Shop-made Ball Turner - with a Question

[tjb]

I made a ball/radius turner by copying the one made by Ade Swash in the attached youtube video. I had a Criterion boring head that wasn't in the greatest condition, and had an opportunity to buy one that, after thorough examination, appears to have never been used (still in original case with sealed package of instructions and Allen wrenches). I decided to convert the old boring head into a radius turner for my lathe. The build turned out great (pictures below). My first project was the black Delrin ball on the end of the handle. I'm waiting on some round inserts similar to the ones Ade showed in his video, and I will need to make a holder after they arrive. I'm very satisfied with the result.

But I do have a question. At about 14 1/2 minutes into his video, Ade describes how he took the adjusting screw out of the boring head, drilled and tapped the end of it, made a spacer, and threaded a grub screw into it so that he could adjust the boring head from the opposite side. I tried a Plan B that was almost guaranteed to not work (it didn't) by cutting the threaded section off a socket head screw and epoxying it to the end of the adjusting screw. First time I used it, the epoxy gave. I was hoping to avoid trying it, but it seems the only feasible approach is Ade's. I soon discovered that the screw is very hard. Any tips on how to drill and tap a hole into that screw? (I posted a question on his video, but haven't heard anything back.) As he explains, adjustments are much easier from the off side, but I think I need some guidance on how to accomplish that in a hardened screw.

Thanks for any advice.

Regards,
Terry

 

[JimDawson]

Cobalt or carbide drill bit maybe?

 

[tjb]

Cobalt or carbide drill bit maybe?

I'd be willing to spring for one of those if that's what it takes. What about threading?

 

[JimDawson]

I would drill for 50% thread depth, and use a high quality spiral point tap. Maybe 2 of them, standard spiral point to start, and bottoming to finish.

McMaster-Carr

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[tjb]

I would drill for 50% thread depth, and use a high quality spiral point tap. Maybe 2 of them, standard spiral point to start, and bottoming to finish.

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

Thanks, Jim.

Not sure what you mean by 50% thread depth? I'm anticipating using a 10-32 socket head screw.

Regards

 

[markba633csi]

Most tap drills specify for a 75% depth of thread, but you can cut less by using a tap drill a size or two larger than the recommended size
-M

 

[JimDawson]

In that case a #18 drill (0.1695'') would give you 50% thread depth vs. a #21 drill (0.1590'') which would give a standard 75% thread depth. That way you are removing less material with the tap and the threads will still be plenty strong for the application.

 

[benmychree]

Can you file the screw? If not it is not likely that you could tap it.

 

[tjb]

Can you file the screw? If not it is not likely that you could tap it.

No. It's very hard. But it's the same screw that Ade describes drilling and tapping in his video.

 

[tjb]

Okay, Mark and Jim. I thought that's what you meant, but I wasn't sure. Earlier today, I mounted the screw on my lathe, made sure it was running true with a live center, attempted to drill with a #21, and it became very clear very quickly that wasn't going to happen. I'm using Hertel bits that I think are cobalt, but I'm not positive. Would carbide be better?