Picked up an 80% lower for an AR-15 build? Want to know how to get that fire control cavity looking clean-cut, like it was milled by a manufacturer? We're covering how to safely and effectively finish your 80% lower receiver using a drill press like a mill. Most of your work can (and should) be completed with vertical drilling and cutting. We will focus on using our drill press for only light milling to clean up the receiver once most material has been removed.
Drill Press Diagram
Before we dive into machining a firearm, you should familiarize yourself with the drill press's components. More importantly, you should be aware of the most likely points of failure while machining. Let's start with the most important stuff.
The drill chuck holds your drill bits and end mill bits. On the side of the chuck is a small hole for a key. This key locks and unlocks the chuck, allowing bits to be seated or removed. Turning the key tightens the chuck, which clamps the bit into place.
Jacobs Taper #33 (JT33): This is the type of chuck your drill press probably comes with. It's the most common chuck found on entry-level drills at most hardware stores. The JT33 is a female tapered press-fit connection. That means it uses friction to hold the chuck to the machine.
The arbor is a long metal rod that connects the chuck to the quill, which is the rotating mechanism responsible for doing work via the motor. The arbor on a drill press is usually a tapered press-fit connection, meaning the chuck is held onto it with only friction. This is called a press fit and it's why using a drill press as a milling machine must be done carefully.
MT/JT33 Arbor: The most common arbor you'll find on a drill press is one with a male JT33 connection for the Jacobs chuck, and a male Morse Taper (MT1 or MT2) connection that holds the arbor inside the quill/spindle with another press-fit connection. Higher-end drill presses and milling machines may use a threaded connection and drawbar to hold the arbor inside the quill. This is a safer connection that can more easily accept side loads without risking the arbor and chuck falling out of the machine.
The quill, also called a spindle, is connected to the drill press's motor via a belt. As it spins up, it rotates the arbor and chuck to do work. The spindle's fasteners (one large bolt with washers) typically secure both it and the adjustment handle to the head.
Head, Motor, and Switch
The drill press's motor is an electric high-torque motor that drives the machine using belts inside the head. The head is little more than a plastic cover for protecting the belt and machinery inside. Simple drill presses operate at one speed and have only an ON/OFF switch. Mid-grade and high-end drills will also come with controls for adjusting RPM.
Column and Handle
The column supports the entire drill and connects the head to the base. The column is made of a solid outer tube with a solid inner tube that can be raised and lowered via the handle. This adjusts the depth of the drill relative to the table. You'll use the handle every time you drill to control the depth of the bit. The handle has measurements to indicate your bit's depth.
The drill press table holds your work. In the case of fabricating an 80% lower, the table holds your vise steady, which clamps your lower and jig together. The table's height can be adjusted to accommodate different work, but it stays static once you begin machining. The table has slots cut into it so you can bolt your vise in place.
The base holds the drill press to your workbench or floor. It also has slots cut into it for bolting the machine in place. This is required to ensure minimal vibration and runout while you work. These two factors play a bigger role than normal when machining a lower receiver, so be sure to secure the drill with the base before working.
Challenges of Milling with a Drill
There are two major concerns when side-loading a drill press's chuck and arbor by cutting like a mill:
- Creating excess load on the arbor, causing it to fall out.
- Creating excess load on the chuck, causing it to fall out.
Basically, side-loading the machine too much will cause lateral pressure and vibrations to overcome the tapered friction fit of the chuck and/or arbor. If either unit comes loose, it will fall from the drill while spinning. This can cause serious injury to you, and serious damage to your jig, 80% lower, and machine.
Best Practices / Tips
So, how do you minimize risk and get your drill press to do some light milling without falling apart or exploding?
1. Only Use Conventional Milling
This is one of the most important tips you can use to ensure you're milling with your drill as safely as possible. When you mill any work, you're not just placing 'side-to-side' forces on the end mill. You're also placing 'up' or 'down' forces on the bit as it spins and cuts into the work. Depending on the direction of the cut and feed of the work, these forces will try to push the bit up into the machine or pull it down and away.
Conventional/Up milling is accomplished when the direction of the cutters on the end mill move against the direction of the work being fed. This rotational force drives the bit up into the machine. This is critically important when trying to cut work with a tapered/press-fit chuck and arbor that could be pulled out of the machine.
Climb/Down milling is done when the end mill rotates in the same direction of the work's feed. This pulls the bit away from the machine, potentially separating the tapered connections between the chuck, arbor, and spindle.
Benefits of Up Milling
- The bit is pushed up into the machine, preserving tapered connections.
- Opposing forces push the work and vise down, enhancing stability on table.
2. Remove most aluminum with drilling
The best way to mill with a drill press is too, well, simply avoid it as much as possible. This sounds counter to everything we're saying but you'll come to find that using drill bits to remove most of the aluminum from your 80% lower is actually just as easy, and probably faster, than milling altogether. You can complete most of the lower's fire control cavity with drilling alone, as you can see below.
Once the receiver has reached this stage of fabrication, milling with a drill press becomes much safer, easier, and faster. This is because you're only removing very small amounts of material and not placing any substantial lateral loads on your drill press. Once finished with this "polish" milling, your receiver will look just as clean and squared away as any commercially fabricated AR-15 receiver:
3. Cut the floor of the receiver vertically
You can even complete the bottom of the receiver with vertical cuts, and you'll get the same results above. This can only be done with the right bits: Center-cut end mills can cut vertically like this. With a center-cut bit, the floor of the receiver can be plunge-cut perfectly flat without the need for side-to-side milling. Essentially, you'll use the end mill bit on your drill press exactly like a drill bit, plunging it down to the final depth to make a flat surface on the floor. The receiver above was completed with a particular 80% jig we discuss below.
4. Upgrade that chuck
If you bought a basic drill press from the local hardware store, the chuck it came with is probably inadequate for this operation. That's okay, because a decent upgrade will run you less than $50. It's a great safety measure to bulk up the chuck by grabbing one with a stronger lock. A keyless knurled chuck provides a better bit clamp and makes for easy tooling changes without the need for a key. Most importantly, an upgraded chuck will reduce runout and vibration, lowering the risk of having the chuck fall off the arbor while cutting.
5. Consider a threaded arbor
Even better, you can also upgrade your Morse-tapered arbor to eliminate one potential point of failure entirely. Some tool makers and lathe/mill manufacturers have developed affordable threaded arbors for drill presses. This replaces the press-fit JT33 connection between the chuck and arbor with a threaded connection. This effectively makes the arbor and chuck one unit, which will dramatically reduce runout and vibration.
6. Make shallow cuts
There are no hard and fast measurements here. Generally, you should maintain a shallow cutting depth (0.2" or less) when milling with your drill. A deeper cut invites more runout and chatter, increasing the risk of slipped tooling. Shallow cuts also extend the life of your bits and produce a cleaner finish on the work itself.
7. Go very slow while milling (low SFM)
If you're not familiar with machining, one of the most important factors in milling safely is SFM, or Surface Feet per Minute. This is how fast you're cutting the work. On a typical mill (which can spin at thousands of RPM) billet or forged aluminum can be milled quickly, around 280 to 300 feet per minute. When you're operating a much less stable drill press that's probably spinning at few hundred RPM, you must go slowly. It's recommended you cut about one foot per minute. That sounds too slow, but you're only milling a few inches' worth of aluminum on an 80% lower. Doing it safely won't take long.
Most of All, Get The Right Jig
Most of all, it's the jig you use with your drill press that'll make the most of your makeshift milling operation. Some jigs require more cutting than drilling, which obviously increases the inherent risks of machining. If you're using only a drill press, you want a jig that removes most of the aluminum from your 80% lower using drill bits and vertical machining. Let's look at some options that help accommodate 80% builders who use a drill press.
1. Broken Arms 80% Jig
The Broken Arms Jig is our best recommendation for 80% builder using a drill press, because it takes full advantage of our best practices above:
- It completes most of the 80% lower vertically, requiring only light milling to polish the work.
- It uses center-cut end mills, allowing you to make plunge cuts when cleaning up the receiver's floor.
- It uses open template guides, allowing your end mill bits to ride along the edge of the jig for stability.
That last part is important: The Broken Arms Jig's guides allow you to make clean, shallow cuts with "conventional/up" milling while keeping a close eye on your feed rate and cutting depth. The jig's plates guide the bit along the receiver's walls to make the final polishing cuts. If you want to complete most of your receiver with drill bits and avoid any and all milling save for light polishing, this is the jig for you.
2. Elite Builder Jig
The Elite Builder Jig is the option for builders who have either a drill press or a drill/mill machine. It works largely like the Broken Arms Jig and allows the builder to complete the 80% lower largely with vertical loads. The primary difference between the Broken Arms and Elite Builder are the cutting templates: The Elite Builder uses more conventional cutting guides and fewer pilot hole templates. This doesn't prohibit you from completing the receiver mostly with vertical holes and cuts.
Compared to Broken Arms, Elite Builder Jig may allow you to work more quickly. You have the choice of only drilling the first set of pilot holes before using conventional milling. Because of this, the Elite Builder's the better choice if you own a more robust drill press or drill/mill combo that's capable of both machining methods.
- A drill press isn't designed to mill, but lightly milling an 80% lower can be done safely.
- Completing the receiver requires only polishing the walls and floor with some "drill" milling.
- Your lower can be substantially finished using a drill press and vertically drilled holes and cuts.
- You should maintain a low feedrate (SFM) of 1 foot per minute when milling with your drill.
- Always use conventional or "up" milling to prevent the bit from falling out of the drill.
- Use plenty of machining oil and lubrication to keep the receiver and bit cool while cutting.
- Let the jig's guide plates stabilize and direct where the cutting bit goes, reducing chatter.
Ready to get started? Check out the guide "How to Complete an 80% Lower Receiver".