Custom Wrench Material | Practical Machinist

I have a need to design a custom wrench for my company's assembly line. My first prototype was made from 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.

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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. 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. 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. 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.

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• Tool on nut close up.jpg

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. 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 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.

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. 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.

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!