Hex bit shank conversion from straight shank to retained

[mbatarga]

I have a set of 7 hex shank bits that are nut driver style. The issue was they are straight shank and fall out of the driver whenever gravity exists.
As I recently got an estate sale purchase of a Craftsman lathe that I've posted on this forum previously

Recent vintage Craftsman Lathe purchase ...

I went to a local estate sale that I saw on the EstateSales.NET app Friday. Lots of "junk" and most of the power/hand tools were older and lower quality than what I usually would purchase. There was a Craftsman metal lathe (Model 101.21200) with lots of tooling that caught my eye - I spent over...

www.garagejournal.com

I thought it would be rather easy to machine a recess in the straight shank and make them retained in the driver. That was a false assumption I've posted a picture below of the machined bits where I've added the cove recess that closely mimics the radius which i measured on existing bits that work.
Anyone have any experience on this conversion (?) - as I can't get any of the 7 bits I've converted to actually be retained fully. I pretty much free handed the modification just based on measuring current working hex shank bits. None of the 7 revised bits worked in a Ridgid driver or in my Snappy hex shank chuck.

 

[KnurledNut]

@mbatarga
Is the round portion of the shank preventing the bit from fully inserting?
Other than that, the machine work is a little crude with tolerances varying greatly. The fit would probably be sloppy even if they did lock in.

I have done several myself, all freehand like this stubby nutsetter.

 

[whateg01]

Second that! Insert one of the factory ones and see how far it goes in. You'll need to turn down the round section to 1/4" or make it a 1/4" hex for that length.

 

[vjquan]

Shouldn't be difficult, it's just a couple of ball bearings locking into two opposite corners of the shank. I modified a hex shank with a round file with no issues. What is not working?

 

[mbatarga]

Okay - thanks to @KnurledNut's comment about the round shank section - I took another look at these in the shop today. I took one of the bits and turned the diameter down and tested it, turned it down some more - and realized the diameter has to be reduced almost the full length of the shank after the recess. It was not obvious that is where there was interference in the fit. Once that was done they all are now retained in either of the two chucks I used for fitment.

 

[KnurledNut]

Okay - thanks to @KnurledNut comment comment about the round shank section - I took another look at these in the shop today. I took one of the bits and turned the diameter down and tested it, turned it down some more - and realized the diameter has to be reduced almost the full length of the shank after the recess. It was not obvious that is where there was interference in the fit. Once that was done they all are now retained in either of the two chucks I used for fitment.

It would have been a little stronger and provided better centering to leave the hex portion in front of the notch instead of turning it all down. Like the one I pictured in my previous post.

 

[Qualitytools]

Okay - thanks to @KnurledNut's comment about the round shank section - I took another look at these in the shop today. I took one of the bits and turned the diameter down and tested it, turned it down some more - and realized the diameter has to be reduced almost the full length of the shank after the recess. It was not obvious that is where there was interference in the fit. Once that was done they all are now retained in either of the two chucks I used for fitment.

Great work, thanks for sharing.

 

[KnurledNut]

Here’s another one I did. Just put it in a drill and freehanded it.