Using a steady rest on lathe

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I have been told not to let my work stick out past the chuck more than 1.5 times the diameter of the work. So a 2" bar can 'safely' extend 3" past the chuck. But what is the rule when using a steady rest? If I have an 18" bar chucked up (not through the headstock), and I use my steady rest, where does the rest need to be placed? It seems using the 1.5 times the diameter past the steady is not needed. I would think that using a steady would be like having two 'bearings" on a shaft, rather than just the chuck acting as a single bearing point.

Am I on the right track? School me please.
 
Greetings, that is a good question, my-self i use the steady as close to the end as possible.
 
As with most things, it depends... But, typically, I feel that the work piece will be the most rigid when I can put the steady as close to the tail stock end as possible. The chuck holds one end, the steady the other.

Ted
 
I was taught the same 1.5 diameters but have to admit to violating it all of the time within reason depending on the operation.

Drilling on the lathe. The concern with sticking too much material out is run out causes an imbalance which could cause the material to bend and start whipping. Lots of videos out there showing what happens when too much unsupported material sticks out of the back of the head stock.

Cutting on a lathe side loads the material being cut. So it really sets you up for buckling the material and getting it to whip. Parting is naturally a lot more side load than cutting. Bump knurlers too, that's why a lot of us swear by the scissors type.

Liability aside (long time since I had the classes in school), equations are:

deflection = (force x length) / (cross sectional area x Young's modulus).
Stress in a simply supported beam = (moment x distance from the neutral axis) / moment of inertia

Young's modulus for steel is around 30,000,000 psi. Moment of inertia for a beam with a rectangular section is = ( base x (height)^3 ) / 12. Yield strength varies depending on the alloy, but 30,000 psi would be a pretty safe number for mild steel. Moment is the force applied times the distance. Distance from the neutral axis is the edge you're touching relative to the center of the part.

Cutting to the quick, stress (force per unit area) is what buckles the steel. The material acts like a spring (elastic region) with a side load until you stress is above the yield strength, then it buckles (plastic region). Double the thickness and you increase its stiffness by a factor of eight. So you are "safer" hanging a 2" round 12" outside of the chuck than you would be with a 1/4" round 12" out.

I keep work really tight to the chuck when parting, go less than 1.5 diameters hanging out. Other than that, I'll play with the depth of cut and let the chips tell me if I'm at risk. Lots of chatter, be really careful. Nice curly Q's, pretty safe though naturally with each cut you're making the diameter smaller and increasing the risk of buckling.

Bruce
 
I have a project that has caused me first to build a steady rest. I'll write that up later, I'm planning ahead now.

What I plan on doing, provided it is safe from the point of using the steady rest, it to chuck up a bar of 1.625" 17-4 H1150. the bar I have is 24" long. I was planning to place the steady at about 18". I basically making cones, profile the inside at 60* then the outside at 60*, .065" wall. I need to make about 20 of these. Basically, I was hoping to make 4-5 pieces before having to move the steady then repeat.

Does that sound reasonable, or am I pushing to hard?
 
Place the steady as close to where the work is being done as possible.

Every steady rest that I have ever used opens to allow part changes and is clamped to the ways with one screw, set it right at the chuck then slide it to the end of the stock then turn a part and slide it for every part. Once it is set you never have to adjust it again anywhere along the ways.

Be aware that if the steady is off center it will sometimes walk a part right out of the chuck, this is very unpleasant at best.

Moving it will be the fastest part of the job.

I used a cats head on this part in order to avoid leaving roller marks on the OD , you would also have to do this on hot rolled stock because it would be to rough and out of round for a steady. If possible you may also turn a band on the stock where the rollers will ride, you will often find such a band at the open end of welded hydraulic cylinders like this one. https://cdn.shopify.com/s/files/1/2134/2653/products/71unkyUVZaL._SL1500_large.jpg?v=1527546846

 
Due to the way things bend::
And comparing the bend of a part sticking out 1.5X its diameter::

a) when you hold a long part with the tailstock center the bar can be 4X as long (sticks out 6X = 1.5X*4) and the unsupported center (3X) will be bending just as much as the end of the unsupported part sticking out 1.5X its diameter.

b) when working (facing, boring) on the end of a long part, place the steady rest as close to the end as practicable. (As P. Walther above)

c) when turning a part longer than 6X and 12X its diameter, place the steady rest in the middle and use a center to support the end far from the chuck. Make a bullnose if necessary to support any work already performed on the part end near the tail stock.

d) when turning a part longer than 12X its diameter, place the steady rest no more than 6X diameters from the tailstock center and turn the part in sections (sometimes one can use a follow rest in these situations.) En-the-Large one sections up the turning so that one is working no more than 6X its diameter at any step.
 
Exactly, when turning a part with a large L/D ratio place the steady at the center and a center in the far end and turn it in 2 operations, 1/2 the length from each end to center.

This is a shaft repair that was welded by the Millwrights and I simply had to turn it to the original diameter, it was not even close to straight after welding, this did not effect its use when returned to service, it is slow moving and in a crude industry. It is also 304/316 SS, getting through the surface of the weld used about $50.00 worth of carbide insert tooling, my boss loves me (-:

 
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One thing make sure you have the shaft concentric where you set your steady or you will find the stock might walk out of your chuck jaws.
Jim
 
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