Machining aluminum and heat

Discussion in 'General Engine Discussion' started by Philipintexas, Sep 12, 2019.

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  1. Sep 12, 2019 #1

    Philipintexas

    Philipintexas

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    I’ve always known that materials expand with heat, but recently while machining 2.5”+ OD 6061 aluminum involving deep large diameter drilling followed by boring and turning to diameter.
    I was making internal bearing pockets in parts that needed the OD press-fit in a cylinder so the dimensions were fairly critical.
    I rarely aim for .0005” accuracy but this project needed it to hold bearings in pockets and an OD to fit snugly in a cylinder.
    Some parts became hot enough that I couldn’t comfortably handle them during the process (lots of heavy turning cuts).
    I was happy with my results, until the next day, NOTHING fit perfectly as they had while in the lathe.!

    WOW! I found some OD’s changed by as much as .005”, and my internal slip-fits were now tight press-fits. That blew my hopes for .0005 accuracy out the window.
    On the remaining parts, I tried removed them periodically and cooled them in water before final cuts and got much better results.
    The moral is, keep your material near room temperature during machining if you really need accuracy....

    This is probably old news to most here, but it may be of help to some, like myself, than never really saw the actual effects of heat buildup... Keep it cool!
     
  2. Sep 13, 2019 #2

    kwoodhands

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    Make a bushing that will be sized for Loctite . I would make the bushing from bronze , cast iron or brass instead of aluminum. You could also use a press fit instead of Loctite. I haven't had much luck with press fits, that's why I suggested Red Loctite.

    mike
     
  3. Sep 13, 2019 #3

    Philipintexas

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    Good suggestion. I also wanted all my parts to be relatively easy to disassemble so I was trying to get snug fits but not too tight. The bearings were commercial steel so all I was making was the holders.

    I will say, after this experience, heating a part & chilling the other should make “Press” fits easier.
     
  4. Sep 13, 2019 #4

    TSutrina

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    The aluminum expanded due to heating and the tool you measured the diameter was cold in comparison. Thermal coefficient of expansion of 6000 series aluminum is 13.0 in/in/F 23.4 mm/mm/C both time 10^-6. The bearing is steel ~ 8.5 in/in/F 15 mm/mm/C also 10^-6. The approach that is used by many companies that use aluminum housing is to shrink fit in a steel ring and machine it to a tight fit for the bearing. The ring should not loosen at the maximum temperature of the housing.
     
  5. Sep 13, 2019 #5

    BaronJ

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    Hi Philip, Guys,

    Wait until you are parting aluminium or brass rod and it expands grabbing the parting tool. :)
     
  6. Sep 14, 2019 #6

    Cogsy

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    I can't see why this would happen. The part will expand in all directions as it heats, effectively it becomes 'scaled-up' so the groove you're cutting will increase in size and provide clearance for the parting tool, assuming that the part is heating evenly. In reality, I would expect more heating to be occurring in the spot where the parting is happening so the groove width should expand even more than the surrounding material. Either way, although I know parting tools can get very grabby, I don't believe thermal expansion is the cause.
     
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  7. Sep 14, 2019 #7

    BaronJ

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    Hi Al,

    I find that heat from parting is conducted into the large mass of the chuck jaws and quickly lost causing unequal expansion. Whilst I always try to part off as close to the chuck jaws as possible, parting off 10 or 15 mm away does feel different.

    Whilst I agree with your comment that a part will expand in all directions the narrower part wont expand as much as it gets narrower.
     
  8. Sep 14, 2019 #8

    Cogsy

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    That's the bit I don't agree with. Using the coefficient of linear expansion, call it α, the increase in length of a material is α * length* change in temperature. This holds for any direction. So if the part is increasing temperature uniformly then it will be uniformly growing in length as we are not removing material from the length until the piece finally parts off. Now as our cut diameter decreases, the amount the uncut diameter increases will be greater as we have more material expanding, but the length of any portion of the part, and therefore the width of the groove we are cutting, will be increasing uniformly.

    I also think that the hottest part of the workpiece would be directly under the tool, especially as it has less mass than the surrounding material as it gets cut away. So the narrower point will/should be the hottest point and will expand (linearly) the most and increase the groove width. I know it seems counter-intuitive for the groove to increase in size as the workpiece heats up, but unless the part of the workpiece that is directly under the cutter is somehow the coolest part of the piece then the cutter heat will increase groove clearance rather than decrease it.
     
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  9. Sep 14, 2019 #9

    mcostello

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    The tool heats up also.
     
  10. Sep 14, 2019 #10

    BaronJ

    BaronJ

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    Hi Al,

    I follow your logic completely. But how do you account for the flow of heat ?
    The whole work piece doesn’t heat up uniformly. Basically the heat flow is from the hotter part into the colder part, so the hotter part expands more.

    I fully agree that the hottest part would be the point at which the tool would actually be cutting. I wish I still had access to the FLIR camera. With that you can actually see the way the heat flows.
     
  11. Sep 14, 2019 #11

    goldstar31

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    John et All

    Considering the fact that if care is not taken( and we all have ideas about cooling), the tip of carbon and hss cutting tool gains a welded piece- which makes a a hell of a mess of a finish cut.
    Logically( what I learned when I went to school:p) aluminium has a much lower melting point than steel, copper and so on. Hence the risk of welding.

    Well, dammit someone said that I had an IQ of 135 or summat and I did sometimes listen instead of looking out of the school windows for German Bombers carrying ferrite( mixture of aluminium and rusty steel turnings. ;)

    Beats hasty retreat back to the old people's home and those nice people in white coats

    Oh yes- I'm sort of norm- or so they say
     
  12. Sep 14, 2019 #12

    BaronJ

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    Hi Norman,
    I trust that you are keeping well.
    I'm sure that you meant thermite ! Nasty stuff that. Nowadays commonly used for welding railway lines together. Spraying water on it only makes it burn better.
     
  13. Sep 14, 2019 #13

    goldstar31

    goldstar31

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    Yes John. I was thinking of getting a ferreteria and keeping ferrets;)
    or senile debentures!:confused:
     
  14. Sep 15, 2019 #14

    Cogsy

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    Hi Baron, I'm not sure what you're getting at about the flow of heat. If the tool tip is producing the heat, which it should be unless the sides are rubbing for some reason, then the heat will be 'entering' the workpiece at the point it is being cut. As we cut, this point becomes the thinnest diameter of the piece (assuming we start with a cylinder just for ease, it doesn't really matter). So the thinnest diameter is the hottest point and the heat flows into the rest of the part, not necessarily uniformly but again it doesn't make a difference. If we now had a FLIR (which I love playing with but don't have my own) and we scanned the linear length of the part, the highest temperature would be our groove and thus it would have the highest linear expansion making our groove wider than it was.

    Making up some figures for ease, let's say α is 0.0001 and our groove is 2mm wide and has heated up by 100 degrees over ambient. Let's also say the part each side of our groove has heated to 98 degrees over ambient. So our 2mm groove is now 2+ (2*0.0001*100) = 2.002mm wide. Now even without doing the math, as the material each side has not increased in temp as much, it cannot have expanded as much as our groove. Even if the heat flow was perfect, it could not be hotter than our grooved area so, at worst, it would have expanded at the same rate and our part is just 'scaled up' uniformly.

    The diameter of the part would be increasing by a larger amount in other areas, simply because there is more material to expand in that direction, but this diameter increase makes no difference to our groove clearance.

    Finally (phew) I can see that of one end of our workpiece is constrained by the lathe jaws and thus cannot move, then all the linear expansion that occurs between the jaws and the cut would be to the right (assuming normal lathe orientation) so our groove would be displacing to the right as the piece expands. In this case, I can see the left side of the parting tool rubbing on the side of the cut. Realistically though, as we part reasonably close to the chuck, coefficients of expansions are small, and the chuck is a heat sink, I can't see this having much effect. (I just did a quick calculation, for aluminium being parted 20mm from the chuck and increasing in temp by 100 degrees C, our cut would shift 0.044mm to the right in the above scenario, using the actual figures).

    That's what I'm thinking, but if I've missed something I'm happy to be corrected.
     
  15. Sep 15, 2019 #15

    goldstar31

    goldstar31

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    Gentlemen, I understand what you are discussing but respectfully disagree that expansion has much bearing on the act of parting off.
    I belong- and have belonged to the George H Thomas School. He wrote extensively and especially in Volume 142 of Model Engineer and this was later incorporated in Dr Bill Bennet's 'Model Engineers Workshop Manual' by GHT.
    John has a Myford Super7, so had GHT and so have I. Apart from anything else the lathe suffers from a weak saddle. In truth, it is so weak that it can be bent if care is not taken.
    I hand scraped one back to normality. Again it belonged to the narrow guide principle which also lends itself to going in and out of cut rather more than merely expanding that tiny fraction from heat.
    Importantly, Ian Bradley, part of the Duplex Partnership with Dr Norman Hallows and then GHT kept improving the Myford parting problem by moving the cutting tool from the front to the rear and the 'finally' tilting the blade 7 degrees down wards and grinding a female kerf of 140 degrees along the cutting edge and the grinding the front edge of the tool to a 'male' angle again of 140 degrees inclusive. This is sold as a kit for two sizes of lathe by Kit Burswell of Hemingways kits. I've had no problems since mine was made in 1973 and it has lived quite successfully on a variety of lathes, Chinese and others.

    Now turning to Chinese lathes and the 7X and including my Sieg C4, the difficulty in incorporating what can be called GHT's parting tool is the size of the saddle although the SC4 is better and bigger/beefier. However, the fitting to the lathe bed is inherently weak and the internet is full of ideas for improvement ( or something). Again the rigidity and play of 7X spindles has featured largely over the years and Arc Eurotrade( maybe others) sells better bearings and fits them to 7X lathes on payment if required.
    Sadly, the bigger and beefier SC4 cannot without modification adopt the rear parting principle. Mine, I modified to take a subplate to use my GHT tool mentioned earlier.
    I'm of the old school and use lard oil extensively and this helps to literally roll the swarf from parting off into neat and narrowed coils which drop away from the work.

    So very little if any importance can be gained in long discussions on coefficients of expansion.
    Thinking of my school days and fishplates to take up heat variations in railway lines, this technique is obsolete with the advent of thermic welding.

    Got it right this time John!!!!

    So Not just my thoughts, copied by thousands of satisfied model engineers


    Regards


    Norman
     
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  16. Sep 15, 2019 #16

    BaronJ

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    Hi Al,

    Whilst I don't disagree with your maths, I hate maths...

    An interesting discussion, but is the real question here, why does the parting tool try to bind in the groove !

    You don't believe that heat is the issue, but I do based on observation.

    The chuck will be much colder than the workpiece and conduct the heat away from the work reducing expansion on that side, simply because its a bigger mass than the work. The other side of the groove will be the part that is expanding towards the groove as the point where the tool is cutting is getting smaller in diameter and the amount of expansion at that point is no more than the tool cutting width.

    There is also another consideration, the cutting tool is getting hotter as well, as it cuts, so theoretically the cut should be getting wider. It all starts to depend upon how quickly the heat builds up and how rapidly it can dissipate.
     
  17. Sep 15, 2019 #17

    BaronJ

    BaronJ

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    Hi Norman,
    I think that I've opened a real can of worms here :cool:
     
  18. Sep 15, 2019 #18

    goldstar31

    goldstar31

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    Moi aussi!:D

    Whether it is going to be the 2nd Norman Conquest is quite another matter. Hence the French- or a pun my word??o_O
    It keeps what little is left of the old grey matter working.:)
     
  19. Sep 15, 2019 #19

    TSutrina

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    Thermal conductivity of aluminum 6000 series is ~112 Btu * ft / (F * hr * sqft) 193 W *m /(sqm*C) steel and steel ~ 27 Btu*ft/(F*hr*sqft) 47 W*m/(sqm*C). Thus the temperature of the aluminum part if small is ~ uniform where the chuck and tool with a small cross section has a significant temperature gradient, poor it carrying the heat away. Parting tolerance compared to the expansion in generally meaningless (10^6 / temp change) But the tolerance of the bearing press fit is not meaningless.

    Stop the lathe and start touching things. Can not touch the aluminum and the actual cutting insert. Can touch anything on the lathe including the tool holder and the measuring tool, as you move away from the aluminum quickly temperature drops off fast. So within an inch the lathe is at room temperature. It is the effect of poor conduction of steel and relatively small contact area and the significant difference in the thermal mass.
     
  20. Sep 15, 2019 #20

    Ozwes007

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    Just my 2 cents.
    6000 grade Aluminium is one of the best grades at machining, machines cool, doesn’t stick to the tool often, and produces close tolerance parts all day long. Aluminium of any type will stick to tooling if you don’t use a lubricant and a coolant. The reason is aluminium when heated oxidises and will bond at a molecular level to its host(the tool). This oxidisation is as hard if not harder than diamond! This build up, once started, will produce an ever larger stalactite of metal rapidly that will destroy the job and tool very quickly. The moral here is to use a coolant and a lubricant(usually one and the same.)
    On a lathe or mill if you don’t have a pump make one out of a car windshield washer, they work great and cost next to nothing from a retail outlet. BTW quick way to cool Aluminium while cutting is to use a mist coolant nozzle, just don’t inhale the vapour. And one of the first rules of metrology is to make sure everything is at a std temperature(usually 24 degrees) and that includes your mics, telescopic gauges, verniers etc etc. you will never get an accurate measurement if it’s not at the right temp.
     

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