Page 1 of 4 123 ... LastLast
Results 1 to 10 of 38

Thread: Newbie needs help with mini-lathe

  1. #1
    Join Date
    May 2019
    Posts
    9

    Default Newbie needs help with mini-lathe

    I'm a newbie when it comes to machining and I need a little help.

    I have a MicroMark MicroLux 7x16 mini-lathe. I've been trying to machining some brass (C360) on it and I'm getting what I would call "catches" or "grabs" when I'm turning. The work seems to catch and either leave a deeper than wanted gouge or the work jams and the motor stalls. I'm looking for why this is happening. It happens on face turning and length-wise with a slow feed rate.

    I'm taking very light cuts (0.003" - 0.005") and the RPMs are low (not sure the exact amount). Everything seems tight on the lathe. All gibs are tight without play as far as I can tell. I'm using HSS bits that I'm sharpening myself on a high-speed grinder. I'm fairly certain that the bit is very close to the center line of the work.

    Any suggestions?

    Thanks!

  2. #2
    Join Date
    May 2007
    Location
    Live Oak, TEXAS
    Posts
    1,815

    Default

    Brass is just about the easiest material to machine, and it likes high RPM's.
    If your RPM is low, then you should definitely ramp up your RPM's to at least 1000.
    Turning .005 with a hss tool shouldn't even stall a 1/4 Horsepower motor, unless your RPM is too low.
    Also make sure the chuck is tight, to prevent the part from spinning (Gouging), and your motor isn't trying to die.

  3. #3
    Join Date
    May 2019
    Posts
    9

    Default

    Okay, maybe my RPMs are too low. However, the brass piece is 1-1/4" in diameter. Is that still okay to spin at 1000 RPMs?

    I'll check the chuck as well.

  4. #4
    Join Date
    Feb 2012
    Location
    Metcalfe, Ontario, Canada
    Posts
    1,273

    Default

    What shape have you ground on the tool?

    Too much back or side rake can cause the tool to pull into the work when turning brass on a light lathe.

    If you're not using a tool with a flat top, that is, zero back and side rake, you might want to try one.

  5. #5
    Join Date
    Oct 2012
    Location
    San Antonio TX, USA
    Posts
    2,572

    Default

    also - how is the work held and supported? 3 jaw or 4 jaw? How much stick out? 1 1/4" is pretty chunky relative to the lathe, but it can still flex. Essentially the cutting tool pushes the metal away or up, so cutting less, then it springs back causing the cutter to take too big of a bite.

    Double check everything again, including the tool post to make sure nothing is loose or letting the cutter get pulled down relative to the work.

    Also might be worth honing the cutting edges after grinding, depending on how coarse your grinder wheel is.

    plus everything above

  6. #6
    Join Date
    Dec 2018
    Posts
    227

    Default

    your OP sounds like chatter to me.
    chatter comes from a myriad of things:
    something is loose: tool bit, the work, the spindle bearings, or anything else
    something has too much overhang: the tool bit sticking too far out of its holder, the work sticking out too much from the chuck, too much distance between chuck and tailstock
    are you using tailstock to steady up the work? using center rest?

  7. #7
    Join Date
    Jun 2001
    Location
    North Central Texas
    Posts
    2,508

    Default

    As Cameron said, brass doesen't like top rake, else it tends to grab.

  8. #8
    Join Date
    Dec 2015
    Location
    Chilliwack, BC, Canada
    Posts
    5,251

    Default

    Maybe not as high as 1000 for that size of bar stock but you want it to run faster than for steel. Or at least it CAN run faster than for steel.

    As mentioned the bad finish and catching is likely due to your setup. Minimize the overhang of everything with the tool bit, any holders for the tools and even make sure that the whole tool post is seated on the compound solidly and with maximum area. And don't let the compound hang out. Keep it back but with the dovetails fully engaged for maximum support and the shortest path from the tool tip down to the bed.

    And just because you can't feel any play in the gibs it doesn't mean that they are not loose. I tend to snug up the gibs on the compound so there's a slight drag. Takes a bit of fussing but pays off in many ways. I set up the cross slide so there's just a barely felt drag compared to how the hand wheel feels when it's actually loose. Here again a slight noticeable drag ensures that the cutter is not able to flex things and move where you don't want it to move. Everything metal is actually a spring after all.

    And finally a hearty DITTO on the HSS tools for brass being left flat on the top. Just make the front and end faces of the cutting portion sharp and relieved by around 4 to 5. If you do anything to the top side it should be to simply run the tool on a stone to remove any grinding burrs and ensure keenly sharp edges. But if you have a good free cutting grinding wheel this should not be needed.

  9. #9
    Join Date
    Mar 2005
    Location
    Toronto
    Posts
    10,754

    Default

    Quote Originally Posted by devils4ever View Post
    Okay, maybe my RPMs are too low. However, the brass piece is 1-1/4" in diameter. Is that still okay to spin at 1000 RPMs?
    .
    No. Speed is a function of material, tool material and DIAMETER. I can't ever recall brass benefiting from speed, exept you're done more quickly. Except with carbide, most of time in most materials if it works at X rpm, it works just as well at 1/4 rpm etc so cranking the speed up won't solve the problem.

    Tool geometry is likely the issue. As has been noted, zero rake and I bet you get a perfect finish.
    .

  10. #10
    Join Date
    May 2019
    Posts
    9

    Default

    I'm using pre-ground bits that I've been touching up on my grinder trying to keep the original angles. They are all positive angles. I guess it is pre-ground for steel?

    The tool is protruding a little from the tool post. I can probably pull it in.

    I do have some blanks, that I can try to grind all described.

    The work is being held in a 3-jaw chuck and is only protruding 1/4" or so since I'm doing a face cut now. It measures 1-1/4" diameter x 0.750" length. No tailstock since the work is not projecting much from the chuck.

    Everything seems tight, but I'll check again.

    I tried using Tap Magic to see if it helps. Should I be using a cutting fluid on brass?

    I'll try to post some pics tonight.

    Thanks to all!

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •