Bad thread machining results. What am I doing wrong?

[air8]

I consistently get results like the attached picture. Seems I am only cutting one side of the thread. I have tried threading towards the head stock and away from it. I am using a fishtail gauge. I have tried feeding the compound at 29½ deg. and 30 deg. and tried Zero degree infeed with the cross slide. I have several types of inserts and tried all of the above with the different inserts. Also have tried many, many sizes of threads. I have machined aluminum down to practice on but still get results similar to the attached picture.

Just can't seem to get them to cut properly. I have nearly memorized a webpage on Smithy that has amazing, detailed information about machining threads. I'm either not getting it....or the lathe is totally screwed up. LOL.

 

[The Cobbler]

looks like the compound angle is wrong. but it's been a long time since I did threading, so I may be wrong .

 

[waynem]

What type of lathe? Sure looks like the tool is not being returned into the thread in the correct place each pass. Have you tried threading without disconnecting the half nut? Some lathes you have to Change a gear to thread metric. Could you be trying standard threads with the metric gear in place? Don't know if it would cause this.

 

[The Cobbler]

are you infeeding with the cross or compound?

 

[no704]

+2 for leaving the half nut engauged. Reverse the machine out for the next pass

 

[bullnerd]

Wrong 29-1/2*

Pics of the compound?

 

[larry_g]

I consistently get results like the attached picture. Seems I am only cutting one side of the thread. I have tried threading towards the head stock and away from it. I am using a fishtail gauge. I have tried feeding the compound at 29½ deg. and 30 deg. and tried Zero degree infeed with the cross slide. I have several types of inserts and tried all of the above with the different inserts. Also have tried many, many sizes of threads. I have machined aluminum down to practice on but still get results similar to the attached picture.

Just can't seem to get them to cut properly. I have nearly memorized a webpage on Smithy that has amazing, detailed information about machining threads. I'm either not getting it....or the lathe is totally screwed up. LOL.

I'm looking at the cutter bit and see no relief on the leading edge? (assuming your cutting right to left toward the headstock.) Are you using a threading insert of a cutting insert. I would suggest that you get a HSS bit and grind your own cutter. Keep at it till you get ALL the angles ground correctly and it cuts a proper thread. Then you will know what a proper bit looks like, and cuts like. From there go back to store bought tools.

lg
no neat sig line

 

[air8]

In this picture I had just attempted feeding left to right with the insert upside down and lathe running reverse.

I have attempted leaving the half nut engaged with similar poor results.

The lathe is a Jet GH1660-ZX and all other operations seem to be precise and consistent.

I'll look for some HSS blanks and try grinding my own insert. I have been using the threading inserts from Kennametal. At least that is what Kennametal's website lists them as. However I have also tried some insert that habe no markings on them. But they fit perfectly in the fishtail gauge.

I have tried infeeding with the compound at Zero infeed angle...straight in. Have tried that a lot actually.

Also I have run the lathe slow and fast. Slow as in 25, 65, 120, 185 RPM, and up to 315 RPM.

I'm definately not giving up. I have some tools I want to make for work, to speed up repairs and touch-ups for parts that wear or break.

 

[davewo]

Your compound is 30 degrees from parallel to the spindle instead of 30 degrees perpendicular to the spindle. I can tell because the trailing edge of threads look like 60 degrees.

 

[kazlx]

Your compound is 30 degrees from parallel to the spindle instead of 30 degrees perpendicular to the spindle. I can tell because the trailing edge of threads look like 60 degrees.

That’s what it looks like to me. He’s got the wrong 30*. If your compound is marked, you might have to use 120 or whatever else. Look at where the comopound mark is when the compound is at 90* to the spindle axis. Then move 30 off of that. Your problem is probably that you are reading the 30 and setting it there and it’s not the correct angle, hence the saw teeth. That’s pretty much the dead giveaway is the cut isn’t even. No matter what the tpi is set at, speed, etc. If you feed with the cross slide and get threads like that, your angle is wrong.

 

[lilredex]

Have a look here:

For some excellent info on lathe threading.

 

[air8]

Ohhhhh ****. I bet you guys are right. I'm setting the compound by the 30-deg. mark which is relating to the longitude of the machine. That makes perfect sense.

Gonna go try that as soon as I get a chance. I will post my results. Daaaaaannnng it. I feel humbled and dumb now. LoL

 

[bullnerd]

Your compound is 30 degrees from parallel to the spindle instead of 30 degrees perpendicular to the spindle. I can tell because the trailing edge of threads look like 60 degrees.

Bingo.

That's why I asked for pic of the compound.

You'll get it now.

Have fun, chasing threads is a blast.

 

[air8]

Bingo.

That's why I asked for pic of the compound.

You'll get it now.

Have fun, chasing threads is a blast.

I was planning to take that picture today when I get over to the lathe. Good call on that one.

 

[air8]

Ok quick attempt and quick pictures after I was finished. One heckuva better looking thread. I didn't have a chance to mic them with the wires yet but they sure are much better.

Thanx guys for helping me see the error.

 

[kazlx]

That looks better . Eh, wires are fine, but unless it's for something super specific, just thread a nut on there for a cheap and easy go/no go gauge.

 

[bullnerd]

Boom!

Also, don't run your fingers over the threads while the lathe is running! See a lot of people do this and it will shred your finger tips.

Try 59 and a half too, just for fun so you can see the difference.

 

[air8]

Right on. I will give that a try tomorrow morning.
I am still kind of in shock I didn't realize I was setting the compound at 60-deg. instead of 30. LoL.

 

[2oolhound]

Thanks for posting, I still haven't tried to cut threads on my lathe but this will help overcome the fear and uncertainty.

 

[ndnchf]

I'm sure you're not the first person do make this mistake, and you won't be the last. By sharing your experience, you are helping other aspiring home machinists. Good on ya.

 

[air8]

So here is another practice run on 1/2" cold roll. I am super pleased. I am confident now to make a mandrel for some small belt rollers so they can be turned back to level.

 

[The Cobbler]

that looks real nice!

 

[Bigblue&Goldie]

So here is another practice run on 1/2" cold roll. I am super pleased. I am confident now to make a mandrel for some small belt rollers so they can be turned back to level.

Out of curiosity, what size/pitch is this?

 

[air8]

Out of curiosity, what size/pitch is this?

1/2"-13 thread

 

[matt_i]

I did this once machining a part that had stripped external threads, it was an M24 x (??) or something like this, was welded up first, but first time using a lathe I was unfamiliar with and the half nuts could not be opened due to inch-measure leadscrew. Luckily I saw the non-symmetric thread form early on and corrected the compound in time to recover.

It appears some lathes have the -0- degree mark parallel to the cross-slide's axis and other lathes have the -0- degree mark parallel to the main bedway's long axis.

 

[bullnerd]

Congrats dude! Thread away!

 

[OccupantRJ]

Since you got the threading dialed in, another way to make a mandrel is to turn a shoulder on a piece of stock that will closely fit the bore of the part. Drill and tap the turned area for a taper pipe thread to accept a pipe plug. Split the turned area with a saw. Slide part onto arbor, tighten plug. You can machine a face and OD with this type arbor.

 

[air8]

Since you got the threading dialed in, another way to make a mandrel is to turn a shoulder on a piece of stock that will closely fit the bore of the part. Drill and tap the turned area for a taper pipe thread to accept a pipe plug. Split the turned area with a saw. Slide part onto arbor, tighten plug. You can machine a face and OD with this type arbor.

That's a great tip. I like it.

Now that I think about it, there are two mandrel type parts at the shop that are very similar to what you described. They have a large o.d. though. Something like 2" and 3 or 4 inch.

 

[jmarkwolf]

Deleted by author.

You got it sorted.

 

[ilovevocs]

Just throwing out my .02 even though you sorted your initial issue.

As a hobby machinists with hobby quality tools when I want a real nice thread I will cut it on the lathe and leave just a little material to make a final pass with a die.

Leave your part in the chuck and get a holder for the tail stock. Keeps things in alignment.

 

[lilscorpion]

Nice job, those look great!!

For those of you who want to thread but aren't into single point, check out a Tailstock Die Holder. I enjoy single point but find the die holder approach to be a really fast way to cut a really nice thread on occasion.

 

[OccupantRJ]

I don’t single point unless I feel a need to do so. For most utility items it is a tailstock die holder.

 

[air8]

Yea using a die in the tailstock would be easier.

There are some bolts I need to replace which are custom cut. They have threads that are machined up to a shoulder. Maybe .050" short of the shoulder.

My first attempt with a die to fab new ones, resulted in the threads not going far enough for a nut to hold the parts. I will take a picture to show them. I also purchased some bolts to replace the broken ones but they didn't work for the application either. Those bolts are a very rare case of needing to be custom machined.

 

[OccupantRJ]

A standard threading die can be ground on the leading face to produce a die that will thread right to a shoulder. At work, I kept two of each die to allow me to thread to the limit of the standard die, then switch to the ground die to get that last thread up to a face. I had 3phase power, so just used plug reverse at the right time to reverse right before the shoulder. Alternately, the lathe could be stopped, placed out of gear, and the chuck rotated by hand to achieve the same.

 

[air8]

Oh nice!
I never would have thought of grinding a die down. I do have a surface grinder.

 

[OccupantRJ]

On dies that had removable thread inserts, we also sharpened them on a surface grinder to renew the cutting edge face. That can be done up to a certain point before geometry changes occur.