# Thread: Deflection calculation - 20mm rod

1. ## Deflection calculation - 20mm rod

I would like to ask for some help calculating the deflection of 60" long unsupported rod of 20mm hardened steel. I have used the BeamBoy as well as the formulas from SKF Linear Material documentation, and my results are unrealistic.

I am looking to see the deflection of a 20 lb force hanging from the dead middle of a 60" long piece of 20mm hardened steel clamped at both ends. My computations so far give me deflections way below 0.1mm - which would be great, but it would be too good to be true.

I am a week-end woodworker and am not very handy with metal. I don't know what to expect. Tolerances of 0.5 mm are acceptable for me, maybe even 1mm. I don't want to go to 25mm or 30mm rods unless I have to.

thanks for anyone who would venture to start helping a noob

cheers,
wm_crash, the friendly hooligan

3. Moment of inertia (I) = pi*r^4/4
Modulus of Elasticity (E) for steel = 30,000,000PSI
l = 30"
P = 20lb

I=3.14*.394in^4/4
I= .0189

Ymax=20lb*30in^3/48*30,000,000PSI*.0189

Ymax=.0198 inches or .504mm

BTW - this is for a simply supported beam, not a clamped one so the actual numbers will be less.

4. I really appreciate your help! My situation is far better than that . . . I simplified things a little. I am going to use two rods and will have a router (less than 20 lbs) travel on a fixture with linear bearings.

I am planning to build some drums and was looking for a solution to round the inside of rather long and narrow shells. I make the shells from staves, like a barrel. My plans are to have the shell rounded on the outside first. Then I place it on some bearings (well secured). I place the 2 rods through the shell and ride the router while I slowly turn the shell. With a router angled at 60 degrees, and using a 120 degree V-bit, I can (at least on paper) have a smooth finish on the inside.

Thanks again for the help!

cheers,
wm_crash, the friendly hooligan

5. No problem, nice little refresher for me

1. ###### Thread Deflection calculation - 20mm rod | CNCzone.com-The Ultimate Machinist Community | BoardReader
03-20-2012, 06:46 PM