Making a male & female thread in Aluminium.
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    Default Making a male & female thread in Aluminium.

    Hi,

    I'm new to machining and just got myself a lathe. I am trying to internally thread some 50x4mm aluminium pipe and make an end cap to suit.

    I have made a few but they are always too tight and I have to go back and cut the threads deeper and skim some of the top of the threads to make it fit together.

    Would someone mind going through the exact process on how they would machine both parts (cutting depths etc...)

    Thank you in advance.

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    Before we get into cutting the threads, how are you measuring them?

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    And Alu on Alu is a very bad idea.

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    OT: Not wishing to get too woke here, but are there still male and female threads?

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    Quote Originally Posted by matyp View Post
    Would someone mind going through the exact process on how they would machine both parts (cutting depths etc...)
    It would be better to have a drawing with all dimensions.

    Then look at the Machinery Handbook for things like class-of-fit.

    For me, I always make the female side first. Then I use the female part as a gauge when making the male thread. You could go either way but I find it easier
    to examine the exposed male threads.

    With aluminium I use some anti-seize compound.

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    Quote Originally Posted by matyp View Post

    Would someone mind going through the exact process on how they would machine both parts (cutting depths etc...)

    Thank you in advance.
    Yes, we would.

    Try cracking a book first, do some test cuts, and then ask here for help solving a problem your having.

    You want someone to write you a personalized essay just for you ?

    Your profile says "IT" so why not explain how the internet works, just for me ?
    And don't leave anything out, cite your sources too.

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    In addition to the above, some corrosion-resistant aluminum pipe is made of aluminum grades with relatively poor machinability. If you're tearing up the threads, then finding them galling (Al on Al), making a looser fit really isn't the answer.

    Also, shocking news to some posters above. Aluminum is often alloyed with trans elements. Right smack dab in the middle of the Periodic Table. Who knew?

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    If you're new to machining then practicing with aluminium is what I would do. If you can work some practice into making something useful. That is the goal.

    Many times I make stuff and stick the part on the dinner table and gloat over it as I watch the evening virus news reports.

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    A little off topic but...

    During a BMW R60/2 restoration I made many of the metric fasteners out of stainless steel. I still have a peanut butter size jar full of the original fasteners.
    I would not have done this without first practicing in aluminum. The goal of the practicing was to make the procedure so boring and easy, such that there would be
    no mistakes. To compound that, it's no fun turning down hex bars to machine threads.

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    I think my main problem is tolerances, where and how much I should be adding to my cuts. I'll give a full run down of my method.

    1. First I started on the Female, I square the piece up on the lathe checking with gauge.

    2. Using a boring bar I take off .2mm on the inside of the pipe, about 20mm. ( I am measuring with a DRO and then taking a measurement with a telescoping gauge and measure this with my vernier (Mitsutoyo, not junk) I then cut a 3mm wide groove to give me time to disengage the half nut.

    2. I have chosen a 1.5mm thread pitch and most tables on the show a depth of cut for this is .812mm (I added a bit and cut the thread to .820 (1.64 on the DRO in Diameter)

    3. I do about 5 passes cutting starting deep and getting smaller (I do 3 passes at the end at 1.64 without changing depth.)

    4. Now I cut the Male side.

    5. I face up the bar, and cut a 20mm length at the Female Max Diameter (which I calculated at the internal female diameter + 2x depth of Female depth Cut. I dint add the tolerance.

    6. I cut the male thread .92mm + .2mm tolerance deep.

    The threads start but just barely. So I end up cutting the male thread deeper and turning down the diameter further to get them to fit. But I would like to know where I am making the mistake and it is also a pain in the ass to recut metric threads if I have had to go and do another cutting operation.

    My method for cutting a metric thread is.
    1. Engage half-nut on dial indicator "1".
    2. Disengage after cut.
    3. Turn machine off and take cutter away from surface.
    4. Reverse lathe and re-engage half nut on same number "1"
    5. Turn lathe off, leave half nut engaged and cut cut again...

    I really need info on cutting depths and diameter calcs I think...


    Anyway any input would be appreciated.

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    Large fire fighting hoses Al hose end fittings. They have some kind of teflon or anodized finish so they do not gall up and sieze.
    Bill D

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    A common beginner mistake is to overlook the truncation in the crest and root of the thread. The root will have the same radius as the top of the tool, but the crests will be left with sharp points.

    Those sharp crests will foul the radius in the root and if you cut the thread deeper you'll have a sloppy fit in the rest of the thread. The first thing is try is running a file over your male threads to shorten the crests.

    It would be well worth your time to read up on thread forms and understand concepts like pitch diameter. This will help you both in making threads to match existing parts and in fitting your own parts together. Can't do any problem solving until you identify the problem.

    I also would not rely too heavily on calculated depths. There are a lot of variables that can affect the actual depth, primarily flex in the tools and the machine. Calculation only gives you a starting point, you need to measure the part or check it against its mate to see where it has actually ended up.

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    Quote Originally Posted by matyp View Post

    2. I have chosen a 1.5mm thread pitch and most tables on the show a depth of cut for this is .812mm (I added a bit and cut the thread to .820 (1.64 on the DRO in Diameter)

    3. I do about 5 passes cutting starting deep and getting smaller (I do 3 passes at the end at 1.64 without changing depth.)

    4. Now I cut the Male side.

    5. I face up the bar, and cut a 20mm length at the Female Max Diameter (which I calculated at the internal female diameter + 2x depth of Female depth Cut. I dint add the tolerance.

    6. I cut the male thread .92mm + .2mm tolerance deep.

    The threads start but just barely. So I end up cutting the male thread deeper and turning down the diameter further to get them to fit. But I would like to know where I am making the mistake and it is also a pain in the ass to recut metric threads if I have had to go and do another cutting operation.

    I really need info on cutting depths and diameter calcs I think...
    You're doing 60 degree tool cuts. The tool movement into the male thread valley is going to be 0.625 * pitch.
    cos 30 = depth / (0.625 * pitch) , depth = 0.8118. So your depth of 0.812 is correct.

    Not sure what you are doing with those 3 passes at the end.
    Also not sure if you are using a full or partial profile cutting bit. I myself would use a full profile bit if I was doing the same threads over and over again.
    Otherwise a partial profile is more versatile.

    If using a partial profile then the valleys of the threads will not be wide enough and the tool will have to be moved in the X direction during operations.
    The crests can be cut with a file using light strokes. Crests and valleys should have the same flat length.

    You got a thread center gauge?

    I'm using a Hardinge and the method described in the book Machine Shop Practice is how I do things. There are two volumes, get both.
    Not sure why you have to turn power off. I never do until I screw something up or have a perfect job.

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    Quote Originally Posted by rons View Post
    It would be better to have a drawing with all dimensions.

    Then look at the Machinery Handbook for things like class-of-fit.

    For me, I always make the female side first. Then I use the female part as a gauge when making the male thread. You could go either way but I find it easier
    to examine the exposed male threads.

    With aluminium I use some anti-seize compound.

    Typical Male

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    Joe pieczinski has a video of how to dia in thread depth. Very simple and it works. As cautioned watch for male thread tip profile.

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    Thank you everyone for your input.
    I will read up more and have another go tonight. I will do a mock up in inventor and post all my dimensions and cuts so better to pinpoint my problems.

    Thanks again.

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    The table I have for metric threads show the major diameter for the male and female as the same.
    Is this right and you just file down the top of the male thread after or should I cut the male diameter smaller? and if so by about how much?

    Thanks again.

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    "I wrote this essay on the fundamental geometry of modern screwthreads a few years back, and this looks to be a good time to repost it:

    There are three different forms of screwthreads that have been in widespread use in the industrialized world in the last century or so that we should talk about here, and probably several dozen more forms that I'll tactlessly ignore. All three of these threadforms -- the Sellers (aka Franklin Institute, National, American National, and US Standard), the Unified, and the ISO Metric -- are descended from an earlier form, the "60 degree Sharp V".

    The 60 degree Sharp V threadform has been obsolete since the 1800s, but it provides a good place to begin our discussion.

    Let’s start by imagining a bolt with a 60 degree Sharp V thread. Now imagine that we cut that bolt lengthwise, so that the plane of the cut contains the central axis of the screwthread, and then look closely at the profile of the thread in the plane of the sectioning cut.

    If we were having this discussion over a cup of coffee in the breakroom, I'd be drawing sketches to show you what I'm trying to explain. Since we aren't, though, you might want to get out a pencil and paper and try to make your own sketches while I talk.

    The Sharp V screwthread profile looks like a row of equilateral triangles, each with one side resting on a straight edge with their points pushed together. All sides of these triangles are the same length, so the “points” of successive triangle away from the straight edge are separated by the length of the triangle side . . . we'll call this distance The Pitch of the screwthread.

    There is another row of these little triangles on the opposite side of the sectioned bolt, offset along the length of the bolt by a half Pitch, with their not-on-straightedge points pointing in the opposite direction from the first row's not-on-straightedge points.

    Following so far? Ok, now lightly draw two parallel lines, one connecting the away-from-straightedge triangle points on one side of the bolt and the other connecting the away-from-straightedge triangle points on the other side of the bolt. These two line are separated by the Major Diameter of the screwthread.

    The on-straightedge sides of the two rows of triangles are separated by the Minor Diameter of the screwthread. The away-from-straightedge points of each row of triangles are off of the straightedge by the Single Depth of Thread, which is The Pitch x Cosine 30 degrees.

    The Double Depth of Thread is twice the Single Depth of Thread, 2 x The Pitch x Cosine 30 degrees. The Double Depth of Thread is also the difference between the Major Diameter and Minor Diameter.

    If you were going to cut this thread on a lathe using a single-point toolbit with the compound rest slewed to feed along the flank of the screwthread, and assuming that you zero the compound when the sharp point of the toolbit just touches the already-cut-to-Major-Diameter workpiece, you'd have a complete threadform when you'd fed the tool into the workpiece by a distance equal to The Pitch. After all, all three sides of each triangle are the same length.

    While the geometry of the 60 degree Sharp V screwthread is nice and simple, it has practical problems. The sharp point on the toolbit breaks or wears very quickly, the sharp ridges at the Major Diameter of bolts and Minor Diameter of nuts get banged up very easily, and the sharp grooves at the Minor Diameter of the bolts are "stress risers" weakening the bolt. By the 1860s the American industrialist and machine tool builder William Sellers proposed a modified version of the earlier threadform, one with 1/8 Pitch flats at both the Major Diameter and Minor Diameter, as a new standard.

    This new threadform, the Sellers threadform, was fairly well accepted, but it didn't actually become the official US Standard threadform until well into the first half of the 20th century.

    So let's modify those sketches. The general spacing and angles stay the same, but the new profiles have flats instead of sharp points at the Major Diameter and same-size flats instead of sharp grooves at the Minor Diameter. Both flats need to be 1/8 Pitch long, which reduces the length of the angled flanks AS MEASURED ALONG THE AXIS OF THE SCREWTHREAD to (Pitch - 1/8 Pitch at the Minor Diamter - 1/8 Pitch at the Major Diameter) = 6/8 Pitch = 3/4 Pitch.

    The length of the flank is reduced by the same ratio, and the other Pitch-dependent calculations are adjusted accordingly.

    Flank length (along-flank infeed using slewed-to-feed-along-flank compound rest) for Sellers threadform = 3/4 Pitch.

    Single Depth of Sellers Screwthread = 3/4 Pitch x Cosine 30 degrees.

    Double Depth of Sellers Screwthread = 2 x 3/4 Pitch x Cosine 30 Degrees.

    The Sellers threadform served the US's needs well enough until World War II, when the difference between the United States' and the British Standard threadforms created major logistical headaches . . . British equipment could only be repaired with British-standard hardware while US equipment could only be repaired with US-standard hardware.

    Once WWII had been won, the US, Great Britain, and Canada (which, interestingly enough, had fifty years of experience struggling to supply the appropriate British Standard and US Standard hardware when and where needed) put their collective heads together to develop a single standard that all three nations would use. To "share the pain" of forsaking a traditional standard screwthread, a new threadform was developed that both the US and Britain would need to learn to use. This new threadform was called the "Unified" threadform, and it incorporates the easier-to-tool 60-degree angle with flats at the Major and Minor Diameters of the Sellers screwthread, but with different proportions.

    A decade later, the fundamental geometry of the Unified threadform was incorporated into what could be considered a metric version of the Unified form intended to replace the various European national standard threadforms. Since the International Standards Organization developed and promoted the new metric standard, it was christened the ISO Metric threadform.

    The major difference between the Sellers and Unified threadforms is that the length of the flat at the Minor Diameter of the Sellers threadform was doubled to 1/4 Pitch for the Unified threadform. The flat at the Major Diameter of the Unified and ISO Metric threadforms is the same as the Major Diameter flat of the Sellers threadform, 1/8 Pitch.

    So, for both the Unified and ISO Metric threadform the axial length of the flanks is reduced still further to (Pitch - 1/4 Pitch at Minor Diamter - 1/8 Pitch at Major Diameter) = 5/8 Pitch.

    Flank length (along-flank infeed using slewed-to-feed-along-flank compound rest) of Unified and ISO Metric threadforms = 5/8 Pitch.

    Single Depth of Unified and ISO Metric threadforms = 5/8 Pitch x Cosine 30 degrees.

    Double Depth of Unified and ISO Metric threadforms = 2 x 5/8 Pitch x Cosine 30 degrees.

    Minor Diameter = Major Diameter - Double Depth of Thread.

    Well, that's the basic geometry of these screwthreads.

    As you've already pointed out, the dimensions of real hardware are properly perturbed by allowances and tolerances. External screwthreads cannot be larger than the dimensions derived from the basic geometry, and internal screwthreads cannot be smaller than the dimensions derived from the basic geometry if the external and internal screwthreads are to fit together. The along-flank infeed calculated from the basic geometry assumes that the Major Diameter of the to-be-externally-threaded workpiece is right at the basic Major Diameter AND the flat on the toolbit is the proper width (1/4 Pitch for Unified and ISO Metric threadforms, 1/8 Pitch for the Sellers threadform) . . . or that the Minor Diameter bored into the to-be-internally-threaded workpiece is right at the basic Minor Diameter AND the flat on the tip of the toolbit is the proper 1/8 Pitch width (any of the three threadforms we've talked about).

    As for the charts . . . even though the Unified threadform replaced the Sellers threadform as the official US standard threadform a half century ago, many of the tables in various "reference works" have been carried forward from edition to edition with values appropriate to the Sellers threadform.

    I realize that my posting is long-winded, but I hope it's clear enough for you to follow. If not, post back and I'll try to answer your questions."

    by john garner

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    Like others said, you'll have galling trouble with aluminum on aluminum if you don't get them anodized. Galling will weld the two pieces together eventually. If you decide to just use a lube and the pieces are unscrewed/screwed repeatedly, you'll need to regularly clean the threads and relube.


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