Cutting mild steel with a high speed spindle dry. Please advise.

[boydage]

I have a very rigid gantry CNC Router with a 2.2KW variable speed spindle. I want to have a go at making some money cutting mild steel with it as it was offered today. Can I have some advice please on where to start with regards to cutters, air supply, misting etc.

Has anyone had success doing this and wondering if it is a worthwhile project to pursue.

What about cutters? Any recommendations for dry cutting mill bits?

I hope to hear from someone I see a thread without any replies ha.

Regards from NZ
Boyd

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[pippin88]

Really is not the right tool for the job.

[boydage]

Really? Do you think so? What about using cutters like this one?

G8A02 - X-5070 Blue | Milling Tools Supplier | Cutwel Ltd

Or there are other ones I have seen that just ask for a good air supply.

Hey do you think you would mind expanding on your answer? I am not a newbie to my CNC router and have actually cut some mild steel once but it was dirty and hot chips were flying. I am keen to read your thoughts.

Regards
Boyd

[Louis_Cannell]

Cutting mild steel well relies more on Torque and Rigidity than most routers have, I can cut steel with my build but can cut steel far better on less than a 1/4 of the power and RPM on a really cheap converted micro mill. Its horses for courses my router can compete with the micro mill eating aluminium but can get no where close to it when milling mild steel, as a rule routers are less rigid and resonance flex and lack of torque come into play in a big way. If you can do the job then fair play I admire anyone earning a honest living your statement about hot chips is a pointer to fast on RPM and chips to hot sounds like one of problems I would expect. Calculate some feeds and speeds and run some tests good quality end mills are a must and generally come with recommendations. Small jobs I have done for myself have been fine but regular stress above a machines true capabilities would quickly cause problems and damage machine.

I am not trying to be negative just my limited experience and the fact my 2.2kw water cooled spindle performs poorly under 6000rpm and my mill tops out at 5000rpm which is in fact quite high for a mill.

[Hezz]

I have a very rigid gantry CNC Router with a 2.2KW variable speed spindle. I want to have a go at making some money cutting mild steel with it as it was offered today. Can I have some advice please on where to start with regards to cutters, air supply, misting etc.

Has anyone had success doing this and wondering if it is a worthwhile project to pursue.

What about cutters? Any recommendations for dry cutting mill bits?

I hope to hear from someone I see a thread without any replies ha.

Regards from NZ
Boyd

Generally cutting mild steel is almost always done with coolant and at modest speeds. You can certainly cut dry if you keep the speeds down and are not planning on doing a lot of cutting. Most routers are not designed for cutting steel in large quantities since they don't facilitate coolant and are significantly less rigid than a mill is. Heat is the biggest problem when cutting steel and the coolant is very important for longer cutting times to keep the cutting tool cool. You will waste a lot of money on tooling trying to do what you are talking about. But how fast is high speed. You can probably cut mild steel dry with a small end mill at 5000-6000 RPM but that isn't really considered a high speed spindle. You will need to do light cuts and have the feed rate dialed in just right. But you will probably destroy tools very fast doing this. They will probably only last 1/3-1/4th of their normal cutting life. since you will have to have a variable speed spindle to find the right RPM for the feed rate you are using you can dial this in to get the correct balance. This is very important as the chips needs to be the right size to reduce cutting forces as much as possible. If you are doing a small amount of mild steel look up the recommended speeds and feeds in your machinists handbook and start from there. These values usually assume a pretty rigid setup with coolant. so your speeds and feeds will most likely be less.

You have to understand the concept of chip load and feed rate and how it relates to cutting speed and RPM. These have to be calculated for an approximate starting place and then adjusted from there. You will want carbide cutting tools but they are expensive and need to be kept cool.

[konrad.nc]

Generally cutting mild steel is almost always done with coolant and at modest speeds. You can certainly cut dry if you keep the speeds down and are not planning on doing a lot of cutting. Most routers are not designed for cutting steel in large quantities since they don't facilitate coolant and are significantly less rigid than a mill is. Heat is the biggest problem when cutting steel and the coolant is very important for longer cutting times to keep the cutting tool cool. You will waste a lot of money on tooling trying to do what you are talking about. But how fast is high speed. You can probably cut mild steel dry with a small end mill at 5000-6000 RPM but that isn't really considered a high speed spindle. You will need to do light cuts and have the feed rate dialed in just right. But you will probably destroy tools very fast doing this. They will probably only last 1/3-1/4th of their normal cutting life. since you will have to have a variable speed spindle to find the right RPM for the feed rate you are using you can dial this in to get the correct balance. This is very important as the chips needs to be the right size to reduce cutting forces as much as possible. If you are doing a small amount of mild steel look up the recommended speeds and feeds in your machinists handbook and start from there. These values usually assume a pretty rigid setup with coolant. so your speeds and feeds will most likely be less.

You have to understand the concept of chip load and feed rate and how it relates to cutting speed and RPM. These have to be calculated for an approximate starting place and then adjusted from there. You will want carbide cutting tools but they are expensive and need to be kept cool.

cutting mild steel dry(air blow) all the time with HSM toolpaths. tool life is great.

try to do some test cuts with very small radial engagement, deep cuts and smaller cutters. Lets say 8mm or 6mm.
i run my small cutter on mill at 12k rpms. taking up to 20% stepover.
drop stepover to 5% and cranked up feed by 30-40%

[ger21]

[mactec54]

boydage

As you can see by the video it can be done, but be prepared, to replace the spindle Bearing on a regular basis, if you already have one of these spindles, pull down on the spindle, if it has not seized up, the spindle will float about .010 to .015,up and down, this is the nature of how these spindles can run at high speed, so using a end mill will pull the spindle down, this then beats the Bearings to death,because they are loose then with no preload, that is why any one that does this will have a lot of noise, in the video he was blaming the noise on other things, but in reality it was the Bearings beating around because the spindle preload can not overcome the downward force, from the cutter, these spindles are marginal even to mill aluminum, lots do it, but in reality these spindles are just not designed to machine metal

[Hezz]

boydage

As you can see by the video it can be done, but be prepared, to replace the spindle Bearing on a regular basis, if you already have one of these spindles, pull down on the spindle, if it has not seized up, the spindle will float about .010 to .015,up and down, this is the nature of how these spindles can run at high speed, so using a end mill will pull the spindle down, this then beats the Bearings to death,because they are loose then with no preload, that is why any one that does this will have a lot of noise, in the video he was blaming the noise on other things, but in reality it was the Bearings beating around because the spindle preload can not overcome the downward force, from the cutter, these spindles are marginal even to mill aluminum, lots do it, but in reality these spindles are just not designed to machine metal

This is really the issue. You can do it but your machine was not designed to hold up to that kind of harsh cutting environment. If you are planning to cut a large amount of material for several hours your machine will likely tear itself apart. Both the spindle and perhaps the frame. IF the frame does not have sufficient mass for vibration dampening the welds will eventually start to crack. You also need to setup at least the Air/mist coolant spray as shown in the video. It's almost as effective as flood coolant and in some case it works better because it moves the chips away from the cutting area better.

Also, these high speed spindles with very little preload should prabably be run for a while to heat them up before you start cutting steel. One of the reasons they have very little preload is because a high speed spindle will generate a lot of internal heat and expand a lot so the initial preload on the bearings needs to be loose. This reason alone might be the main reason that steels are cut at more moderate spindle speeds in most production environments. The spindles need higher preload on the bearings to retain rigidity through the machine. So this pretty much limits how much heat can be tolerated in the spindle assembly which limits RPM of the spindle on all but the most expensive and sophisticated machines with external spindle cooling.

If you really want to do a lot of this steel on your machine you might want to invest in a spindle that is more suited to doing steel.

[Hezz]

cutting mild steel dry(air blow) all the time with HSM toolpaths. tool life is great.

try to do some test cuts with very small radial engagement, deep cuts and smaller cutters. Lets say 8mm or 6mm.
i run my small cutter on mill at 12k rpms. taking up to 20% stepover.
drop stepover to 5% and cranked up feed by 30-40%

Ya, these are the kinds of things that will need to be done to retain decent tool life. Instead of doing like typical 50-70% step overs you can go to small step overs and up the cutting speeds.

[mactec54]

Also, these high speed spindles with very little preload should probably be run for a while to heat them up before you start cutting steel. One of the reasons they have very little preload is because a high speed spindle will generate a lot of internal heat and expand a lot so the initial preload on the bearings needs to be loose. This reason alone might be the main reason that steels are cut at more moderate spindle speeds in most production environments. The spindles need higher preload on the bearings to retain rigidity through the machine. So this pretty much limits how much heat can be tolerated in the spindle assembly which limits RPM of the spindle on all but the most expensive and sophisticated machines with external spindle cooling.

If you run one of these Spindles to warm it up first, then it would have less Preload on the Bearings,they are already water cooled, and don't have an internal heat problem, you obviously don't know anything about these spindles, and how they are designed and assembled

[Hezz]

If you run one of these Spindles to warm it up first, then it would have less Preload on the Bearings,they are already water cooled, and don't have an internal heat problem, you obviously don't know anything about these spindles, and how they are designed and assembled

You are right, I haven't looked at the exact design of these spindles. I was making an assumption on how many spindles are designed and ones that I have had experience with. I would need to look at an exploded diagram and get a firm grip on the exact design. But I think we can both agree that this spindle is not ideal for production steel cutting.

[mactec54]

You are right, I haven't looked at the exact design of these spindles. I was making an assumption on how many spindles are designed and ones that I have had experience with. I would need to look at an exploded diagram and get a firm grip on the exact design. But I think we can both agree that this spindle is not ideal for production steel cutting.

Any endmill used will try to pull the cutter down into the metal, even light cuts, this unseats the AC Bearings in the front of the spindle, allowing them to rattle around in the Bearing seat/race, and eventually destroying the Bearings, I have modified some to handle a high loading, which seem to be working ok, the other problem is they run out of torque at the low speeds that is needed to cut metal, even with torque boost to the max

They are just not designed to cut any type of metal, I have been repairing these spindles for some time, they are a $150 to $260 spindle, suitable for doing wood & plastic, although a lot have good success with doing aluminum, which is stretching the limits of these spindles, there purpose was meant to replace the standard hand held wood routers for CNC use, which have been very successful when used correctly

[Hezz]

Any endmill used will try to pull the cutter down into the metal, even light cuts, this unseats the AC Bearings in the front of the spindle, allowing them to rattle around in the Bearing seat/race, and eventually destroying the Bearings, I have modified some to handle a high loading, which seem to be working ok, the other problem is they run out of torque at the low speeds that is needed to cut metal, even with torque boost to the max

They are just not designed to cut any type of metal, I have been repairing these spindles for some time, they are a $150 to $260 spindle, suitable for doing wood & plastic, although a lot have good success with doing aluminum, which is stretching the limits of these spindles, there purpose was meant to replace the standard hand held wood routers for CNC use, which have been very successful when used correctly

Agree with you about the metal pulling the mill down into the work. In my spindle design which I haven't yet built I have two mirror image AC bearings which are captured at both ends at the front of the spindle and also a double set at the rear of the spindle. Both bearings are captured at both sides. One by the spindle and the other by the housing cap. When one bearing looses it's preload due to heat or the work pulling it down the other bearing loads up so they sort of balance each other. But I understand in less ambitious designs there are fewer bearings and simpler spindle geometry to reduce machining cost. So you can't really get this effect. Actually, I'm amazed at how well those 150-260 dollar spindles perform compared to using just a hand held router motor. It's good you have some work rebuilding those spindles and I'm sure you have gained some valuable insight in optimizing them for their intended purpose. They seem to work so well with wood and plastic and even to some limited extend with aluminum that some seem to think it's just a small jump to do steel.

[boydage]

I remember refurbishing gas turbine engines when I was with air nz long time ago. We used to install a stack of bearings, roller and ball together to hold the lateral and longitudinal loads of the compressor. And I still remember using gloves apparently the bearing on the end, the really hot one at 44krpm. Was NZ$40k to replace.

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