# Thread: Large End Mill for Bridgeport

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Originally Posted by vpt
I never put much thought into sfpm or cut speeds, I just kind of know what sounds and feels right.

But to get to know this math anyhow, what number are you starting with all the time to figure out rpms? "cut speed" is mentioned to be multiplied by 4 then divided by the tool diameter. What is "cut speed"?
200m/min for coated carbide in normal steel, half of that in stainless and quarter in hardened steels.
HSS tools about one tenth of the above, 20m/min for low carbon steel.

Metric values so multiply with 3

I calculate the speed hardly ever, I have large cheatsheet printed from excel that shows various materials, diameters and tools all in one.

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Originally Posted by wyop
In your first posting, you mentioned mill scale. If you were taking a shallow cut on hot-rolled steel and cutting in the outer layer of mill scale, you probably took the edge off the end mills.

Mill scale is some rather abrasive stuff, and it takes the edge off HSS end mills pretty quickly. As I see it, there are three options to clean the scale off hot-rolled steel:

1. Use a HSS end mill, but make your depth of cut deep enough that you're fully under the scale, and the scale is just peeling up with the chip you're taking under the scale. Keep your HSS end mills down, fully cutting in the steel instead of doing most of your cutting in the layer with the scale. When you get under the scale, you'll often find that your HSS end mills last longer.

2. As many have suggested, you could get yourself a face mill that uses carbide inserts. This is my solution to getting rid of mill scale most of the time.

3. As others have also suggested, you could use a fly cutter. A single-point HSS toolbit could be used, or you could use a cemented carbide tool bit. Be careful to tram in your mill closely, or set it up for "tailing", where the leading arc of a fly cutter is cutting a thou or so higher than the rear arc. If you tram a mill for tailing, you can run your work in only one direction to get a good finish.
Yes I messed up the end mills some how, maybe a combo of both speed and rubbing on rusty scale. It was fine when I was running on the saw cut part but after a few passes on the scale that was it for both endmills. This thread grew so I think most people missed the post where I did one side with a fly cutter and HSS bit. It was slow, I had to resharpen the bit and anything more than a skim cut would result in Whomp, Whomp so definitely not for taking off material. Its still the side with the best surface finish though.

I do have a 2" face mill that uses SEET carbide inserts coming. The inserts are a bit beat up but the tool looks to be in usable shape. https://i.ebayimg.com/images/g/5k4AA...IU/s-l1600.jpg

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Originally Posted by vpt
I never put much thought into sfpm or cut speeds, I just kind of know what sounds and feels right.

But to get to know this math anyhow, what number are you starting with all the time to figure out rpms? "cut speed" is mentioned to be multiplied by 4 then divided by the tool diameter. What is "cut speed"?
Here's a simplified cutting speed chart

On a mill, the calculation is done with the diameter of the cutter, on a lathe it is the diameter of the stock. In each case it is simply the speed at which the cutter and the material move past each other. Speeds above the recommended range will cause accelerated (or sudden) tool breakdown.

On a Bridgeport (which has sufficient HP and rigidity), I use (cheap Chinese) brazed carbide tools in a fly cutter. This allows higher RPM, which reduces the pounding. Using HSS in a fly cutter is an exercise in frustration.
Last edited by MrWhoopee; 06-12-2019 at 10:41 AM.

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Originally Posted by vpt
I never put much thought into sfpm or cut speeds, I just kind of know what sounds and feels right.

But to get to know this math anyhow, what number are you starting with all the time to figure out rpms? "cut speed" is mentioned to be multiplied by 4 then divided by the tool diameter. What is "cut speed"?
This is one of those situations where we need the data when we're a beginner. But after some time doing different jobs we get a feel for what looks and sounds right and don't need to check the data other than when we do something a bit out of the usual comfort zone.

It's like using a torque wrench. A great idea for a newbie but as we do enough of this stuff we get a feel for what is a good torque for each size of screw and only use it for special cases.

And even if we are good at doing the math in our head we still need the chart for getting the right SFM for each material. And an expanded version that includes SFM for both HSS and carbide is nice.

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Originally Posted by BCRider
This is one of those situations where we need the data when we're a beginner. But after some time doing different jobs we get a feel for what looks and sounds right and don't need to check the data other than when we do something a bit out of the usual comfort zone.

It's like using a torque wrench. A great idea for a newbie but as we do enough of this stuff we get a feel for what is a good torque for each size of screw and only use it for special cases.

And even if we are good at doing the math in our head we still need the chart for getting the right SFM for each material. And an expanded version that includes SFM for both HSS and carbide is nice.
I have this printed to full A4 size at shop wall:

If someone wants to modify their own here is the excel file:
https://www.dropbox.com/s/6ujdafhnn3...able.xlsx?dl=0

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Originally Posted by vpt
I never put much thought into sfpm or cut speeds, I just kind of know what sounds and feels right.

But to get to know this math anyhow, what number are you starting with all the time to figure out rpms? "cut speed" is mentioned to be multiplied by 4 then divided by the tool diameter. What is "cut speed"?
Cutting speed mild steel is 100 .. I learned that in school in 85 and have remembered it since.
I calculate spindle speed in my mind daily when working. .
And I don't have trouble remembering 400 for 1 inch, 800 for half inch.. the rest well it's easy to figure if out.....but up to 1 inch.. that covers A LOT of TOOLING and situations..

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Originally Posted by bborr01
What I'm hearing is that he has a couple of HSS end mills that are dull and he is planning on replacing them. He can probably get a facemill with carbide inserts for less than the price of a couple of large endmills with smaller shafts but the carbide will outlast the HSS ones by multiples. So I stick by my advice. But the carbide facemill and skip the HSS.

Does anybody use HSS router bits anymore. Not that I know of. If we buy carbide to cut wood why would we continue to use HSS for all steel cutting?

Brian
This is actually quite easy if you think it out.
First, it's not wood... altho working with 6061 can be very similar and tools might swap.
Second, a lot of router work is short edge work. Less than 1.5 inch, open side.
A lot of machining is not. They don't make 4 and 6 inch long end mills so that you have an extra 2.5 inches of tip sharpening.
Maybe you never mill a 3 inch deep pocket or 5 inch high face edge, but many do on a daily basis.
Price out a 4 inch long endmill in HSS, then carbide, and you will know why both are sold.

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Originally Posted by bborr01

Does anybody use HSS router bits anymore. Not that I know of. If we buy carbide to cut wood why would we continue to use HSS for all steel cutting?

Brian
I use carbide most of the time when milling wood, especially wood for gunstocks. Gunstock blanks are taken from the part of the tree right above the roots, and the wood has significant amounts of silica in it, which will dull HSS cutters quickly. There are times I use HSS tooling on wood, (eg, checkering), but I know I have to stone it frequently. I also use O-1 chisels, gouges and scrapers on wood - those need sharpening as well.

As for why to use HSS for steel (or other) machining, yes, HSS has a place in machining steel. Allow me to explain both perspectives. Let's start with carbide inserted tooling and what it does well.

Carbide removes material quickly. My machining instructors used to tell me "Carbide is hungry. You have to feed it."

In industrial machining, where the industry has mostly gone to CNC machines with vastly more rigid and massive machines than home machinists use (ie, NOT a Bridgeport-style mill, and not South Bend 10L lathes), industry vastly prefers inserted carbide tooling. But they don't use the cheap carbide tooling that many home shop machinists use - no, the way the large shops work is that they buy into a particular tool supplier's line of tools. Some of the large shops around here use Sandvik tooling exclusively, some use Iscar, some use Kennametal. The inserts come with tooling for the inserts - ie, you're buying the whole insert tool product line. Many shops then have inserts for their "standard" tools in vending machines, where you punch in your employee ID and it vends inserts, so that the shop can maintain some tracking on who is using how many inserts.

But carbide tooling of this type wants to be fed. Many of these carbide insert product lines are not for taking light cuts. One local shop, where former students of mine work, has an informal policy my former students tell me: if the management comes by and sees the load on your machine isn't 50% or better while taking a cut or operation, the make a mental note of it. Then they might walk by later that day or week, and if they see your machine in a cutting operation but not loaded to at least 50%, they might stop and ask you what you're doing. As one former student put it: "They bought that line of tooling and a \$100K CNC mill to see metal get cut off fast - in big cuts." If they happened to catch you both times doing finishing cuts, they understand. But they do tell new machinists to "load the machine down - we didn't buy a mill with a 30HP spindle to use only 2HP of that capacity."

HSS tooling cannot do this, even when ground perfectly, even when the best HSS tooling has been chosen, even when flood-cooled. Carbide will remove material at least 4X as fast as the best HSS. You buy the best carbide inserted tooling to make money - and make no mistake: You make money in a machine shop when you're doing your rough cuts. The faster you can take off the excess material, the faster the part is done. Time is money. You want as much material taken off as fast as possible, with as few insert changes, with as little fuss or muss. That's what carbide does and does well when you choose the right insert family and tooling for your machine, material and situation. An example of "right tool for the material" is that four-flute carbide end mills actually slow down removal rates in aluminum. In aluminum, you want to use a 2 or 3 flute end mill, so you can clear the chips faster.

OK, so where does HSS tooling still have a place? As I see it, HSS still works well in several situations, even in a big shop where they're pushing their machines and tooling hard:

1. Custom, one-off tools that you need "now" - not when the carbide tooling sales rep gets to your shop next week. A machinist who knows how to grind a custom HSS tool can get the job done today. The job might not get done as quickly as if they had the correct carbide tooling, but it gets done - today. That matters.

2. When you want to take very light cuts - like less than 0.002" depth of cut. Many carbide inserts have geometries that make very light cuts a problem. Unless the insert is put into the work deep enough, the insert might not cut; instead, it will tend to rub and burnish the workpiece. With HSS tooling, you can make a tool sharp enough to shave with, and on some materials this will allow you to take a 0.0005" (or less) DOC and actually cut, not rub, assuming your machine is tight enough to pull this off.

3. Types of interrupted cut machining. Some carbides don't tolerate interrupted cuts well or at all.

In a home machining shop situation, you're probably not going to invest huge money in carbide tooling - just enough to get by. There will be some times that you won't want to invest more money in carbide tooling for a particular material or a particular job. And then the HSS comes out again.

I use carbide most of the time, but there's plenty of HSS tooling in my shop, especially on my lathe.

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Originally Posted by donf
Yes I messed up the end mills some how, maybe a combo of both speed and rubbing on rusty scale. It was fine when I was running on the saw cut part but after a few passes on the scale that was it for both endmills. This thread grew so I think most people missed the post where I did one side with a fly cutter and HSS bit. It was slow, I had to resharpen the bit and anything more than a skim cut would result in Whomp, Whomp so definitely not for taking off material. Its still the side with the best surface finish though. https://i.ebayimg.com/images/g/5k4AA...IU/s-l1600.jpg
By my reckoning (watching students make mistakes), over half of the reason why tooling fails is the RPM is too high (on either a mill or a lathe). I've seen students melt carbide tooling. Some people say "carbide won't melt." I have seen small boxes filled with evidence to the contrary that have been sent out to carbide recyclers. Most of the time, people won't "feel" or "hear" a RPM that is too high, until the tooling fails. What they'll see early on are chips that come off blue or only slightly blue. Many people don't understand that steel, above about 650 degrees F, goes from blue back to a light silver-blue. Someone who is pushing the RPM very, very hard might not see a "blue chip" which will happen from about 540 to 600F.

Feeds too fast will often result in machine vibration and bucking more quickly. These signs, most people understand: "that machine ain't happy."

If your end mills were expensive, you can get them re-sharpened. The geometry will be correct, the mills will be sharp, they'll just be under-dimension.

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Originally Posted by wyop
By my reckoning (watching students make mistakes), over half of the reason why tooling fails is the RPM is too high (on either a mill or a lathe). I've seen students melt carbide tooling. Some people say "carbide won't melt." I have seen small boxes filled with evidence to the contrary that have been sent out to carbide recyclers. Most of the time, people won't "feel" or "hear" a RPM that is too high, until the tooling fails. What they'll see early on are chips that come off blue or only slightly blue. Many people don't understand that steel, above about 650 degrees F, goes from blue back to a light silver-blue. Someone who is pushing the RPM very, very hard might not see a "blue chip" which will happen from about 540 to 600F.

Feeds too fast will often result in machine vibration and bucking more quickly. These signs, most people understand: "that machine ain't happy."

If your end mills were expensive, you can get them re-sharpened. The geometry will be correct, the mills will be sharp, they'll just be under-dimension.
Thanks for the feed back and tips. From the speed calculator it looks like I was at 196 SFM. So that was not good. The chips were not blue or past blue. I was taking .010 to .015 cuts feeding by hand, then I turned on the power feed still I feeding slow and cutting both ways. I was operating the mill in a short sleeve shirt. I don't know much, but I already know what a hot chip burns like and am not in that big of a hurry to warrant standing there getting hit by smoking chips. After thinking about it, I really think the light cuts on the rusty mill scale that did them both in or at least helped the process along. The metal is from the scrap yard and has set out for a long time. Either way the ends of the mills are done and I will not guess on speeds again. Its not a big loss both were used off eBay and under \$25 each. The reason I purchased them was for cleaning up tubing cuts by sizing the OD of the end mill to the OD of the tube. Since I was only just touching the tip, I still think they can be used to improve the fit tube on occasion. When I fly cut I slowed it down in low range and fed by power feed set to #1. That was the best finish but was really slow even for hobby standards. I had to resharpen the HSS bit even though I just sharpened it. I had no more trouble once the scale was gone. I still have a few more sections of the metal to mess around with, but the tips here gave me some ideas for next time.

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