Acrylic chipping on threads

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PatrickR

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I have been having a problem threading acrylic. intermittently i get chipping on the threads (using a die). I have never had this happen with alumilite or ebonite. Anyone else seen this? I think it could be the temperature. I need to test that theory.
 
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rixstix said:
I've had trouble with some Berea AA blanks actually being Poly.
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I have found the only way to be sure is the smell when machining it. This is acrylic. A little research shows I may need to undersize the tenon more than usual, switch to a soap and water lubricant and go slower.
 
Usually in the pen turning world, "acrylic" frequently gets used synonymously for the word "Plastic" and not in reference to a specific material or polymer based on Acrylic acid. Generally speaking, the two materials you named, Alumilite and Ebonite can be less brittle than several of the other typical pen blank plastics.

Ebonite is a brand name for elastomeric polysulfide or styrene-butadiene rubber, materials made by vulcanizing natural rubber. It usually contains a relatively high percentage of sulfur and some linseed oil. It got its name because the early use was as a replacement for ebony wood.

Alumilite that is used to make pen blanks is a urethane rubber based resin. Urethanes sort of fall in between rubber and plastic materials. Alumilite typically falls in line with the hardness and brittleness of Ebonite, so I would expect them to have similar workability.

Other "acrylics" aka plastic pen blanks are made from a variety of plastic materials such as epoxies, polyesters, cellulose, acrylics, acetates, and combinations thereof.

Of the "acrylics" I have used, by far the most brittle has been Inlace Acrylester which I think is owned by WoodTurningz. They are gorgeous blanks and polish to a fantastic high gloss, but they can be quite difficult to work with because they are so hard and are prone to chipping. I have found that using a negative rake angle on my tool helps prevent chipping.

Right behind Inlace Acrylester I find Rhino Plastic blanks which are made from a polyester resin. Although they are still quite hard and they polish to a very nice gloss much like Inlace, I have found them to be less brittle and easier to turn without chipping.

Considering how brittle Inlace Acrylester can be, if that is one of the "acrylics" you are using, it would not surprise me that you would have problems with chipping.

Can you share the trade name of the "acrylic" you are using?

Regards,
Dave
 
Thanks. Yes, Acrylic Acetate. (I hate how the term has become generic) I worked with a lot plastics especially Plexiglass in the sign trade but never any threading.
 
PatrickR said:
Thanks. Yes, Acrylic Acetate. (I hate how the term has become generic) I worked with a lot plastics especially Plexiglass in the sign trade but never any threading.
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I have made pens with the Acrylic Acetate from WoodTurningz before, but not too many. I made a few with #48 Hog Wild, an orange and black blank. Those were the school colors of Doane University where my oldest son attended. I don't remember them being as brittle as the Inlace Acrylester, but assuming that the amount of Acrylic is significantly more than Acetate in the material, they could be quite brittle. Acetate is a natural polymer like what is found in rayon and photographic films which is less brittle where Acrylic is a petroleum based polymer like Plexiglass and Lucite and they can be very brittle. - Dave
 
I destroyed 3 out of 3 inlace acrylester lowers I was working on last weekend. Broke at the tenon every time during shaping while mounted on a mandrel.
 
You don't mention the specs of the threads you are having trouble with, nor the size tenon you have been using, but you might be correct about needing to under-size the tenon a little more.

I typically aim for 0.10 to 0.15 mm when I'm making Mx0.75 or Mx0.8 threads - but have often taken it further than that for cap threads so that there are no sharp peaks where writers grip the pen. It really doesn't take much of a groove for the cap threads to get a firm grip (provided of course that you haven't drilled the hole too large).
 
duncsuss said:
You don't mention the specs of the threads you are having trouble with, nor the size tenon you have been using, but you might be correct about needing to under-size the tenon a little more.

I typically aim for 0.10 to 0.15 mm when I'm making Mx0.75 or Mx0.8 threads - but have often taken it further than that for cap threads so that there are no sharp peaks where writers grip the pen. It really doesn't take much of a groove for the cap threads to get a firm grip (provided of course that you haven't drilled the hole too large).
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I'm using a 13x.8 triple start and think this tenon was 12.9 I'll be doing some testing soon.
 
I also use M13x0.8x3 frequently. I just checked a couple of the barrels I'm working on, the outer diameter of the threads on one of them is 12.6mm, and the cap holds tight on it. The "peaks" of the threads are well below where the die cuts, so even if there's a little irregularity in the thread it's "down in the valley" and the outer surface is still smooth and polished. (I polish the tenons before threading them.)
 
duncsuss said:
I also use M13x0.8x3 frequently. I just checked a couple of the barrels I'm working on, the outer diameter of the threads on one of them is 12.6mm, and the cap holds tight on it. The "peaks" of the threads are well below where the die cuts, so even if there's a little irregularity in the thread it's "down in the valley" and the outer surface is still smooth and polished. (I polish the tenons before threading them.)
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Thanks. No chance to play with it today but soon.
 
Patrick for acrylic I make my tenons a touch thinner, but also take into consideration that the die can be loosened with the small hex nut. If you feel it really tight as you start to thread, loosen the screw a touch
 
I did some test today, even though the shop was in the low 30s. First warming the blank helped but wasn't perfect. Next test I cut the tenon down to 12.8, warm blank and used copious amounts of lube (mineral oil). I only saw one chip. I'll probably not use the AA until it warms up some. Then try again at a slightly small diameter.
It's odd because I have threaded it before with no issues. I have never had this problem with alumilite or ebonite EVER.
@CjG78 - thanks, I always start with the die fully open. Then compress it if needed to get a smooth fit to the cap.
 
PatrickR said:
I did some test today, even though the shop was in the low 30s. First warming the blank helped but wasn't perfect. Next test I cut the tenon down to 12.8, warm blank and used copious amounts of lube (mineral oil). I only saw one chip. I'll probably not use the AA until it warms up some. Then try again at a slightly small diameter.
It's odd because I have threaded it before with no issues. I have never had this problem with alumilite or ebonite EVER.
@CjG78 - thanks, I always start with the die fully open. Then compress it if needed to get a smooth fit to the cap.
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Have you tried warming the die? A hot-air gun or hair dryer might make a difference.
 
PatrickR said:
I did some test today, even though the shop was in the low 30s. First warming the blank helped but wasn't perfect. Next test I cut the tenon down to 12.8, warm blank and used copious amounts of lube (mineral oil). I only saw one chip. I'll probably not use the AA until it warms up some. Then try again at a slightly small diameter.
It's odd because I have threaded it before with no issues. I have never had this problem with alumilite or ebonite EVER.
@CjG78 - thanks, I always start with the die fully open. Then compress it if needed to get a smooth fit to the cap.
Click to expand...
I just finished a couple of pens, the other day, using Acrylic Acetate. The green one I used threaded fine, while the black was a real pain. It was very tight, even snapping a tenon, which I haven't done in quite some time.
 
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