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Thread: Rookie lathe user needs some advice

  1. #1
    Join Date
    Nov 2018
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    Tampa Bay, FL
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    4

    Default Rookie lathe user needs some advice

    I'm brand new to machining and working on getting familiar with my Logan 11" lathe. I'm doing my first cuts and could use some advice.

    I'm using a home-ground 1/4" HSS right hand turning tool in an Armstrong tool holder. Cutting an unknown type of steel, 1-1/2" bar that I got from a scrap yard. No coolant, just some cutting oil before a pass. Turning to the left using power feed at 0.002" per rev at 535rpm.

    The cuts are producing very thin, curly chips. I'm assuming that's because my feed rate is so low. The surface feels like it has very tiny grooves in it. I don't think I rounded the tip of the tool enough. Also, when cutting 0.020" (0.040" off diameter) the lathe starts to bog down. It makes it, but you can tell that is about the limit.

    I'm guessing that my tool shape is not ideal, and that I'm likely not running at a good feed rate/RPM. Any advice on feed/RPM and depth-of-cut and what I should expect? BTW the lathe has a 1/2hp motor with flat-belt drive.

    Thanks!
    Mike M.
    Palm Harbor, FL

  2. #2
    Join Date
    Mar 2017
    Location
    central MA
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    282

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    The cutting speed(rpm) is a little fast. When using HSS cutters the *approximate* cutting speed for mild steel is 100 feet per minute. To convert that to rpm multiply by four and then divide by the diameter you are cutting: (100x4) / 1.5 = 267rpm. If you are cutting hig alloy steel use 75 feet per minute. For aluminum up the speed to 200 feet per minute. For brass, about 150 fpm. Note that these cutting speeds are approximates.

  3. #3
    Join Date
    Jan 2011
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    Cinti,Oh
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    To get a feel, buy some Known material from a reputable supplier....use the recommended SFM for that material...if your results are not good, look to your cutting tool.

    Ps-don't get 12L14...it cuts so easy almost any random tool and SFM will seem right.
    Last edited by 1200rpm; 02-03-2019 at 03:08 PM.

  4. #4
    Join Date
    Jan 2004
    Location
    Missouri
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    Default

    Turning inherently creates a grooed finish, that's just how a lathe cuts. The tool shape can improve that, but there is some of that effect regardless of shape. Grinding, lapping, etc are usually needed for items such as piston rods that must slide.

    Your speed is a little high, but being high does not bog the machine, exceeding the machine capability does. Capability is affected by motor power, drive type (belt or geared-head), the grind of the tool (flat topped or high rake) and so forth, including the type of material (which you do not know). Speed of the cut, in both feed and RPM, obviously afect the power needed.

    Can you tell if the "bogging" is due to belt slip?

    What shape is the tool? Does the top of it slope, or does the holder produce a sloping top?

    A tool with a definite slope down from left (cutting edge) to right will cut easier, and require less power.

    Some materials are just harder to cut.
    Last edited by J Tiers; 02-03-2019 at 07:10 PM.
    1601

    Keep eye on ball.
    Hashim Khan

  5. #5
    Join Date
    Jul 2018
    Location
    SE Michigan
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    Ah, a fellow newbie! 6 months after buying my hobby 10x22 lathe, I am still working on grinding, and refining basic HSS tools. Watched half a dozen YouTube videos on the subject and numerous articles. Still, it was many versions and tweaks later that I finally got a simple facing and cutting tool to nearly eliminate those ugly ridges. I found that a healthy radius on the tip helped a lot but that meant another tool grind to finish the newly created face as the original tool left too much radius there. I guess that is why everyone says the machine is a small cost compared to the tooling. .

  6. #6
    Join Date
    Mar 2010
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    1,075

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    For small lathes using HSS, a Diamond toolholder is a big improvement compared to the old style Armstrong set-up. https://eccentricengineering.com.au/...=32&Itemid=297 Sharpening is simplified and much faster. https://www.youtube.com/watch?v=xUAPrkC7Q-Q

    RWO
    Last edited by RWO; 02-03-2019 at 05:26 PM.

  7. #7
    Join Date
    Dec 2018
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    101

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    I am not a pro by any means, but my feeling is that if you are grinding your own tool, getting those curly chips, and NOT chattering, and chips not too hot, then you are doing good.
    If you just got that lathe, a Logan 11", then that lathe has been out of production for 40 some years. What size motor is that powered with??
    you say it is bogging down, my feeling is that it is too small a motor power.

  8. #8
    Join Date
    Oct 2013
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    USA MD 21030
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    Searching for "Effects of rake and clearance on lathe bits" I found this study. Only briefly skimmed through, but it may have some valuable info:

    http://psrcentre.org/images/extraima...%201211736.pdf (rake and feed rate vs torque)

    https://www.researchgate.net/publica..._Turning_Tools (rake vs tool life)

    http://www.minaprem.com/machining/cu...-and-function/ (clearance angle, and links to rake and other factors)

    http://www.engineeringenotes.com/ind...ineering/27210 (General information)

    I have found that I got good results with a very small clearance angle, close to the work, and an almost square cutting edge. I haven't done much lathe work lately, and I've been experimenting a bit with carbide insert tooling, while I've had most experience with HSS bits on my 9x20 lathe. It may help to know that the swarf from a turning operation with a fairly large DOC and small feed will be something like a slinky spring, so the left side of a right hand turning tool does most of the cutting and the nose follows the cut and helps shave the ridges as the operation continues.

  9. #9
    Join Date
    Nov 2008
    Location
    SF East Bay.
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    In general, you want small chips that are curled. You will see them referred to as "6s and 9s".

    If you are finding that it's bogging down at .020 DOC it would seem to point to too little FRONT clearance. While you are at it check the side clearance. 7 to 10 degrees is a usable general purpose angle for both front and side clearance in steel.

    While an 11 inch lathe is fairly good size, the 1/2 HP is kind of wimpy. Is it enough? Well, I used the HP calculator at http://www.custompartnet.com/calcula...ing-horsepower to figure that out. All values are from the first post. Given 1.07 IPM, .02 DOC, 1.5 inch cut diameter gives us 0.101 in3 /minute. Plug that into the next calculation with a value of 1 HP needed for cutting mild steel and you have spindle HP of 0.101 HP.

    Obviously your lathe should be able to handle both .020 DOC and .04 DOC ( .25 hp) without any problems.

    The other problem that you may be running into is top rake that is sloped to the nose or flat. That will increase the HP needed to remove the metal. A positive rake (slopes down below the cutting tip) uses less power but the edge needs sharpening more often. http://www.difference.minaprem.com/m...negative-rake/

    Dan
    At the end of the project, there is a profound difference between spare parts and extra parts.

  10. #10
    Join Date
    Jul 2017
    Location
    Buffalo NY USA
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    Seems like you're doing mostly OK as a fellow poster noted. Maybe just some time to refine your setup, maybe slow down the RPM's. I can highly recommend you download and read the Lathe operations manual, published by Atlas way back in the day. They spend a good deal of time with illustrations on tool grinding, using the Armstrong tool holder, speeds and feeds. Even though its for a different brand of lathe its an *excellent* all-around reference. Here's a link:
    https://coffeeshopmath.files.wordpre...ts-tables1.pdf

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