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  1. #1
    Join Date
    Apr 2020
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    Melbourne Aust
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    Default Newby question re spindle speeds

    Please be gentle!

    firstly if this statement is wrong , don't bother reading any further lol

    so if I want to turn something, anything, I think I know this:
    firstly i need to know the type of product, ie ally, steel etc
    Then the type of tooling,
    then I need a diameter of the stock,
    then I need to calculate the best rpm to run the lathe at.
    I have a Hercus 9 CR and I also have a Sheraton 9,
    how do I calculate what rpm the machine is putting out on each pulley?
    If I physically measure the revolutions per minute where do I measure it from?
    If I put in a piece of stock and measure the revolutions, wouldn't the diameter of the stock vary the amount calculated?
    Am I way off here??
    This sounds like a ridiculous question to me!

  2. #2
    BobL is offline Member: Blue and white apron brigade
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    Perth
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    7,182

    Default

    I bypassed most of the above by installing a VFD and a tacho on my Hercus https://metalworkforums.com/f189/t20...ght=BobL+tacho . However, I don't use the tacho that much ahead of time, ie work out what rpms I should be using and target that - instead I tend to use it in reverse. In that thread you will see where on the machine I measure the speeds.

    In most cases I usually make sure I have a sharp tool and just get the workpiece turning and tweak speeds and feeds from there.
    When the curlies are coming off nicely I occasionally note the RPMs for next time.

    I find it really helps having a VFD because then you can change the RPMs on the fly - this is especially hand when parting as you can increase the speed as you get closer towards the centre. If a bit of vibe of chatter starts to creep in then often its just matter of a slight tweak either side of that speed will reduce or eliminate that.

    Same on the mill.

  3. #3
    Join Date
    Nov 2017
    Location
    Geelong, Australia
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    57
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    2,651

    Default Newby question re spindle speeds

    If you’ve got the ability to measure the lathe rpm then that will be easier than working out the gearing. The spindle and chuck are physically connected so you can measure anywhere on those. I’ve got an optical tacho that works off a bit of reflective tape stuck to the rotating item.
    I’d use the chuck.

    You can calculate the surface speeds etc but if calcs aren’t your thing then just find a chart online somewhere that has material, diameter, and RPM. If you have both HSS and carbide tooling just find a chart for HSS and multiply the RPM by 4 when using carbide.

    Steve

  4. #4
    Join Date
    Oct 2008
    Location
    Cairns, Q
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    666

    Default

    QUOTE: how do I calculate what rpm the machine is putting out on each pulley?

    With a belt driven lathe this is easy. It is correct for a flat belt driven lathe, and close enough for practical purposes for a V belt driven lathe. The speed of each driven pulley is proportional to the ratio between the two belt pulley diameters. Just measure the diameter of each pulley, the outside diameter on V belt pulleys.

    Say your motor speed is 1425 RPM, the motor pulley is 2 inches in diameter and the countershaft pulley is 6 inches in diameter. Then the countershaft speed will be 1425 x 2 divided by 6, i.e. 475 RPM.

    Say you have matching three step pulleys on the countershaft and lathe spindle whose diameters are: Countershaft 4", 3" and 2" and on the lathe are 2", 3", and 4". Then the lowest speed will be 475 (i.e countershaft speed) x 2 divided by 4 = 238 RPM, the middle speed will be the same as the countershaft, i.e 475 RPM since the pulley steps on countershaft and spindle pulleys are the same, and the highest speed will be 475 x 4 divided by 2, i.e. 950 RPM.

    Frank.

  5. #5
    Join Date
    Apr 2020
    Location
    Melbourne Aust
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    21

    Default

    Quote Originally Posted by franco View Post
    QUOTE: how do I calculate what rpm the machine is putting out on each pulley?

    With a belt driven lathe this is easy. It is correct for a flat belt driven lathe, and close enough for practical purposes for a V belt driven lathe. The speed of each driven pulley is proportional to the ratio between the two belt pulley diameters. Just measure the diameter of each pulley, the outside diameter on V belt pulleys.

    Say your motor speed is 1425 RPM, the motor pulley is 2 inches in diameter and the countershaft pulley is 6 inches in diameter. Then the countershaft speed will be 1425 x 2 divided by 6, i.e. 475 RPM.

    Say you have matching three step pulleys on the countershaft and lathe spindle whose diameters are: Countershaft 4", 3" and 2" and on the lathe are 2", 3", and 4". Then the lowest speed will be 475 (i.e countershaft speed) x 2 divided by 4 = 238 RPM, the middle speed will be the same as the countershaft, i.e 475 RPM since the pulley steps on countershaft and spindle pulleys are the same, and the highest speed will be 475 x 4 divided by 2, i.e. 950 RPM.

    Frank.
    Thanks Frank, I will now go and check if there is a spec plate on the motor with its rpm!

  6. #6
    Join Date
    Sep 2012
    Location
    York, North Yorkshire UK
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    6,439

    Default

    Hi AGeckoCan,

    Usually lathe induction motors are four pole and run at around 1440 rpm, though it could be a two pole motor and will run at 2880 rpm, twice the speed. Six and eight pole motors are much rarer.

    The rating plate will give you speed information as well as other data.
    Best Regards:
    Baron J.

  7. #7
    Join Date
    May 2019
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    New Zealand
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    13

    Default

    Quote Originally Posted by AGeckoCan View Post
    Please be gentle!


    If I put in a piece of stock and measure the revolutions, wouldn't the diameter of the stock vary the amount calculated?

    Hey buddy, maybe I misunderstood you, but the revolutions won't vary depending on the diameter of your stock. The surface speed varies, but not the RPM.
    Eric

  8. #8
    Join Date
    Jan 2004
    Location
    Mackay North Qld
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    6,446

    Default

    Hi, AGeckoCan
    Welcome to the forum
    Further to the post directly above this one. Surface travel speed is a big factor .Feet per minute or metres per second is usually how it is referred to. Once you place your stock-usually round stock -in the chuck the rpm set up is calculated.

    Each type of material whether it be steel, brass ,aluminium or whatever it is, has it own "sweet spot" at a cutting speed relative to a particular diameter.ie the recommended FPS.

    To explain, the metal is chucked ,the diameter known or measured is used to work out the required FPM and the lathe spindle rpm is set to give the closest figure to that FPM recommendation.

    The nomogram chart below is from the Tafe handbook and is marked in Metres per minute.

    It has worked very well for me over the years for my metric based Chinese built 12 x 36 ( 305mmx 900mm ) lathe.

    I have my copy laminated and close by to the lathe. After a bit of lathe time it becomes easy to assess the material and diameter and mentally calculate the RPM range and select a spindle speed.

    As you can see the materials and their recommended M/Minute are selected on the right ,the diameter on the top and the RPM is shown at the LH side.

    Example : Mild steel needs 30M/Minute, diagonal line is followed and at the point where the line intersects the vertical for the diameter turned-lets say 25mm, the horizontal line is followed out to the left where it indicates 400 rpm. Remember you can almost never get the exact figure as the lathe spindle speeds are close, but not exact to what you need.


    The other facets of the set up, the cutting tool, the cutting tool angle , the tool center height.the tool rigidity are all very important as well.

    Grahame


    Tafe feed & speed chart.jpg

  9. #9
    Join Date
    Apr 2020
    Location
    Melbourne Aust
    Posts
    21

    Default

    Quote Originally Posted by Grahame Collins View Post
    Hi, AGeckoCan
    Welcome to the forum
    Further to the post directly above this one. Surface travel speed is a big factor .Feet per minute or metres per second is usually how it is referred to. Once you place your stock-usually round stock -in the chuck the rpm set up is calculated.

    Each type of material whether it be steel, brass ,aluminium or whatever it is, has it own "sweet spot" at a cutting speed relative to a particular diameter.ie the recommended FPS.

    To explain, the metal is chucked ,the diameter known or measured is used to work out the required FPM and the lathe spindle rpm is set to give the closest figure to that FPM recommendation.

    The nomogram chart below is from the Tafe handbook and is marked in Metres per minute.

    It has worked very well for me over the years for my metric based Chinese built 12 x 36 ( 305mmx 900mm ) lathe.

    I have my copy laminated and close by to the lathe. After a bit of lathe time it becomes easy to assess the material and diameter and mentally calculate the RPM range and select a spindle speed.

    As you can see the materials and their recommended M/Minute are selected on the right ,the diameter on the top and the RPM is shown at the LH side.

    Example : Mild steel needs 30M/Minute, diagonal line is followed and at the point where the line intersects the vertical for the diameter turned-lets say 25mm, the horizontal line is followed out to the left where it indicates 400 rpm. Remember you can almost never get the exact figure as the lathe spindle speeds are close, but not exact to what you need.


    The other facets of the set up, the cutting tool, the cutting tool angle , the tool center height.the tool rigidity are all very important as well.

    Grahame


    Tafe feed & speed chart.jpg
    Thanks for that Grahame,
    I have downloaded that pic, cropped it, enlarged it to suite my failing eyes and have sent it off with the boss to her work for laminating lol.feed & speed chart.jpg

  10. #10
    Join Date
    Apr 2020
    Location
    Melbourne Aust
    Posts
    21

    Default

    Hello again brains trust,
    so based on the information I have learnt from your replies, I have done a
    drawing on what I believe the chuck speed would be on my Hercus.
    Could someone check my math please.
    I calculated it as follows:
    motor RPM 1425 x motor pulley dia 2.48" / countershaft main pulley dia 9.45" = 374 RPM round up to 375 RPM
    Then calculations continued:
    375 RPM x first pulley dia 5.40" / first headstock pulley 2.87" = 705 RPM at the chuck
    Is this correct?
    DazzUntitled.png

  11. #11
    Join Date
    Nov 2007
    Location
    melbourne australia
    Posts
    3,228

    Default

    Hello Dazz,
    A few years ago I fitted a tacho to my Hercus 260. I removed it shortly after I fitted a VFD because because I realised it didn't really matter what the actual RPM was, but rather whether I was getting the result I wanted. Now I set RPM by feel/experience. If I'm getting good chips with no chatter I'm happy. If not I adjust the speed.

    I see you are in Melbourne somewhere. If you can be bothered driving to the Mornington Peninsula you can have my tacho free of charge. I mounted the hall effect sensor inside the headstock side cover and put a magnet in one of the holes in the spindle.

    PM me if you want my address.
    Attached Images Attached Images
    Chris

  12. #12
    Join Date
    Apr 2020
    Location
    Melbourne Aust
    Posts
    21

    Default

    Quote Originally Posted by jack620 View Post
    Hello Dazz,
    A few years ago I fitted a tacho to my Hercus 260. I removed it shortly after I fitted a VFD because because I realised it didn't really matter what the actual RPM was, but rather whether I was getting the result I wanted. Now I set RPM by feel/experience. If I'm getting good chips with no chatter I'm happy. If not I adjust the speed.

    I see you are in Melbourne somewhere. If you can be bothered driving to the Mornington Peninsula you can have my tacho free of charge. I mounted the hall effect sensor inside the headstock side cover and put a magnet in one of the holes in the spindle.

    PM me if you want my address.

    G'day Chris,
    thank you so much for the generous offer, if im down that way i will definitely take you up on it.
    I'm in Macleod so a bit of a hike for me.
    I'm just starting out in metal work so my "feel" is still being developed!
    I really just wanted to get an idea of the RPM each pulley gives me so I have a base line to start from.
    If i change as per the recommended speeds it will give me more experience and familiarity with the machine.
    Thanks for your response mate. appreciate it
    Dazz

  13. #13
    Join Date
    Oct 2007
    Location
    Alexandra Vic
    Age
    69
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    654

    Default

    One small thing that appears not to have been mentioned when working out spindle speed vs motor speed is the backgear system which I believe will be present on the Hercus, if not on both machines.

    Your calculations above seem to be spot on for when the spindle and final step pulley are locked together, but the final step pulley can run free on the spindle by pulling a pin. Then moving the backgear lever forward will engage backgear and introduce a further reduction ratio of about 3:1 into the system, dropping your calculated speeds accordingly.
    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.

  14. #14
    Join Date
    May 2007
    Location
    Sth Gippsland Vic
    Posts
    130

    Default

    Quote Originally Posted by AGeckoCan View Post
    I have a Hercus 9 CR and I also have a Sheraton 9,
    how do I calculate what rpm the machine is putting out on each pulley?
    Good questions, I'm wanting to know the same things .

    This may be of some help . If we have the same pulleys and motor speed . I have an AR 9" Hercus and the other day bought an electric tachometer .
    I checked the spindle RPM speed for each pulley with back gear engaged and disengaged . I worked out the rev per second as well and stored it all on my phone .

    IMG_4637.PNGIMG_4638.PNG

    Motor speed (1420 rpm) and the $25 Tachometer. Handy little thing that Ill have some other uses for around the workshop with a milling machine .
    IMG_4639.PNGIMG_4640.jpg

    That's My lathe and pulley set up . Have you got the same Pulley set up on the CR?

    IMG_6806.PNGIMG_6807.jpgIMG_6808.jpg
    Edit . I just saw and read your post no 10 . Maybe Ill see if my pulleys are the same diameters as yours.
    Rob

  15. #15
    Join Date
    Apr 2020
    Location
    Melbourne Aust
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    21

    Default

    Quote Originally Posted by malb View Post
    One small thing that appears not to have been mentioned when working out spindle speed vs motor speed is the backgear system which I believe will be present on the Hercus, if not on both machines.

    Your calculations above seem to be spot on for when the spindle and final step pulley are locked together, but the final step pulley can run free on the spindle by pulling a pin. Then moving the backgear lever forward will engage backgear and introduce a further reduction ratio of about 3:1 into the system, dropping your calculated speeds accordingly.


    You are correct, I completely forgot about the back gears lol
    So with the back gear engaged I would just divide by 3?

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