# Has anyone tried burnishing on the lathe?



## btboone (Dec 12, 2004)

I'm checking into burnishing tools for my titanium rings and wondered if the technique might also work on wood or plastic.  In burnishing, there's a tool that has a roller like a ball bearing under constant pressure with the part.  The idea is to "iron out" all the small machining marks caused by the cutter.  When properly done, metals can be absolute mirror finish right off the lathe.  I just got information for one system that works with super high pressure coolant to "float" the ball where it follows the contoured surface automatically.  It's almost like a ballpoint pen in the way it works, but the system is horrendously expensive because of the super high pressure coolant system.  I've also seen single tools that look like they are nothing more than a spring loaded ball. Perhaps a tool like this could be made to work on pens.  It seems like plastics and maybe wood could possibly work because the materials are so relatively soft.

I'll try to get a ball detent to make a tool, or make my own tool from a spring and ball bearing.  I was wordering if anyone has tried something like this?  It could potentially save a lot of time on finishing if it works.


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## Fred in NC (Dec 12, 2004)

Wood burnishing is not very common.  It does not cut the wood fibers, but kind of pushes them down, and folds them, etc. to even out the surface.  A polished metal rod is a good wood burnishing tool.  

However, my favority burnisher for pens is . . .
a piece of brown paper bag (kraft paper)...

Try this: turn a barrel in common wood, maple, cherry, etc. and sand to 600.  Apply a coat of shellac, let dry for a minute, and sand 600 again. Apply another coat of shellac, and let dry a minute or two.  Then burnish with the piece of kraft paper.

Shellac is not a good pen finish, it is more of a sanding sealer. You can put a finish on top of that, and wax with TSW.


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## btboone (Dec 12, 2004)

Fred, My issue is that my CNC doesn't allow me to open the door when it is spinning.  I take a final pass only a couple thousandths of an inch deep at 7000 rpm and .0005" per rev feed, so I'm real close.  I just need to sand with around 8000 grit against the direction of the machining marks and do a final polish.  I need to do that on my manual metal lathe (a 1985 Jet 14" x 40") if I want to access it when it's spinning.  I was wondering if there's anything I could do to eliminate that step. The rings take up so much time that I need to be as efficient as possible wherever I can. I'm thinking it would be more for plastics than wood to get the glasslike finish.

For your Kraft paper, do you lubricate it with anything?


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## Fred in NC (Dec 12, 2004)

Bruce, first the kraft for WOOD.  No, I just use a clean piece every time.  I cut about 1x4".

The other question is beyond me.


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## Paul in OKC (Dec 13, 2004)

Hey Bruce, I love to mix machine talk with pen talk[].  I'm not to sure about the burnishing of wood or plastic with the type of tool you are talking about. It takes some pressure to use.(not that it wouldn't work). I would think that with the rpm and feedrate you are using you would have little to 'fix'. Is your lathe not able to jog the spindle with the door open for sanding? Maybe there is a bypass switch or some 'code' to override the open door?
   I burnish wood with the shavings held in my hand against the blank as a last step before applying finish. With plastics, I wet sand with up to about 600 to 800 grit and apply a little friction polish.


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## btboone (Dec 13, 2004)

Hi Paul, the jog speed when I open the door is very slow, something like 15 rpms is all you get, and you have to be pressing the jog button.  You can't rotate the spindle automatically like the older lathes.  They are pretty serious about not opening the door when it runs.  All tool motions are locked out, and if you are to jog any axis by hand, you need to reposition a switch.  I had a larger Mazak at one time, and it was fine to run it with the door open.  You just had to be careful not to get a face full of coolant.  Being that this lathe is so quick (1950 inches per minute rapid), and so tight on space (the drill misses the chuck by less than 1/4" when the next tool is turning),  I would just as soon leave the safety switch as it's designed and never mess with it because that would allow the axes to travel as well.  I could do the friction polish on my manual lathe afterwards, but I was wondering if there might be a way to eliminate that step altogether.  The ridges left after machining plastic might be about .001" or less tall, so the surface could pass as done, but in the right light, you can see the fine lines going around it.  It certainly doesn't take much to finish them to a perfect finish, but as an engineer, I just wonder if there's a way to make it more efficient.

The burnishing tools have my interest piqued.  I've seen hardened steel do a 2 microinch finish right off the lathe.  It doesn't get a whole lot better than that.  That's hip joint perfect mirror finish.  I've been looking to find techniques like that for my rings in titanium and tungsten, both of which take a lot of hand finishing time.  I got to thinking that maybe that can work on plastic as well.  As you say, the pressure will have to be backed off quite a bit, but it should be fun to experiment with.  I can get the hardened ball bearings, springs, and some stock to make a tool for a couple bucks, so I think it would be worth trying.  It will probably have to wait until January, after the Christmas season to get some time.


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## Paul in OKC (Dec 13, 2004)

Can't say no to making tools and playing[], that's at least half the fun.  Definitely don't want to bypass any safety features. The lathe I run at work is a little older and I can jog at any speed without holding the button, but I have a lot more room toget tools out of the way.
 I have never done external burnishing, but have internal on some hydraulic vavle bodies. Like you said, mirror finish and tolerances to .0002-.0003. Cool stuff, plus surface hardening as well.  
Look forward to seeing what you come up with.


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