Lathe speed advise
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  1. #1
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    Default Lathe speed advise

    Hi Everyone.

    My 1st post here.

    I'm based in UK and have a small workshop at the end of my garden where I like to spend as much of my free time as possible.

    All my tools and machines are DIY rated at best. I'd love bigger better gear but that's never going to happen.

    Over the years I've got bored with woodworking and moved more into metalworking.

    About 12 yrs ago I managed to get a very old (100yrs) small lathe... a RandA, which appears to be based on a Portass.

    It was just a siezed up pile of rust and although I stripped, cleaned and rebuilt it I never knew enough to know how worn out some of the parts were.

    Over the years I've tried to 'improve' it but it's gradually dawned on me that it's me that needs improving as much as the lathe.

    Some of the 'improvements' I've made have actually been the complete opposite. So It's now getting a full overhaul, or more accurately a budget overhaul.

    It was previously mounted on just a piece of kitchen worktop that I now realise offered the rigidity of a wet blanket. That's being addressed.

    The 4" 3 jaw Burnerd I have discovered has 1 hard jaw and 2 soft jaws... and they are all very bell mouthed. New 3 and 4 jaw chucks (and backplates) are planned.

    The head stock bearings are not original. They've been remade (badly... even by my standards). New oil filled phosphor bronze bearings have just been fitted to the best standard I can achieve.

    The leadscrew nut was so worn that the backlash was around 270'. I made a new one from brass years ago but that has failed completely now and the original one is back in. That's terrible and needs replacing.

    I've got a small 3 phase motor and a single to 3 phase inverter for the drive but I have no idea what speed range / RPM to aim for with the pulley drive system.

    So, I guess the 1st question I'd like to ask is.... what's the RPM range I'm looking for considering this is purely a hobby set up turning ally / mild / stainless up to a max of 4" (4" will be for the backplates initially) but generally smaller diameters. I've got the back gears to get down low for screw cutting if / when needed.

    Thanks for any advise and I look forward to asking more questions (if I may) as my lathe adventure finally really begins.

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    I am going to assume that you are using High Speed Steel cutting tools (HSS) This material is appropriate for use on a 100 year old lathe with the stiffness of old bubble gum.

    Each material has a speed that works most productively HSS tool bits.
    That speed is the surface speed of the material, not just rpm. Like many things in life, size matters.

    Stainless steel wants to turn at 60 SFPM (Surface feet per minute)
    Plain carbon steel 100
    Aluminum 200

    Faster will wear out or destroy the cutting edge, slower means it takes longer to do the job.

    SFPM is the circumference of the item being cut* Rev/min. converting the circumference to units of feet.

    Largest diameter SS part you will want to cut will drive the low end of your gearing.
    The lubrication and design of your spindle bearings will drive the top end of your speed. I would not shoot for more than 1500 rpm.
    The rest is left as an exercise to the reader

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    Thank you Erich.

    Looks like I need to do some maths.

    After work this evening I broke down the whole rig as I've got some 20mm x 100mm steel bar that I want to use to add some rigidity.

    But first I did some checks and my rpm is lower than I thought.

    Without accounting for the back gear I have 85 to 680rpm.

    The pulleys I have to play with will allow me to (approximately) double or quadruple this.

    I think to quad this would ask a little too much of my miny little 1/4HP motor though.

    I have been trying to use carbide insert tooling but from what I read about speeds I think I was way too slow for this.

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    The size and quality of the spindle and bearings and their lubrication will limit the maximum speed if you want to get a reasonable life out of them.

    Larry

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    Hi Larry.

    The headstock / spindle bearings are brand new oil filled bronze, and of course I'll keep them well lubed.

    I'm not looking for this to perform like a beast, just be somewhere near the mark required for light hobby use with the occasional slightly heavier job.

    I bought spare bearings as I was amazed how cheap they are.... just £12 sterling for a set and some minimal work to fit them.

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    If you are doing one-ups and few-ups 800 to 1000 will be Ok, not always the best but cutting time is not a serious factor, but saving your machine is.

    if my machine I would think 800 plenty.

    Qt QP[New oil filled phosphor bronze bearings have just been fitted to the best standard I can achieve.]

    Likely I would add an oil dripper or two.
    https://www.amazon.com/drip-oiler/s?k=drip+oiler

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    As Erich Suggested I would hope to add.
    A formula for determining spindle speed.
    Rpm = 4 X Surface Ft per minute / diameter.

    Surface Feet/Minute using HSS Tool Bits.
    Low Carbon Steel 60 - 80
    Aluminum 200
    Brass 80
    Tool Steel 40

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    Quote Originally Posted by michiganbuck View Post
    Likely I would add an oil dripper or two.
    https://www.amazon.com/drip-oiler/s?k=drip+oiler
    There are 2 oil ports (1 above each bearing) already on the machine and I do lube every job.

    I re-arranged the pulley setup today uning what was available and should now have approx double thhe previous speeds, so up to about 1400 top end. Won't know for sure untill the whole rig is fully rebuilt.

    Made up a base (of sorts) using the 20mm x 100mm bar sections and this should rigid the whole thing up massively. Certainly doubled the weight anyway!!

    Found the manual for the inverter also, and there appear to be some settings in there that look like they'll give a torque boost. Also some settings to reduce the 'metallic' sound from the motor. Looking foreward to playing with them

    I had the max freq set to 80hz (uk standard is 50hz). Wonder what the max can safely be set to?

    I tried to up load a pic of the original flyer showing my old lathe but the site wouldn't let me upload it.

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    I thank you for the advise given to me in this thread.

    However, I have just read this in another thread while looking for answers to some of my problems...

    This place can be harsh, but it’s pretty simple.
    There are rules, and new folks don’t read them(or ignore them) and post forbidden topics.
    It’s a forum for professional machinists with professional problems, not for home-shop conversions and “ how can I do this cheapest possible way” topics

    There are other forums that specialize in hobby shop activity.


    So it looks very much like I've joined the wrong forum.

    My apologies.

    I'll take my amateur / hobby questions to another forum that's more my level.


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