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Boucher
12-03-2010, 10:03 AM
If the top face of a cutting tool is polished to a finish of four r.m.s. or better, all else being unchanged, tool life will usually double.

Like a puzzle the big picture of HSS tool sharpening is composed of lots of small pieces. This important piece is in one of the references that whitis posted in another thread.

randyc
12-03-2010, 11:30 AM
If the top face of a cutting tool is polished to a finish of four r.m.s. or better, all else being unchanged, tool life will usually double.

Like a puzzle the big picture of HSS tool sharpening is composed of lots of small pieces. This important piece is in one of the references that whitis posted in another thread.

That's an interesting observation and it certainly wouldn't hurt to try it. I once read a recommendation to buff wood chisels and plane irons with jeweler's rouge after lapping. I tried it, using a little buff in a dremel tool and the difference in cutting ability was noticeable. (Gotta' be careful though - the technique is important because the buff can just as easily take off the cutting edge instead of the wire edge.)

I think I'll try that the next time I have to lap a HSS lathe tool, thanks for the tip. Maybe it would be worth conducting a longevity test, lapped versus buffed -

Cheers,
Randy C

Carld
12-03-2010, 11:55 AM
Hmm, polishing the top surface to a 4 rms lengthens cutter life.

Could that possibly be related to the fact that when you polish the top surface your also putting a fine sharp edge on the cutter?

OTOH, a fine sharp edge breaks down faster under a load and dulls somewhat to a lasting durable edge.

Also, there is the false edge that builds up on the cutting edge that somewhat protects the edge of the cutter.

Geez, so many things to consider, so many things to think about. :eek: :D

Boucher
12-03-2010, 11:59 AM
After John Stevenson posted his information regarding diamond wheels and their use on HSS, I began to just touch the ground surfaces to the diamond wheel. It only takes a second to significantly improve the finish. Next I use a set of three inch square stones that quickly produce a mirror finish.

I have a 10 inch buffer permanently set up in the shop and use it to finish wood chisels and pocketknives. This works great but I question its use on the HSS.

lazlo
12-03-2010, 12:05 PM
Could that possibly be related to the fact that when you polish the top surface your also putting a fine sharp edge on the cutter?D

I think that's exactly what's happening.

Wear resistance is related to the material toughness (fracture strength), and the hardness differential between the cutter and the workpiece.

noah katz
12-03-2010, 12:28 PM
Also removes the fine scratches that are stress risers that would shorten life.

randyc
12-03-2010, 12:52 PM
Yes, I agree too - the surface finish is important because of its relationship to the keenness of the cutting edge.


...I have a 10 inch buffer permanently set up in the shop and use it to finish wood chisels and pocketknives. This works great but I question its use on the HSS.

I'm curious - if it works for knife and chisel blades, why wouldn't the buffer also work for HSS (assuming that the buffing technique doesn't radius the cutting edge) ? The difference between heat-treated high carbon and high-speed steel shouldn't mean much to abrasive media. Are you using a felt wheel, by the way ? (Wouldn't the lack of density and the size of individual fibers in a cloth wheel be too coarse to produce 4 microinch finishes ?)

It's all personal preference but I like a small, felt bob in a dremel tool or die grinder when small parts are worked (parts stuck to a benchblock with double-back tape). This technique has been useful for freeing up small, case-hardened parts in tightly-fitted areas (e.g. trigger sear, hammer sides in revolvers). Big cloth wheels have flung small parts across my shop, not to mention burning my thumbnails more than once. A large, flat pencil eraser can also be useful as an intermediate stage in lapping to a fine surface finish.

Cheers,
Randy C

Carld
12-03-2010, 01:46 PM
I posed the question about polishing the top surface because that is my belief as well. Having tried buffing I find I prefer a leather strop to remove the knife edge burrs. A buffer wheel tends to round everything and is hard to localize the buffing action. I haven't tried using a buffer in a dremel yet but that would be better than a large felt wheel.

Having read some articles about a false edge produced on a lathe cutter leads me to think the sharp edge is not doing as much work as the smooth surface the removed metal can flow over. It's amazing the things I can think about while I watch a lathe make a cut. Have you ever taken a heavy feed and DOC at 75 rpm or less and watched with a magnifying glass? It would really be interesting to actually look inside as the metal is being chiseled off the work wouldn't it. :D

To be honest I don't polish my cutters to a mirror finish that a 4 rms is. I just lap the surfaces with a stone or diamond lap. I only use a leather strop on knives or wood chisels.

I don't try to get my lathe tools sharp enough to shave with, just a good sharp edge does it for me. I have found a fine surface finish on the cutter does not always produce a fine surface finish on the work. There are other issues involved that affect the works surface finish but a good sharp edge and slight tip radius helps.

As far as a machine finish surface on a lathe I use a shear cutting tool now on the last finish cuts. If I don't mind filing and using emery cloth I just machine over size and file and sand to size.

I think a lot of people go to extremes when they sharpen cutters. To put that in perspective, at home in your own shop with no time limits it's no issue to spend an hour sharpening a lathe cutter. However, at work for a living I didn't have that option. That's when I learned how to compensate and one regret is that I didn't learn of the shear cutting tool while I was employed. It would have been a real asset at work in a job shop on manual machines with time and quality an issue.

Paul Alciatore
12-03-2010, 01:53 PM
Could it be a reduction in friction when the chip slides over the top surface? Less friction equals less heat and less abrasion. Also less heat equals less wear by keeping the tool harder.

lazlo
12-03-2010, 02:17 PM
I have a 10 inch buffer permanently set up in the shop and use it to finish wood chisels and pocketknives. This works great but I question its use on the HSS.

This comes up in custom knife making a lot. John Verhoeven, a materials engineer at Iowa State University, has written several knife-related technical papers, including one entitled "Experiments with knife sharpening." Most of the blades he tested were tool steels (normal practice for custom knives).

He addressed the custom knifemaker's common practice of grinding a convex edge on the blade with a slack belt grinder, and then polishing the final edge with a buffing wheel. The polishing rolled the fine edge over and effectively ruined an otherwise perfect cutting edge.

He has a bunch of fascinating micrographs of the blade edges in various stages of the sharpening experiments -- very impressive work.

randyc
12-03-2010, 03:00 PM
Wouldn't it be great to be an academic ? All those grad students that have to work for you with no pay - performing cool experiments like this topic suggests would be so simple. The kids do the work - then the prof gets to publish and take the credit, LOL. Not to mention infinite job security once you publish enough and get tenure !!!

lazlo
12-03-2010, 03:17 PM
John Verhoeven is a neat guy. He worked with Al Pendray, a Master Smith, for over 10 years to re-discover how to make real Wootz Damascus.

He and Al published a great paper in Scientific American a couple years back showing their results, including explaining how they discovered that vanadium "impurities" in the raw Indian iron ore cause the beautiful carbide dendrites:

http://farm1.static.flickr.com/54/112903257_62e6f8aeee_o.jpg

randyc
12-03-2010, 03:21 PM
... to re-discover how to make real Wootz Damascus.

Lovely stuff ! Is that the more-or-less forged finish or has it been etched ?

noah katz
12-03-2010, 03:34 PM
As far as a machine finish surface on a lathe I use a shear cutting tool now on the last finish cuts.

Didn't we recently agree that all bits are shear tools?

What differentiates the one you use for finishing?

Carld
12-03-2010, 04:12 PM
Actually there is a difference, and that is a shear cutting tool is a modified left hand turning tool turned upside down in the holder. When you get to the last .005" to take off you switch to the shear cutting tool and take two passes at .002" to .003" DOC at about 100 rpm and .004" feed. The modified part is the leading edge is at about 30 deg and the front angle can be about 10 deg or it could be "0" deg. You can see in the photo how it shears the metal off.

http://i82.photobucket.com/albums/j276/yeathatshim/P2210016.jpg

This is a photo of the end of the cutter.

http://i82.photobucket.com/albums/j276/yeathatshim/P2210014.jpg

It will produce a near ground finish. It is a very old trick of early machinists, possibly around the time they started using square lathe cutters with the lantern tool post in the Armstrong holders. The earlier cutters were forged on the end of a rectangular bar of carbon steel that was held in the lantern post so it's unlikely it was used then.

AiR_GuNNeR
12-03-2010, 04:15 PM
This comes up in custom knife making a lot. John Verhoeven, a materials engineer at Iowa State University, has written several knife-related technical papers, including one entitled "Experiments with knife sharpening." Most of the blades he tested were tool steels (normal practice for custom knives).

He addressed the custom knifemaker's common practice of grinding a convex edge on the blade with a slack belt grinder, and then polishing the final edge with a buffing wheel. The polishing rolled the fine edge over and effectively ruined an otherwise perfect cutting edge.

He has a bunch of fascinating micrographs of the blade edges in various stages of the sharpening experiments -- very impressive work.

Here is a link to his paper
http://www.scribd.com/doc/10044505/Experiments-on-Knife-Sharpening-John-Verhoeven

randyc
12-03-2010, 05:09 PM
Actually there is a difference, and that is a shear cutting tool is a modified left hand turning tool turned upside down in the holder. When you get to the last .005" to take off you switch to the shear cutting tool and take two passes at .002" to .003" DOC at about 100 rpm and .004" feed. The modified part is the leading edge is at about 30 deg and the front angle can be about 10 deg or it could be "0" deg. You can see in the photo how it shears the metal off...

Carl,

That's a new one on me and I want to make sure that I understand it because I LIKE that surface finish. The spindle is still rotating in the normal direction, right, but the cutting edge is now the front ANGLED edge of the tool rather than the left side (in the case of cutting toward the headstock) ? So it's almost a scraping operation, right ?

Why the very low spindle RPM ?

Thanks,
Randy

noah katz
12-03-2010, 06:41 PM
...the cutting edge is now the front ANGLED edge of the tool rather than the left side (in the case of cutting toward the headstock) ?

My question too - what part of the bit is on center.

How does this work on thinner more flexible parts?

whitis
12-03-2010, 06:55 PM
Anything that makes the chip glide smoothly over the face of the cutting edge is likely to reduce tool temperature, cratering, and built up edge, whether it is a good surface finish, a plated on coating, or lubricant. In addition, it will reduce the chip thickness (chip is always thicker than the cut) and improve the shear angle.

As far as removing cracks and other heat damage, a cool finishing grinding pass probably does more as polishing doesn't cut very deep.

If you use something flexible to polish with, you will round off the edge, as mentioned above, while something rigid will help hone a straight edge.

A very slight radius on the edge might turn out to be a good thing in some cases as it is less prone to chipping. Tool bits wear very rapidly at first, then wear slowly, then finally deteriorate rapidly. Putting a very slight radius on the edge might be equivalent to pre-wearing the edge, but with a more controlled (straighter) outcome. In the case of negative rake carbide, they do sometimes round the edges slightly:
http://maja.uni-mb.si/files/apem/APEM5-1_013-024.pdf
Tool wear goes down with a very slight radius (more radius does not improve much) but cutting force and radial force go up with any radius and considerably with radius. Makes sense for production CNC machines but probably not a good idea on less rigid manual hobby machines. Besides, in the home shop, you can quickly kiss a HSS tool with the grinder as it wears.

lazlo
12-03-2010, 07:07 PM
He addressed the custom knifemaker's common practice of grinding a convex edge on the blade with a slack belt grinder, and then polishing the final edge with a buffing wheel. The polishing rolled the fine edge over and effectively ruined an otherwise perfect cutting edge.
Here is a link to his paper
http://www.scribd.com/doc/10044505/Experiments-on-Knife-Sharpening-John-Verhoeven

Thanks! That's the one:


Conclusion

These results suggest that buffing on cloth or felt wheels is not the best method for finish sharpening of knife blades.It appears that evena light buffing action removes a significant amount of metal near the thin edge of the blades which leads to a bur along the edge

miker
12-03-2010, 07:20 PM
Here is a link to a previous discussion on the Finishing Tool.

http://bbs.homeshopmachinist.net/showthread.php?t=39862&highlight=contrary+ground

Rgds

Bob Ford
12-03-2010, 07:24 PM
This is the link that goes into detail. http://bbs.homeshopmachinist.net/showthread.php?t=39796&highlight=shear+tool

Bob

randyc
12-03-2010, 08:55 PM
Perfect - thanks for confirming my understanding. Just one more question: why the low spindle speed or does it make a difference ?

Cheers,
Randy C

Carld
12-03-2010, 09:29 PM
Randy, noah, the cutting is on the front end of the cutter. You can move the tool up or down as long as the front edge of the cutter is against the work. I found the 30 deg angle to do the best job and the cutting action is much like a lathe file with the steep angle of the file cut. It cuts the metal off in slivers is the best way to describe it. Here is a photo of the tool near the work in cutting position. The photo was taken about 3" from the work and the piece was rebar because I wanted to use trash metal.

http://i82.photobucket.com/albums/j276/yeathatshim/P2230010.jpg

I was amazed at the surface finish it gives and the important thing is you can machine a smooth straight finish with a perfect diameter the length of the cut. To say I saw a lot of good use for that cutter is an understatement. When I want a fine accurate finish out comes the shear cutting tool. I can take a better photo if needed.

The spindle speed has to be slow because it is shearing or scraping the metal off. If you look at the angle of the cutter as it would be cutting you can see it looks like the angle of a lathe file cut. I found anything much over 100 rpm does not work well and you can't take more than .002" to .003" DOC. I usually leave .005" .010" oversize depending on the finish I am getting and then go to the file, now I use the shear cutting tool to finish it out. The feed has to be slow because you don't want to rush across the surface but I guess you could use up to .006" but .002" to .004" gave me a better finish and finish is what I want.

randyc
12-03-2010, 10:06 PM
<sound of appreciative whistle>

RE-BAR ? That's amazing. I get the slow spindle speed now, it's like draw-filing. The finish on that part is darned near bearing-journal quality - a slight kiss with 400 paper and it's ready. Thanks very much, I'm amazed that I've never heard of that technique before. Of course the old-timers always kept a card or two up their sleeves from us youngsters, LOL !

Noah also posed an interesting question regarding the "slenderness" of the workpiece. What's the consensus about cutting pressure and workpiece deflection using that cutter geometry ? Can the .002 DOC be applied - for example - to a 1/2 diameter x 2 inches long. (I'm estimating that the aspect ratio is about what your picture depicts.)

Cheers, and I'm still marveling at that finish -
Randy C

Carld
12-03-2010, 10:22 PM
Yes, it's 1/2" rebar about 3" long. Since your only taking a few thousandths there may not be much deflection on smaller stuff but anything smaller than 3/8" I would use a live center in the tail stock to support it.

Randy, grind you one and try it, I think you will like what it does. Unless I needed a shinny finish for the bearing journal I wouldn't even hit it with emery cloth. It really did look almost like a ground shaft you would get with a tool post grinder.

noah katz
12-03-2010, 10:34 PM
Thanks, Carl, very cool, and I have just the job to try it on this weekend.

Carld
12-03-2010, 10:35 PM
It would be interesting to know how the first machinist came to try that. I wonder what he was machining and what lead him to even try using the cutter upside down. That is one man I would love to talk to.

I have only been around a few old time machinists and they seldom told you anything unless you asked and they thought you really wanted and needed to know. They would always ask me how I was going to do it before they even offered any info.

One old timer, born in the late 1800's, was probably the best machinist I have ever met face to face. Some of the tools he made were of extremely high quality and I wish I had bought his tool box when they auctioned off his shop. I bought some stuff but for some reason I didn't bid on his tool box, maybe it was going to high for me, I can't remember now. He did job shop work and built heavy duty dozer blades.

randyc
12-03-2010, 11:27 PM
...Randy, grind you one and try it, I think you will like what it does. Unless I needed a shinny finish for the bearing journal I wouldn't even hit it with emery cloth. It really did look almost like a ground shaft you would get with a tool post grinder.

It's near the top of the list, Carl - IF the shop EVER gets warm !

Last week, I needed to turn a couple of bearing journals - arbors for the horizontal mill - within a few tenths and with a fine finish. (The arbor outboard ends are supported in a tapered bronze bearing, as you may recall from the original post.) I used a similar procedure to the one you mentioned earlier. Because the arbors were hard, I turned them .0005 oversize with a carbide tool then polished to size with three different grits of paper. I had plenty of time so I didn't mind spending ten or twenty minutes polishing, measuring, polishing, measuring.

But it was COLD out there, LOL ! The point of THAT little story is that I'm envisioning the usefulness of a holder that positions a carbide insert upside down, about ten degrees off horizontal axis and maybe twenty degrees off vertical. (Wonder if I can diamond-lap the insert to a keen enough edge to produce that nice scraped/sheared finish on hard material for finishing cuts ?)

Thanks,
Randy

Jpfalt
12-03-2010, 11:50 PM
Using the high shear finishing tool is the same sort of effect as draw filing with a hand file. The shear angle tends to lay the chip to the side rather than piling it up in front of the tool. The chip laid to the side is thinner and takes less force to move.

Concerning polishing the rake of the tool, the surface finish could make things better or make things worse, depending on the metal you are cutting. If the material tends to weld to HSS, then the finer finish promotes the welding. For material not prone to welding, the finer finish reduces friction between the tool and the chip.

If metal is prone to welding to HSS, then a rougher surface with plenty of contamination, such as sufonated cutting oil works better because the chip has less actual surface contact with the HSS and breaks away easier during chip flow.

Boucher
12-06-2010, 12:42 PM
I am not buying that. Poilishing the surface the chip tends to rub on is allways going to improve the cut. Good cutting oil will also improve the cut surface finish. I simply have not seen any tendancy for BUE to occur with this shear tool. I have primarily used it on steel and a little brass. There is another version of this shear tool for brass it was the first one that I ever encountered. It is the same principle with the cutting face at a small angle off horizontal. Comparing the two has been on the to-do list. Just have not got it done. The last time that I looked for it I didn't find it. Will look again and post some pictures.

It is surprising how really good tool configurations are forgotten and not passed to the next generation. I am really glad that this one was rediscovered. The discussion is benefical even if we arrive at different conclusions.

charlesb
12-06-2010, 03:05 PM
Also removes the fine scratches that are stress risers that would shorten life.

That's why I keep a stereo microscope in the shop. It lets me see small items at 10-40x in 3-D.

The great thing to do with a stereo microscope ( besides looking at bugs, or your fingertip ) is to take a HSS tool that looks all mirror-like, fresh from the grinder and look at it at 40x for a tour of the grand canyon.

Knife edges are interesting too. Magnified, a fine edge on your knife looks like a jagged, crudely made tree saw. - That's why it cuts so well.

I found my ancient but still usable Swift Instruments stereo microscope at a pawn shop for five bucks - but you can get a nice new one on eBay for around 100-150 bux that will serve you much better... Using my old Swift is a real challenge as it has seen better days and only works if you hold your mouth just right - kind of like its owner!

A finely finished tool will run cooler and last longer.

There's less chip friction, and that means less heat and wear. The edge is better, but the smoother tool surface near the edge makes a difference too.

Tool wear begins in microscopic 'canyons' in the metal which drag and wear, or sometimes drag and build up. - In either case, if these micro scratches in the tool are fewer and smaller to start with, it takes longer for the erosion ( or buildup ) to begin, and the wear progresses more slowly when it finally does begin.

There are dozens of other fun and educational things that you can do around the shop with a stereo microscope. It's hard to beat as an inspection tool.

It is kind of counter-intuitive - but the lower powers like 10x are useful in more situations than the higher powers like 40x in a stereo microscope.

There are some stereo microscopes that are made to work by sliding on a steel rod, much like a familiar dial indicator with a magnetic base. - These microscopes can be used anywhere in the shop that you can use a dial indicator, to inspect tools or work pieces that are still set up in the lathe, milling machine or surface grinder.

Everybody wants to look at bugs though, so try to keep a dead bug on hand for visitors in the shop... That's what really wows them.

I am going to make one of the shear tools being discussed today, to see if I have the idea for making one right or not. I look forward to trying one out here on my lathe.

noah katz
12-06-2010, 03:17 PM
There is another version of this shear tool for brass it was the first one that I ever encountered. It is the same principle with the cutting face at a small angle off horizontal.

Do you mean vertical?

Boucher
12-06-2010, 07:49 PM
Couldn't find the tool. From memory set the grinding table at 20-30. Lay a 1/2" blank on its left side and grind the top surface so that when it is in the holder it slopes up from left to right. When presented to the work it cuts at the tangent point lifting the chip upward. Like the other shear tool if it wears a little you just adjust the height a little.

Someone on this foreum posted this back a couple of years. It was used for sizing and finishing brass rod. I didn't ever get it to work on 18" long 1" brass rod without a follow rest. I may have just never got in the right ballpark on speed, feed, and depth of cut. For the shorter pieces it worked ok but so did a regular right hand turning tool. After posting this guestimate the tool will suddenly appear tomorrow from its hiding place.

whitis
12-06-2010, 11:16 PM
A straight edge cutting tool presented at an angle to the work on a lathe has the interesting property of acting a bit like a tool with a nose radius of from anywhere from half the part diameter (at 0 degrees from vertical) to infinity (at 90 degrees from vertical). As viewed from above (direction of work travel), the radius is reduced to 0 to infinity but still quite large at the angles in question. If we ignore the angle of the bit to the travel of the work aspects on cutting, the effect is somewhat similar to a hand scraper blade with its very large nose radius. If you avoid chatter, a large nose radius combined with the feed per revolution set a theoretical limit on the quality of surface finish achievable - i.e. the ripple between the valleys and crests of the cut. This will vary with the diameter of the work.

It would be interesting to compare the effects of the shear bit with a lathe tool with a very large radius, such as what you would get by holding the rear of the tool stationary on a pivot point while you swivel the front against the grinding wheel. For a fair comparison, you want to generate a smooth arc comparable to the quality of the straight edge on the shear bit. This would help separate the effects of the cutting angle vs the effective nose radius. The large radius tool, however, will cut/rub on a wider area than removed, effectively recutting the same spot more than necessary as if feeds so the tool should probably be fairly narrow (but still wider than the feed per revolution. Perhaps shaped like a v-nose tool with a "flat" on the front which is a wide radius; this gives good rigidity and heat dissipation vs a narrow tool while possibly reducing the rubbing. V-nose would allow you to use the tool for roughing, as well, but then surface finish would be limited by tool wear and since the nose radius is not tangential to the sides the intersections would be weak. Note that the depth of cut doesn't need to be much deeper than the ripple depth left by the previous tool.

A 2" radius fed at 10mils/rev gives a surface finish limit of 25microinches peak to peak, 4 microinches at 4mils/rev, and 0.25microinches at 1mil/rev, vs 401, 64, and 4 microinches for a 1/8" nose radius - i.e. 16 times better (conveniently proportional to the improvement in nose radius). Optically, there is another affect of using a large radius vs a finer feed as the scattering angles are reduced giving a less diffuse finish. Downside is it involves much rubbing as it is producing many "chips" with a precut thickness similar to the depth of the surface finish so it is like continuously hand scraping at extreme speed. Rapid wear is an issue (and this may be what is happening on the shear bit, as well).

This is only true on the lathe, not on the shaper, as the nose radius affect comes from the diameter of the work unless a deliberate radius is put on the tool which might well have been done to avoid sawtooth effects on the surface finish from the edge of the tool.

Carld
12-07-2010, 09:41 AM
That's an interesting idea. I guess it wouldn't be to hard to grind a segment of a 2" radius on the end of a 1/2" sq cutter and try it. My thought is that the cutting edge applied to the work horizontal to the axis of the work would not produce the same finish as the shear cutter which is applied at an angle.

I'll have to try your idea but I still think the surface finish of the shear cutter is because of the angle of the cutting edge. Another thing is when you look at the edge of the shear cutter there is a worn area that is about .040" wide but I suppose it would be the same on a radius cutter. I have used a 1/4" radius cutter to smooth a finish but never have gotten as good a finish as the shear cutter.

What rpm and feed are you recommending for say a 1" shaft?

Carld
12-07-2010, 12:32 PM
I didn't have a 2" radius gauge but I did have a 1 3/16" gauge so I ground a 1/2" sq to that radius.

I had some 3/4" CR laying in the shop so I used it. First I made a clean up pass of about .010" then I made a pass with the radius cutter at 120 rpm, .004" feed and .004" DOC for the length of about 3". Then I made a pass with the shear cutting tool at 120 rpm, .004" feed and .004" DOC for about an 1 1/2".

The shear cut is on the right and the radius cut is on the left. At the extreme left there is a short area where the clean up cut is shown. To me the clean up cut is better than the radius cut and the shear cut on the right is even better than the clean up cut or the radius cut. I still like the shear cutter better than anything I have seen or tried.

http://i82.photobucket.com/albums/j276/yeathatshim/PC070001.jpg

http://i82.photobucket.com/albums/j276/yeathatshim/PC070003.jpg

I don't know if a 2" radius would produce a better finish or not, perhaps someone wants to try.

madman
12-07-2010, 12:41 PM
I have 3 m deburring polishing buffing wheel on one side of grinder. The other side has my grinding wheel. I usually grind my cobalt or hss tool to finish shape then buff the cutt edges shiny and lightly retough(regrind) the cutting edges. I t does make a big differance especially in aluminumn to have a shiny top and side surfaces.

randyc
12-07-2010, 12:44 PM
You beat me to it, Carl, I was going to try a 1/2 shank ball-nose HSS router tool this morning in a 1/2 boring bar holder, LOL. I did make a few experiments yesterday with a CARBIDE insert, configured in a round boring bar (so that I could rotate the bar in the holder and adjust the shear angle).

Results were not so good but I didn't expect them to be without lapping the insert edge (it's not real sharp, out of the box). I'll try it again after I get a chance to hone the edge. I'm interested in trying this with carbide because I see an opportunity to get a good journal finish on hardened/semi-hardened stock. Would save a LOT of time if it works.

That photo you posted is mighty impressive !

Cheers,
Randy C

Carld
12-07-2010, 04:47 PM
I used a diamond lap to polish the cutting edges of both cutters but I didn't try for a mirror finish. The photo was from about 4" and holding it in your hand at about arms length the right side cut looks like a polished or ground surface.

The cutter for the radius is Congo HSS and the shear tool is HSS Cobalt.

Randy, I have a piece of triangle shape carbide stick that I have used as a shear cutter and it does a nice job. That could be used on a hard shaft I suppose.

ckelloug
12-07-2010, 04:53 PM
I think Noah has it right about stress risers. When you look at the tool in terms of fracture toughness, each scratch is a flaw. The larger the flaw, the lower the stress required to cause a crack to propagate from that flaw.

--Cameron

KEJR
12-07-2010, 06:52 PM
I agree with polishing reducing aluminum cold welding. We regularly polish punches and things at work that form soft materials. Without the polishing we get galling (cold welding) very rapidly. Lubrication can help, which is why oil on aluminum seems to help.

Polished micro grain carbide works the best :o)

whitis
12-08-2010, 11:50 AM
carld: Thanks for trying that. I don't think you can really test it by grinding to fit a radius gage (accuracy about 6tenths) when you are talking about surface finish limits in microinches. I think you need a pivot. Positive back rake might also improve the results.

As far as speed, I don't know what would be best but I would suspect, due to rubbing, that lower than normal recommended speed would be better, especially if it is a plain round nose rather than a V with a radius. Depth of cut at 0.004" was huge for this type of tool and feed was moderate. Theoretical limit for that nose radius would be 6.7microinches. With a depth of cut of 0.004", The metal is probably recut about 25 times. This is because the width of the tool engagement at that depth of cut is about 0.100" to the left of the center where the final cut is made or about 25 revolutions. This is why I was thinking a combination of V and radius edge might be an improvement. Ideally you would have a radius a little wider than 0.004" (the feed/rev), then a small arc that rounds of the corner a bit for tool strength, then the V angle. The corner radius would be tricky to grind.

Surface finish looks worse in the pic for the radius tool. However, looks can be deceiving, especially with surface finish samples. I wish I had a profilometer. I have surface finish specimens where the turned samples with quite poor surface finish look better in some ways than the ground ones with fine finish. I think part of it is that tooling marks hide irregularities in the underlying material and that grinding tends to look a bit more random than ordered. The splotchiness of the radius turned sample could be underlying properties of the metal, chatter, galling, burnishing, built up edge, tool wear, etc. There is some splotchiness in the other sample (shear cut) but it looks hidden by the tooling marks. I will note that in the radius cut sample, you can resolve three lines of the reflected overhead lighting while in the shear cut one you can only resolve two but this could be due to less camera saturation.

Like I said before, the numbers are simple geometric surface finish limits. Actually achieving them in the real world, is another matter. I think the shear tool has a potential advantage in reducing the rubbing area while still giving the effect of a large nose radius.

One thing you can do easily with the shear tool is raise or lower the tool to use a non-worn spot. This doesn't help you on a long cut on a manual machine but can be done between cuts.

Have you tried putting something like printed text behind the rod to see if either is anywhere close to a mirror finish? One judges a mirror by looking through it, not at it. How do they feel to the fingernail?

Carld
12-08-2010, 04:10 PM
I had concerns about not using your suggested 2" radius but I didn't want to make one to grind the cutter to so I used what I had. I just now tried a fingernail test and the radius cut felt smoother than the shear cut but not by much. Then I used some worn out 180 emery cloth on both with four back and forth strokes and the fingernail test was the same for both but the surface texture by eye was still better for the shear cut. The shear cut still looks smoother than the radius cut.

My personal opinion is the shear cut seems to be easier with a better visual finish and I have a feeling it is better because of the angle of the cut. It's a lot like draw filing and that always produces a better finish.

In the end I don't think it would matter which you used except for the fact the radius is harder to grind to resharpen. As you said, the shear cutter can be moved up and down to get a new cut area until you have to resharpen it and it is easier to sharpen.

I have tried HSS, HSS Cobalt and Carbide and of course the carbide lasts longer, the Cobalt comes in second and the HSS wears the fastest. For a long cut I would use the carbide cutter I made. Even then I would mic it the length of the work to be sure.

Then there is the visual aspect, it does look more pleasing to the eye. I always hit a finish surface with emery cloth to remove any burrs.

Here is a photo of the cuts at 12" away in the dark with only flash before the emery cloth polishing. Neither cut would be anyway near to a mirror finish, they both produce a satin finish of sorts. I think you would have to burnish it to get a mirror finish or polish the heck out of it. It just has the texture of a ground finish.

http://i82.photobucket.com/albums/j276/yeathatshim/PC080002.jpg

Forestgnome
12-08-2010, 05:51 PM
That really seems to reduce the "smear". Might be good on softer stainless too.

Carld
12-08-2010, 07:03 PM
Forestgnome, have you looked at the photo's on page 4? Do you mean changing the distance from the shaft? I can still see a difference in the surface finish of the two cuts at 12" and looking at it in hand is much different from a photograph.

whitis
12-08-2010, 09:21 PM
Yeah, I am thinking that on the shear tool the equivalent nose radius helps minimize the tooling mark ripple and the unusual shear angle helps shear the metal cleanly.

The difference between the visual appearance and the fingernail test illustrates that there can be a difference in the mechanical and cosmetic smoothness.

Using the 1-3/16 vs 2" radius probably wasn't a big deal. It increases the theoretical limit of surface finish ripple by about 70%. In actual practice, it might not make a lot of difference. It might even work a little better due to less rubbing. There comes a point where the things which aren't easy to calculate become more important than what you can easily calculate. I used 2" as a starting point because it is about the length of a small lathe tool and somewhere in the same ballpark as the effective radius of the shear tool, then did the math.

I was noting in one of the experimental results given in Application of Metal Cutting Theory (1987) how fast the side flank on a lathe tool can wear. 20mils of wear was the limit before cutting quality fell off. On a carbide tool, this was achieved in 3.16 minutes at 600sfm and 50minutes at 300sfm (this being about the normal tool life). That much wear over the useful life of the tool could lead to some part dimension surprises (especially for CNC). I don't think the front wore that fast, fortunately, so change in diameter isn't as bad. Table 5.1, Fig 5.7, Table 5.22, Fig 5.8. Unspecified alloy steel, 0.100" DOC, 0.008inch feed/rev.

Forestgnome
12-09-2010, 09:03 AM
Forestgnome, have you looked at the photo's on page 4? Do you mean changing the distance from the shaft? I can still see a difference in the surface finish of the two cuts at 12" and looking at it in hand is much different from a photograph.
Sorry. Guess I jumped in with an "out of sequence" comment. I only meant using the shear tool rather than a standard tool.

lazlo
12-09-2010, 09:09 AM
I think Noah has it right about stress risers. When you look at the tool in terms of fracture toughness, each scratch is a flaw. The larger the flaw, the lower the stress required to cause a crack to propagate from that flaw.

Agreed, but I don't think it will be easy for an amateur to polish a cutting edge without rounding it over, as Verhoeven found.

If you're going to polish an edge, Carl's approach (using a diamond flat-lap) seems the least likely to round over the sharp, freshly ground edge.

Polishing the cutting edge with a buffer seems like the worst-case scenario for a cutting edge.

Bob Pastor
12-12-2010, 01:27 AM
I used a diamond lap to polish the cutting edges of both cutters but I didn't try for a mirror finish. The photo was from about 4" and holding it in your hand at about arms length the right side cut looks like a polished or ground surface.

The cutter for the radius is Congo HSS and the shear tool is HSS Cobalt.

Randy, I have a piece of triangle shape carbide stick that I have used as a shear cutter and it does a nice job. That could be used on a hard shaft I suppose.

This is the first time I've seen mentioned "polishing HSS with diamond lapping compound". I'm going to try it on some new HSS inserts I have from the A.R. Warner Co. I just made a video on using HSS and I wish I had included this polishing idea in the video. http://www.youtube.com/watch?v=LKGkkGFsF50

I'm going to try polishing my inserts and report back on how it worked.

Bob

lazlo
12-12-2010, 10:49 AM
This is the first time I've seen mentioned "polishing HSS with diamond lapping compound".

I think Carl's just talking about honing the edge with a DMT dipstick-type diamond hone.

Many of us here do that. I use the same DMT hones to touch-up carbide cutting tools as well.

Carld
12-12-2010, 11:27 AM
Yes, it's a stick with diamond grit on it. There are three grades in the kit fine, medium and coarse. I also use them to sharpen my pocket knife and other cutting tools. I refused to buy some for a long time and after reading about what a good job they do I got some the size of business cards and really like them for finishing a tool out. Then a member set me a set that look like finger nail files for doing a favor for him and I use them more than the other set.

MichaelP
12-12-2010, 01:51 PM
Carl, how do you deal with running the tool to a shoulder? Did you try raising the tool so that it cuts by the lower end?

P.S. I assume the cutting edge is slanted from the upper right to the lower left. Is it correct? If it's vise versa, we could lower the tool to get closer to the shoulder.