Striped finish problem

[Dobermann]

First a question: Are we looking at the result of hand-feeding the carriage or are we looking at the result of carriage feed by the lead screw? If not hand-fed, try that to see what you get.

Second, I have seen a pattern like that caused by a "buzzy" or electrically imbalanced single phase motor. The offending motor was not built in the USA if that's any hint as to its origin. We switched to a VFD driven 3-phase motor and the pattern went away. I'm not saying that's what's causing what you're seeing but it is possible.

 

[Scottl]

I think its caused by the single phase motor........these motors dont run at constant speed unless fully loaded,lightly loaded they accelerate /decelerate .

I don't think it would be that rythmic though. To me that looks like something that is very repetitive such as a bent screw, eccentric shaft or pulley or even a gear with uneven wear that gives it an eccentric action. I wonder if a dial indicator running on a follower might show such erratic motion.

 

[SLK001]

I don't think it would be that rythmic though. To me that looks like something that is very repetitive such as a bent screw, eccentric shaft or pulley or even a gear with uneven wear that gives it an eccentric action. I wonder if a dial indicator running on a follower might show such erratic motion.

It just looks rhythmic. The pattern is just a Moiré pattern made from the 60 Hz variation in the cut. The pattern will change due to the diameter of the work and the feed rate as the 60 Hz "bump" goes around the work and appears next to the last revolution series of bumps. If you could straighten the pattern out, it would just be a series of equally spaces bumps at a distance of the surface speed rate (in inches per second) divided by 60 Hz. If you slow down the rotational speed, the bumps will get closer and closer until they almost disappear.

 

[chipss]

Slides have a little drag? Good.
RPM? Faster is better with carbide.

I just put a piece of scrapbinium steel in my SB9 and used a brazed tool similar to what you are using. The cut is smooth as it gets especially when I belted it up for max revs. The drawback of that is the chips fly everywhere.

Aww c'mon, it's Sunday and you have nothing else to do. Flop a dial indicator on it and pull up on the spindle for a test.

 

[Scottl]

I agree with car2 that a 1.5 hp motor is too large for such a light lathe and may simply be exposing weaknesses of the design made worse by wear.

 

[camaro_dan67]

I get the same finish hand or power feeding. Not sure whether to try a treadmill motor or a small 3 phase motor. The one now makes a constant hum while running.

 

[jim rozen]

"I get the same finish hand or power feeding." Well there goes the moire pattern idea out the window. How about trying something not carbide:

 

[SLK001]

Yes, these Whiz-Bang Gizmos work really well on small lathes. After you grind them, hone them on a stone to remove all the grinding marks around the cutting area. Then hone a small radius at the point.

 

[Dobermann]

DO NOT try a treadmill DC motor unless you provide a PWM (Pulse Width Modulated) power supply for it. If you use one of the cheaper SCR DC supplies for it you'll be right back where you are right now.

 

[chipss]

Ok three phasers answer this. My SB9 with a lousy inferior 1/2hp 120 volt single phase motor does not show the problem the OP is having. Why. Am I doing something wrong? Am I a flat earther? Should I immediately change over to a 3 phase motor and drive?

edit:
Dan67, drill a center hole on your test part and run a center in the tail stock in to support the part. Then repeat the test.

.. Mental image of judge smails muttering 'well, we're waiting'..

 

[michiganbuck]

Cutting forces can stress a machine, you might use the tool geometry recommended for the material, a bent lead screw can cause such marking. We had a Carroll Jamieson lathe that had a bad gear that made similarly. a chip setting on a gear that needs picking out.
*Check to see that your tool bit is well seated /flat bottom in the tool holder.
*Could be too big a radius, or not sharp enough.
Good to look at wear land with a loop and if the wear lane is +.010,,or showing chips that could be a problem, not enough clearance under radius, or on the lead side cutting edge.

 

[michiganbuck]

9-inch South Bend is a little light of a machine for carbide, Carbide is often not as sharp-edged as HSS. For mild steel, HSS is/can be better than carbide.
Carbide or ceramic for hard stuff with higher RPM and light cuts.

 

[SLK001]

The one now makes a constant hum while running.

This could be your problem. Perhaps it is not wired correctly. Does it get hot?

 

[tylersteez]

Am I crazy for thinking that as long as the rpm of the motor is correct, using a 1.5hp motor in place of a .5hp wont do anything besides extra power when pushing the machine hard? Sure the machine may not be designed for the extra power but you wouldn't really see any side effects from it unless you're doing a heavier cut than the .5hp motor was already capable of doing. It's not like that bigger motor is turning the belts any faster or crazier than the original smaller motor was capable of if the rpm is the same.

 

[Illinoyance]

Banding is often the result of irregular feed rate due to wear or poor fit between the feed pinion and the rack.

 

[michiganbuck]

Q:
Did the machine give a better surface finish before the motor was changed?
Can you feel a difference in vibration?
Did you pull the bearing caps installing the new motor/belt?
Did you ever try a HSS tool bit?

 

[dalmatiangirl61]

My 12" unmentionable lathe had a 1.5hp single phase motor, it sometimes produced the finish the OP is seeing, the cure was resharpening the HSS tool bit and getting it razor sharp. Ditch the carbide, learn to grind HSS.

 

[jim rozen]

"Carbide is often not as sharp-edged as HSS."

But it *can* be, when properly done on a diamond wheel. Hardly any novice with a south bend 9A has a diamond wheel. Hence the reliance on pre-'sharpened' brazed carbide widgits.

Standard bench grinder, 3/8 inch square HSS tool to start. Do NOT round or hone the edges. First go is with the tool right out of the grinder.

 

[michiganbuck]

"Carbide is often not as sharp-edged as HSS."

But it *can* be, when properly done on a diamond wheel. Hardly any novice with a south bend 9A has a diamond wheel. Hence the reliance on pre-'sharpened' brazed carbide widgits.

Standard bench grinder, 3/8 inch square HSS tool to start. Do NOT round or hone the edges. First go is with the tool right out of the grinder.

Likely 120gt running wet would be just ok.> 220 wet or smoother would be much better IMHO
Fingernail the along the edge and shave your fingernail proves sharp.
* But, magnify the last tool bit you showed 500% and it looks very, very poor. I would not expect that to run a good finish on an ee lathe

 

[camaro_dan67]

Here is my latest attempt at fixing this problem. I went to a hss bit and changed the chuck to the small chuck that looks like the hand drill style. While the chuck was off I put the lathe in back gear and with a 2x4 put some pressure on the front of the spindle. I got about .0005 deflection. The stripes are just as bad so I'm thinking of buying a 3 phase motor today to try that.