Help an old Shop Hand please

[DP68]

Few questions, if anyone can teach me something please. I work with this stuff everyday, but am confused on a few points.

1. What makes different steels harder to cut? And cut different ways. As in...I can zip through a pc of ss304 round rod with an abrasive disc, but takes quite a bit longer with a portable band saw.

2. What is the term for hardness to cut?? I see rockwell is the 'denting' and the tensile is the 'stretching'...correct? Which applies to cutting?? As in...Everyday carbon and stainless have roughly the same rockwell...so why is stainless SO hard to drill? Is the tensile, which seems twice that of carbon?

3. We are testing 6" cut-off wheels here to find a good one. How is this possible....Taking 1 wheel until it's used up, we cut the following items:
1" SS304 Solid Round Rod, 3/4" A106 CS Pipe SCH80, A36 Angle 1x1x1/4, and GR8 Bolt 1". The wheels are getting exactly the same amount of cuts on each type I just listed. It struck me as odd, and I would have thought we included a wide range of "hardness-es" there. Yet, as I said, they cut all about the same...??

Thanks if someone can teach a guy something.

 

[ornerynorsk]

1. Alloying elements and hardness make ALL the difference in the world. Some materials respond better to abrasive, rather than mechanical cutting. Some materials also work-harden. 304 does this very easily.
2. Machineability. Rockwell (c scale for most metals) is an indicator of hardness. Lower tensile materials, usually less hardness, (yes, tensile is stretching) tend to be more "gummy" and difficult to machine. Think of cutting a steak or a roast. If it's frozen solid = difficult to cut. Completely thawed = not quite as difficult to cut. Somewhat frozen = easiest to cut. Maybe not the best analogy, but it works for me.
Stainless steels are hard to drill because they generally have a lot of nickel and chromium, AND, they work harden. Peck drilling with a lubricant helps considerably. If your chips are coming out straw going to blue, you're hardening the material, even though 300 series SS is not heat-treatable.
3. I swore off ever using abrasive cutoff unless absolutely necessary over 20 years ago, so can't offer any good theory on this one.

I'm not an engineer so some of my answers may be a bit simplistic, but I've got better than 30 years making chips and doing a variety of other things, and you just can't help but learn a few things along the way.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[1gibson]

Work hardening is a big part of it.

 

[DP68]

So the abrasive wheel "over-rides" the work hardening? Because, as I said, I'm getting the exact number of cuts out of similar diameter cs vs ss with the exact same wheel. It also seems to over-ride whatever hardness. That grade 8 bolt, in our minds, should have eaten up our wheel much quicker than the a36. But, id didnt. Weird.

Appreciate the comments.

 

[dgallup]

Some materials have elements added to them specifically to make them easy to machine. 12L14 is one, it has both sulfur and lead added. Typically a material data sheet will give you a machinability index as well as suggested speeds and feeds for different types of tools. Some materials respond differently to different types of machining. Some of the super alloys are very sensitive to correct process setup. Try to cut them slowly and they will work harden and kill your tool.

There have been many books written on the subject, it's a complex field.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[MintJulep]

I suspect that the difference between abrasive wheel and saw with respect to work hardening is the chip size.

A saw creates a relatively large chip, so there is a large amount of material deforming before the chip breaks off, and therefore more material work hardening.

The chip created by grinding is much smaller, so less material deformation and less work hardening.

 

[swall]

My take is that with band sawing, you are shearing the material on a small scale as you cut and the shearing action creates a work hardened edge on the kerf. With an abrasive cut off wheel, you are removing material via abrasive wear in a localized area. Testing abrasive wheels is more art than science. You can have the same grade of wheel give different results from lot to lot.

 

[redpicker]

Cutting with a band saw is really a machining operation (as has been mentioned, above). The characteristics that improve machinability will improve band saw performance. Lower materials hardness, lower work hardening, lower ductility, all improve machinability for the material being cut. For the cutting tool (bandsaw blade), the sharpness of the cutting surface, blade set, pitch, and chip relief will all have an effect, and to get the best solution, these variables need to be optimized for the noted characteristics of the material being cut. The 304, in this case, will be cut faster with a coarse pitched blade with a heavy load run at a low speed, with adequate coolant to keep the cutting edges cool and sharp. This will allow the cutting edge get below the work-hardened surface and cut a good chip. This will create a lot of heat, hence the need for heavy coolant flow. The coarse pitch will allow for a larger chip to prevent the chip from clogging the relief and dulling the cutting edge.

Cutting with an abrasive wheel is more of a grinding operation, and grit size and hardness, as well as binder hardness need to be matched for the material being cut. For soft materials, a coarser grain with a hard binder will perform best with the optimum binder hardness such the binder will break-down at a rate to match the dulling of the grit. For harder materials, a finer grain and softer binder will perform better since the binder has to break down faster to expose new grit sooner since the harder material will dull the grit faster. Higher work hardening will cut better with a coarser grit. For your example, the A106 and A36 materials will be nearly the same hardness, and even the Gr 8 bolt will close to the same hardness. I would expect the 304 to get quite a bit hotter during the cutting, but I wouldn't be that surprised the cutting rate to be about the same. If you tried to cut a piece of hardened drill rod, or tool steel, I think you'd find poorer performance, but a blade optimized for the drill rod would not cut the A36 angle iron or the stainless very well.

Being observant and asking pertinent questions is the key to learning. Never stop since nobody ever knows enough.

rp

 

[tomwalz]

Don't worry about being confused. As least you know you are.

There is an aboslute defintion for Rockwell as a test from the Rockwell people. However the ramifications of Rockwell hardness are immense and incredibly complex.

There is no exact and universally agreed upon defintion for “Tensile” and many other terms. E.G. Is it the point where it breaks or just before where it breaks? Does it break or part? Etc.


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[DP68]

Thanks for the input...

So basically...In our attempt to find a good cut-off wheel, I have wasted a bunch of material! We are a mechanical piping contractor, and use grinders/wheels more than any other item...cutting run of mill pipe, structural stuff, threaded rod...padlocks...whatever the case may be. SO...if I am getting roughly the same life out of a wheel on all these different types of materials...I could have simply picked one and been done.

But I'm too deep into it now, I still have a few to go. Note we are testing just about every cut-off wheel I can get my hands on, up to 37 different manufacturer's/models at this point. I plan to put it all into a spread sheet, to determine the best price per cut, best speed, etc. It's kind of cool actually, as we are constantly bombarded with Sales Reps having the Ultimate Blade in all the lands. Now, we will know.

Also note, the tests are fair. We are using a gravity fed (no human interference) system...with each wheel cutting exact materials vs each other, with exact placement on the jig, same size, same angles...etc. It's a fair playing field, in our humble testing option.

I will come back here and post a link to the finished product in a few weeks, just for fun. Some of the results are a little surprising, and ultimately you too can tell the random Sales Rep...Sait for example (I happen to know)...no sir, sorry, your cut-off wheels really isn't that great, at least according to a true testing chart I saw once....have a nice day.

Thanks again.

 

[ornerynorsk]

Doesn't sound like a waste to me! You'll have a good solid database of what works for you. You'll reap the reward just in time saved when dealing those pesky salesmen!

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.