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Depth of cut for lathe????

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  • Depth of cut for lathe????

    Hi all, I have been away for a while (and no, not prison..college... lol).

    Anyhow, I have been working on a very important project (turning a big bar into a small one..lol) And I am having some problems.

    I have a Chinese 9x20 lathe, that i just made a 4 bolt tool post clamp for. I have modified it with a variable speed motor, and just recently bought a live center for the tail stock, and I am using a 5/16 cobalt tool bit that I sharpened with an oil stone. I have tried all kinds of different speeds.

    I am turning between centers, with no lube.

    Its a Home Depot 1" x 12" bar, and from what I gather, that could be the problem, but...

    How deep should I be able to cut before it starts to chatter.

    Currently i can get to about .025 inch before it starts 'screeching'.

    Is this about all I can expect? Or am I doing something wrong?

    I thought I could cut .100 per pass with no problem, but.....

    Is .100 only reserved for the 'big boys'?

    Thanks, Phil
    \"I see\" said the blind man, as he picked up his hammer and saw.

  • #2
    Phil:

    What speed is the spindle turning at?

    Also, steel bought at HD may be of some dubious origin.
    Perhaps a pic of your tool bit may help...if you can get away w/ .020 per pass and finish the job, try that. .100 is probably pushing a 9x20 CL, I've never run that particular lathe maybe some owners may chime in.
    I bury my work

    Comment


    • #3
      A sharp and properly positioned tool bit should never screech. It may if your spindle speed is too high and it's burning the bit.
      It should be too busy cutting to screech. JT will tell you how to sharpen that bit.
      I use his methods for HSS now and it's pretty amazing how deep the cuts can be.
      Think .125 doc (.250 total)with an underpowered SB9..in backgear
      I have tools I don't even know I own...

      Comment


      • #4
        Crappy CRS steel can be a nightmare to turn. Cut deep and feed fast. Try not to take light cuts or it will tear and chatter.

        Comment


        • #5
          recoilless,

          I do not have an rpm meter (yet), so I can not give you the exact rpms, but trust me, I have tried all kinds of different speeds.

          As far as finishing the job, I am just experimenting. I would just like to know what the capabilities of the machine/me are.

          I have found occasionally, that if I start at .030 it will do o.k. for a while and then start chattering after a few inches.
          \"I see\" said the blind man, as he picked up his hammer and saw.

          Comment


          • #6
            "JT will tell you how to sharpen that bit."

            I would like to talk to JT. Does sharpening the bit have that much to do with it?

            I thought I did a decent job.

            I know I am ignorant in these matters, but doesn't the bit basically get "dull" after just a few seconds anyway?

            Hell, I cant sharpen my steak knives without my wife/kids using them for cutting open packages, Barbie packaging tie downs, screwdrivers, marble carvings, ice sculptures,.....etc.. and ruining the edge.
            \"I see\" said the blind man, as he picked up his hammer and saw.

            Comment


            • #7
              The machine is a bit light and a bit more flexible than some, so that will limit depth.

              Sharpen the bit to cut on the side, and not the end, with a 30 deg rake from left to right, you should get maximum depth of cut....

              Minimize the radius on the end, and relieve the bit away from the work on the end.

              Works for me.....

              The major issue is how much edge is in the cut, and how much cutting force is needed. With a flat top, "neutral rake" you have a lot of force. With strong positive rake, that is drastically reduced.

              Then also, radius on the end is all cutting edge, in the metal, but some of it is much longer than it needs to be per MM of cut depth.

              So a straight cutting edge, at 90 deg or so to spindle axis, gets the max depth of cut per mm of edge in the cut. Minimal radius, and raking away from the work on the end, both help.

              I can cut leaving an 0.090" "stem" when starting with a 0.312 diameter rod, in one pass, and let the stem stick out an inch.... With a radiused cutter, no way, you will push over the stem. (I use a 10" Logan, which admittedly is stouter than the 9 x 20).

              Your 9 x 20 needs all the help you can provide, because the flexibility encourages chatter. So use lots of rake, etc, and you will do the best it can.
              1601

              Keep eye on ball.
              Hashim Khan

              Comment


              • #8
                Tonight I found a rod of HRS. Ground a 1/4" hss bit, and tried a cut with my new lathemaster 8x14. Depth of cut was around .050, made very nice BLUE chips, no screeching.
                The 8x14 though IS a beefier machine than your 9x20, not a whole lot but enough for me to say that it is very close to my South Bend 9".

                If you are getting screeching, I would first look at your tool bit geometry, make absolutely sure nothing is rubbing and that the cutting edge is the only part of the tool touching the workpiece. A finishing bit has a large radius so more of the bit is contacting the workpiece, you must use a small doc or it will chatter.

                Comment


                • #9
                  See..I told you
                  You got pics of your cutter?
                  I have tools I don't even know I own...

                  Comment


                  • #10
                    There's a guy on Ebay that sells electronic optical RPM counters and also has an online store. I bought one long ago and it's excellent, and I received it within days of ordering it.

                    http://www.meter-depot.com/

                    Comment


                    • #11
                      I assume JT is short for J Tiers. Thanks for replying.

                      When you say "30 degrees from left to right", does this mean that that the bottom of the bit should be 30 degrees behind the top(a 30 degree slant from top to bottom)?

                      What does "relieving the bit from the work" mean?

                      The only example I can give, is in the book I bought. For Aluminum, it tells me to grind a very, very steep angle on the bit. Is this what you are talking about? However, for steel it says only 10 degrees or so.(almost flat-slightly tilted back).

                      I bought pre-made bits from my local supplier and the 'rake' is not very steep, and from the book I have, the 'rake' for steel is also very shallow. However I don't think they had my machine in mind when the went to print.

                      Is this the price I pay for a small machine?
                      \"I see\" said the blind man, as he picked up his hammer and saw.

                      Comment


                      • #12
                        "you must use a small doc or it will chatter."

                        What is a doc?

                        OMG, I am learning so much, it's like am back in class.


                        DP, no money for rpm counters right now, but trust me, it's on the list.


                        Torker, yea you were right.


                        BillH, My 9x20 could beat up your 8x14...LOL
                        \"I see\" said the blind man, as he picked up his hammer and saw.

                        Comment


                        • #13
                          DOC = Depth of Cut

                          How about this picture



                          This is a good read too... http://en.wikipedia.org/wiki/Tool_bit
                          Last edited by KiloBravo; 12-22-2008, 12:29 AM.

                          Comment


                          • #14
                            Originally posted by Phil McCrackin
                            "you must use a small doc or it will chatter."

                            What is a doc?

                            OMG, I am learning so much, it's like am back in class.


                            DP, no money for rpm counters right now, but trust me, it's on the list.


                            Torker, yea you were right.


                            BillH, My 9x20 could beat up your 8x14...LOL
                            Well size does matter, measure the bed width of your lathe, also measure the width between the dovetails of your crosslide. Now compare that to the 8x14 lathe. Hey, you just learned something new

                            Seriously, post some pictures so we can help you better.

                            Comment


                            • #15
                              Phil,

                              This will help with the angles.

                              http://littlemachineshop.com/Reference/RakeRelief.php

                              J are you saying 30 degrees of side rake?
                              -Dan S.
                              dans-hobbies.com

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