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Help for beginner with lathe questions on unusual material

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  • Help for beginner with lathe questions on unusual material

    Hello Everyone!

    Hopefully this is the right place to post questions. I recently purchased a Central Machinery 7x10 Mini Metal Lathe from Harbor Freight to start out and learn with and I'm having some difficulty understanding what I'm doing wrong with my first project. I also picked up a set of C6 Carbide Tipped Tools from McMaster-Carr.

    I'm attempting to create a ring out of Synthetic Opal and while I have been able to create a donut shape matching my general desired shape, my inaccurate drilling has resulted in an off center hole. After centering an AL-4 tool on my piece I attempted to even out the center circle with no success and stopped before doing serious damage to my material. I can't tell if my piece isn't mounted properly in my lathe chuck, if I've incorrectly positioned my cutting tool, if I'm using the wrong kind of tool, or any other manner of mistakes I'm more than likely making. Any advice or insight to push me in the right direction would be very appreciated!

  • #2
    Thats a lot of questions with very little detail to answer them. A picture of your work setup would help immensely, otherwise any answers will literally be a blind guess. Speaking of blind guesses though, sounds like you were trying to bore out the center with an incorrect tool. Looking at it, an AL4 is a left-hand OD tool, while you could probably internally bore with one it would have to be a pretty good sized bore. I wouldnt wanna run brazed carbide on a mini lathe either, not for boring. The higher forces required usually exceed the rigidity of the machine, plus brazed tools dont seem to be the sharpest things in the world out of the box

    Also, might wanna throw in a preemptative "stuff it" to the incoming crowd of "asian machines are garbage you wasted your money rah rah rah" types. Nothing wrong with the 7x10s for a first lathe, they just take a bit of tuning.

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    • #3
      Better read up on what cutters to use on opal, I have a few decades experience, but none on this.
      pat attention to cutter material rake or clearances and cutting speeds, then we can help more.

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      • #4
        Yeah, there's a lot of unknowns here without seeing the setup.

        I'd also venture that few of us ever worked with synthetic opal either. My first fear is that the dust or chips created from machining might be highly abrasive and damage the lathe or not. Right off the bat you might have a situation where you will need to totally break down the lathe and clean all the sliding ways to ensure the machine does not lap itself into oblivion in a relatively short time.

        It sounds like you drilled out the stone and are trying now to bore out the hole with a cutting tool instead of a boring bar. If this is the case check that the heel of the tool below and behind the cutting edge is clear of the side of the bore or if it is rubbing.

        Turning stone might also call for tools that have a negative rake angle on the top of the cutters so they scrape away the material instead of cutting into it. If you're getting a lot of chipping out ahead of the cut then this may be an issue for you in your attempt to machine this synthetic stone material. This importance for top rake angle shows up in metals and even plastics too. Brass tends to turn best when used with a zero rake or slightly negative angle. Some plastics also work better with a zero or slightly negative rake top face angle.

        This is all just tossing ideas out and you'll need to pick up on which you need. No idea really as I've only once used some hand tools on some soap stone. That's the sum total of my own work with minerals.

        Chilliwack BC, Canada

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        • #5
          Would be good to know the shape of your raw material. Is is cylindrical? A bar? What dimensions? Is it brittle? Also approximately the shape you are shooting for.

          As said, some pics would help the more experienced give solid advice. I'm also a beginner. Best to learn from these guys who have already made most of the mistakes. It will save you a lot of time, money, and frustration.
          S E Michigan

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          • #6
            An AL tool is for cutting on the LEFT side of a part or the INSIDE REAR of a cylinder. I suspect the tool doesn't have the clearance angle for cutting inside your cylinder (ring). AND you will have to be running in reverse. Better to use a boring bar for your operation. Nothing wrong with learning on that lathe. I made a lot of bushings with that vary same lathe. I still have it even though I moved to a larger one.

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            • #7
              I forgot to mention (Edit doesn't work): Most ALL those carbide tool bits will need to be ground before use. None I've seen ever have any clearance angle and most are as sharp as a rock.

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              • #8
                Not sure of the material, but synthetic opal sounds like a hard material. Perhaps you need diamond tooling, and perhaps the lathe is not the right machine to use- or you need to invest right away in some kind of toolpost spindle. Use the lathe to turn the part, but use something like a Dremel to turn a diamond cutter at high speed, and perhaps that material needs a constant water wash while the cut is going on.

                Would you be working with this material often? How much 'making tools to make tools' are you willing to get into?
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  Welcome to the forum.

                  Operating a machine, be it lathe or mill, like any endeavour, is a learning process. As with any learning process it's best taken in steps. Starting off making a part out of synthetic opal might be a bit ambitious for your first time on your new lathe. I'm going to recommend that you start by making the exact same part out of an easier to machine material like aluminum. Once you get comfortable with speeds, feeds, and tool geometry you can advance to your desired material making slight adjustments as needed for that material.

                  As it's been said before, a picture of your setup would be helpful for providing more specific guidance.

                  This forum is home to a wealth of information and encouragement just waiting for you. Ask many questions, analyze answers, and enjoy your new avocation.

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                  • #10
                    Try some CCGT and CCMT tooling. Better yet, pick up some HSS blanks and learn to grind proper tool bits, get sufficient at that, move to positive insert carbide like CCMT and CCGT.
                    Become a student of the craft, watch plenty of YouTube, mimic what the Pro's do. Granted, your 7x10 isn't a Monarch, but it will still cut metal.

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                    • #11
                      I think your going to have trouble trying to cut synthetic opal with carbide. That stuff is about 80% silica. It's like glass.
                      You might want to look into PCD inserts. Maybe there are some jewelers forums that may have more info. Us guys here usually work with metal or plastics.
                      It sounds like most of your problem is your trying to cut something with something that is almost the same substance rather than not being familiar with your new machine.
                      I've drilled glass before with carbide spade bits but that's slightly different and it's not the easiest thing to do.

                      JL..............
                      Last edited by JoeLee; 05-22-2020, 09:45 PM.

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                      • #12
                        If synthetic opal is similar to glass, conventional turning will only shatter it. It needs to be ground with diamond wheels and points.

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