Sticky ebonite threads

[spindlecraft]

I've been making a few kitless pens out of ebonite lately – and I've noticed something with this material that I don't really see with other materials I've used. When I go to screw the cap onto the body, the threads get reallyyyyy tight and "sticky". I've included a video for reference.

I know that often times the threads of taps and dies start off smaller to get a clean start – so I am making sure to tap my threads beyond what I need, to make sure the threads are getting enough "bite". But beyond that, I'm not sure what else I could be missing. I am using plenty of lubrication when I tap the threads, and starting with the appropriate sized hole prior to tapping. And it only seems to be a problem with ebonite. When I turn acrylics like alumilite, it never seems to be an issue.

In watching the video that I've attached, you might conclude that the problem isn't the threads, but the unthreaded part behind the threads. However, I can verify that the threads really start sticking & feeling tight before it gets to the unthreaded section.

So, really, I have two questions.

1.) Any idea what is causing my threads to be this tight?

2.) Is there anything I can do to loosen them up a bit? I'd hate to have to scrap a perfectly good pen (with an expensive blank), just because you can't screw the bloody cap on.

Any help/feedback would be greatly appreciated.

Thank you!

 

[duncsuss]

Ebonite is sometimes called "hard rubber" - which gives a clue about some of its properties, one of them being that it kind of squirms out of the path of the cutting edge, only to snap back after it's gone by.

Result is that taps and dies remove slightly less material than if you were making the pen parts out of a less flexible material (such as acrylic acetate).

I leave as much material as I can in place before cutting threads, and drill holes a smidge larger, and make tenons a smidge smaller, than I would with acrylics.

I never drill the hole in the pen barrel before I cut the outside threads with a die (figuring that thin walls distort more easily than a solid rod); and I always screw a threaded washer onto the outside threads (I make these washers from delrin or acrylic) before drilling and tapping internal holes - again, to stop the walls from flexing out the path of the cutting tools. For pen caps, which only have an internal thread, I clamp the blank in a collett chuck before drilling and tapping, again to stop the walls flexing outwards as the tap passes by.

Finally - a dab of cutting/polishing compound (or diluted tripoli compound) on the threads then work the parts against each other for a while can smooth out any roughness.

Hope this helps.

 

[hooked]

It looks to me that the relief you cut into the back is maybe catching the cap threads? It sounded like in the video that the relief was rubbing on the cap threads. Possible? Maybe it just appears that the relief is above the threads when freezing the video.

 

[PatrickR]

I think you need to thread the barrel further. You have 4-5 threads and the first couple are probably shallower due to the chamfer on the die. Run the die up to the stop.

 

[Dannv]

Are you reversing the die after your initial cut os the threads aren't tapered?

 

[Penchant 4]

Ebonite is sometimes called "hard rubber" - which gives a clue about some of its properties, one of them being that it kind of squirms out of the path of the cutting edge, only to snap back after it's gone by.

Result is that taps and dies remove slightly less material than if you were making the pen parts out of a less flexible material (such as acrylic acetate).

I leave as much material as I can in place before cutting threads, and drill holes a smidge larger, and make tenons a smidge smaller, than I would with acrylics.

I never drill the hole in the pen barrel before I cut the outside threads with a die (figuring that thin walls distort more easily than a solid rod); and I always screw a threaded washer onto the outside threads (I make these washers from delrin or acrylic) before drilling and tapping internal holes - again, to stop the walls from flexing out the path of the cutting tools. For pen caps, which only have an internal thread, I clamp the blank in a collett chuck before drilling and tapping, again to stop the walls flexing outwards as the tap passes by.

Finally - a dab of cutting/polishing compound (or diluted tripoli compound) on the threads then work the parts against each other for a while can smooth out any roughness.

Hope this helps.

The washer of acrylic or delrin is a "Eureka" idea!

 

[Penchant 4]

Lots of good suggestions above!

One that you might also attempt, if you are using an adjustable (round) die to cut the threads on the barrel/body, is to back out the adjusting screw to reduce the diameter of the die. When threads are too tight for me, this is the first method that I try in order to correct the issue. Do it by small...small increments...not more than 1/8 of a turn at a time, re-cutting the threads with each adjustment.

Of course, if the adjusting screw is already backed out completely, then this suggestion will not be helpful.

 

[Paul in OKC]

Couple of thoughts. It seems to act like either the cap is not threaded deep enough, the counterbore in the cap is not deep enough, or the tenon is not threaded back far enough. Hard to tell from a video and not seeing all the parts clearly.

 

[spindlecraft]

Thanks for the tips all! Going to give this another go this weekend, and see if I can't remedy the situation.

 

[Psychmike22]

Ebonite is sometimes called "hard rubber" - which gives a clue about some of its properties, one of them being that it kind of squirms out of the path of the cutting edge, only to snap back after it's gone by.

Result is that taps and dies remove slightly less material than if you were making the pen parts out of a less flexible material (such as acrylic acetate).

I leave as much material as I can in place before cutting threads, and drill holes a smidge larger, and make tenons a smidge smaller, than I would with acrylics.

I never drill the hole in the pen barrel before I cut the outside threads with a die (figuring that thin walls distort more easily than a solid rod); and I always screw a threaded washer onto the outside threads (I make these washers from delrin or acrylic) before drilling and tapping internal holes - again, to stop the walls from flexing out the path of the cutting tools. For pen caps, which only have an internal thread, I clamp the blank in a collett chuck before drilling and tapping, again to stop the walls flexing outwards as the tap passes by.

Finally - a dab of cutting/polishing compound (or diluted tripoli compound) on the threads then work the parts against each other for a while can smooth out any roughness.

Hope this helps.

I agree with everything here. I'm going to try the polishing compound trick. It's like lapping the threads! When this happens to me, I drill a hole in a piece of wood to the outside diameter of the cap. Then, with the cap inside the wood hole, I tap the cap again by hand this sometimes cuts out that little bit of extra that squirmed away without allowing the cap to stretch.