Turning Brittle Acrylics

[mikefoye]

I turn a lot of acrylic blanks and I use carbide tools exclusively for all of my wood and acrylic turnings. I ran in to chipping and breaking problems some time ago trying to turn Mica blanks from PSI and got so frustrated that I just don't buy them anymore. I recently had a similar problem with Rhino blanks but I sped the lathe up a little and took much shallower cuts and took my time (after reading reviews) and was able to get the desired result. Now I am REALLY having a problem with Stars and Stripes blanks (also from PSI). When I read reviews they all say how hard these are to turn without chipping and cracking. To try something new, this weekend I purchased new Woodpecker tools because they have a 45 deg. bevel on the shaft and I thought that it would help me "cut" the blanks instead of "scraping" them. I have the lathe at around 3,000. The Woodpecker carbides are really very sharp and if I go really slow I can get them close. I read a couple of reviews that suggested using dull tools! I don't know how to do that because I buy carbides because they are always sharp. The last two blanks that I made I roughed them with the tools and they were chipped a bit. I then resorted to a Dremel rotary sanding disc to get them down close to size and below the chips and then very slowly and with really shallow cuts I was able to use the 2 deg radius scraper to finish them good enough for final sanding. Every single one of these I turn has chipping problems no matter what I try. I bought a bunch of these for my American Patriot and Salute to Troops pens and am hoping that someone out there has a magic bullet idea that will keep me from breaking these things because they are strikingly beautiful when they are done.

 

[Bill in Buena Park]

Mike,
My only luck with the brittle acrylics is to use a REALLY sharp HSS spindle roughing gouge (my normal pen turning tool of choice), not carbide, and use on an angle to cut and not scrape. Also, high RPM helps, but not trying too push to fast or hard on the tool; there is a certain "feel" when the cut starts to slice shavings. I still might get sections that chip out, but its very minor and after sanding through the grits, a little CA will eliminate. I would practice on some less-critical acrylic blanks to get the feel with the roughing gouge before finishing your expensive blanks.

 

[Ted iin Michigan]

Hmmmm - interesting experiences. I've turned a lot of the same type of materials - hope my following comments help.
After doing many I've settled in on using a non-high speed steel spindle roughing gouge (not a skew) on those type materials. I sharpen on a Work Sharp with a jig I made or on my low speed grinder. Either way gives a decent edge. If it's a material that I know is particularly sensitive, I may sand the edges and get the blank to a very rough round shape. Then I'll turn at about the same speed you do. I keep my tool rest at a height that presents the edge pretty well centered - but not high. I try to keep the bevel as close to the material as I can.
I've been pondering why I prefer this gouge (just a HF that was part of my first set) and I think it might be partly due to familiarity, partly because it has a pretty short handle and maybe partly because I can get a nice edge on it that doesn't stay super sharp for very long. I'll resharpen every other pen or so.
I know it's not the carbide you are using but my experience is that carbide works nicely on things such as TruStone but I prefer steel on just about everything else.
Hope this helps

 

[McKenzie Penworks]

Don't resist using a skew chisel... Right tool for the job. Once you get it down you won't look back. I love my carbide tools, I've used them for 10 years... But when I turn acrylester and a few others I use a gouge and skew and they get the job done without drama

 

[WriteON]

No Mica's for me. Not my faves. However if you strike like a snail with air brakes with sharp tools they are manageable.

 

[elyk864]

I turn a lot of acrylics and use carbide tools with very little problems now. I would experiment with the angle of attack. I will have the tool at almost a 45 degree angle up, to the point where I am making contact below the center line of the blank. If that all makes sense. You will then need very very little pressure to make cuts.

 

[raar25]

I knock the corners off and round using a round carbide with the dished top side not just a flat carbide cutter. If I go slow it gives me nice ribbon after the blank is round. Than I use a skew on its edge as a negative scraper to get rid of the chipping and chatter. Be careful on two part pens because the middle will vibrate as you get far away from the head stock. Than I finish up with my 80 grit gouge to get to final size. Hope that helps.

 

[TattooedTurner]

...
I've been pondering why I prefer this gouge (just a HF that was part of my first set)...

I use an ultra cheap HF spindle gouge that came in my first set of tools on really brittle stuff. I don't know why, but it seems to do the best job in spite of the fact that it's useless for just about anything else. I reground the tip to more of a detail gouge shape (by accident) and dang if it didn't start slicing through the brittle stuff like that's what it was made for.

 

[Woodchipper]

raar25, I might do the blanks as a separate operation for turning.
TatooedTurner, your post has me thinking. Scary, isn't it?

 

[TattooedTurner]

raar25, I might do the blanks as a separate operation for turning.
TatooedTurner, your post has me thinking. Scary, isn't it?

I hear you. Between casting and turning, I've gotten very lucky at times completely by accident. It seems Ted and I have both gotten lucky using cheap HF tools on the brittle stuff. Maybe because they don't sharpen to as good of an edge?

 

[robutacion]

I turn a lot of acrylic blanks and I use carbide tools exclusively for all of my wood and acrylic turnings. I ran in to chipping and breaking problems some time ago trying to turn Mica blanks from PSI and got so frustrated that I just don't buy them anymore. I recently had a similar problem with Rhino blanks but I sped the lathe up a little and took much shallower cuts and took my time (after reading reviews) and was able to get the desired result. Now I am REALLY having a problem with Stars and Stripes blanks (also from PSI). When I read reviews they all say how hard these are to turn without chipping and cracking. To try something new, this weekend I purchased new Woodpecker tools because they have a 45 deg. bevel on the shaft and I thought that it would help me "cut" the blanks instead of "scraping" them. I have the lathe at around 3,000. The Woodpecker carbides are really very sharp and if I go really slow I can get them close. I read a couple of reviews that suggested using dull tools! I don't know how to do that because I buy carbides because they are always sharp. The last two blanks that I made I roughed them with the tools and they were chipped a bit. I then resorted to a Dremel rotary sanding disc to get them down close to size and below the chips and then very slowly and with really shallow cuts I was able to use the 2 deg radius scraper to finish them good enough for final sanding. Every single one of these I turn has chipping problems no matter what I try. I bought a bunch of these for my American Patriot and Salute to Troops pens and am hoping that someone out there has a magic bullet idea that will keep me from breaking these things because they are strikingly beautiful when they are done.

Well, I call it the "Flap Disc System", not everybody likes or accepts the system which is OK with me, try it and if it works for you, problem solved...!

Good luck,

Cheers
George

 

[mikefoye]

tattooed turner: can you tell me or describe what a "detail gouge" shape is? I have a 1/2" HSS fingernail gouge that I don't use at all since switching to carbides. I am still a relative novice and I have yet to learn the proper use of a gouge and a skew.

 

[TattooedTurner]

tattooed turner: can you tell me or describe what a "detail gouge" shape is? I have a 1/2" HSS fingernail gouge that I don't use at all since switching to carbides. I am still a relative novice and I have yet to learn the proper use of a gouge and a skew.

They have a shallow flute which you can't duplicate, but you can copy the grind. The cheap HF spindle gouge has a shallow flute so that wasn't a problem. I ground it to a long fingernail shape, and decreased the angle which made the tip pointier than a regular or fingernail grind gouge - which allows it to get into tight spaces like between beads. This was entirely by accident, it was the first time I ever sharpened a spindle gouge and had no idea what I was doing! Using the wing with the tool rotated about 45 degrees slices through the brittle stuff like butter. As usual for the cheap carbon steel tool, it need frequent sharpening, easily 2 or 3 times on a blank. FWIW I'm pretty sure it's a 3/8".