# Ridigity of boring bars..



## Hal (Nov 5, 2010)

With out stepping on any one's toes, I would like to set the record straight.

You CAN NOT make something stiffer by cutting flutes in it......

If you have 2 rods, of the same material and diameter, and cut flutes in one. The one with flutes WILL Flex more under the same load......

If having flutes made something made it stiffer, boring bars would come with flutes in them...

If you go by weight, such as rifle barrels, the fluted barrel will be stiffer because the material,(weight) removed fluting will allow for a larger diameter barrel to be used and still make the weight limit.


Hal


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## Ned Ludd (Nov 5, 2010)

Much like the old saying "a tube is stiffer than a solid bar", no it isn't if it has the same dimensions, but a tube with the same weight of metal and length as the bar will be. 
If some of you are thinking of saying "this is not true if the wall thickness was paper thin", forget it, I am thinking real world situations.
Ned


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## Lew Hartswick (Nov 5, 2010)

Ned Ludd  said:
			
		

> If some of you are thinking of saying "this is not true if the wall thickness was paper thin", forget it, I am thinking real world situations.
> Ned


I'm not so sure it isn't even then. Need Marv and a bit of calculus to verify. 
  ...lew...


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## Ned Ludd (Nov 5, 2010)

All right, taken to an extreme, if you had a foot long bar of aluminium say 1" in diameter and the same amount of metal made into a foot long tube but with a wall thickness of cooking foil, how do you think the stiffness would stand up? :noidea:
Personally I think it would not even be stiff enough to be self supporting, even when standing on end. 
Now if the same metal foil were to be made into a sausage or ball shape and filled with air at greater than atmospheric, that would be a different ball game, but we are only talking of a plain open ended tube. 
Ned


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## Blogwitch (Nov 5, 2010)

I don't know where this has popped up from. 

Why are steel I or H beams used in construction if a square one of the same size would be stronger?

I was under the impression it is the surface area that counted, so a tube (of course not like the paper thin one mentioned above) which has roughly double the surface area of a solid bar would resist bending much better than a solid bar.

Bogs


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## Jasonb (Nov 5, 2010)

Bogs if you wanted the same properties from a solid square or hollow for that matter it would require more actual steel per linia meter or foot so the I or H sections are used as these give the best performance for a given amount of material.

Surface area has little to do with it, but cross sectional area and its position from the central axis of the beam is what counts. thats why if you slice through a beam the two flanges are usually thicker than the central web as that is really just there to keep the two flanges apart.

Its a long while since I did structures at college but a 152x152 Universal column weighing 37kg per meter would have similar strength charicteristics to a 150x150 Square hollow weighing 66kg. So you can see that the most efficient use of materials is the H or I section. If you were not constrained by the same "size" then using a Universal beam would get the weight down to 25kg/m but you would need a depth of 200mm. Or if you wanted to keep teh weight at 37kg then 200x100 sq hollow would have similar strength

Jason


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## Ned Ludd (Nov 5, 2010)

Hi Bogs,
In simple terms the "stiffness" of an H or I beam can be likened to a foot steel rule. If you try to bend a rule in one direction it is floppy but if you try to bend it the other way it is difficult to bend. The end pieces that make our rule into either an H or I operate at right angles thus they rules on their side and stop any bending the other way.
I don't know if that makes any sense but i know what I mean.
Ned


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## Lakc (Nov 5, 2010)

Its actually a pretty simple FEA exercise to show the stiffness of a fluted v regular bar. I expect you may find the solid bar bends slightly more under static conditions of gravity due to its increased mass (thus load), the fluted bar, would show more deflection versus any applied load. It does not seem to make sense that you take can something away and gain strength or stiffness.


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## Maryak (Nov 5, 2010)

From memory, ships propeller shafts are hollow.

Obvious is weight reduction, less obvious is the ability to carry more load in the form of torque. This increase in torque is only true if you core the shaft by around 1/3 of the cross section area, any more and it is weaker than a solid shaft. Shaft Horespower in Turbine ships is determined by a torsion meter fitted to the propeller shaft which measures the twist in the shaft and hence the load being generated.

Hope this helps.

Best Regards
Bob


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## Lew Hartswick (Nov 5, 2010)

Ned Ludd  said:
			
		

> All right, taken to an extreme, if you had a foot long bar of aluminium say 1" in diameter and the same amount of metal made into a foot long tube but with a wall thickness of cooking foil, how do you think the stiffness would stand up? :noidea:
> Personally I think it would not even be stiff enough to be self supporting, even when standing on end.
> Now if the same metal foil were to be made into a sausage or ball shape and filled with air at greater than atmospheric, that would be a different ball game, but we are only talking of a plain open ended tube.
> Ned


I'm not at all sure you are right. The problem you would run into in that scenario is 
NOT stiffness in the bending mode but the ability to maintain shape with gravity 
force. Suppose we do it in space with no gravity and remember only measure 
"stiffness" (as in bending). It will require some rather interesting method to insure 
the ends don't deform. ie. they must remain circular. I think it is one of those things
that just don't "LOOK" like it should be. 
  ...lew...


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## Ned Ludd (Nov 6, 2010)

Hi Lew,
Even in space Newtons laws apply, but as you say testing would be near impossible without some very clever way of holding the thing, without adding to stiffness.

Let us not take this too far away from real world situations, I can sense some hostilities from some in the group when things head that way. *knuppel2* *bang* :wall: and perhaps :shrug:
Ned


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## Tin Falcon (Nov 6, 2010)

Lets keep this practical here folks.
the best boring bars are solid carbide because it is stiffer than steel . and I have read where hollow steel bars filled with lead shot can improve the finish over a solid steel bar due to vibration dampening. Most people here IMHO want to know practical ways to make a boring bar and practical limits and rule of thumb for depth of holes and minimum diameters. etc.After all a 3 inch diameter hollow boring bar may be stiff and rigid but it will not fit in a 1/2 in hole and will not fit on a Sherline or mini lathe very well. lets keep it real.
Tin


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## steamer (Nov 6, 2010)

I've been watching a bit.

This spells out the "pertinent" science well enough that it doesn't bear repeating it.

http://www.eisc.com/support/boring.htm

There are some boring bar designs that have the cutter clamped via an internal bar that puts the boring bar in tension.  While this won't increase the stiffness of the boring bar, as is claimed, it may raise it's natural frequency....making it a little less likely to chatter.  Grind your tool so most of the cutting is done on the front face, not the side.

http://www.hemingwaykits.com/acatalog/_Dore__Small_Boring_Tools.html

Dave


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## Ned Ludd (Nov 6, 2010)

Thanks Dave, that link says it all.

I wrote the following before reading the above but some of it might still be relevant.

With manual turning, I think you have to know your tools and listen and watch how they work. As a wise old chap, when talking about woodturning, said "cut the wood the way she wants to be cut". The same can be said about metal, but you can only know how by observing and reacting accordingly.

The simplest advice must be use the biggest diameter, shortest bar you can and make sure it is sharp, but that does sound like stating the bl**ding obvious, 
doesn't it.

I might not agree about solid carbide being "best" because of its very brittle nature (horribly expensive if you have a dig-in due to a senior moment) but carbide tipped, with tips intended for boring, I would wholeheartedly recommend.

One boring bar that I use quite a lot is in fact a 10mm replaceable tipped drill, it looks rather like 2" long, single lip end mill and when bought, as surplus, it came with a box of new triangular tips and was quite cheap. I have it mounted in a blank end No1 Morse taper, fitted in an otherwise unused Dickson holder. It is perhaps not as rigid a set-up as I would like but when used within its "comfort zone" it cuts cleanly enough.
Ned


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## Blogwitch (Nov 6, 2010)

You will have to bear with me here as it is nearly 45 years since I did structures and things like moments and stresses, and of course things have progressed a lot over that time, and a lot of my original learning has been lost along with a few brain cells, so I will not have an up to date understanding.

Just a couple of questions.



> the I or H sections are used as these give the best performance for a given amount of material.



So what you are saying, is that if you took a 1" square bar, and formed it into a much larger I or H beam, would it have more or less strength and resistance to flexing than the original 1" bar?

The way I see what you are saying is that if you took a 1/2" square bar, and formed it into a 1" top to bottom and 1" side to side shaped I beam, even though it is 1" in size, it will only have the same strength as a 1/2" bar.

and



> If you try to bend a rule in one direction it is floppy but if you try to bend it the other way it is difficult to bend



So in theory, you weld 4 rulers along their edges to form a square tube, it will be more rigid because it resists bending no matter which way you try to bend it.

You might be wondering why I am asking such stupid questions.

Over the last few years, engineered lumber is starting to be used for building houses here in the UK, and they make a special point that the shaped and bonded (basically I beams) are much stronger and more rigid than a solid beam would be if it was the same external size, and they can be used for longer spans for a given size of solid beam.

Who is right and who is wrong? I am very confused over this issue.

Bogs


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## Jasonb (Nov 6, 2010)

> So what you are saying, is that if you took a 1" square bar, and formed it into a much larger I or H beam, would it have more or less strength and resistance to flexing than the original 1" bar?



It would have more resistance to flexing than a solid bar. Lets say your 1" square could make a 1" wide by 2" deep x 1/4" thick I beam, this would resist bending loads applied to the top far more than the solid square of the same cross sectional area. If we also made a beam with 1x1/4 top and bottom and a 4x1/8 central web, this would still use the same amount of metal but be even more resistant to load applied to the top



> The way I see what you are saying is that if you took a 1/2" square bar, and formed it into a 1" top to bottom and 1" side to side shaped I beam, even though it is 1" in size, it will only have the same strength as a 1/2" bar.



Again the H (equal height and width would be termed H not I) would be more resistant to the load than a solid square of same cross sectional area.

In both these cases and also your timber I beams the bulk of the materal is spaced at the top and bottom of the beam where it has the most effect. 

JasonThere are other reasons that the timber I beams are used, they can use odd bits of smaller faster growth trees as most use OSB or other manmade board for the central web, as the important parts of the beam are at the top & bottom and the web is just a spacer it can be drilled for services far more than solid timber or the ones with the zig-zag steel to space the top & bottom members allow you plenty of room for services.

Jason


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## steamer (Nov 6, 2010)

OK guys.  The magic number is the "Moment of Inertia", not to be confused with the MASS moment of inertia.  For this conversation, we'll consider this a static analysis. Nothing is moving.

The moment of inertia is the second moment of area. It has units of length to the 4th power.  I'll spare you the derivation.....google is your friend here.

Bogs any beam has stiffness that is proportional to the moment of inertia.

For a rectangular beam, the calculation is I = (bh**3)/12

Notice that the Moment of inertia is proportional to the cube of the height of the beam and is linear to the width.  If you double the height of a rectangular beam, you will increase it's stiffness by a factor of 2**3= 8.

So if I took a 1" square bar, and rolled into a bar that is .5" wide and 2 inches tall ( same cross sectional area, so the same amount of material)
the beam would be 4 times as stiff! (B/2 x 2**3)= 4

That is engineering reality.

"I", the moment of inertia, is a function of the cross sectional shape of a beam.

Now interestingly enough, the round bar is more sensitive to the OD than the ID (I is proportional to the difference in diameter to the 4 power). So a hollow bar doesn't give up that much stiffness as compared to a solid bar.  but it does give up some.  However the hollow one is much lighter.

As to engineered beams, you will notice the I section, that shape is taking advantage of the geometry to make a stiff yet light beam. All the stress is at the top and the bottom.

Dave


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## Blogwitch (Nov 7, 2010)

Many thanks Jason and Dave, I think I have my head around it now.

I wasn't trying to be argumentative at all, but I just couldn't grasp what was being said. 

It is just one of those things that catch you up in later life.

I suppose I will have to follow a little rule my wife gave me the other day.


Bogs


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## steamer (Nov 7, 2010)

Well Bogs...MY wife gave me that very sign, I think I have it hanging my study! :big: ;D

I'm glad its cleared up.

Dave


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## Ken I (Nov 10, 2010)

The problem of chatter with all tools stems from a fundamental rule of physics :-

The rules of physics are out to get you.

Cutting metals requires an almost constant unit of power for the amount removed - a good rule of thumb for mild steel is 3H.P. per cubic inch per minute. (The "K" factor)

If the cutting speed is increaced, the force required to perform the cut diminishes - force x velocity = power - more velocity = less force and of course vice versa. This seems counter intuative but I assure you it is true - in fact if the force increaced with velocity, chatter would not occur at all.

So if a tool (or any other part of the machine for that matter) deflects then the force increaces - and the deflection worsens - until the cutter gains the upper hand and the process reverses.

If the natural resonant frequency of any part of your set up falls into line with this the problem goes from bad to worse.

What to do - obviously the more rigid the tool and support structure, the less the problem is going to be.
Any form of damping is going to help and of course changing speeds and feeds might move you out of resonance.

Obviously a hollow boring bar is less rigid than a solid one - but its resonant frequency is different so you might be lulled into thinking it works better.

A hollow boring bar with a tie rod down the middle allows you to pre-stress the boring bar - thereby changing its natural frequency - this allows you to "tune" the bar.
The addition of a close fitting lead sheath between the tie bar and the bore in the bar or lead shot which is also compressed by the tensioning is said to be good for damping (although I haven't tried it).

Using the biggest, shortest bar that will do the job is generally the way to go.

Use "D" bits ground from soild carbide or HSS round stock clamped in a split sleeve with the shortest stick-out length you can live with (access & visibility).


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## steamer (Nov 11, 2010)

Hi Ken,

Used lead shot in precision grinder base castings to dampen things. Usually with some oil.  Worked great.  Epoxy Granite works better though.


Welcome!

Dave


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## Ken I (Nov 11, 2010)

Dave,
    Thanks for the reminder of epoxy granite - that's a material that's fallen out of my memory bank.
At one stage it was popular amongst certain machine builders. I don't know if it still is.

Ken


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## steamer (Nov 11, 2010)

I can assure you , epoxy granite, as well as real granite, is used every day...

http://www.mooretool.com

Dave


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