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PStechPaul
09-20-2014, 04:15 AM
I had mentioned to my instructor that I had found a problem with a 3/8" lathe bit I had ground for square threading, where the tip sheared off while attempting to cut some 4140 3/4" round stock, and he said that there are various grades with different hardness, and also susceptibility to losing their hardness if overheated while grinding. I may have overheated the narrow tip, and I think it did start to turn blue before I quenched it, but he said that it would be a problem if it became straw-colored. But I think the entire piece in question was softer than normal, as I was able to gouge it with an Exacto knife blade.

I had other tool bits that looked similar, with a fairly rough gray and rusty finish, and I was able to just barely scratch them with the blade, but not actually gouge it. And I had other bits with a smooth shiny finish that I could not even scratch with the blade. I think perhaps the gray bits are a lower grade of HSS, while the shiny ones may be M-2. The instructor said he prefers M-6, but I could not find much information on that. But I did find some interesting references:

http://www.crucibleservice.com/eselector/general/generalpart1.html

http://www.simplytoolsteel.com/

There did not seem to be a very large range of hardness for most of the tool steels:

http://www.crucibleservice.com/eselector/general/generalgifs/choosehardness.gif

But toughness (impact resistance) seemed to vary greatly, and somewhat inversely proportional to hardness:

http://www.crucibleservice.com/eselector/general/generalgifs/chooseimpacttough.gif

There is also interesting information about CPM (powder metallurgy) versus conventional methods of steel production, especially for those with carbide content.

I had hoped to test the hardness of some of my lathe bits at school, but I think the instructor said their hardness tester was not operational. So perhaps the scratch and gouge test may have to suffice for now. I may buy some 3/8" or 1/2" blanks for my lathe and I would like to know what grade would be "best" for most work.

I have found a lot of M-2 and some M35 (which seems to be a cobalt alloy), but not much else.
http://www.ebay.com/itm/Altai-M35-HSS-blank-1-2-x-4-cobalt-4-bits-square-lathe-cutting-milling-boring/111372040758

I found some labeled HSS-Co:
http://www.ebay.com/itm/5-pcs-Cobalt-Lathe-Tool-Bits-1-2-x-4-Tougher-than-HSS-/220563112764

and some just HSS:
http://www.ebay.com/itm/3-8-Square-x-3-Long-High-Speed-Steel-Tool-Bits-Lathe-Bits-Ground-5-pieces-/321513697256

And this says M2 in the description:
http://www.ebay.com/itm/3-8-X-3-8-X-4-HSS-TOOL-BIT-SQUARE-LATHE-FLY-CUTTER-MILL-BLANK-/121398564662

McMaster also has M42 (Cobalt) steel rods, but not lathe bits. They only have presharpened HSS, and carbide and diamond tipped. I think I have enough lathe bits for my purposes for a while, but you can never have too many, and if I find a real bargain I may get some. Maybe on Craig's List. And I wonder if it would be good to do a scratch test for hardness, perhaps using some known good bits?

[edit] I found a thread on the PM forum that discusses this pretty well:
http://www.practicalmachinist.com/vb/general/all-hss-lathe-bit-blanks-created-equal-257038/

I also looked closer at a couple of my rough-looking lathe bits that I have ground as standard RH and LH cutters, and one is Rex 49 (the LH tool), and the other is Rex "aaa", which appears to be T4. I found that the cutting tips of each would scratch the rough surface of the shank, but not the ground surfaces. My shiny lathe bits would just barely scratch the Rex tools. My carbide bits would just barely scratch the shiny ones. So I seem to have a range of hardness.

Richard P Wilson
09-20-2014, 05:03 AM
You can go mad trying to work that lot out. Stick to High Speed Steel (HSS) bits they come in a range of sizes, square and round, are always 'bright and shiney' and usually say 'HSS' or 'HSS Cobalt' on them If you can find a known US brand, thats all the better. No way should you be able to gouge or even scratch anything you want to use for metal turning. HSS is pretty immune to softening during grinding unless you really abuse it.

Peter N
09-20-2014, 08:40 AM
Yep, M2 (HSS) or M42 (HSS-Cobalt) are what you need for cutting tools.

Regarding (some) of the other steels in that chart, I can tell you H13/S7/A2/D2 are what is known as "Hotwork Tool Steels", and we use these for making mould cores and cavities and components for injection moulding and diecasting tools, and you can also use them for making punches and dies.

So in this respect they are indeed "Tool Steels" but not necessarily "Cutting Tool" steels.

quadrod
09-20-2014, 09:54 AM
I admittedly have little experience being the typical hobbiest , but in your chart I have only hear of the cr, m4, m42, and T15 as metal cutting tool steals. All the others are tool steels but for molds, shafts, dies and the sort of high wear resistance tooling. I have uses some T15 on my lathe and found it to be excellent.

RussZHC
09-20-2014, 10:20 AM
Nearly all of my HSS has been off EBay purchases so a thought from that would be if you are paying only say 50 cents more per lathe tool bit of a known/brand name compared to some steel claiming to be XYZ, why would you not buy the known?

I am not saying anything you buy could not be very good and function as stated but my chief concern would be about consistency, either good or bad and IMO one piece of older Mo Max or Rex of a given grade is likely to be nearly identical to that next piece of the same grade. There is some very good stuff out there that is a sort of "off brand" but "off" in this case usually means either very small company (or part of a larger company who had other specialties but who could make very good stuff) or from a company gone so long few know of them.
The other option is to go high end import, import as in not of North American mfg, had good results with the WKE series but it ain't cheap and sometimes buying metric helps...I mean your lathe doesn't care and you set the bit height...

doctordoctor
09-20-2014, 11:36 AM
Excellent research! This is what separates the fakes from the people who actually know what they are talking about!

I would suggest that unless you know the precise hardness of the exacto blade, you probably shouldnt use it for any kind of testing.

And is the rockwell scale linear? In other words, is an RHc of 67 harder then 52 by (67/52)-1 = 28% or is it non linear and actually much harder than that?

dalee100
09-20-2014, 11:47 AM
Hi,

All decent HSS pretty much share the same characteristics. The ability to hold their temper to a red heat without losing it on cool down. The addition of cobalt adds a bit of higher red heat resistance and more toughness than plain M2. (This is why cutting speed and feed rates are up to 25% faster than plain M2). Quenching a piece of HSS while forming a delicate tip can destroy the temper by making the affected area glass hard and prone to chipping and breakage. Because of this, I tend to let the tool air cool naturally when forming delicate tools to preserve the proper hardness. Perhaps this is why the nose of your tool sheared off after quenching when grinding it. But if you can actually carve off slivers of steel, there is probably something hokie with the tool bit. Toss it!

For lathe tool bits, I tend to buy HSS with 4% cobalt. A noticeable improvement over plain M2, but also easier on the wallet than the 8% cobalt. Now the cheap, cheerful, Chinese stuff can be variable in quality, but I have yet to get any bad HSSCo myself, YMMV. Stick to 5/16" sizes and smaller. They grind faster and are plenty stiff enough for all but the heaviest of cuts.

Oh, any steel can be used as a cutting tool. You just won't like the glacial cutting speeds and feeds you may well be forced to operate at. Remember, a cutting tool at the basic level only needs to be harder than the material being cut and should last long enough to complete that particular job.

Dalee

outback
09-21-2014, 01:24 PM
What was your cutting speed turning 4140?

I always look for high speed tool bits made of M2 with Cobalt. Always use USA or European made tool bits. The Chinese and India made high speed tool bits have been of poor quality in the past. I doubt they are really high speed. The poor quality surfaces when the tool bits are sharpened because they grind way to easily and they make a redish spark when ground. They do not hold a sharp edge for very long.

The appearance of the tool bit blank does not tell you as much as markings on the tool. If there are no markings try grinding it. My experience, the more difficult to sharpen the better the high speed tool bit will hold a sharp edge.

In addition, Tungsten is the alloy that makes high speed steel. Tungsten gives high speed the ability to
retain hardness even when red hot. Most other tool steels will loose their heat treated properties when they become red hot. While I try to keep tools cool when sharpening them, I have never seen a
high speed tool bit become annealed while sharpening.

Jim

J Tiers
09-21-2014, 01:57 PM
Hi,

But if you can actually carve off slivers of steel, there is probably something hokie with the tool bit. Toss it!

Dalee

Not necessarily..... you can usually carve off pieces easily from a carbide toolbit shank.... because the shank is not the same material.

Likewise, some HSS was made with an HSS end welded (I assume, but might be brazed in some cases) to a "plain steel" shank, from back when HSS was not cheap. (they knew you could never use the last bit of it, probably wouldn't sharpen back very far, and didn't waste material on the shank) Same with Stellite. The HSS looks more-or-less like regular steel, and you might never notice the join. Stellite has a bit different look, and you would probably see it if you looked carefully, especially since it couldn't be welded to steel as far as I know, and would be a smaller piece, brazed..




Always use USA or European made tool bits. T
Jim

Israeli tools are also usually very good.

dalee100
09-21-2014, 03:10 PM
Not necessarily..... you can usually carve off pieces easily from a carbide toolbit shank.... because the shank is not the same material.

Likewise, some HSS was made with an HSS end welded (I assume, but might be brazed in some cases) to a "plain steel" shank, from back when HSS was not cheap. (they knew you could never use the last bit of it, probably wouldn't sharpen back very far, and didn't waste material on the shank) Same with Stellite. The HSS looks more-or-less like regular steel, and you might never notice the join. Stellite has a bit different look, and you would probably see it if you looked carefully, especially since it couldn't be welded to steel as far as I know, and would be a smaller piece, brazed..





Israeli tools are also usually very good.

Hi,

I haven't seen something like that sold commercially for nearly 30 years mself. I've made special tooling for one-offs like that a time or two. HSS is so cheap these days that it would almost cost more to attach a small piece of HSS to mild than just use all HSS. Still it could be possible I suppose.

Dalee

mickeyf
09-21-2014, 08:02 PM
With an Exacto? No chance some joker slipped you a piece of 3/8" keystock? Things like that have been known to happen...

PStechPaul
09-21-2014, 08:33 PM
This was one of the lathe bits I got as an eBay lot sale quite some time ago. Probably the one on the middle right side, shaped like a single point thread cutting tool:

http://enginuitysystems.com/pix/Lathe_Bits_0815_800p.jpg

Here is the defective bit, showing how the tip was apparently sheared off as it tried to cut the 4140:

http://enginuitysystems.com/pix/tools/Lathe_bit_bad_1342.jpg

It has some writing etched into the shank, but I can't quite make out what it is, except some sort of numbers. Perhaps it is key stock or mild steel or "Fool's steel"?

http://enginuitysystems.com/pix/tools/Lathe_bit_bad_1343.jpg

J Tiers
09-21-2014, 10:40 PM
This was one of the lathe bits I got as an eBay lot sale quite some time ago. Probably the one on the middle right side, shaped like a single point thread cutting tool
...............................
Perhaps it is key stock or mild steel or "Fool's steel"?



if it cut metal, sheared off, and did not seem to bend, "mushroom" or the like, it probably was some type of hardened tool steel.

But the tip looks like it was actually worn down by a rough workpiece.

It is short enough that it might have been teh shank from a carbide tool, EXCEPT for the angle cut on the backside. If it was not made to fool someone, that cut marks it as probably HSS right through, since that is commonly put on HSS as a starting pace for more grinding. It does seem like a different angle from what is commonly seen on HSS, though..

You apparently ground it, what were the sparks like?

PStechPaul
09-21-2014, 11:06 PM
I don't remember the sparks, but it seemed to grind a lot more easily than other tool bits. I think it sheared off as I attempted to cut through the rough surface of the 3/4" round bar. Here is a picture of it with the piece I was working on, so you can see the black coating:

http://enginuitysystems.com/pix/tools/Lathe_bit_bad_1339.jpg

This was the first time I had used this bit, so I think it was soft when I got it. The ~45 degree end angle is similar to other bits in the lot, although the others seem to be more like 30 degrees. I think I'm pretty well set for lathe bits, but if I find some at a yard sale or such, perhaps the exacto knife test would be able to detect junk, or maybe a known good lathe bit would be better.

J Tiers
09-22-2014, 08:22 AM
I've bought plenty of cutters and blanks at sales, and I have NEVER found a bogus part. You (as you have found) get all sorts of shapes usable as-is or can be modified. Very handy and saves hours of time at the grinder.

JCHannum
09-22-2014, 09:02 AM
Your assortment of cutters is fairly typical of what you would find in an older, say 40's & 50's vintage, tool box. There are some forged carbon steel cutters, the black ones that appear to be bent, as well as reground HSS bits. Also, HSS bits were more commonly then found unground, with a black outer coating.

The writing on your bit was done with an electric pencil, probably in the tool room where the bit was ground for a specific job. The previous owner had since reground it for other purposes.

Forrest Addy
09-22-2014, 10:13 AM
Free hand tool grinding is a masochistic passtime. You grind until the tool gets so hot it fries your fingers, then quench. Ahhh! And repeat. The teensy cast iron water cups usually furnished with the better industrail grade bench grinders are plain too small. You need one holding as least a gallon and it should be deep enough to dunk your whole hand. A restaurant sized (#10) can is ideal. Have your favorite coffee shop hold a few for you. The standing water mixed with swarf gets icky fast ( thiobacillus ferrooxidans and leptospirillum ferrooxidans, they're in the air) and should be changed every day in a busy shop. That was one of the helper's chores where I worked.

The reason why tool bits crack on grinding is they are brittle as shown to the right of the second graph in the OP. They are very hard but not tough. They have to be ground and quenched with care. It's all too easy to quickly quench a tool whose tip is at 700 degrees and start a small contraction crack that fails completely when the tool is used.

The solution is that big #10 quench can of water I mentioned. It's big anough you can sink the tool shank first slowly into the water so, as its level approaches the hot zone, it starts to sizzle. You can keep the slow sizzle going as you lower the tool in the water. This has the advantagle of quenching your frying fingers first yet allows the tool to cool at a rate that doesn't promote cracks.

Hold the tool like a movie tough guy holds a cigarette - with the cool end between the thumb and forefinger and the hot end cupped near your palm. An inch of water does you no good. You need immersion. The water has to be 5" deep at least. The water displaced by your hand may rise to the rim so you can't over-fill it either. The can is 7" tall so 5" of water is about right.

OK that's the water can and how to quech a hot tool without cracking it.

Another point is the cutting edge. Bench grinders roughly dressed with star dressers produce poor finishes and the finish striations tend to go across the chip flow like teeth on a file. The finish of a cutting tool is not particulatly important. Rough grinding is plenty good enough except at the cutting edge and areas of chip flow. They need to be stoned smooth - preferably glossy if finish is important. The stoned surface seldom needs to be wider than 1/32" but it does need to be there if good clean cutting action is desired.

Also many grind too much clearance on their tools. I've found that 7 degrees side and end clearance is right in the sweet spot. More for soft materials (up to 10 degrees for aluminum) and less for harder materials 5 degrees for Rc 40 alloy steel). Top rake: more for soft less for hard. The topic of free hand ground cutting tool configuration can grow into a three volume set of tool design manuals, but briefly, as above.

Of the tool steels charted in the OP, M2 and M4 are used for general purpose machining operations. They are not particularly expensive and hold up well in most common materials. Not listed is M42, a cobalt HSS noted for its extended hardness vs temperature range used for tough and hard materials but it is more expensive. T15 is lovely stuff that seems to hold an edge forever in tough materials but alas it's quite brittle and very sensitive to thermal shock. There are others in the HSS family of tool steels. I've probably used many in my checkered career but was mostly unaware of their subtle distinctions.

M2, M4, M42, and T15 are common tool bit materials. I've hand ground them and used them all extensively. Treated right they give good service. None are resistant to thermal shock and many a time tools have failed me because I got in a rush and quenched them too fast in the rough grinding stage.

If I was to choose a essential attribute for tool bits I'd pick "edgeholding:" how long the tool keeps its keen edge. T15 would be my first choice BUT it's very brittle, difficult to grind, and expensive. M42 is probably the best all around material were it not so expensive (more than double the price of M4 which is expensive enough) I prefer M42 for oddball tools; I can silver braze it without sacrificeing much in hardness - there's a lot of difference in the edge holding of Rc65 tool steel and Rc62, far more than you'd expect for the small difference in tensile strength. M2 and M4 are my woirkhorse tool steels. They grind easily and hold up well in most materials.

Selecting a tool steel for a machine shop application is a paper, scissors, rock thing. There is no single all-around best choice.

PStechPaul
09-22-2014, 03:37 PM
Lots of good information, comments, and advice. Thanks.

We will probably be grinding tool bits and using the lathe in class this Thursday, so I'll see what the instructor recommends.

Forrest Addy
09-22-2014, 05:04 PM
Paul, Great original post. Text and images were apt and the links ilustrative.

Refer your instructor to my earlier words in Post 17 regarding the size of the water can by the grinder and how to dip the tool slowly shank first. I re-posted the relevent paragraphs below. Your instructor has my permission to use this material without attribution in any way as he sees fit. The words are mine but the lore behind them was passed on to me by Forrest Spencer in 1967.

"... The teensy cast iron water cups usually furnished with the better industrial grade bench grinders are plain too small. You need one holding as least a gallon and it should be deep enough to dunk your whole hand. A restaurant sized (#10) can is ideal. Have your favorite coffee shop hold a few for you. The standing water mixed with swarf gets icky fast (thiobacillus ferrooxidans and leptospirillum ferrooxidans, they're in the air) and should be changed every day in a busy shop. That was one of the helper's chores where I worked.

"The reason why tool bits crack on grinding is they are brittle as shown to the right of the second graph in the OP. They are very hard but not tough. They have to be ground and quenched with care. It's all too easy to quickly quench a tool whose tip is at 700 degrees and start a small contraction crack that fails completely when the tool is used.

"The solution is that big #10 quench can of water I mentioned. It's big anough you can sink the tool shank first slowly into the water so, as its level approaches the hot zone, it starts to sizzle. You can keep the slow sizzle going as you lower the tool in the water. This has the advantagle of quenching your frying fingers first yet allows the tool to cool at a rate that doesn't promote cracks.

"Hold the tool like a movie tough guy holds a cigarette - with the cool end between the thumb and forefinger and the hot end cupped near your palm. An inch of water does you no good. You need immersion. The water has to be 5" deep at least. The water displaced by your hand may rise to the rim so you can't over-fill it either. The can is 7" tall so 5" of water is about right."