Lathe RPM ?

Discussion in 'Horological Tools' started by Sterling, Jan 5, 2018.

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  1. Sterling

    Sterling Registered User

    Jun 10, 2009
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    jeweler, carburetor builder
    NY
    Sorry, verrrrry basic question here.
    But I just set up my Moseley 8mm with a pulley system and a 1920's sewing machine motor, and shot it with an RPM laser gun.
    It was THROUGH THE ROOF, I thought!

    What's the right RPM for cutting brass?
    Steel?
    Burnishing?
    Polishing with compound (on my junk lathe)?

    Thanks, all.
     
  2. Firegriff

    Firegriff Registered User
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    Feb 22, 2013
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    If you hooked the motor directly to the lathe be careful with the speed most from the old days had a secondary pulley reduction system to regulate lathe speed and keep the motor RPM up so the motor will not overheat. I am also a newbie and I have pondered speed. I also have the electronic tachometer and have wondered proper speeds. Also I have seen a lot of balances with spread out arms and I bet somebody put them in to remove the balance staff in and started the lathe at too high a speed destroying the balance.
     
  3. karlmansson

    karlmansson Registered User

    Apr 20, 2013
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    A countershaft is a good idea. It gives you more inertia in the system and allows you to run the motor at high speeds and still get a good rpm with higher torque at the headstock.

    The spindle speed is dependent on the type of material you are cutting and the diameter of the work. Surface feet/min in imperial measurements. You can look it up in the machinerys handbook or multiple online resources.

    Or you can arrive at you own conclusions by trial and error. It's also a matter of control if you are just starting out.

    Best of luck!
    Karl
     
  4. Sterling

    Sterling Registered User

    Jun 10, 2009
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    Thanks. I have a pulley set-up to give me 4 speeds. But the high speed was 4000 RPM! So i set up a larger pulley (2x) to reduce the speed by half. Still seems too fast.
     
  5. glenhead

    glenhead Registered User
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    Nov 15, 2009
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    Ok, quick and dirty, here's a wiki with the speeds: Speeds and feeds - Wikipedia
    Note that these are *maximum* speeds (surface feet (or meters) per minute (SFM). Slower is good, too. If you're turning too fast you're polishing, not cutting - the cutting edge doesn't have time to shave into the surface. (Yes, that's a gross oversimplification - let's not get all pedantic, mkay?)

    Just to keep from having to do multiple iterations of math, the rest of this is going to discuss feet and inches, not meters. (Old money, for you Brits.)

    To figure out surface feet per minute you're turning out, the formula is pi x diameter x RPM / 12.

    Examples:
    1-inch diameter stock turning at 1000 RPM = 262 SFM
    1/8-inch stock turning at 1000 RPM = 33 SFM

    Hardenable steel needs to turn in the 50-60 SFM range. Brass is 300-700 SFM.

    As someone else pointed out, all this is grand, but it's really all a matter of feel. You just have to get enough experience with cutting (by turning big metal pieces into tiny metal pieces) to get a feel for the correct speed, feed, angle of attack, etc. It's easy to get caught up in "analysis paralysis" trying to figure out exact numbers for this stuff.

    Hope this helps. Happy cutting!

    Glen
     
  6. Jim DuBois

    Jim DuBois Registered User
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    And perhaps we should also note the feeds and speeds all change with the type of material being cut, steel being the more important. Tool steel, water hardening, air hardening, oil hardening, mild steel, all will need different cutting speeds. And then there is the tool being used. Speed for carbide cutting tools is a lot faster than normal tool steel tooling.....I think glenhead is correct....trying to figure all this out carefully will give you headaches...how fast does my lathe need to turn to cut a .007" pivot in blue steel pivot material using a carbide tool?

    By the way, I recommend buying and using a Sherline drive motor and speed control on any smaller "jewelers lathe". The variable speed, the very high torque, and the modern motor will all give far better results than old sewing machine motors limping along with resistor foot speed controls and the like. They can be had directly from Sherline for under $150, or at least that is what the last one cost me...no commercial tie in to them by me, just trying to identify what I think is a better solution.

    IM000372.JPG IM000366.JPG derby 2.jpg
     
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  7. Sterling

    Sterling Registered User

    Jun 10, 2009
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    Thanks, guys.
    I've had a Sherline, and I have a Southbend for big work.
    I guess my question should've been, "What's the FASTEST RPM the Moseley should go?

    I think the fastest my 9" South end goes is about 700, though I'd have to look in the book. But chucking up .25" brass rod in a 3c collet on that tank and doing the same on the Moseley as it is now at the biggest pulley reduction is too crazy of a difference.

    I'll do the math for small diameter brass rod and use that as my high RPM. Worse case is I'll match the SB high RPM since I use the compound holder on the Moseley.
     
  8. Sterling

    Sterling Registered User

    Jun 10, 2009
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    Jim Dubois, I like how you'v mounted the Sherline motor.
    And is that mill a Levin?
     
  9. Jim DuBois

    Jim DuBois Registered User
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    Jun 14, 2008
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    The mill is an out of production Derbyshire. It came in vertical and horizontal models. Very handy for a lot of smaller clock work.....10mm collets..here are both models

    IMG_0847.JPG IMG_0848.JPG IMG_0849.JPG
     
  10. Tbucket

    Tbucket New Member

    May 7, 2016
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    When turning any metal on a lathe there are a few relatively simple technical features to understand. first is SFM (surface feet per minute). think of the SFM value as the temperature that the RPM will be producing. all metals require a certain temperature range, generated by the cutting action, where they machine the best. the factors involved are RPM, cutting material such as carbide or HSS (high speed steel) & the material being cut. Here is a simple formula for calculating recommended RPM:

    SFM X 3.82 divided by the diameter being cut = RPM
    SFM SFM
    Material Carbide HSS
    Aluminum 500 125
    Brass 500 125
    Steel 400 100
    Stainless 350 87

    If you are turning 1/2 diameter steel with a HSS turning tool, the calculation would be:
    100 x 3.82 = 382
    382 divided by .50 (1/2") = 764 RPM

    If you are turning 1/2 diameter steel with a Carbide turning tool, the calculation would be:
    400 x 3.83 = 1528
    1528 divided by .50 (1/2") = 3,056 RPM

    This all very general, but will give you a good idea where to start. when using pulley driven motors just try to get as close as possible.
     
  11. karlmansson

    karlmansson Registered User

    Apr 20, 2013
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    I think different cutting tools are picky to different degrees in regards to SF/M. I understand that carbide inserts need a higher SF/M and deeper depth of cut than HSS and carbon steel.

    If you are interested in learning more about the theory and mechanics involved in cutting with plastic deformation I recommend checking out a YouTube channel called That Lazy Machinist. A very charismatic and very teaching oriented man named Marc LeCuyer runs it. He has a couple of videos called "Chips, speeds and feeds" that was a real eye opener for me. What tbucket says above is absolutely correct, it's about the temperature and pressure (closely correlated) and the rake of the cutting tool (angle of the cutting edge in relation to the tool).

    In my own experience, this boils down to being more conservative when machining steels as it will have a tendency to work harden and polish if cut too fast, plus you will dull your tool. And I'm less conservative when it comes to brass. Brass work hardens when plastically deformed but doesn't burnish to the same extent as steel.

    Best of luck!
     
  12. Jim DuBois

    Jim DuBois Registered User
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    Where the formulas and SFM and the like break down is in some of the smaller work we do. Using the same formulas on high speed steel with carbide cutting tools we get something like 15,280 rpm for .010" dia work. Not a speed that sleeve/steel lathe bearings or for that matter most ball bearings will tolerate for very long. If we can even achieve that speed with our set ups? Very recently I was milling some very fine work in a fairly soft material using a .030" dia mill. Pretty big sizes for watch work. Recommended speed from the machinist manual calls for a spindle speed in excess of 50,000 rpm. Not do-able, my spindle max is 4000. Just pointing out that our smaller work will cause us to use a series of compromises.
     
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  13. karlmansson

    karlmansson Registered User

    Apr 20, 2013
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    Just saw a clip from a fairly new manufacture in Switzerland where they actually used those spindle speeds in production. Super specialized, high speed machines are needed though. So yes, I agree with what you are saying.
     
  14. Jim DuBois

    Jim DuBois Registered User
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    I had one machine that used water cooling of the spindle that was rated in excess of 50,000 rpm. It was a CNC circuit board drilling machine.....at least some companies seem to have moved on to laser drilling of such boards these days. Extremely high speed spindles can be a pain...for our purposes most of our machines stay under 5000 rpm because of drive and bearing limitations.
     
  15. Sterling

    Sterling Registered User

    Jun 10, 2009
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    This. The steel bearing was the concern.
    When I had the Sherline motor set up on my Geneva Wolf Jahn, the pulley reduction I used gave me 1/6th motor RPM, which equals a theoretical max of judt over 1,650, but with a LOT of torque.
    I've since sold the Sherline (and with it, the motor). That motor had a load comp circuit built in to the control, which I'm sorely missing now. So the installation of a fat flywheel on the transfer axle is in order.
    I reduced the pulley further yet, which should give me a no-load max RPM of about 1.5k.
    (I don't run them both at once like the picture shows.

    20171208_190713.jpg
     
  16. Jim DuBois

    Jim DuBois Registered User
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    Nice Levin Micro Drill....had one of those also, miss it....I am not as big on Sherline lathes and mills as I am their motors and controllers. Nothing wrong with Sherline mill and lathes, there are just others I personally like better. But dollar for dollar Sherlines are impossible to beat.
     
  17. Dushan Grujich

    Dushan Grujich Registered User

    Jun 20, 2003
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    #17 Dushan Grujich, Jan 7, 2018
    Last edited: Jan 7, 2018
    G'Day Jim!

    I cannot but concur with You. Different set of rules applies when we are using small sized watchmakers' lathes for turning small diameters. For driving any of my watchmaker's lathes I am using German Multifix motors with no additional counter shaft drives, the only pulley with multiple grooves is the headstock one, motors are with a single groove pulley.

    However, all the Multifix motors are repulsion induction motors, and are equipped with a lever which rotates the angle of brushes in relation to the armature thus effectively changing the RPM and direction of rotation by positioning the lever onto the opposite side from the zero speed position, one side being CW and the other CCW direction of motor shaft rotation. These motors provide almost constant torque over their working RPM range with full current flowing through the windings at any speed, hence their larger size. Perhaps it should be noted that on my small lathes I have never used spindle speed exceeding 2000-2200 RPM, at all. For the most part when turning tempered blue steel, with TC graver, spindle speed is between 800 and 1500 RPM

    When turning, I do not really adjust lathe spindle speed according to the tables, as they have shown to be pretty much useless. I strive to adjust the speed according to the quality and the looks of swarf that comes out from under the cutter or the graver. Of course, swarf differs from one material to the other as well as it depends on the geometry of the cutter. Nevertheless, swarf produced by hardened and tempered steel should be shiny, not dull in colour, also coming out curly in the long 'ropes'.

    Thus, I do not even bother to check the RPM of the spindle, after years of practice just the sound is enough of the indication of correct RPM, and if needed easily adjusted for the quality of swarf, and, what is the most important the quality of turning finish of the part itself. The aim is to turn a part in such a manner to end up with a piece that requires minimal additional work to get it to a finished state, often no more than just burnishing being the final step in finishing it.

    One does learn through practice to adjust the cutting (peripheral) speed for different materials as well as the cutter angles, which does not apply to the use of gravers as their angles can be easily adjusted by changing the position of the graver. Also one should keep in mind that some materials need some sort of lubrication i.e. cooling media during cutting, or the cut i.e. swarf produced shall not be optimal, particles of metal sticking onto the cutting edge and dulling the cutter.

    This would be a short description of my procedure of setting up the cutting speed, it also applies to milling and drilling.

    Cheers

    Dushan
     
  18. Jerry Kieffer

    Jerry Kieffer Registered User
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    While I do not own a Multifix motor, I occasionally use one at the NAWCC School. Your description is accurate in my experience and I have had no issues with this motor. However, I do prefer the Sherline motor and control. The Multifix was designed and offered well before solid state DC motor controls were refined and greatly reduced in price. As such, the Sherline offers the the same characteristics such as full load torque from 0 rpm on up at a lesser cost now days. It also allows me to power thread with a Tap and Die on the lathe at speeds that are very comfortable. An electric reverse switch allows me to instantly reverse the spindle by flipping the switch while under full load.

    As far as cutting speeds for Micro items, I completely agree with your comments. I personally adjust speed under load to achieve the best results. This often ends up being speeds well under 2000 rpm.

    Jerry Kieffer
     
  19. Sterling

    Sterling Registered User

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    Thank you everyone for your input, and Hello to you, Jerry Kieffer!
    I've been absent from the forum for quite some time, so I don't know if you recall, but you answered a lot of questions for me when I first acquired the Moseley lathe.

    The very small sewing machine motor (about 1930, It believe) works actually quite well now that it has been reduced using an additional pulley. However, it's still subject to belt tension and load. Temporarily, I used a Sherline 4 jaw chuck as a flywheel on the transition pulley, and though it made a noticeable difference in keeping inertial motion, it was only at higher speeds.

    I believe (although I was hoping to keep the period aesthetics of the lathe) that a Sherline motor and control may be in order for the best functionality. (Though I'll look into Dushan's described motor as well.)

    However, the thought has occurred to me that, since the motor I have is in fact an AC-DC motor, might a load compensation circuit board be used to maintain RPMs while cutting, or am I just looking to burn the house down?
    I don't know if the Sherline motors have dual field windings.
     
  20. Jerry Kieffer

    Jerry Kieffer Registered User
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    I seem to remember several conversations on watchmakers lathes, but, to honest I cannot remember what happened yesterday since I turned 60. Not knowing exactly what you have, you can experiment with anything as long as its fused for its rated output. The current Sherline motors are strictly DC motors, unless you have a early one without brushes designed for only AC. These AC motors and their controls are unstable and undesirable at least from my personal perspective.
    The current Sherline DC motors use a version of a Pulse width modulation wave form circuit. The motor is at full voltage and full strength at the slowest speed that is very smooth and allows full width cuts at very slow speeds per the attached photo. While shown in brass because I happen to have the photo, its just as easily done in steel. On watchmakers lathes with oil film cone bearings, its best to avoid heavy drive belt tensions that can cause bearing issues in some cases. Always properly adjust and lubricate at all times.

    Welcome Back
    Jerry Kieffer

    DSCN7218.JPG
     
  21. John MacArthur

    John MacArthur Registered User
    NAWCC Member

    Another solution. I bought this setup from Grainger some 25 years ago, and adapted it. It's a D.C. motor and controller - full torque at all rpms, with instant reversing. I, like others, usually turn at slower speeds than would calculate from sfpm, often much slower. It's a great setup.

    Johnny

    NCM_0183.JPG
     
  22. wefalck

    wefalck Registered User

    Mar 29, 2011
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    Didn't we have the same discussion a short while ago ?

    Anyway, I also go by the look (of the swarf and the turned surface) and feel (when feeding), rather than by the book.

    Theoretically, these hard steel bearings seem to have been rated for up to 4000 rpms, considering what kind of motors some of the manufacturers offered for them.

    I have both, a nearly 20 year old Sherline motor on the lathe and a Multifix repulsion motor on one of my mills. I actually quite like the repulsion motors, because they run very quietly. A motor with brushes is always more noisy. The rotor of the Mulitfix has also nearly twice the diameter of the Sherline, which gives it a lot of inertia.
     
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