Help with Thread cutting

[Tangoman]

G'Day Folks and Happy New Year to you all !

I need some help with cutting the external threads on pen bodies where the cap screws onto.

I got a lovely Christmas present from SWMBO of a 13mm triple start tap and die. I've successfully cut threads on harder type plastics i.e the cheaper swirl types - in fact no problems at all cutting this material !

But I really want to use some beautiful blanks that I have that seem to be made from a very different material. I cant say for sure exactly what type of material it is, they are from Bruce at Woodnworkshop. This just cracks and splinters into pieces when I try and cut the thread ( tapping the internal hole is a lot easier with only very little cracking).

I turn the tenon to just below 13mm (approx12.9mm), lubricate it with either WD40 or engine oil and it just splinters or cracks off pieces when I try and turn the die. I tried using a die holder as well as handheld die wrench but the same result each time.

What do I need to do ? What am I doing wrong ?

HELP !!!!

Regards,
Cam

[dai sensei]

The product is probably a very brittle polyester cast. Not all polyester casts are like this but it is easy to get one that is very brittle due to the amount of catalyst used.

Not sure if you die is adjustable, if so, perhaps try loosening it to creep up on the thread.

Hopefully Big Shed will add comment as he has the experience in threading

[artme]

Would it be better to cut the thread while the blank is still solid and drill later??

[QC Inspector]

First off congratulations on having a wife with such good taste in gifts.

As already stated it could just be the blank material. While I have a 13mm set too I haven't used them yet but have done so with my 12mm and 14mm sets. You might try with the barrel turned to 12.6 or so and see how you get on. I use cooking oil or spray rather than the WD40 or worse the motor oil. Petroleum based products tend to craze (surface cracks) plastics down the road so I keep them off mine. Ammonia also is bad for plastics so I avoid any cleaning with that too. I'm not sure what is in the WD40 but it does have solvents of some kind and they cause crazing too.

If you are cutting the 13mm threads over the section threads the wall thickness might also be giving you grief depending on the size. Sometimes cutting the outside thread while solid (already mentioned) and then threading on a previously made collar (brass or thicker soft plastic) to provide support for the drilling and threading of the inside section thread portion will work. That also helps with section and feed threads if making a fountain pen.

Although not likely, it is possible that you haven't noticed that the die has a side with the treads ground back a little to help start the cutting. A slight chamfer cut at the start of the barrel also helps get the die cutting.
One more thing, more of an issue for me this time of year, is that the plastics work better when they are warm.

Keep playing and it will come together.
Pete

[Tangoman]

Fella's thanks for your input.

I think it's just a brittle blank - shame it's a stunner when polished!

I've poured a couple of blanks using 2/3rds of the recommended catalyst (Diggers) and I'll turn them to see if its made a difference as my previous homemade blanks where also very brittle. Might have to get more serious about blank making !

Cam

[salty72]

Fella's thanks for your input.

I think it's just a brittle blank - shame it's a stunner when polished!

I've poured a couple of blanks using 2/3rds of the recommended catalyst (Diggers) and I'll turn them to see if its made a difference as my previous homemade blanks where also very brittle. Might have to get more serious about blank making !

Cam

Those blanks can be difficult to thread - the trick is not to drill first and don't cut full threads
ie if your using a M13 die make sure your diameter is smaller then recommended try say 12.5mm
also as suggested before expand your die to cut over size and then recut the thread to the correct size if possible
since your using a triple start the threads should be fine and this will also help - course threads are deep and remove more material causing greater chipping also I found that when you cut the threads don't go tooo far before backing off that 1/4 turn to break the chip this will also crack the blank..sometime I will only cut 1/8-1/4 of a turn before clearing out the swarf .. Veggie oil is a great lub and spray Version is better
good luck and don't forget to post photos ...