Breaking shafts issue

For the Experts out there.....Ok, so I started to use a QR 3/8x10 flat minor pin on some break cues recently. I installed phenolic inserts that are 1.250" long at .560" dia. Drilled/bored to a .308 hole, then tapped with a tap with .308 pilot and the threads come out firm, but not tight. I have used brown linen and black canvas phenolic. most customers have had no problems, but 2 customers have bought them and they are the only ones that break in the 23-25mph range consistently. Both have come back within days with broken shafts, right at the collar...see pictures. The maple shaft sheared right at the collar, AND split the phen insert right down the middle, the purpleheart shaft split the wood slightly, but the phen still looks solid, just the part under the collar sheared off. Both sheared right where the threads stop on the pin, just by coincidence, I made the collar the same size as the thread length.
Now, has anyone else had this happen? I know that others use phenolic inserts without issues, is it the pin not being threaded all the way thru the insert, or is there something else going on? I love these pins, but can't have this keep happening.

Any insight is greatly appreciated,
Dave

I have thought that this maybe the case, I also did try to make the hole to be the same as the unthreaded pin, like the unilock is done, but I think the tap bored it slightly wider leaving a slight gap. I will try a smaller dia. like .500" and change the pin to a full thread and see how it goes. The first one, the maple shaft, I made a new shaft and just tapped the wood threads and it has been fine so far.
Thanks Mario,
Dave

Michael Webb said:

Your plug is to big for starters. 9/16 plug. 5/8 collar. Basically a .030 wall. Not good. What kind of glue holding them?

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The phenolic plug is .560" dia. 1.250" long glued in with West 205 with a 1/2" sticking out and a 1/2" thick phenolic collar bored to .560" then glued to the plug using West 205. The finished joint dia. is .850, so leaving a wall thickness of the maple/purpleheart to be around .140" per side, give or take a few thousandths, right where the wood meets the collar. I measured the brass inserts I have, and they seem to run around .450 ish, which adds .050 per side, but still doesn't sound like a lot. I may just go back to using the full threaded pins for breakers and just use these on the regular playing cues.
Dave

Wilk said:

Your second picture shows it all. The thread is only going into the ring section if the insert., and not the rest of the way. Thus you have 100% of the axial force of the break going into just the section of threading between the ring part of the insert, and the actual insert into the maple. I can't measure photos, but it looks like a 1/8 at best trying to handle all that force.

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It is actually 1/2" of threads, which by coincidence, just happened to be the same size I made the collars. I may try using a .300" thick collar, but not sure if that would be enough to solve the issue.
dave

Another thought, would carbon fiber or G10 be better insert materials?
dave

Thanks for all the responses guys. I will incorporate all this into the next ones. I went with just threading the new replacement shaft, and replacing the QR pin with a fully threaded pin.
My theory behind using a collar glued to the insert was to have a phenolic to phenolic joint to try to achieve as stiff of a hit as possible. The collar on the butt was a fully capped collar that I then drilled/bored and installed the pin, and tried to get almost the same on the shaft collar, and it seems, there lies the issue...lol.
Again, thanks,
dave