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Jim Stabe
03-09-2010, 09:34 AM
I'm cutting 3/4-10 threads on each end of a 1" bar for a buffer I am making. I can cut the right hand threads with a die if I have to but I need to single point the left handers. Does the compound need to be turned 29* pointing away from the chuck to do the left hand threads or does it stay the same as if I were cutting right hand threads (pointing towards the chuck)? Also any tips on getting a good surface finish? On a practice piece (used to be the real shaft) using a fresh point on an inserted tool the finish was really crappy (I had the compound pointing towards the chuck).

Carld
03-09-2010, 09:38 AM
To cut a left hand thread cut from the chuck to the tailstock. Set the compound so the crank is on the left side toward the chuck. Set the threading tool on center line perpendicular to the work run the spindle in the forward direction. Set the levers in the correct position for the thread you want.

Ian B
03-09-2010, 09:46 AM
Some are of the opinion that setting the compound over is not totally necessary:

http://bbs.homeshopmachinist.net/showthread.php?p=501360#post501360

:)

Ian

Mcgyver
03-09-2010, 09:52 AM
Also any tips on getting a good surface finish? .

make sure your side clearance is correct.....the side clearance needs to accommodate the helix angle which, for a left hand thread, is slanting in the opposite direction to what you'd have for a right hand thread

ptmachine
03-09-2010, 09:55 AM
I cut left hand threads for the well drillers in my area all the time. I made a left hand threading tool which I place in my tool post upside down. I then set my compound at 29 deg. I rotate the spindle backwards and thread towards the head. Either way you do it, you will have to make a left hand tool. I feel that by setting up the way you would normally thread (except the rotation) your reactions will be normal in case of emergency. Neal

vpt
03-09-2010, 09:59 AM
Some are of the opinion that setting the compound over is not totally necessary:

http://bbs.homeshopmachinist.net/showthread.php?p=501360#post501360

:)

Ian


I would set the compound over. If the leadscrew is worn trying to cut the left side of the threads while moving to the right may cause the carriage to shimmy over and not cut right. To take up backlash and slop you should be cutting the side of the thread you are traveling toward.

Carld
03-09-2010, 10:01 AM
Yeah, that was an interesting thread and mardtrp didn't understand what he thought he did. Plunge cutting is nothing new and for fine threads it works well but for coarse threads it has issues. As to a CNC lathe the program moves the threading tool at a 30 deg angle for each pass but I guess you can program it for plunge cut threading.

I hope this thread doesn't to downhill from here. It seems like threads about how to thread always go astray. It's a real simple process that some like to make complicated.

Jim Stabe
03-09-2010, 10:45 AM
Thanks for the replies. I hadn't thought about the clearance for the helix angle nor had I thought about turning the tool over and reversing the rotation. I'll practice some more before tackling my next shaft.

drof34
03-09-2010, 11:18 AM
It seems to me that swinging the compound around to cut only on the leading edge is all about not making plunge cuts, which feeding straight in is. Plunge cuts like to induce chatter. Rigidity can eliminate chatter. A locked down compound is a lot more rigid than one that is not(so you can feed it).

Most any thread that a home shop machinist will likely do will probably never go straight in very far and that will be done in very small increments.

My 10EE operators manual states that the last thousands or two should be done by feeding the cross slide straight in to ensure a properly formed thread(just at it's deepest point and most likely to chatter). Why not just do it all straight in?
It's worked for me for over 50 years, but then I have never run a 7" lathe either.

Carld
03-09-2010, 11:52 AM
Well, fine threads are mostly cut in one or two passes so they can be a plunge cut but most threads are best done with the compound to feed in for each pass. I know there are a lot of machinists that use plunge cut a lot but I get better thread finishes with the compound feed for most threading.

I too use a plunge cut of .001" sometimes to clean up a thread but most the time my threads come out just fine without a final plunge cut. Once you learn how to thread you usually find what works best for you and keep doing it that way. I use a little bit of every method as fits the situation except for using the cutter upside down. I see no good reason to do that but others love the hell out of that way. What ever blows your dress up I guess.

Oldbrock
03-09-2010, 12:51 PM
All good advice except you need to make an undercut with a parting tool if you are going to thread in forward rotation. Reversing the lathe with reverse rotation and upside down tool is ok if you have a camlock or LOO chuck mount. Threaded chucks can come loose with results you don't want in reverse. You should have no problems, why not single point both? You get a much nicer thread single pointing. Peter

Cheeseking
03-09-2010, 02:02 PM
Don't forget threading with tool upside down and in reverse will have cutting forces tending to lift the carraige off the ways. May or may not be a problem esp with heavier machines. Just a thought.

Alistair Hosie
03-09-2010, 04:15 PM
I was taught to cut threads without setting over the topslide and thats how I do it I know both ways give results so what's the biggy.Grandpa Alistair

Carld
03-09-2010, 04:33 PM
No biggy at all you just do what blows your dress up or makes you whistle Dixie or blows your whistle or toots your horn so on and so on.

Walter
03-09-2010, 04:42 PM
Cutting left is generally a pleasure (in my opinion) especially short run threads. I do jobs like what the OP is doing on a regular basis as I have a fleet (so to speak) of grinders, buffers and sanders to keep alive.

Undercut to max minor dia. Start tool in undercut and don't even bother with the compound, just straight feed it. Every 3rd / 4th pass, run a spring pass then move on. Sharp tool with no build up on edges and it goes like a dream. A light (brush applied) coat of yer favorite oil on the threads. My common thread is 5/8 11 Lh for the sanders. Only thing you need to do is note at each start what yer actual depth is. once the tool approaches the minor drop final depth to a few thou and finish it. As long as your cautious on hitting that max minor, then you know what you have for leeway for excess depth.

Personally I like to go the average between max and minimum minor dia. I've had excellent results this way, also do the same with acme threads. Hrm, I generally tend to straight feed internal threads. In fact the only thing I use the compound for is cutting standard threads, now that I think about it it's probably more from habit than anything =)

It's funny how threading kind of becomes something different for all of us. Teachings, habits (good and bad), etc...

Carld
03-09-2010, 05:00 PM
The only thing I plunge cut is square threads. Everything else is feed in with the compound.

Boucher
03-09-2010, 05:14 PM
There are two kinds of people that post in these threading discussions. Those that turn threads and yes we all have our different techniques. The second group can tell you all sorts of therotecial procedures and have never actually done any of them. They are not bashfull about putting them forth but what they need to do is get on the machine and try to do what they propose. It never hurts to ask but the bottom line is cut some practice pieces.

John Stevenson
03-09-2010, 06:01 PM
Now to put the cat amongst the pigeons.

I normally plunge cut everything and often use Coventry die insets for tools as they give a full form thread, i.e. correct roots and crests at one go.

I do use the compound if I'm doing big course threads as it's easier on the machine and you usually get a better finish on the thread.

Now I make worms for a particular job, hundreds of the bloody things over the years, 5/8" diameter, 8 tpi ACME, left hand, about 2" of worm thread.
I cut with home ground up carbide cutter made from a trapezoid inset freehand on a diamond wheel. Chuck running normal and cut left to right.

Compound is rotated to in-feed at 14 degrees as you would expect.

Now the kicker, because I do this virtually daily I set the compound without thinking. So after making these worms for years I suddenly realised that I was feeding in at an angle but on the TRAILING edge because it's a left hand thread !!!!

So did it cut crap ? , no it cut just fine and still does so because I still set the compound wrong :p

.

Your Old Dog
03-09-2010, 08:43 PM
Some are of the opinion that setting the compound over is not totally necessary:

http://bbs.homeshopmachinist.net/showthread.php?p=501360#post501360

:)

Ian

It simply means you'll be cutting both sides of the thread at the same time until you reach depth. If you make the 29 '/2 degree angle then you are only cutting one bank, either the left or right pending on which side of center you are on.

Cutting one side of the thread has to be easier for the machine hence less chance of vibrations that can cause a ratty looking cut.

I'm just a rookie machinist here but someone can correct me if I'm wrong.

bollie7
03-09-2010, 09:04 PM
I hope this thread doesn't to downhill from here. It seems like threads about how to thread always go astray.
So what you mean is, you hope this thread on threading doesn't get cross threaded? LOL
bollie7

darryl
03-09-2010, 09:50 PM
Well, how about cutting left hand and right hand on the same stub. Cut everything that way, and you won't have to wurry if you have the right nut or not. You'll go crazy though trying to remember which way is on which way is off- :)

Ian B
03-10-2010, 01:48 AM
YOD,

I was also using the 29.5 degree method for many years. I think it originated in the days of UK model engineers, cutting all sorts of threads on underpowered, undersized, worn out light duty lathes. Cutting on one flank must reduce the chance of chatter in these circumstances.

I now have a much heavier lathe, and for small (ie. anything up to M24 or so) I just plunge cut. Seems to work fine - the biggest variable as far as surface finish seems to be the metal you're cutting. I always get a good finish on brass, never get a good finish when threading rebar, everything else seems to lie somewhere in between...

Ian

Evan
03-10-2010, 01:53 AM
Here is a handy chart:

http://ixian.ca/pics7/threading.jpg

gambler
03-10-2010, 02:03 AM
thanks mr. evan, I saved that and will print it out.

beanbag
03-10-2010, 06:26 AM
I said this before and I'll say it again. (One of) the reasons for the compound angle is so that whatever side of the thread you cut on pushes the carriage back against its direction of movement. This way, even if there is backlash in the lead screw, the position of the carriage is repeatable.

The reason that CNC's can do all these different infeed patterns is because there is no backlash in the carriage (I hope).

Mcgyver
03-10-2010, 07:41 AM
Evan, I don't do things like left have threads often enough that it happens by rote, takes some pondering each time...that's a handy graphic, thanks

Carld
03-10-2010, 09:20 AM
Beanbag, when your cutting a left hand thread the conventional way the carriage is moving toward the tailstock and pulling the work away from the headstock. That contradicts your belief that the force should always be toward the headstock. It has been done that way for about 200 years with no problems or damage to the lathe or the operator or the work.

The reason the compound is set at an angle of 29 to 30 deg is so the V of the thread is cut on the leading side of the V as the carriage moves unless your plunge cutting the thread. There is no reason to always cut toward the headstock for any operation in a lathe.

Plunge cutting a thread is an acceptable method and the force can be away from or toward the headstock as needed.

There is nothing magic about cutting threads and I can assure you that any lathe that is set up for threading can cut the thread toward or away from the headstock with no damage to the lathe.

Paul Alciatore
03-10-2010, 09:30 AM
For best finish concentrate on:

1. Dead sharp tool

2. Slow speed can help - sometimes I use a hand crank on the spindle instead of power

3. Good cutting fluid - I like TapMagic but there are other good ones

4. Of course, the alloy can help.

And as others have said, watch for sufficient clearance. If the tool rubs against the flank of the thread, it will not look good.

I often use a brass brush to clean up a freshly cut thread. Or a die if I have the proper one.

Jim Stabe
03-10-2010, 11:23 AM
Thank you everyone that posted, I have learned more than I hoped for when I asked the original question.

I was looking through a box of small tooling that I got with the mill I bought looking for tool blanks to grind a left hand threading tool. In the process I ran across several notched round discs that have a 60*point ground into the outside and there is a mounting hole in the center. This looks like a threading cutter that can be mounted to a post with a flat head allen screw. It appears that resharpening the tool is done by grinding the top of the notch to dress up the point and cutting edges. I did a search on Google but apparently I didn't have the right key word. Can anyone shed some light on these and tell me what they are called?

Jim

chipmaker4130
03-10-2010, 04:06 PM
Sounds like the old Armstrong thread cutter. I think most of those were carbon-steel, not HSS, and you're correct about sharpening.

beanbag
03-10-2010, 05:09 PM
Beanbag, when your cutting a left hand thread the conventional way the carriage is moving toward the tailstock and pulling the work away from the headstock. That contradicts your belief that the force should always be toward the headstock. It has been done that way for about 200 years with no problems or damage to the lathe or the operator or the work.


My agree with your originally recommended configuration, and my previous post also confirms that. I never said you should always cut moving towards the headstock either. what I did say was that the force on the cutting tool should always be against the direction of motion. In the case of the left hand thread, you HAVE TO cut moving from the headstock to tailstock if you want the tool in front of the piece (as Evan's diagram shows). That means that the force on the carriage (due to the cutting action) HAS to be towards the headstock. This means that you HAVE to cut the thread on the tailstock side of the V. This means you have to set the compound such that the handle faces the headstock. This is your recommended configuration.

davidwdyer
03-10-2010, 05:34 PM
EVAN,

Thanks so much for graphic. I already printed it out to keep with other cool and interesting stuff.

I confess to cutting my first thread on the lathe yesterday. Until now, I've been using dies and just letting the lathe turn the part. But I ran into a bigger shaft and needed to use a single point tool.
This was on a 1" stainless rod.

I watched a couple of videos by Tubal Cain on threading, then did it my own way, plunge cutting and never disengaging the half nut. I just backed out the cross slide a little and then reversed the lathe to return the cutter for another pass. No doubt this is all wrong, but it worked great for me.

vpt
03-10-2010, 08:03 PM
My agree with your originally recommended configuration, and my previous post also confirms that. I never said you should always cut moving towards the headstock either. what I did say was that the force on the cutting tool should always be against the direction of motion. In the case of the left hand thread, you HAVE TO cut moving from the headstock to tailstock if you want the tool in front of the piece (as Evan's diagram shows). That means that the force on the carriage (due to the cutting action) HAS to be towards the headstock. This means that you HAVE to cut the thread on the tailstock side of the V. This means you have to set the compound such that the handle faces the headstock. This is your recommended configuration.


I said that in post 6 but with less words.

bollie7
03-10-2010, 08:57 PM
I watched a couple of videos by Tubal Cain on threading, then did it my own way, plunge cutting and never disengaging the half nut. I just backed out the cross slide a little and then reversed the lathe to return the cutter for another pass. No doubt this is all wrong, but it worked great for me.
Not wrong at all. In fact sometimes the only way to cut a thread if its an odd ball lead or cutting a metric pitch on an imperial machine for example.
Do you know the difference between the lead (pronounced leed) of a thread and the pitch of a thread?

bollie7

Evan
03-10-2010, 09:13 PM
I became tired of changing change gears so I taught myself to cut threads by eye. I do not jest. :D

darryl
03-10-2010, 11:40 PM
If not jesting, then shirley you're joking :) Ok, I won't call you shirley again-

I don't think anybody has mentioned it yet, so I will. If you cut on both flanks of the cutting tool, wouldn't the svarfe tend to pile up in the center of the cutter, rather than flow across it and curl away? Seems like simple plunge cutting a thread would be a tougher job because of that, and harder on the tool.

I was going to suggest that it might not be an issue for a very shallow depth of thread, as in a large number of TPI, but the effect would still be the same and you do want to preserve the cutting tip in any case. Feeding in from the 29 odd degree angle should get you mostly one strip of material coming off, then whatever is actually being removed by the nose radius is less consequential. It does also seem equally important for high thread count and shallow depth of the final vee, because your nose radius would be smaller and the tool more fragile at that point. Seems to me a pretty good reason to adopt the practice of using the compound and setting that angle reasonably closely.

Carld
03-10-2010, 11:59 PM
darryl, everything you said is true and yes plunge cutting puts a heavy load on the tip of the cutter. Fine threads can be plunge cut in one or two passes. A man I worked with cut 1/2-13 threads in one pass all the time but they didn't always look good and sometimes the cutter moved and destroyed the thread. I have done 20 tpi in one pass but not when the quality of the thread was important.

When you plunge cut a thread the load is very high and the more coarse the thread the higher the load. Personally I don't like plunge cutting and rarely do it but a lot of people plunge cut and prefer it.

I have never been it that big of a hurry to do a thread and would rather have a better finish and I would never condone plunge cuts for a class 2 or 3 thread fit.

Cutting a square thread is plunge cutting but I have my own way of doing that to relieve the pressure and danger of breaking or moving the cutter while threading. Acme is a plunge cut with the tip of the cutter.

Evan
03-11-2010, 12:55 AM
I just yesterday cut inside and outside matching 1 5/8 x 8tpi threads in aluminum by plunging the cutter which I ground up from a stick of HSS. They turned out nice and fit well. I rarely use the compound to advance the cutter. Tonight I cut a .0625 x 20 tpi worm in hardened stainless steel using a carbide high rake insert, also using the cross slide and they also look just fine. I did however break the tip off the insert in a moment of inattention as I was threading at 210 rpm and went a little too far on the very last pass.

I really do cut threads by eye, mainly camera threads. They are easy to do because there are only a few threads in engagement. It takes a little practice but it isn't difficult since most camera threads are the same 1mm pitch and can be cut in a single pass in aluminum or acetal.

Alistair Hosie
03-11-2010, 04:37 AM
Is there anything you can't do?

Evan
03-11-2010, 05:17 AM
I can't cure your Parkinson's or my Fibromyalgia. I would give everything I have to be able to do that.

davidwdyer
03-11-2010, 06:50 AM
Not wrong at all. In fact sometimes the only way to cut a thread if its an odd ball lead or cutting a metric pitch on an imperial machine for example.
Do you know the difference between the lead (pronounced leed) of a thread and the pitch of a thread?

bollie7

No, I confess that I don't know the difference between the lead and the pitch. But I'm always ready to learn.

Black_Moons
03-11-2010, 06:59 AM
I can't cure your Parkinson's or my Fibromyalgia. I would give everything I have to be able to do that.

Clearly you just need a bigger mill.

Evan
03-11-2010, 07:15 AM
No, I confess that I don't know the difference between the lead and the pitch. But I'm always ready to learn.

It's a bit of a trick question. For a single start thread the lead and the pitch are the same. But, a screw can have multiple thread starts. The pitch is the distance between thread crests and the lead is the distance between thread crests of the same thread start. A two start screw has a lead twice the pitch.

davidwdyer
03-11-2010, 07:30 AM
That's interesting.

So, just to complicate things, how do you do a two or three start screw? Is it related to that counter that attaches to the lead screw? (South Bend lathe).

beanbag
03-11-2010, 07:58 AM
That's interesting.

So, just to complicate things, how do you do a two or three start screw? Is it related to that counter that attaches to the lead screw? (South Bend lathe).

I... I don't know that!
[falls into Gorge of Eternal Peril]

Mcgyver
03-11-2010, 08:16 AM
you set the feed to be twice the state tpi...for a 20 tpi double start you set the feed for 10 tpi, cut one thread, then index the work 180 degrees and cut the second. there are some pics and a bit more in this thread. The result is the lead is twice that of 20tpi single start, its used on say moving parts when the motion you'd get from a single start would be too slow.

http://bbs.homeshopmachinist.net/showthread.php?t=29782&