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Bad surface finish (manual lathe)

buicamarius

Plastic
Joined
Nov 26, 2021
Hello everyone, this is my first thread

I'm getting poor surface finish on my lathe (SNB 400x2000 ARAD ROMANIA)& not sure what I should be looking into to fix it. I'll show you a pic of what I'm seeing 20211126_134028.jpg

I tried different feed rate, cut depths and speeds from 60 rpm to 1200 rpm with the same results. I use carbide inserts CNMG 120408 & 120404 and brazing carbide P30.
 
Have you tried taking a skim pass...after you back the tool to the beginning on the piece leave all the adjustments the same and let it run again.
 
Before moving the carriage back to the right, retract the crosslide one full revolution, move carriage to right, then dial crosslide back to its original location, then take a spring pass. How much stickout do you have? Might need to support material with the tailstock.
 
Hello everyone, this is my first thread

I'm getting poor surface finish on my lathe (SNB 400x2000 ARAD ROMANIA)& not sure what I should be looking into to fix it. I'll show you a pic of what I'm seeing View attachment 335409

I tried different feed rate, cut depths and speeds from 60 rpm to 1200 rpm with the same results. I use carbide inserts CNMG 120408 & 120404 and brazing carbide P30.

What Material ?

Looks like too light of a cut with material that work hardens.
 
The banding may be due to improper engagement of the carriage pinion and the rack on the bed. It is common on old lathes thaat have a lot of wear.
 
I looked at the google pictures for your machine. A heavy weight gear driven lathe. You can rule out rubber drive belts with a set in them.

Are your compound and cross slide adjustments tight? I would suggest some other things but I think your are past all of that (such as tool height).

I would think going all the way down to 60 rpm is too slow. Somewhere in the 500 - 1000 rpm.
Look at the tables in Machinery Handbook and follow them. When I do it always helps.

There are some bars I have that I can not get a good finish on. No matter what. A lot of heathen pieces from the past that I don't identify with
a marking pen.
 
today I brought a mechanic and the carriage descends on a length of 500 mm to the chuck, 0.5 mm.
thanks all for reply!
 
On my very first lathe I used to get a banded finish that looked like the same pitch as the lead/threading screw. I bought it in 1976 and sold it to a friend that still uses it. I bought it new and would say the brand name but I don't want to get banned. Just to get my point across I'll either call it a "Charles" (the original mister universe) or "one of those big sheets of paper that has a picture of the entire world laid out on it". When I sold it I thought it was finally gone 'at last, yes, at last'. (snicker!)
 
On my very first lathe I used to get a banded finish that looked like the same pitch as the lead/threading screw. I bought it in 1976 and sold it to a friend that still uses it. I bought it new and would say the brand name but I don't want to get banned. Just to get my point across I'll either call it a "Charles" (the original mister universe) or "one of those big sheets of paper that has a picture of the entire world laid out on it". When I sold it I thought it was finally gone 'at last, yes, at last'. (snicker!)

ATLAS!!!! do I win a prize:D
 
Bent feed rod. Causes the carriage to lift. Mount an indicator on the bed, and check the carriage as it traverses.


I'd bet you can see the rod wobble as it turns.
 
Not to my eye. The pattern is too regular, with equal width fairly wide highs and lows. Like an Acme threaded leadscrew...

Feed rods generally move pretty fast and I should think would generate more of a squiggly surface.
 
Has the lathe ever cut a good finish?
Can you notch the material with a file?
Is your insert/bit close to the part centerline?
Is the part sticking way far out of the collet of chuck?
Is it raining outside?

Too many possible variables to even make a guess.

You might throw a short 25mm slug of mild steel in the chuck, run at 200 rpm, take about .25mm cut/feed x .75 deep, and use a sharp HSS tool bit as a starting place for troubleshooting.
 
today I brought a mechanic and the carriage descends on a length of 500 mm to the chuck, 0.5 mm.
thanks all for reply!

.020 descending in 20 inches is .001 per inch so that is poor, but still on a short part of one inch long or so not very much.

*It Seems descending would not be the cause of that pattern.

We had an old Carrol Jamieson that would give a wavey pattern in only one gear, and the other gears were Ok.

Will a pass over a part with crocus cloth take it out to an acceptable finish?

Have you checked to see if a bent lead screw is lifting the saddle?

I have run clunker lathes where 5 or 10 minutes with a file or crocus cloth was expected on a fussy part.

Out near the tail, many clunkers are straight, so fussy work/features can be run there.

You might blue up the feed screw so to see if it is riding on the threads or on the Od, and check for bent.
Indicate the saddle to see there is not a chip under it.
Change your gears to exclude every gear you are currently using.
 








 
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