Can a 1/2 pitch in 1" Thread be Cut?
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    Default Can a 1/2 pitch in 1" Thread be Cut?

    I'm curious whether or not a 1 pitch thread in 2" can be cut where the lowest pitch thread on my lathe is 4 pitch
    The thought here is can it be faked out to believing its cutting a 1/2" pitch somehow?
    So I'm looking to apply a 1 full thread over 2 full inches of 2.5" dia. round pipe...
    I'm sure that it can be done on a CNC but can a standard lathe without the CNC features cut this thread?
    Thanks
    Paul

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    The cutting torque is pretty high on the spindle output gear when trying to pull the carriage that fast. For this reason, manual lathes equipped for coarse lead thread cutting have an extra set of 'speed up' gears to increase the speed of the spindle output gear. Without this feature, you might just break that shaft off and then have to fix your headstock.

    A way around this is to rig up a motor drive system directly to the input shaft of the quick change gearbox and apply the torque there. This then back drives the spindle (which is not under power) so that the thread can be cut almost as usual. I've never rigged up such a thing.

    It is really no fun at all trying to machine threads that coarse on the lathe. It is very slow going, because you're forced into trying to control the tool position by riding the spindle clutch (if the lathe is so equipped) in order to not overrun the tool's working zone. Trying to operate the half nuts is next to impossible at any kind of normal cutting speed. Leaving the half nuts engaged means that any spindle which coasts a bit might mean your tool over travels by an inch before you can do much about it.

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    Thanks for the reply
    I'm new at this and this sounds like it would not be a function I would wish to perform at this time...
    Thank You for the reply though

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    1/2" pitch is 2 TPI

    "one full thread over 2 inches" is 1/2 thread per inch

    Since you stated your coarsest thread is 4 TPI - you would need an 8 to one "speed up compound " in the left end gearing

    This is because your proposed 1/2 thread per inch is eight times coarser than your built in limit of 4 TPI


    Naturally the loading on the parts in play on that left end and in the QC gearbox will also go up by a factor of 8

    Here you go

    image1.jpg
    Last edited by johnoder; 06-16-2021 at 04:48 PM.

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    You could make that with a lead screw driven dividing head on a vertical mill.

    I ass-u-me that you are not looking for a traditional thread form but if you are I'm having trouble visualizing how that would look, let alone not cut through a piece of pipe.

    20170531_143937.jpg20170821_145846.jpg
    Last edited by kenton; 06-17-2021 at 11:52 AM.

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    Quote Originally Posted by kenton View Post
    You could make that with a lead screw driven dividing head on a vertical mill.

    I ass-u-me that you are not looking for a traditional thread form but is you are I'm having trouble visualizing how that would look, let alone not cut through a piece of pipe.

    20170531_143937.jpg20170821_145846.jpg
    To do it on a lathe you would not only have to cobble together a special gear train but you would also need a small lathe mounted mill head. Trying to cut that steep a spiral with conventional lathe bits IMO would be an exercise in futility.

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    Quote Originally Posted by Texasbowhunter View Post
    I'm curious whether or not a 1 pitch thread in 2" can be cut where the lowest pitch thread on my lathe is 4 pitch
    The thought here is can it be faked out to believing its cutting a 1/2" pitch somehow?
    So I'm looking to apply a 1 full thread over 2 full inches of 2.5" dia. round pipe...
    I'm sure that it can be done on a CNC but can a standard lathe without the CNC features cut this thread?
    Thanks
    Paul
    Please indicate what your "Standard Lathe" is.

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    Is this an actual screw thread? Will there also be a female thread for this part?

    It sounds like you are trying to cut an oil groove? If it is an oil groove, do it by hand with a file. I have had to do this a number of times as my lathe also only goes down to 4TPI.

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    Quote Originally Posted by Scottl View Post
    Trying to cut that steep a spiral with conventional lathe bits IMO would be an exercise in futility.
    You just grind the tool for the helix, but that doesn't solve the feed problem.

    This is why they invented thread mills, but most likely he doesn't have one of those.

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    Any time I've seen the topic of long lead threading mentioned most seem to agree that cutting any thread pitch less than your lead screw pitch isn't recommended due to that high torque load others have already mentioned. The D.H. Chaddock book about building the Quorn cutter & tool grinder addresses this topic The Quorn Tool & Cutter Grinder By D. H. Chaddock : Free Download, Borrow, and Streaming : Internet Archive His solution involved back driving the lathe spindle through the lead screw as again others have already mentioned. It's one of the few times where a plain change gear lathe is more versatile than one with a built in gear box. Chaddock also used a cobbeled together power feed and milling head set to the thread pitch angle to cut a 2 tpi thread on a part with a lathe having an 8 tpi lead screw. The details of how that was done start on page 16 of that link. Compounding the problem is the slow rotation of the part being cut that's required since the feed rate is so high for each complete revolution on the part. You basically set up the lathe to do what a mill can with a universal dividing head geared to the X axis is capable of as Kenton's pictures show.

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    Quote Originally Posted by Laverda View Post
    Is this an actual screw thread? Will there also be a female thread for this part?

    It sounds like you are trying to cut an oil groove? If it is an oil groove, do it by hand with a file. I have had to do this a number of times as my lathe also only goes down to 4TPI.
    Another way to do an oil or grease groove inside a bushing is to use a ball shape carbide burr held on a die grinder mounted in the lathe toolpost. You can set this tool at a suitable angle to make it clear the bore and yet not have to be concerned about making helical clearance. I use a coarse thread setting (like 3/4 TPI which my Summit will do) and just roll the chuck over by hand while the burr does it's work. This makes it easy to stop the groove before the tool comes out either end of the bore. I usually switch from RH to LH threads so I get a set of crossing grooves.

    You can also make blind T intersection grooves, with an axial followed by a radial component with mere hand feeding movements.

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    That is a very coarse pitch. Larger lathes may have the capability, but it is quite possible that you can "trace it".

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    Thank You guys for your input...I realized that my usage of pitch was incorrect...I should have used TPI and yes I needed a 1/2 thread per inch so when the 2" of the shaft has a full 1 TPI...
    I believe I have this now covered...I don't have the means or the skill level so I have out sourced this to a Machinist that works out of his garage and he has a CNC to program and make it happen...
    I'm still just a student learning what I can accomplish and learning along the way...
    Thanks for all the suggestions and guideness on this mater...
    paul

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    Holy smokes! I just looked at my American threading chart. It goes down to ½. Maybe I'll try it someday...........in plastic first.

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    Quote Originally Posted by johnoder View Post
    1/2" pitch is 2 TPI

    "one full thread over 2 inches" is 1/2 thread per inch

    Since you stated your coarsest thread is 4 TPI - you would need an 8 to one "speed up compound " in the left end gearing

    This is because your proposed 1/2 thread per inch is eight times coarser than your built in limit of 4 TPI


    Naturally the loading on the parts in play on that left end and in the QC gearbox will also go up by a factor of 8

    Here you go

    image1.jpg
    dam what is the swing of that beast ?

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    likey your side cutting edge clearance would best be modified with traveling so into the part rather than around.
    Last edited by michiganbuck; 06-18-2021 at 03:28 PM.

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    Quote Originally Posted by michiganbuck View Post
    likey you side cutting edge clearance would best be modified with traveling so into the part rather than around.
    For damn sure. I had it in back gear at 30rpm and it "flew" sideways. Looks like most of the cutting edge will be on the side.

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    1/2 TPI on a 2+ inch diameter? Sure, why not? It is just a case of having the correct gears and grinding a tool with enough clearance. His lathe probably has a QC gear box but change gears can change the numbers on it.

    Oh, and you may want to have the motor drive the lead screw and let the gears drive the spindle. That would take a bit of a modification.

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    Some old lathes had a quick threading facility,where the train was driven by the cone gear. Example:2tpi in normal,4" per turn driven from cone assuming a back gear reduction of 8


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