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southbend 13"

dbljets

Plastic
Joined
Feb 21, 2004
Location
ft mohave arizona
how will you correct a rough surface finish that causes steps and grooves. i have had the spindle bearings adjusted but i still get rough finish. the bedways are hardened and no damage or scorings. i need your expert opinion on this matter. I also have identical problem on a colchester engine lathe. please share your expertise. thanls a million.
 
Use a round nosed turning tool that has a 1/32" to 1/16" radius and well honed with an oil stone and a fine automatic feed. Got this from a SB booklet.
 
tom,
thanks for the quick superb answer. on a finishing cut the round nose worked well but on a heavy cut it magnifies the vibration. my other specific question is? what is the culprit of these staggard rough surface finish? I am axious to know exactly what is the the specific part to be repaired or corrected on a lathe if these situation is extremely evident and upsetting without throwing in an arm and leg or rebuilding the whole machine?. (as a reminder, the bedways and saddle has very minmal wear, bearing and spindle has been re-adjusted to the closiest tolerance recommended) is it the rack and apron pinion? lead screw? feed worm gear? key or keyway on the lead screw? this ordeal has been eating me up. objective is just to let the old baby SB cut a consistent quality finish. which head of the nail to hit? anyone?
 
THe round nose tool will give a smooth finish with lighter cuts but is prone to chatter with heavier cuts. Are you using a lantern style holder? If so, you'll get much better results with an Aloris tool holder - they are much more rigid. Also, keep the amount the tool extends from the tool holder as short as possible.

Experiment with different coolants. I get good results with a mix of kerosene+lard for 6061 and many steels.

Finally, be sure you're turning at the correct speed, and too shallow a cut can burnish your tool.

Some of the real machinists will have better suggestions, but these are things that have work for me.

John
 
"I also have identical problem on a colchester engine lathe."

Since you said you have the identical problem on another lathe, my first question would be: What is the common denominator?
I don't believe in coincidences when it come to trouble shooting.
I'm afraid one common denominator is the operator (that's you!) It could also be the cutter geometry if you are sharing cutting tools between the two machines. The operator also affects tool height, gib adjustment, choice of cutters (and their geometry), material used, speeds, feeds, and depth of cut.
I'd put my money on one of these before assuming there is the same mechanical thing wrong with two different lathes.

Regards,
Dave
 
Good deduction method Dave.
Dbljets - how about posting a picture of a sample piece? Follow link from main page on how to do it. Also, are the steps evenly spaced apart?
Tom
 
What is happening with your chips?

If they are getting between the point of the tool and rubbing against the freshly cut work piece then they will mess up the finish.

Put some back rake on the tool bit so the chips curl around and get free of the tool point right away.

On a South Bend you want a free cutting tool that makes spring like chips that come off as one continuous tightly coiled chip that runs down off the tool post, over the carriage wings and into the chip pan.

Not all mateials will do this, but in High School we used to have chip races - see who could get the longest chip to run across the floor on the right side of our lathe.

Ordinary cold rolled steel was great for it.

Invariably a tool bit that gave a good chip like that also yielded a good finish. The secret is the rake.

The Aloris tool post makes the tool bit jut right out horizontally and that makes you have to grind quite a lot off the top of the bit to get good back rake.

The Armstrong tool holders give the bit an upward angle and that automaticaly gives some back rake. You can still add more. oh, yes, don't forget the side rake either.

South Bend lathes aren't Throbbing Animals with 900 hp motors. you can and should use very keen cutting tools. Rake that Rascal!
 








 
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