lead screw versus power feed on a lathe?
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  1. #1
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    I have not used a big lathe in a long time and I am confused about the difference between the lead screw and power feed. I have a book which says the following:

    "The lead-screw should be used only when cutting threads. For all other operations, motion is transmitted through the feed-rod. This practice is to preserve the accuracy of the lead-screw."

    On the mini lathes it seems there is only a single rod, the lead screw, which turns and drives the carriage. There seems to be no other way to drive the carriage. According to the above, however, lathes have two rods (?) both threaded (?) that can drive the carriage. On pictures of lathes I only see one threaded rod though.

    I know the half-nut engages the lead-scew. In the same book it also describes something called a "friction clutch". Is this how the "feed rod" is engaged? Is the feed rod threaded?

    Thanks for any assistance clarifying how these things work.

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    When moving the carriage manually, this happens by turning the carriage handwheel, which through a gear system and cog wheel moves the whole thing. Power feed basically provides power to this gear arrangement, usually through a hex shaft running parallel with the leadscrew.

    The leadscrew is a shaft with precision threading which through the half nut moves the carriage with much higher precision for threading work.

    The statement you quote is correct, but on the smaller lathes a compromise has been made by using the leadscrew also for normal feed. As you would normally not do "heavy" work with such a machine, the leadscrew will not be loaded heavily.

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    Lead screw has pitch for thread cutting, not suitable for feeding lenghtwise and turning down part diameters.

    However, my old Logan only has one screw which is used for both by proper gear selection.

    Threading with the lead screw is a thing of the distant past, in my opinion that is, today we have self-opening die heads and CNC machines that regulate the relationshp between the different spindles to be driven by individual motors and computer programs, not mechanical linkage.

    I would say, forget about looking for a leadscrew if you want to focus on doing work in today's environment.

    Cutting against a shoulder using a leadscrew takes a lot of skills that only a few people possess. I am not one of them.


    You can buy a self-opening geometric head and all kinds of chasers, and cut any thread, including acme, that you can cut using a leadscrew. You can cut right against a shoulder without even making an undercut and the nut will screw on touching the shoulder with the proper chasers, even without the undercut. Once set up, it doesn't take a lot of skill to do quality threading. The die heads can be adjusted to cut as sloppy or tight as you like.

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    On a lot of smaller lathes the leadscrew has a keyway cut along its length. This keyway drives the normal cutting feeds, and even though the leadscrew is turning, its threads are not being used to drive the carriage. I think both Logans and South Bends are made like this, but I'm not sure about the imported mini's. If your leadscrew has a full length keyway, that would be how the turning feeds are driven. If not, then its using the screw itself for drive, but that would be sorta unusual since thurning feeds are usually finer and closer together than threading feeds.

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    Actually, Benta is only telling you part of the story. In the smaller sizes, only your really high-class lathes have a totally separate feed shaft. However, not having a separate feed chef does not tell you the whole story. The reason you would normally not use the lead screw for feed is because the thread forum form get wo microphone off rn and become inaccurate, which would not be good when it comes time to cut threads with it. What I've seen on most of the smaller lathes, like the Logan's and South Bends, is they use a lead screw with a keyway in it. With this design, the threads are only used when you're cutting threads, normally lead screw drives the carriage through the keyway. This gave too much of the benefit of separate shafts, meaning that you don't incur much where on the threaded section of the lead screw, but is still a little bit cheaper than using a completely separate shaft. Of course there are few laths out there, like some of the Atlas models and the extremely small(7x12, etc.) Chinese lathes, which actually use the threads for feed. These lathes will in fact where their lead screws quicker than others, but you must understand that those models were built strictly from a price point of view.

    Ed

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    Two replies just while I was typing? Man, I gotta learn to type faster

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    Well, I agree on the additional points.
    But the original question was on "mini lathes", where you usually only have the leadscrew. The other reference was to BIG lathes, where I'd expect a separate power feed (plus a shaft for auto reverse etc.).
    I was just trying to keep it simple

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    On my Logan the leadscrew is slotted. A key in that slot drives both carriage and crossfeed, through a clutch. Halfnuts are only used for threading, which is as it should be.

    An an Atlas 10" or 12", the leadscrew is also slotted, but the gear that uses the slot drives only the power crossfeed, not the carriage travel. Carriage travel runs on the halfnuts, wearing them unneccessarily. Atlas halfnuts are a hot item.

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    Seems to me that John just wants to know the mechanics of the different drive methods.

    Firstly, some cheap smaller lathes provide only a lead screw to move the carriage. On these machines, the handwheel is on the tail-end of the lathe and you move the carriage by just turning the crank and spinning the lead screw directly. Power feed (if any) is identical to threading, you just have to set up the gearing for a fine feed.

    A step up is lathes that provide a rack along the bed and a hand crank on the carriage, which drives a pinion along the rack for hand feed. The only way to power feed is still through gears driving a lead screw and the handwheel justs "coasts" while power feeding. A lathe so equipped will have a half-nut.

    Both of these methods use the lead screw for any power feeding and wear it out much faster. The first type won't even have a half-nut and the screw is constantly engaged (and wearing out).

    The next step up is lathes that have a lead screw with a keyway along its length. On these machines, there are two separate mechanisms in the carriage to pull it along the bed: 1) lead screw and half nut used only for threading 2) bevel gear driven by the key/keyway and chain of gears that essentially turns the handwheel and moves the carriage via the rack.

    Yet another step up are lathes that are built with separate lead screw and feed rod only have the one threaded rod (lead screw). The other rod is just an un-threaded shaft with a keyway. It operates similar to the lathes in the previous step. This (arguably) makes for more accurate threading since there is no missing portion of leadscrew for the keyway slot. Really though, you'd probably never notice any difference unless there are burrs or some kind of damage to the keyway.

    These separate mechanisms in the last two types greatly reduce the wear on the lead screw since you only use it to cut threads. Depending on the lathe, you might have to be careful though. Some lathes have a "lockout" that won't let you engage the half nut and feed lever at the same time (which would quickly break something in one or the other mechanisms) and some don't.

    I hope this helps.

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    That helps a lot. You are right: I am trying to figure out how the different mechanisms work so I can compare different kinds of lathes to one another.

    I understand how the half nut works and also how the rack and pinion work with for a manual feed.

    The only part I still don't get is how you could drive a carriage with a keyway in a rod. I assume this is a longitudinal keyway. If the rod is rotating how could anything go into the keyway?

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    Hi John,

    Last summer I had the carriage if a chink lathe taken apart to repair it. It had a feed shaft and a lead screw; but I saw how the whole thing worked by having it apart. The feedshaft has a keyway cut down most of its length. On that keyway rides a gear that has a captive key in it. So, it's always turning with the feed shaft. When you engage a clutch lever; another gear comes into contact with the turning gear which then transmits into motion of the carriage or crossfeed as the case may be.

    Andy Pullen

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    Ok, I think I am getting it. A gear is centered on the lead screw inside of the apron and the gear has a square protrusion in its hole that mates with the keyway. This gear transmits rotation to the rack to move the carriage. As long as the hole in the gear has a wider diameter than the lead screw threads all the wear from the power feed will be on the sides of the keyway.

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    Donning my ‘Captain Irrelevant’ hat, and not offering a useful answer, the question has reminded me of an article I saw in ‘Model Engineer’. Someone had done some experiments using automatic feed without using either a layshaft or the leadscrew. He had a small, cheap pre-WWII ‘Winfield’ lathe, 7” swing, I think. He fixed a piece of string to a pulley on the carriage handwheel (which was geared down to the rack pinion). He hung various weights on the string, and found that he could take, for that lathe, a good big cut, 0.15” deep, with a regular coil of swarf coming off, absorbing all the motor’s horse power (1/3 HP), with the tool feeding itself at a regular rate.

    Another advantage was that there was no fear of overrunning at the end of the cut – he just rigged up a simple mechanical stop for the carriage. It made him appreciate what large forces the leadscrew nut would be subjected to if he’d been using that for the automatic feed instead.

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    Asquith-

    One of the lathe hands here told me a trick whereby they cut a long taper by similarly fixing a big pulley to the crossfeed wheel, then engaged the longitudinal feed while the gravity-fed X axis did it's thing. They were making fishing rods out of titanium bars, I think.

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    Ferrous
    Thanks for that - very interesting use of the principle.

    I've since dug out the copy of 'Model Engineer' (Vol 168 No. 3916) so that I could credit the author, who was a Mr I Davidson.

    I was a bit surprised that the cut was so regular, but re-reading the article, the author says he had subsequently looked at the theory of metal cutting, and found that there is a definite relationship between depth of cut, feed rate, and side force on the tool. Since the depth and the force were fixed, it follows that the feed rate must obey the equation, so it, too, is fixed.

    He added that he had never before succeeded in turning off metal at the rate he could achieve with that device, and 'found it distinctly alarming at first'.

    It's good to see the results of amateurs' experiments in their shops.

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    While the discussion on the proper way to design the feed-mechanism for a lathe has been of great interest, I really do think that it overlooks one of the main technical points. That machine design is a conservative business, where basic designs stays unchanged for decades, rather than optimal technical solutions. It is therefore most unlikely, that anything firm may be derived from simple design-features alone.

    Then again, my comment may only reflect the dire disappointment of knowing that the Schaublin 102-VM screwcutting lathe, that I use for small work, is of low quality as it has only one leadscrew for both screwcutting and feed

    Jokes aside, it is worth to mention, that the leadscrew is 40mm (1.57") in diameter and placed in the center of the closed bed (the swing over the bed is 102 mm, 4"). Thus it is fully protected from both metalparticles and cutting fluid - a neat design feature, more than 50 years old, that I have not seen used in other designs.

    Best regards

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    richardny;
    why i am totally surprised at your remarks.
    thread cutting on a lathe a thing of the distant past?
    cutting to a shoulder isnt in your bag of tricks?
    and i thought from reading of you 50+ years of experience with no roadkill you could do everything, now i am enlightened!
    if you in fact did start 50 years ago threadcutting would be one of the most common procedures on an engine lathe.
    i cut bastard threads all the time, yes right up to a shoulder.
    just use an indicator to see where you stop. its easy but butt puckering...jim

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    In the Mar/Apr '05 HSM, page 51 photo #102 is of the backside of a Monarch 12"CK lathe's apron. The feedrod passes through the worm(lower center), which has a key in it. The feed rod turns,it is driven off the spindle through gearing, it turns the bronze worm gear, which turns the internal gearing. The spur gear on the right side, about mid height, meshs with the rack gear that is attached to the bed. When the longitudinal feed clutch is engaged, the carriage will feed. The cross feed clutch shaft is to the left and just above the worm gear, and to the right of the lead screw carrier/guide. The half nuts are on the extreme left. The bar just above the worm is the threading interlock. This prevents the half nuts from being engaged at the same time the longitudinal feed is engaged, and vice versa. In the photo, the half nuts are shown in the closed postion(engaged), and the threading interlock is preventing the feed being engaged.
    All lathes, that I am aware of, from about 1930 on, that have threading and feeding capabilities, either through a leadscrew and feed rod, or splined leadscrew, will have the interlock feature.
    Harry

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    Threading with the lead screw is a thing of the distant past
    There are one hell of a lot of shops doing repair work, prototypes, one off, and short run production that are going to be very surprised at that statement. I was in a hydraulic repair shop a while back. There were several good old American built engine lathes there being used to thread the ends of many different sizes of hydraulic cylinders.

    An engine lathe will cut ANY pitch it is capable of on ANY diameter it is capable of. According to the text: SCREWCUTTING IN THE LATHE by Martin Cleeve, a set of twenty change gears can be set up in different combinations of 2, 4, or 6 gears to cut 750,000 different pitches of threads. That would be one hell of a set of chasers.

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    With chaser sets for the various die heads costing around $100.00 a set for "v" threads for each size, thread,I'll keep doing it the old way.


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