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Lamination on threads (CNC machining)

Discussion in 'Manufacturing and Related Processes' started by Emil Hortea, Oct 1, 2015.

  1. Emil Hortea

    Emil Hortea Member

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    Hello,

    I have some trouble with small lamination on a threaded hole of M8x1, after tapping the surface of the thread is very rough (caught on microscope). Parts are from casted Alu – AlSi9Cu3(Fe) and after machining and tumbling undergo galvanic plating.

    The problem with these parts is that after plating the GO gauge will not enter (before it did). I’m sure one of the problems is due to plating but there may be some (for sure it is) impact due to machining as well.

    I’m looking for any specs on lamination of threads, if anyone knows where I can find please let me know. Is this due to machining, casting, tools (shape only, I already excluded ware or wrong tool placement), coolant… there are a lot of variables to take in consideration. Also it may help to know that the GO will not enter only from one side, the side the drill is made from.

    Did any of you encountered this problem before? Any help would be greatly appreciated.

    Best Regards,
    Emil
     
  2. Miner

    Miner Moderator Staff Member

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    Are you certain that it is lamination? While fairly common on rolled threads, it shouldn't occur on cut threads. Are the threads fully formed? Where is it occurring? Could it be a burr on the exit side of the hole that is folded down onto the thread?
     
  3. Andy Nichols

    Andy Nichols Moderator Staff Member

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    I'd agree. "Lamination" doesn't happen to threads, in general terms. It might be that the plating material is not adhering correctly, as throwing plating down a hole is a) difficult and b) almost impossible to get good adhesion - especially with a thread form. The peaks may well "over plate", which is probably what you're seeing.
     
  4. Emil Hortea

    Emil Hortea Member

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    Hello and thank you for your replies,

    I don't know if "lamination" is the correct word, on the microscope the sides of the threads look like snake scales or flakes.
    Thread is done by cutting tap and yes, threads are fully formed. The flakes appear on the sides of the thread, the upper edge is clean, no burrs on the thread.

    Is it possible that this is causing the problem of the GO gauge not entering or can it be plating? This would be the last issue with machining process, if I can eliminate it I have to find root cause in plating or raw material. Can raw material be an issue?

    Our machining expert here doesn't even want to believe that we have this issue, he said it's impossible but still, looking through a microscope I see them (true enough I multiplied x46).

    I also attached a picture, it's not very clear but you can still see some irregularities on the thread.

    Best Regards,
    Emil Hortea
     

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  5. Andy Nichols

    Andy Nichols Moderator Staff Member

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    How many times has the tap been used? I'd suggest that it's close to being worn out. Certainly, if you plate down a threaded hole without using a tap which is made for the job (slightly oversize on cut) then the gauge will not go into the hole, since, (in effect) you've closed up the hole by putting extra material in there. You should run the tap and check the hole before plating. As I say, unless you know how many "hits" the tap has been used for, you should anticipate it's worn and get a new one. Then monitor how many times it's used to avoid it wearing out.
     
  6. Miner

    Miner Moderator Staff Member

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    Was this taken post tapping and pre-plating? It is difficult to tell for certain, but I think Andy is on the right track. This might be galling caused by a severely worn tap, which would also make the threads oversized.
     
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  7. Emil Hortea

    Emil Hortea Member

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    Hello and thank you again for your replies,

    On cast materials is very hard for us to check how many hits a tap made because of the raw material. If the tap hots a hard point or a porosity in the raw material it will get destroyed even if it's new.
    Yes, the picture is before plating but the thing is, before plating the thread is good according to the go - nogo method of inspection. It's only after plating that the parts have problems.
    I know for sure that alternate ways of inspection must be implemented for future production but, at the moment, considering after machining parts are good, after plating they are not should I find the root cause in machining or in plating or a bit of both?

    Also, is there a way to measure these "flakes" or a standard that may put some light on it?

    Best Regards,
    Emil
     
  8. Andy Nichols

    Andy Nichols Moderator Staff Member

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    If you plate after tapping then the gauge won't go! You're adding material. You have to use a specially made tap to allow for the plating to add material and bring it to the correct size. An alternative is to use a gauge which has an allowance.

    I think you need to study the taps, how frequently you use them, what they're made from, lubricant etc to improve the cutting. BTW you shouldn't be measuring/counting "flakes". It's a symptom, not something which you can fix.
     
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  9. Miner

    Miner Moderator Staff Member

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    Ditto on Andy's comment. The tapped hole dimensions must be designed to allow for the thickness of the coating. Note that the coating will tend to be thicker in certain areas such as the thread root.
     
  10. Jamie Lill

    Jamie Lill Member

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    What type of plating are you using? You might have to have before and after plating go-nogo gauges and also make your holes and threads oversize to allow for the plating
    This happened to me when we were chrome plating certain parts
     
  11. Andy Nichols

    Andy Nichols Moderator Staff Member

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    I'm not the OP
     
  12. QMSmaster

    QMSmaster Active Member

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    I have seen this on turned threads. Sometimes aluminum will gall or "tear' during machining and can give a rough surface. Kind of like how aluminum is brittle and will crack when formed. I solved my threading problems by adjusting parameters (speed, feed, tc), tooling, coolant, etc.

    Emil, Do you see this detail on parts both before and after plating? Plating is like painting in that it magnifies surface imperfections. What ever you see after plating is likely there before plating. That is my experience.
     
  13. gaugestools

    gaugestools Member

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    My reply is late, but this is a common question, maybe others also has this problem, so I try to share my opinion. No matter internal thread or external threads,manufacturer of threaded parts need to make suited allowance to the threads before plating, the manufacturer should know the specific plating requirements in advance. If the allowance is not controlled well, then which will leads to the threads will be too tight or too loose.GO gauge will not enter (before it did), I think you haven't controlled well the allowance. please view this article, it is very valuable to answer this question Pre-plate thread and gauges' calculating
     

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