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debequem
12-09-2002, 07:49 PM
I am trying to build a device that requires a rod about 3/8 to 1/2" diameter and is a little less than 4" long. The diameter will need to be sized to give the right amount of deflection for a given force applied at one end. Essentially, I am making a specialized bending beam which will be outfitted with strain gauges or load cells.

The rod is press fit into a base about 1.25" deep and a 7" shaft fits over the other end by about 1".

That leaves about 1.440" between the two parts.

The rod must be able to flex about 1 degree when a force of about 25 lbs is applied to the end of the 7" shaft. The base will be held stationary.

The hard part is that the rod must return to its original position and have a return accuracy of about 1 degree. That is, which ever direction force is applied, when the force is removed it must return to zero and be within 1 to 2% of the total deflection.

So far I have tried untreated steel and hardened drill rod. Both exhibit the same amount of accuracy, that is, about 5% at best.

My test setup is a dial gauge clamped to a Bison 6" chuck in my lathe. The rod is chucked tightly and a force is applied to the rod about 8.5" from the chuck. A dial gauge is placed 1.44" from the chuck end and a measurement taken.

If I apply 25 lbs in one direction and then the opposite direction I see about +/- 0.007" (with 1/2" drill rod or 1/2" steel rod - both are the same).

Both rods will return to zero with an accuracy of +/- 0.0005". The further you measure out on the rod, the larger the numbers, so I feel the rod is the source of the problem.

The weird thing is that the amount of force required to cause this inaccuracy is low. 5 to 10 lbs at the end of the rod will be all that is needed. Adding forces above 50 lbs does not exhibit a proportional change in accuracy.

I am thinking of testing spring steel, but I thought drill rod would be close. The fact that the drill rod is no better than plain steel disturbs me.

Would spring steel give me the accuracy desired or do I need to get a sample of unobtainium from an alien space craft on their next flyby?

Marv

L Webb
12-09-2002, 11:48 PM
I make formed parts out of 1044 1/4" flat bar which is then heat treated to a spring temper. It is a bear to resize the parts after H/T. Takes a whole lot of force to resize.
Les

Thrud
12-10-2002, 12:45 AM
Marv
NiTi alloy might be able to do it, but I have only seen this stuff in wires. It was developed by the USN to be able to be heat shaped to its "set" form then deformed and then electrically returned to the set shape. In the wire form it is called "Muscle Wire"

A Titanium rod might work, it is very flexible and can be stronger than steel. Ti(13V-11Cr-3Al) alloy has a 280,000 psi Tensile Strength. You might have trouble finding this stuff unless you are lucky enough to live near Boeing. Be warned that it is as difficult to machine as hardened die steel.

Copper/Berylium alloy is a spring material, and very well know for that property, but it is toxic to machine and handle.

Dick Payne
12-10-2002, 01:17 AM
Marv:
You were measuring .007" deflection with a dial indicator and said it came back to within .0005. You will need a dial indicator with .0001 resolution to measure the spring back to within 1 part in 70 which is 1.6% and you wanted to have it spring back to 1%. It is going to be tough to measure to better than .0001' without using some sort of light beam and mirror or other way of amplifying the movement.



------------------
Dick

Forrest Addy
12-10-2002, 01:51 AM
25 lb is nothing. Is it foolish to suggest a plain old spring or are you cooking up a dynomometer having greater capacity but using 25 lb as a tare load?

I don't know what you're building but it seems to me you're going at it backwards. Instead of designing something having a fixed performance to close tolerences why not design something that's close but provide adjustment to close tolerences.
All measuring devises not intrinsicly self checking (like a square) have to have a means of calibration.

Treven Baker
12-10-2002, 03:48 PM
Could'nt alot of the problem be in the chuck and the spindle?

docsteve66
12-10-2002, 03:52 PM
TO kind of ride on what forrest says:
first: I thought anything used as a "spring: returned to its original position unless (is it Hookes law) stressed beyond its elastic limit. For small amounts such as you are seeing, i would expect that that material would slowly (what ever that means) "creep" back to zero when the stress is removed (think of the spring in a automatic magazine). See if that happens.

Second: you might put a "stop" (per Forrest) to keep the device preloaded and use that as your zero, then you are adding, subtracting to the forces but never letting the force be zero. MHB has a section on pre-loading bolts and why it is imporatant to never allow the forces to reverse (to avoid fatigue failures).

INteresting problem to say the least
Steve

Treven Baker
12-10-2002, 05:15 PM
Preloading would remove the back-lash of the chuck and the spindle.

debequem
12-10-2002, 05:52 PM
The spring can be pulled in any direction. Max force is 25 lbs, but it may see more severe duty and should be rated at 100 lbs min.

The spring should deflect about 5 to 10 thousands with 25 lbs applied. this is measured 1.5" from the secured base. There is a 9" handle on it that does not bend.

I will use a strain gauge in each axis to measure X and Y deflection.

I have made measurements on long sections of rod (about 10") and apply 25 lbs at the end. I measure the deflection near the end and see a proportional amount of movement off of the null position after the force is removed.

You can try this for yourself and see the results.

I might be doing something wrong, but data from a dial gauge and a strain gauge attached to the rod both indicate that the rod does not return to zero and I don't know why.

Marv

Forrest Addy
12-10-2002, 06:57 PM
Steel in the low end of it's stress strain reange is almost perfectly elastic otherwise it would be impossible to tune a piano.

I think there is undetected movement to account for the system hysteresis you detect. I suspect a mechanical joint.

Treven Baker
12-10-2002, 09:05 PM
Yeah? like play in the spindle and chuck. Am I crazy? How come no one else has responded to this? Is'nt there some deflection of the spindle when It is loaded due to running clearance made more pronounced due to the rather long overhang not supported by the tail stock? When It is not spinning the oil would not be evenly distibuted around the bearing thus more deflection.

debequem
12-10-2002, 09:12 PM
Hmm, I think that I will change my test set up.

I'll clamp the base end of the dial gauge to one end of the rod and measure deflection at the other. The longer the distance between the two the better rsolution I should have with my measurements.

This way I should eliminate any issues with the chuck.

What do you think?

Thrud
12-11-2002, 12:27 AM
debequem:
Do your measurements on v-blocks on a granite plate. The stationary end can be clamped to the flats edge. Remember to restrain the rod in at least two places on the stationary end or the measurements will be off.

Forrest
The piano wire is in constant tension. A concert grand is tuned often two or three times a day during days with performances. This is because the wire stretches. The wood moves with moisture content in the air as well.

docsteve66
12-11-2002, 11:15 AM
debequem: for small measurements, think of your machinery, including the part you are measuring as all being made of rubber. Do your checking from place to place with the minimum amount of "Rubber" in the middle. Sounds to me as though you are now getting rid of lots of Rubber with your new test set up.

I agree with forrest- except the grand piano is not applicalbe in this case. But then I can't read forrests mind so it may be applicable. Wife has a "baby" grand, there is a LOT of force involved when you look at adding the individual string tensions. Same with a fiddle http://bbs.homeshopmachinist.net//smile.gif
Steve

debequem
12-11-2002, 06:27 PM
Yes, lots of force, indeed.

I broke the cord that holds the bridge of my violin. that act had won me numerous awards (the silencing of my attempts to play it, that is).

docsteve66
12-12-2002, 01:28 PM
I said i disagreedwith forrest using piano as example. Forrest is rigth, the example is good. He says "...Steel in the low end of it's stress strain reange is almost perfectly elastic ..". I interpeted his statement to mean with no stress to very little. I now think he meant what he said and therefore a baby grand is a good example, so long as the strings are not stressed beyond elastic limtis. BTW glass is one of the most elastic things going- its just that when the elastic limit is exceeded the stuff cracks. I was told the cracks propagate at the speed of sound- I find this hard to believe. any one know or have good links ?