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  1. #1
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    Default plus or minus on recommended speeds

    See attached photo. Small live centre for home made wood lathe. Made from a scrap of mild steel.

    Theoretically, my lathe isn't capable of the required RPM to cut this mild steel down to 1/4" diameter at 100fpm. I was doing maximum of 720 rpm so speed at 1/4" was 47fpm.

    However my parting tool works very well, and I thought it would be worth using it to plunge down to required diameter to see what the results looked like.

    After several plunge cuts to create the 1/4" stalk I traversed the tool to smooth out a couple of ridges.

    Anything silly or dangerous about doing that ? Yes I know its a cruddy finish.

    So ..................... does this note inspire any tips or tricks regarding cutting smaller diameters when lathe is theoretically incapable of the RPM ?

    It seems to me that when any operation - parting tool, or facing, or cutting a point - gets close to the centre then the speeds are miniscule compared to the recommended cutting speeds -is there a big plus/minus on the recommended cutting speeds for different materials ?

    Can I cut at lower speeds as long as I have a correspondingly lower feed rate ?

    Any good web pages to look at regarding this ?


    Bill
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  3. #2
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    Default

    Once again this is theory.

    The cutting speed/rpm you use will be governed by your machine.

    If you have say 1000 and 600 available and you worked out you need 800 you could try either.

    If on the other hand it worked out to be 700 I would 1st try 600.

    Over time you will be able to judge RPM based on experience and the type of cutting tool,length of job material you are using.

    The same can be done for feed rates.

    You have to do what works best for you with the tools you have.

  4. #3
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    Default

    Bill, when anyone quotes cutting speeds, they are quoting the speeds that are used industrially that are the best speeds to optimise tool life for maximum metal removal rates. While I would not go faster (good way to shorten tool life because of heat build up) slower is always an option.

    With traversing the parting tool remember that the tool will flex if pushed sideways, so
    • don't have too much material that a traversing tool needs to take off - the tool may flex enough that it will snap if the load is high enough
    • because a parting tool has a width when traversing the flex means that the corner of the tool will cut very slightly deeper than a plunge cut. If you need a diameter spot on you need to experiment a bit to find the right infeed for the tool geometry (I could never work out why doing that would not get the right diameter then saw the explanation in a book . Book learning is a wonderful thing)


    Motor speed will dictate how fast you can spin the job, motor power will dictate how big a cut you can take. Regardless of the speed you can get, feed and depth of cut (DOC) will always have to be adjusted to suit the power available at the time.

    Michael

  5. #4
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    Default

    Bill, you can always go slower with surface speed, just not faster.

    The speeds given by tables are intended for industrial use, on modern and heavy machines. Not for typical home shop machines, which are usually either very light or very old or both.

    Also, the speeds given by tables are a commercial compromise between tool life and time it takes to do the job. If you have no reason to value your time the same way as industry does, then bey all means go slower.

    Further, speeds given by tables assume you are using todays prevalent type of industrial tooling. A home lathe often does not have the rigidity to use such tools (especially not negative rake tools), nor the speed range required to get "dark blue and smoking" chips. In a home lathe one typically uses positive rake tools to minimize cutting forced and thus vibrations and chatter.

    No reason not to use a parting tool for finish turning small diameters. If in your special case it does the job and does it well, then do not feel bad about using it. What counts is the end result. The downside is that parting tools can only take small side loads, so you need to go slow.

    If you want more flexibility with spindle speeds, get a VFD variable speed drive. It will double your top speed. Among many other advantages. You will never look back. VFD variable speed is the best and most useful single modification you can do to a lathe. Chris

  6. #5
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    Default Book

    hi Bill

    I would recommend a little book written in the 1940's , original price was 3 shillings and sixpence . ( they turn up on EPAY )

    LATHE AND SHAPING MACHINE TOOLS : author DUPLEX published by Percival Marshall

    The book is aimed at hobby and hobby engineering people .

    The tools described in the book are meant for low powered hobby lathes .

    Mike

  7. #6
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    Default Thanks - understanding things a bit better now

    Thanks for responses - am understanding things a bit better now - I believed the speeds were critical and must be obeyed. Will learn more as I try different things.

    Quote Originally Posted by Michael G View Post

    Motor speed will dictate how fast you can spin the job, motor power will dictate how big a cut you can take.
    Michael

    Thanks Michael, If I try too big a cut can I do severe damage to the machine or simply stall the motor, break the tool, mess up the current job and give myself a bit of a scare ? I think that I’m currently very conservative doing fairly fine cuts.

    Quote Originally Posted by cba_melbourne View Post

    If you want more flexibility with spindle speeds, get a VFD variable speed drive. It will double your top speed. Among many other advantages. You will never look back. VFD variable speed is the best and most useful single modification you can do to a lathe. Chris
    Good One Chris, I have forwarded your reply to my wife for consideration at future birthdays/Christmases/lottery wins

    Have seen VFD control units on ebay for as low as $150ish but then I have to get a 3 phase motor as well so $300-400ish would do it


    Mike, am off searching for that book now.

    Regards

    Bill

  8. #7
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    Default

    Quote Originally Posted by steamingbill View Post
    If I try too big a cut can I do severe damage to the machine or simply stall the motor, break the tool, mess up the current job and give myself a bit of a scare ? I think that I’m currently very conservative doing fairly fine cuts.
    Severe damage is unlikely. If the V belts on your drive train are loose then all that will happen is that they will slip - a good reason to not have them too tight.
    Most people start with fine cuts and as confidence and knowledge build, increase the size of the cuts.

    Michael

  9. #8
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    Default

    For a reply for someone in a similar situation not long ago. I worked in engineering nearly 30 years ago but starting off fresh is still a big learning curve after all that time.

    Thanks Michael, If I try too big a cut can I do severe damage to the machine or simply stall the motor, break the tool, mess up the current job and give myself a bit of a scare ? I think that I’m currently very conservative doing fairly fine cuts.
    If you are being conservative and doing fairly fine cuts I doubt you will have much of a problem with over large cuts. You will gradually get more confidence and increase the work the lathe does and along the way learn its limits. This is largely dependant on the size lathe you have and whether it is worn. I am using a very old but very worn lathe. It can take some pretty heavy cuts but I have not reached its limit yet except when trying to cut ACME thread but that is another story.

    A diamond tool holder from Eccentric Engineering is a good tool to learn with because if an oversize cut is taken all that will happen is the tool bit will be pushed down in the holder. The tip will probably be damaged but that is not a big deal because they are very easy to sharpen.

    If I try too big a cut can I do severe damage to the machine or simply stall the motor
    Unlikely!

    break the tool, mess up the current job and give myself a bit of a scare ?
    These can happen. Call it a learning experience.

    All info given by other members here is good advice. Take your time and get confident at what you are doing.

    Regarding good Web pages, I think you are looking at the best one right here!

    Dean

  10. #9
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    Default Abe

    Quote Originally Posted by steamingbill View Post


    Mike, am off searching for that book now.

    Regards

    Bill
    FOUND A FEW COPIES ON abe BOOKS ...

    Lathe & Shaping Machine Tools. by Duplex: Percival Marshall & Co, London Paper Cover, First Edition - Tony Hutchinson

  11. #10
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    Default

    Just be carefull you don't blow the whole thing up,better to stop now rather than experience the ins and outs of the mechanical trade.

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