Wire EDM high performance gears
Close
Login to Your Account
Results 1 to 7 of 7
  1. #1
    Join Date
    Aug 2016
    Country
    SWEDEN
    Posts
    27
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    4

    Default Wire EDM high performance gears

    Hi,

    I'm about to make a low serie of performance gearbox gears. And my question is to you gear mfg guys out there. If there is a better way than for example wire edm the gear in EN36 steel and then send it out to case hardening, and then shootpeen it. With todays special steels would it be possible to use a more expensive steel that could be through harded before the wire edm and that way have better precision due to no distortion from hardening process?

    Don't say gear hobbing shaving etc. This is not about time. its becouse I have a decent makino wire edm and time is not a problem. And the material price is not very sensitive either.

    How much distortion would a case hardning do? What class gears should I expect? I mean this way it shuld be as good as hobbing and then case hardning. My idea was to shootpeen them to avoid cracks from the edm surface.

    Anyone with input on this?

  2. #2
    Join Date
    Jul 2006
    Location
    Hillsboro, New Hampshire
    Posts
    7,394
    Post Thanks / Like
    Likes (Given)
    1743
    Likes (Received)
    5140

    Default

    There's guys here with much better gear knowledge than me, but I'll give this a shot...

    With a HiPo gear, what's going to matter is tooth profile accuracy and final fatigue properties of the gearset. The profile accuracy sets the efficiency of force transfer and ultimate load capacity, the fatigue life sets how long the forces can be transmitted before tooth failure.

    With EDM there's usually a risk of mechanical or metallurgical damage to the surface of the part, this tends to be less with with newer, more sophisticated controls. But unless you've got a new machine with such capacity, you'll have a white or HAZ layer on the tooth profile that will degrade the metal surface to some degree. This will lower both load capacity and fatigue life.

    How accurate the tooth profile is will be a function of how well you set up the blank (perpendicularity to wire path) as well as the movement accuracy of your machine. If it's new and well maintained, you should get micron-level control of profile. Worn, damaged, or otherwise impaired means less tooth uniformity. Non-uniform teeth means lower efficiency, more risk of tooth failure, and less load capacity.

    As to alloy steel choice and post treatment, hopefully others will advise. Precision shot blasting of the teeth can help from a improved fatigue standpoint as well as potentially helping with oil film retention, but may not make up for EDM-caused damage to the metal.

  3. #3
    Join Date
    Jun 2002
    Country
    CANADA
    State/Province
    British Columbia
    Posts
    2,543
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    1981

    Default

    Good morning panscan:
    The step that will create the most problems for you is the case hardening step, because of the distortion it produces.
    It doesn't matter how you cut the gears; as soon as you harden them they will move.
    Traditionally, this is dealt with by grinding the gears after heat treat; and if you want both case hardening AND precision, you cannot eliminate the grinding step if your goal is an accurate gear.

    Thankfully there is an alternative, and that is to through harden a tool steel plate, and then cut the gear teeth after hardening.
    Of course, as you know the wire EDM won't care that the plate is hardened; so long as you deal adequately with stress release as you wire cut the profile and the bore, you will get a very accurate gear; as accurate as a ground gear and provided you give it the correct profile to follow (another whole subject), you will get a gear that performs well as Milland points out in post #2.

    What you will not get, is the same surface finish as a ground gear and that may become a problem if the gear must perform under extreme conditions, as Milland also points out.
    The other thing you will obviously not get is the hard case and soft core of a case hardened and ground gear, and there are desirable physical properties that come with that process compared to a tool steel gear that's been through hardened.
    The last thing you will not get, is the economy of case hardened and ground, but for low numbers of gears who cares!
    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

  4. #4
    Join Date
    Jul 2006
    Location
    Hillsboro, New Hampshire
    Posts
    7,394
    Post Thanks / Like
    Likes (Given)
    1743
    Likes (Received)
    5140

    Default

    Quote Originally Posted by implmex View Post
    Good morning panscan:
    The step that will create the most problems for you is the case hardening step, because of the distortion it produces.
    It doesn't matter how you cut the gears; as soon as you harden them they will move.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining
    Marcus, if he roughs the blanks (say, to within 0.5mm of finish geometry) then has them case hardened, would the risk of variations of case depth be too great, even if actual geometry after final wiring is good?

  5. #5
    Join Date
    Jun 2002
    Country
    CANADA
    State/Province
    British Columbia
    Posts
    2,543
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    1981

    Default

    Hi Milland:
    The difficulty with what you propose, is that he creates a helluva fixturing problem for himself if he roughs the blanks, then cases them and then has to find a way to set them back up on the wire for finishing.
    If he cuts all the way around the periphery of the gear in the roughing, he has to somehow capture the roughed distorted non-round gear in a fixture, pick up the tooth orientation and then finish wire the tooth flanks and the bore.
    If he leaves it tabbed on the plate from which it was cut, and finishes the teeth then cuts the tabs he ends up with a discontinuous case right at the tips of two teeth...right where they can flex and potentially crack the case off the core.

    The only way he could make this work, (and it would be a lot of work) would be to rough the blanks and case them as you suggest, then hard turn or grind the tooth tips and the bore to make them round again, then fixture the part on the tooth tips, pick up the tooth orientation, write and run a new "tooth-flank-only" program that cuts a bit past the tips of each tooth into the jig but not around the tips themselves, and hey, presto we're done, but at the cost of a wire break and re thread at each tooth space.

    That's a lot of screwing around, but yes...it can be made to work if cost is no object.
    With your comment regarding case thickness variation...yes there will be variations, but that is also true for cased and ground gears, and appears to be no impediment to their performance, so long as you don't go right through the case anywhere.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

  6. #6
    Join Date
    Jul 2006
    Location
    Hillsboro, New Hampshire
    Posts
    7,394
    Post Thanks / Like
    Likes (Given)
    1743
    Likes (Received)
    5140

    Default

    Good points, Marcus.

    With newer machines, shouldn't it be possible to finish the bore (however fixtured), then clamp from there and pick up a few teeth through capacitive sensing (or however it's done), then macro the path to correct it?

  7. #7
    Join Date
    Jun 2002
    Country
    CANADA
    State/Province
    British Columbia
    Posts
    2,543
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    1981

    Default

    Hi Milland:
    The problem with fixturing from the bore is that the lower head that guides the wire is connected to an arm which must be able to pass under the part.
    So the part needs to be "hung" into the worktank.
    Similarly, the upper head must be able to pass over the part.

    So if you imagine what the fixture must look like to clamp from the bore, you can see the problem.
    You can cut the teeth from the 9:00 position through the 12:00 position to the 3:00 position but you run into trouble as you approach the 6:00 position, so you must either spin the part or be able to access everything without interference, and that typically means making a holding fixture that in this case would be simply a plate hung from one or two edges of the table with a hole the size of the gear OD accurately wire cut into it, and a split so you can expand the bore enough to slip the part in, then collapse it again to clamp the part.

    Once you've dropped in your part, you can interrogate it with the wire to find out the radial orientation of a tooth, then write your code for finishing with that radial orientation factored in (or else just rotate it in the machine control)
    Often the whole path is coded, but sometimes it's better to simply code one tooth and loop the code for each tooth in succession...I've done it both ways successfully.
    Important to note that the finishing code is NOT the same as the roughing code with a new wire offset...it has to be completely different code because it cannot be allowed to traverse across the tooth tips...that's the parts you're using to hold the gear into the fixture.
    So you get to write code twice in order to make your part.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

  8. Likes Milland liked this post

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •