Mild steel machining dilemma
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  1. #1
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    Default Mild steel machining dilemma

    Hi All,

    I've had this problem in machining mild steels when it comes to the surface finish. There will always come these stripes like material get dragged between the tool and part making deeper grooves and rough finish. No matter if I use carbide insert or sharp HSS tool. Harder steels like tool steel, stainless or aluminium this doesn't happen. How to prevent this from happening on mild steels like 11SMn30+C / SAE 1213 free cutting steel and S235JR / AISI 1018 mild steel?

    soft_steel.jpg

    Thank you.

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    IME. Finish passez need to be 2 or 3 times as deep as other materials. Also run as many RPM as you can. 2000 SFM is not unrealistic.

    R

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    As Litlerob1 says, increase your finish pass material. I have found leaving 0.05 (0.025 per side) for finish works well with C1018

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    Looks like buildup on the cutting bit causing welding and tearing. Mild steel likes oily flood cooling.

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    Dead sharp tool bit.

    Coolant, preferably an OIL based one.

    Higher cutting speed - within limits.

    Use a leaded alloy.

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    I had the same problem turning mild steel with my ancient Stark #4. Felt like hard spots or crap in the material was causing the cutter to jitter or something. Leaded steel cuts great, but i felt quilty about using a material that supposedly rusts easily, so i switched to 1144 Stressproof. Great surface finishes and cuts like butter. Definately worth the price.

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    Get the speed cranked right up like Rob said. That right there's your problem. After you do that run just a trickle of coolant right at the tip of the cutting tool to prevent the work getting marred by any stray chips. Soft steel with good carbide you are going to need 1000+ SFM. Up it until you get a good finish or you can't anymore because you ran out of RPM.

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    Quote Originally Posted by MrStretch View Post
    I had the same problem turning mild steel with my ancient Stark #4. Felt like hard spots or crap in the material was causing the cutter to jitter or something. Leaded steel cuts great, but i felt quilty about using a material that supposedly rusts easily, so i switched to 1144 Stressproof. Great surface finishes and cuts like butter. Definately worth the price.
    LOL! It didn't ship that way. The softness conspires to CREATE those as you go.

    Low RPM budget? Sharp HSS, slow and wet. "Neat" oil or old stinky brown Sulfurized. Lousy for "cooling" ability. Basically a lube and barrier to chip adhering.

    And "Oh, By the way..." there was always a long-angle lathe bastard file to-hand "back in the day" when Carbides were scarce, costly, and dreadfully shitty, as well. One part to make, tool wasn't perfect, file-finish wasted less time than f*****g with that for small gain.


    More RPM on-tap? Modern Carbides, fast and usually "dry". Coated? I'm good with HSS, so somebody else's call.

    Book answer, any tooling, is still a more cooperative alloy. Pays yah back in f**k-with TIME saved. Steel is cheap and recyclable. Time is dear and gone forever.

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    Sharp carbide inserts work really well. Average steel specific insets are useless unless you get enough speed and doc. Aluminium specific ones are almost always enough sharp and work with tiny cuts and low speeds as good if not better than hss.

    Some steel specific inserts work well also, ie Sumitomo T1500Z coated cermets.

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    As already said: higher rpm, cutting fluid, sharp tool,and leave more material for your final pass (not to much as depending on the application you could end up out of round. You also may want to lower your feed rate. It sounds like your having issues with build up or rubbing so make sure your tool has lots of clearance on all trailing sides of the cutting edge.

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    Corn,

    When I'm boring on the mill, I usually use a very slow downfeed, probably 1/4 the standard feed.

    Works on the lathe as well.

    Chuck
    Burbank, CA

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    That looks terrible.
    Now how can I get a bit of a clean cut and then so many grooves. What was cutting the grooves and gashes as my tool did the other outside nicely?
    Why the trenches? What the heck?
    It's easy mild steel, how can this go so wrong?
    Bob

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    Quote Originally Posted by CarbideBob View Post
    That looks terrible.
    Now how can I get a bit of a clean cut and then so many grooves. What was cutting the grooves and gashes as my tool did the other outside nicely?
    Why the trenches? What the heck?
    It's easy mild steel, how can this go so wrong?
    Bob
    Correct me if I'm wrong Maestro.

    But the chip that's being peeled off the bar needs to be hot enough to break, and heavy enough to curl around the cutting edge. That way it doesn't stick to the cutting edge. The chips stuck on the cutting edge are non-solicited mini Groove tools. Lots of ways to accomplish the basic task of creating 6's and 9's.

    R

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    Liltlerob, hit on what is happening. Material is welding to the top of the cutter then letting go. My home shop lathe was a South Bend 9C and I could get a good finish on 1018 even at the low speeds and feeds that machine developed.
    For finishing my old fore man said .003"(.1mm) per side (I have always done close tolerance work), it is a rule I followed since and works in almost all situations were good finish and close tolerance are needed.
    Your 1213 will finish very similar to the 12L14 recommendation, the free machining additives should give you a very good finish even if not optimal on speeds and feeds.
    Since you are not getting good finishes on the 1213, I would check to make sure your tools are on center and your feed is smooth and not feeding in fits and starts. Finishing 1018 and 1213 with carbide I would use a positive rake tool that is designed for aluminum and definitely use cutting oil as many have suggested.
    My likes on feed and depth of the finish cut are way different than most of the suggestions so far, try both.
    I have seen some people hone off the cutting edge after sharpening high speed tools. I want the edge sharp enough to shave with for finishing.

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    Taking the edge off HSS (carbide too) in a controlled manner is something that helps when doing heavy roughing. For finishing with HSS always go for a dead sharp edge. OP is almost certainly running into chip weld issues caused by too low of a finishing speed; that has already been pretty well established.

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    You might put an oil wick, perhaps a felt stopper in an oil can so the part stays wet but does not drip to run ahead of a very sharp tool bit with it having some side cutting edge positive rake angle and a .010 to .030 radius. HSS is good for mild steel and some aluminum inserts are sharp enough but may not have desires rake attitude.
    Very short tool bit hang-out is good.

    I like a bit sharp enough to shave your fingernail.

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    You need a lot of surface speed to pull off a finish on mild steel. Proper insert geometry is important and cutting oil helps.

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    My guess is the part is only about 8-10mm dia, under 1/2 inch?
    Makes it super hard to get the high surface speeds you guys are talking of doesn't it?

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    Quote Originally Posted by rokbottom View Post
    My guess is the part is only about 8-10mm dia, under 1/2 inch?
    Makes it super hard to get the high surface speeds you guys are talking of doesn't it?
    Looks like a good job for a Star JNC-10 I was looking at with a 10k spindle

    Seriously grind up a contrary tool and use that.

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    Quote Originally Posted by Hardplates View Post
    ...
    Seriously grind up a contrary tool and use that.
    Grind a contrary tool? Please explain.
    Bob


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