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Old 10-30-2017, 09:59 AM   #1 (permalink)
 
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Default Smooth finish with Carbide?

When I turn an acrylic blank using my round carbide cutter, and the lathe at full speed around 4000 rpm, I inevitably end up with "ribs" around the blank. Some are deeper than others and I'm able to sand them all down using 320 for a few minutes. However, is there a way to get rid of these radial high/low points with the carbide cutter? I've tried using very light slow cuts but that doesn't seem to be helping.

Is it possible to get a smoother finish with carbide? And if so, how?

P.S. I know this can be accomplished with a skew, and I have done so. However, I'm still working on my skew technique because my results are still too sporadic. I have a hard time keeping an even thickness across the blank.
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Old 10-30-2017, 10:39 AM   #2 (permalink)
 
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I'm certainly not an expert on this subject, my my limited experience is that the larger the diameter of the cutter, the smoother the surface that it will leave.

This is where having multiple tools can be helpful - a small diameter circle for roughing, a medium diameter circle for general purpose smoothing, and a radiused square for finish work.
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Old 10-30-2017, 10:55 AM   #3 (permalink)
 
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Do you know what kind of plastic you're working with? I have run into this before with a soft plastic camouflage blank I got on Amazon (though I don't know the exact material). Was able to mostly smooth it out with the round cutter by holding it about 45 degrees and taking *very* light passes, almost like I would a skew. What I couldn't get completely, I was able to sand down to flat with 150 grit paper.
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Old 10-30-2017, 11:03 AM   #4 (permalink)
 
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I use a round carbide tool (11 mm diameter) securely mounted on the carriage of a metal working lathe. I take very light cuts (0.002" or less) and move the carriage slowly for the final 2 or 3 cuts. . I can achieve no observable ridges and very little sanding is required.

I have done AA acrylic, Alumilite, polyresin, polyester, etc. blanks this way and hardwoods as well. . Desert ironwood works particularly well.

I know that most people here don't have a metal working lathe but that is what I bought because I have been familiar with its use (which is really quite easy) for years. . For me it is much easier, although perhaps slower, to shape pen blanks this way.

I just mention my experience for what it's worth and not expecting everybody can relate.
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Old 10-30-2017, 11:08 AM   #5 (permalink)
 
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Sounds like you are moving the tool too fast. I use a round nose or spindle gouge. Haven't mastered the skew for pen turning.
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Old 10-30-2017, 11:52 AM   #6 (permalink)
 
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It helps to have a very smooth tool rest and tool bottom. When I get nicks on the tool rest, I sand it smooth and polish it up. If the bottom of your tool is nicked or rough, do the same procedure. When both glide easily, you'll get much better results. Round carbide can take more practice.
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Old 10-30-2017, 12:10 PM   #7 (permalink)
 
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We do not do much acrylics but you can end up with ridges and waviness in wood as well.

We wax the tool rest periodically. Make sure your carbide is sharp. Because you don't have to sharpen it often some let it go too long.

We end everything with a sharp skew. We touch it up on the leather strip or diamond card between barrels. If it won't shave the hair on my arm it needs to be touched up. We actually end using a 3/4 skew in a negative rake.

Almost never have to sand with anything but micro mesh polishing.
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Old 10-30-2017, 09:25 PM   #8 (permalink)
 
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IME a radius square carbide will leave a much smoother finish than a round. As mentioned, a skew used as a negative rake scraper also leaves a very smooth surface on acrylic, but it dulls the tool quickly. Keep a diamond card handy.
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Old 10-30-2017, 09:34 PM   #9 (permalink)
 
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Quote:
Originally Posted by TattooedTurner View Post
IME a radius square carbide will leave a much smoother finish than a round. As mentioned, a skew used as a negative rake scraper also leaves a very smooth surface on acrylic, but it dulls the tool quickly. Keep a diamond card handy.
I agree. I find a round cutter too aggressive and prone to leave gouges. I use a radius square at a small angle for plastics, acrylics and wood. All my carbide is on round shanks.
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Old 10-30-2017, 09:38 PM   #10 (permalink)
 
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Quote:
Originally Posted by monophoto View Post
I'm certainly not an expert on this subject, my my limited experience is that the larger the diameter of the cutter, the smoother the surface that it will leave.

This is where having multiple tools can be helpful - a small diameter circle for roughing, a medium diameter circle for general purpose smoothing, and a radiused square for finish work.
I use the corner of my radiused square carbide for roughing, the round carbide for shaping, and the edge of the radiused square for finishing, if needed. I have found that if I am very slow with the round carbide, I don't get the "rings".

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