Weight specification for 5C collets?

[opscimc]

Imagine a 1"-thick flywheel on a 7/8"-dia. shaft that is inserted the full length of a 5C collet, with face of the flywheel itself sitting almost against the nose of the collet (so the weight is at an effective length of 1/2" from the nose). Is anyone aware of a specification for the maximum weight the collet should be able to hold without what the manufacturer of the collet considers "unacceptable" distortion?

To be even more specific, imagine the flywheel is made of some super-heavy kryptonite that only has to be 1" OD to have the required weight and inertia. How heavy could it be such that the TIR at the outer edge of the face of the flywheel would be no more than 0.0005"? While you're imagining things, imagine that the lathe can start slowly enough the the inertia of the flywheel wouldn't cause the shaft to slip so that's not an issue, only the weight is.

Within the limits of drawing with ASCII, this is the geometry:

______|...........| <--- (0.0005" TIR)
______............|
............|...........| W=flywheel weight

(ignore the dots -- I had to add them because blank spaces are deleted)

 

[moonlight machine]

1" diameter? Any lathe that can hold a 5C collet will start instantly with no slippage even if your 1" diameter weighed 100 Lbs. 100Lbs. at 10" diameter would start slowly on a small lathe. It may slip as well if the collet does not have enough tension on it. I have never heard about a maximum weight capacity for a 5C collet.

 

[opscimc]

I only mentioned slipping to say ignore it. I only am asking about a maximum weight. I, too, don't remember ever seeing a maximum weight for a 5C, and I couldn't find any relevant information using google, but that doesn't mean there isn't a maximum weight. I'm hoping someone here has the information.

 

[moonlight machine]

The weight capacity of the collet will be governed by the capacity of the lathe spindle.

 

[sfriedberg]

I am too lazy to look up the proper formula in Marks Handbook, but the elasticity in a drawn-up steel collet is going to give you an insignificant deflection for any sane weight. Probably about the same stiffness as a decent set of preloaded angular spindle bearings, maybe stiffer.

 

[opscimc]

the elasticity in a drawn-up steel collet is going to give you an insignificant deflection for any sane weight.

The length of the taper on my 7/8" (0.875") 5C collet is only 0.62", and even though the 7/8" bore continues for a total of 1.1", once it's past the taper there's nothing but the 1.25" OD of the collet body to support the weight (i.e. a gap between the 7/8" shaft and the ID of the collet, followed by 0.188" walls). Given this, and depending on what you consider to be a sane weight, it's not clear to me there wouldn't be a measurable deflection. However, ideally I'm hoping someone will be aware of a manufacturer's specification for weight that I haven't been able to locate myself.

 

[APD]

Imagine a 1"-thick flywheel on a 7/8"-dia. shaft that is inserted the full length of a 5C collet, with face of the flywheel itself sitting almost against the nose of the collet (so the weight is at an effective length of 1/2" from the nose). Is anyone aware of a specification for the maximum weight the collet should be able to hold without what the manufacturer of the collet considers "unacceptable" distortion?

To be even more specific, imagine the flywheel is made of some super-heavy kryptonite that only has to be 1" OD to have the required weight and inertia. How heavy could it be such that the TIR at the outer edge of the face of the flywheel would be no more than 0.0005"? While you're imagining things, imagine that the lathe can start slowly enough the the inertia of the flywheel wouldn't cause the shaft to slip so that's not an issue, only the weight is.

Better question: What is the swing of your lathe?

 

[Cole2534]

You trying to enrich uranium in your South Bend? Haha

 

[opscimc]

Better question: What is the swing of your lathe?

I'll be happy to tell you the swing, but first I'd love to know why that's relevant to my question.

 

[triumph406]

I'll be happy to tell you the swing, but first I'd love to know why that's relevant to my question.

Well your question doesn't make any sense. There isn't a weight limit for 5C collets, but there will be for the spindle.

You have all the hallmarks of being a 'researcher' asking questions that would only make sense to a 'researcher'

 

[Cole2534]

Well your question doesn't make any sense. There isn't a weight limit for 5C collets, but there will be for the spindle.

You have all the hallmarks of being a 'researcher' asking questions that would only make sense to a 'researcher'

Yep. This line did it for me-

imagine that the lathe can start slowly enough the the inertia of the flywheel wouldn't cause the shaft to slip so that's not an issue

Just imagine, a lathe with a clutch!

 

[jaguar36]

Is anyone aware of a specification for the maximum weight the collet should be able to hold without what the manufacturer of the collet considers "unacceptable" distortion?

No, there is no maximum weight. Keep in mind that the 5C collet, or Cataract No. 5 draw back chuck as it was originally known, is over 100 years old. Every type of working holding will have some amount of "distortion". If it is acceptable or not to you depends on what you are working on and what the rest of your setup looks like.

 

[APD]

I'll be happy to tell you the swing, but first I'd love to know why that's relevant to my question.

Sure:

Let's say you have a 12" swing lathe.
A 1" thick solid Osmium disk with a 12" diameter will weigh 98 pounds.
Osmium is the heaviest naturally occurring metal.
A 5C collet and 7/8" shaft, and your lathe spindle bearings can support that.

If you're using a 5C collet in a 30" lathe to turn a 30" disk, then you're fired

 

[4GSR]

I'd wouldn't want to be around when that 30" disk comes loose and starts across the shop. Yikes!! Through the wall, down the street it goes!

 

[Nmbmxer]

I like a 3:1 rule on stick out, for chucking work with a collet and no tail support I wouldn't turn something larger than 6" using a 1" collet unless it was very slow with sharp tools. I think you are going to run out part of rigidity well before any weight rating on the spindle. I think cutting forces are also a lot higher than typical part weights. If that weren't true then you'd rarely need tail support.

 

[Scottl]

I don't think it's going to be much of a problem with high quality collets running in a precise spindle. There are straight shank mounted small lathe chucks sold for use in collets and they certainly are not light.

 

[triumph406]

There could be an issue with bending of the shaft projecting out of the collet. A heavy overhanging load, and an inadequate shaft diameter could lead to fatigue and failure of the shaft.

 

[opscimc]

You have all the hallmarks of being a 'researcher' asking questions that would only make sense to a 'researcher'

I suppose that is meant as an insult, although I'll take it as a compliment. However, I've gotten as much useful input from this thread as I'm likely to get so thanks to everyone who responded.

 

[Scottl]

There could be an issue with bending of the shaft projecting out of the collet. A heavy overhanging load, and an inadequate shaft diameter could lead to fatigue and failure of the shaft.

In his first post the OP said there is only one inch projection of the heavy part, which is one inch in diameter.

"Imagine a 1"-thick flywheel on a 7/8"-dia. shaft that is inserted the full length of a 5C collet, with face of the flywheel itself sitting almost against the nose of the collet (so the weight is at an effective length of 1/2" from the nose)."

My guess is that the weight is one inch diameter tungsten welded/brazed to a 7/8 shaft of lighter material. I have in the past had fabricated parts of tungsten brazed to copper for plasma electrodes so I am familiar with the concept.

 

[EPAIII]

This is easier to visualize with a drawing so I made one. The collet is in green and the flywheel/shaft in red. The forces are in blue.



The first thing is, this IS to scale. 1" x 1" flywheel on a 7/8" shaft. It also accurately shows just how a 5C collet would grip this part. That contact between the collet and the shaft is only 0.75" long. The rest of the shaft, all the way to the rear of the collet, as specified by the OP, is not in contact with the 1" bore at the rear of the collet. It is just hanging in mid air. The force of gravity is only shown as acting at the center of the flywheel section of the part, but it would also act on the rear of the shaft which is hanging in mid air. But the OP only said that the flywheel was super heavy, not the shaft so perhaps this should be left out of the consideration.

The OP wanted to know about the TIR at the "face" of the flywheel but did not specify where on that face. From the drawing it is clear that either gravity or the force of cutting would produce deflections in that face in opposite directions at it's top and bottom. But, if we are machining it, we would be primarily interested in the deflection at the height of the lathe's center line so I drew the TIR arrows there. This is also close to the point which would change the least due to these forces.

I drew the two most likely arms of rotation at the top of the drawing because I suspect that any forces due to cutting the flywheel will be a lot greater than those due to gravity for any practical material. So when considering the effect of gravity you should imagine they were in a mirror reverse position at the bottom. The These two arms are at angles of 19.4 and 23.7 degrees to the horizontal axis. And while the ends of those two arms would move approximately on circular arcs, the TIR would only consist of the horizontal component of that motion. So the TIR would be the total motion multiplied by the sine of those two angles (sine(19.4) = 0.33 and sine(23.7) = 0.40). This implies that the ends of those two arms would need to move almost twice the 0.0005" that the OP specified.

That's the geometry but it says nothing about the amount of rotation that would result from either gravity or the forces generated while cutting. It does show that those two forces, both of which would definitely be present, ACT IN OPPOSITE DIRECTIONS. Therefore they would tend to cancel out. With no actual data, I strongly suspect that the cutting force will be a lot greater than that due to gravity for any real material. I also strongly suspect that the 5C collet and the geometry that holds it are not the weakest link in this situation. I suspect that the spindle bearings will give a lot more than anything else. Other factors may be things like the draw bar and how much is will stretch when loaded in this manner.

And things may vary from one lathe to another. So actual measurements on real lathes would be needed to settle the question for each one individually.