Single phase motors can effect lathe finish cuts
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  1. #1
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    Posted by Jack. Erskine. on December 25, 00 on the WWWMachine Shop board

    Dear Forrest. I remember a short time ago you gave a very explicit explanation about a Lathe that was producing a consistant irregular pattern on the finishing cuts ?. I have a machine that at a certain rev range and feed rate produces a very strange wavy type pattern on free maching steel, & I mean uneven & wavy, yet this does not appear when I increase the cutting speed,with out changing the feed rate. Also the strange situation is that this problem does not appear when I turn Aluminium or 304SS. materials ? any where near as bad, also this pattern is evident in a spiral form when I run a facing cut on a piece of 2ins. Alum. at the same settings. This machine is one of the better quality,??? 12.x36. Taiwanese Lathes which other wise performs great, it turns parrallel to .OOO2" over a 12", & faces dead flat after a lot of tinkering & setting up.It is a Geared Head Model & another strange thing is that if I tighten the Drive Belt from the Motor to the Headstock to tight , but not too tight by normal standards the Geared Head makes Quite a lot of increased gear noise to the point of being annoying. I stripped the the complete Gearbox assembly & replaced all the Roller & Ball Bearings with Top Quality Replacements, Very carefully rebuilt all the Gear box ,removing any excess clearances, shimming every thing up to very fine tolerances, reassembled, run the Lathe & it sounded exactly the same as before I started, slackened the belt off to the point where it would not slip when starting at the Top Speed of 1400RPM. The noise level was acceptable. I spoke to the Importers & Distributos of these Machines & they told me that both these problems could be caused by the Tawianese Electric Motor Cycling ? & advised me to try a better Quality Motor such as Aust, USA. or British . Do you think that the Motor could be contributing to both of my problems ?. I seem to recall that you made mention of this Frequency Situation ?. In your earlier posting on a similar problem. Also I have been advised that the situation could be improved by substituting a round section Polyeurathane Rounb Drive Belt instead of the current V Belt being used. I have checked & the Largest Diameter that I can purchase is 12.5.MM. Diameter. What is your opinoin of this modification. ?. The logic put forward for this argument was that the Quality of the Taiwanese Belts was very poor & could be contributing to the problems. I tried a New Gates Toothed Belt thinking this would help , but made little if no improvement, they said that the Poly Belt was a lot more forgiving, & could possidly Soak-Up this Surging Problem. I forgot to mention this machine is the same as the Lathe Sold By Grizzly in the USA. as their Model.G4003., just in case any other site visitors have had the same problem with a similar machine as I'm sure you dont use these Asian Machines Forrest. Another point which may throw some light on the problem, even though I get a quite acceptable finish you can still see a distinct consistantly repeating similar circular pattern on the length of the turned sample.( again mainly Steel.) I hope you can help me ? In anticipation I thank you in advance for your valued advise & I also wish to Sincerly Thank all those who took the time to reply to my earlier posting regarding 5C. Collets, I tried to post a Thank You To All but Could not Transmit at the time. Wishing Each & Every One OF You , A Safe & Happy Holiday Season. Best Regards Jack Erskine ,. in Sunny Australia.

    Reply by Forrest Addy

    Those of you who have bought small Asian built lathes equipped with single phase motors or if you converted a three phase machine to single phase by replacing the motor with a single phase unit, you may have encountered a finish problem. If the feed marks are closely examined they seem to resemble the grooves in an old fashioned record, that is they may have varying depth or an irregular appearance much like chatter. Extreme cases may look like hell.

    It's sometime difficult to distinguish this problem from another -- tool chatter. If the finish problems cannot be eliminated by the usual chatter remdedies, you might be coping with vibration "phonographing" the finish.

    The problem may be caused by a torsional vibration in the single phase motor, sometimes called "cogging." The armature of a motor is tightly gripped by the magnetic fields that cause it to rotate. A single phase motor acts something like a single cylinder engine where the power pulses may be several times the average torque of the full revolution. This pulse is transmitted by the belt to the spindle and the opposing torque transmitted by the motor stator to the machine. The consequent vibrations may meet where the tool contacts the work, recording itself on the finish like on a phonograph record.

    Here's a trouble shooting method: Slack the belts so they will barely drive the lathe while it's in a light cut. If possible, isolate the motor by slacking off the mounting bolts and slipping hose washers between the motor base and the mount and between the bolt head and whatever it bears against. Tighten the bolts just enough to hold the motor in position. The object is to inject as much compliance and damping as possible in what is usually a fairly rigid drive. If things wobble around a little it's OK. That's our plan. Take a couple of trial cuts and see if there's a significant improvement in the finish quality. This suggestion is intended to support diagnosis not a fix.

    If the pattern disappeared when the machine was practically de-coupled from the motor, that indicates the problem is definitely motor, not belts or tool chatter.

    The only solutions are to either to de-couple the motor via rubber motor mounts or procure a new motor smoother in operating characteristics. Often, there is something about in a lathe/bench assembly that propagates motor vibration. You might try placing sandbags here and there on your lathe bench or motor mount. Inert weight like sand or pea gravel blots vibration to an amazing degree. If sandbags cure the problem, chances are the bench or whatever need stiffening or reinforcement at those points.

    If your checkbook can stand it, a new motor might be the best solution because you can upgrade in power at the same time -- that is if your electrical system can stand the extra load.

    The best solution is naturally expensive: either a DC motor and solid state drive or a 3 phase motor preferably with a solid state drive if not a rotary converter.

    I also suggest a softer acting V-belt known as "Link Belt." This is a V-belt comprised of interlocked rubber/fabric links. Links can be added or removed to secure the desired length. Their principal advantage is their forgiving nature and vibration mitigation. Another big advantage is you won't have to dismantle your spindle to replace the V-belt, just cut the old belt off and link together the new belt.

    Needless to say, the smoother the drive the better. when I bought my lathe new in 1971, it came direct driven by a 3 phase motor but I replaced the motor with a 5 HP single phase to suit my power. I had the same perplexing "phonographing" problem. I solved it after a fashion by relocating the motor from the machine to the floor nearby using a V-belt drive. I got good productivity and good finishes but naturally it wasn't as smooth as a Monarch 1760, one of the last and best engine lathes built in the UScosting $75,000.

    A few years ago, I upgraded my lathe motor to 10 HP 3 Phase with a variable frequency drive. The motor happened to be precision balanced but I don't think that was a factor in performance improvement. It made an incredible difference not to have the 60 Hz torsional impulse or cogging. Where I used to see faint ripples in a cold drink set on the headstock, the machine now runs dead smooth. Where the gear noise used to be annoying, it's now a smooth musical whir.

    If anything, I now get better finishes with my machine than on the Monarch, thanks to a smooth drive free from torque impulse and vibration.

    A cautionary note. A 3 phase motor running on a static converter still develops significant torsional vibration and 2/3 rated HP. It will run smoother on a rotary converter and develop full HP. Therefore I suggest if you wish to change out single phase motors you obtain two three phase motors. One to run your lathe and another the next nominal HP larger from which to build a phase converter. I'd really like to suggest a variable speed drive but that might be too expensive for most people.

    I'm sure your own creativity will solve your vibration problem. I suggest using found materials where possible. Perhaps a small automotive engine mount or suspension component.

    Here is blubbery USA there's a large traffic in exercise machines. Naturally, they are seldom used and may be bought cheap at yard sales. They have smooth running 1 HP DC motors with a cheap variable speed drive that might be suited if your lathe is used exclusively for light work or for very small lathes like 6" and under.


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    Can someone suggest where one of the "Link Belts" for a Bridgeport J-head can be obtained?

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    Have a look on the web for "Brammer Belt". I think that this may be what you are talking about and if so it is certainly still available in the UK and probably in the US.

    Hope this helps

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    Runs in my mind the name to look for here in the States might be Fenner. I would try MSC or McMaster Carr. James

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    Thanks for the inputs. I'll take a look.

    JP

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    A 3 phase motor running on a static converter still develops significant torsional vibration and 2/3 rated HP.
    Actually more like 1/3 of rated HP in my experience. Excellent post otherwise.

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    I am getting a static 1to3 phase converter hooked up to run a Hardinge HLVH 220V/ 3phase lathe in my garage. Should I upgrade to a rotary converter instead? I dont need all the hp for the parts I make, but surface finish is a concern.

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    Quote Originally Posted by LatheWerks View Post
    I am getting a static 1to3 phase converter hooked up to run a Hardinge HLVH 220V/ 3phase lathe in my garage. Should I upgrade to a rotary converter instead? I dont need all the hp for the parts I make, but surface finish is a concern.
    Static is not the way to go........ If at all possible place a rotary phase converter outside of your shop... or at least the motor part. the static converters are not quiet either. they are sometimes called "buzz boxes".

    I am using a scrounged motor and a Pannel from Anderson converter. I would recommend looking for a 10 Hp motor to use for a Ideler and buying a pannel and wire t up. that would give you 3 phase for any 3 phase equipment you might want to drag home in the future. there are many thoughts and views on this but in the end most people use rotary phase converters or run VFd's on their 3 phase lathes.

    Just my 2 cents worth
    Happy Hunting
    archie =) =) =)

    P.S. no connection to anderson converter just a satisfied customer.
    my setup is posted here. http://jeep534.smugmug.com/gallery/1...09117250_g64MX

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    "Can someone suggest where one of the 'Link Belts' for a Bridgeport J-head can be obtained?"

    There are only two domestic manufacturers, and I believe even HF buys from one of them.

    The red ones are Fenner. The green ones are the other guy's.

    Both are essentially the same.

    Both of these replace the much, much earlier riveted, leather segmented belts.

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    McMaster-Carr, page 1024 has 3 different types of adjustable length belting.

    Mike


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