Cast aluminum vs extruded?

[FoxCNC1]

First off guys, my terminology could be wrong but when I say extruded I am talking about T6 6061 sheets and bar etc that you can get online, and how it differs from mic6? Cast aluminum?

I read somewhere that that "regular" 6061 will warp if machined too thin due to it being prestressed? Wereas cast will not and be more true to size?

If someone can plaese shed some light and correct me that would be great. I am looking to mill some flat fixture plates in the near future and i was told to look at cast alum.

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[peteeng]

Hello Mr Fox - Firstly extruded aluminium is intended for shapes not plate whereas plate is plate and Mic6 is a "tooling plate" something that can be machined into moulds or mould parts or other things. Mic6 is a 7000 series alloy which means its hardening agent is zinc. 6061 is a 6000 series alloy and its hardening agents are magnesium and manganese with a dash of silicon to taste. Now 6061-T6 is stronger then Mic6 if you need strength. The T6 means its been solution heat treated & aged. This means its been held at about 600deg C for a set time then cooled quickly (which puts it into a particular metallurgical state) then aged (which hardens it) in an oven at 180deg for about 8 hrs. This heat treatment also means its internal stress is pretty low.

The Mic6 people do not describe its heat treatment history but some docs do say its stress relieved. This means its held at 350degC for a few hours then cooled slowly. This puts it into -0 temper or annealed. This means its unlikely to change metallurgically or mechanically in various applications.

https://www.designnews.com/materials...aluminum-plate

The whole aim of stress relief is so when you machine it, it does not change shape. Any metal can warp when machined very thin from a thick plate. In regards to your fixture plate a cast billet eg Mic-6 or other tooling plates are good if the price is good. If you use 6061-T6 for instance it may not warp but it is not sold as a tooling plate, it's usually a structural specification with no guarantee of staying straight whilst being machined. To guarantee no warping occurs (steel or aluminium or other metals ) it's often roughed to near nett shape then stress relieved before final machining. This makes sure no last minute changes occur in critical expensive parts for instance. Cheers Peter

[FoxCNC1]

Are you saying that as long as machine 6061 and allow it to cool down I can re machine it and not have it bow?

[NIC 77]

Are you saying that as long as machine 6061 and allow it to cool down I can re machine it and not have it bow?

No. He's saying that you would need to put it in a special high temp oven, following a specific temperature profile for many hours to relieve the internal stresses. Essentially you'd have to pay for someone else to do it.

What is the temperature profile for stress relieving 6061 T-6 Aluminum? It would obviously need to be heat treated again afterwards.

I've seen large 3/4" thick aluminum plate (not cast) cut into smaller pieces and then have most of one side machined down to 3/8" along it's entire length and it bowed like crazy. I've never seen the cast stuff used, so I can't comment.



Look at the pic from this video.....does that not look a bit warped to you? You can see the shadow along the edge of it, makes me think that the middle is bowed up a bit. It would have been nice to see him put a straight edge on it after machining in the video Also, I didn't catch what kind of aluminum was used.

[peteeng]

Hello Fox - If you use 6061-T6 plate there is no guarantee that it will not bow on you as it is sold as structural plate not tooling plate. The heat of machining is not important in this case. Mic-6 is sold as a tooling plate and it has been stress relieved specifically so it does not change shape during machining. During machining there should not be a significant change in temp. Unless you need the extra strength of 6061-T6 use the tooling plate....Cheers Peter

[NIC 77]

This is interesting:

[peteeng]

Hi Nik - Yes VSR is used for structures that are difficult to put in an oven at 650C for several hours. Either due to size or shape. I do analysis work for a company that makes large mining structures and they VSR all their structures. VSR deflects the metal so far it yields it so is mechanically stress relieving. Trouble is it can crack welds if your not careful!! Peter

look up Bauschinger Effect its one of the reasons this process works.

[mactec54]

First off guys, my terminology could be wrong but when I say extruded I am talking about T6 6061 sheets and bar etc that you can get online, and how it differs from mic6? Cast aluminum?

I read somewhere that that "regular" 6061 will warp if machined too thin due to it being prestressed? Wereas cast will not and be more true to size?

If someone can plaese shed some light and correct me that would be great. I am looking to mill some flat fixture plates in the near future and i was told to look at cast alum.

Correct cast does machine better as far as warping goes but if you don't have a lot of machining to do on the parts then regular extruded 6061 works fine, Cast like Mic6 also warps if you have a lot of machining to do on the part something to be aware of with Mic6 is it is softer than 6061 and can be easily marked, a lot use it in it's ground state for convenience of having a better surface flatness than extruded 6061 bar

[FoxCNC1]

So if I wanted to make the head sink shown in the video above, of even better a fixture plate for my machine, is there any value to cast over formed/extruded (Is that what the other kind is called?
Thank you

[mactec54]

Are you saying that as long as machine 6061 and allow it to cool down I can re machine it and not have it bow?

Cooling down has nothing to do with it, the part should not get hot to start with, it will warp hot or cold if best machining practice's are not used

It all depends how you machine the part, and how much you take off ( 1 ) side as to the other, and if your cutters are sharp a dull, a cutter will induce stress into the cut material or heavy cuts will also, there is an art in knowing how to machine without inducing stress into the material most warped material is from bad machining practice's

[FoxCNC1]

tell us about that art so I can start practicing it

[bostosh]

Please show us the part so that the answer will become evident.
I have seen 110" x 6"x 4" thick 6061 billets warp and 1/4" plate warp

[metalmayhem]

I've run plenty of large aluminum plate and holding up parallelism and flatness isn't much of a problem if proper procedures are taken . #1 thing , don't over tighten , Most bowing is due to user error and not stress relief . When in doubt put a dial to the part while clamping , it's surprising how little clamping force it takes to bow a plate .

6061 is a pretty stable material and stress relief isn't much of a problem unless there is a lot of material being hogged out of it . If thats the case then the proper thing to do is hog it out leaving some material , then re-clamp and run finish passes .
As mentioned mic6 or other cast aluminum is soft , it is also brittle . If you plan to have threads that are repeatedly being tightened and un-tightened then helicoils or other thread inserts are a must . Otherwise you face stripping out the threads . If your excessively tightening then you may also face fracturing a chunk out of the plate even with inserts .

If a cast fixture plate is meant for the long haul then sending it out for hard anodizing is wise . The stuff is always a pleasure to machine and cuts like butter and it really does work great for making fixture plates .
Overall , 6061 will out perform cast plate , is cheaper and more readily available . Only thing to remember is that not all 6061 is the same , be sure to use quality materials vs offshore crap

[FoxCNC1]

I've run plenty of large aluminum plate and holding up parallelism and flatness isn't much of a problem if proper procedures are taken . #1 thing , don't over tighten , Most bowing is due to user error and not stress relief . When in doubt put a dial to the part while clamping , it's surprising how little clamping force it takes to bow a plate .

6061 is a pretty stable material and stress relief isn't much of a problem unless there is a lot of material being hogged out of it . If thats the case then the proper thing to do is hog it out leaving some material , then re-clamp and run finish passes .
As mentioned mic6 or other cast aluminum is soft , it is also brittle . If you plan to have threads that are repeatedly being tightened and un-tightened then helicoils or other thread inserts are a must . Otherwise you face stripping out the threads . If your excessively tightening then you may also face fracturing a chunk out of the plate even with inserts .

If a cast fixture plate is meant for the long haul then sending it out for hard anodizing is wise . The stuff is always a pleasure to machine and cuts like butter and it really does work great for making fixture plates .
Overall , 6061 will out perform cast plate , is cheaper and more readily available . Only thing to remember is that not all 6061 is the same , be sure to use quality materials vs offshore crap

I had a small project that started with 0.125" stock. I want to face mill it to 0.100" What would be the best way to do this then without warping it?
The plate is 15 x 8

[jessefranco211]

T6 6061 sheets or bar is best for tooling fixture, just be conservative in machining and use new sharp tools.

[peteeng]

Hi Fox - There is no best way. You will find out when you do it. But to minimise warping you will have to remove some from both sides, not all from one side. Peter

[FoxCNC1]

Thank you, I will try that next time.

[mactec54]

tell us about that art so I can start practicing it

Here are some good examples that I machined started at 1" thick and have a .125 wall thickness top and sides, the part size finished at 14" x 12" the smaller parts are for a Power module have a 1.5mm wall by 19mm deep pocket

The Black anodized part was 1.250 thick machined to .375 on the flanges all these parts where flat on the back side when completed if not they where machined

If you have a few years to do a Tool and Die or Mold making apprenticeship you will learn how to machine like this it's not something you can do with a few Emails or examples posted here

[mactec54]

I had a small project that started with 0.125" stock. I want to face mill it to 0.100" What would be the best way to do this then without warping it?
The plate is 15 x 8

You would have to hold it with a vacuum table being that thin or there is special sticky sheet paper that some use

[metalmayhem]

I had a small project that started with 0.125" stock. I want to face mill it to 0.100" What would be the best way to do this then without warping it?
The plate is 15 x 8

under assumption that the 1/8 plate isn't warped to start with then a vac table , double sided tape will also hold for facing . knocking .025 off shouldn't cause warping
Depending on the finished project then there may be a better approach