Why does thicker metal take longer to cut?

MontanaLon

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What I mean is if you are cutting it on the short edge. Clearly if you are cutting through a thicker piece through the thickness it will take longer but say you have 2 pieces of metal 5 inches wide. One is 1/4" thick and the other is 1/2" thick. But if you cut along the 5" width it should take the same amount of time to cut and it doesn't. The thicker piece will take longer. This is all given using the same tool to do the cut.

It just doesn't equate for me. Equal length strokes should remove equal amounts of material. But it appears to not be the case. Anyone have an explanation for my challenged mind?
 

mikey

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Your description is unclear to me so maybe I'm seeing it wrong but if you mean that you are cutting two 5" long pieces of material, one 1/4" thick and one 1/2" thick, and you are cutting through the first one and half way through the second one with an end mill then yes, the thicker material takes longer. The reason is that you are cutting the thicker piece with both side and end, while the thinner piece is cutting only with the side; much smaller contact patch with the through cut, plus the chips clear easily so not much re-cutting is happening.
 

pstemari

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Assuming you're cutting with a saw of some sort, it depends on the tooth and gullet size. If you use a fine-tooth saw with thick stock, the gullets will fill up with chips before they get to the other side. If you use a coarse tooth saw with thin stock, you strip the teeth right off.

I have three different blades for my metal cutting bandsaw, each of different TPI, and they get swapped depending on what I'm cutting. Thin angle iron or tubing gets the high TPI blades; large-diameter bar stock gets the low TPI blades.

Ideally you should have at least three teeth in contact with the work at all time. Not sure about an upper bound, I think my coarsest blade is 6-10 TPI, and that does a decent job cutting 3 inch round bar.

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JimDawson

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Under ideal conditions for a given horsepower you can remove some maximum volume of material per minute. If I understand your question correctly, then the volume of material removed in the 1/2 piece is twice that of the 1/4 inch piece. Given that the conditions are probably not ideal, then the cut time may not be directly linear.
 

mikey

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Perhaps a better description of the cut or a picture would help.
 

MontanaLon

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Your description is unclear to me so maybe I'm seeing it wrong but if you mean that you are cutting two 5" long pieces of material, one 1/4" thick and one 1/2" thick, and you are cutting through the first one and half way through the second one with an end mill then yes, the thicker material takes longer. The reason is that you are cutting the thicker piece with both side and end, while the thinner piece is cutting only with the side; much smaller contact patch with the through cut, plus the chips clear easily so not much re-cutting is happening.
I was thinking more along the lines of a saw. If a piece of stock is 1/8" thick and 5" wide and the other is 1/2" thick and 5" wide and you lay them both on edge so you are cutting across the width (making the cut 5" long) then both pieces should cut through in the same amount of time. 5" cut is a 5" cut.

If you were cutting across the wide edge then you would be making a cut through 1/8" and 1/2" which is 4 times as thick so it should take 4 times as long.

Conversely if both pieces are the same thickness at 1/8" but different widths, say 3 and 5" if you cut through the thickness so each cut is 1/8" so should take the same amount of time all other things being equal.

It would bee like if you where drilling a hole through 2 pieces of stock of the same thickness but different widths. It would take the same amount of time to drill the same size hole as the width has nothing to do with the thickness.

It was just something that occured to me while cutting a piece of 1/2" plate with a hack saw. Too much time to think.
 

Latinrascalrg1

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Are you talking about "rip cutting" the piece along the 5" edge length wise through the thickness so that for example if I cut the 1/2" thick by 5" long (missing the width measurements) piece in half as you are trying to describe i would end up with 2 pieces that are 5" long by 1/4" thick minus the blade kerf by however wide your piece started out as will remain the same length. Is this what you are asking?
 

MozamPete

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I was thinking more along the lines of a saw. If a piece of stock is 1/8" thick and 5" wide and the other is 1/2" thick and 5" wide and you lay them both on edge so you are cutting across the width (making the cut 5" long) then both pieces should cut through in the same amount of time. 5" cut is a 5" cut.
The saws downward force would be across 4 time the area when cutting the 1/2" section so he pressure per tooth would only be a quarter - hence less metal removal per tooth pass.
 

catsparadise

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A saw blade removes a volume of material, just like any other cutting tool. The volume in the first case is 5 x 1/8 x the saw kerf width, the second is 5 x 1/2 x the saw kerf width, ie 4 times more, so I'd expect the second to take at least 4 x longer, provided the saw gullets don't fill up as @pstemari said.
 

RJSakowski

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Cutting metal requires pressure to make the cut. This is very obvious when drilling or turning. Without pressure a drill won't cut which is why drilling manually ytakes longer than on a drill press. When turning, we see deflection of the part due to the cutting pressure. The deeper the cut, the greater the pressure and the deflection.
For a saw blade the applied force is divided between all the teeth in contact. If you increase the number of teeth, the pressure per tooth drops and it takes a smaller cut. If you increased the pressure per tooth to equal what the thinner stock sees by increasing the force, it would cut just a quickly, assuming you have the horsepower to move the blade.
 

MrWhoopee

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Regardless of which way the stock is turned, the thicker piece requires the removal of a greater number of cubic inches of material. Assuming a constant rate of stock removal, the thicker piece will always take longer to cut. A piece 4x thicker will take 4x longer (all other factors being equal).
 

Latinrascalrg1

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Regardless of which way the stock is turned, the thicker piece requires the removal of a greater number of cubic inches of material.
Not exactly true. If the stock material is set on edge so that the 1/2" or 1/4" side is parallel to the blade so that it cuts into that presented 1/2" or 1/4" wide edge length wise the blade will have the same number of teeth in contact with the material for both pieces. Although I dont know why someone would choose to do this however In a case such as this, the blade should cut both pieces at the exact same rate because it is cutting through the exact same amount of material....assuming the blade kerf falls completely within the span and all other measurements remain identical. The only other factor i can maybe see having an effect is heat dispersion being much different between the 2 pieces regardless of which way its cut.
 
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Paul in OKC

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Not sure of the thinking here. Thicker will always equate to longer times. Looking at your theory, it should take the same amount of time to cut a 5" wide piece regardless of thickness. Unless I am missing something, or this is some of that 'new' math....... Also how can it be said that whether 1/4 or 1/2 thickness you will have the same number of teeth in the cut when set on edge. Unless you have like 2 teeth per inch. Again, wider material can only mean more teeth in the cut. Am I really missing something here?
 

MrWhoopee

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Not exactly true. If the stock material is set on edge so that the 1/2" or 1/4" side is parallel to the blade so that it cuts into that presented 1/2" or 1/4" wide edge length wise the blade will have the same number of teeth in contact with the material for both pieces. Although I dont know why someone would choose to do this however In a case such as this, the blade should cut both pieces at the exact same rate
Yes, it will cut "at the exact same rate". The "rate" being measured in cubic inches per minute, the thicker piece will take twice as long. It will cut thru the first 1/4 inch of the 1/2 inch plate in the same amount of time as it cuts thru the first 1/4 inch of the 1/4 inch plate. Let's assume a .035 kerf width and 5 inch width. The amount of stock removed for the 1/4 inch plate =.035 x 5 x .25 = .04375 cubic inches. For the 1/2 inch plate = .035 x 5 x .5 = .0875 cubic inches, which is, of course, twice as much stock. ALL OTHER FACTORS BEING EQUAL, it will take twice as long to cut the 1/2 inch plate.
 

pontiac428

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Work equals force times displacement. If you're displacing chips, you are doing more work when the thickness increases. Power is work over time. If you are doing more work, either you need more power or it takes more time. It takes either more power or more time to displace those chips.
 

RJSakowski

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Cutting speed is not proportional to cutting pressure. I cut a 3/8" x 3" piece of aluminum in 57 seconds by cutting the 3/8" side but it took 135 seconds to cut it cutting the 3" side.
 

Aaron_W

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Cutting speed is not proportional to cutting pressure. I cut a 3/8" x 3" piece of aluminum in 57 seconds by cutting the 3/8" side but it took 135 seconds to cut it cutting the 3" side.
Thanks for doing the test, that is exactly where this question led my brain, which way cuts faster, thin side or wide side.

I suspected what you found, although I am surprised that the difference is so significant a little more than 50% longer to cut through.
 

ezduzit

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...if you cut along the 5" width...
You would never cut across the widest dimension--always the narrowest. You want to minimize the number of teeth contacting the material at the same time, maximizing the pressure on each tooth.
 

matthewsx

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Work equals force times displacement. If you're displacing chips, you are doing more work when the thickness increases. Power is work over time. If you are doing more work, either you need more power or it takes more time. It takes either more power or more time to displace those chips.
Physics, the laws you can't disobey ;)
 

RJSakowski

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You would never cut across the widest dimension--always the narrowest. You want to minimize the number of teeth contacting the material at the same time, maximizing the pressure on each tooth.
Actually, I usually cut the widest dimension because my cut will be truest. I sacrifice speed for accuracy. Sometimes, I will start a kerf along the wide side and flip it vertically to finish. The kerf helps to guide the finish cut. Another reason would be if the saw limits the cut. A 4 x 6 band saw won't cut a 5" wide piece if mounted vertically.
 

ezduzit

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Actually, I usually cut the widest dimension because my cut will be truest. I sacrifice speed for accuracy. Sometimes, I will start a kerf along the wide side and flip it vertically to finish. The kerf helps to guide the finish cut. Another reason would be if the saw limits the cut. A 4 x 6 band saw won't cut a 5" wide piece if mounted vertically.
The kerf should be completely unnecessary if your saw is properly setup; this just prematurely wears out the blade. And, if the material won't quite fit in vertically, or is too thin for the tooth pitch, you would clamp it in the vise on an angle off horizontal.
 

RJSakowski

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The kerf should be completely unnecessary if your saw is properly setup; this just prematurely wears out the blade. And, if the material won't quite fit in vertically, or is too thin for the tooth pitch, you would clamp it in the vise on an angle off horizontal.
The saw is a Buffalo brand, Taiwan manufactured almost 40 years ago. The saw has always cut at a slight angle vertically and there is no means for adjustment. Believe me, I've tried.
I do clamp at an angle, usually when a piece is too large to fit (although the saw is nominally a 4 x 6, the largest horizontal cut that I can make is 5"). I prefer not to since it is not very stable and the work can pull out of the vise, potentially doing nasty things to the blade.
 

nnam

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Say cut 1x5" metal. If cut 1", only the front few teeth cut it. So it takes less time for the teeth to go through the 1". Same for 5", it takes a lot of time to go through 5" at a same speed. The rest of the teeth just dangling there. In reality, maybe they all sink in a little, but the force is just not enough to push the whole row of 5". So maybe the front is pushed down a bit and the back. Saw cutting is somewhat similar to lathe cutting, a few thous here and there make a big difference. So if the saw blade don't bite down, it wouldn't cut much. For a wide piece, it's hard to force too much, and the teeth wouldn't bite down either. Like lathe, it starts out below the surface to cut. If we do a plunge cut, it works, just not so good, and that's an equivalent of 1 tooth. Plunge cut of 20 or 50 teeth must be very hard, and that's why it's much slower. It just doesn't cut deep enough. I think if people want to experiment, they can try both and compare the shavings. I don't think they're the same.
 

epanzella

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A single tooth at a constant speed and constant down force into material will cut at a constant rate. Double the amount of teeth in contact with the work and you need twice the down force at the same speed to keep each tooth cutting the same amount. Ten teeth will cut twice as fast as 5 teeth if down force (per tooth) and speed is held constant even though EACH TOOTH is cutting the same amount as before. Being as nothing is free in this world, more teeth cutting and more down force will require more power or the speed will drop and all then bets are off. I hope this helps you because I didn't actually understand your question.
 

magicniner

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A couple of guys have hit the nail on the head, to answer this question is is useful to imagine theoretical extremes.
If a saw blade of infinite strength, length and rigidity and with a set 40lb cutting pressure is set to cut a 1/8" metal work piece cutting will be very rapid, make the work piece close to the length of the blade and the pressure per tooth will barely mark the metal.
 

whitmore

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The tooth design (and edge character) on the blade will determine the results. A twist drill sharpened for steel neither pulls into nor pushes
out of the cut, but self-feeds with relatively low applied force, in steel. It dives into copper, however, or brass or lead.
Similarly, a slightly dull blade requires pressure (and lacking pressure to take at least a few mils per tooth, it will just heat
the cut and not penetrate at all). So a dull blade won't bite along a long kerf (as in the five-inch dimension)
though at similar feed pressure it might cut the 1/4" dimension adequately.

A blade on a bandsaw won't apply uniform pressure on all its teeth for the whole length of a five inch cut, unless
the blade is stiff (like a 3/4" blade depth, rather than a 1/2" blade depth) . That's what blade flexure does to you.
 

cathead

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Rocking(changing the angle) the hack saw every few strokes allows one to cut thick material easier. This is because the angle
change reduces the number of teeth cutting at any one time. Less teeth cutting increases the pressure on each cutting tooth
resulting in more material removed with each stroke.
 
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