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Thread: What steel to use for expander?

  1. #1
    Boolit Master

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    What steel to use for expander?

    I have a set of .38-55 rcbs cowboy dies. Neither expander is large enough to accommodate my 0.383 bullet.

    I am a new lathe owner and am considering turning and threading a custom expander.

    What steel is appropriate? A2 tool steel? Stainless? Should I flame heat treat it?


    Edited I posted in gunsmithing as I am likely to find more lathe experience here than in the reloading equipment forum.

  2. #2
    Boolit Master
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    You can use low carbon steel and case harden it with this stuff:
    https://www.brownells.com/gunsmith-t...aspx?rrec=true
    I've made small wood gouges/scrapers out of cut nails and used that stuff...
    It's what I'd do,

  3. #3
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    A2 would work just fine if you already have it , if you need to buy material I would get 02 instead .
    The air hardening tool steel can work harden fairly easy if you aren't careful . The oil hardening is much more beginner friendly , plus it seems to give a very slightly better surface finish .

    Both would likely outlive you without hardening

  4. #4
    Boolit Grand Master

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    I have turned up expanders from 4140 pre hard, A6, o1, S7, Allen cap screws, and made one with a carbide pin soldered in. All worked well even cold roll left soft will last a long time here. I find finish is more important than hardness. A good smooth polished surface will hold up a long time and reduces friction also.

    If you want to harden it then leave it .010-.015 large with a false center on the end so it can be finished after hardening. use carbide to turn to +.002 and finish with fine sand papers to a chrome like finish. When finished cut off the false center.

  5. #5
    Boolit Grand Master Nobade's Avatar
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    I use a lot of 1144 for that. Cheap, machines well, and easy to harden if you feel the need. Those expanders don't need to be hard though, they last a long time anyway.

  6. #6
    Boolit Master

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    Thanks guys.

    Finish on the surface with stainless I thought might be a bit tougher to get smooth.

    False center. Good idea.

    I have been practicing threading on scraps just to learn the techniques better. I last did lathe operations in the 80’s.

    Turning on unknown mystery scraps is a bit hit and miss on finish looking for the right speed and feed.

    I was wondering if hardening was necessary. It’s brass it’s contacting after all.

    You have been most helpful.

  7. #7
    Boolit Buddy Reverend Recoil's Avatar
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    I have turned similar parts from automotive cylinder head bolts. I do no know for certain of the alloy of manufacture but they are strong, tough, wear resistant, and cost nothing.
    DRB #2276 President's Hundred 2021

  8. #8
    Boolit Master

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    And if used they’re heat treated.......a lot.

    I am prone to recycle bits myself.

  9. #9
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    Nobade is giving you good advice.
    You don't have to over think or engineer this. Mild steel will outlast us. I made a 0.409" expander about 20,000 - 24,000 rounds ago from 12L14 and it is still giving the exact same diameter necks as the day I made it.
    The use of hex stock makes up a nice nut on top that you do not need to knurl like round stock needs.
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    Last edited by Chill Wills; 09-19-2018 at 10:09 PM.
    Chill Wills

  10. #10
    Boolit Grand Master

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    For the easiest turning a false center on each end, this allows for the pin to be turned between centers with a small lathe dog. The expander leads and flare section along with the threads and head can be turned in one setup start to finish. For small parts we had the welder tig weld a small dia rod to the head for a dog. Get the lather running straight and turn the head dia then the thread dia here you can thread now or wait to thread after other dias are done. turn expander end and belling angle if one is desired. You can also chuck round stock long and do this. One trick when turning the expander end is to use HSS cobalt tool bit and grind it with the front angle at the angle you want the bell. Simply turn with this tool to length the angle is formed as the part is cut and a perfect blend from expander to bell is achieved.

  11. #11
    Boolit Master

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    I am taking all these in. The experience shown certainly kickstarts my thinking like a you put a jato bottle on my head. Even carpentry i stare at things a while. *(JATO jet assisted take off)

    I measured my A2 round stock is too thin. So picking up some hex stock is simple. I saw some at fasten all, they’re pricey but I drive by several times a week.

  12. #12
    Boolit Master

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    Quote Originally Posted by Chill Wills View Post
    Nobade is giving you good advice.
    You don't have to over think or engineer this. Mild steel will outlast us. I made a 0.409" expander about 20,000 - 24,000 rounds ago from 12L14 and it is still giving the exact same diameter necks as the day I made it.
    The use of hex stock makes up a nice nut on top that you do not need to knurl like round stock needs.
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    Ok I am convinced heat treating is excessive for the job at hand. I wasn’t sure. I don’t know all that I don’t know!

  13. #13
    Boolit Master

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    I have regular Hss tool blanks. I have ground one cutting tool so far. It wasn’t too hard. I also have 1/2” carbide Chinese set with replaceable triangle 60 degree bits.

    Grinding a cutter with the bell angle leading edge is a doable project.

  14. #14
    Boolit Master


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    I have used grade 8 bolts of the proper thread, 9/16-18 I have think it is to make expanders.

  15. #15
    Boolit Grand Master

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    Another nice touch on a expander bell stem is a witness lone on the top. this allows you to see and count revolutions easily when adjusting. if 16 threads per inch the expander moves .060 per revolution. The mark makes seeing this easier

  16. #16
    Boolit Grand Master

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    Grade 8 bolts ae good material as are allen head bolts. I have used allen set screws or odd projects but most of these require carbide to cut. Hex stock is quick and easy to machine and gives a good surface to hold on to. The other big thing is to use you file, break all the sharp edges and remove burrs from the piece.

  17. #17
    Boolit Master
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    I will second 12L14.

    Odds and ends of scrap are ok for some things and practice. But I prefer a known alloy for things I deem important and don't want to have to do again.

    I also had not messed with a lathe since the 80's. Messed around around trying to thread on some unknown round stock. Threads were ****. I did a whole lot of checking and measuring(lots of time), cut again on the same piece of scrap stock, same result. Went to a piece of known alloy,mproblem solved.

  18. #18
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    I made some out of grade 8 bolts for my "M" die. I put a nut with an Allen lock screw on it that way I can use the same die body with several different expanders (also do the same for compression dies).

  19. #19
    Boolit Master

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    Great thoughts. I have done some similar little projects with other stuff. Use a bearing collar with the grub screw replaced with a cap head screw with the tip turned down for a sight pusher.

  20. #20
    Boolit Master

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    Quote Originally Posted by country gent View Post
    Another nice touch on a expander bell stem is a witness lone on the top. this allows you to see and count revolutions easily when adjusting. if 16 threads per inch the expander moves .060 per revolution. The mark makes seeing this easier
    I do this with a paint marker on my dies. A yellow stripe on the side to a reference mark on the press. Mostly for accidental loosening and loss of adjustment.

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