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  1. #1
    Join Date
    Feb 2014
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    Newcastle NSW
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    78

    Default Thread cutting with a Hercus C model

    Hi guy's wondering if there is a guide or tutorial somewhere on how to begin cutting threads with the C model?

    Maybe i should have waited and bought a 260 but would love to know if it's possible as i have a full set of gears and a threading dial that came with the lathe but a chart that is confusing to me.

  2. #2
    Join Date
    Sep 2018
    Location
    South Australia
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    88

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    Quote Originally Posted by Zumanity View Post
    Hi guy's wondering if there is a guide or tutorial somewhere on how to begin cutting threads with the C model?

    Maybe i should have waited and bought a 260 but would love to know if it's possible as i have a full set of gears and a threading dial that came with the lathe but a chart that is confusing to me.
    Well, I am a long, long way from being an expert, but you could try what I did. Although I have a 260 with a gearbox, I was still a bit confused by the charts and what have you. So I bought a cheap thread pitch gauge on ebay, turned up a nice smooth finish on a piece of round stock, blued it up and started running scratch passes, moving nice and slowly, comparing the charts to the measurements I took.

    It made it feel like I was achieving something, and pretty soon it started to make sense and I was confident enough to start cutting threads. Of course I was just working with tumblers whereas you'll need to mess around with change gears, but the principle is the same.

    There are far wiser people on here who will have more info for you, I'm sure. Have fun!



    Moz

  3. #3
    Join Date
    Oct 2007
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    Alexandra Vic
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    69
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    654

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    My father had a 9in Southbend which is what the Hercus is based upon, with a full kit of gears. He bought this in the 60's secondhand and used it extensively until his sight failed just before he passed twelve years ago. His machine was an imperial one (US made immediate post war model, they didn't do metric then). He cut a multitude of threads with it over the years, from tiny BA series up to quite large and coarse threads, including metric threads courtesy of translation gears to build in the conversion to cutting metric with an imperial lead screw, so most things should be possible with your lathe.

    It helps to understand what is supposed to happen which is fairly simple really.

    The headstock spindle rotates, along with the material. The leadscrew has a fixed pitch and each turn of the leadscrew will move the carriage toward the headstock (or away from it depending on the position of the tumbler gears) a fixed distance once the half nuts are engaged. A gear train connects the back of the spindle to the leadscrew, and determines the relationship between the rotational speed of the spindle and the leadscrew. With a full set of change gears, you can set the gear train to provide a set ratios to achieve most of the standard thread pitches required for the measurement system (imperial or metric) that your particular lathe was built for. (An imperial lathe will have imperial lead and feed screws, and imperial calibrated dials, while for a metric lathe these will all be metric).

    The threading dial (if fitted) on the carriage has a gear driven by the leadscrew. The face of the threading dial displays the relative movement of the carriage and leadscrew threads. If the half nuts engage the leadscrew, there is no relative movement between the carriage and leadscrew as the half nuts are locking the two together, and the threading dial is stationary, although the carriage is being moved by the leadscrew. If the half nuts are not engaged, the carriage can be stationary while the spindle and leadscrew rotate, or the carriage can be moved with the handwheel and rack. In either case the threading dial would be rotating at a speed relative to the difference in leadscrew rotational speed and physical carriage movement.

    The halfnuts may be engaged whenever the leadscrew threads match up with the half nut threads, but that itself may not guarantee that the carriage would be driven in sync with a part formed thread in the work, as it may advance at the same speed but with a different starting point on the thread splitting it (causing a second thread to start on the work). By watching the threading dial and always engaging the half nuts at the same indication on the thread dial, you are guaranteed that the new tool passage will be in sync with the previous tool passages, provided that you are working in the measurement system that the lathe was made for.

    This means that you can disengage the half nuts when you come to the end of the thread you are cutting, manually wind the carriage back clear of the thread start, advance the feed the required amount for the next cut, watch the threading dial and engage the half nuts at the same point on the dial to make the next pass, without needing to stop the spindle.

    If you are cutting metric threads on an imperial lathe (or vice versa), the threading dial won't retain the relationship between leadscrew rotation and carriage position along the bed because of the non integer ratio involved with the conversion gears. In that instance, or if your lathe does not have a threading dial, you need to engage the halfnuts once before you start the first pass of the thread and keep them engaged until the tread is complete. You then need to be able to stop the spindle completely at the end of each threading pass and reverse the spindle to wind the carriage back beyond the thread start, either by reversing the motor, or manually rotating the spindle at the chuck or drive belt. This is the only way to keep spindle position and carriage position in sync to avoid splitting the thread being cut.

    I haven't discussed the actual gear ratio that you create to link the spindle and leadscrew, as the manufacturer has normally calculated that for you and affixed a chart to the machine to try to provide this information, and initially supplied the gear set to create these ratios. Hercus supply a DVD available through ebay that I believe explains the complete operation of the lathe including where the gears go in the gear system and how to set them up.

    I strongly recommend getting a thread pitch gauge as suggested previously, and always cut a light pass after setting the gear train and checking it with the thread gauge to verify that the machine is cutting the correct pitch before proceeding the cut the thread.

    I haven't used a Hercus since high school (50 yrs approx) and haven't used the Southbend for probably 20+ years, so I am not going into details about gears and their position, based on distant memory, bit can assure you that with what you have, you should at least be able to cut threads in the machines native dimension system without too much trouble. The simplest way to get used to this is to do it in a practice situation until it becomes much less of a challenge for you. Threading in the alternate dimension system to your machine is possible with conversion gears if you have the gears available, but I suggest that you get the operation fully sorted in your native system before you attempt it.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  4. #4
    Join Date
    Oct 2008
    Location
    Cairns, Q
    Posts
    666

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    If you want to know almost anything you are ever likely to need about cutting threads on small manual lathes I would suggest getting a copy of "Screw Cutting in the Lathe" by Martin Cleeve.

    https://www.google.com.au/search?sou...4dUDCAc&uact=5

    A lathe with loose change gears instead of a threading gearbox can be annoying if you have to cut a number of different thread pitches over a short time period because of the need to keep changing the gears in the gear train, but it really comes into its own if you need to cut unusual thread pitches. Cleeve explains how to calculate the gear combinations needed for this from first principles.

    I have a 9" Brackenbury and Austin lathe with an 8 TPI leadscrew similar to the South Bend. Over the years I have made a few extra gears for it when needed to cut an unusual thread. It can now cut:
    BSW, BSF, BSC, Brass thread, UNC and UNF threads up to one inch.
    The finer BSP and NPT threads (except 27 TPI)
    A very close approximation to all the ISO metric thread pitches from 0.4 to 3.0 using a 47T+37T compound transposing gear
    A good approximation to DP threads down to 24DP using a 55T+35T (pi/2) compound translation gear (44T+28T would work using the South Bend gears too)
    A reasonable approximation to all the 0BA to 10BA threads, with one exception.

    This range would be difficult to cover with the usual type of threading gearbox.

    Frank

  5. #5
    Join Date
    Apr 2016
    Location
    New South Wales
    Posts
    45

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    Quote Originally Posted by franco View Post
    I have a 9" Brackenbury and Austin lathe with an 8 TPI leadscrew similar to the South Bend. Over the years I have made a few extra gears for it when needed to cut an unusual thread. It can now cut:
    BSW, BSF, BSC, Brass thread, UNC and UNF threads up to one inch.
    The finer BSP and NPT threads (except 27 TPI)
    A very close approximation to all the ISO metric thread pitches from 0.4 to 3.0 using a 47T+37T compound transposing gear
    A good approximation to DP threads down to 24DP using a 55T+35T (pi/2) compound translation gear (44T+28T would work using the South Bend gears too)
    A reasonable approximation to all the 0BA to 10BA threads, with one exception.

    This range would be difficult to cover with the usual type of threading gearbox.

    Frank
    Question:
    Apart from 47T and 37T can you indicate which other atypical (non-standard) change gears you made or needed ?

  6. #6
    Join Date
    Jun 2007
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    sydney ( st marys )
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    64
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    Is there any particular part of the process that is causing concern or the whole process from selecting gears assembling the train and starting to cut a thread.

  7. #7
    Join Date
    Oct 2008
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    Cairns, Q
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    666

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    Quote Originally Posted by Donaldo View Post
    Question:
    Apart from 47T and 37T can you indicate which other atypical (non-standard) change gears you made or needed ?
    Hi Donaldo,

    The original gear set was 2x20T, then one of each gear from 30T to 65T, going up in 5 tooth steps. In addition to the 37T and 47T gears I have added one of each of:
    25T, 28T, 48T, 70T, 76T and 80T. In the unlikely event that I ever need to cut a 22DP thread I will have to cut a duplicate 35T gear.

    Frank.

  8. #8
    Join Date
    Apr 2016
    Location
    New South Wales
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    45

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    Quote Originally Posted by franco View Post
    Hi Donaldo,

    The original gear set was 2x20T, then one of each gear from 30T to 65T, going up in 5 tooth steps. In addition to the 37T and 47T gears I have added one of each of:
    25T, 28T, 48T, 70T, 76T and 80T. In the unlikely event that I ever need to cut a 22DP thread I will have to cut a duplicate 35T gear.

    Frank.

    Thanks muchly.

    I think the 35T, 55T, 65T, 70T, 76T (as well as 37T & 47T) would not be commonly found in connection with Hercus. I think I had both a non-Hercus 55T and a non-Hercus 65T once, until I sold them because the tooth profile was slightly different from all the other Hercus change gears I had (i.e. 55T and 65T could have originally belonged to another similar make like Sheraton or South Bend, I never found out).

    I have also come across 108T and 116T. The 108T is half of a 108T/18T 6:1 Hercus compound.

    Where I am typing this there is a brass electric clock on the wall, the brand is "Genfa". I mention this because it has a second hand which ticks 53 times per revolution. This suggests to me that someone, probably Chinese, figured out it is cheaper to make clocks with 53 seconds per minute. There could be a 53T change gear being heavily utilised in a clock factory somewhere in China. If it works for 53T it could be even cheaper to make 47 second per minute or 37 second per minute clocks with the 47T or 37T change gears.

    Donald

  9. #9
    Join Date
    Jan 2009
    Location
    Holbrook, NSW
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    73
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    490

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    Quote Originally Posted by Zumanity View Post
    Hi guy's wondering if there is a guide or tutorial somewhere on how to begin cutting threads with the C model?

    Maybe i should have waited and bought a 260 but would love to know if it's possible as i have a full set of gears and a threading dial that came with the lathe but a chart that is confusing to me.
    There are lots of videos on thread cutting on a lathe on youtube, if the physical process is the problem. If it's deciphering the thread chart that has you stumped, pick a thread and we'll go through the chart selection. You should have the chart in the attached pic for imperial threads on the end cover of your machine. All models of Hercus lathes A, B and C are capable of thread cutting, so don't get discouraged.
    Mal
    BS108 9c gear plate.JPG

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