Out of Roundness After Cut-Off Using Lathe Machine

I expect stresses in the tube are relaxing when it is cut off. Try stress relieving the metal before machining. This is usually done in an oven.

If that is not possible, try centerless grinding to take the metal to finish diameter after it is parted off. Once it is round and to correct OD, chuck it in a collet to finish the ID in a lathe.

The cut-off process is damaging the part, either by warping the part during cut-off or relieving built up internal stress in the machined part that is a result of material removal before cut-off.

Why are you doing this if you have no experience?

What does your supplier say?

Cut the part to length before final ID/OD finishing.

Edit: Or, what mike k said.

both mike k and lyn :

what you've mentioned make sense. It is possible the internal stress relieved when the part had been cut-off.

FYI, the one doing this machining work was not me, but a lathe operator. I gave the drawing to him and he complained that the part became out-of-roundness after being cut-off.

Most probably internal stresses are deforming. The most simple way is to turn with a material reserve about 2 to 3 x the ovality you obtained on the 2 surfaces and then cut. Let the tube rest for about 24 h if possible in VERTICAL position and then turn to final dimensions. The first turning operation will give the possibility to internal stresses to manifest and the final turning being relatively thin will not release important stresses even if some remain. Which is the length ?

A nice idea. To clarify :

1. we turn with a reserve material, cut-off, wait for 24-h, turn it into final diameter; OR

2. we turn with a reserve material, wait for 24-h, turn it into final dimension, and then cut-off

which one did you mean?

Turn

Cut

Wait

Turn again

The hole has to be a bit longer before you cut I would say about ID/4

The reserve has to be more than the ovality you got.

You did not give an important answer how slim is the part ? L/D ? May be you should bring a support on the end ?

I found the wall thickness of the tube before parting off to be 19.49 mm. After averaging out the 4 OD measurements and the 4 ID measurements, the wall thickness came out to be 19.59 mm. Something is not right. It seems that if it is out of round, the wall thickness shouldn't change. Are you sure of your measurements?

I was sure about the measurements though there were difference of measuring points between ID and OD. The point was when the part being cut-off, the "round" cylinder became "oval".

Exactly how are you holding the part in the lathe? What are the tolerances? Let me know and I can help you.

Sir Anrava,

Are the forces of the lathe chuck jaws flexing the rough stock prior to and during turning?

Consider a chucking plug to intercept the jaw forces. The chucking plug should (1) be rigid, (2) make a good fit inside the raw stock, (3) not extend into the parting-off zone.

Mark Bingham
Relativity PL

There could be many factors at work here but in some order of probability:

1. The work-piece springing back when released from chuck > try holding the piece in a 6 jaw chuck or with soft jaws in a 3 jaw chuck. Note that if one machines something while it is deformed it will revert to its stress free state when set free.
2. Neither the I.D. or O.D. are cylindrical from clamping pressure or machine harmonics > measure with a CMM capable of plotting roundness and providing both minimum inscribed circle and maximum circumscribed circle. This is the only way of truly knowing the part geometry. Plug and ring gauges will also provide the same conclusions but one is not likely to have that complete a set.
3. Spring-back due to pre-machining stress. This would move up on the list if is welded tubing and the discoloration is visible.

Steps for a solution:

1. Be sure of speeds, feeds, and sharp tools.
2. Carefully look at work holding and consider a soft or 6 jaw chuck. Use a steady rest and or a follow rest if the part is long.
3. Cut over size blanks and stress relieve before machining.
4. Depending on the use of the part consider "Maximum Material Condition" tolerancing. Back to the plug and ring gauges that replicate the way the part is used.

You have your homework assignment. Let us know the results.

Hi,

There are 2 factors which will impact the circularity of the part being machined.

- The Holding of the Part in the Chuck. I assume you are using a hollow Pipe or Ring as Raw material for machining and NOT a solid Rod. When you use a Hollow Pipe as a Raw material and its Held in the Chuck of the machine, some stresses are induced in the material. The removal of the job from Lathe can cause the distortion. Use of a solid material may be a possible solution.

- Second factor would be the stresses induced during parting but this seems less likely.

My suggestion would be as follows :

- Use a Pipe/ Ring as raw material for saving on material and machining time.

- Machine the ID First.

- Cut the part by Parting operation

- Now HOLD the part from its ID.....this is IMPORTANT ..and machine the OD with this as the referance. Keep the stresses minimum when you hold the part.

Hope it HELPS

Need more info on fittment of tube.........

1.Does the od shrink fit into a housing

2 Does the id shrink fit onto a shaft

Well how long are the pieces you are cutting off? Sounds like they get deformed either by the sawblade or to much pressure on the holding vice !

Consider " parting " them off on the lathe. Using the righth speed and feed as well as proper tooling will keep them round.

What saw blade? Holding vice??? What am I missing here?

I thought we were working in a lathe.

Could a close fitting mandrel be fashioned to mount in the tailstock and slide inside the part during parting off to support it both sides of the parting off location with a tapered recess at the parting off location to allow the parting off toll to cut through the finished product without touching the mandrel and thus minimize distortion in the product.

The mandrel could be mounted in a bearing in the tailstock (like a live centre) allowing it to spin with the product being supported.

I'd use a wood lathe to turn a hardwood cylinder equal to the ID and place it inside the tube before cutting. It'll absorb the heat without much expansion and prevent the cutting tool from distoring the object. Or pack it with sand. Or cut an additional cylinder of metal to fit the ID of the first and insert that before cutting.

This tube has a 10mm wall thickness, so chuck pressure should not be a real big factor. An easy thing to try is to saw or part the tubes off, then turn them. I would hold by the roundest dimension first. The OD is probably roundest, tube makers probably shoot for that.

When the tubes are parted, stresses will relax some. I don't recognize the ID, what metal is this?

I'm not sure why no one has asked you the condition of the 420 martensitic stainless steel tubing, nor its method of manufacture, so I will. Is the material annealed, process annealed, or heat treated and if so please report the hardness. Was it stress relieved? If it was stress relieved, at what temperature (have supplier show you actual temperatures from process, not the "range").

Was it quenched in water or oil? If it was in full hardened condition, it is unlikely to me that the chucking was the issue. If it was hardened, was it cooled in air or quenched in oil. If it was annealed, the chucking pressure may indeed be a factor.

Is the material seamless? Or produced by another process, in which case does the ovality orient in some way to the weld?

As for cutoff, what is the metalworking fluid used or was one used? The cutoff process if the material was hardened and if heat was not removed during operation could create distortion.

My order of factors: 1)material not stress relieved and should have been; 2)material cut off not done with metalworking fluid; 3)if material was annealed, collet pressure springbuck tension a factor;

Another thing to consider is to use of a rotary cut off saw instead of a cut off blade, and plenty of cutting fluid.the lower chip load per tooth will result in lower temperatures and less distortion and less of a cutoff burr thrown off inside the ID(which you didn't mention but I suspect is there.)

Also, you might want to confirm that the material is in fact what you specified.

Milo I apologize for the run on paragraph- macbook and safari don't play nice with this site.

I would add a question to make sure: is the "tube" made by turning a tube or a full steel round ?

And again how long is the part after cutting ?

There are a LOT of good questions if you do not bring any answer you cannot obtain a solution for your problem.

Assuming you have well fitted jaws, (pie jaws would be best) move the turning and parting off operation as far away from the chuck as possible, within the limits of achieving acceptable finishes. Stress that may be induced in the material during turning, will be distributed evenly on all sides and would likely only affect the diameter size while roundess effects would be balanced due to the even distribution. Be sure to make a finish/spring pass the last cuts on both diameters ID/OD.

sorry for the late reply. I will read and "learn" all the advices (thanks for many advices :) ). I attach a rough sketch of the process below

The out of roundness is caused by the stock removal to create the shape removing the straightening stresses imparted when the product was straightened as a bar. Your description of this as s tube was incorrect, leading all of us on a best effort wild goose chase. Milo

One possible problem you are having indicates the CNC machine needs to be leveled. You should do a ball bar test on the CNC. That will insure the levelness.

There has been so much bad advice and misinformation that I am going to say goodbye. I really do not want to participate in all of this foolishness.

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