, I learn something new every day.
Mr.Ron from South Mississippi

earlgo
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Re: Leveling feet ?

Post by earlgo » Sun Jan 20, 2019 2:31 pm

I did similar calculations for a living in a galaxy far, far away.
To continue:
Bending stress on the stud sticking out. Stress = M*c/I
Where M = bending load x distance = P x L where P is the allowable bending load and L is the length of the stud in this case
From that: Stress = (P*L*c)/I
manipulation gives 1/P = (L*c)/(Stress*I)
Inverting: P allowable = (Stress allowable * I)/ (L* c)
Assume Stress allowable in tension Sa = 40000 #/in^2 This depends entirely on the steel used Grade 8 is tougher, hardware steel not so much
I = moment of inertia = 3.14* D^4/64 for a round section. = (3.14 * .4^4)/64 = .0013 in^4 ( D is the stud minor diameter)
L = 2.5 in
c = distance from the centroid to extreme fiber = .405/2 = .202 in
All together: P allowable = (40000* .0013)/( 2.4* .2) = 104.7 # This would be easy to bend if conditions were not perfect.

You might consider reinforcing the extended stud with an internally threaded sleeve long enough to stiffen but short enough to allow some adjustment.

--earlgo (sorry, but I got carried away in the moment)
Before you do anything, you must do something else first. - Washington's principle.

Russ Hanscom
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Re: Leveling feet ?

Post by Russ Hanscom » Sun Jan 20, 2019 2:50 pm

I used to do those calculations too, once upon a time, now I just go with a size that I know will not fail and let someone else worry about moving it!

John Hasler
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Re: Leveling feet ?

Post by John Hasler » Sun Jan 20, 2019 4:12 pm

Also note that because the stud is threaded there will be a stress concentration at the root of the topmost exposed thread.

jcfx
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Re: Leveling feet ?

Post by jcfx » Mon Jan 21, 2019 8:44 pm

HOLEY MOLEY Earlgo !
I need to digest your answer after I get some sleep from a late night project .
Thank you for the insanely detailed explanation.

Liveaboard, I agree that you can't make something too strong, I may go up in threaded stud size or really break the bank
and go acme thread ( which means buying an acme tap ).

Now that a job went out the door, I re examined what my potential stick out would be and it's less than what I initially thought it
would be, so rather than 2 3/8" it's 1 1/4" .
IMG_5688.jpg
Top nut on the stud will be the bottom of the leg, marked area is to be cut and a new plate to be welded.

John Hasler
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Re: Leveling feet ?

Post by John Hasler » Mon Jan 21, 2019 9:02 pm

Why would you use acme thread?

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liveaboard
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Re: Leveling feet ?

Post by liveaboard » Tue Jan 22, 2019 3:32 am

I'm not numerate enough to understand the calculations above, but I don't think the threads will fail.
But they will be prone to deflection; your machine will wobble.
That will be unspeakably annoying.
If you offset the mounting points, perhaps you could use the feet you have with very slight thread protrusion, and still have clearance with the castor wheels?
It looks like the interference is slight in the picture.
Or move the wheels a little.
Making my own adjustable feet was a lot of work, I made 12 of them for 2 tables and it took at least a day, maybe 2 with the nuts. and they're way down on the floor where no one will see them, too.

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