Question about threading

[NewLondon88]

Here's one for you metal lathe gurus..

I've read that when you cut an external thread, you're supposed to turn
your compound to a 29.5 degree angle. Why 29.5? Why not 30?

If you use a 60 degree cutter, wouldn't half of that be 30 degrees on
each side? And if your internal thread is cut perpendicular to the ways,
wouldn't that interfere with the 29.5 degree cant of the external threads?

I'm just trying to fix this in my brain.. Most of the manuals or instructions
I've found tend to treat the issues as if you already know this stuff.
But I don't..

 

[OKLAHOMAN]

Beyond me

This help?

 

[NewLondon88]

Whew .. I thought you were taking my picture..

 

[skiprat]

Charlie, if you are cutting the thread in just one pass then it makes no difference what angle the topslide is. As long as the 60deg cutter is perpendicular to the work.
If you make the cut with multiple passes ( for a deep coarse thread ) then it is only the left side of the cutter that does the actual cutting if the setting is just less than 30degs. I believe that this is the prefered method because if the cutter was also cutting on the right hand side, then it would be cutting with a negative rake. If the topslide was set to exactly 30deg then the rhs would be rubbing. the 29.5deg just gives that little bit of clearance.

As the threads that we use for pens are very rarely coarser than 1mm, I simply ignore this 'preferred rule' even if I take two passes to cut the thread.
When making multi start threads, I use the cross slide to control the 'depth' of the cut and I use the topslide ( set parallel to the bed) to advance or move the cutter for the next thread.

Lets assume that I want to make a three start thread. I'm metric so I decide that I want 1mm between each of the grooves.
I set the lathe gearing to cut a 3mm pitch on the work.
Normally one would make multiple passes over the same place so that you end up with a very coarse 3mm pitch thread.
But if the cutter is only advanced into the work to cut 1mm grooves by advancing the cross slide and then the topslide is advanced by 1mm the second thread can be cut. When this second cut is finished, the topslide is simply advance 1mm again to do the third one.

But even a 1mm depth cut is a bit hard on the material and perhaps lathe, so I do two passes to get the depth.

There is a really fantastic book about threading on the lathe from the Workshop Series collection. It explains many methods of cutting threads and goes into great depth on how to use the gears.
I can get one for you if you want.

When I get home, I'll get the ISBN number. I have mentioned the number here before so you might be able to search for it.

Hope this helps

 

[skiprat]

Here's a link to the book.

http://www.amazon.co.uk/Screw-cutting-Workshop-Practice-Martin-Cleeve/dp/0852428383

scroll down a little and there is even a bargain on three very popular books

 

[skiprat]

I'm sitting on site waiting for the client to get here so I knocked up this drawing to show the negative rake on the right hand side of the groove.

Look at the pic....on the left of the nut is the normal way cutters are shown in books and stuff. This is looking from above the work. But on the left of the nut I've drawn the cutter from the side ( or front ) of the work.
It's not a great pic but you can see that the left side of the cutter will have positive rake while the rhs (blue) will be a negative.

Like I said, I just ignore this on fine threads. Hope this helps

 

[workinforwood]

And you are using that gauge you showed in the Advanced section to set the angle of your cutter Steve? Where did you find that gauge at ?

 

[NewLondon88]

I'll order the book .. thanks! I've got some older books from South Bend, but
they are geared toward machinists who already have some working
knowledge.

I'm not 100% sure what I'm seeing, but it seems like the left hand side of the cutter
would cut the thread and on the next revolution the right hand side of the cutter
would shave it down?

Would a side cutter (in the pic) accomplish the same thing as the 29.5 degrees
if I left the topslide parallel to the ways? (or offset by .5 degrees?) If the
cutter is held perpendicular (coming straight out of the tool holder) it looks
like it would have 0 degrees already .. as if the left side of the cutter is
making a parting or facing type cut.

My internal threading tool came in the mail yesterday, so I'm anxious to
go make some metal dust.. :tongue:

Thanks for the help..

 

[j.d.sackett]

Would a side cutter (in the pic) accomplish the same thing as the 29.5 degrees

no, because youre still going straight in and cutting with both sides of the cutter. the reason for the compound at 29.5 is so you have some relief on the back side of the cut, you are actually cutting more on the feed side leaving some room on the backside. less chatter and better cut. when you get close, go straight in with the cross slide for a thou or so to clean things up. once you do it a couple times it will make some kind of sense for you. personally, i dont like carbide inserts for thread cutting. i prefer to grind them out of hss. thats my preference though and a lot of people use em. a good thing to practice on is some pvc pipe. soft, easy to cut and disposable. main thing is to have fun and dont hurt yourself. regards, j.d.

also, go to youtube and do a search on thread cutting. several good videos that will help you out.

 

[NewLondon88]

Thanks for the replies..
I just got the QCTP for this lathe, and found I need to alter the
bottom a bit. .it sits too high and needs a detent cut into one spot.
Good thing the machine shop likes me.. :biggrin:

I've been practicing on brass with old cutters from yard sales and
such.. but have new tools now. The carbide indexable cutter made
some nice looking threads, but I didn't have anything to thread onto
it, so it just looks nice for now. Don't know if it works.

I've got some PVC though.. sounds like a good idea. I also think you're
right .. much of it will start to make sense after doing it.. even though
I'm not very good on 'just do it' first.. I like to understand it before I
make the mistakes..

 

[workinforwood]

I'll interject Charlies post because it all relates and I am being informed here too. I have those same cutters he has there plus this other set. I think the second one from the bottom is a thread cutter for outside threading. It comes to a skinny point but also it is tapered on the bottom side. Am I correct that this is a thread cutter? I am pretty sure I know what a couple of the other cutters are for, but how about someone list what they all are, for me and anyone else starting at the top. Thanks!

 

[skiprat]

As long as the cutter is 60 deg it doesn't matter what you use, carbide insert or HSS. We all have our preferences. I really don't like the carbide inserts as they are very unforgiving. :redface: Sometimes you will want to cut a thread simpy by rotating the chuck by hand ( power off ) Well I've broken more carbide tips than I'd care to count. Only cos of my own stupidity really. If you turn the chuck backwards with a carbide cutter still touching the work, then it'll snap the tip off.
I love HSS, it slices stainless steel like butter and you can make all sorts of cutters to suit any requirement.
Some people say that carbide cutter give a superior finish. Mmmm, perhaps, but I'm gonna polish the finish anyway:biggrin:

Jeff, the gauges are dirt cheap and you should easily find them at any tool shop. But let me know if you really can't find one and I'll post you one.

Charlie, the book will really clear things up for you but don't 'overthink' it. It all sounds more complex than it really is and sometimes explanations just make it worse. You will eventually see that for the fine threads that we use, you can bend the rule book quite a bit.

One thing though...and this is a bit difficult to explain clearly. Sorry, I'm no machinist.

I've already said that I cut a thread ( doesn't matter how many starts ) in (say) two passes. Using my method above, the tip of the cutter cuts the second pass directly on top of the first one, just a little deeper. This will cut both sides of the V on each pass.

The 'correct' way is when the cutter is advanced into the work at an angle ( 29.5deg). So this means that the first cut is done where both sides of the cutter are doing the cutting. But now the cutter is advanced deeper by adjusting the topslide ( or compound) so that the left side cuts while the right side just rides down the side of the previously made groove. But as the cutter is ground to 30 deg, the bottom of each pass will still produce a 30 deg groove.

OK, perhaps now you will see that on the second pass the cutter is cutting a 1mm wide slither on the lhs and just a 0.5mm slither on the rhs?
This is less strenuos on the tool, the work and even the lathe I suppose.
But lets be realist, a fine pen thread is hardly about to wreck a reasonable sized lathe is it?

One day when I need to cut a really coarse thread then I'll do it 'properly'
but untill then, I'll simply take lighter cuts:biggrin:

 

[NewLondon88]

"Don't overthink it" might be good advice..

Here's a visual of what I was wondering.. turning the compound 29.5 degrees..
Would it do this to your threads? First one is going straight in with a
60 degree cutter. Second is the same cutter turned 29.5 degrees.

Is that right? Or am I missing something pretty basic?

 

[j.d.sackett]

yep, what skiprat said. on external threads you said you didnt have anything to check your threads on. just turn down some brass or aluminum to any given size standard thread and use a nut to check it. or you can bore a chunk of something, cut some internal threads and then use that to check for fit. there are various tables on the net for depth of cut, minor and major diameters etc and those will make it easier for you. you didnt say what kind of lathe you have, but if it will reverse you may want to do that to practice on internal threads. you can reverse the spindle and the lead screw, feed the cutter in as deep as you want the cut and engage the half nuts. then you will be cutting away from the chuck, left to right. no worries on crashing the cutter into the bottom of the hole. heres an example. anyway, like skiprat said, dont overthink it, its easier than it sounds.

 

[JerrySambrook]

Probably the best reason to cut at between 28 to 29.5 degrees is for chip control.
This helps to roll the chip out to the side rather than straight up. This in turn will help to reduce poor thread finish and chipping of the cutter due to swarf (chip contamination).
Also, as skiprat pointed out, the backside will now get some relief in the cutting action on the second and subsequent passes which helps to produce a cleaner thread.
There are some other technical resons that have to do with friction reduction and "better" centering results when parts are assembled, but those are probably not as applicable here

Once again though, the biggest reason is to reduce chip contamination.

just my .02
Jerry

 

[ldb2000]

Charlie , the cutter should be perpendicular (like your picture on the left) to the work piece it's the top slide that is turned to 29.5 degrees , then by advancing the top slide you will be cutting mainly on the left side of the cutter and slightly to the left and a little deeper .

 

[NewLondon88]

so .. the tool will still be perpendicular to the workpiece?

Keep in mind, I've cut exactly ONE thread in my life.. just to see how it worked
and to make sure I could change out the gears OK.

 

[ldb2000]

Yes and by advancing the top slide you are just cutting with the left side of the cutter . I do it the same way skippy does for fine threads and it seems to work well .
Get a copy of the book that skippy mentioned , it's written for a person with little metal lathe experience and is easy to understand , not that I understood most of it , it does explain allot .

 

[NewLondon88]

I ordered a copy from Amazon.

I don't mean to be a pain or overthink things.. I just like to understand the "why" of
things .. that way I won't have to ask "what" again..

 

[Paul in OKC]

Cutter perpedicular to the axis, compound set to 29.5 degrees. This is technically to allow the tool to cut on one side, making for less tool pressure and better chips. The last pass or two is to be made with the cross-slide so to clean up both sides at the smae time with a minute cut. The depth of threads on pens would not require this in my opinion. Heck, I don't even do this unless I am cutting an 8 tpi, and sometimes not even then! Just give it more rpm and hang on!!!!!

 

[NewLondon88]

OK, I keep seeing the "set the cross slide to 29.5 degrees" after setting the cutter
perpendicular. Won't that give me the thread shown in the figure on the right?

Or am I setting the crossslide to put the CUTTER at 29.5 degrees? (rather than 30) ?
.. half of the 60 degree cutter head = 30 degrees

Setting the cross slide to 29.5 gives you a 0.5 angle on the left and 30.5 on the right.??

One of these I understand.. the other, I don't.
Sorry if I'm sounding dumb here..

 

[JerrySambrook]

Charlie,
a) Set your toolpost perpendicular to the workpiece (axis of lathe centerline).
b) Set tool in toolholder so that tip has same angle on both sides when measure to workpiece (axis of lathe centerline)
c) Rotate your toolpost betweem 1/2 and 1 degree TOWARD the headstock.
d) Lock toolpost in this position.

This will set up your cutter to have the 29.5 degree lead angle you are looking for

If you are having a December get-together up there, then maybe I can come by and show you what I mean

Jerry

p.s. The other thing we have been discussing is somthing I will be working on this weekend unless I HEAR from you.
j

 

[NewLondon88]

Ok.. that fixes it in my mind now. Thanks.

On the other issue, I'm not really getting feedback yet.
Hard to say..

 

[skiprat]

Charlie, watch this video.
http://www.youtube.com/watch?v=9y0MmvscBzg

Jerry, sorry but I can't agree with the sequence of the points you made. I think point C is not correct:wink:

 

[NewLondon88]

Ok .. something just clicked.
You NEVER move the crossfeed. Only the compound is moving. And that's driving
the cutter toward the headstock AND into the work.

So .. is the cut really only being made by the left side of the cutter tool?
Looks like it would be a scraping cut?

One of the things that was throwing me is that my tool holder locks in 90 degree
stops .. so if I turned my compound to 29.5 degrees, the tool would also be at
29.5 degrees. I'd have no choice.

I've now changed that out for a QCTP, so I can do this..

thanks!

 

[skiprat]

Here is your sketch. I've shown three passes to do the thread.
If this doesn't sink in after this, then I'm gonna come over there and slap you!!!!!:biggrin:

 

[NewLondon88]

Yep.. the one with the X through it is what would happen on my lathe if I tried
to change the compound to 29.5 degrees.. since the tool holder was locked to
the position of the compound, the tool could NOT be perpendicular to the work.

But now (with the QCTP) it can.

 

[Paul in OKC]

Good work Skippy. Most tool posts can be made to lock in at any angle. They may have 'detent' areas to lock at certain degrees, but can be set in between so your cutting tool is always perpendicular to the work.

 

[skiprat]

Good work Skippy. Most tool posts can be made to lock in at any angle. They may have 'detent' areas to lock at certain degrees, but can be set in between so your cutting tool is always perpendicular to the work.

I thought the same thing Paul. My toolpost indent thingy is spring loaded and so, like you said can be locked in any position.
LOL, but I think poor old Charlie has more than enough to give him nightmares:biggrin:

 

[NewLondon88]

Naah.. this cleared it up. I could "just do it" because it says to, but I wouldn't
really learn anything that way. Now that I understand it, I've got it.

The tool holder on mine could only be set at the 4 detents (90 degree angles) but
when I got it to the machine shop to mill down the 'stop' that fits in the detent, they
told me it was only stuck. They heated it up and pulled it out. It was supposed to be
removable, but it was wedged in there pretty tight.

If I tried to move the tool holder to another position, it sat above the compound by
about 1/8th of an inch.. and crooked as my smile.

All is well now. :biggrin:

 

[JerrySambrook]

Jerry, sorry but I can't agree with the sequence of the points you made. I think point C is not correct:wink:

Fair enough that you don't agree.
However, for my knowledge sake, why don't you agree.
There are other ways to do this, but this is by far the simplest.

Thanks
Jerry

Charlie,
I will see what I can do to get a few cutters that already set with a 29.5 lead angle, and you only have to put them in square to get the proper angle
This should help

 

[NewLondon88]

Jerry.. I think the idea isn't the angle of the cutter, but the angle of the compound.
You want it to move into the work at 29.5 degrees. The cutter still stays perpendicular
to the axis.

It looks like each successive pass takes more off of the left side ,but not really anything
from the right hand side.

 

[JerrySambrook]

Okeedokee,
I disagree, but hey, what the heck, that is why therre is always more than one way to skin a cat

 

[NewLondon88]

well.. I'm still playing..

I accidentally learned how to make a left hand thread today..

 

[skiprat]

well.. I'm still playing..

I accidentally learned how to make a left hand thread today..

:biggrin::biggrin::biggrin:

Crashed the cutter into the chuck or tailstock yet?
You'll learn some new words to go with your new found skill:biggrin:

 

[Paul in OKC]

Can we make this any more complicated?????????? Threads are 60 degrees included (total angle). The purpose for the 29.5 is for tool pressure and chip making. If you run a deep thread with just the cross-slide moving in, you will have the tool making a chip on each edge of the angle. Lots of pressure, tip loads up, galling, tearing, etc., etc., etc.
That is the theory. As a machinist, I am not one of those that say this is THE way, but for making threads for pens, you are not going to run a deep enough thread to be a problem. Just run a 60 degreee tool, and use the cross-slide and just go straight in for each pass.

 

[JerrySambrook]

Paul,
Thanks for repeating what I said earlier.
1, that itis for chip removal
2, I tried to explain the easiest way.

With the replies both here and pm that I was not right, I was wondering how I did it since the 70's as well

Once again, thanks

Jerry

 

[NewLondon88]

:biggrin::biggrin::biggrin:

Crashed the cutter into the chuck or tailstock yet?
You'll learn some new words to go with your new found skill:biggrin:

Oh.. I re-learn the words every time I try to move the tail stock
or adjust gib screws. There might be enough room for hands in there..
.. but not human hands.

No crash yet.. I'm paranoid about it. That's why I thought going in reverse
was a god idea.. moving the tool away from the headstock. Oops..

I've still got some fine tuning to do. There's a bit of rocking in the saddle
and when there's any pressure on the cutting tool it can slip under the
work. (ex. final 1/32" with the cutoff blade) I'm trying to find the balance
between a firm carriage and smooth movement.

 

[Chuck Key]

:biggrin::biggrin::biggrin:

Crashed the cutter into the chuck or tailstock yet?
You'll learn some new words to go with your new found skill:biggrin:

How about catching you knuckle or finger nail with a spinning chuck jaw? Only took me a few of those to learn that trick.

Chuckie

 

[workinforwood]

It's all making sense to me. I bought a 5c collet chuck so that I don't have to break my hands on the jaw chuck. Can't wait till I have time to play with this great info myself!