Announcement

Collapse
No announcement yet.

Newbie needs help with mini-lathe

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • 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
    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.

    Comment


    • #3
      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.

      Comment


      • #4
        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.

        Comment


        • #5
          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

          Comment


          • #6
            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?

            Comment


            • #7
              As Cameron said, brass doesen't like top rake, else it tends to grab.
              Location: North Central Texas

              Comment


              • #8
                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.
                Chilliwack BC, Canada

                Comment


                • #9
                  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.
                  in Toronto Ontario - where are you?

                  Comment


                  • #10
                    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!

                    Comment


                    • #11
                      Welcome to the HSM gang!

                      Here are some video clips showing my experience machining brass. Might give you some ideas:

                      http://enginuitysystems.com/pix/Slee...tting_1817.AVI (Turning 59 MB)

                      http://enginuitysystems.com/pix/Slee...tting_1818.AVI (Turning - light cut 81 MB)

                      http://enginuitysystems.com/pix/Slee...tting_1821.AVI (Grooving 61 MB)

                      This is the part I was making, before final clean-up. It's an air fitting for a Sleep Number bed:



                      The plastic male hose fitting from the bed had broken off inside the receptacle on the air hose:






                      My lathe is a Harbor Freight 9x20.
                      http://pauleschoen.com/pix/PM08_P76_P54.png
                      Paul , P S Technology, Inc. and MrTibbs
                      USA Maryland 21030

                      Comment


                      • #12
                        Originally posted by devils4ever View Post
                        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?
                        steel, Al, really anything but brass. no coolant needed with 360. Just take a blank and grind some side and front clearance on it (leave the top flat). If you just want to get on with what you've got, take one of your tools with positive rake and stone the top of the cutting edge flat, doesn't need to be very much to stop the digging in. btw, all your cutting will benefit from stoning after grinding
                        Last edited by Mcgyver; 05-13-2019, 03:41 PM.
                        in Toronto Ontario - where are you?

                        Comment


                        • #13
                          Most of the other guys have covered about all of the good pointers. I would like to add one point not yet covered. Cross slide and compound screw backlash removal before entering the cut.
                          There is another little "trick" that is known to work in turning copper and its alloys, and that is to set the cutting edge of the lathe tool a few thou (0.005 to 0.008) above the work center-line, which makes up for tool downward deflection and reduces the clearance just below the cutting edge.
                          Harder to take a big bite out of a BigMack or Whopper if your chin is in the way....heh heh
                          BTW, Tap Magic doesn't do much for copper and its alloys. Save your money.

                          Good Luck,
                          Amigo

                          Comment


                          • #14
                            Anothe +1 for what Cameron said. Flat top tool is best.

                            I never have much trouble with speed cutting brass, but maybe the mini-lathe is different. I am sure it is a lot lighter than my 10" Logan.
                            1601

                            Keep eye on ball.
                            Hashim Khan

                            Comment


                            • #15
                              If you are grinding tool bits you need to hand stone/polish the grinder marks out with a fine hand stone until the grinder marks don't show any more when looking for them with a 5 or 10 power magnification.

                              Comment

                              Working...
                              X