What's new
What's new

How does stress relief affect part measuring stability (aluminum 6061)

MULTUSB200

Plastic
Joined
Feb 13, 2016
Hello all,

I am looking for material regarding stress relief and it's affect on dimensional stability.
I am currently making a very precise part from round stock on a MTM Okuma Multus machine.
There is a lot of material that is being removed from the part and I am curious as to how this can
affect the part:
1. how is the part expected to hold it's measurements?
2. if the part is subsequent to move - at which phase will it stabilize?
3. will the part ever stabilize (given enough time) without thermal treatment?
4. is a stress relief process in a 170 degreee oven for 4 hours equal to letting the part sit
around in the shop before final machining for X amount of time (what is X)?

These are just example questions I think of.
If anyone knows where I can find an article/paper about this subject?

Hope I was clear!

Thank you.
Vyacheslav.
 
I believe it was 7075 but we use to vibratory stress release round bar stock prior to machining parts had a small bore on one end about a .625 ish dia the other end had about a .900 ish. And just under 1 inch od. Around 3 inch long. (Don't remember specifics) with a window cut out of the middle. The 2 bores were extremely tight tolerance. Like +- .0002 and cylindrical call out equally as tight. with a finish call out of like a 3 to 5. They were hard anodized after machine and honed after for finish and size again. So yes there are ways to stress relief for low cost and still have high accuracy.
 
For precision parts out of 6061 I use cryogenic stabilization, usually after after rough machining and before final. Parts are immersed in liquid nitrogen and left to reach the temperature - when all boiling activity stops. Sometimes I repeat the process after reaching room temperate.
 
Hello all,

I am looking for material regarding stress relief and it's affect on dimensional stability.
I am currently making a very precise part from round stock on a MTM Okuma Multus machine.
There is a lot of material that is being removed from the part and I am curious as to how this can
affect the part:
1. how is the part expected to hold it's measurements?
2. if the part is subsequent to move - at which phase will it stabilize?
3. will the part ever stabilize (given enough time) without thermal treatment?
4. is a stress relief process in a 170 degreee oven for 4 hours equal to letting the part sit
around in the shop before final machining for X amount of time (what is X)?

These are just example questions I think of.
If anyone knows where I can find an article/paper about this subject?

Hope I was clear!

Thank you.
Vyacheslav.

This really depends on many factors.
"Tight Tolerance" can mean different things to different folks. Please be specific - are we talking a few microns or what? If microns, then yes, stress relief is very critical to success.

1. The part should be at measurement at a stabilized 20°C - how long it takes to determine stabilization depends on section thicknesses. I do not know of any specification regarding how long after machining the part must remain at dimension. (And it really depends on what "tight tolerance" is......)

2. The part will move with *every* machining and clamping operation, how much depends a lot on shape, tool, cutting parameters, clamping system and placement, etc. Every time you remove material you change the shape of the part and the internal / external stresses. i.e. a dull tool can put stress into a part, while just removing material can change internal stresses.

3. Yes, it will. In the natural world all things come to equalization. When all of the stresses have equalized it will quit moving - given a stable temperature. How long that takes depends on a lot of factors.

4. Stress relief should be at 230°C for a minimum of 12 hours (at saturation temp) with natural cooling to room temperature. This is basically the same as tossing it in a field for a year or two and letting nature do it's thing.

Again, it depends on the order of magnitude of your measurements required. I deal with microns / sub microns and material stresses play a very large role.
 
This really depends on many factors.

4. Stress relief should be at 230°C for a minimum of 12 hours (at saturation temp) with natural cooling to room temperature. This is basically the same as tossing it in a field for a year or two and letting nature do it's thing.

At what point does stress relief temperature/soak time start messing with the heat treat? The numbers I've seen for 6061-T6 say not more than 400F (200C) for 30min before you start running into partial annealing.
 
I've seen parts measure correct on the lathe and when parted off, measure big. Chuck pressure was crushing the bar/part.
Also, parts measure correct with a tailstock being used, and then when removed again the dims 'move'.
Both were nothing to do with stress relieving as in mtl removal, but bad methodology

Well, it is stress. The poor clamping method is just introducing an unacceptable amount of stress to the part.
 
At what point does stress relief temperature/soak time start messing with the heat treat? The numbers I've seen for 6061-T6 say not more than 400F (200C) for 30min before you start running into partial annealing.

Temperature is the determining factor of whether it is precipitation heat treating (artificial aging)or annealing, not time. The range for precipitation heat treating for Al/Cu alloys is ~180-~240°C. Around 250°C you get into the annealing range which extends to about 410°C.

The time @ temperature is determined by section thickness, specific alloy, part shape and other factors.
 
Quote Originally Posted by MULTUSB200 View Post
Hello all,

I am looking for material regarding stress relief and it's affect on dimensional stability.
I am currently making a very precise part from round stock on a MTM Okuma Multus machine.
There is a lot of material that is being removed from the part and I am curious as to how this can
affect the part:
1. how is the part expected to hold it's measurements?
2. if the part is subsequent to move - at which phase will it stabilize?
3. will the part ever stabilize (given enough time) without thermal treatment?
4. is a stress relief process in a 170 degreee oven for 4 hours equal to letting the part sit
around in the shop before final machining for X amount of time (what is X)?

These are just example questions I think of.
If anyone knows where I can find an article/paper about this subject?

Hope I was clear!

Thank you.
Vyacheslav.
This really depends on many factors.
"Tight Tolerance" can mean different things to different folks. Please be specific - are we talking a few microns or what? If microns, then yes, stress relief is very critical to success.

1. The part should be at measurement at a stabilized 20°C - how long it takes to determine stabilization depends on section thicknesses. I do not know of any specification regarding how long after machining the part must remain at dimension. (And it really depends on what "tight tolerance" is......)

2. The part will move with *every* machining and clamping operation, how much depends a lot on shape, tool, cutting parameters, clamping system and placement, etc. Every time you remove material you change the shape of the part and the internal / external stresses. i.e. a dull tool can put stress into a part, while just removing material can change internal stresses.

3. Yes, it will. In the natural world all things come to equalization. When all of the stresses have equalized it will quit moving - given a stable temperature. How long that takes depends on a lot of factors.

4. Stress relief should be at 230°C for a minimum of 12 hours (at saturation temp) with natural cooling to room temperature. This is basically the same as tossing it in a field for a year or two and letting nature do it's thing.

Again, it depends on the order of magnitude of your measurements required. I deal with microns / sub microns and material stresses play a very large role.



Thanks for your reply.

First of all, the tight tolerance discussed is at a 0.001" true positions between one hole and several features at an angle to it.
We usually stress relief at 170c for 4 hours, then slowly reach room temp. for about 8 more hours.

The problem with this part is that I am machining it in a Multi Tasking machine which leaves no room for Rought machining, removing to be stress relieved and then finishing, like we normally do with other parts.

So I understand that from your experience oven stress relieving (or aging as it called?)is equal to about 1-2 years of nature's work.

But in that 1-2 years of the part being stressed - is it more sensitive to temp. variation while being measured?
(Compared to a relieved part)
 
Tony yes it is stress as in where there's stress there's strain!
But what I was getting at is the differentiation between this (as in clamping/holding) stress, opposed to the part distorting because of the natural residual stress 'relieving' itself when the part gets machined

I know, I was just trying to be humorous........

After the OP's later post, this seems plausible for part of his/her situation. My gut feeling without seeing the set-up is it is a stack up of several things. Removing lots of material can drastically change internal stresses on a piece of stock as we have all seen. (Anyone ever mill 0.02" off of one side of a piece of 3" x 1/4" x 2' long 6061 and seen it bow up to 1/8"- or worse? - A lot have seen similar would be my guess.)

To the OP:

My suggestion is to:

a) Firstly, you need to determine WHICH type of stresses are having the most impact on your tolerance. To do this, you need to TEST.
There are 4 basic types of stress inducers (for lack of a better term) that need to be addressed to determine which has the most influence on tolerance - 1) Clamping / fixturing - 2) Machining - 3) Material - 4) Tooling.

b) Devise testing routines that isolate as best as possible each type of inducer. i.e. chuck pressures, clamping methodologies, etc for item 1. Process for 2 (change operation order, direction of paths, DOC, etc), different heat treats for 3 or different batches of material for 3 and different types / geometries / coatings for 4.

c) After determining which type of stress inducer has the most influence on the tolerance in question, then you need to do MORE TESTING to determine what controls withing that variable you have available and can control and write these controls into your work plan / procedures.

It may be something as simple as changing the order of operations or a tool type. - worst case is that all have equal influence and you have to do a lot more testing and controlling to fix the issue.

It all comes down to a well defined manufacturing process and the only way to get a good one is with proper testing.

Be sure to keep accurate records of your testing and what was changed with each test. Change ONLY 1 VARIABLE at a time!
 








 
Back
Top