Slot Cutter Vs End Mill

4/4/20224/4/2022

Machining is one type of manufacturing process where excess material is removed from the workpiece to impart intended shape, size and finish. It follows the principle of subtractive manufacturing approach to build 3-D components. Conventional machining processes employ a cutting tool that contain one or more shape edges, and layer by layer material is removed from the workpiece in the form of solid chips. In order to process a wide variety of materials in different ways, there exist several conventional machining processes, namely straight turning, taper turning, internal turning, facing, threading, grooving, knurling, chamfering, face milling, peripheral milling, end milling, shaping, drilling, boring, reaming, broaching, etc. Each of these processes employs a specific cutting tool and can produce specific type of feature. Drilling is performed to originate a hole on solid surface using a cutting tool called drill. End milling is performed to cut features like slot, walls, fins, columns, webs, etc. The cutting tool employed in end milling operation is called end-mill. A drill is somewhat similar (apparently) to an end-mill. However, there exist several differences between a drill and an end mill.

The cutter diameter is the dimension that will define the width of a slot, formed by the cutting edges of the tool as it rotates. Selecting a cutter diameter that is the wrong size – either too large or small – can lead to the job not being completed successfully or a final part not being to specifications. A slot drill is just a 2-flute end mill used for cutting a slot, as per the name, more clearance for the chips to come out of the back without clogging then, there'll be one flute cuts over centre so it can be used to plunge cut small distances too, to start slots in the middle of material.

Metal cutting drills (twist drills) typically have two cutting edges that are located only at the bottom cone of the drill body. There exists no cutting edge at the periphery of the drill. So entire cutting operation is performed by the bottom face, and thus feed is always given in axial direction to make a hole. It is not possible to give feed in any direction other than drill axis. Thus drills are suitable for vertical plunging only. On the contrary, end mills have main cutting edges located on the periphery of the tool body and auxiliary cutting edges located at the bottom of the tool. An end milling can contain up to 16 cutting edges. For flat end mills, ideally the feed can be given in a direction perpendicular to the tool axis. However, for rounded-corner end mills or ball end mills, small feed can also be given in axial direction. Thus, end mills are not suitable for only plunging, rather they are suitable for peripheral cutting with a small amount of plunging. Furthermore, all cutting edges present on the drill body remain in continuous contact with the workpiece during entire operation. End milling is one intermittent cutting process where each cutting edge simultaneously engages and disengages with workpiece in every rotation of the tool. The engagement angle, however, depends on radial immersion. Several similarities and differences between drill and end mill are provided below in table format.

Both are multi-point cutters as more than one active cutting edges participate in material removal action. It is worth noting that micro-end mills (tool diameter is about 10-50 μm) may also have only one cutting edge; however, such tools are not commonly used.
In both the cases, material removal takes place in the form of solid chips.
In both the cases, the cutting velocity is provided through the rotation motion of the cutting tool (drill or end mill). Such velocity is also proportional to the tool diameter and rotation speed.
Due to physical contact between tool and workpiece, several problems such as tool wear, residual stress, heat generation, etc. are encountered in both the operations.

Drill (Drilling)	End Mill (End Milling)
Drill is the cutting tool used for drilling operation. Drilling is one machining process used to originate a hole on a solid surface.	End mill is the cutting tool used for end milling operation. End milling is a machining process used to cut features like slots, channels, walls, fins, columns, webs, free form surfaces, etc.
The cutting edges on a drill are located on the tapered (conical) bottom of the cutter.	Main cutting edges on an end mill are located at the periphery of the cutter. Auxiliary cutting edges are present at the bottom.
Metal cutting drills (twist drills) typically have two cutting edges. However, wood cutting drills can have 3 or 4 cutting edges.	End mills can have a wide variety of cutting edges (or flutes), ranging from just 1 (in micro end mills) to as high as 16.
Feed direction for drilling is always axial to the cutter.	Feed direction in end milling with flat-end mill is usually perpendicular to the cutter. With ball end mill, feed can be given at any angle to the cutter axis.
Drills are suitable for vertical plunging, but cannot cut in side direction.	End mills are particularly suitable for peripheral cutting. However, small plunging is permitted in ball end milling.
Drill cannot generate plain surface. It generates one cylindrical surface at periphery and one conical surface at bottom (in blind hole drilling only).	End milling can generate plain surfaces. With flat end mills, one flat surface is generated at bottom and two vertical flat surfaces are generated at two sides.
Conventional drill cutters are mostly made of HSS (High Speed Steel).	End mills are commonly made of WC (Tungsten Carbide), coated or uncoated, insert type or plain.
Drills can produce features (holes) of high aspect ratio (hole axial length to hole diameter), even up to 10:1.	End milling is commonly not used when aspect ratio exceeds 2:1.
Usually low rotational speed is used in drilling as high rpm of the drill can create problem in chip evacuation and can lead to breakage of the drill.	End milling can be carried out at high rotational speed of the cutter.
During drilling, all cutting edges of the drill remains in continuous contact with workpiece. Thus all cutting edges cut material throughout the entire drilling time.	End milling is one intermittent cutting process. In every rotation of the cutter, each cutting edge simultaneously engages and disengages with workpiece. So each cutting edge cuts material from workpiece only for a portion of time for every rotation of the tool.
Uncut chip thickness for each cutting edge remains constant during entire drilling operation.	In end milling, the uncut chip thickness does not remain constant, rather varies in between zero and a maximum value during the engagement period in every rotation for each cutting edge.
In drilling, feed motion can be imparted either by moving the tool or by moving the workpiece.	In end milling, feed motion is usually imparted by moving the workpiece.
Drilling can generate long continuous chip as cutting edges remain in continuous contact with workpiece.	End milling inherently generates discontinuous chips owing to repeated engagement and disengagement of cutting edges with workpiece.

References

Machining and Machine Tools by A. B. Chattopadhyay (Wiley).
Saha et al. (2020); An analytical approach to assess the variation of lubricant supply to the cutting tool during MQL assisted high speed micromilling; Journal of Materials Processing Technology. https://doi.org/10.1016/j.jmatprotec.2020.116783
Saha et al. (2020); An investigation on the top burr formation during Minimum Quantity Lubrication (MQL) assisted micromilling of copper; Materials Today: Proceedings; https://doi.org/10.1016/j.matpr.2020.02.379

Pintu, 'Difference Between Drill and End Mill – Drilling vs End Milling', http://www.difference.minaprem.com/machining/difference-between-drill-and-end-mill-drilling-vs-end-milling/ (accessed on 17/12/2020).

MQL application in micro-milling
Burr formation in micro-milling
Laser surface re-melting process

Related differences:

I got a note recently from a G-Wizard user who wanted to know how to decide on best depth and width of cut when milling. It’s a great question. Most machinists, I suspect, use rules of thumb and habit more than anything else unless the situation dictates something in particular based on the dimensions of the feature being machined. They’re used to using some fraction of the cutter’s diameter or some figure that they got to some other way through habit (40 thousandths or some such is what they’ve always used). Perhaps their CAM program has a hardwired default that is a percentage of the cutter’s diameter.

But these values, while they have worked over time, are not necessarily optimal figures with respect to material removal rates, tool deflection allowances, or a host of other variables we might choose to consider. What’s a more systematic way to approach the problem?

First thing is we have two variables (width and depth of cut), so it’s hard to make progress unless we can nail one of the two variables down and focus on the relationship of the other. It’s usually pretty easy to nail down one of the variables based on the situation. Let’s divide our work into two categories:

– Slotting: I’ll generalize this to be any situation where the material to be removed is very close to the cutter’s diameter. It may be a slot, or it may involve interpolating a hole or pocket that’s only a little bit larger than the endmill’s diameter.

– Pocketing: Here again, I will generalize this to be any situation where the cutter’s diameter is quite a bit smaller than the dimensions of the material to be removed. That doesn’t mean there isn’t some inside radius or other feature that isn’t more like the slotting example, but for the most part, we have some room to work in. Note that profiling will be considered to be the same as pocketing for this discussion.

Okay, so now we have to take the task before us and decide whether it is closer to slotting or pocketing. The reason I’ve defined these two the way I have is that it informs our choice of which variable to work on first. If we are slotting, the cutting width is the first variable. If we are pocketing, the cutting depth is the first variable. Why?

When slotting, the feature is very close to the cutter’s diameter in size. We can’t take a 1/2″ endmill and use it to make a 1/4″ slot. In general, we want to use the largest diameter endmill that fits the feature, and then we pretty much have to make at least one cut that is full width. Once we’re cleared that cut, anything remaining is handled the way we would under pocketing. So, when slotting, we focus first on cut width and make that the cutter’s width to get started.

When pocketing, our limitation will be the smallest inside radius we have to deal with as well as the depth of the pocket. Remember, it may be advantageous to make two passes. The first with a cutter that has a diameter too large for the smallest inside radii we have to deal with. That’s a roughing pass that uses a larger cutter just to get done faster. The second pass is a finishing pass, and must use a cutter whose diameter is less than or equal to that required to reach into the smallest internal radius the pocket holds. Note that we can go around an outside radius (a boss) with any diameter cutter, it is the inside radius that limits us.

So, we pick a cutter that is either as big as the smallest radius, or we choose to go two passes and go with a larger cutter. Let’s leave the two pass issue aside for the moment, because figuring out when that is optimal can take a bit of trial and error. Its similar to think of one pass. Given that the cutter is chosen, we can choose just about any width of cut we want. So how do we nail down a variable when pocketing? On the slotting case, I like to nail down cut width. On the pocketing case, I prefer to nail down cut depth.

In general, we get a nicer finish if we cut the pocket in as few layers as possible. CAM programs are good at layering down into the pocket, so we can pick arbitrary depths of cut. If I can, I like to do it in one layer for a pocket that isn’t two deep. If not, I prefer the depths of the layers to be equal. In other words, I wouldn’t go down 1/4″, 1/4″, and then 0.19″ on the third layer. So pick a layer depth that satisfies that criterion.

Now, in both cases we have locked one of the two variables–slotting locks width, pocketing locks depth. We need to determine the best value for the variable we left floating based on the value of the one we locked. This is where the G-Wizard Cut Optimizer makes it easy. Enter the values you know for the cut and let the Optimizer figure the value for the floating variable.

For example, let’s suppose we need to cut a pocket that is 3/4″ deep in 6061 aluminum. The smallest internal radius is 1/4″, so we’ve decided to use a 1/4″ 3 flute carbide endmill. Here is the problem set up in G-Wizard:

Material, Tool, Tool Diameter, Flutes, and 3/4″ Cut Depth Entered…

Now we can invoke the Cut Optimizer just by pressing the “Rough” button:

As you can see from the red arrows I added, for a 3/4″ depth of cut, this endmill can handle no more than 0.1799″ width of cut when roughing. Let’s round that down and go with 0.170″

Press the finish button to see what sort of finish allowance we should have the CAM leave for our finish path and we get 0.0052″. That’s a pretty light pass, but 3/4″ is deep for this 1/4″ endmill. Here’s an interesting thought: if we reduce tool holder to tip length to 0.9″ instead of 1″, we can increase that cut to 0.0095″. That gives you an idea of how important it is to keep the tool stick out as little as possible. I’d be inclined to go with choking up on the tool and a finish width of cut of 0.009″ were this my job. The other thing to consider is two levels of finish pass. If we don’t mind taking two levels and still choke up on the tool, we can get 0.015″ width of cut for the finish. That’s about as much as I like to take on a finish pass.

The problem with this cut is its a little bit deep for our 1/4″ endmill. That’s a 3:1 ratio of diameter to depth. We can tell its straining because the max recommended widths of cut are so light. If I had a CAM program that made it easy to make the roughing pass with a bigger cutter, I would be tempted to jump in with a 1/2″ endmill (or maybe even larger) for the roughing pass and then go to the 1/4″ for finishing, but you get the idea.

The slotting case is pretty similar, except for that case, instead of trying to compute the width of cut, we want to use the optimizer to figure out the depth. For example, if we continue with our 1/4″ 3 flute, let’s say we need to cut a slot 0.300″ wide to a depth of 3/4″. Our plan is to cut a full slot 0.250″ wide down the middle, and then finish it up by cutting the remainder on each side. How deep can we make our full slot passes? Once again, dial up the initial parameters, and this time, hit the “Slot” button. For roughing, the Cut Optimizer tells us we can cut to a depth of 0.3466″ before we get too much deflection. Two passes at that depth will get us to 0.6932″ deep. That leaves 0.0568″ on the bottom for us to finish and 0.0259″ on either side for the finish pass. Remember, we’re not cutting a full slot for the finish pass, so we treat it just like we did our pocket to figure out the width and depth of cut.

That’s all there is to it. To summarize:

1. Decide whether you are slotting or pocketing.

2. When slotting, pick a value for width, and use Cut Optimizer to decide depth.

3. When pocketing, pick a value for depth, and use Cut Optimizer to decide the width.

If you approach the problem this way, you’ll maximize your MRR’s while minimizing your tool deflection as appropriate for either roughing or finishing. That’s a much more optimal approach than the old wet finger in the wind!

For more thoughts on cutting parameters when milling, check out the Milling Surface Finish page.

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