What causes warp during heavy machining? 1

[mlarmond]

The subject line says it all, but for those of you who prefer the question in the body of the post:

What causes warp during heavy machining?

i.e. When removing significant amounts of material from billet blocks via machining, what causes warp?

I believe that warp is caused by non uniform thermal stresses induced by the machining process, however my question is does grain direction of the materail have anything to do with it?

This is my first post so if this should be in another area - I apologize!

 

[looslib]

The warp is caused by both the machining process and the manufacturing process of the billet. When removing material, you disrupt the natural stresses of the billet. Grain direction will affect the outcome of the machined part, if enough material is removed.
One of the ways to avoid warpping parts is to cut them in two steps. First rough the cut down to close to your finished size. Then remove the piece from the olding device, which will remove other stresses on the part. You can then do a heat treat/stress relief cycle, if needed. Finally do the finish cuts to produce the part per the drawing.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[rb1957]

good reply above ...

also try to keep maching cuts symmetrical (ie on both sides on the billet). you'll also hear of people flipping the billet during the process ... primary cut side 1, flip, primary cut side 2, flip, 2nd cut side 1, ... generally manufacturing types don't like flipping the billet (takes time).

machining spped is also important ... traditionally low speed reduces heat transfer into the billet; modern ideas also go with high speed ... i guess medium speeds are the worst !!

 

[mfgenggear]

in addition to the good answers above

heat & forces from machining induce heavy stresses.
thick thin section just as in heat treat will cause the
material to pull push in diffrent directions.

to prevent it is the best medicine.
rough out, then due a thermal treatment if possible.
then semi or finish machine, of course there are to many, many varibles to mention so each job or parts have their specifics.

it takes experience to know how to handle each case.
in many cases the pressure is on to finish in one wack.
there lie some of the problems.

roughing then finishing is normally better for complicated
thin walled or web parts.

 

[arunmrao]

I assume that you are to remove sufficient material from billet stock in order to achieve the desires shape and dimensions.

Can you not resort to a primary material removal by flame cutting,power saw cutting etc to near shape and size and then load the job on the lathe for further processing?

Learn the rules,so you know how to break them properly.
Dalai Lama

_____________________________________

 

[mlarmond]

arunmrao - not for the intricate geometry required for this product.

The previous posters have sufficiently answered my question though, thanks everyone!

 

[flash3780]

Metal forming processes (rolling sheet, etc) tend to cold work material, creating plastic strain. This can be quite a good thing, as cold working increases the yield strength of material. However, the processes tend to also create residual stresses within the material.

When material is removed, often the material will want to move due to the residual strains from the forming process. This can be greatly reduced by rough machining material in an annealed condition, then finish machining after heat treatment. Be careful, though, some materials (such as 17-4SS, etc) tend to gum up when they're in an annealed state.

Often, simply machining your workpiece slowly and making symmetrical cuts will allow you to meet tolerances. It takes a bit of "doing it" to know which situation calls for what. If it were me, I'd consult a gray-bearded machinist if at all possible.

 

[Cockroach]

Residual stresses imparted by the machining process in the application. I've noticed this significantly in the production of Colets, the fingers spring open once the webbing is removed.

Not a problem, heat treat to stress relief prior to cutting the finger web manually. This solves the problem.

But residual stress at the surface is the source.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada