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  1. #1
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    Default Custom Wrench Material

    I have a need to design a custom wrench for my company's assembly line. My first prototype was made from 4140 steel, hardened to RC 50-53 and black oxide coated. I thought for sure this would be strong enough as it the same material we use for other custom wrenches. However due to the high torque load on this wrench (220 ft-lbs), it broke after just 3 uses. See attached picture. Unfortunately, due to the size constraints of the product, I cannot significantly change the geometry of the wrench in order to help with strain relief. Anyone have any suggestions of another material that may be more durable for this application? Obviously we want to keep the cost to produce relatively low as well.
    Attached Thumbnails Attached Thumbnails 20210726_082815.jpg  

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    Find you chrome-vanadium steel such as a grade AISI 6150. After making the wrench, Q & T to around 40-45 HRC. Ken

    Edit: May try A-500 plate, still Q&T to around 40-45 HRC. Your original Heat treat is too high, make things too brittle. Your after toughness not hardness.

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    Looking at the photo, the fracture started at a sharp corner, which is what I expected before even clicking on the thumbnail.

    Sharp corners like that are known as stress raisers and are often the starting point for fractures. You need to relieve the corners with a small radius. If you look at most six point wrenches made today you'll see what I mean. While high volume manufacturers grind their broaches to do this, for small volume the best way is to mill into the corner with a small diameter end mill until the sharp edge is gone. Then temper for toughness as suggested above and you should be fine.

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    I concur with 4GSR on tempering to a lower hardness.

    Some years ago I was futzing with making some replacement Maserati valve shims so had an original tested for hardness. I figured it to come in somewhere in the 50's.. To my surprise it barely touched 50 Rc on the scale. I remembered then reading something about one of their race cars having trouble with valve shims cracking. They don't need to be that damn hard to do the job, but you sure as hell don't want them breaking altogether.

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    In a case like a valve shim, you want some reasonable core hardness with a further hardness at the surface for wear resistance if it's directly taking cam lobe wiping. So a core hardness in the upper 40's to 50 with a skin hardened by nitriding into the 60's might be ideal. Nitriding usually gives additional anti-wear properties when the right steel is used.

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    It might just be the photo angle, and you don't state the size of nut, but the jaws look really shallow on that wrench to handle 220lbs. If you're trying to turn a nut that's 1/2" across the flats with an open end wrench....never gonna happen.

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    If a standard size. 1. Impact socket 2. Box wrench 3. Flare nut wrench. If the space is too tight for a hex wrench switch to a socket head cap screw. At 220 torque must be 3/4 or larger screw.

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    Thanks everyone for the feedback. I really appreciate the advice! I will try a chrome-vanadium steel and heat treat to a lower hardness rating.

    In terms of the radii on the wrench end, they are currently at 0.025" I will try to increase those as well for some more strain relief.

    For those of you asking about the size constraints see the attached pictures. Basically our product engineering designed a part in which the nut is impossible to get to with an access hole that is not centered over the nut location. Currently our operators use pry bars to stop the nut from spinning while they torque the bolt. Not ideal!

    Edit: I guess I should clarify too, in the second picture I made the one wall of the part invisible so you can see the nut straight on. The only way to access the nut is through the oval access hole.
    Attached Thumbnails Attached Thumbnails close-up.jpg   tool-nut-close-up.jpg  

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    What about, forget about the wrench --- How about a custom nut shaped to fill the void / key into / jam against the "V" shaped recess?

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    Quote Originally Posted by AckshunW View Post
    What about, forget about the wrench --- How about a custom nut shaped to fill the void / key into / jam against the "V" shaped recess?
    Or.....make the mating part with a flat for the standard nut to set against.

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    Quote Originally Posted by cmax View Post
    Thanks everyone for the feedback. I really appreciate the advice! I will try a chrome-vanadium steel and heat treat to a lower hardness rating.

    In terms of the radii on the wrench end, they are currently at 0.025" I will try to increase those as well for some more strain relief.

    For those of you asking about the size constraints see the attached pictures. Basically our product engineering designed a part in which the nut is impossible to get to with an access hole that is not centered over the nut location. Currently our operators use pry bars to stop the nut from spinning while they torque the bolt. Not ideal!

    Edit: I guess I should clarify too, in the second picture I made the one wall of the part invisible so you can see the nut straight on. The only way to access the nut is through the oval access hole.

    You need better engineers.

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    Quote Originally Posted by digger doug View Post
    Or.....make the mating part with a flat for the standard nut to set against.
    I wish! Unfortunately, as an manufacturing engineer I am at the mercy of what the product engineers design. It normally doesn't make sense in terms of manufacturability.

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    12pt or spline drive nut with it's smaller head may lead to a better wrench design?

    Added bonus- they cost more. You can prepare a quick email saying I'd like to solve this by using a different nut that will accept standard (hopefully) wrenches. It will cost an additional $/unit.

    That may lead them to re-evaluate the design.

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    Quote Originally Posted by cmax View Post
    I wish! Unfortunately, as an manufacturing engineer I am at the mercy of what the product engineers design. It normally doesn't make sense in terms of manufacturability.
    So the engineers don't submit their design to you for "producibility review" first ?

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    Quote Originally Posted by tdmidget View Post
    You need better engineers.
    Yeah, I agree sometimes what is designed boggles my mind.

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    Quote Originally Posted by digger doug View Post
    So the engineers don't submit their design to you for "producibility review" first ?
    They do, but sometimes when we question things their response is "well that's not our problem figure out how to build it." This product change however was completed long before I was with the company. Nonetheless I have already pushed for a product change on this, my assumption is nothing will happen. Hence the custom wrench to save our operators time and risk of injury.

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    Quote Originally Posted by digger doug View Post
    So the engineers don't submit their design to you for "producibility review" first ?
    I wish there were a "funny" mod on this forum.

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    If the handle was removed could you slip in a round OD Hex box wrench, just the head through that opening?..with it having a serrated OD, yes notches all the way around, insert it with a magnet might be good.

    Then insert a spanner-type wrench with a notch catch end through the opening to catch the serrations (notches).

    With the spanner having only one side it could be made more heavy duty on that one side.

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    Quote Originally Posted by michiganbuck View Post
    If the handle was removed could you slip in a round OD shaped Hex box wrench, just the head through that opening?..with it having a serrated OD, yes notches all the way around,

    Then insert a spanner-type wrench to catch the serrations (notches).

    With the spanner having only one side it could be made more heavy duty on that one side.
    I will look into that as an option. I am not certain it will be possible with how tight the part is and the offset from the access hole to the nut itself. But worth a shot. Thanks!

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    With notches, you would only need perhaps a 1/8" draw/turn for each tightening movement so it should work.

    by AckshunW View Post
    What about, forget about the wrench --- How about a custom nut shaped....

    Like a round-shaped nut with notches at the OD ..and then use a catalog spanner wrench.

    Suggestion reward $20. each to me and Ackshunaw.


    oh, Having square notches so it might be tightened and loosened.


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