Polishing metal on a lathe

[joezmam367]

I hope this is still active, I'm in a rut here in a new shop and need advice. Been polishing but we only have 400 & done oil that smells like mint? Think it's a rust preventive, either way, after literally hours of rubbing from hard to softer, with fresh oil and a wipe down often, i can't seem to get these "flecks" out.
Spinning around 150-200 rpms, the parts close to 300lbs.
Any ideas here? Any help is appreciated!

Also, i always use thumb and forefinger to hold my paper, but it still scares the bejezzus outta me on some sharp edgrd finish parts

 

[SamI]

What's the material? In some cast materials you can get porosity or other imperfections that could show up like that. Hot rolled plate can also have imperfections near the surface.

 

[Bob Korves]

My guess from the photos is that the "flecks" are residual tear outs from the earlier machining. I have not been professionally taught on achieving great surface finishes, but what I think I have learned over time is that you must get ALL the imperfections out with the coarse media before changing to a finer media, and not by working on individual spots, take the whole surface down evenly until it is as good as it can be with the coarser abrasive. Then go on to a finer abrasive, and not in too big of a step. Repeat, getting it as good as possible with each step. So, something like starting at 60 grit, changing to 100, then 220, 400, 1000, then changing to successively finer polishes. Otherwise, you can end up with a beautiful high polished surface that still has imperfections that were not cleaned up in the earlier steps. Amazingly it takes less time overall if corners are not cut, and the final piece turns out better. Sadly, what you are seeing usually means going back and starting at the beginning, or near to it. If you start trying to get a few individual defects out while preserving the polish you have already achieved, it just does not turn out nice.

Of course, having a better surface finish and less defects from the machining is the major cure, well worth working at to save lots of time and effort in the finishing work.

 

[mmcmdl]

If you're looking for an 8 or 4 or mirror finish , it's a grinding or lapping job . You could polish on a lathe till the cows came home , and you wouldn't get it . Machining tears or shears metal and leaves imperfections .

 

[joezmam367]

The material is "4140" is what I'm told
@Bob Korves i think that's what will end up happening, going back to start and getting everything out completely. Thanks for the input!

 

[vtcnc]

There is a guy on u tube that works on the lathe and mill with gloves. Not latex but real gloves. Seem to finish his project with fingers intact. Amazing.

...but mostly lucky!

 

[Mitch Alsup]

2 centers with no dog??

I made some bushings that required the hole to be absolutely parallel with the outside surface. The dimensions were 0.5 by 0.45. I machined them on 2 centers and there was no room for a dog. {You had to drill the center hole 0.246; because no boring bar would fit and then turn the outside edge on centers.}

 

[Mitch Alsup]

I polish on the lathe all the time. But never wrap around the part. I hold the emery or scotch barite between the thumb and for finger. That way when it grabs, it just pulls it out of my hand. No harm no foul.

I do too, I hold the 2"×2" sandpaper (typically 1500 grit) under the part while turning, and use the other hand to drizzle on WD-40 (aluminum) or cutting oil (steel). I hold the paper UNDER the turning part right at the bottom using upward pressure on my fingers only.

If the paper catches on the part, it comes right out of my fingers.

 

[macardoso]

I think the lack of any drive force other than the two centers probably makes the original video safer than polishing on a workpiece held in a chuck. Anyways here is a good safety PSA for lathe safety:

 

[682bear]

We were taught in tech school to never let the 2 ends of the sandpaper strip touch. Use both hands, one on each end of the strip, and keep the ends at least 45 degrees apart... this naturally eliminates the 'strap wrench effect' that Joe is talking about in the video.

Also, I have used my fingers, WITH CAUTION, to lightly polish aluminum... at lower speeds... but, IMO, doing so with any type of gloves, including thin latex, is foolish.

Even latex, when it catches, will tighten up on your fingers before tearing... at 900 rpm, chances are, you get hurt before the latex gives way.

Again, this is my opinion, but I don't understand the desire to do metalwork while being afraid to get your fingers dirty...

Hands are too easy to wash to risk being injured.

-Bear