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Old 10-28-2012, 11:57 PM   #1
Tin Falcon
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Default Tips on selecting a lathe -will it thread

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.




Quote:
Quote:
Why does the #1001 SB need quick-change gears if it says that it can thread ?
I have never threaded and do not know anything about the QC gears.
Can someone enlighten us ?
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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Old 11-04-2012, 03:04 PM   #2
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Default Accessories and Tooling

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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Old 11-04-2012, 03:15 PM   #3
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Default The spindle nose

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.
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Old 11-04-2012, 05:13 PM   #4
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Default Size ?

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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Old 11-04-2012, 05:53 PM   #5
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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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Old 11-04-2012, 07:06 PM   #6
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Default Work Holding or why so many accessories?

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.

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Old 11-04-2012, 08:34 PM   #7
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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.

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Old 11-11-2012, 09:28 PM   #8
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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.

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Old 11-11-2012, 11:08 PM   #9
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Default Lathe Selecting Tips

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

we can discuss workarounds an modifications in there popper places.

http://www.homemodelenginemachinist....05/#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

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Old 11-11-2012, 11:46 PM   #10
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Default Power and speed control

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



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