Homework help: machining/tooling class questions

Factual Questions
1

I’m taking a Machine Tool course this semester, and have my first homework assignment, and I am completely stuck! We haven’t had any lectures about this material yet, and the assignment is due prior to the start of class tomorrow. I have been googling like crazy, but I just can’t seem to figure out what the questions want, let alone trying to put together an answer from the wealth of information out there! I don’t want anyone to actually do my homework, of course, but I’d appreciate any links or explanations which could help me reach it myself.

One question is two parts, and reads as follows:

I have no idea how to go about solving this! I know that, for the first one, the 20 TPI refers to the number of Threads per Inch, but I don’t know what the 1/2- means (thread depth?). How does the angle come into play? I’ve gathered that infeed is the rate at which the workpiece will be tooled… is that even right? As I said, we have had NO information about this provided to us in class! I have found the following 2 things, which might be more or less on the right track:

  1. I found a couple of web pages that say that Infeed Depth = pitch x 0.6. One website specifies that this is for UN threads, but says nothing about TPI units. I know that Pitch = 1/TPI (but how does that mesh with the website that says it’s for UN??) Also, if this is even remotely what the question might be asking me to solve, where does the 30 degree angle and the handedness of the thread come in?

  2. Other websites say that Feedrate = rpm x tpi, to yield inches per minute. How do I get rpm out of the question I was given? Again, what does handedness or the angle have to do with it?

I am just very very confused, and don’t even know where to begin to look to solve this.
The other question is the following:

I have found information on different types of tapers, such as Morse, Brown and Sharpe, R8, etc, but there are way more than 8 of these and they don’t seem to be what the question is asking (though it is so vague, who knows what the prof wants?). All the websites I’ve read don’t seem to mention “three ways” to do anything, and besides, it all seems to depend on the actual type of lathe being used. I guess I’m just not using the right keywords, but I don’t know what questions to ask in order to get a useful answer. Could it be referring to something like how the workpiece is placed, either in a chuck or between centres or ??? (what would the third one be?). If anyone has any clue about what type of information I should be looking for to answer this question, I’d really, really appreciate it!

I am looking forward to actually learning how to use the lathes and milling and other machines over the semester, but I have a feeling these “assignments” are going to be somewhat ridiculous seeing as we only have 1 hour a week with a lecturer, and he shows up late every time and doesn’t cover much material (if any!) in the time he does have!

2

!/2 is the dia. Classically 1/2 =13 is normal. 1/2 -20 is finer thread.

3

Could the professor be trying to asses what level of knowledge the class already has?

Are there prerequisites for the class that you haven’t met?

4

Not machinist, although I have played with a lathe to make scrap. :smiley:

I think in the second question they are asking for technique to be used. For example, you could angle the cross feed to match the desired taper and use the cross feed to advance the cutter. An alternative would be to offset the tail stock and use the screw feed to advance the cutter. I can do that on my wood turning lathe.

5

This is a first-year course, with no prerequisites at all. The rest of the assignment was relatively straightforward, but these two questions seem to be giving everyone a lot of trouble!

GaryM - that’s possibly the kind of answer he’s looking for, but I don’t know how/where we were supposed to find information about it! I don’t even know what a screw feed is (though I can certainly look it up, now that I’ve come across the term!) I found this in wikipedia: Machine taper - Wikipedia

Meets the requirements of “three things”, but gives no info about what they are, how they work, and what the advantages/disadvantages could possibly be. And it’s hard to filter through all the different search results, since there is so much terminology we’ve never seen! I’ve ordered the textbook (found one for 5$ on Amazon’s Marketplace!!) but it hasn’t arrived yet. :frowning:

6

I’m not trying to be a killjoy, just trying to dispel my confusion. I thought homework help was not allowed/appropriate.

7

If the mods feel this is asking too much, then they are welcome to close the thread. I don’t want someone to just give me the answer, but perhaps to provide any sort of hint/general explanation/definitions which may help me in figuring this out. I can’t learn if someone does the work for me, but I also have trouble learning when I’ve never been presented with the material before!

I guess I was hoping that the answers were obvious to someone with machining experience, and they could point me to a (or a couple of) webpage(s) that I could read to solve the questions. I don’t know enough about this material to even know what questions to ask (e.g. to help me google!)

8

1/2 - 13 is normal, for a coarse thread, while 1/2 - 20 is normal, for a fine thread. Usually denoted as UC and UN, respectively, IIRC.

9

Mnemosyne

I am a home metal worker, don’t call myself a machinist.

Do you have access to a copy of Machinery’s Handbook? Machinery’s Handbook is the source for all kinds of information regarding metal removal.

Also your local library will have a selection of books on metal working.

There are many sites having to do with various aspects of metal working, here is the one that I frequent most (homeshopmachinist.net/www)

I don’t think the handedness of the thread makes any difference to the amount of metal that needs to be removed to form the thread.

The method your instructor is asking about is usually called “single pointing”, that’s where the tool bit is sharpened into the desired thread form and feed into the work the proscribed depth and made to move along the axis of the work according to the desired threads per inch.

Most threads forms use 60 degrees as the included angle and you set your top slide at either 29 or 30 degrees. For the thread forms that use 60 degrees the tool bit is sharpened into a 60 degree point. It is then feed in along a thirty degree path relative to the work.

When making a thread the depth of the thread is measured from the outside diameter of the stock, this is a right angle while the the top slide is moving in at 30 degrees. OH yes you change the depth of cut with the top slide, not the cross slide. So, turning the vernier of the top slide 0.001 inch moves the point of the work into the stock by an amount less then 0.001 inch.

Male threads are made just a little small (called tolerance or clearance) and female threads are made just a little big so that they turn freely. Less clearance (closer tolerance) = harder turning. While a zero tolerance could be specified in theory, in practice it’s impossible.

Z

10

I am a machinist but it’s been years since I thought about this stuff.

I agree that the Machinist’s Handbook is the first thing that came to mind for reference for evrything regarding most of this. My copy 28th edition from a few years ago has 95% (or so) of the same information as the one I have from the 1950’s. Nice to know some thing don’t change and need updating.

I digress, thread size is determined by diameter and threads per inch. You know the TPI. The diameter is measured by the average of the thread depth. If you measure the major diameter (across the tops of the threads) and the minor (the smallest part at the valley of the threads) and average it you get the size.

What he is asking is how much material needs to be removed to get to the nominal size. The 30deg and right hand thread are extra info that doesn’t matter.

11

Thanks everyone! I did order a book, as I said (though it’s not the one you’ve mentioned, I’m sure it has much of the same info!)

A classmate told me that for the first question, he just wants us to solve the pitch = 1/tpi, which doesn’t make much sense, but since that seems to be what everyone is doing, that’s what I’ll do too!

For the second question, the same classmate indicated some random info which, again, seems to be what most students are writing down, so it’s what I’ll do. It doesn’t seem to really address the question directly, and there is a lot of stuff that I’m copying without knowing what it means, but in the end, it isn’t worth all that much overall, so I’ll just live with it. I hope today’s class goes a little better and he has more time to tell us actual pertinent information! 1 hour lecture, 3 hour lab/machining time… there isn’t enough lecture time, IMHO!

Thanks again for the explanations - they were still helpful to me!

12

Nitpick:

Pitch is indicated by tpi. 1/tpi is the thread’s modulus. Imperial threads are specified by pitch, metric by modulus (in mm).

These terms are so frequently misused as to make the distinction near meaningless, but Machineries Handbook, which is THE BIBLE in the American machine shop, maintains the distinction, and some of the explainations in the gearing and threading sections are much easier to understand if you keep it in mind.

The infeed along one flank is the modulus, thus the need to take the reciprocal when working in imperial units. This is not required for metric threads…which make up for it by negating the function of the threading dial on the lathe.

13

The screw threading question omits so much needed specification that one assumes only nominal or perfect situation data is required. The answer is simple math concerning sides of triangles.
Consider the 1/2-20 in screw form. This denotes a threaded rod with a nominal diameter of 1/2" with 20 threads to the inch, often called “fine” thread.
When machining you need to know where you’re starting from; in the real world 1/2" stock could vary from a grand (.001") or two over to several grand under, depending on material and manufacture. Precision ground stock is typically one half thousandth under the basic 1/2". So you’re starting at .4995, which will be the crest of the thread.
You need a target or endpoint, in this case the root of the thread. A web search for "2B thread " will yield many tables to extract data from. 2B is common garden thread spec in the U.S.
The difference between root and crest will give the “infeed”.

As for the second question, cutting tapers, meaning, how to turn cones instead of cylinders.

  1. Using the compound

  2. Offsetting the tailstock

  3. Via taper attachment

    Of the three, the taper attachment has the least limitation but the first two can be done with the lathe as is.

14

Hey, two out of three. Not bad for someone who’s only made scrap on a metal turning lathe. :smiley:

15

In class, after we handed in the assignment, of course, he went over the three tapering options - that is what I had put down, luckily!

As for the first question - well, I, and many others, will just take the hit, grade-wise! He still hasn’t talked about that! For both, many of us had simply put 1/TPI (or equivalent). Oh well. It isn’t worth all that much, but it is still very frustrating. The textbook I ordered has shipped, but it hasn’t arrived yet. I’m sure it will be very useful once I get it!

Thanks so much for all your replies - it’s been very educational, and, at least, has given me a good starting point to try and understand this stuff better.

And sooner or later, I’ll actually get to USE a lathe, at which point I’m sure it will all make a lot more sense! I learn better by doing than by answering random questions on things I haven’t seen yet!

Thanks again!