Burnishing

[darryl]

Thanks for all the replies. Couple of things- as I mentioned I'll often take a file to the surface I've just machined to knock down the high spots. I do this under power so the entire surface gets 'treated' equally. Sometimes I go for a dull file so I'm not really taking material off, and sometimes I'll reverse the spindle to apply the treatment from both directions. I don't know if this is beneficial or not, but I do it anyway. Using a file I'm typically taking off about 2 tenths, so about 1/2 thou overall from the diameter. I factor this in when I'm turning to allow for this 'shrinkage'. I would imagine that burnishing would require about the same allowance- and I wasn't expecting to be able to shrink a diameter by two thou, more like 1/2 thou perhaps. Just enough to turn a too-tight press fit into a useless sloppy fit, if you know what I mean No, I've never done that before, nope-

By the way, I've tried over the years to get a good feel for how much extra to leave on a work piece so that my file method would bring it to the exact desired size. Now I pretty much always use the file before the caliper or micrometer measurement is made. I think I do pretty much get a more consistent reading this way as well.

As far as burnishing, I might just make up a dual ball bearing setup where I can adjust the spacing between them for whatever diameter I'm working with at the time. I'd probably set the gap slightly small- then I could hand-hold this tool over the workpiece and let it do its thing. If the gap is too small I won't get enough pressure between the rollers, and if too large it will just pass right over without doing anything. There will probably be a 'sweet spot' where I'll get lots of pressure between the rollers without having to apply too much side pressure against the work piece. Using ball bearings as rollers is probably not the best, but it should be somewhat effective anyway. Maybe I could find some roller bearings that would be more suitable- perhaps I could cannibalize some roller lifters to make this tool.

Or- maybe I could use smooth rollers in a knurling tool-

[Rich Carlstedt]

I bet I'm not the only one who'd like to hear more - can you tell us/show us more about it?

Well My photobucket account is shot so I will try to explain a few things without pictures.
The Process was invented by Chrysler Automotive back in the 1930" s to improve crankshaft bearing life, and it did dramatically by producing a surface finish far finer than turning and grinding did and reduced wear measurably.
So much so that is is used throughout the world today. It involves presenting a fine abrasive to a turning surface and oscillating that abrasive cross-wise to the rotating metal. Industrial machines are very expensive and I have given links at the bottom below to show you the commercial machines and some commentary. Basically you have a non repeating oscillating abrasive move irregularly across a rotating workpiece to remove offending surface protrusions created by earlier machining /grinding work. It is not a heavy force application like milling or grinding (!) ( OR Burnishing !). Low forces ( hand pressure) are all thats required to remove the high points and it does not remove "valley" irregularities
Now let me tell you how to make one at home .
One of the secrets to Superfinishing is variability. you do not want a oscillation that is controlled time wise ( think metronome ) and the more irregular, the better. While commercial machines use rolls of abrasive or stones , a home machine can use a small stone...like a 600 India stone or finer .
Let me tell you what I did as I am not sure whats available today. I found a old electric razor at a flea market 20 years ago. This was a Palm unit that is similar to this eBay sales item

https://www.ebay.com/itm/IN-CASE-Vintage-SCHICK-SUPER-3-SPEED-ELECTRIC-RAZOR/123215344075?hash=item1cb034f1cb:g:TnwAAOSwClxanvVg:rk:34:pf:0

The important part is that it is not a rotary unit shaver, these old ones oscillate quite fast ( 500 CPS ?) and vary with load as the vibrators are not strong
Throw the head away and make a adapter to hold a stone ( 1/2 x 1/2 x 4 ?) . My adapter was a small piece of walnut (1 x 2 x 3/8" thick ) that was milled to hold the stone . make the slot about .002 smaller and the wood will hold it. the wood milled so it is retained by the 2 vibrating arms that formerly moved the shaving blades L & R by oscillating them maybe .010-.020 "---perfect for superfinishing
Now when you have a piece in the lathe , rotate the work piece about 100 RPM and set the stone that is exposed from the end of the shaver head on the surface with a bit of oil and watch the surface get really smooth. The stone moves both forward and backward and also oscillates side to side. this criss-crossing knocks all the irregularities down to give a superior surface . If you find an old electric razor , it can be pretty cheap addition to the shop for journal turning .
Keep the rotary speed down so you don't crash and run the lathe backward, so any force throws the offender upward and not downward into the carriage/ways. Obviously CARE must be used to keep the stone moving left to right and not hitting a crank throw.
I wrote an article for Model Engine Builder magazine a few years ago about this in magazine issue number 29 which is available pretty cheap on line

https://www.modelenginebuilder.com/

Rich


commercial machines

https://www.aaabrasives.com/shop-manufacturers/dynabrade/dynabrade-stationary-machines/dynabrade-superfinishers

Description of the process

https://www.supfina.com/products/technology/superfinish/

results

https://www.supfina.com/products/superfinishing-machines/

[Mike Burch]

The only knife steels I've used have been very old, and if they were files, they would have been worn out years ago.

I don't think they are meant to cut, but the fine grooves reduce the area of contact between the knife and the steel, increasing the unit pressure. I believe they work by drawing out the edge and thinning it. This is somewhat similar to the way scythe blades were sometimes sharpened, by hammering the edge out on a small, slightly domed anvil ( I've never had any success in sharpening a scythe that way!).

I haven't seen any evidence of "filings" from a knife steel, but of course if the edge is drawn out thin and worked back and forth, the wire edge can break off as a fine sliver.

Edit: We speak of "grooves" in a knife steel, but of course, they are ridges.

I use a steel on my kitchen knives pretty frequently, and I always wipe the blade clean before using the knife. If you use a piece of white paper towel, for example, you will see a smear on it, indicating that a small amount of material has been removed.

[boslab]

I’ve seen machines for burnishing what we call track rod ends, you know the ball on the end of the steering rod, it was just rollers, think they were steel not carbide, probably nitrided or somthing, suppose hardened smooth rollers in a knurling tool would smooth out a surface
Mark

[fixerdave]

"Simple and efficient, Cogsdill DBFM tools are designed to produce mirror-like finishes on any surface; from carbon steels to tool steels, cast iron to alloys, and most ferrous and non-ferrous metals."

I really like the sound of that, shinny brochure-speak or not. So, I'm thinking I'll take that old non-scissors knurling tool I have that never gets used and replace the wheels with some appropriate bearings. Maybe steal another one of my kid's fidget spinners*, might have to make some bushes to resize to whatever pin the knurler uses.

I figure I can angle the toolpost a bit to get a corner of the bearing doing the work, and see how it goes. If it does anything, besides raise my frustration level, then I can think of a few other ways to incorporate the process... grinding the teeth off a thicker slitting saw and mounting in some holder with bearings. Stuff like that.

Yeah, I know, some of you get that mirror finish using stuff like tools that cut. Me... something besides filing sounds interesting.

David...

* No, I'm not really that mean. The local store was dumping fidget spinners for $0.99 each a while back. I bought 1, took it home, pulled the bearings out, looked them up on ebay to find the cheapest I could get them was $0.49 each, so I went back and bought 10 more spinners. I can do the math and the size is useful. I threw them in my kid's toybox, so I don't have to store them and they're there when I need 'em.

[MattiJ]

Budget version of Rich's superfinisher is wooden or plastic stick covered with 1200 grit sandpaper and 2 extra cups of coffee so that your hands are randomly shaking around

[plunger]

I wonder- has anybody here burnished a turned surface to further prepare it for use? Perhaps you turn a shaft to about a thou or two large, then you burnish it with rollers to knock down the high spots and bring it to size.

Seems it would improve the wear life of a plain sleeve bearing, and it could facilitate the mounting of a press-on ball or roller bearing. It potentially could become shinier- probably depending on how smooth and precise the rollers are.

I usually do a careful dressing with a file to level the surface a bit more- and now I wonder about burnishing instead, or in addition to.

You learn something every day. Never heard of this. At first thought the heading made me think of lapping process. I have tried this now on a piece of en 8
I machined a shaft to clean it up and then polished it too a high polish using 320 grit emery.
I then took my ball bearing tool used to align work in a three jaw. I miked the shaft and it gave me 29,77mm.I then ran the ball bearing so just the corner rubbed the workpiece at at the slowest feed let it travel along the shaft with enough force to just get the bearing spinning.

It immediatley gave a better finish or at least left a tell tale line as it traveled along the length of the shaft.I miked the shaft and got 29,75mm.
I was suprized it made the diameter less even though not by much.
This could be useful for making nice sliding fits (think air bearing ) but to get a consistent measurement would be a challenge. I would not know how to make an internal diy tool for this though.
AS an external tool I think it would help to springload the tool to take pressure of the crosslide leadscrew Any thoughts.?

[MattiJ]

I machined a shaft to clean it up and then polished it too a high polish using 320 grit emery.
I then took my ball bearing tool used to align work in a three jaw. I miked the shaft and it gave me 29,77mm.I then ran the ball bearing so just the corner rubbed the workpiece at at the slowest feed let it travel along the shaft with enough force to just get the bearing spinning.

It immediatley gave a better finish or at least left a tell tale line as it traveled along the length of the shaft.I miked the shaft and got 29,75mm.

This could be useful for making nice sliding fits (think air bearing ) but to get a consistent measurement would be a challenge. I would not know how to make an internal diy tool for this though.
AS an external tool I think it would help to springload the tool to take pressure of the crosslide leadscrew Any thoughts.?

I think the tool/link I posted on previous page also had some sort of spring loading.
Many tools also seem to have dial indicator to indicate the force ie:


0,02mm reduction in diameter after you already smoothened the surface with 320 grit emery paper sounds a lot or measurement error to me? It's like 10-fold what I would expect.
320 grit should give you surface roughness something like Ra = 0.3um.
Now depending on what Stetson-Harrison conversion factor you use that would be Rz = 1.5 to 3 um (between highest peaks and deepest valleys).
So I wouldn't expect the diameter to change more than 1-2 micrometers even if you flatten it completely.

[Doozer]

David...

* No, I'm not really that mean. The local store was dumping fidget spinners for $0.99 each a while back. I bought 1, took it home, pulled the bearings out, looked them up on ebay to find the cheapest I could get them was $0.49 each, so I went back and bought 10 more spinners. I can do the math and the size is useful. I threw them in my kid's toybox, so I don't have to store them and they're there when I need 'em.

You do realize that this is the definition of hording, don't you????

--Doozer

[plunger]

I think the tool/link I posted on previous page also had some sort of spring loading.
Many tools also seem to have dial indicator to indicate the force ie:


0,02mm reduction in diameter after you already smoothened the surface with 320 grit emery paper sounds a lot or measurement error to me? It's like 10-fold what I would expect.
320 grit should give you surface roughness something like Ra = 0.3um.
Now depending on what Stetson-Harrison conversion factor you use that would be Rz = 1.5 to 3 um (between highest peaks and deepest valleys).
So I wouldn't expect the diameter to change more than 1-2 micrometers even if you flatten it completely.

I too was suprized.Thats why I mentioned it. I think I should go read the measurement again.

[Mcgyver]

Well My photobucket account is shot so I will try to explain a few things without pictures.

great post - thanks!

[cameron]

You do realize that this is the definition of hording, don't you????

--Doozer

You're not suggesting there is something wrong with hoarding, are you????

[cameron]

Budget version of Rich's superfinisher is wooden or plastic stick covered with 1200 grit sandpaper and 2 extra cups of coffee so that your hands are randomly shaking around

I've been doing that, with and without the coffee. But I think the paper should be backed with a flat strip of steel, or replaced with a fine stone where possible.

[MattiJ]

I've been doing that, with and without the coffee. But I think the paper should be backed with a flat strip of steel, or replaced with a fine stone where possible.

Why is that? Commercial superfinishers seem to use rubber wheel.
Rubber block backing for low cost solution..

[reggie_obe]

So, I'm thinking I'll take that old non-scissors knurling tool I have that never gets used and replace the wheels with some appropriate bearings. Maybe steal another one of my kid's fidget spinners*, might have to make some bushes to resize to whatever pin the knurler uses.

I figure I can angle the toolpost a bit to get a corner of the bearing doing the work, and see how it goes. If it does anything, besides raise my frustration level, then I can think of a few other ways to incorporate the process... grinding the teeth off a thicker slitting saw and mounting in some holder with bearings. Stuff like that.

Yeah, I know, some of you get that mirror finish using stuff like tools that cut. Me... something besides filing sounds interesting.

David...

* No, I'm not really that mean. The local store was dumping fidget spinners for $0.99 each a while back. I bought 1, took it home, pulled the bearings out, looked them up on ebay to find the cheapest I could get them was $0.49 each, so I went back and bought 10 more spinners. I can do the math and the size is useful. I threw them in my kid's toybox, so I don't have to store them and they're there when I need 'em.

You do realize that this is the definition of hording, don't you????

--Doozer

* No, I'm not really that mean. The local store was dumping fidget spinners for $0.99 each a while back. I bought 1, took it home, pulled the bearings out, looked them up on ebay to find the cheapest I could get them was $0.49 each, so I went back and bought 10 more spinners. I can do the math and the size is useful. I threw them in my kid's toybox, so I don't have to store them and they're there when I need 'em.

What quality low-buck bearing do you you think are used in these "toys"? Especially the outer three which serve no purpose other than to add mass? I would be very surprised if the bearings weren't all rejects from "Q/C"?l

Hording...from the guy who says he wants to downsize his shop into an old bread delivery van....