Chatter Marks on Single Point Threads

[macardoso]

Hi all! I haven't posted here in a while as life got a little busy and its been tough to find time to be in the shop. I just got back to some projects, and I figured I would ask for some advice on a reoccurring problem I'm having.

I do a lot of single point threading and I'm quite comfortable hitting precision thread fits, however from day 1 of using this lathe I have been getting chatter marks on the flanks of my threads. These chatter marks are periodic and typically only appear on the flank of the thread opposite the direction I am cutting (if I am cutting towards the spindle they will be on the left flank of the thread, and if I'm going towards the tailstock then they will appear on the right). The opposite flank of the thread is usually very nice looking with no discernible chatter. You can feel the chatter when threading the components together, however this doesn't seem to impair the function of the thread.

I first thought that the tooling I was using has some flex in the insert pocket (MTVOR holder for "on-edge" threading inserts), however I have seen this on all threading attempts including "lay down" style insert tools and internal boring bar threading tools. I have also tried threading both with the compound slide at 60* as well as pure radial threading (which I do most of the time) and neither of these seems to change the amount of chatter. It appears pretty much the same on aluminum, steel, and brass.

My lathe is a 12x36 and is fairly tight (although no paradigm of rigidity compared to the monsters that some of you have). I have thought about slop in the leadscrew or half nut as a potential culprit, but I'm not sure how to verify. I also thread with the back gear engaged, however I'm not sure this is a problem since other machining with the back gear doesn't show chatter.

Any thoughts on how to get really clean single pointed threads would be great! Thanks.


~M20x6 in 41L40 steel. "Laydown" style threading insert


Same thread in an aluminum practice piece


2.25"x8 TPI in brass. "On Edge" style insert (there are other issues with this thread, but you can see the chatter marks periodically)

 

[4ssss]

Dull tool or trying to take off too much at a time will do that.

 

[macardoso]

Dull tool or trying to take off too much at a time will do that.

Tool is not dull, however it is of course possible that the insert geometry isn't good for what I'm doing.

What would you consider an acceptable depth of cut? I usually do .002" radial DOC on steel and .004 on aluminum. Pure radial feed.

 

[4ssss]

Internal threading is one of the more difficult things to do on a lathe. Your depths seem reasonable enough, so maybe the relief angle needs a little more. Don't forget you're trying to cut 1 side and the bottom of the thread together, and if you're using your compound with the cross slide instead of just the cross slide, you're basically doubling the amount of free movement they both have and need to move. Try tightening the gibs a bit, and make sure you're far enough away from the backlash of the screws. The cross slide and compound screws wear mostly in one spot and getting away from that spot by moving your tool in or out will also help. Other than that I'd point a finger at your spindle play as a last resort. Try the easy stuff first. Once a chatter is cut on your thread, it's usually deeper than the amount of stock your trying to cut, and your tool will just follow the lead of the previous cut.

 

[benmychree]

Try using HSS cutting tools. You should not need to use back gears when using carbide. Backlash in screws should not cause chatter.

 

[middle.road]

Half-Nut on lead screw not staying engaged fully? Possibly.
I had that problem on my 14x40 Birmingham.
The other thing I o that I picked up off of videos, is that I installed an extension spring to keep tension on my lead screw.
It moves ever so slightly at the tail stock end.

 

[Cadillac]

Just some observations the steel ID threads are the worst. Seems to get alittle better with the aluminum but still there. The relief cut on the brass was that done with the thread tool or something else? Because that is heavy chatter.
What are you using to cut the ID threads?
What speeds are you going?
Could you have your change gears to tight “no backlash”?
Tool not on center?
Have you tried HSS to see if it changes?

 

[BaronJ]

When threading I don't bother setting the top slide at an angle at all and plunge cut. I also try to always cut towards the tailstock at a high speed.
I've just cut some internal threads, 1.125 x 12 tpi in Nylon 60 at around 300 rpm with a home ground HSS tool bit.


Very difficult to get a good picture of this stuff.

 

[Bob Korves]

Check that the tool post is actually clamped to the compound, not just to the t-nut. If the upper narrower part of the t-nut is too tall, the t-nut will tighten to the tool post, but not tighten to the compound. If not that, wiggle stuff, using a dial indicator in multiple directions to figure out where the looseness is.

 

[macardoso]

Thank you all for the ideas. I'll have to do some testing here to verify some things, but in the meantime I will try to respond to everyone:

4ssss: I started to plunge cut with the cross slide only since it is easier and I never noticed any issues with doing so. That being said, I have noticed the same chatter when using the compound at 60 degrees. The machine is new-ish (c.a. 1994) and has very little slop. I tend to keep the gibs quite tight but well lubricated. Spindle play has been checked within the last 3 months and is roughly .0001 or a tad more under 50lbs of axial force. The bearings are in good shape, and the spindle oil is clean. I totally agree that once the chatter is there, you're probably never going to get past it. The chatter appears even on the first pass, so I'm wondering if I'm cutting too lightly?

benmychree: I do not have any HSS internal threading tools but I can look into that. I agree a faster speed would be preferable, however I keep it at a pace that I can manage. This cut is up against a shoulder internally with a very small relief, so I am using a LH tool and threading to the tailstock. The return motion is done under power and I don't want to miss the stopping point and crash into the shoulder. I suppose I could thread at high speed then reverse at low speed, but that would be quite annoying to change the backgear constantly.

middle.road: My biggest concern is the leadscrew. I keep my hand on the halfnut lever at all times with firm pressure. My lathe has a spring that tends to disengage the halfnuts if I don't do this. The leadscrew has roughly 1/16th of an inch of play axially which is taken up as soon as the half nuts engage (The carriage takes a brief moment to actually start moving once the half nuts are engaged). I'm wondering if the cutting forces end up pushing the carriage/leadscrew axially, thus causing chatter. I would tend to think the forces are small, but I'm open to ideas. Maybe that could explain the chatter on only one flank of the thread.

Cadillac: The steel part looks better in person and the aluminum part looks worse than the picture was able to capture. The brass part was complete junk and was just going to be used to lap a chuck thread. That brass tube rang like a bell and was just awful to cut. I really shouldn't have even posted the picture. ID threads are cut with a 3/8" SILxxxx indexable boring bar with a carbide laydown insert (partial profile). The tool overhang is minimal, however it could be reduced further. The threads were cut at a whopping 100 rpm. Change gears are set with .003-4" of clearance using a piece of paper. They run pretty smoothly so I think they're ok. As stated above, I don't have a HSS tool to do this particular cut. Tool is on center, erring on slightly too high rather than too low. The chips form nice curly q's.

BaronJ: Nice threads! I should try cutting faster. Maybe that will do something. I also feed with the cross slide.