Swiss Lathe Question regarding Big depth of cut

[Rk1989]

Hi Everyone,

I am trying to turn a part in our KSI 38S swiss lathe, which is titanium Ti-6AI-4V grade Ø12mm bar. the finishing diameter is Ø4.45mm with step OD of Ø 11.5mm, and 160mm long. I used Walter DCMT11 R0.4 insert to cut the part in single pass by supporting with sub spindle. the cutting parameters were DOC- 7.5MM, SFM 200 and Feed is 0.05. I was able to finish the part, however, the part has runout/bend around 0.30 mm after machining.

is there any way to reduce the flex and decrease the cutting load?. The current load in Z axis varying around 50 to 60 percent.

The machine does not support Pinch turning, and also I cannot go multiple passes because the guide bush length is smaller than part length. so someone kindly advice regarding tooling or process.

someone told me to use 25*25 CCMT R0.2 tool/insert after milling it down to 16*16. But does C type inserts not flex the part more than D or V type inserts?.

Does anyone have experience with similar parts?

we have production requirements for 1000 pcs, any suggestions would be very helpful.

Thanks in advance.

 

[JonFinn]

I would go C type. It just has more carbide and better support for the insert. The stock size is more critical so that you can get the guide bushing nice and tight, that way the stock cannot deflect.

 

[wmpy]

Why are you supporting with the sub-spindle? Is it possible that the sub is putting the bend into the part? Is the bar stock straight before machining?

To reduce the Z load, you might want to try a ground insert like a DCGT instead of a DCMT. This should be freer cutting than the sintered insert you're using.

I don't think that switching to a CCMT will help with the problems you are having.

 

[Rk1989]

Why are you supporting with the sub-spindle? Is it possible that the sub is putting the bend into the part? Is the bar stock straight before machining?

To reduce the Z load, you might want to try a ground insert like a DCGT instead of a DCMT. This should be freer cutting than the sintered insert you're using.

I don't think that switching to a CCMT will help with the problems you are having.

without sub spindle support the parts tends to deflect more, that is why I used the sub spindle to support the part.

the bar stock is straight before machining, let me try with DCGT and see.

Should I use bigger insert or DCGT 11 should be ok?.

 

[wmpy]

without sub spindle support the parts tends to deflect more, that is why I used the sub spindle to support the part.

the bar stock is straight before machining, let me try with DCGT and see.

Should I use bigger insert or DCGT 11 should be ok?.

As long as the insert has a cutting edge that is long enough for the depth of cut you want to take, you should be fine.

 

[guythatbrews]

It's not 100% clear what the problem is. You say bent/runout. Do you mean the axis of the part is not straight or IOW the part is banana-shaped? That's what I envision.

I don't think insert type is the problem but I would use the most free cutting c style you can. V and d will deflect more under bigger force.

Can you rough and finish step turn in segments shorter than guide bushing? If you do this and the part is still warped could be asymmetric stress in the material. Could be crooked guide bushing, collet, misalignment, or a combination. Check stock runout an inch or two away from a loose guide bushing and recheck same spot with guide bushing properly adjusted. Much variation and you are inducing the runout. You'll have to straighten things out.

I suspect you are getting better results supporting with pickoff spindle because you are actually inducing some straightening. Deflecting a bent shaft while rotating it can straighten it. You might try feeding the pickoff in x a small amount then feeding back into alignment. STRICTLY a guess here maybe 1000 rpm and .15mm/rev offset by 4mm then feed back to center .02mm/rev. Write a little program to do this and see if straightness improves. Vary parameters you may be able to dial it in. Assuming you are grabbing pretty shallow with the pickoff.

 

[Rk1989]

It's not 100% clear what the problem is. You say bent/runout. Do you mean the axis of the part is not straight or IOW the part is banana-shaped? That's what I envision.

I don't think insert type is the problem but I would use the most free cutting c style you can. V and d will deflect more under bigger force.

Can you rough and finish step turn in segments shorter than guide bushing? If you do this and the part is still warped could be asymmetric stress in the material. Could be crooked guide bushing, collet, misalignment, or a combination. Check stock runout an inch or two away from a loose guide bushing and recheck same spot with guide bushing properly adjusted. Much variation and you are inducing the runout. You'll have to straighten things out.

I suspect you are getting better results supporting with pickoff spindle because you are actually inducing some straightening. Deflecting a bent shaft while rotating it can straighten it. You might try feeding the pickoff in x a small amount then feeding back into alignment. STRICTLY a guess here maybe 1000 rpm and .15mm/rev offset by 4mm then feed back to center .02mm/rev. Write a little program to do this and see if straightness improves. Vary parameters you may be able to dial it in. Assuming you are grabbing pretty shallow with the pickoff.

Sorry for the misunderstanding, I meant to say the part is warped.

I tried segmented turning also, the part is still warped around 0.02" thou, and I could not clear the band/step marks on OD either. The tolerance is 0.001 thou only, so cannot leave any marks.

The guide bush is properly adjusted already. l will try to modify the program like you mentioned.

and I am also thinking about using 12 mm CCGT R0.2 insert to see how it reacts to the same cutting condition.

 

[guythatbrews]

You might consider straightening if you just can't get the warpage under control. 1000 pcs is a lot to straighten I know. But flogging yourself and making no headway is no fun either.

If you have a small arbor press set up 2 vee blocks with an adjustable anvil midway underneath. Adjust the anvil, deflect down to the anvil, check, and repeat until straight. Position the indicator on the top so you don't have to move it each time. Once the anvil is adjusted you should be pretty good if all the parts are warped about the same. It can be a quick process if you are lucky.

You can also use a bridgeport quill for a press since likely very low force required. Stops in the vee blocks to locate the shaft will help speed things up.

 

[dcrace]

A few things to try that usually help with deflection..

• Use a turning holder with a 0° lead-angle as a 3 or 5 ° positive angle wants to "pull" the material into the tool.
• Use a high shear ground insert to reduce cutting force Arno make a nice one with a ASF chip-breaker. NTK also makes great inserts for titanium.
• Try to use an insert with a smaller included angle if you can.. a DCGT or even a VCGT as they will give you less tool pressure also.

 

[latheman78]

You have to be careful not to heat up titanium, is the material staying cool?

 

[wmpy]

You have to be careful not to heat up titanium, is the material staying cool?

I was wondering this as well. With the high Z-axis loading and un-ground insert, maybe it's doing funny things to the material.

 

[latheman78]

The only fire I ever saw in a Swiss lathe involved titanium, the set-up operator ran away, screaming, I grabbed an extinguisher, opened the machine guard and stood there. The fire went out on it's own. Glad I did not have to pull the trigger on a dry extinguisher, they make a big mess. I am not a metallurgist so I don't know if heating up titanium makes it worse to machine. I am open to be educated.

 

[guythatbrews]

My gut feeling is asymmetrical stress in the material, not machining induced stress. I'm assuming the curve is in one plane so the machining-induced stress would have to be all on the same side of the bar more or less and that seems unlikely.

Another feeling is misalignment. You say all the chucking members are in line but have you verified this? Take a short piece of stock and put it in the guide bushing but not the collet. How much does it run out a few inches away from the guide bushing? You might have a bad guide bushing. Do the same for the collet.

Have never turn Ti only milled but seems if you were getting it hot enough to hurt you'd have a fire, which you don't want. Only fire we had was broken down insert on carbon steel. Luckily only damage was dry extinguisher stuff in the oil. Now they all have fire trace installed.

 

[EMTech]

Several others have already said this, but it sounds like it may be a material stress issue to me.

Freer cutting inserts can help not introduce more stress into the cut, but if there is stress already in the material, there is not much you can do to end up with a straight part after turning.

As far as SFM goes, that recommendation is going to vary by insert, but please keep in mind that 200 SFM at the 4.45mm diameter is over 500 SFM at the 12mm stock diameter. Swiss turning with large depth of cuts produces substantially higher SFM at the stock diameter than at the programmed cut diameter.

 

[Rk1989]

You might consider straightening if you just can't get the warpage under control. 1000 pcs is a lot to straighten I know. But flogging yourself and making no headway is no fun either.

If you have a small arbor press set up 2 vee blocks with an adjustable anvil midway underneath. Adjust the anvil, deflect down to the anvil, check, and repeat until straight. Position the indicator on the top so you don't have to move it each time. Once the anvil is adjusted you should be pretty good if all the parts are warped about the same. It can be a quick process if you are lucky.

You can also use a bridgeport quill for a press since likely very low force required. Stops in the vee blocks to locate the shaft will help speed things up.

Thanks a lot for the idea, l will look it up.

 

[Rk1989]

A few things to try that usually help with deflection..

• Use a turning holder with a 0° lead-angle as a 3 or 5 ° positive angle wants to "pull" the material into the tool.
• Use a high shear ground insert to reduce cutting force Arno make a nice one with a ASF chip-breaker. NTK also makes great inserts for titanium.
• Try to use an insert with a smaller included angle if you can.. a DCGT or even a VCGT as they will give you less tool pressure also.

Arno and NTK are not available here, l will try to get those from either ISCAR or SECO.
Thanks.

 

[Rk1989]

You have to be careful not to heat up titanium, is the material staying cool?

The coolant flow is abundant, so I believe it doesn't heat up.

 

[Rk1989]

My gut feeling is asymmetrical stress in the material, not machining induced stress. I'm assuming the curve is in one plane so the machining-induced stress would have to be all on the same side of the bar more or less and that seems unlikely.

Another feeling is misalignment. You say all the chucking members are in line but have you verified this? Take a short piece of stock and put it in the guide bushing but not the collet. How much does it run out a few inches away from the guide bushing? You might have a bad guide bushing. Do the same for the collet.

Have never turn Ti only milled but seems if you were getting it hot enough to hurt you'd have a fire, which you don't want. Only fire we had was broken down insert on carbon steel. Luckily only damage was dry extinguisher stuff in the oil. Now they all have fire trace installed.

I already checked it out, the run out is less than 0.002 thou at 8" inch away from guide bush face.

 

[Rk1989]

So thanks everyone for the assistance, I have noted down all the points.
I will post an update once I tried it.

 

[Nerdlinger]

I’ve never machined titanium before but are you certain your guide bushing is tight enough? Maybe put in a bar of 12mm plain steel and see if you get the same results? Good luck!