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Ruined Your 80 Lower? The Good News is You Aren’t Alone

Correcting mistakes while machining an 80 lower

So, you’ve started machining your 80 lower into a stripped AR-15 receiver and in the process, you made a rather catastrophic mistake. Maybe you pushed your end mill bit through the floor of the lower, maybe your drill bit went in canted and now your safety selector won’t do much good. Ruined your 80 lower? The good news is you aren’t alone! If you need some jig replacement parts or new tooling, you can find everything you need here. Now, here are some of the common mistakes we’ve all made when machining an 80 lower – and here’s how you can avoid them!

80 Lower Drilling and Tooling Mistakes

#1: Rough cuts, uneven surfaces

  • Set the right speed. To get a clean, precise cut on your 80 lower with a handheld router and end mill bit, you’ll need to mill your 80 lower using the highest allowable cutting speed. Most handheld routers operate at 2,000 to 30,000 RPM. We recommend machining at 25,000 RPM and adjusting as necessary.
  • Reduce speed to reduce chatter. Running your tooling too quickly can cause chatter. Chatter occurs when your end mill bit or drill bit creates vibration while cutting. Chatter will cause noticeable, rough marks to appear. Reduce your tooling speed to eliminate chatter.
  • Reduce your feed rate to reduce heat and runout. Runout occurs when a tool is bent and deflected from its intended cutting surface. If you’re experiencing excessive heat or runout while cutting, reduce how quickly you push your end mill bit through your lower while you cut.

The Porter Cable 6430 Handheld Router - Blog Photo 1

Not all routers are created equal. If you’re experiencing issues with getting your end mill bits and router to play nicely, consider trying a router designed for this type of work. We recommend the Porter Cable 6430 Handheld Router. Most jig manufacturers use this router when designing their 80% jigs!

#2: Misaligned 80 lower holes or cuts

  • Get a drill vise. Most builders will use a handheld drill to makes the holes necessary for completing an 80 lower. A handheld drill is naturally imprecise and without the right setup, your holes will be mis-aligned. A drill press vise is strongly recommended to ensure your 80 lower’s properly seated so you can drill correctly aligned holes. If available, a drill press will also guarantee alignment better than a handheld drill.

80% Lower Receiver Jig (Drill Press) Replacement Parts - Blog Photo 2

  • Tighten your guide plates and collets. If your end mill bit or drill bit is cutting imprecisely, ensure the guide plates included with your 80 lower’s jig are properly secured to the jig and router. You should also periodically check that your drill’s and router’s collets (the steel sleeves holding your bits) are tight. Ensure the bits are flush inside the sleeves, too.

80 Lower Milling Mistakes

#3: Too little metal removal

  • Increase speed. Running an end mill bit too slowly can also cause problems, like premature tool failure or heat build-up. This will also cause rough cuts and a poor finish. If you find milling is taking too long per pass, increase your tooling speed.
  • Check the health of your tooling. End mill and drill bits quickly wear out while finishing an 80 lower. Cheap tooling from the local hardware store can even fail before you’ve finished your lower. You should only use the bits provided with your 80% jig.

#4: Too much metal removal

  • Check your depths. Ensure your end mill bit’s depth is set properly using the depth measurement guides provided with your 80% jig. If you’re cutting too quickly, make shorter passes until you’ve reached the appropriate depth. If your depths are correct, reduce your tooling speed until the desired rate of removal is achieved. Completing your final cuts with a shallow tool depth and high rate of speed will also provide a cleaner, smoother surface.

#5: Premature 80 lower bit failure

80 Percent Lower Easy Jig GEN 1 Tooling - Blog Photo 2

  • Double-check everything above. Heat, runout, excessive metal removal, too little metal removal, cutting depths, poor tool alignment, and all the other concerns described above will ultimately lead to tooling failure. Keep a close eye on your collets, feed rate, and cutting speed to prevent premature bit failure. We recommend investing in multiple end mill bits specifically designed for 80% lower machining, like Easy Jig’s End Mill Bit 3-Pack.
  • Use machining lubricant. We also strongly recommend cleaning your lower while your drill and cut. Removing debris and using a proper lubricating fluid while working will dramatically reduce heat buildup and increase tool life and cutting precision. Do not use WD-40 or PB Blaster as machining lubricant.

DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At AR-15LowerReceivers, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.


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