Turning Speed for PR

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KDM

KDM

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Joined
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Redditch, England
I've cast some PR for the first time. When I turn it, there are shiny gouges in the turning. I'm not sure if these are air bubbles, or if the cutting tool is biting into the material and taking chips out.

Assuming they're not bubbles, I must have the speed wrong. What turning speed woud you lot recommend?

Thanks.
 
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As fast as your lathe goes and you are comfortable.

If you are chipping the material, it sounds bad. If you are making ribbons, and get bubbles, they were there before you turned it.
 
I have my lathe set at 1800rpm and run that speed for eveything from turning to sanding.

It sounds more like your tool needs sharpened and a less aggressive cutting angle. It sounds like the tool is grabbing instead of cutting.
 
I'm with Ed... My lathe sits on the top belt for most pen turning, especially acrylic... 3900 rpm. That and the skew for acrylics is the best advise I have heard/give.
 
KDM said:
Assuming they're not bubbles, I must have the speed wrong. What turning speed woud you lot recommend?
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Once the blanks are round, you want to make absolutely sure you are doing a shearing cut, not even a little bit of scrap. You should get fine ribbons like Ed said. A skew is the preferred tool for this but again, make sure you are using an tool in a cutting orientation, not like a scraper. The point of the skew should extend beyond the blank. If the point of the skew intersects the blank the cut is a scrap, not a shear cut.
 
My lathe has two speeds for turning , Full and Off . The only time I go slower is for drilling . Shiny gouges in the surface sound like chip out , sharpen your tools and take lighter cuts .
 
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i use a negative rake scraper and have really good success. ALlthough, as mentioned the skew is quickly gaining steam to be my favorite!
 
ed4copies said:
If you are chipping the material, it sounds bad. If you are making ribbons, and get bubbles, they were there before you turned it.
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Ribbons, no. I'm getting very fine dust. No visibly chips. Hold on: I'll take a photo. Maybe it's not that bad...
 
DozerMite said:
I have my lathe set at 1800rpm and run that speed for eveything from turning to sanding.
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Erm. OKay. Mine was set to 800. I've cranked it up to 1600 now by moving th V belt. I can go to 2000, but that needs me to change the drive wheel on the motor. Not ready to go there quite yet!

DozerMite said:
It sounds more like your tool needs sharpened and a less aggressive cutting angle. It sounds like the tool is grabbing instead of cutting.
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Tool is 100% sharp. It's a brand new DCMT, so the rake on it is 12 deg. By what you're saying, that's probably too steep.

It's probably relevant that I'm using a metal turning lathe rather than a wood turning one. What sort of angle does this skew tool cut at relative to the horizontal?
 
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randyrls said:
You should get fine ribbons like Ed said. A skew is the preferred tool for this but again, make sure you are using an tool in a cutting orientation, not like a scraper.
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I can't understand how the heck you can get ribbons when turning anything at 2000RPM+!!
 
I like ldb2000's Full and off speed response (sounds like how I like to drive my car). For any project on the lathe - as the diameter decreases, the spindle speed increases. For diameters as small as pens are, feel free to turn up the speed of your lathe as high as it will go. The cuts will be smoother as well as effortless. The second part to this would be keeping your cutting tools as sharp as possible. I've always found a skew chisel to be the best tool to turn plastics with. I know it's the "evil" tool in your turning tool arsenal, but my advice would be to make freinds with it. A sharp skew turning at 3000RPM or so will produce a very nice surface that requires little sanding. As your skills improve with the skew, you'll find youself starting with 220 or even 320 grit sandpaper rather than 120 or 180. Nobody likes sanding so learn to use your skew... Jason
 
The tools you linked to are metal cutting tools and are very aggressive , they are designed to cut metal not plastic . Are they being used in the tool holder or in a handheld tool on a tool rest . If you are using the tool holder that could also be part of your problem , you have no tactile feedback and no idea how aggressive you are cutting . While you are turning are you getting a sound like that of breaking glass , a shattering sound . If you are then that is chip out happening . There are several good videos in the library on how to use a Skew , a handheld tool on a tool rest . If you don't have one of the conversion tool rests I would advise you to get one and a Skew or at the very least a round nosed scraper and crank up the speed and you will start getting the ribbons everyone is talking about . You will not get ribbons with those tools you linked to by the way .
 
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ldb2000 said:
The tools you linked to are metal cutting tools and are very aggressive , they are designed to cut metal not plastic. Are they being used in the tool holder or in a handheld tool on a tool rest .
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Yes, mine is a metal turning lathe and the tools are in the toolpost.

ldb2000 said:
If you are using the tool holder that could also be part of your problem , you have no tactile feedback and no idea how aggressive you are cutting.
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No, I have no real control over the rake of the tool. Ony the depth of the cut. I can control it pretty well to the nearest 0.02mm, which should be good enough!

ldb2000 said:
While you are turning are you getting a sound like that of breaking glass , a shattering sound . If you are then that is chip out happening .
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No. It's more of a grinding noise.

ldb2000 said:
There are several good videos in the library on how to use a Skew , a handheld tool on a tool rest . If you don't have one of the conversion tool rests I would advise you to get one and a Skew or at the very least a round nosed scraper and crank up the speed and you will start getting the ribbons everyone is talking about . You will not get ribbons with those tools you linked to by the way .
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I decided to grind my own tool. It should perform in a similar way to a skew. I presume we use teh skew to kind of shave or scrape the PR rather than cutting into it? Anyway, I'm happier with it now. Below are the results. Not a great photo, but the bottom polished blank has pits. The topmost one isn't polished, but is turned at a much higher speed with a freshly ground tool.

 
Looks like a fairly clean cut . Your moving in a better direction .
The skew , if used properly is used at an angle to the work . It performs a shearing cut instead of a scraping cut and will kinda peel off long streamers or ribbons and give a cut that requires almost no sanding .
 
Yeah - I been looking at the way skews are used and to tell the truth, I think the rake on the is substantially greater than teh DCMT tool I was using! They seem to take much more brutal cuts!
 
Rake is important when using a tool in a scraping manner but a skew used properly isn't used as a scraper . A skew is used more like a knife peeling an apple , this is why you hear about ribbons coming off the blank . It is held at a very oblique angle and slices the surface of the blank which is much less brutal then scraping the surface off . Ed4copies has a video somewhere in the library that shows the proper use of a skew . A gouge because of it's shape performs a similar cut but not to the same extent , this is one of the reasons why it is not the best tool for turning acrylics . Turning metal and turning acrylics require two totally different techniques .
 
Alan Batty has an AWESOME video on the skew. Taught me how to use one. I think a big skew is helpful at first as well. Something at least 1" wide.

Someone here also mentioned a brilliant idea. Mount a carrot between centers with the lathe unplugged. Leaving the lathe unplugged, skin the carrot with a skew, taking long cuts down the length of the carrot.
 
PS. That looks like some chipout going on in the pics. Hard to tell really. Are there tiny little shiny bubble looking spots, usually in a ring all the way around the blank? If so, then it is chipout. The grinding sound you are hearing is the tool bouncing off of the material and slamming back into it over and over. Each time this happens, it knocks a little tiny piece of material out.

Hope that helps. I dont know much at all about metal lathes, so i would definately go with what Butch and the gang say!
 
KDM said:
I can't understand how the heck you can get ribbons when turning anything at 2000RPM+!!
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Sharp Sharp tools in used in the right orientation.

I turn all my pens at full speed on the jet 1014, using just the 3/8 spindle gouge once round, i get nice long ribbons, to the point it annoys me when they wrap themselves around the end of the mandrel!
 
KDM said:
I can't understand how the heck you can get ribbons when turning anything at 2000RPM+!!
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Not only do you get ribbons, if you are aggressive enough the ribbon shoots up in the air about 2 feet. I am usually not that aggressive.

It also depends on exactly which kind of acrylic you are turning. Some produce more and better ribbons than others. With some of the acrylics, I have to stop the dust collector after each blank and clean the ribbons off the debris screen in my dust collector. The ribbons clog the screen and the DC drops noticeably in effectiveness.
 
RAdams said:
Are there tiny little shiny bubble looking spots, usually in a ring all the way around the blank?
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Difinitely "bubble-looking spots." That's wht at first I did't know if they were bubbles inteh blank or chips. Not all teh way round. When I turn, I tend to move all teh way along before going back to the start, so I wouldn't get a ring of them, but I think what you;ve described is what I was seeing before I ground my new tool.

RAdams said:
Hope that helps. I dont know much at all about metal lathes, so i would definately go with what Butch and the gang say!
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...and I don't know a damn thing about turning wood/plastics!!
 
alphageek said:
Although I disagree with slowing it down... Yes melting is a possibility... However, I use them wet and light! when I use them and still at full speed!
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Good, because my lathe is kind of in the large hobby / small industrial size bracket and changing the belt takes a few miuntes!

I've tried on fast speed with light touch and it works a real treat.
 
DennisM said:
once round, i get nice long ribbons, to the point it annoys me when they wrap themselves around the end of the mandrel!
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Heh heh! Like this?

1_20100509PR_Ribbons.jpg
 
I find that the ribbons are annoying but a reminder of how sharp my tools are. I do have to clean off sections when I do not move fast enough to avoid as much build up as possible, good luck
 
I noticed from the workshop photos that some of the folk here have lathes which can go in excess of 3000RPM.

For info, I'm getting really good results at about 1600RPM. My lathe can go to 2000, but that involves taking a spanner to the grearbox, so I'm resistant.

Does anyone think it might be worth trying the higher speed?
 
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