Messing up acrylic

Signed-In Members Don't See This Ad
StanBrown

StanBrown

Member
Joined
Jul 24, 2018
Messages
90
Location
Waller,Texas
I seem to be having a terrific problem with acrylic. I have watched the videos and turned a few nice pieces. Lately, however, I haven't been doing so well.

The problem is that the corner of the skew will suddenly "grab" the turning piece. This cuts a groove in the work piece all the way around. Sometimes it is pretty deep and always has a line of chipped-out places in the groove. Sometimes it is salvageable, most often it is not. Sometimes when trying to salvage a blank after this happens, it happens again on the same piece, rendering it good only for trash status.

Here are the solutions I have tried, all to no avail:
- brand new edge on a carbide tool
- freshly sharpened HSS tool
- turned at high speed (~2,000 rpm)
- turned at various lower speeds down to ~500 rpm

I have turned around 100 or so pens but still consider myself to be a newbie so any suggestions or possible remidies will be appreciated. Learning is always a good thing.
 
Signed-In Members Don't See This Ad
Have to say you do not present the skew the right way then. What size skew are you using. The center to lower half of the skew should be touching the round blank. The roundness of the blank makes the area to cut so thin and small there is no way to get a catch. Unless you are using the skew as a scraper than you twist the skew and cause the catch. There are so many videos on the net that show the proper use of a skew. It really is an easy tool to master and the things you can do with it makes it so useful. Practice practice and you will not be sorry you did.
 
First of all, I suggest more speed. I do all my (pen sized spindles) turning at 3,700 or thereabouts. Don't know what you've got for a carbide tool, but the 15mm R2 cutter works very well for me (MUCH less apt to catch than the square cutters). It is on a "Magical Skew", which has a hex shaft oriented so that the cutter can easily be help at a skew-like angle (can also be held flat for scraping). I tend to start with a skew-like cut, then once the piece is round, switch back and forth.

I did the same thing with the HSS skew back when I was using that. Hope this helps, as always, YMMV.

Michael
 
Gary Beasley said:
I learned recently that a negative rake scraper does a good job on plastics. If you have one or can make one try it. Theres been negative rake carbide inserts available lately, something to check out.
Click to expand...

Now my curiosity is in full bloom. I have looked at the negative rake carbide tool heads and wondered how they are used. Do you use them in the same way as conventional tools? Is any adjustment to the tool rest position needed? I.e. should it be higher or lower than when using conventional tools, and should it be further away from the work piece?
 
StanBrown said:
Gary Beasley said:
I learned recently that a negative rake scraper does a good job on plastics. If you have one or can make one try it. Theres been negative rake carbide inserts available lately, something to check out.
Click to expand...

Now my curiosity is in full bloom. I have looked at the negative rake carbide tool heads and wondered how they are used. Do you use them in the same way as conventional tools? Is any adjustment to the tool rest position needed? I.e. should it be higher or lower than when using conventional tools, and should it be further away from the work piece?
Click to expand...

Have a look see.

how to use negative rake scrapper turning tools - Bing
 
I prefer to turn wood, but have had good luck with acrylic blanks. I use both high speed steel (HSS) and carbide turning tools. Here are a few tricks that have helped me:

* Make sure your tools are extra sharp. Sharpen the HSS before you begin and again while turning if needed. Rotate the carbides to expose fresh sharp edges. You already tried this, but it may help others.

* Round off the edge "corners" before you begin turning. I prefer to drill the blank first, to avoid issues where the rounding is not symmetrical.

To round off the corners, I put the blank end-to-end in a clamp, then use the clamp to hold each "corner" of the blank against a belt sander for a brief moment. I roll the blank slightly to round the edge. Keep contact brief between the blank and the belt sander to avoid friction heat. (Before I tried the belt sander, I made a jig with a V-groove to cut off the corners on a bandsaw or table saw. It worked okay, but not great. The belt sander is faster and easier.)

* Work slowly, patiently, and carefully. It takes me much longer to turn acrylics than wood. Don't push it. Don't rush it. Don't press hard. Take small cuts.

* Run the lathe on the highest speed it will go. I tried many different speeds. I thought that slower should be better, but what I found was that the highest speed works best for me.

* Find the right tools that work for you. I use the HSS tools for roughing and shaping, but the carbide tools for the final work, matching the bushings, etc.

* Some acrylics are much easier to turn than others. The easiest acrylic blanks for me are the "acrylic acetate" and also the various "Glow Stonez" are similar material. (I buy acrylic blanks at Rockler.)

Blanks with "Acrylester" and/or "Inlace" in the name are more challenging to turn. Frankly, they feel like turning concrete. After finishing, they look very nice. Stay away from "acrylester" and "inlace" until you have achieve success with other acrylics. (There are a few unusual, specialized blanks that are notoriously difficult to turn, like the coffee bean blank.)

* You may need to use (or learn) different turning techniques for acrylics. Some acrylics can be bevel cut with turning tools, but some acrylics just insist that you present the tools in scraping mode.

* If the skew chisel isn't working out with acrylics, you are not alone. I avoid the skew chisel on some acrylics because it doesn't work on them, at least not for me. On other acrylics, the skew chisel peels off beautiful strips that wrap themselves around the blank like cotton candy. Here is a great YouTube video about using the skew chisel: https://www.youtube.com/watch?v=KfeLAHQSbqk
 
penicillin said:
Some acrylics are much easier to turn than others... Stay away from "acrylester" and "inlace" until you have achieve success with other acrylics... Some acrylics can be bevel cut with turning tools, but some acrylics just insist that you present the tools in scraping mode.
Click to expand...
Inlace Acrylester is NOT Acrylic. See http://www.penturners.org/forum/f14/not-all-plastics-acrylic-148328/ for more information.

Acrylic is one of the easier plastics to turn, sand, and polish.

To anyone having trouble learning to control the skew, I recommend trying pen blanks made from opaque (not clear) Alumilite. It is the most forgiving material I have turned. Once you learn how to cut ribbons with opaque Alumilite, you'll have the confidence to move on to more challenging blanks.

Regards,
Eric
 
I turn almost only acrylic or "plastic". (I do use wood for segmenting).
I use a skew (HSS) and carbide. I use negative rake only for plastic....I am not going to say its a miracle cutter, but it makes what I would call a "noticeable" difference.

I buy them from Woodcraft....they are a little pricey...but worth it.
Turn "wide open" for speed, and try a negative rake insert.
I also have round negative rake that I use on the bottom of a bowl to clean it up. It does not take off much material, but cleans up chatter and tear out, eliminating the need for some sanding.

As mentioned, different materials, turn differently. Some drastically. When I started, I labeled the material I bought with sticker when it came in, and then how it turned in a little log. If you just throw your blanks in a box when you get them, its easy to loose track of what they are. Or just always buy the same stuff.....
Just my .02.
 
Last edited:
Stan - I agree with Michael/mecompco - Bob Jackson's Magical Skew, 4,000 rpm, light touch.

I round the corners of the carbide cutter a tiny bit on a diamond stone (or with a diamond bit on a Dremel tool).

With some practice, you'll get to the point where the mesh pad sequence is unnecessary, and you can go straight from skew to plastic polishing.

Have fun.

Best regards - Bob
 
The stinging cost of carbide cutters drops quite a bit once you realize how easy it is to sharpen them on a 600 grit diamond card. People around here recommend Trend, I read that after I had already bought a different one, Ive had no problems.
 
I think I'm probably in the same skill level with about the same experience. What I do is use a carbide finishing gouge (round insert) and cut slightly above the center line with the tool rest backed off of the work a bit. It seems to me that some blanks that you get or make are more prone to "dog in" like that, so when I get close to within 100 thousands or so of the diameter that I want I switch over to the "80 grit" chisel and start being mindful of the heat. It takes a little longer that way, but you have less risk of a blow-out. If you ever turn mica (ie. pot of gold from PSI) it's so prone to fracture that I start off with the sand paper. Takes a while, but it makes a beautiful pen.
 
I use a carbide-tipped tool that I made myself. For the carbide, presentation matters. I present the cutter flat to the work, slightly below the centerline. Above the centerline doesn't cut and too far below it starts to catch. A radiused-edge square cutter seems to work better than a flat edge one. I don't even need a skew when I'm on my game, I can go to polishing just using the cutter. You'll know you are doing it right when you get long ribbons peeling off the acrylic.
 
I use a round skew and cut with the long tip down, ie the tip not the heel of the skew it peels it but take gentle cuts. Sometimes if it chips or catches a freshly sharpened roughing gouge cutting on the wing works best as. the extra weight dampened any vibrations caused by the chipped area.
Other times I confess to using it flat as a scraper having diamond honed it first. I have not tried carbide , even though i've lots of tips for my metal work lathe . That's on the to make list currently .

good luck and keep trying
 
Hi Stan.
I had much the same problem. Spent a few internet hours watching other people demonstrate their techniques and then practiced on offcuts and scrap wood. This did lead to a big improvement in my scrappage rate but now and then still get one of those nights when it all goes wrong. However when it's going right, the finish that a nice sharp skew gives will make the hours of practice worthwhile.

Also I would echo the comments of Penicillin; not all acrylics are the same. Sometimes a scraper will be the thing to use. In the end it's down to what works for you, on the day, with the particular blank.

Anyway, good luck with your turning and let us know how it's going.
 
You must log in or register to reply here.
Back
Top Bottom