Useful spindle speed range for a Lathe ?
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  1. #1
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    Looking at buying a new lathe. Rimax and Steelmaster have what looks like a good product. Their 13X40 lathes, using single phase power have spindle speeds in the 70 to 2000 rpm range. Should I be looking for even slower speeds like 25 or 40 rpm ? What should help me in making my decision ? Are really low speeds useful and if so for what ?

    Sincerely,
    Tom [img]smile.gif[/img]

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    70 to 2000 is very workable. For generations cone pulley and back gear lathes did fine work with less range then the gear head lathes you refer to. You might want to run 80 ft/min at the largest swing and 300 ft per minute at your smallest practical stock reduction size. Something like that. High end engine lathes might have a spindle speed range of over 100 to 1.

    If you get a three phase motor with your new lathe you can install a VFD between the single phase and the lathe motor and get a much higher spindle speed range. I have one on my lathe and I can go from 1 RPM (20 RPM on the geared spindle speed or 6 Hz on the motor) to 3000 RPM (2000 RPM on the geared spindle speed at 90 Hz). I've never used the very low ranges but the higher speeds are nice for small diameter work.

    Call it to suit your line of work.

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    Tom.

    For general turning, drilling, boring work, you will never need such a low speed as 70 rpm. Even with big parts – the larger the work, the slower the rpm, to keep your surface speed at the tool correct. But on a 13” lathe, even if you were turning a 13” diameter that’s only 238 feet/min. Roughly speaking that’s a third of what HSS will put up with in steel.

    The slow speeds really come in when you are thread cutting. The slower the spindle speed, the slower the carriage travels. That would mostly be a problem with flying – course leads. You have to stop, or retract the tool, particularly cutting up to a shoulder. The faster the spindle is going, the quicker you have to react.

    All in all thou, 70 rpm’s is not that fast, and would be pretty normal on that size lathe. It’s still only just less than 1 turn per second. You would be hard pressed to find any thing big enough, that could be turned in such a machine that would require any thing slower.

    Vic, doesn’t cut it as a location. If you are any where near the big town. Your welcome to come and have a play with a Nardini, I have down the back. It has 25 – 40 & 63r.p.m in the bottom end. I’m in Thomastown, and in all day Saturday. Stuffed if I’ve ever used the bottom, end, except for maybe shaving the last little bit out of a acme thread, with a polished tool, and dark oil, just to bring the shine out on the flanks. My email is in the profile.

    I’m guessing with those brands, the vendors don’t have them under power. You’re welcome to come and have a look and play with 25 rpm. I doubt you will feel the need for it, against 70rpm.

    Regards Phil.

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    ........I've only been home hobby machining for a bit over 2 years now. I have a 11x36 Logan/Powermatic that is variable speed from 40 to 2000 rpm. I've never had it over 1100 rpms and do most of my work between 8-300 rpms. I don't know what I'd be doing to need 2K rpms.

    I have been doing most of my threading for the past several months with the spindle turning clockewise and the tool on the backside so the slow speed isn't really used that much anymore, either. Before it was real handy for threading.

    Rick

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    Forrest, Re your VFD suggestion: Running your motor at 90 Hz in top gear, I assume that your spindle is turning 50% faster than originally designed. Do you have any concerns about the gear train and spindle bearings when you do this? I have the same situation and am wondering where to set my high frequency limit.

    Brad

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    "Do you have any concerns about the gear train and spindle bearings when you do this?"

    Nope. I looked at the factors (gears, oil, bearings) It's a two taper roller bearing lathe so if the spindle warms it tends to reduce the pre-load. When I installed the VFD I ran the machine through the gears with a blank back plate on the spindle. I got to top gear, ran it for a while monitoring bearing temps, then incremented up with the speed pot. Above 100 Hz the rear bearing got a little warm so I set the Hz/max parameter to 90 Hz.

    The only time I come close to 2000 RPM is on small work like little knurled screws etc. They take only a few minutes to make and thread - too little time for the spindle to warm up. I max the RPM only to polish them. While I have 1 1/2 times rated RPM and it is handy it turns out I use it lightly. I made some bored aluminum sleeves for friends fishing reel. I drilled them a full 3" or so deep using milk and a 3/32 drill at 3000 RPM. That took a few minutes and the machine didn't suffer at all.

    In Hollywood when the pilots are incapacitated from bad food and the hero comes forward to fly the plane, as soon as he clicks off the autopilot the thing instantly dives to the ground. BS! That's stupid Hollywood "dramatic effect." The real world doesn't work like that. An airplane trimmed for altitude and power setting will fly straight and level gaining altitude slowly as fuel burns off for as long as there's fuelt to burn. Airplanes are really stable especially the big ones.

    It's mostly the same with any engineered product. Often the engineers and designers have good reason for setting limits but they can be tinkered and their performance exploited provided an incremental and logical approach is used where the various factors are brought up, considered, assessed, and provided for.

    So it worked on my lathe. Many low speed early WW II lathes were equipped with 500 RPM head stocks to suit the HSS tooling then available. Some of these have been re-pulleyed to double or even triple their RPM and the only change necessary was to back off the bearing preload.

    Increasing the spindle speed might work on your lathe too if it's equipped with roller bearings. If it's furnished with plain bearings there may be a definite limit to the max spindle speed attainable without re-clearancing.

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    only 238 feet/min. Roughly speaking that’s a third of what HSS will put up with in steel.


    Must be a mis-statement.

    John

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    John:

    That goes to show ya how far we Old Farts are behind the times.

    238 fpm is for their new Atomic high speed steel tools and it's for roughing. As he says, they go three times as fast for finishing.

    These newfangled lathes go 2,000 RPM in back gears.

    Whyyyyy - If I was a young 'un I wouldnt want any lathe that couldn't whip up to at least 38,452 RPM!

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    But on a 13” lathe, even if you were turning a 13” diameter that’s only 238 feet/min. Roughly speaking that’s a third of what HSS will put up with in steel.
    To my eye that's approaching the limit of carbide in alloy steel, and about 2 times what I'd expect HSS to take for too very long.

    Most production folks would put an upper limit of something like 100 fpm for low alloy and maybe 140 fpm for something easier machining with HSS. The carbide numbers would be all over the map but something like 600 fpm for a low alloy, maybe half that for higher alloy.

    On the original question - my 10EE has a speed range of 40-4000 rpm, and I've used it to both ends but not on the same day. The low end is handly when threading and occasionally when setting up, the high end when you're working on really small stuff, in my case when I was drilling some really small holes. I don't care to work on the lathe at 4000 rpm, there's something unnatural about a 3500 pound lathe doing 4K. I don't have the same problem with a friend's Levin at that speed.

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    If you ever need to wind springs, low end (very low end) is useful. Even small lathes like my Chippies should and do go down to 30 rpm and I do occasionally use it.

    Cheers, Stan.

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    My Rivett goes about 20 to 4000 and I can't say that I have used either end. 99% of my turning is in the 200-1500 range as I would expect it is for most any lathe with a 13" swing.

    I was turning some 1/2" 4140 yesterday afternoon, 1500 rpm, or about 200sfm, with carbide. Thats middle of the suggest speed range for the insert I was using. Could I have got faster? Sure, lots faster, but the surface finish was fantastic and the feed isn't overly hard to keep up with at that speed so thats what I used.

    With my first lathe, a 13x40 Colchester knock off, it had something like a 70-1800 range. At first I was always running stuff real slow, but I didn't know it then. After a while I started to get comfortable with higher speeds and liked the reduced time and improved finishes. This was a result of getting a copy of Machinist Handbook and learning about SFM cutting speeds.

    Look at the bright side, its a lot easier with a lathe than a mill, at least with a lathe you only have one cutting tooth to worry about. I still have to get out a calculator to figure speeds and feeds when milling.

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    Regarding the overspeed - I think your biggest concern should be the chuck and whatever is in it, not the spindle bearings. Just because I was curious about this, I looked up the PV and rpm limit ratings for NSK tapered roller bearings with a 75mm bore - I'm guessing that's about the size that would be found on a typical 13x40. The rpm limit with oil lubrication is 3800-4300 rpm, depending on the radial load. No worries there.

    I have a couple Bison chucks; the instructions that came with them were very clear about the max safe rpm of the chuck. I think that's your real "limit" on speed.

    Bison show quite a difference in max rpm between so-called "semi-steel" and steel chucks. For example, the max recommended rpm for an 8" semi-steel chuck is 2500 rpm. An 8" chuck with a steel body is rated to 4000 rpm. (I looked that up, so those are the actual numbers. If you download the Bison catalog from the TMX site you will find the rpm chart on page 152.)

    I think we can assume that those figures are conservative to establish a safety factor. But that is Bison and we can probably trust their numbers. When we start talking about the no-name chinese chuck that comes with an asian lathe I would be a lot more leery of testing its limits.

    Everyone has a risk level that they are comfortable with. My risk tolerance dictates that I would replace the no-name chuck with a chuck from a reputable company before running the lathe much past it's original upper speed.

    - Glenn M.

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    GlennM 15 years ago Bison was Polnisch junk
    Now it is a reputable company
    How long will it take the Chinese

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    Peter,

    The question of whether or not the Chinese are capable of making quality tools is beside the point, as is the history of Bison.

    My point is this: If you're trying to determine a max safe speed for the lathe, you have to look at the entire drive system. The chuck is potentially the weak link in the system, and the chuck is _definitely_ the piece that could do the most damage to the human portion of the system if it did fail.


    - Glenn M

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    Well, an enquiring mind could center up a cheap 8" Chinese chuck on a piece of 1/2 CRS about 2 feet long, remove it from the lathe, grip the other end of the 1/2 CRS in the collet of a 25,000 RPM router equipped with a tach, and slowly increase the RPM with a motor control until a loud noise happens. You might want to do this inside a scatter shield of some kind.

    Lemme know how it works out. By snailmail, preferably.

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    Forrest,

    I don't think that is a test for a novice. Only a seasoned, skilled machinist - such as yourself for example - should perform that sort of experiment.

    I think you should give it a whirl and see what flies out, then let us know the result.

    - Glenn M.

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    Yes, I suppose it could be interesting to spin up a cheap cast iron chuck, and see how fast it would run before it turned to shrapnel......but....y'know, some really careless person just might find out 'the hard way' with the wrong combination of lathe and chuck.....some of the '50's and '60's 14" and 16" Monarchs, Lodge & Shipleys, etc., were set up with a 2000 or 2500 rpm top speed....now, imagine a truly clueless operator fitting a new cheap cast iron 12" 3 or 4 jaw chuck, and spinning it up to 2500.

    Now, I've never seen such a thing happen....but I have seen, first hand, the wreckage after a 12x2" grinding wheel came apart at 1750 rpm. Fortunately, in that instance, the operator was standing to one side of the wheel and was not killed or injured.

    There is one very real safety consideration when using lathe chucks at a relatively high spindle speed. Its possible for the chuck jaws to be forced outward by centrifugal force (think about the weights of a governor) to an extent which will release a workpiece if its held with a relatively low chucking pressure, as in holding a delicate workpiece for polishing.

    I got to hear the 'bang' when this actually happened, to an inexperienced operator.

    The workpiece was released, at which point a chuck jaw struck it with sufficient force to send it across the shop at a high rate of speed...dunno how fast, exactly, but...lets just say it was travelling 'very fast'....not quite like an actual small-arms bullet, but plenty fast enough.

    The workpiece just 'grazed' the operator's head....if he had been standing another few inches to the right, he may well have lost his life.....no joke.....needless to say, the incident had a substantial adverse affect on the operator's morale level (to put it mildly).

    cheers

    Carla

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    A very real consideration for over revving a chuck is having the part fly out. The speed rating are for jaw grip strength at speed not so (much) that the chuck will fail. JRouche

  22. #19
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    John.

    Must be a mis-statement..
    Your caught me out cold. I shouldn’t even try to think in Imperial any more. Last thing on a Friday afternoon, while trying to get out the door. 30 years as a metric country. Clearly those figures are ambitious and wrong. My bad.

    I did think of another use for the very low ranges last night when driving home. They effectively lock the spindle, by not being able to back drive the transmission. That’s handy to hold the spindle, when running a tap or die by hand.

    Regards Phil.

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    Should I be looking for even slower speeds like 25 or 40 rpm ?
    The lower speeds are really nice for power tapping. Everything happens a lot slower at 40RPM than at 80RPM (imagine that :rolleyes: ). Unless you do a fair amount of power tapping, this shouldn't be much of a concern.

    Large OD work on hardened materials & large ID boring. Smaller lathes don't have 10,000# of mass & headstocks the size of a Volkswagon. A part that would run without chatter in a Pacemaker or L&S will need to run more slowly in a 13" machine. Boring, especially as you get farther away from the headstock, usually benefits from slower than book speeds - slowing down usually eliminates chatter & textured finishes.

    Drilling, especially as the drill size approaches 1" and over. Most lathe drilling is done dry, using HSS drill bits. The drillpoints are pretty sensitive to heat, more so as the depth of the hole goes past one or two diameters. 100RPM is a good speed for a 1" drill (25SFM), 50RPM for 2".
    ---------------------
    Barry Milton


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