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View Full Version : Is a grade 8 bolt a good material to make an insert toolholder out of?



beanbag
11-01-2011, 03:03 AM
I want to make an insert boring bar, but I don't want to deal with that heat treating stuff. How would a grade 8 bolt work? I would need to do a little off-center turning to make the head vs shank, and then CNC a little insert pocket in it. My other option is that I have a bunch of 1/2" square 4140 annealed.

macona
11-01-2011, 03:06 AM
Does not even need to be heat treated. Any steel will work.

But have you looked how cheap a basic boring bar is? Even the American made ones are stupid dirt cheap. I can't justify making one. The borite ones are pretty good. Pretty cheap inserts too.

Black_Moons
11-01-2011, 03:20 AM
Grade of steel will not change how rigid the bar is. It will somewhat change how easily the pocket wears, mainly due to damage/failure events. And mainly determin if your boring bar snaps, Or bends, when a serious failure happens. (Come to think of it, Snaping would likey be the better failure mode then bending for a boring bar)

That said just about any steel is fine.

beanbag
11-01-2011, 03:42 AM
more specifically, one that takes a cpgt insert. Not ccgt.

beanbag
11-01-2011, 05:29 AM
Grade of steel will not change how rigid the bar is. It will somewhat change how easily the pocket wears, mainly due to damage/failure events. And mainly determin if your boring bar snaps, Or bends, when a serious failure happens. (Come to think of it, Snaping would likey be the better failure mode then bending for a boring bar)

That said just about any steel is fine.

But the grade 8 bolt is STRONGER than my stick of 4140. That means it will take much more force before it bends/breaks, and the pocket should be harder to deform as well.

Forrest Addy
11-01-2011, 06:01 AM
I've got a couple dozen home brew boring bars in my collection. Most are intended for HSS or brazed-in HSS or carbide. Most were crude works of the moment but a few are carefully made with built in dampers. Except for the fancy ones most all were made from un-heat treated drill rod or plain vanilla mild steel.

A boring bar sees no great stress in service. It's made stout to be stiff; to resisst deflection in the cut. The factory made boring bars are made hardened to make them more durable to abuse, chip wash, and mild collisions.

When pressed, back in the day, I could make a quick and dirty boring bar in about a quarter hour - unless it was made for boring Acme threads in a lead screw nut. Sometimes those had to be a works of art.

cuemaker
11-01-2011, 07:30 AM
But the grade 8 bolt is STRONGER than my stick of 4140. That means it will take much more force before it bends/breaks, and the pocket should be harder to deform as well.

Stronger is necessarily the right word. It has a higher tensile strength, but along with that higher tensile strength comes a higher likelihood that when if it fails, it will snap as opposed to bend.

The grade 8 bolt will have less flex and be more ridged, that is for sure.

Beanbag, I have a crap load of B7 threaded bar you can have.. Its 4140 heat treated to below gr 8, but higher than a 5. B7 is used for holding all your local refineries/chemical processors flanges together. You need carbide to turn it and its cheaper to use coolant than burn up inserts.

philbur
11-01-2011, 08:29 AM
They are both made of steel therefore they have the same modulus of elasticity, each has the same flexibility/rigidity for the same geometry.

Has anybody ever actually "broken" a steel boring bar?

Phil:)


The grade 8 bolt will have less flex and be more ridged, that is for sure.

Lew Hartswick
11-01-2011, 08:59 AM
They are both made of steel therefore they have the same modulus of elasticity, each has the same flexibility/rigidity for the same geometry.

Has anybody ever actually "broken" a steel boring bar?

Phil:)
Isn't it amazing how various people interpret some words? :-)
I think the TV ad people have about ruined the English language.
(American version at least, sorry Brits, Ausies,etc.)
...lew...

SGW
11-01-2011, 10:34 AM
philbur is correct. The slope of the stress/strain curve for steel (i.e. its "stiffness") is virtually independent of hardness in the elastic region--the range of stress in which the steel returns to its original form when the stress is relieved.

Hardening increases the elastic region --- the steel can withstand a greater stress before it take a permanent set , i.e. bends, but that's all.

Evan
11-01-2011, 01:37 PM
But the grade 8 bolt is STRONGER than my stick of 4140. That means it will take much more force before it bends/breaks, and the pocket should be harder to deform as well.

To reiterate, all steels and steel alloys have about the same resistance to bending (given the same size of material) up to the point of permanent deformation.

The "strength" of the alloy determines how easily it deforms permanently. It has no significant effect on how easy it is to deflect before deformation.

Different materials have a different modulus of elasticity. That is what determines how easy deflection is. Tungsten carbide is about three times more resistant to deflection than steel alloys. That means it is three times stiffer which is why the best boring bars are made from tungsten carbide.


HOWEVER, as usual, there are some exceptions. If steel is prestressed far into the work hardening portion of the stress-strain curve it changes the modulus of elasticity. The material will not return back down the curve to where it began during deflection and following deformation. This effect occurs just before failure.

If you look at the stress strain curve for steels it rises linearly until the point of deformation. When deformation occurs the curve climbs for a bit and then falls as the applied stress required to produce further deformation decreases. As work hardening begins the curve then rises again. If the stress is removed just before failure the steel comes back down the curve but not back up over the hump. This moves the bottom of the curve to the right.

That will make it harder to deflect than usual. The effect is quite pronounced in some alloys, especially ones that are not considered hardenable such as mild steel.

Boucher
11-01-2011, 01:59 PM
Original Question: Is a grade 8 bolt a good material to make an insert tool holder out of? Answer: Yes

OP: My other option is that I have a bunch of 1/2" square 4140 annealed. Answer: That would work also.

Other things being equal, I tend to use what ever I have the most of or what is more available.

I also stock some 12L14 which machines so nice.

jep24601
11-01-2011, 02:48 PM
The grade 8 bolt will have less flex and be more ridged, that is for sure.
.

As others have pointed out - the grade 8 will flex just the same as 4140 and be no more rigid.

Evan
11-01-2011, 02:49 PM
Yes, a grade 8 bolt will work well. Here is how to make an insert tool holder from one:

http://ixian.ca/pics9/cutter1.jpg

And this is how it works:

http://ixian.ca/pics9/cutter4.jpg

And yes, it will also work as a boring bar.

BigMike782
11-01-2011, 02:55 PM
"Has anybody ever actually "broken" a steel boring bar?"

I crashed one into the chuck and broke the end off:( .

Evan
11-01-2011, 03:00 PM
Here is a good example of a boring bar.

http://ixian.ca/pics9/boringbar.jpg

macona
11-01-2011, 03:10 PM
Has anybody ever actually "broken" a steel boring bar?

Phil:)

I had one break off at the thread for the insert screw.

PixMan
11-01-2011, 07:24 PM
IMO - The degree of how well a boring bar cuts is primarily the insert geometry and secondarily the density (or stiffness) of the bar itself. That density helps damp vibrations, and is why solid carbide and heavy metal (high tungsten content steel) bars seem to cut smoother.

What diameters and length-to-diameter bores are you looking to finish with this homemade boring bar? And is there some reason you've chosen CPGT over the more-common CCGT or the more-edges WCGT? I don't know what equipment you intend to use for milling the insert pocket sides, but a 7º (14º included angle) cutter for a CCGT side clearance angle is probably easier to find than an 11º (22º included) cutter.

beanbag
11-01-2011, 07:39 PM
I got a ccgt bar and returned it immediately because the degree that it needs to tilt the insert downwards in order to provide enough relief for small bores (<.5"). A CPGT needs to be tilted less.

The aim is for bores from slightly under 1/2" and larger. The length will be adjustable because I will also make a holder for it that it can slide in and out of.
I'm going to CNC out the pocket, so its particular shape isn't an issue.

Evan
11-01-2011, 07:44 PM
secondarily the density (or stiffness) of the bar itself.

Density and stiffness are not related. Think lead.

PixMan
11-01-2011, 10:36 PM
True, that. Carbide is pretty dense though, and remarkably stiff too. A lead boring bar, not so much.


I got a ccgt bar and returned it immediately because the degree that it needs to tilt the insert downwards in order to provide enough relief for small bores (<.5"). A CPGT needs to be tilted less.

The aim is for bores from slightly under 1/2" and larger. The length will be adjustable because I will also make a holder for it that it can slide in and out of.
I'm going to CNC out the pocket, so its particular shape isn't an issue.

The inclination angle of the insert, especially with high-positive screw-down inserts intended for boring operations, is easily offset by the chipbreaker/top form geometry design. I've never seen any great advantage to CPGT over CCGT in performance, and there sure are a LOT more CCxxT inserts out there to choose from.

beanbag
11-02-2011, 03:49 AM
The inclination angle of the insert, especially with high-positive screw-down inserts intended for boring operations, is easily offset by the chipbreaker/top form geometry design. I've never seen any great advantage to CPGT over CCGT in performance, and there sure are a LOT more CCxxT inserts out there to choose from.

For a bore of just under .5", a ccgt insert needs an inclination of -9.x degrees whereas a cpgt only needs -5. Besides, I only need one cpgt insert, namely a Kennametal cpgt-HP 21.51 which can be had on ebay. The KC730 coating seems to work on all materials. There is also a TiB2 coated version that is supposed to work for aluminum, but I have that coating on another insert, and it doesn't seem all that great.

beanbag
11-02-2011, 03:52 AM
Even though all steels are supposed to have the same stiffness, I can just pick up a stick of stainless 304 and it somehow "feels" easier to flex (even before taking a permanent set) than a hardened steel, like an aircraft drill.

DATo
11-02-2011, 04:07 AM
The other day I needed a small boring bar and couldn't find what I needed in the tool room so I took a piece of water hardening drill rod and made my own. Rough machined the shape, then took it up to cherry red, swished in a bucket of water to cool then took it up to pale gold to draw and finally polished and sharpened. Made two of them and you can't tell them apart from a Bokum brand tool of the same size. For the cost of a set of quality purchased boring bars you could make half a bucket of your own.

Kiwi
11-02-2011, 04:14 AM
I make my boring bars out of what ever it don't matter for the stuff I do the small bar 1/4 X 1/4 is quite good I don't pack it up in the tool holder (non height adjustable) I just bend it up to the right height the sharpening of the tip is the critical thing The one on the far left is broken i think it had too much heat when brazing
http://i355.photobucket.com/albums/r457/Thorcalmac/Workshop001.jpg

philbur
11-02-2011, 04:22 AM
There is a small difference in Young's modulus between stainless and carbon steels but not one you would notice by "feel".

It's all in the mind. Within their respective elastic limits reinforced concrete and granite are 3 to 6 times more flexible than steel.

Phil:)



Even though all steels are supposed to have the same stiffness, I can just pick up a stick of stainless 304 and it somehow "feels" easier to flex (even before taking a permanent set) than a hardened steel, like an aircraft drill.

J Tiers
11-02-2011, 08:38 AM
Regardless of the modulus, you can "push harder" with hardened tools, and that is why everyone thinks they are more rigid.....

Because they ARE more rigid......

At least in the definition most people use. I can guarantee you that an unhardened lockpick, prybar, or similar tool will not work as well as a similar hardened one, if it works at all.... with hardening, the ultimate force that can be applied is larger, because you go right past the stress that causes deformation in the unhardened tool.

Sure, it eventually breaks, but.............

As for grade 8 bolts, grade 8 or grade5 bolts are often a material that workhardens ferociously, and can be a bear to work with if you try to sneak up on dimensions.

vpt
11-02-2011, 09:07 AM
I've made one out of a cummins valve.

http://img254.imageshack.us/img254/9068/boringbar015.jpg

philbur
11-02-2011, 09:16 AM
Trouble is a boring bar doesn't have the same functional requirements as a lockpick or a prybar so the analogy is a bad one.

You can push harder because they are stronger not because they are more rigid.

Phil:)


Regardless of the modulus, you can "push harder" with hardened tools, and that is why everyone thinks they are more rigid.....

Because they ARE more rigid......

At least in the definition most people use. I can guarantee you that an unhardened lockpick, prybar, or similar tool will not work as well as a similar hardened one, if it works at all.... with hardening, the ultimate force that can be applied is larger, because you go right past the stress that causes deformation in the unhardened tool.

Sure, it eventually breaks, but.............

As for grade 8 bolts, grade 8 or grade5 bolts are often a material that workhardens ferociously, and can be a bear to work with if you try to sneak up on dimensions.

Chris S.
11-02-2011, 11:02 AM
Here is a good example of a boring bar.

http://ixian.ca/pics9/boringbar.jpg

Evan, this is one of the things I like about you. You have the ability to go so far in subject depth as to cause eye glazing if coffee isn't close by. Then there's the other Even... brief and funny! :p

Evan
11-02-2011, 01:21 PM
Because they ARE more rigid

Rigidity is generally accepted to mean the ability to resist deflection rather than deformation. In this case especially, correct nomenclature is essential to highlight the difference between deflection and deformation. It isn't nit picking since the difference is all important. The word "stronger" is a poor choice as it has no specific meaning or measurement unless qualified.

It is necessary to use terms that many people are not familiar with to correctly describe the difference between deformation and deflection. "Bending" also doesn't make the grade.

The hardened bar "feels" "stronger" and more "rigid" because it takes more deflection and hence greater applied force to bring it to the point of permanent deformation. There is even more to that story but it does involve possible eye glazing in respect of the shape of the stress/strain curve for different treatments and alloys of steel. Once into the deformation range of the curve all steels are not the same and that is what produces the impression that some steels are stiffer than others.

DR
11-02-2011, 02:11 PM
Anybody know what alloys are commonly used for grade 8 bolts?

I wondered when we had a couple jobs to modify large quantities of them.

I decided to have the bolts annealed for easier mnachining. After machining they would be sent back to the heat treater for re-hardening.

Called the US manufacturer to get the alloy which the heat treater needed for the annealing and re-hardening.

The response I got from the manufacturer was "none of your business"!!

Huh? They explained the bolts could be made of bubble gumn as long as they met the requirements of the grade 8 spec.

I paid $75 extra to the heat treater to do a chemical analysis and we were off and running.

PixMan
11-02-2011, 02:46 PM
If you paid an extra $75 to get an analysis, shouldn't you have gotten a report showing that the material was 4340, 4150, 8620, or ??

Evan
11-02-2011, 03:35 PM
Anybody know what alloys are commonly used for grade 8 bolts?

Bolts aren't specified by alloy, only by properties. Each grade has minimum specifications for tensile strength primarily and a few other properties too.

strokersix
11-02-2011, 04:15 PM
I know that 1038 is a common composition for bolt heading wire. Not sure if 1038 can produce Grade8 spec or not.

DR
11-02-2011, 05:00 PM
If you paid an extra $75 to get an analysis, shouldn't you have gotten a report showing that the material was 4340, 4150, 8620, or ??

I didn't even think about it at the time. It wasn't something I needed to know, only the heat treater needed the composition.

Now, I'm just curious, wondering if they use alloy steels or plain high carbon.

interrupted_cut
11-02-2011, 08:15 PM
I've got nothing to add to the OP's question, but Evans picture of the boring bar reminded me of my machinist friends shop. This guy is 86 years old and is a little crusty. Always willing to answer an honest question, but doesn't suffer fools gladly. He was letting me use a lathe in his shop, and told me to take any tooling I needed from the cabinet next to it. I opened the cabinet, and the top shelf was labelled "Boring Tools". The next shelf down was labelled "More Interesting Tools". I almost fell on the floor laughing!

J Tiers
11-02-2011, 09:52 PM
Trouble is a boring bar doesn't have the same functional requirements as a lockpick or a prybar so the analogy is a bad one.

You can push harder because they are stronger not because they are more rigid.

Phil:)

phooey.....

Perhaps YOU like your boring bars to permanently deform..... To each his own...... I prefer mine to spring back after deflecting, and you may suit yourself.

In case you do NOT want your boring bars to bend and permanently deflect, I suggest that the analogy is quite decent.... both tools are commonly long with respect to their diameter, both have a working end to which force is applied more-or-less perpendicular to the axis of the tool, and both must resist that force without deformation.

Perhaps you might like to reconsider your hasty statement............

Stronger? Well of course they are, but I deny that the "strength" is what is responded to.

People are not responding to the ultimate breaking strength, which is not being approached closely. They are responding to the fact that one can push much harder because the tool does not deform...... it has a *spring constant*, along with a larger elastic deflection, and that allows a larger force to be applied at the cost of a bit more temporary deflection.





Rigidity is generally accepted to mean the ability to resist deflection rather than deformation.

Um.... I did say that I described what was as most folks understand it... pointing out thereby that it is not necessarily the technically correct description.....

but obviously deformation is an extreme form of deflection, insofar as the physical results in a structure or machine are concerned....

You quibble about the names, I see that the lifting arm is not lifting, but is laying on the ground, bent out of shape. Since that is presumably not the result I had in mind, I am considerably less interested in words, and more interested in the fact that it "deflected" plenty...... and not in a good way.




The hardened bar "feels" "stronger" and more "rigid" because it takes more deflection and hence greater applied force to bring it to the point of permanent deformation.

I offer you the alternate explanation that it "feels stronger and more rigid" because you can push harder with it. Something that bends out of the way and stays bent (elastic limit was exceeded) is not called "strong" as far as whatever usage it was put to....

JoeLee
11-02-2011, 10:23 PM
I made these little boring bars out of a soft steel, not sure what it was, it machined beautifully like lead alloy steel does. They work fine for what I've used them for which is in the lathe and my BP boring head. I use a good sharp carbide insert in them and have never had them chatter.

JL......................

http://i911.photobucket.com/albums/ac317/JoeLee09/Mini%20Boaring%20Bars/Image003.jpg
http://i911.photobucket.com/albums/ac317/JoeLee09/Mini%20Boaring%20Bars/Image009.jpg

beanbag
11-03-2011, 04:01 AM
Nice work, Joe.

cuemaker
11-03-2011, 07:29 AM
A SAE Grade 8 Bolt is supposed to be made from alloy steel (per IFI)... which would mean typically 41XX range.

I cant remember exactly what, but for steel to be an "alloy" there are certain chemical mix between 1 or 2 of the elements.. I am pretty sure its carbon and them maybe something else.

When I get to work I will be able to say for sure as I have the IFI book

Edit to add... Right from the IFI book 7th ed. Carbon steel is classed as an alloy steel when the mx of the range content specified for manganese is greater than 1.65% or for silicon .60% or for copper .60% or when the chromium content is less than 4.0% (if greater it approaches stainless) or when the steel contains a specified min conten of aluminum, boron, cobalt, columbium, molybdenum, nickel, titanium, vanadium, zirconium or any other element added to achieve a specific effect. Litterally dozens of differnt carbon alloy steels are usef r fastener manufacture. Some of the more poopular are AISI 1335 (Mg) 4037(Mo), 4140 (Cr,Mo) 4340 (Ni,Cr,Mo) 8637 (Ni,Cr,Mo) and 8740 (Ni, Cr,Mo)

Goes on to say more. Also in the charts of the book it calls for medium carbon alloy steel, the product is quenched and tempered. Grade 5 calls for medium carbon steel that is quenched and tempered

philbur
11-03-2011, 09:34 AM
The most common functional failure of a boring bar is chatter. Chatter is a result of the applied force causing elastic deflection. The stress level needed to cause this type of failure is way below that required to cause permanent deformation (bending). You will stop a boring operation due to chatter long before you get even close to bending the bar.

The most common functional failure of a lock-pick or a pry-bar is permanent deformation, it got bent! This permanent deformation is due to the applied force taking the material past its elastic limit.

So to perform their respective functions a boring bar needs a large modulus of elasticity to resist elastic deflection (to avoid chatter) and a lock-pick and pry-bar need high strength to resist permanent deformation (to avoid getting bent). One needs low elasticity the other needs high strength. The functional requirements are therefore not the same or even similar, so the analogy is in fact a bad one.

It is interesting to note that tungsten carbide boring bars (the ultimate boring bar) have a similar tensile strength as low carbon (mild) steel but also have an elastic modulus 3 or more times greater than that of any carbon or alloy steel. This in itself should tell you that resistance to elastic deflection is much more important than ultimate failure in any boring bar design.

If this were an old wives knitting circle (some might say that’s what it feels like at times) I would agree that a common understanding of the meaning of strength and rigidity might be a problem. However as this is a forum with a strong engineering lean I would propose that the most logical definition of these two words would be that assigned by the engineering community. Also it would be interesting to know what sample size you used to determine that it “is as most folks understand it”, just saying it doesn't make it so.

You can deny and propose alternatives all you like but it doesn’t change the engineering facts by one jot.


phooey.....

Perhaps YOU like your boring bars to permanently deform..... To each his own...... I prefer mine to spring back after deflecting, and you may suit yourself.

In case you do NOT want your boring bars to bend and permanently deflect, I suggest that the analogy is quite decent.... both tools are commonly long with respect to their diameter, both have a working end to which force is applied more-or-less perpendicular to the axis of the tool, and both must resist that force without deformation.

Perhaps you might like to reconsider your hasty statement............

Stronger? Well of course they are, but I deny that the "strength" is what is responded to.

People are not responding to the ultimate breaking strength, which is not being approached closely. They are responding to the fact that one can push much harder because the tool does not deform...... it has a *spring constant*, along with a larger elastic deflection, and that allows a larger force to be applied at the cost of a bit more temporary deflection.

Um.... I did say that I described what was as most folks understand it... pointing out thereby that it is not necessarily the technically correct description.....

but obviously deformation is an extreme form of deflection, insofar as the physical results in a structure or machine are concerned....

You quibble about the names, I see that the lifting arm is not lifting, but is laying on the ground, bent out of shape. Since that is presumably not the result I had in mind, I am considerably less interested in words, and more interested in the fact that it "deflected" plenty...... and not in a good way.

I offer you the alternate explanation that it "feels stronger and more rigid" because you can push harder with it. Something that bends out of the way and stays bent (elastic limit was exceeded) is not called "strong" as far as whatever usage it was put to....

Abner
11-03-2011, 10:03 AM
Evan, You made my morning. Boring bar...The exciting ones were where I almost got in a fight. Boring or exciting and not much else, might explain my machining hobby....

Evan
11-03-2011, 11:17 AM
If this were an old wives knitting circle ....

But it is. :D The only difference is which way we position the toilet seat.

See here about Knitting with wire (http://knitting.about.com/od/knittingskills/qt/wire-tips.htm).

dian
11-03-2011, 11:45 AM
@joelee: what inserts are theese?

J Tiers
11-04-2011, 12:26 AM
You can deny and propose alternatives all you like but it doesn’t change the engineering facts by one jot.

The problem here is that you-all are insisting that everyone think the correct way. Most folks are not engineers...... a fact which ensures that engineers get paid.

I know ALL ABOUT the biz of the modulus..... don't be fooled into thinking that I am arguing about that.....

What I am saying is that there ARE "properties" of hardened material which vary significantly from those of non-hardened, and that the properties folks are responding to are not what you think.

Specifically, a piece of soft wire may simply fold up and is unable to be used as a "spring" to, for instance, push some heavy object on a table.

But that same wire, if hardened properly, may be perfectly able to do that......

It is obvious from that sort of demonstration that the "maximum force exertable" through the hardened wire is in excess of that exertable with the unhardened.....

(The ultimate strength is not involved here, but the stress necessary to cause a permanent deflection IS directly involved.)

For most folks, that "means" the hardened wire is more "rigid"..... they say, with some justification, "after all, one folded up and the other didn't..... what else would you call it?" And as far as they are concerned, your techie mumbo-jumbo is so much double-talk and hooey unless you can make it clear.

If your intent is to educate, the first thing you must do is observe what the wrong beliefs are. It does no good to repeatedly spout the same technical words...... That is like one who believes a foreigner will understand the local language better if the local citizen repeats the same words but speaks more slowly and much louder.....

You need to understand why your audience believes what they (wrongly) believe, before you can effectively correct them.

My statements were aimed at showing the basis of "most folks" misunderstanding..... NOT, as you apparently presume, at denying engineering facts..... Engineering facts get me paid on a regular basis, so I rather like them.

philbur
11-04-2011, 04:26 AM
You have exceed the yield strength, which, in the context of this particluar discussion, is effectively the same thing as untimate strength.

Permanent deflection (correct term deformation) is a direct consequence of exceeding the yield strength.

Phil:)

PS: You appear to be switched to the engineering definitions of untimate strength and yield strength.


(The ultimate strength is not involved here, but the stress necessary to cause a permanent deflection IS directly involved.)

J Tiers
11-04-2011, 08:40 AM
You have exceed the yield strength, which, in the context of this particluar discussion, is effectively the same thing as untimate strength.

Permanent deflection (correct term deformation) is a direct consequence of exceeding the yield strength.

Phil:)

PS: You appear to be switched to the engineering definitions of untimate strength and yield strength.


"Breaking strength" is not exceeded, obviously.......

The deformation point (yield point) is equally obviously exceeded... at least "locally", but clearly not throughout the thickness of the material.

if the breaking strength (ultimate strength, etc) had been exceeded, the wire would have, well, BROKEN......

Again, you are arguing correctness in a case where I have specifically stated I am NOT TALKING ABOUT technical definitions, but about the "observables" obvious to the non-technical person.

If you pull test a sample, the yield point is reached and the curve has a "knee " in it. But after that, the material does not actually "break" until a somewhat higher "pull" is applied. It usually necks down, work-hardens, and eventually breaks at some point a few percent higher.

jep24601
11-04-2011, 09:35 AM
J Tiers is addressing "Rigidity" which is not necessarily the same as "Stiffness".
Stiffness is very clearly defined as ExI (modulus x Moment of Inertia)
Rigidity is a more general term often used to relate to assemblies rather than individual members or parts. One for instance would normally refer to how rigidly a boring bar is held rather than how stiffly it is held.

Ah! Semantics.

Chris S.
11-04-2011, 11:36 AM
J Tiers is addressing "Rigidity" which is not necessarily the same as "Stiffness".
Stiffness is very clearly defined as ExI (modulus x Moment of Inertia)
Rigidity is a more general term often used to relate to assemblies rather than individual members or parts. One for instance would normally refer to how rigidly a boring bar is held rather than how stiffly it is held.

Ah! Semantics.

A good explanation for us old guys. :rolleyes:

lazlo
11-04-2011, 06:31 PM
I made these little boring bars out of a soft steel, not sure what it was, it machined beautifully like lead alloy steel does.

Easy to tell if it's 12L14 -- have they rusted already? :)


They work fine for what I've used them for which is in the lathe and my BP boring head. I use a good sharp carbide insert in them and have never had them chatter.

As has been said, all steels have the same stiffness, regardless of hardness. You're good to go ;)


A SAE Grade 8 Bolt is supposed to be made from alloy steel (per IFI)... which would mean typically 41XX range.

That was my understanding as well. That although AISI/SAE doesn't specify the exactly alloy, they do indicate:

Grade 8: "Medium Carbon Alloy Steel [AKA, Chromoly], Quenched and Tempered"
Grade 5: "Medium Carbon Steel [AKA, 1045], Quenched and Tempered"
Grade 1: "Low or Medium Carbon Steel"

http://www.americanfastener.com/technical/grade_markings_steel.asp

precisionmetal
11-04-2011, 09:06 PM
I think it's probably fair to say that the majority of us have never broken a boring bar.

It's probably also fair to say that not many of us have ever bent (permanently) a boring bar.

That said: I'll add something else to the fray: a softer (as in Rc) boring bar will "act" different than a hard one -- and I'm speaking about using it well within its yield limit.

I've been involved with go-kart racing for over 35 years, and all the modern sprint racing go karts have changeable axles for the conditions. The manufacturers all label them from "soft" to "hard". Despite all of them having the same ID and OD, the vast majority of racers believe that a "soft" flexes (deflects under a given load) more than a "hard" axle will under the same load. Technically that is of course impossible, however I can assure you that they most definitely "feel" different on the racetrack. Different axles for different conditions. I can hang one up holding it by one and and give it a smack with a wrench (so it rings), and can just about tell you how it will feel on the racetrack.

A long story, but point being that boring bars (if they are not overly huge so there is no movement whatsoever) will "feel" different if they are fully heat treated or dead soft.

my 2c.

beanbag
11-05-2011, 03:47 AM
Today in a CNC mishap, I managed to jam a tool into my insert toolholder made out of annealed 4140. It cracked the insert and put a gouge in the pocket. If it had been hardened, it probably wouldn't have gouged.

philbur
11-05-2011, 05:29 AM
If you want variation in flex why not just play with the ID or OD or both. How do the manufacturers designate the difference. Do they just call then soft, medium and hard etc?

Phil:)


The manufacturers all label them from "soft" to "hard". Despite all of them having the same ID and OD, the vast majority of racers believe that a "soft" flexes (deflects under a given load) more than a "hard" axle will under the same load.

precisionmetal
11-05-2011, 11:04 AM
If you want variation in flex why not just play with the ID or OD or both. How do the manufacturers designate the difference. Do they just call then soft, medium and hard etc?

Phil:)

That was essentially my point: the Italian chassis manufacturers (where literally 95% of the innovation comes from in kart racing) discovered a long time ago that it really isn't the "amount" of flex in the rear axle that fine tunes the feel of the chassis -- rather it's the "springiness" or "deadness" of the axle that is used to adjust the feel under various conditions.

Boring bars can act similarly -- if you have one that is vibrating/ringing during cutting, you can change its tuned frequency (and/or damp it) by clamping a piece of metal to the bar. Clamp a piece of steel to it, or clamp a piece of lead to it... and the bar will respond differently.

fwiw... :)

lazlo
11-05-2011, 11:20 AM
I think it's probably fair to say that the majority of us have never broken a boring bar.

More importantly, wouldn't you want the boring bar to yield (fracture) before the cast iron dovetail on your topslide?

That's the nice thing about carbide boring bars -- very stiff, but very brittle. So if you have a crash, the boring bar gives, instead of the lathe.
You can easily snap a thinner carbide boring bar from chatter, or even advancing the saddle into a blind hole. Ask me how I know :rolleyes:


I've been involved with go-kart racing for over 35 years, and all the modern sprint racing go karts have changeable axles for the conditions. The manufacturers all label them from "soft" to "hard". Despite all of them having the same ID and OD, the vast majority of racers believe that a "soft" flexes (deflects under a given load) more than a "hard" axle will under the same load.

No offense to the racing community, but my experience has been that they're very superstitious. The racing community is the biggest market for cryogenic treatment, for example, and it's not for conversion of retained austenite immediately after heat treatment, as done for high alloy tooling.

The hardness/stiffness fallacy is common in bladesmithing too -- many folks intuitively think that a hardened blade is less flexible than a differentially hardened (hard edge, soft spine) blade. But several eminent metallurgists will take out an old-school double-edged "safety razor", which is around 64 Rockwell, and bend it end to end. It snaps back to flat with no plastic deformation at all. A rapier or a filet knife is just as flexible, fully hardened or annealed. It's the geometry that makes the blade flexible, not the hardness.

PixMan
11-05-2011, 04:32 PM
@joelee: what inserts are theese?

I'm not JoeLee, but I can take a good guess. Without scale in the photo it's impossible to be certain about size, but the shapes are easy.

They appear to be a 7º side clearnace angle. If so, the ones with the chipbreaker around the edge are usually referred to as TPMR, while the flat-topped one is TPGT.

In ANSI desgination it would be as follows:

TPMR21.51-PM2 (the "PM2" being PURE speculation as a chipbreaker designation is manufacturer-specific.)
TPMR32.51-PM2
TPGT21.51
TPGT32.51 (no chipbreaker)

In ISO designation the possibility would be:

TPMT110204-PM2
TPMT16T304-PM2
TPMW110204
TPMW16T304

They may also use a G or H in the 3rd character position for tighter size tolerance inserts (ground periphery, usually). K is also used, but usually for milling inserts.

If the inserts are the 5º side clearance angle, then:

TCMT, TCGT, TCMW, TCMM, etc.