Tips on selecting a lathe -will it thread

Discussion in 'Tools' started by Tin Falcon, Oct 28, 2012.

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  1. Oct 28, 2012 #1

    Tin Falcon

    Tin Falcon

    Tin Falcon

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    On another thread no pun intended someone asked about the threading capability of the new south bend 8 x 18 lathe the following is based on my answer with a few more details added in.

    One serious consideration when purchasing a lathe is threading capability. How does the lathe create the proper ratio between the lead screw and head stock.




    Yes it will indeed thread!
    In early days of of the screw threading lathe the only way to change the pitch of a screw being threaded is open the head stock cover, if it had one.( sometimes they were exposed) and change out the gears to set the ratio of gearing between the head stock spindle and the lead screw. these gears are furnished as a set with the lathe, Hence the term change gears and change gear set.
    a set will allow for all common threads to be made either imperial , metric or both.depending on the lathe.

    Modern production lathes have a gear box with a series of levers or knobs that allow to change the ratio of gearing in the head stock quickly .
    this gearing also sets feed rate for automatic feeds.

    Modern hobby lathes may be a hybrid you can quickly change thread pitches with levers or knobs but within limits like imperial threads or feed rates or metric threads . if you need to change MODE open the box and switch change gears.

    the old way saves money on production cost but takes time to change out gears.

    A full featured QC gear box is an expensive option. but in a job shop where things change fast an furious it save production time and setup cost.

    the hybrid is a compromise it makes threading setup easier and faster and keeps costs reasonable.

    Some of the small hobby lathes like taig and sherline have no head stock gearing at all . this is an added option.

    Single point threading was one of the basic skills learned on the lathe in tech school for me. As a hobbyist one needs to decide how important the skill is and how important the threading capability of his or her lathe is.

    Added info:
    Metric vs imperial most lathes I have seen are either designed to measure and function and are built either to be a metric machine or a imperial machine. the difference is the lead and feed screws are made to either metric or imperial standards. so if one has an imperial standard machine and wants to thread you need a transposition gear of 127 teeth or in the case of the 7 x mini lathes a 21 tooth gear. and this gives a close approximation.

    So the lesson here if wanting to do serious threading get a lathe built for what you work with or are comfortable with. some machines are built metric and only aproximate us decimal inches. The newer imports have corrected this and use inch based screws.

    CNC can be a horse of a different color. Look ma no gears. If the head-stock of a cnc lathe has a spindle sensor the computer can set the ratio of the spindle speed to the lead/feed screw electronically so you program in the numbers and the computer does the threading. just swith working units to metric or inches and the computer does the math
    I/we still recommend learning to thread with a manual machine.


    These are some things to keep in mind when selecting a lathe . Threading capability is one consideration. and selecting a hobby lathe can be tough. I learned to machine on a real lathe a modern south bend and a standard modern. full featured lathes with all the capabilities thereof.
    My first home lathe a grizzly 7x10 champagne tastes on a ginger ale budget to say the least. The one advantage I had was a target of what a real lathe will do.I had to get what I could afford and not give away all capabilities . If I bought a taig (one of my first considerations) it would not thread at all.

    One must still consider things like budget accessories size of work to be done old iron vs import, swing and center to center size available workshop space shipping weight shop location (Garage basement attic) and a few other things. I hope this helps the new folk.




    Tin
     
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  2. Nov 4, 2012 #2

    Tin Falcon

    Tin Falcon

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    One of the factors to consider when purchasing a lathe is what accessories and tooling comes with the lathe. Check everything and assume nothing.
    It is hard to define standard tooling.
    IMHO a well equipped lathe should have:
    3 jaw scroll chuck
    4 jaw independent chuck
    face plate
    Quick change tool holder
    machinable center
    live center
    drill chuck or tail stock chuck

    the Basic Seig C2/C3 aka 7xs come with a three jaw and a center and a basic tool post.
    a 9 x 19 has a, face plate, steady rest, follow rest, and a woodworkers cheap 4 jaw .

    The point here is if a lathe costs say $ 600 and adding the steady ,the follow rest the face plate and a 4 jaw these will likely add up to another $ 200. then add shipping and in the case of Harbor freight add the extended warranty cost. so you are now close to the cost of a similarly equipped 9x19.

    The other question is how much do you need or want the steady rest and follow rest. IMHO there is rely no place to put a steady on a 7 x 10 a 7 x 16 maybe.
    Other stuff: if you are turning on centers you need a machinable center and a drive dog as well as a drive plate or face plate with slots. You may also need mandrels.

    When using a face plate you need clamps a angle plate, and often a counterweight.

    You will need cutting bits. IMHO HSS is best for hobby machines
    right hand bit for turning LH bit for facing and a parting grooving bit.

    Drill bits. Screw machine length will save spce on short lathes but are more expensive than jobbers length at least in sets.

    Buy or make a boring bar or bars generally use the largest that will fit.

    reamers will help smooth and size bores of cylinders .
    laps will refine cylinder bores.

    A scissors knurl will save on bearing wear. as apposed to a plunge knurl.

    A used lathe can come from stripped to well equipped so when looking at a used lathe take careful inventory.


    Tin
     
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  3. Nov 4, 2012 #3

    Tin Falcon

    Tin Falcon

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    Another consideration in lathe selection is how the chuck face plate etc mounted to the spindle on the head stock.
    1: theads. there are many sizes here. some are metric. make sure you can get upgrades and spares to fit the lathe unless you are prepared to make your own back plates. been there done it.
    2: bolt on. Works but do not expect quick set up changes. found on the seig C2/C3.
    3: D camlock there are several sizes these are rare on import hobby lathes . Nice if you can find a lathe with this mounting system.
    4: L taper mount found on older industrial lathes. Do not recall seeing this mount on new imports.

    Warning a screw in chuck can screw off if the spindle direction is reversed unless there is some sort of locking feature to prevent this.

    also a spindle that has a 5 morse taper will take 5c collets.
    a 3mt will take 3c collets.
    Tin
     
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  4. Nov 4, 2012 #4

    Tin Falcon

    Tin Falcon

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    lathes are sizes in terms of swing or the largest diameter one can turn. this is USA definition. The other important measurement is center to center lengths.

    As mentioned earlier 7x10,12,14 and 9 x 19 are common hobby sized lathes.
    is bigger better. in most cases yes in general you can make small parts on big lathe but not the other way around. but beware. a 1/8 in brass rod requires a speed of 9,000 rpm this may exceed the speed limit of a large chuck. and a large lathe may not run that fast. so there are always trade offs. A good rule of thumb is get a little bigger that what you think you need in swing . as far as cts it is hard to have too much. .
    If you choose a larger lathe you may need a couple different sized chucks face plates etc.
    Also in general the larger the lthe the more real lathe features it will have and the less hassle changing tooling.

    get what you can afford.
    what you have room for
    what you can move into your space safely.
    Tin
     
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  5. Nov 4, 2012 #5

    kvom

    kvom

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    The tailstock drill chuck should take at least 1/2" bits, and up to 3/4" is better if the tailstock taper is large enough. Many 3/4" chucks only go down to 1/8", and in this case you'd need a second chuck for the smaller drill bits.

    For threading you need to either grind a 60-degree HSS bit, or buy a toolholder with inserts. You also want a 60-degree thread gauge and a set of pitch gauges.

    Pay attention to the spindle hole size; larger is better. If you think you'll be machining long bars then position the lathe so that you can feed the bar through the back of the chuck through the spindle.
     
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  6. Nov 4, 2012 #6

    Tin Falcon

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    One of the most basic but confusing aspect of machining is holding the work. It is important when choosing a lathe what choices you have in this respect.

    3 jaw chuck . this is probably the first work holding device that comes to mind for a lathe. It is quick and easy to use. It will never run true unless you have a set true chuck. a set true is more expensive and not common in the home shop.
    a 3 jaw is perfect for hex stock and general machining of round stock. good for a simple part like a hat bushing.

    4 Jaw chuck: more versatile easy to dial in a part. good for many odd shaped parts. and is great for square stock. It can be offset for eccentrics etc. Take a bit more time to clamp in the part.

    A face plate allows one to clamp a part down . you will need clamps often a counter weight to balance things and sometimes a angle plate to hold the work. an old school but effective way to hold model engine parts.

    Turing on centers. the part is held between a center in the head-stock. A machinable center will ensure you are truly on center. and a center in the tail stock. you may need to adjust the tail stock for true center to prevent taper or kick it over to set a taper. a lathe dog is clamped to the part and the dog leg is inserted into a face plate or drive plate slot. turning on centers is great for shafts. a mandrel may also be placed on centers . a mandrel hold a part with a a hole in the middle like a gear blank or a cylinder.

    Collets. Indusrial lathes have a 5 morse taper in the head stock . this allows for 5c collet holders to be mounted in the head stock and allow for 5C collets. . smaller lathes with 3 MT tapers can accomadate 3c collets.
    5c collets are more common and have a greater size range . 3C only go to 1/2 in round. collets come in various sets usually in 1/32 or 1/64 increments.
    you can only use a collet for the specified size with a little tolerances built in.

    collets save Z axis space hold the work truer than a 3 jaw hold the work all the way around and not just 3 or 4 points so good for tubing.
    collets are also made for hex stock and square stock as well as round.
    so a collet adapter is something to consider when purchasing a lathe.

    A steady rest hold a long part so it can be machined .

    a follow rest sits behind the cutting tool and supports long thin work as it is cut. it follows the cutter.

    Tin
     
  7. Nov 4, 2012 #7

    kvom

    kvom

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    Some other options for workholding:

    1) Soft jaws. Some 3-jaw chucks have 2 piece jaws, so that the outer jaw can be removed and replaced with aluminum (or other soft material) versions. These can then be bored out to hold a specific diameter workpiece. The work can be removed and replaced without affecting runout as long as the chuck is not removed from the lathe. This method is one of the best ways to hold and machine thin parts.

    2) Jacobs rubberflex collets. These are an older collet style where the collet is a stiff rubber with some metal "jaws" embedded. Unlike 5C collets, the rubber allows a wide range of sizes per collet, and larger stock can be held. Collets and chucks can sometimes be found on eBay.
     
  8. Nov 11, 2012 #8

    Dunc1

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    While threading capability is great to have, on a practical basis on many hobby lathes it is difficult to practice. I have a mini & a 10 inch (5 inch center height) lathes. Both have threading capability (lead-screw, change gears, etc); however, their minimum speed (for the mini: add with adequate torque) is much too high to be able to thread. There is a reason that a Myford (and many others) has a back-gear to achieve speeds that are slow enough that everything is not happening quicker than I, at least, can think - let alone react.

    For the mini there are a number of plans out there to add a spindle crank to turn the lathe manually. For the larger lathe I am toying with the idea of adding a jack-shaft to the motor drive to reduce the speed. In the meantime, I have been using taps/dies.
     
  9. Nov 11, 2012 #9

    Tin Falcon

    Tin Falcon

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    Dunc:
    lets confine this thread to tips on selecting a lathe.

    we can discuss workarounds an modifications in there popper places.

    http://www.homemodelenginemachinist.com/f25/threading-mini-lathe-19305/#post199866

    I invite others to add to this thread on lathe selection.

    If you want to share workarounds to the limitations of your lathe please post in the tips and tricks section.
    If you have modified your lathe to improve it lets see that in the machine mod section
    I would like this to be resourse for those wanting to buy a lathe and hope to avoid a tangled thread.

    Tin
     
  10. Nov 11, 2012 #10

    Tin Falcon

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    Another consideration in selecting a lathe is what kind of motor is mounted on the unit. also how does the operator control the spindle speeds.

    motors can span from 1/4 HP to several HP on an industrial lathe . a 7x import lathe may have a 350 watt motor or a 500 watt motor depending on who is selling it. these lathes have variable speed dc motors to control speed. so one can change speed on the fly.

    Another common set up on hobby lathes is belt and pulley stop the machine loosen the belt move it an re tighten to get speed changes.

    Gear head machines use gears to set speeds.

    Large industrial machines use a variable frequency drive to set motor speeds.
    and these vfds will translate 220 two phase power to industrial 3 phase.

    More power will increase productivity and production rates but beware while it is a nuisance to have a motor stall or a belt slip when overloaded a Multi hp motor will gladly bend things and suck an operator into the machine if something stupid happens.
    So more to think about.
    Tin
     
  11. Nov 18, 2012 #11

    Bastelmike

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    Some notes on Tins excellent explanations about lathes:

    1. The distinction between faceplate and independent 4-jaw chuck isn't common in Europe. Most lathes have a European style combination faceplate, i.e. a faceplate with slots and 4 independent jaws that are easy to remove. This tool serves for most clamping needs and is usually the only clamping accessory found on large industrial lathes with swing > 30".

    2. Turning between centers:
    Besides using a lathe dog there are 2 other possibilities. The first is to install the workpiece between the centers and then clamping it with the jaws of an independent 4 jaw chuck or (European style) faceplate. Again, this is standard on large lathes - I've never seen a 6" lathe dog.

    There is another tool available for turning between centers, a "Stirnseitenmitnehmer", dunno an english word for this. Its attached to the lath spindle and it consists of a center and several spring loaded hardened wedges which grasp into the face of the workpiece. Advantage is, You can machine the complete OD of Your part in one pass; disadvantage it damages the face of your part.

    3. Threading capability
    Tin has written a lot about this subject, though I wonder whether its really very important for the small threads most of You usually cut. I find it very difficult to machine small internal threads with the single-point method. I once have cut an internal M10x0,8mm thread and found it very difficult to make. For small threads I use taps and dies, they are reasonably cheap.
    If You need to machine large threads like M30 or larger or need something like ACME threads, You need threading capability - taps/dies this size are pretty expensive and only real large lathes develop the torque for these tools.

    Mike
     
  12. Nov 18, 2012 #12

    Bastelmike

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  13. Nov 18, 2012 #13

    Tin Falcon

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  14. Nov 18, 2012 #14

    Bastelmike

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    Hi Tin,

    sorry, this is NEVER used in the tailstock. These are dead centers for the main spindle. Believe me there is no ball bearing inside.

    The conical parts are to drive the part, they grip into your workpiece. You just need one of these tools in the main spindle and a live center in the tailstock to tun between centers.

    Mke
     
  15. Nov 18, 2012 #15

    MuellerNick

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    Also not knowing the proper term in English for it, I'd call it a "face driver".
    There are no springs in them for the driving part, but three cylinders that allow the driving part to adjust to the maybe-out-of-straight face of the part driven. The only spring inside of it is for the center.

    Also, you need a live center in the tailstock with a pressure gauge (at least, it is highly recommended) or any other means to adjust and control the force.


    Nick
     
  16. Nov 19, 2012 #16

    David Merrill

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  17. Nov 19, 2012 #17

    dman

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    this is no entirely true. dead centers can be used in the tail stock if you have no other choice. tooling is expensive as we all know and you may not have a live center. you can use a dead center to support long work when you hold the part in the chuck if you use lube and light pressure. also, I was actually instructed in tech school to never use a live center for knurling. the pressure can damage the bearings. instead we used dead centers and a high pressure grease that was high in molybdenum content. some metals in lubes such as phosphorus, molybdenum, and I believe zinc form a chemical bond to metal parts. under pressure after the oil wedge breaks down the bonded metals will protect the surfaces. the part and center won't wear till the moly cote scrubs off. you still don't want to do this with your only dead center but if you have a couple of them and one is damaged or less true than the others its acceptable to use it in the tailstock if the situation calls for it. just remember the lube, run the rpm slow, and don't force the center into the part or it may bind, use light pressure or even leave slight slack. aka use common sense.

    that said I don't think I'd ever use two dead centers to run parts between centers. although the part can still be driven with a lathe dog and drive plate leaving the tailstock slightly loose to keep it from binding could mess up the roundness and part Finish.

    CNC threading is mostly for production. its a pain to program, setup and qualify the tool offsets for a single part so its not always good for hobby environments. and there is no good way to chase threads. if the part cant index to the spindle the same way every time you have no way to fix it if the part is removed from the spindle. you need to be able to gage the thread before you remove the first part (unless the part is expendable). also single point can be a good way to repair fasteners. not so easy on a CNC.

    with a manual machine picking up old threads is pretty strait forward. you can engage the QC gear box, lock the half nut and turn the spindle in the proper direction to pull out backlash. then with the compound rest set to the proper angle work it and the cross slide till the cutter is properly engaged in the threads. zero the cross slide and back off the compound rest and you're ready to chase the threads. on a cnc you have no way to know where the carriage is supposed to be for the spindle position to check alignment and no way to program the Chuck phasing to the thread. its a matter of balancing the tool z axis offset and the thread start z position. this also means doing multiple threads or other helical features on a lathe you have to do so by setting different start positions for each groove. for things like oil grooves in a bearing you may need to start the cycle several inches from the part to get multiple grooves (mills don't have this limitation if you are not tapping but lathes usually only have one or two commands to index the carriage to an exact spindle angle at full spindle speed and it doesn't have any spindle angle arguments, the hardware is there to do more but not the software.)
     
  18. Nov 19, 2012 #18

    MuellerNick

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    It is entirely true, because bastelmike wasn't talking about dead centers, but a drive center (or face driver). And that one makes no sense at all in the tailstock. Unless you want to prevent work from rotating. And that would be quite uncommon for lathe work.


    Nick
     
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  19. Nov 19, 2012 #19

    Tin Falcon

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    sorry for the confusion I see now what bastelmike posted is indeed a driving device face driver drive center whatever we call it and it is used in the head stock. to drive soft material that the teeth will grab. I know these are used for wood. What I posted is a different animal interchangeable live center used in the tail stock.
    Both of these are valuable tools for industry and enhance the capability of a lathe..
    Tin
     
  20. Nov 19, 2012 #20

    Noitoen

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    You usually see those markings on shafts an rotors that were subjected to a balancing operation.
     

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