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Strange finish on turned aluminum

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ChrisAttebery

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#1
Hello,

I'm still getting used to the Yangzhou 13x40 Gear Head Lathe I bought a few months ago. I turned a 2.125 - 16 thread gauge over the weekend and it has a weird finish. I did a finish pass on the center section of the gauge from right to left, but the finish has a pattern that goes from left to right. I turned it at 400 rpm, .003" IPR feed rate, .010" DOC. What would cause this? It seems counter-intuitive to me.

I'd appreciate any input you could give me.


Chris
 

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Cadillac

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#2
Looks like the lathe traverse speed is set to fast. Your basically still cutting a thread?
 

Bob Korves

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#3
Double check your complete setup for the feed. Also looks like you are using a pointy tool.
 

ChrisAttebery

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#4
It looks like it, but the "thread" is reverse of the direction the carriage and spindle were traveling.

Looks like the lathe traverse speed is set to fast. Your basically still cutting a thread?
 

Cadillac

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#5
Were you getting a lot of chatter. The bevel on left side of threads looks chattered. Could be reflection. Even the far left looks bad. Like bob asked is your tool a sharp point? Did you back out when movin the carriage? Is that the spring pass on return?
 

ChrisAttebery

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#6
I'm using a TCGT 32.51 insert. This pass was at the low end of the recommended feed and DOC. I'm not sure where to start on the feed setup that's why I was asking.

Double check your complete setup for the feed. Also looks like you are using a pointy tool.
 

ChrisAttebery

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#7
I wasn't getting excessive chatter. It was a light finish pass. I backed the tool off before I returned the carriage.

The far left end was cut at a high feed and DOC. I can understand why the finish is poor at that end.

Were you getting a lot of chatter. The bevel on left side of threads looks chattered. Could be reflection. Even the far left looks bad. Like bob asked is your tool a sharp point? Did you back out when movin the carriage? Is that the spring pass on return?
 

Bob Korves

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#8
Do you have the manual for the machine? Is there a threading chart on the lathe? Is the lead screw metric or imperial?
 

ChrisAttebery

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#9
This machine appears to be the same as the Grizzly G4016. I've been using their manual as a reference. There are charts for feed and threading on the lathe. It was in feed mode when I did the finish pass.
 

pstemari

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#10
Those reverse thread patterns are a form of resonant chatter. 400 rpm is way slow for carbide (210 fpm) , and the cut is shallow. Try burying the end radius of the insert in the work and running at 2000 RPM. With a 32.51 insert, you want depth of cut to be at least 0.016.

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higgite

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#11
It’s hard to tell without a close up of the threads, but it sort of looks like you were cutting the threads by moving the tool in with the compound and the compound was set at a 30 degree angle off the spindle axis instead of 30 degrees off perpendicular to the spindle axis.

Tom
 

ChrisAttebery

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#12
Ok, I was wrong it was 750 rpm.

The machine does have a vibration or resonance. I put a new Gates belt on it tonight hoping it would make the vibration go away but I can see it isn’t very even. There’s a high spot that rides up in the pulley about 3/32”.

The resonance has about a 3 second period and makes the machine thrum. I’m wondering if there’s a bad bearing somewhere in the headstock. I’m going to be out of town for a couple weeks but when I get back I think I’ll pull the spindle and do an inspection.


Those reverse thread patterns are a form of resonant chatter. 400 rpm is way slow for carbide (210 fpm) , and the cut is shallow. Try burying the end radius of the insert in the work and running at 2000 RPM. With a 32.51 insert, you want depth of cut to be at least 0.016.

Sent from my Pixel XL using Tapatalk



Sent from my iPhone using Tapatalk
 

petertha

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#13
Once you have sorted through the 'cutting' aspects - speeds, feeds, tooling, in-feed orientation... another aspect may be the threading drive-line itself. If your threading lead screw rod has a slight bow, or the clam shells are not set up quite correct & displace the bar off axis, or the bushings/bearings on either end of rod are scratching, or the gear train is set up with improper (too tight) backlash.... any of those things can translate partial interruption to the rotating drive & therefore show up on fine feed cutting. A bit harder to diagnose, but it also wouldn't surprise me. There are lots of adjustment subtleties to these Asian machines.

I had issues like that under straight power feed mode which engages a different drive rod, apron etc. It was leaving a 'superimposed' resonance looking pattern. Ultimately discovered my PF rod was bowed & related setup issues.
 

ChrisAttebery

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#14
This is my 4th lathe. The speeds and feeds I used the other day may not have been ideal, but I was able to get good finishes on my 12x36" lathe with a top speed of 1200 rpm with the same tooling I have now.

I don't think that the feed rod is the problem. I disengaged the feed system and I could still feel the resonance. It gets worse with RPM. The lathe is pretty loud at 1200 and 2000 rpm so I tend to use it at 750 or below. I'm pretty sure that there's something going on in the headstock. It may have a chipped gear or bad bearing. I just haven't had the time to tear it apart yet.
 

ChrisAttebery

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#15
I just got back from vacation last night. I pulled the spindle this afternoon and sure enough there are a few rough bearings in the head stock. Both of the spindle bearings sound rough. The bearings in the feed idler were really rough.

Here's a video I took of the spindle bearings:
 

stupoty

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#16
Hello,

I'm still getting used to the Yangzhou 13x40 Gear Head Lathe I bought a few months ago. I turned a 2.125 - 16 thread gauge over the weekend and it has a weird finish. I did a finish pass on the center section of the gauge from right to left, but the finish has a pattern that goes from left to right. I turned it at 400 rpm, .003" IPR feed rate, .010" DOC. What would cause this? It seems counter-intuitive to me.

I'd appreciate any input you could give me.


Chris
you might need to take a bigger bite out of it , I would normaly push a much hevier feed rate for .010 DOC in aluminium.

Stu
 

ChrisAttebery

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#17
Over the weekend I turned a couple 2-3" parts. I used 1255 rpm for the 3" part, (985fpm) and 2000 rpm for the 2" part (1047fpm) with .006 -.014" ipt feed and .030"-.060" DOC. At first the parts had a similar finish to before the bearing swap. I tightened the spindle bearings up about 1/16th of a turn and the finish improved. I think they still need a bit more preload.

I was able to take spring cuts as light as .001" DOC, .003" IPT to improve the finish. These inserts are high polish carbide with a 25 degree rake angle.
Dragon TCGT32.51FX Inserts


I posted most of this in my machine specific thread here:
https://www.hobby-machinist.com/threads/new-to-me-yangzhou-ty-co632-13x40-gear-head-lathe.65871/

"After I got the lathe back together I measured the noise level at 110dB at 3', before it was 115dB at the same distance. It sounds like it's all gear noise. If I put the carriage feed in neutral it drops down another 3dB or so.

I did a couple hours worth of turning with it over the weekend. At first the finish didn't seem much better. Then I tightened the spindle bearings about 1/16th of a turn and it seemed to help. After doing some reading on the web I think they might still need to be tightened a bit more. I can turn the 6" chuck by hand and it will make 2-3 revolutions before it stops.

There is still a bit of a harmonic somewhere in the drive system. I think it's because the belt isn't running true. I'm going to pick up a Gates Tri-Power belt today and see if it will run a little smoother."
 

pstemari

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#18
Those are some nice looking inserts. I hadn't seen anything like that before in a TC_T format.

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ChrisAttebery

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#19
I put the new belt on the lathe and tightened up the bearings another 1/8th turn. I let it run at 2000 rpm for an hour to break the bearings in again. Then I measured the temperature of the head stock right next to the front bearing. It was ~135F. I turned another part last night and the finish was much better.

I was taking cuts at .060" DOC, .014"IPT and ~1000 FPM. The chips were tight little springs about 2-3" long and about 1/4" OD. I tried using a slower feed rate but the springs turned into bird's nests. The chips were hotter than hell though.

Disregard the dB readings from above. The app I used to take them seems to read about 18dB higher than a couple others I tried out. It's now around 91dB at 2000 rpm with the carriage feed engaged. It drops to 88dB at 1255 rpm.
 
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