What is making these odd striations

Fellows,

This is a lathe question but not a Southbend lathe question. I feel more comfortable asking in this forum as I'm not a professional machinist.

The lathe is a Cincinnati Tray-top 12 1/2 x 48".

I have attached photos of the markings I'm getting.

I have not seen these on this lathe before but I have never turned anything this long before. I'm turning between centers using a drive dog and face plate. The bar is about 15" long. I have used three different live centers thinking this was the problem but it was not. I've tried twisting, lifting, pulling and pushing on the saddle and compound rest without effect. I have tried tightening the saddle lock some with no effect. The lead screw has some slop at the quick change gearbox bushing but putting a pry bar against the lead screw at that junction while running at a slow speed has had no effect. The lead screw is eight thread per inch but I cannot correlate the cutter pattern to the eight thread per inch or a multiple of eight. If I feel the end of the bar at the headstock it is vibration free. If I feel the bar at the tailstock end the cut is vibration free for about an inch and then starts vibrating as the cut progresses towards the headstock. This pattern will occur at all headstock speeds and leadscrew feed speeds. The striation spacing will change with speed changes. The striations will also occur if hand cranking the saddle or using the half nuts. I have also tried multiple tool bits with different grinds to them.

Do you fellows have any thoughts about the cause?

Thank you,

Vlad

Hint: Think guitar string.

You've got resonance in the part, it's moving much like a guitar string would under slow motion.

Slow Motion GUITAR Strings -2/4% slower - YouTube

it's a little different because you're influencing the vibration with the contact of the cutting tool, but as the tool moves along the part the resonance (also called chatter) changes, so you get new marks from the tool as the bar vibration changes. Once the marks are introduced they can self energize to continue the pattern (or a variant of it) as you try to cut deeper.

Shorten the length of stock you're cutting by half and try again, then move outward and cut to blend. Or significantly slow the spindle RPM. A very sharp tool with a light cut will lower the energy input of the cutter and make it less likely to vibrate the bar, at the cost of very slow work output.

You refer more than once to lead screw - yet you are obviously not trying to thread anything

TELL US what system you are using to cause carriage to move along automatically

Lead screw and half nuts or apron clutch for longitudinal feeds?

Even though the TT is a simple lathe, I'll bet it has one system for FEEDING and another for THREADING - they are not the same

Vladymere gr said:

Fellows,

This is a lathe question but not a Southbend lathe question. I feel more comfortable asking in this forum as I'm not a professional machinist.

The lathe is a Cincinnati Tray-top 12 1/2 x 48".

I have attached photos of the markings I'm getting.

I have not seen these on this lathe before but I have never turned anything this long before. I'm turning between centers using a drive dog and face plate. The bar is about 15" long. I have used three different live centers thinking this was the problem but it was not. I've tried twisting, lifting, pulling and pushing on the saddle and compound rest without effect. I have tried tightening the saddle lock some with no effect. The lead screw has some slop at the quick change gearbox bushing but putting a pry bar against the lead screw at that junction while running at a slow speed has had no effect. The lead screw is eight thread per inch but I cannot correlate the cutter pattern to the eight thread per inch or a multiple of eight. If I feel the end of the bar at the headstock it is vibration free. If I feel the bar at the tailstock end the cut is vibration free for about an inch and then starts vibrating as the cut progresses towards the headstock. This pattern will occur at all headstock speeds and leadscrew feed speeds. The striation spacing will change with speed changes. The striations will also occur if hand cranking the saddle or using the half nuts. I have also tried multiple tool bits with different grinds to them.

Do you fellows have any thoughts about the cause?

Thank you,

Vlad

Click to expand...

That is the sign of classic chatter. Something is not stiff enough. I don't see the whole length of the bar in your photos, so I can't tell the diameter-to-length (L/D) ratio, but the workpiece itself may not be stiff enough for your turning conditions. Check everything that affects the rigidity of the setup - tailstock quill stickout, tool stickout, center looseness, and the headstock bearings' looseness. Verify that your tool edge is at or below center. You should be able to turn a 1-1/4" dia x 15" long bar between centers without chatter, if the rest of the setup is decent. Things to try include faster feed, lower speed, higher depth of cut (DOC), smaller tool nose radius, sharper tool, more axial rake, and less radial rake. You should be able to turn a 1" dia x 15" long bar without chatter, but with some trial and fiddling with speeds, feeds, and DOC. The main idea is to put more load on the tool and work so that it springs to a steady position and then stays there. Everything deflects under load. You want to control the deflection so that it is steady instead of oscillating, which is chatter.

Classic case when turning between centers. Support the headstock end with a chuck or collet and the stiffness doubles (approx). One thing to help out is holding something against the work to help damp out the vibrations. Somethings just a finger on the work is enough or try a piece of soft plastic like HDPE. Usually works best with the dampener at right angle to the plane of cut.

Tom

Just a thought, did you wedge your dog in the drive plate? I got some weird chatter one time when my wooden wedge
fell out of the drive plate while turning. I pushed it into place but didn't tap it with my plastic hammer. Maybe something simple. PB

Thank you all for your responses. I will have to reread them to fully understand as I just skimmed them for now.

Additional photos have been added to show the setup, the lathe and the cutting tools used in testing.

The bar is a little under 15" in length and started out at 7/8" diameter. I did make sure that the cutting tool was on center or lightly below by pinching a six in scale and looking to see if it is truly vertical.

I have been referring to a leadscrew as the leadscrew drives the carriage via a slot cut in the leadscrew length driving carriage gearing or half nuts when threading. The system works like a Southbend carriage drive. I.E.- you select either linear carriage drive/compund rest drive or you select threading. These marks occur with either linear drive or threading drive.

These marks occur with any headstock speed from the slowest (45 rpm) to the fastest (1800 rpm) and any carriage speed from the slowest (.0019” to .1215”). I tried turning this bar at these speeds and multiple setting in between.

I’m confident that the headstock bearings are OK as when I first brought this lathe home I did a detailed strip and clean and headstock bearings are something I adjusted. I also verified that the spindle run out is with in specs at that time.

Packard Bill, I did not wedge the drive dog in the face plate. I will give that a try.

TDegenhart, I was wondering if a follower rest would be the ticket to resolve the harmonics but I do not have one. I could purchase materials and make one though. I can also try using a chuck at the headstock instead of a dead center and drive dog.

Thanks for all the tips. I’ll put the suggestions to use and report back whether the problem is resolved or not. It might be a few days though as life gets in the way.

Vlad

When you develop harmonics like this, try wedging the dog or use a 4 jaw (flywheel effect) and cut from the head stock to tail. this tends to over ride what you've created. Ive also used a magnetic stand on the carriage with a piece of plastic pressed against work piece to change the harmonics.

Vlad,
That situation is close to hopeless given how skinny the shaft is relative to its length. You need more support, either from a follower rest, a steady rest, or by chucking the piece and turning the length incrementally.

I'm sure you'll hear from some supposed "pro" who will claim to have turned a shaft twice as skinny and twice as long on a 1889 lathe without chatter while hitting the diameter to within +/-0.0001" blindfolded. Don't believe them!

RKlopp

A rest would be ideal, failing that, very light cuts with a tool that doesn't need a lot of pressure might get you there. HSS with positive rake maybe.

Do you have to run it between centers?

Your later pictures show a fairly broad nose on the cutter. Change that to a more "conventional" left-hand turning tool that comes to a point and see if that helps. You may need to try cutting from left to right (using a mirrored right-hand tool), to help interrupt the already present chatter on the OD.

Sometimes more more back and side rake on the cutting tool to make it a sharper edge can lessen the vibration. Most times when vibration strikes I'll use my fingers on the workpiece to dampen the vibes. I've even used a weighted piece of leather when I needed both hands free. When sharpening small diameter perforators in a punch block a rubber band stretched around all of them can prevent the vibration that usually occurs. I've seen brake rotors get turned with a weighted rubber strap around the periphery of the rotor to hold down harmonics. Hope this was of some help. YMMV

Replace the live center at the tailstock with a dead center. Set the pressure of the dead center against the work piece to be just high enough to prevent the drive dog from being able to flop back and forth in the slot on the drive plate. Take light cuts and go slowly. As mentioned previously, you may need to add some additional damping to the system by resting your hand lightly on the work piece but be really careful about doing that. Any loose clothing could wrap around the the thing and tear your shirt off....or worse.

It’s an inox alloy, am I right?

Edison would have been proud of me, I once had to turn a conical inner taper that was about 5 inches long, 1.4404, it became sort of a sound drum.

When roughing it ain’t an issue. Take two identical finish passes to do away with the harmonics. A sharp tool, 140 sfm, coolant. Try to shear with the steel vertically mounted or with an appropriate steel holder.

7/8” x 15” is a tall order & definitely would like a follow rest. The dead center advice is good, very light center pressure is also wise with a live center. The tip about tossing that broad nose tool is also good. If I think the self excitation is from the tool shape I’ll shave the front and/or side clearance angles to 5 or maybe 3°.

When I’m stuck without a follow rest or box tool I’ll usually put a normal steady up & use the top & back fingers for roughing with the steady locked closed. For finishing you can just unlock the steady & lower the top finger a turn with the back finger just touching. This makes for moving the steady around some to complete the shaft but sometimes you do what you gotta do.

The ultra high shear grinds will kill chatter but it's very limited on depth of cut & feedrate.

Good luck,
Matt

Swiss lathe works by sliding part out of headstock and cutting tool is close to headstock
.
some jobs you stick out say 3" turn to dia, then stickout 3" more and turn that, and then stickout 3" more and turn that, at last 3" might turn part around and put previously machine part in chuck
.
if often leaves a mark between 3" sections but often it will work for many jobs

I thank all who responded to this thread.

Both wedging the drive dog in the drive plate and regrinding the tool shown in the tool holder (in the photo showing all of the cutting tools I tried) made a tremendous difference. Headstock speed also played a part as did placing a piece of 1" x 2" wood against the inside of the ways and levering it against the work to dampen vibrations but toolshape and wedging the drive dog made the most difference. Though I did not totaly eliminate the pattern I think with a little more work on cutting tool geometry I could completely eliminate the harmonics.

Again, thank you,

Vlad

What’s the material now, if I may ask?

Mechenola,

If you are asking for my finished dimensions, I don't know. Perhaps original length, a little under 15" and a guestimate diameter of .650". It started at an O.D. of .785".

If you are asking the composition then again, I don't know. Some kind of mild steel. It was a jack shaft in a machine I used to service, before being declared disabled, that I brought home for future use. It is again residing in my bucket of scrap shafts.

Vlad

I have seen patterns from a bad gear, chip in a gear, poor tool bit geometry mostly rake angle, part hardness needing more support like a steady, chip in center, poor center angle on point or part at one end or the other, , bad bearing in live center, drive dog clicking.
Guess recheck the part centers, machine centers, the dog, sharpen bit and shorten bit length, center bit more over carriage, and try dead, perhaps put a sound rod to the tail to see if a noise the bit. could blue centers and see how they hit.

Some materials and the bit sharpness, point radius and rake angle can have a certain ability to cut.. pressure builds until the material can part..the with release a chatter is (may be) created.. you see this more often on drills indicated by by the outer edge of the chip having course or fine serrations..

Guess file check shaft for hardness.. may need to crank up the speed and use .005 radius carbide tool bit for a .003 or so pass.