Threading problems for Kitless pen

[Tiny James]

Good afternoon all

I'm having a bit of trouble with my threading Acrylic pen blanks.

Size 14mm cap and barrel thread

So I drill with a 12mm drill first to the full depth required to accept the Nib and external thread of Barrel then I drill with a 13mm drill to the depth required for threaded acrylic barrel plus little extra. I place tap Starter and then intermediate into drill chuck add some oil and then pushing on the back of the tail stock holder (banjo I think it's called) and thread my cap I the turn a concave to the end so it will fit with out any obstructions I then turn my attention to the barrel I turn to 14 mm and add my sliding die holder from Beaufort ink fitted with a 14x1 die (writing facing outwards) I then tighten the center grub screw ( as far as it will go) as this acts as I starter thread, I then thread my 14mm tenon. I then untighten my die and tighten the outer two grub screws until they just hold the die in place and finally turn the die around so the writing is on the inside of holder and finish off the threading.

But the trouble I have or think I have is the cap and barrel feel quite lose until it hit the tenon shoulder.

I forgot to say I chamfer and add oil to help thread, I also remove some thread near the tenon shoulder.


So how can I improve the fit of my Cap and Barrel.

James

 

[21William]

Are you using the correct size tapping drill for the thread you’re cutting?

 

[Tiny James]

Yes I think so 14x1 tap drill 13mm

tap and die bought as a set. Should I try a smaller drill bit or turn the barrel a bit bigger say 14.2 ?

James

 

[Phil Dart]

You seem to be doing all the right things James. A few observations from me - a check list really, in vaguely the right order to check them.


A loose fit is subjective. It may be that you are expecting a certain amount of resistance as you screw the two parts together, whereas a sliding fit will have (or should have) no resistance.

Carefully check that your headstock and tailstock are properly aligned. This is the biggest cause of sloppy threads, because the female is likely to end up larger than expected, and the male smaller, and both out of round.

Allow the tailstock to slide forward when tapping, with a bit of assistance from yourself. The impetus for travel in the tailstock should be from the drag caused by the advancing of the tap into the hole, rather than by being pushed. You have to help it, because its a heavy thing to drag, and will otherwise bugger up the thread you're trying to cut, but assistance is what it needs rather than impetus from your hand.

Make sure that your drill chuck is holding the tap by the round part of the shaft, not the square part at the back end.

Double check with callipers that your tap and die are indeed what they say they are. The tap diameter should be marginally larger than the diameter through the middle of the die, but not vastly so.

Tighten the holding screws on your die holder to a bit tighter than "just holding". That will stop the workpiece itself from forcing the die more open than it should be.

Don't drill your hole any smaller - it will just cause the tap to tear our the material rather than cut it. Do check the actual size of your drill bit with callipers though.

Don't make your tenon any bigger (for the same reason as above). If anything, make it a shade under 14mm, to eliminate any risk of tear out. Again though, do carefully check with callipers that your sizing is correct before introducing the die to the tenon.

 

[Curly]

Good afternoon all .................... I then tighten the center grub screw ( as far as it will go) as this acts as I starter thread, I then thread my 14mm tenon. I then untighten my die and tighten the outer two grub screws until they just hold the die in place ............James

I think the problem may be with you mucking with the die screw. That screw controls the fit of the thread, the final size. By opening the die you change the "factory" setting (the class of fit) and when you release that screw the die closes more than it is supposed to, cutting a smaller thread because the screw should have some tension on it. How much depends on what class of fit was required. Squeezing the die with the holder screws makes it even smaller. If you turn the outside diameter of the barrel down to 13.8mm, give or take depending on the thread condition when done, you should with all but the most brittle materials, be able to cut the threads in one go. If the die screw is set correctly you can tighten the die holder screws enough to hold it properly without collapsing the die.

To get the die back roughly to where it should be tap a hole in a piece of scrap to try the test threads with. Make test threads with the die on a some material (something like easy to cut like alumilite rather than a brittle acrylic) turned to 13.8mm and with the die screwed open, cut a thread. Try it in the test hole. If it fits to your liking stop. If too big or tight turn the screw out a bit and recut the thread, checking again. Repeat the process until you have a fit you think is right and then cut a fresh thread on another turned test piece. If the cut in one pass thread fits properly stop, and don't play with the screw again.

The reason for turning the barrel thread to 13.8mm is so the die isn't cutting a full thread possibly tearing the tops off the thread. It leaves a cleaner, slightly flat topped thread. With different materials you will learn how much to turn the barrel thread under the nominal to get good threads. Say 13.85mm for easy cutting material and 13.75mm for a more brittle material for example. Experience will teach you what works for most threads.

If you really want a "starting die" then get a second one and use it for final sizing keeping your original die opened as a starter. Then you don't need to go through the process of reseting it.

Pete

 

[21William]

Apart from the solid ones I’ve come across two different types of split dies, do we know for sure which type he’s using? Just checking.

 

[Penpal]

Surely well served fellas thanks for your insight and knowledge.

Peter.