Made a custom tang screw — three times

While doing a repair on an odd-shaped part that was hard to hold in the lathe chuck, I wished for a tailstock toolholder. ER40 by preference, since I already have ER40 on both ends of the spindle.

I have an MT3 tailstock, so I bought an MT3 ER40 adapter. However, it was meant to be for a mill, so it was intended to work with a drawbar. What I really wanted was a tang, so that when I retract the tailstock it pops out. Seemed easy to make a tang screw, and in the end, it was:

However, the path to a working part was longer than it needed to be. My first mistake was trying to make it before the adapter arrived. It was listed as taking a 3/8"-16 drawbar, so I made that.

Then when it arrived, I found that it didn’t fit at all. The ID of the hole was right for a 1/2" thread, not 3/8". I found that a 20TPI gauge fitted comfortably, so I made a new one with a 1/2"-20 thread. But that only screwed in about a turn and a half, and then stuck. Weird, what’s wrong?

I used a q-tip to put prussian blue on the threads, then used a slip of paper inside the bore to try to impression the threads, and I still found lines that matched the 20TPI gauge. What‽

Eventually, I got a hint to try using hot glue to impression the threads. Now I could see the mucked-up threads well enough to see that the 13TPI gauge would also fit. As far as I can tell, it was partially tapped 1/2"-20, then re-tapped fully in 1/2"-13. (That plus the 0.6mm runout makes me think this was a factory second… No way that is acceptable for a mill!)

I found that I could run the 1/2"-13 tap into the body, so I finally made the third tang screw with that thread, which finally worked.

Here’s the family, in order of making, upper left to lower right:

With the adapter:


Now when I insert the adapter into the tailstock, retracting the tailstock causes it to pop out. This wasn’t true before I added the tang.

And I have several other MT3 chucks and centers that have screws but don’t have tangs and sometimes are hard to remove. I think I might make more of these (but with the ability to easily copy the male thread of the existing screws) and outfit the rest of my tools. :smiling_face:


Looks good and always nice to see trial and error. Not to say that I’m never prone to overthink things or make things more in depth than required, but… Would you have been able to try a few different taps in the hole to find the thread size rather than fiddling around as you did?

The 1/2"-20 tap seemed to work at first, and seeing grooves that measured at precisely 20TPI made me not even seriously consider 13TPI as an option until I got the hot glue with the thread form out and could see it.

Once I had that, yeah I ran the 13TPI tap down it, cleaning up the bad tapping, then while removing the tap found out the hard way that the ER40 nut thread M50x1.5 on the other end hadn’t been finished and had some razor sharp bits sticking out of it. Thank goodness for tegaderm!

If the vendor gives me the promised 50% discount in lieu of return, I’ll have just about gotten what I paid for, I think, given the ~2.5 thou runout…

You may not get what you pay for, but you rarely get more than you pay for! :grin: