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  1. #1
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    Default Machining speeds

    Hi all, I'm a little confused about the speed I need to be turn the work piece on my lathe..

    My lathe is a lovely, well maintained Qualos, which as has a maximum of 648rpm according to the chart on the side. The machining calculators seem to think I need to be running at anything up to 2400rpm spindle speed and this speed goes through the roof if I'm using Carbide, instead of HSS.

    Am I missing something here?

  2. #2
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    Default

    Nope carbide can just handle some insane surface speeds, those calculators are mostly designed to optimise tool life in CNC machines.

    You can run it slower but it won't give the amazing surface finish you get when really pushing it.

    Sent from my Nokia 8 Sirocco using Tapatalk

  3. #3
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    Quote Originally Posted by caskwarrior View Post
    Nope carbide can just handle some insane surface speeds, those calculators are mostly designed to optimise tool life in CNC machines.

    You can run it slower but it won't give the amazing surface finish you get when really pushing it.

    Sent from my Nokia 8 Sirocco using Tapatalk
    Well, the old Qualos isn’t getting anywhere NEAR those speeds!

  4. #4
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    Default

    G'Day Davo,
    Here's a simple formula for spindle speeds; RPM = (300xV) divided by d, where "V" = cutting speed velocity in meters per minute & "d" is work diameter in millimetres. Approximate cutting speed velocities for some common materials using HSS are, Mild steel = 25 to 30 m/min, Brass = 35 to 40 m/min, Aluminium = 50 to 60 m/min, Cast Iron = 20 to 25 m/min. These are starting figures & vary according to the turning operation eg. screw cutting mild steel, the cutting speed would drop to around 5 to 8 m/min. If in doubt start at the lower end speed & increase to suit the application. If you are experiencing a built up edge on a HSS tool, it is often an indication that the cutting speed is too slow. Hope this helps.
    Regards,
    Don.

  5. #5
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    Quote Originally Posted by Don@2480 View Post
    G'Day Davo,
    Here's a simple formula for spindle speeds; RPM = (300xV) divided by d, where "V" = cutting speed velocity in meters per minute & "d" is work diameter in millimetres. Approximate cutting speed velocities for some common materials using HSS are, Mild steel = 25 to 30 m/min, Brass = 35 to 40 m/min, Aluminium = 50 to 60 m/min, Cast Iron = 20 to 25 m/min. These are starting figures & vary according to the turning operation eg. screw cutting mild steel, the cutting speed would drop to around 5 to 8 m/min. If in doubt start at the lower end speed & increase to suit the application. If you are experiencing a built up edge on a HSS tool, it is often an indication that the cutting speed is too slow. Hope this helps.
    Regards,
    Don.
    100% agree with this rule of thumb
    For mild steel with HSS I use 9000/ diameter

    For anyone interested in where this comes from

    s = pi * D *R
    • s = surface speed ie 30 m/min for steel
    • pi = 3.14...
    • D is the cutter diameter
    • R is RPM


    Rearranging we get
    R = s / (pi * D)

    I like to use mm for cutter diameter, so adjust by 1000 gives

    R = 1000 * s / (pi * D)
    R = 1000 * s / (3.1415 * D)
    R = 318.32 * s / D

    And if you work steel s = 30 m/min

    R = 9549/D

    I round this down to 9000 in my head because it's easier to remember and close enough for what I do
    and I triple the values when using carbide as this tends to work with my machines
    I also translate this to lathe work where D is the work diameter

  6. #6
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    Default

    Thanks again, guys!

  7. #7
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    Default

    I've been running the old Qualos in top gear at 648rpm and the finish is MUCH better using the Carbide. so I was thinking as the motor is belt drive to the gear head, could I just change the ratio of the belt drive to get more rpm at the spindle?

    obviously if I double the size of the motor pulley, it would make top gear 1296rpm, BUT, will the old Qualos cop with that?

  8. #8
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    Default

    Many of the earlier light duty lathes used white metal bushings for the spindle, and rely on a drip feed oiler or small oilcups for providing lubrication. This is quite viable if the machine is not expected to operate at maximum loads for entire industrial shifts, and the operator is good at maintaining oil levels. However if you take the spindle speeds beyond what the designer intended, you significantly decrease spindle bushing life, even with good oil maintenance.

    More recent lathes intended to operate at higher loads or speeds tend to have ball races or taper bearings to carry the loads and handle the speeds involved. There are a few models of Qualos lathes that range from light duty to medium duty, so it is hard to guess which model you have, but it would be a fair guess that winding the spindle speeds significantly would be detrimental to spindle bearing life.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  9. #9
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    Quote Originally Posted by malb View Post
    Many of the earlier light duty lathes used white metal bushings for the spindle, and rely on a drip feed oiler or small oilcups for providing lubrication. This is quite viable if the machine is not expected to operate at maximum loads for entire industrial shifts, and the operator is good at maintaining oil levels. However if you take the spindle speeds beyond what the designer intended, you significantly decrease spindle bushing life, even with good oil maintenance.

    More recent lathes intended to operate at higher loads or speeds tend to have ball races or taper bearings to carry the loads and handle the speeds involved. There are a few models of Qualos lathes that range from light duty to medium duty, so it is hard to guess which model you have, but it would be a fair guess that winding the spindle speeds significantly would be detrimental to spindle bearing life.

    It's a 5 1/2" gearhead. tapered roller bearing on the chuck side and ball race on the others...

  10. #10
    BobL is offline Member: Blue and white apron brigade
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    Default

    Davo, this is where a VFD is very useful.

    I tropical operate my plain bearing Hercus at the regular max speed of 1050rpm.
    However to perform a couple light finishing cuts on a job I top up the oil and run it up to 1400, even 1500 on say >12 mm diameter jobs..
    I've been doing that for some time - doesn't seen to have suffered any.

    A VFD helps in a lot of ways like being able to change the speed on the fly when parting.

  11. #11
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    Quote Originally Posted by BobL View Post
    Davo, this is where a VFD is very useful.

    I tropical operate my plain bearing Hercus at the regular max speed of 1050rpm.
    However to perform a couple light finishing cuts on a job I top up the oil and run it up to 1400, even 1500 on say >12 mm diameter jobs..
    I've been doing that for some time - doesn't seen to have suffered any.

    A VFD helps in a lot of ways like being able to change the speed on the fly when parting.
    It's funny, I spent forever trying to find a decent sized single phase lathe, only to find out I could have had 3 phase and just ran a VFD!

    Anyway, upgrading to a 3 phase motor and VFD maybe something I can consider for the future... until then I might just change the pulley on the motor!

  12. #12
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Davo81 View Post
    It's funny, I spent forever trying to find a decent sized single phase lathe, only to find out I could have had 3 phase and just ran a VFD!

    Anyway, upgrading to a 3 phase motor and VFD maybe something I can consider for the future... until then I might just change the pulley on the motor!
    Sure, once you put one on you'll wonder why you didn't do it earlier.

  13. #13
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    Quote Originally Posted by BobL View Post
    Sure, once you put one on you'll wonder why you didn't do it earlier.
    Well, I would have to debate whether I just buy a monster 3 phase unit and sell the Qualos on...

  14. #14
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    Default

    One thing to keep in mind when increasing pulley size to gain speed is the decrease in torque to the spindle. Too large an increase in pulley size could lead to decrease in cutting depth or ability.

    Tony

  15. #15
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    Quote Originally Posted by bts View Post
    One thing to keep in mind when increasing pulley size to gain speed is the decrease in torque to the spindle. Too large an increase in pulley size could lead to decrease in cutting depth or ability.

    Tony
    That’s true... I’ll give it a try, I can always put the smaller pulley back on.

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