Threading logic check

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Harold_V
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Re: Threading logic check

Post by Harold_V »

dbstoo wrote:Don't mean to beat a dead horse, but given the assertion "Observe proper tool geometry and pitch diameter and the other features will be correct" , does it not follow that you will reach the calculated depth if you observe proper tool geometry and pitch diameter? And given that, a tool with the proper geometry that cuts to the proper depth would also result in "the other features will be correct"?
A colt is a horse, but a horse isn't necessarily a colt. Get my drift?

No, it does not. That's due to the relaxed tolerance of less critical features, and is exacerbated by the fact that a minor change in tool tip width (still in tolerance) can equate to a large difference in pitch diameter. For example, if one works to the edge of tolerance for minor diameter (doesn't matter which edge), pretty good chance the pitch diameter will be wrong. It's simple. In a perfect world (show me one, as I've yet to find one), what you propose might work, but in reality, if you judge threads by major or minor diameters, pretty good chance pitch diameter wouldn't be correct. It may not matter in the home shop, but there are situations where it would matter, and a disservice is done to those who are not made aware.

Please note that I am not implying that it doesn't work. My point is, and has been, right along, that if you wish to create proper threads, you can not ignore the pitch diameter, which tends to have the tightest tolerance on thread forms.

On the subject of minor diameters--ever notice that the no go of a plug gauge has a truncated major diameter? It's sole purpose is to gauge pitch diameter (as well as lead, it goes without saying). The go gauge will gauge the major diameter, so it is important that the no go not be restricted from testing the pitch diameter. Rule of thumb, where I was trained, was that a loose thread could accept a no go by two turns, no more.

Harold
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heavyg603
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Re: Threading logic check

Post by heavyg603 »

So what is the correct procedure you should follow if you are presented with the problem of a threaded female part of unknown or unsure size, that you need to make a mated part for ? I realize this is more in the maintenance area vs, new manufacture but would be handy to know for future reference.
dbstoo
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Re: Threading logic check

Post by dbstoo »

Typically, you have two choices.

1) make a mold of the cavity using something like cerrosafe (low melt temperature meta) and use that to determine the major and minor as well as to verify the thread form.

2) Assume that the crest of the internal thread is the minor diameter and reverse engineer the rest of the data based on the theoretical thread form. For instance, if it's a UN thread the major should be 5/8 of the thread height larger. You might need to make a cast of the thread with wax or clay to confirm that it's a UN thread and not some bastard thread.
heavyg603
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Re: Threading logic check

Post by heavyg603 »

dbstoo wrote:Typically, you have two choices.

1) make a mold of the cavity using something like cerrosafe (low melt temperature meta) and use that to determine the major and minor as well as to verify the thread form.

2) Assume that the crest of the internal thread is the minor diameter and reverse engineer the rest of the data based on the theoretical thread form. For instance, if it's a UN thread the major should be 5/8 of the thread height larger. You might need to make a cast of the thread with wax or clay to confirm that it's a UN thread and not some bastard thread.
Great thanks! Going save a link to this for future reference
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Harold_V
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Re: Threading logic check

Post by Harold_V »

As dbstoo suggested, you can make determinations that may be helpful. Often a thread is a common size, and is easily identified. A couple observations, such as the minor diameter and pitch can be quite useful. And, in regards to size, if you're going to single point the thread, there's nothing wrong with using the mating part as a gauge. That way you ensure the thread fits, in spite of the fact it may or may not be within acceptable limits according to the charts. Fact is, I'm going to create one tonight, a bastard size, a replacement part for a surface grinder, and that's precisely how I'll gauge the thread.

If you don't have one, I highly recommend you procure a pitch gauge. They're exceedingly useful, especially if they're not truncated. You can use it to determine pitch of internal threads, assuming the minor is large enough to allow entry, plus they're very useful in ensuring you have dialed the correct pitch when single pointing a thread. With the first pass (which I like to be a scratch pass), you can check to see if you're cutting the desired pitch. If not, no harm done, as almost the entire scratch will be eliminated when the correct thread is created.

I also highly recommend one own a copy of Machinery's Handbook. It need not be a new one, although there may be information in the newer one in regards to threads that is absent in older editions. The thread charts in the handbook are exceedingly useful to those who hope to create threads to specifications, as they provide the allowance (if there is one), major and minor diameters, plus pitch diameters, along with the tolerance of each feature. Aside form the H28 handbook, I expect it may be one of the most complete sources of information for a machinist.

Harold
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heavyg603
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Re: Threading logic check

Post by heavyg603 »

Harold_V wrote:As dbstoo suggested, you can make determinations that may be helpful. Often a thread is a common size, and is easily identified. A couple observations, such as the minor diameter and pitch can be quite useful. And, in regards to size, if you're going to single point the thread, there's nothing wrong with using the mating part as a gauge. That way you ensure the thread fits, in spite of the fact it may or may not be within acceptable limits according to the charts. Fact is, I'm going to create one tonight, a bastard size, a replacement part for a surface grinder, and that's precisely how I'll gauge the thread.

If you don't have one, I highly recommend you procure a pitch gauge. They're exceedingly useful, especially if they're not truncated. You can use it to determine pitch of internal threads, assuming the minor is large enough to allow entry, plus they're very useful in ensuring you have dialed the correct pitch when single pointing a thread. With the first pass (which I like to be a scratch pass), you can check to see if you're cutting the desired pitch. If not, no harm done, as almost the entire scratch will be eliminated when the correct thread is created.

I also highly recommend one own a copy of Machinery's Handbook. It need not be a new one, although there may be information in the newer one in regards to threads that is absent in older editions. The thread charts in the handbook are exceedingly useful to those who hope to create threads to specifications, as they provide the allowance (if there is one), major and minor diameters, plus pitch diameters, along with the tolerance of each feature. Aside form the H28 handbook, I expect it may be one of the most complete sources of information for a machinist.

Harold
Thanks. Totally get the scratch pass, I do that with the layout dye too so I can see it better. Have a pitch gauge, great tool with multiple uses. The Machinery's Handbook (if anything needed a kindle edition.....) as well as that ME ThreadPal software are next on my list for sure.

More often than not I end up with a maintenance task of a female thread and a male part that has either gone missing or crumbled beyond measuring so those tips are great. Working back from the Minimum diameter makes sense to me, but I seldom get it right the first time so I'm trying to get better at this. I don't do aerospace parts for anyone, but I do like turning out the best quality work I can.
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GlennW
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Re: Threading logic check

Post by GlennW »

heavyg603 wrote:More often than not I end up with a maintenance task of a female thread and a male part that has either gone missing or crumbled beyond measuring so those tips are great. Working back from the Minimum diameter makes sense to me, but I seldom get it right the first time so I'm trying to get better at this. I don't do aerospace parts for anyone, but I do like turning out the best quality work I can.
Putting an index mark on the part or stock corresponding to the lathe spindle is invaluable to be able to remove the part from the spindle and then put it back in to continue threading. It doesn't help with indexing the thread to the tool, but it will usually be fairly concentric again. Using a MagnaVisor or similar magnifying device makes it much easier to "pick up" the thread again with the threading tool as you can really see when the tool is aligned properly.

Picking up existing threads can be a bit tricky at first, but with practice it becomes fairly easy.

Turning between centers makes everything easier though if you are lucky enough to be able to do that, as everything lines right back up again! (providing everything goes back in the machine the same way it came out)
Glenn

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heavyg603
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Re: Threading logic check

Post by heavyg603 »

When looking up editions of the Machinery's Handbook I cam across this PDF link https://github.com/aokholm/process-cap- ... strom).pdf

I really need something printed but could be handy for a quick lookup. Not sure of the copyright issues here, just passing along.....
heavyg603
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Re: Threading logic check

Post by heavyg603 »

GlennW wrote:
heavyg603 wrote:More often than not I end up with a maintenance task of a female thread and a male part that has either gone missing or crumbled beyond measuring so those tips are great. Working back from the Minimum diameter makes sense to me, but I seldom get it right the first time so I'm trying to get better at this. I don't do aerospace parts for anyone, but I do like turning out the best quality work I can.
Putting an index mark on the part or stock corresponding to the lathe spindle is invaluable to be able to remove the part from the spindle and then put it back in to continue threading. It doesn't help with indexing the thread to the tool, but it will usually be fairly concentric again. Using a MagnaVisor or similar magnifying device makes it much easier to "pick up" the thread again with the threading tool as you can really see when the tool is aligned properly.

Picking up existing threads can be a bit tricky at first, but with practice it becomes fairly easy.

Turning between centers makes everything easier though if you are lucky enough to be able to do that, as everything lines right back up again! (providing everything goes back in the machine the same way it came out)
That should help! Thanks
heavyg603
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Re: Threading logic check

Post by heavyg603 »

So an update on this if anyone is following. I created an aluminum (figured it was cheaper than messing up in stainless) plug to check fit. Fit like a glove once I deburred it. Then moved onto stainless. First one went fine, thread was beautiful, and had a pitch diameter within the range. Second one did as well but the threads looked a little chattery at the root. Didn't effect the fit but not nearly as pretty as the first so I might do a few more and see what results I get. Soon as I blast off the dirt on my cell camera I'll post a pic.
dbstoo
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Re: Threading logic check

Post by dbstoo »

Glad to hear that you have mastered the process. The bad news is that you will miss all the thrill of trial and error fitting.
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Harold_V
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Re: Threading logic check

Post by Harold_V »

heavyg603 wrote: Second one did as well but the threads looked a little chattery at the root.
That tends to be the result of tip failure. Examine the threading tool, looking for signs of minor chipping and/or rounding of the cutting edge. Stainless, due to toughness, can be hard on threading tools due to poor chip flow. You can limit the tip damage by not taking heavy cuts when roughing the thread.
Note that there's a big difference between hard and tough. Heat treated materials tend to be hard. Stainless, on the other hand, is not generally hard, but tough to machine (nickel and chrome make it so).

Harold
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