Chip Load Calculator for Steel Milling

When a carbide end mill rubs steel at an improper speed, there’s a certain sound. It is a high pitched squeal which typically indicate a lack of cut and more of a rub. Instead of the chip being a crisp curl it becomes a blue, burnt ribbon. Tool life drop to near zero in a matter of seconds.

Steel isn’t about getting to some magic number; it’s about balancing mechanical load with heat and getting the cutter working, not melting. Typically folks begin with a number of feet per minute on surface, and that number depend somewhat on what kind of steel you’re cutting. Hardened alloys require much greater care. Stainless requires it too. But mild steels such as A36 or 1018 can stand up to higher speed.

How to Mill Steel Properly

The calculator does the calculation for you. It takes the rough rules of thumb (surface speed) and turns them into RPMs (revolutions per minute). These RPMs takes into account diameter of your specific tool. Remember, a two inch cutter won’t spin at the same speed as a half inch cutter in order to get the same amount of material removed from the surface. Guessing without the diameter is just that, guessing.

The problem begins here: Feed rate. You must remove a sufficient amount of material per tooth while also shearing steel cleanly. With too little feed, the tool grind on edge of metal. This causes extreme heat buildup and works hardening of the material. This is especially critical with stainless steel. Stainless steel loves work hardening. It’ll wreak havoc on a slow moving or dull cutter in no time flat.

The trick here is that the cutter determine how fast to move based off your spindle speed, number of flutes, and desired chip load. Sounds easy until you’re doing light side cuts. If your radial engagement is less than half of the cutter diameter, what will happen? Even if your programmed feed remains unchanged, the chip becomes thinner. This phenomenon is called chip thinning and it will kill your productivity. You’re thinking you’re cutting with a safe chip load, but in reality, the chip is microscopic. More rubbing occurs which result in premature wear.

Here’s where the software kicks in: It automatically corrects for this. It adds a correction factor to increase the needed feed rate so that the chip remains thick enough to cut cleanly. It is a small detail, but it make all the difference for tool life.

You also need to consider the machine itself. A beefy vertical machining center can runs those aggressive parameters without issue, but a lightweight hobby mill will be rattled to bits. This calculator gives you the option of considering your machine rigidity as well as stickout on tools and whether you are using coolant or not. High pressure mist or even straight flood will push things out just a bit further. It helps not only with chip removal but also with removing heat from the tool. Long tool holders and dry air blasts requires a more conservative approach. This is to avoid deflection and chatter.

On that page there’s a reference table that lays out starting points for all those different steel grades. It provides a starting point for the speed and hardness. But it doesn’t factor in a slight bend in the work piece or a spindle that has seen better days.

The calculator is your place to start with your parameters, but listen to the machine as well. A good cut will sound like a smooth steady roar. A bad one sounds more like a choppy stutter or just plain whine. Reduce the radial depth of cut before dropping the feed rate if you’re hearing the latter. It is often better to keep the chip thick then to slow the feed rate. You risk burning the edge.

All-in-all, milling steel comes down to engagement, speed and feed. You need enough engagement while removing material fast but not too fast as to burn up the tool. By knowing how each of these factors relate to one another, you go from guessing to controlling milling steel. It’s all about getting clean chips, cool tools and smooth surfaces. So don’t let it hit that high pitch squeal, and typically everything else will fall in line.