Turning 4140 with HSS
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  1. #1
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    Default Turning 4140 with HSS

    I volunteer in the back shops for the Hawaii Railway Society, where we have an old 18” x 72” Cincinnati tray top lathe (with an old school lantern tool post and HSS tooling), that I occasionally use. Currently we are replacing a traction motor on a WWII vintage GE 44 tonner road (locomotive). My requirement was to turn a 2”x8”pin out of some 2 1/4” 4140 round stock.

    The pin holds a 2000# traction motor up into a spring buffered mounting bracket on one of trucks - so not exactly a precision surface.

    In making the pin, I had a terrible time getting an acceptable finish on the round stock (using HSS tooling). The rough finish was cottage cheese texture during roughing turning, even though I cleaned up and sharpened the tooling nearly every pass on the work. I finally got a marginally acceptable bearing surface with two shallow finish passes (DOC .005”, at 96 RPM/.0035”feed rate. However the final finish cut left multiple concentric tooling marks where the tooling penetrated an easy 2-3 thou deeper than the rest of the finish surface.

    Wondering if this is a just a function of turning 4140 with HSS tooling, my operator error, or perhaps might the cross feed screw be to wornout to hold the compound rest and tool holder consistent with the axis of the lathe?

    Thanks for any comments.

    Glenn

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    Lots of possibilities for trouble but it should certainly have been possible to turn a good finish with HSS. Your RPM is not the problem, that's okay. What were you using to sharpen the tools? Something was up if you needed to sharpen the tool every pass. If it was just a bench grinder wheel that's part of the problem. A more finely finished tool will impart a correspondingly more finely finished surface. Are you positive the tool was on center? Easy not to be with those lantern tool posts if you're not careful. "Cottage cheese" finish makes it sound like you didn't have enough clearance. What kind of nose radius on the tool? What rake and relief angles were you running? Cutting oil or dry? Like I said, lots of possibilities for trouble here between tooling, setup and machine.

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    Others are better qualified to respond and we obviously don't have direct experience with your machine, but in general carbide inserts are massively better when the going gets at all difficult. 4140 seems to be supplied usually in the T condition which is Brinell 250-300 or say 30 in Rockwell C. That's substantially harder than mild steel at Brinell 130 which is already a challenge for HSS if you're doing a lot of it. I would say getting a modern quick change tool post on the lathe and some carbide inserts would be an outstanding investment if you have to do much more of this type of thing.

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    It sounds as if there is something loose, as the tool holder and or the part of the machine it rides on .Try an 1/8" R on the tool bit with a 6 degree back rake.And a good cutting fluid. I have cut many 4140 parts on a lathe with good results.

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    Are you using the hand wheel or carriage feed to turn the length? Also did you check that the tool height is on center?

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    I use HSS on 4140 all the time with no problems at all. I agree with what rcoope said about a QCTP being an outstanding upgrade, but not having one is not your current problem. Ekretz is probably stating your current problem. Sounds like the tool is dragging or the relief is wrong or something like that.

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    The scuffing during the finish cut sounds like built up edge coming and going. That is the trouble with HSS, is you cannot get anywhere close to a smooth flowing chip because you are so far away from 300 fpm or so, which is the critical speed for turning steel. Lots of oil is about the only escape, if it works. You also need a high positive rake finishing tool for a skim cut.

    I'd usually set my tools a slight bit below center (if there is any doubt, let it be lower). With a high positive rake finisher, it will still have enough rake to work, even if you lose a couple of degrees because the edge is below centerline. The problem with being on centerline is that when the edge dulls the slightest amount, then the edge is rubbing hard because it instantly has no clearance at all.

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    The sharpening every pass sounds familiar. What color were the chips? If you get blue chips with HSS in 4140, especially PH material, my experience is that about 12" of cutting on that general size of part is all she wrote on the edge, if you are depending on sharpness to cut, due to a lighter machine and/or low power.. Carbide inserts last far longer in the same conditions, and brazed of an appropriate grade may also, I don't use much of it.

    I think a larger / more powerful machine might cut using anything as a tool that does not actually melt..... it was common to actually dull the edge of carbide in past times, with maybe a 0.005" wide flat at a 45 deg or so angle. HSS was developed to last on machines with the power to cram it through the material, and I am pretty sure that it did not rely on a razor sharp edge to do it.

    But if you need the sharpness to cut, that amount of sharpening does not sound unusual.

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    fig7-4.jpgThe lantern tool post holder creates an upward angle to the cutter. Therefor you will need more clearance ground on the end relief of the bit. Most of the old manuals show using slightly above center for steel, with that type holder.

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    Your poor finish could also be that your chips are getting trapped between the the tool and the work. If you watch carefully, this can readily be seen. The solution is to increase the back clearance of the tool so the chips fall away. This is a particular problem when making fine cuts because the chips are small. Remember, the number one problem when cutting anything with anything is chip removal.

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    4140 (at least pre-hardened) should be easily free-machining with that set-up.

    I think you're dealing with the symptoms of a little slop in the machine you're running.

    +1 on adding a larger nose radius to the tool.
    Tighten-up the cross-feed and compound gibs just a little, so there's adequate drag, minimizing the consequence of unwanted backlash in cross-slide and compound
    Think about reducing your angle of attack of the tool, relative to the workpiece (to minimize the tendency to dig in)
    Think about traversing the opposite direction (so you're running a right hand turning tool the opposite left-to-right), this will cause the bit to ride out and over a hard spot instead of dig in.
    In a pinch, hold your hand on the lantern and exert a little force back towards you to keep it from digging in.
    +1 on a small doc finish pass, the low load will minimize tendency to dig in.


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