MachIVshooter
Member
Since there seems to be a great deal of interest in 80% lowers lately, I thought I'd take a minute to share what I know.
Anyone who has followed some of my projects is aware that I know a little bit about machining, but even in those threads, the information regarding what one might need to get started is scattered at best. My aim here is to help some of our members who want to go a little further than using drill presses or routers, but are lacking the space or funds to get set up with a full size knee mill.
This thread will focus on the Seig X2, also sold as the Grizzly mini mill, Harbor Freight mini mill and Little Machine Shop (LMS) mini mill. They are essentially the same item, although there are a couple variations. These machines are relatively inexpensive, have a small foot print, and at roughly 100 lbs, are pretty portable as machine tools go.
Harbor Freight is generally going to be the cheapest, running ~$600 before any coupons, but with their 20% or occasional 25% off coupons, you can score one for under $500. LMS and Seig offer variants that use a brushless DC motor, as well as rigid column models. Additionally, LMS has a large table option, which may be of interest to those who anticipate doing slightly larger projects. It is my suggestion that you avoid the solid column models; there are angle cuts, such as serrations, that are much, much more difficult to make if you have to tilt the work piece rather than the mill head.
Anyway, moving on. Most of these machines come equipped with a standard Jacob's drill chuck, which is fine for drilling. To use them as a mill, though, you'll want to run collets. Fortunately, these mini mills use standard R8 collets, which can be had pretty cheaply, such as this 13 piece set. Collets not only hold the cutter more securely, but also reduce the distance that the cutter extends below the quill, further increasing rigidity. On these smaller machines, you need as much as you can get.
On to measuring. These mills have decimal inch marks on the handles, as well as for the fine adjustment on the quill. However, to use mechanical marks, you have to compensate for backlash, which is a headache, and also have to count revolutions. Further compounding the issue is the fact that these mini mills travel 0.0625" per revolution, an odd number indeed. While you can do work this way, you will be far better off spending $100 or so on a set of digital read outs (DROs). Spend more if you can, but the IGaging DRO's honestly work just fine, and I've found that they generally will hold within .001". These 12" scales will work for all 3 axis on the standard mini mill, although you'll need a 24" if you opt for the LMS large table. Installing them is a little bit of work, does require drilling & tapping on the mill (and building a tower to mount Z axis). Plan to spend 2-3 hours mounting the DROs. On my old mini, I set them up in what I felt was the most logical arrangement:
X on the front of the table, Y on the left side of the base (if you're facing the mill), and Z extending up on the left side of the column using a piece of 1/4"x1-1/2" flat bar, and a bracket for the head. #10-32 screws are my preference for mounting, and you will need some spacers to get things where they need to be.
Once you have DROs, everything gets much, much simpler. You will not need a jig for your lowers at all, and your accuracy will be just as good as a CNC'd factory lower if you do your part.
Securing the work piece: You have some options here, but a decent machine vise is the obvious choice. A tilt/swivel as seen on mine has it's benefits, but also detractors; namely rigidity, as it sits pretty high up on the very small table, amplifying movement greatly. A 5" swivel vise probably makes the most sense. 2 axis angle setting can still be done with angle blocks. Your other option for securing work is clamp blocks, but they're kind of a pain to use, and you'll have to indicate everything square each and every time. With a decent vise, once the vise is square, anything that sits square in the vise will also be square.
As for getting things squared up, there is only one right way to do it: a dial indicator. Rather than explain how these are used to square the vise or work piece, I'll defer you to youtube. If you find that the vise does not sit square to the bed, you may need shim stock. A little trick for swivel vises: once you've indicated the jaws exactly, drill a 1/8" hole right where the vise meets the swivel base, and use a 1/8" pin (I like broken 1/8" end mills) in the hole. If you need to hold really, really close tolerances, you should indicate every time, but if the project can be off a thou or so per inch, this is much more accurate and repeatable than trying to eyeball the marks on the base.
Another indispensable item is going to be a smallish deadblow hammer, such as this one, to move work pieces around in the vise or under clamp blocks in small increments.
Tooling specific to the 80% AR lower is:
7/16" end mill
5/16" end mill
5/32" twist drill
3/8" twist drill
I would also highly recommend centering drills, both for starting your holes and for chamfering the finished holes.
Personally, I prefer to use a "U" drill (0.368") on the selector and a #24 drill (0.152") on the trigger and hammer pin holes, then run .3751" and .1555" chucking reamers to finish the holes, but those reamers don't come cheap, and being a couple thou over using standard drill bits honestly won't hurt a thing.
Those items are about all you'll need for AR lowers, but if you've bought a mill, chances are you'll find other projects. You will need some addional tooling for these other things, such as:
calipers
micrometers
vise parallels
angle blocks
1-2-3 blocks
rotary table
rotary indexer
many, many more cutters!
When it comes to actually machining your lower, just remember to measure, measure again, do a dry run, then check a couple more times before you actually start cutting. You really don't need lubricants or coolants to cut 7075-T6. The end mill I listed is not center cutting, so you would want to first drill a hole for that one, but there are center cutting end mills that will work like a twist drill. Otherwise, just take your time, and be sure to keep the gibs tight (lock them on whichever axis you're not moving). It would probably benefit any inexperienced persons to first watch some YT videos of machining aluminum to pick up some tips and figure out how to set speeds, feeds, depth of cut (DOC), etc.
Anyone who has followed some of my projects is aware that I know a little bit about machining, but even in those threads, the information regarding what one might need to get started is scattered at best. My aim here is to help some of our members who want to go a little further than using drill presses or routers, but are lacking the space or funds to get set up with a full size knee mill.
This thread will focus on the Seig X2, also sold as the Grizzly mini mill, Harbor Freight mini mill and Little Machine Shop (LMS) mini mill. They are essentially the same item, although there are a couple variations. These machines are relatively inexpensive, have a small foot print, and at roughly 100 lbs, are pretty portable as machine tools go.
Harbor Freight is generally going to be the cheapest, running ~$600 before any coupons, but with their 20% or occasional 25% off coupons, you can score one for under $500. LMS and Seig offer variants that use a brushless DC motor, as well as rigid column models. Additionally, LMS has a large table option, which may be of interest to those who anticipate doing slightly larger projects. It is my suggestion that you avoid the solid column models; there are angle cuts, such as serrations, that are much, much more difficult to make if you have to tilt the work piece rather than the mill head.
Anyway, moving on. Most of these machines come equipped with a standard Jacob's drill chuck, which is fine for drilling. To use them as a mill, though, you'll want to run collets. Fortunately, these mini mills use standard R8 collets, which can be had pretty cheaply, such as this 13 piece set. Collets not only hold the cutter more securely, but also reduce the distance that the cutter extends below the quill, further increasing rigidity. On these smaller machines, you need as much as you can get.
On to measuring. These mills have decimal inch marks on the handles, as well as for the fine adjustment on the quill. However, to use mechanical marks, you have to compensate for backlash, which is a headache, and also have to count revolutions. Further compounding the issue is the fact that these mini mills travel 0.0625" per revolution, an odd number indeed. While you can do work this way, you will be far better off spending $100 or so on a set of digital read outs (DROs). Spend more if you can, but the IGaging DRO's honestly work just fine, and I've found that they generally will hold within .001". These 12" scales will work for all 3 axis on the standard mini mill, although you'll need a 24" if you opt for the LMS large table. Installing them is a little bit of work, does require drilling & tapping on the mill (and building a tower to mount Z axis). Plan to spend 2-3 hours mounting the DROs. On my old mini, I set them up in what I felt was the most logical arrangement:
X on the front of the table, Y on the left side of the base (if you're facing the mill), and Z extending up on the left side of the column using a piece of 1/4"x1-1/2" flat bar, and a bracket for the head. #10-32 screws are my preference for mounting, and you will need some spacers to get things where they need to be.
Once you have DROs, everything gets much, much simpler. You will not need a jig for your lowers at all, and your accuracy will be just as good as a CNC'd factory lower if you do your part.
Securing the work piece: You have some options here, but a decent machine vise is the obvious choice. A tilt/swivel as seen on mine has it's benefits, but also detractors; namely rigidity, as it sits pretty high up on the very small table, amplifying movement greatly. A 5" swivel vise probably makes the most sense. 2 axis angle setting can still be done with angle blocks. Your other option for securing work is clamp blocks, but they're kind of a pain to use, and you'll have to indicate everything square each and every time. With a decent vise, once the vise is square, anything that sits square in the vise will also be square.
As for getting things squared up, there is only one right way to do it: a dial indicator. Rather than explain how these are used to square the vise or work piece, I'll defer you to youtube. If you find that the vise does not sit square to the bed, you may need shim stock. A little trick for swivel vises: once you've indicated the jaws exactly, drill a 1/8" hole right where the vise meets the swivel base, and use a 1/8" pin (I like broken 1/8" end mills) in the hole. If you need to hold really, really close tolerances, you should indicate every time, but if the project can be off a thou or so per inch, this is much more accurate and repeatable than trying to eyeball the marks on the base.
Another indispensable item is going to be a smallish deadblow hammer, such as this one, to move work pieces around in the vise or under clamp blocks in small increments.
Tooling specific to the 80% AR lower is:
7/16" end mill
5/16" end mill
5/32" twist drill
3/8" twist drill
I would also highly recommend centering drills, both for starting your holes and for chamfering the finished holes.
Personally, I prefer to use a "U" drill (0.368") on the selector and a #24 drill (0.152") on the trigger and hammer pin holes, then run .3751" and .1555" chucking reamers to finish the holes, but those reamers don't come cheap, and being a couple thou over using standard drill bits honestly won't hurt a thing.
Those items are about all you'll need for AR lowers, but if you've bought a mill, chances are you'll find other projects. You will need some addional tooling for these other things, such as:
calipers
micrometers
vise parallels
angle blocks
1-2-3 blocks
rotary table
rotary indexer
many, many more cutters!
When it comes to actually machining your lower, just remember to measure, measure again, do a dry run, then check a couple more times before you actually start cutting. You really don't need lubricants or coolants to cut 7075-T6. The end mill I listed is not center cutting, so you would want to first drill a hole for that one, but there are center cutting end mills that will work like a twist drill. Otherwise, just take your time, and be sure to keep the gibs tight (lock them on whichever axis you're not moving). It would probably benefit any inexperienced persons to first watch some YT videos of machining aluminum to pick up some tips and figure out how to set speeds, feeds, depth of cut (DOC), etc.
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