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Thread: Die Surface Finish

  1. #1
    Boolit Buddy
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    Die Surface Finish

    Iíve been working on my draw dies. Basically building a few different diameters in order to take range brass and turn them into jackets for a calibers I swage.

    Iíve been struggling to determine what the proper surface finish needs to be on the dies. Right now I typically polish put the tool marks with 400 grit lapping compound.

    Iím looking at purchasing finer grits, but Iíve tested out the dies with the 400 grit finish and the jackets/brass cases slide right through will very little resistance. Thereís no tool marks on the jackets, and they look great.

    What do I gain by going with a higher surface finish? It there an ďIdealĒ surface finish for swage dies?

    Thanks in advance for the input.


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  2. #2
    Boolit Master
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    I wonder if the surface is more able to hold lube after being polished with 400 grit, would be interested to hear from those who know more. Regards Stephen

  3. #3
    Boolit Buddy
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    Thatís what I was wondering. I had heard the same thing. Sometimes a rougher surface is better because it retains lube.

    My current approach has been to polish out the tools marks with 400 grit, harden, and then do a finish polish with 1,200 or 2,000 grit lapping compound.

    I am waiting until I have a few dies ready to heat treat, so I can do them all at once. Polishing takes a bit of time and patience, if rougher finish is better or ďGood enoughĒ then I donít mind leaving it at that.

    Thereís surprisingly little written online about when different surface finishes are needed, or when they are appropriate.


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  4. #4
    Boolit Grand Master

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    One other aspect of the lapping compound is as it is used it breaks down so grit stays sharp and cutting becoming finer and finer in the process. SO depending on how long your polishing you end finish may be closer to fresh 600 or 800 grit. A swage or burnish die needs to hold lube to do it job. We sent out a burnish tool to be chromed ground it in and polished it with fine diamond. When done it took way more force to push it thru the brass bushings to size them up. The desired results and finish are not only a factor of grith but the polish lines should look like a figure 8 when done this produces less friction and when filled with lubes it flows in the desired pattern

  5. #5
    Boolit Master 15meter's Avatar
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    Quote Originally Posted by Valornor View Post
    Thatís what I was wondering. I had heard the same thing. Sometimes a rougher surface is better because it retains lube.

    My current approach has been to polish out the tools marks with 400 grit, harden, and then do a finish polish with 1,200 or 2,000 grit lapping compound.

    I am waiting until I have a few dies ready to heat treat, so I can do them all at once. Polishing takes a bit of time and patience, if rougher finish is better or ďGood enoughĒ then I donít mind leaving it at that.

    Thereís surprisingly little written online about when different surface finishes are needed, or when they are appropriate.


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    There are massive amounts of info on surface finish requirements, it's getting the search parameters correct to find it and deciphering it once you find it. There are more surface finish measurement formulas than Carter has liver pills. (sold and trained on electronic surface finish gages in one of my prior lives)

    A too smooth surface does not have, for lack of a better term "nooks and crannies" to hold lube. You end up with metal on metal contact. The scratches are literally little wells that hold oil/lube that the moving surface rides on. Too shallow scratches and there is no where for an oil reservoir.

    Unfortunately surface finish numbers are design parameters that will vary by MFG. I suspect the surface finish numbers for a cheap die are not as well defined as a custom bench rest die. They may be considered trade secrets, or you could call RCBS/Redding/Lyman/whoever and ask what grit they use to hone their forming dies. They just might tell you.

  6. #6
    Boolit Grand Master

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    That was the purpose on machine members being scrapped and flaked to square and true flat. The hills and valleys of the Flaking held oil and lubricated the machines thru travel. But those "gulls wings" on a mill or lathe sure were pretty

  7. #7
    Boolit Buddy
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    I just remember walking through the machine shop and seeing all the dies used on commercial billet presses. They were polished until they are a mirror finish.

    They were also carbide, and I know carbide is a ďslickĒ material compared to a tool steel. So maybe to lower friction coefficient of carbide allowed for a high polish, were as a tool steel you would want something to hold the lube.

    Iíll have to take a closer look at Corbinís dies. Ultimately I am building dies because I canít afford to spend the money on more Corbin dies. I probably should try and get an idea of what being done there and come close to it.

    If thereís no down side, and Iím getting good parts, Iím thinking the 400-800 grit finish maybe my ticket. Iíll remove the tool marks with 400 grit harden, and do a final polish with the 800 grit.




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  8. #8
    Boolit Grand Master

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    How much of that polish is done making them and how much is from 100,000 hits LOL. Our progressive dies after a run or two looked like they were chrome

  9. #9
    Boolit Buddy
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    Brand new, never been in a press


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  10. #10
    Boolit Master



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    Just curious, what compound are you using? I have Permatex gray auto which worked well on homemade casting molds. I too, have flitz which is extremely fine??. I'm guessing something in between should be added for core seating dies.

  11. #11
    Boolit Buddy
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    Quote Originally Posted by just bill View Post
    Just curious, what compound are you using? I have Permatex gray auto which worked well on homemade casting molds. I too, have flitz which is extremely fine??. I'm guessing something in between should be added for core seating dies.
    Hereís the brand. I ordered both 50% concentration.

    To prove the concept I used some Dremel Polishing compound. Works ok. But I donít have much confidence itís going to cut very well once I harden the dies.








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  12. #12
    Boolit Master 15meter's Avatar
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    Quote Originally Posted by Valornor View Post
    Here’s the brand. I ordered both 50% concentration.

    To prove the concept I used some Dremel Polishing compound. Works ok. But I don’t have much confidence it’s going to cut very well once I harden the dies.








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    Brings back bad memories of lapping in a granite plate with a hand lap, when it came in, it was out of spec and instead of sending it back it was faster to touch it up with a hand lap. It was a special order plate with a bunch of stainless steel inserts epoxied into bored holes for mounting fixtures to. Lead time on the plate was like 6 months and the shop boss said it wasn't out THAT much, we'll just back bill them.

    Of course he wasn't the guy with the lap in his hand.

  13. #13
    Boolit Buddy
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    Hopefully this stuff works as billed. Iím doing this all on a pretty limited budget, so the high dollar laps were out.


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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check