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  1. #21
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    To the OP, we hard turn gear teeth diameters all the time with Sandvik's CBN inserts.

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    Quote Originally Posted by Mtndew View Post
    I'm curious why that is (I don't do aerospace).
    It will ruin the treatment. Basically it would get so hot that the previous process for hardening would be reversed.


    Sent from my iPhone using Tapatalk

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    Quote Originally Posted by twjames79 View Post
    It will ruin the treatment. Basically it would get so hot that the previous process for hardening would be reversed.


    Sent from my iPhone using Tapatalk
    Also, I’m sure it would cause stress fractures at that temperature.


    Sent from my iPhone using Tapatalk

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    Quote Originally Posted by twjames79 View Post
    It will ruin the treatment. Basically it would get so hot that the previous process for hardening would be reversed.


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    TW, please get on a computer and update your location, not sure if a phone will do it.
    Welcome also

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    Quote Originally Posted by Rob F. View Post
    TW, please get on a computer and update your location, not sure if a phone will do it.
    Welcome also
    Should be good now.

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    Quote Originally Posted by Larry Dickman View Post
    grinding of heat treated steel is forbidden in the aerospace world.

    I second the CBN.
    Quote Originally Posted by Mtndew View Post
    I'm curious why that is (I don't do aerospace).
    I've never heard that (as in NEVER, sure some prints might specify no grinding) and I have processed a few parts over the years.... I have almost always seen no edm process however. That I can understand be cause it 'disrupts' the material at the cut edges.

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    I mis-spoke a bit. I just looked up the spec, it's covered under non-disclosures so I can't post it. Steels heat treated to over 180kpsi (the op's was 60Rc, so that qualifies) requires post grinding stress relieving. The requirements for stress relief are horribly unrealistic for any small operation, that's why we always just considered it undoable.

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  9. #28
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    Quote Originally Posted by Larry Dickman View Post
    I mis-spoke a bit. I just looked up the spec, it's covered under non-disclosures so I can't post it. Steels heat treated to over 180kpsi (the op's was 60Rc, so that qualifies) requires post grinding stress relieving. The requirements for stress relief are horribly unrealistic for any small operation, that's why we always just considered it undoable.
    They’re doing 2 things there Larry, the post machining thermal treatment is for aging and baking out any hydrogen that’s migrated into the steel during the machining (trust me it’s there).

    It’s true grinding can cause damage, the worst is “re-solution”. It’s not really a HT reversal, it’s raising the surface to near Ac3 temp and the underlying steel quenches the surface violently (a new HT with no temper). It does often cause surface fractures & delamination… The other bad is causing a layer underneath the surface in tension which happens as a result of the material worked past the yield point.

    For that to happen → somebody is really coloring outside the lines with their machining. Formerly is was only abusive grinding and milling that were suspects. Attached shows normal grinding & the tension(and depth) of the surface underneath opposed to a low stress recipe which has a small zone of compression above a small zone of tension.

    That said, people are grinding aviation, cold-service and aerospace bearings and other parts right now. The difference from the auto store bearing is things like carbide balls, special race materials & sometimes 9% nickle steels but they’re grinding the races.

    This tangent really would deserve another thread because hard turning also can cause an under surface layer under tension.

    Good luck,
    Matt
    Attached Thumbnails Attached Thumbnails residualstress4340grind.jpg  

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    Quote Originally Posted by twjames79 View Post
    Use a low content solid CBN.
    Exactly ! Plate it on a 3" cup wheel, spin it about 7500 rpm and just kiss the part. Fat flat flat

    If you do a lead trace of a shaper-cut gear tooth you will see first a little bump where the cutter hits the part and then a second smaller bump where the table compresses against the oil film. Whacking stuff with a tool makes wowees in the surface that grinding does not do.

    I know it's popular and "good enough" for a lot of things but for accuracy, hard turning kinda sucks. If you can grind it, it's better.

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    Thanks for your sharing!

    But i also have a problem here, the customer ask to turn this product in coolant, because the tolerance is too small (5-10 micro). And as i know, most of the hard turning process with interruption is not good with coolant.

    So how do you think about that? i'm stuck at here. yeah and i did make a test that turning without coolant is much better with coolant, but the customer dont want that.

    Do we have any tips to make it through without coolant? Thanks alot!!

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    Quote Originally Posted by thanh tam View Post
    Thanks for your sharing!

    But i also have a problem here, the customer ask to turn this product in coolant, because the tolerance is too small (5-10 micro). And as i know, most of the hard turning process with interruption is not good with coolant.

    So how do you think about that? i'm stuck at here. yeah and i did make a test that turning without coolant is much better with coolant, but the customer dont want that.

    Do we have any tips to make it through without coolant? Thanks alot!!
    5-10 micro tolerance is going to be virtually impossible to hold with a hard turn.

    If the customer is requesting to define the process you should ask them how they want you to do it, and with what tooling.

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    Interrupted cuts = small DOC and slow feed. That applies to both carbide and ceramics.

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    Big round solid CBN - like 3/8 to 1/2" with t-land. Separate round finisher with sharp edge taking the tiniest baby cut to size. Roughing and finishing with same insert is almost impossible to hold a tolerance like that.

    -Gene

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    If I understand correctly, the hard turning dry is giving you trouble with the finished size at room temperature... I've machined large things that moved consistently but never anything that is close to the tolerance you're fooling with.

    Near dry misting helps cutting some, but do any real cooling of the part in my experience.

    Good luck,
    Matt

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    I would use coolant and lots of it. It makes life easier even though you are spitting fire all day long with flood coolant
    5-10 microns is plenty of room in a normal bread basket sized part.
    Biggest I've helped with at this tolerance in hard turn are 40 inch interrupted dias and that was difficult.
    As with anything in this range of sizes setting and cooling between fresh run and measure later add a problem.
    With a good sized run you can calibrate your hot and cold gauges according.

    The entrance and exit around that interruption become points and areas to worry about at this level.
    The machine, and everything will deflect when the load goes down and then spikes, the servos and mechanic systems will attempt to comp at their frequency and time constants.
    Bob

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    Quote Originally Posted by EmanuelGoldstein View Post
    Excuse me ... have you ever looked inside a Pratt & Whitney radial ? Or watched the ballscrews extend the flaps ? There's a ton of grinding on heat treated steel in the aerospace world. That's why there are aerospace procedures for nital etch.
    Correct, grinding is fine. The problem is "burning" from incorrect grinding parameters, which the nital etch will reveal.

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    Hi guys! Sorry for the information.

    I mean the tolerance of the ditch and the geometry inside the mold (yeah, we have many type of products from small to medium size, not only the product in the picture)

    Just like Mr.Matt_Maguire concerned, we afraid of the tolerance inside the products will change if they got hot by dry turning outside.

    And we fail at the holes, like just 1,2 passes, the inserts get break or wear and i think that is the point..

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    I went back and re-read some posts just to make sure that I didn't misunderstand, but I don't know if recommending a low-content CBN grade is the answer.

    Full disclosure - I worked for Seco tools for a while, and really tried to pay attention to the CBN training, including referencing their excellent CBN handbook to this day...

    At least with Seco's grades, (and I would assume other CBN maker's grades as well) low-CBN = high cutting speeds in hardened steels, and high-CBN grades = high toughness in interrupted steels, cast-irons, and super-alloys.

    The reason being, that the boron is an incredible conductor of heat, and would therefore overheat with higher cutting speeds in hardened steels. Thus, they drop the CBN content, and start adding in ceramics to give the mixture more heat resistance.

    By contrast, the high-CBN content grades would often use larger grain sizes as well, for more surface-area per grain to increase with bonding the grains together. They would also employ large/small grain size formula within some grades, so that crack formation within through the mixture became more difficult - crack formation would be stopped as the crack travelled through the binder, but then was stopped by a grain in it's path...



    I leaned on this knowledge to help a customer turn-off carbide from some large roll tools. This part was clad with laser-applied 60% carbide, which had then been run through several heat-cycles, and what I assume was chemical degradation in-service, so that it was severely cracked & interrupted. The cladding was 3.2mm thick per side. We cut this off using a round, coated CBN grade with 90% CBN content, and 22-micron average grain size. The insert took a beating, but we won... We cut all the carbide off successfully. I say all that to illustrate the relationship between interrupted cuts in hardened materials, and grain-size & CBN content percentage.

    cbn-vs.-wc.jpg

    As for other alternatives to CBN, I don't know much about whisker-reinforced ceramics, but perhaps they could work. I know that Seco TH-1000 grade works phenomenally well in hardened steels, including interruptions.

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