Machining deflection 2

[TED7]

Is anyone aware of an anayltical method to estimate tool induced deflection when turning a relatively thin walled cylinder on a vertical lathe? I'm aware I could use FEA or similar software to solve my problem for me but it seems a simple enough problem that there must be a first principles way to get a quick estimate. Roark's wasn't forthcoming with an answer
The cylinder will be ridgidly clamped on a rotary table at the bottom end and unconstrained at the top end where machining will take place. A bracing fixture is being considered to oppose any deflection but I'm not convinced, and neither are many others here, that the component will deflect significantly. However, I can't rely on such non-scientific guessing to say "it will be fine", and I don't fancy spending a chunk of cash on a fixture I might not really need.

 

[tygerdawg]

I'm not sure that you will get an answer for "tool-induced deflection of part during a turning operation."

If you examine & study chip-formation theory and the equations developed from that theory which are used to analyze machining processes, you will see that tool-induced forces apply to shearing off a thin layer of material and making chips. I would think that forces are primarily used to make those chips because that is where the weakest point will be: shearing off a chip along a shear plane and not deflecting the bulk material through bending. This applies to proper machining conditions for your work (speed, feed, depth of cut). Go outside those "normal" parameters for your part and, sure, I suspect that you'll deflect the heck out of it. Wear your face-protection shield and stand back when you do because it may get momentarily exciting.

At the micro level of thought, minimal cutting conditions would (I suspect) produce some deflections in the part, but those deflections would (I suspect) be less than your measuring instrument tolerance and you'd never see them.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[BrianE22]

You should be able to find some machining calculators that will give you radial force and axial force when turning. I've generally assumed both are less than about 200 lbs. maximum for the amount of material my tooling can handle.

A Google search found this:

http://www.kennametal.com/en/resour...ators/turning-calculators/cutting-forces.html

 

[ornerynorsk]

So what are we talking about for dimensions and material?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[3DDave]

I believe this is where the Swiss lathe comes in. It has an opposing feature that resists the force of the cutting tool to provide greater accuracy and avoid chatter. I may be taking liberties with this description so here's another write-up:

http://todaysmachiningworld.com/magazine/how-it-works-why-swiss/

Static deflection might not be much, but if you get a match for the natural frequency you'll make garbage out of the workpiece and maybe the cutting tool as well.

 

[TED7]

Brian - That Kennametal calculator is very helpful.
Ornerynorsk - 2m diameter, 2m length (ish) nickel-chromium-iron alloy with a 20mm or so wall thickness.

Parts used to be made on a horizontal borer and sagged into a slight oval under their own weight - ended up being machined in that deformed shape, recieving a circular cut and relaxing to an oval bore when stood upright again.
They are being cut vertically this time to stop that but it doesn't change the fact they are not really that rigid.

 

[racookpe1978]

Rigging the support is not "routine" on a vertical mill or vertical boring machine, but ANY method of providing the equal to a lathe's mid-point support that will oppose the sideways of into EITHER inside or outside of the wall of the thin rotating part will work. Thus, take a lathe's support, weld a stiff bracket to a frame that can be bolted to the vertical mill's bed, then position the support opposite the machining tool. Done. You don't need to exactly "calculate" (really, just estimate) the force sideways invoked by the tooling, just oppose that force with something stiff enough not to move.

 

[ornerynorsk]

I've used cerrocast to stabilize parts for machining before, but on a part this large it becomes impractical. Clamping at the bottom end and machining at the top end may still cause the material to present like a wet noodle to the cutting tool. Tough part, fixturing may be your best answer, I personally don't think it will machine well unsupported. An ID or OD ring could help stabilize and remove some of the tendency to deflect and chatter.

I'm curious, is the machining to be done on the ID, OD, or end face, and I'm assuming it has a weld seam?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[MikeHalloran]

I'd guess a tooltip force quite a bit greater than 200 lb to make a cut on NiCrFe.

The workpiece proportions are not a whole lot different from those of a common steel drum, but without the reinforcing ring ribs.

I submit that you may need to insert a diaphragm near the top end while machining it; I'm thinking something like the end of a whisky barrel, made from >100m thick timber, jammed in or made with some expandable features, like peripheral elements expanded by driven wedges.

You've got room to frame a big door in the diaphragm, include columns to support it while it's being tightened, and stairs down to access the faceplate inside the cylinder for securing the bottom end.



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

You could finish machine the ID or OD and then lightly press fit a a reusable stiffener to the part before machining to final wall thickness.