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cmjohnson
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Posted: 5/12/2012 11:57:54 PM

THE IMAGE ABOVE IS A PAID ADVERTISEMENT
I finally got off my ass and started a new lower receiver machining project.

It's a zero percent project, starting with raw forgings and the AR lower blueprint that everybody has.

I'd forgotten how frustrating that print is. A lot of important features are NOT dimensioned off of a single specific reference point as they should be.


Has anyone taken the time to redraw that print so that every feature is dimensioned from one reference point? If so, tell me where to get it.

There are features on this print that you have to really look at, and make allowances for different reference points, and it's actually starting to
be a real pain in my ass.

Why can't it be as simple as dimensioning everything off of the rear takedown pin hole? A lot of the features do use this as a reference point,
but others reference off the other end of the receiver or you have to add the distance from the reference point to this hole center, and then subtract
the distance to another hole center, and stand on your head and stick your tongue out while closing one eye for it to all come together.


So, if someone has come up with a simplified print that uses fewer reference points, please let me know.


FYI, I'm using a full sized Bridgeport milling machine clone with quick change tooling and a four axis DRO system. It's been tweaked and tuned and
gives dimensional accuracy of better than .0005" over an 8 inch span. Good enough for a manual mill!


CJ

"Now they will know why they are afraid of the dark.
Now they will learn why they fear the night."....Thulsa Doom

"How are your eyes adjusting to light? It must have been dark under that rock. "..Mosin_nagant
DL1911
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Posted: 5/13/2012 1:50:56 AM
Try taking a look here: Ray-Vin - Zero to Complete Lower
Schleprock
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Posted: 5/13/2012 11:38:09 AM
[Last Edit: 5/13/2012 11:47:48 AM by Schleprock]
+1 on using Ray's dimensions. Easy peasy.
The front pivot pin hole will be your main datum point.

Just make sure you mic the width of your raw forging at several places and jot down the readings as they will vary a bit.

http://arlower.ray-vin.com/ar15
cmjohnson
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Posted: 5/13/2012 1:41:34 PM
Thanks.

I've downloaded and printed all those files out.

One thing I have done that I think makes it easier to machine the lower is that as soon as the takedown pin holes are drilled,
which is my first operation, I bolt the lower to the fixture plate below it. I do this by pre-drilling the fixture plate in the same
spots using a #7 drill and tapping them 1/4-20. Screw the lower to the plate using the takedown pin holes and two suitable
allen head cap screws, and life is good. Plenty of holding strength and NO clamps on the workpiece.

For the deck cut, just spend the money and buy yourself a good 1.5" diameter end mill and the entire deck cut operation
becomes two moves in one path. Straight across and then out and away. Done. No fumbling with trying to interpolate steps
to get the curve right.

I use a modified 1.5" diameter keyway cutter for this job. Its modification consisted of sending it to the local tool grinder
and having the flat face of it turned into a facing cutter. This allows me to plunge cut the corner of the deck in a single operation,
leaving a very smooth finish and it easily handles this in ONE pass.

Since I recently tuned up my mill, it's been an absolute pleasure to use. It has ball screws on the X and Y axes so there's no slop
or backlash. And having a nice four axis (three currently installed) DRO system just makes it as sweet as it can be without having
full CNC capability. It really doesn't hurt at all that it uses QC30 quick change toolholders (which I retrofitted into this formerly
R8 spindle equipped mill) and I have plenty of toolholders.

I also do not skimp on cutters. I don't think I have a single HSS cutter that I actually use, unless it's an inch in diameter or larger.
I only buy and use top brand carbide cutters. Ultratool, Mastercut, G.W. Schulz, etc. No Enco or Chinatrash cutters in my tool cart, please.
The assured accuracy of diameter, long tool life, and surface finish quality is well worth the extra money to me.

I even use a 3" carbide insert face mill with octagonal or round inserts, and also have a 2" version. I wish I had a 4" version.

You'd think that with a setup like this I'd be making parts and money every evening and weekend, but I'm just a hobbyist and
sometimes months pass without me so much as turning the light on in the mill's room, except checking to make sure it's oiled
and nothing's trying to rust.

My setup is really an extreme case of overkill for what I really NEED, but I've always been good with overkill, when it fits within
my budget. It just so happens that the bad economy has made a buyer's market for manual machine tools. You can score
a full size Bridgeport or clone mill with lots of life left for as little as 500 bucks. Maybe even less. I scavenged my DRO system
by getting in touch with the local Acu-Rite dealer and getting some units that they took on trade-in. I then got lucky and scored
several deals on brand new in the box Heidenhain scales, for stupidly low prices, and then found a major dealer of both brands,
and traded Heidenhain scales for the Acu-Rite scales I needed, on a one for one basis.

I'd have used the Heidenhain scales except I was never able to find a Heidenhain DRO that fits my needs at a price I can pay.
Those are expensive. But my Acu-Rite DRO 300M is a very capable unit that will display part diagrams and give step by step
instructions to the operator, if I take the time to program it that way. Which I've only done as a learning experience, and
would be worth the time if I had to do a fairly large run of identical parts.

CJ


"Now they will know why they are afraid of the dark.
Now they will learn why they fear the night."....Thulsa Doom

"How are your eyes adjusting to light? It must have been dark under that rock. "..Mosin_nagant
Schleprock
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Posted: 5/13/2012 3:39:59 PM
Pictures didn't load.
Elwood_Blues
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Posted: 5/13/2012 5:44:30 PM
Originally Posted By Schleprock:
Pictures didn't load.


No kidding. I use a clapped out Bridgeport (yes, they do wear out) with Chinese tooling and Ebay cutters. The Chinese stuff varies from really good to terrible, even from the same tooling set.

ttrmike
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Posted: 5/13/2012 6:08:03 PM
CNC Guns has solidworks and IGS files for an AR lower.

If you havent seen them already:
http://www.cncguns.com/downloads.html
cmjohnson
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Posted: 5/13/2012 9:28:37 PM
I'll be back in the shop tomorrow evening to do more work. I'll get pics then.

There's also a typo on the usual blueprint. The mag release button's slot width is called out as .312. It's really .318.

Just try to find an end mill with a diameter of .312 unless you get one reground!


CJ
"Now they will know why they are afraid of the dark.
Now they will learn why they fear the night."....Thulsa Doom

"How are your eyes adjusting to light? It must have been dark under that rock. "..Mosin_nagant
Schleprock
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Posted: 5/16/2012 12:04:11 PM
Bump for pics.
jodachma
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Posted: 5/17/2012 7:39:40 PM
I just started reading through this and wanted to comment on a couple things.

The blue print is dimensioned and toleranced for the application. I can think of only rare occurrences that you would have a single common datum. You will end up with improperly toleranced features. For example, you would dimension from a datum to a bolt hole pattern and then dimension and tolerance the bolt hole pattern for the mating component and not from an unrelated datum. Also, you do not want to machine a critical slot using the exact size cutting tool. So for your .312 slot you would not use a .312 or .318 end mill. You would use a smaller diameter endmill and machine it to size.

I can tell you that I design in and have machined .312 slots frequently using standard end mills. Don't assume it is a typo.

I'm looking forward to seeing some pictures of your progress and how it turns out.
cmjohnson
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Posted: 5/17/2012 8:28:10 PM
I expect to get back to it this weekend.

I've had a little consultation with a friend of mine who owns a machine shop, and I'm going to get the mag well cut via water jet.

We're looking at what it would cost to take raw forgings and do a minimal amount of work to them, specifically the deck cut and the mag well,
and if the cost is low enough, and those two cuts don't meet the ATF's definition of 80 percent done, I may buy a pile of the forgings, get this
work done, and sell them to builders for a very reasonable price with the hardest part done for them.

I figure that the deck cut, the takedown pin holes, and the mag well operations don't yet equal 80 percent. I might even go so far as to send
an example to the ATF for a ruling on this. With paperwork in hand I would have no worries about selling them.

I'm a big fan of dimensioning off a single reference point whenever it's possible and practical. If I can avoid having to shift my DRO's reference
points in mid-setup, I'm happy about that.


I've now fine tuned my mill to the point that its dimensional error is down to .0001 per inch. And I can get it even closer by adjusting the scale
compensation factor. The error is now at 100 PPM so I shave off 100 PPM in the scale compensation and at that point the error should be unmeasurable
unless you can measure to five digits past the decimal point.

At this point, cutter variations are now greater than scale inaccuracies.

CJ
"Now they will know why they are afraid of the dark.
Now they will learn why they fear the night."....Thulsa Doom

"How are your eyes adjusting to light? It must have been dark under that rock. "..Mosin_nagant
jodachma
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Posted: 5/17/2012 9:19:11 PM
[Last Edit: 5/17/2012 9:35:26 PM by jodachma]
I wouldn't use waterjet unless he has a machine with a fourth axis head, the waterjet will leave a beveled cut over the entire depth of the magwell. Cut a test piece first and you will see it clearly. Also, the tolerance capability of the machine may exceed what is required of the magwell.

I recommend wire EDM.



Sounds like you are having fun with it.

cmjohnson
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Posted: 5/17/2012 9:26:03 PM
[Last Edit: 5/17/2012 9:26:57 PM by cmjohnson]
The pre-cut estimates are that the waterjet will result in a mag well that will be, at most, .008" wider at the insertion end than at
the top surface. I think that would be quite acceptable.

It can also be cut with a slight angle on the cuts to compensate for the slight spreading of the jet.

It'll be tested on scrap before committing to cutting a lower forging.

The reference standard will be that a pmag has to drop free. No tight mag wells.

I'll be happy if a little finish filing is required.


CJ
"Now they will know why they are afraid of the dark.
Now they will learn why they fear the night."....Thulsa Doom

"How are your eyes adjusting to light? It must have been dark under that rock. "..Mosin_nagant
Motor1
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Posted: 5/17/2012 9:40:54 PM
The water jet thing sounds cool. I have set up and ran a CNC water jet. In your first post you stated you had your mill down to .0005" in 8.0" Then in your last post you state you now have it down to .0001" per inch. Well .0005" in 8" is better than .0001" per inch !! How much does the indicator move when you lock the knee or one of the other axis.
I am a machinist with 25 years of experience with both manual and CNC machines. I doubt your Bridgeport type mill is as accurate as you may believe. What kind of read out does it have? Have you ever bored holes with it and confirmed their locations with precision measuring equipment?
I'm not in any way saying you won't make good or even excellent parts. I'm just saying if you can machine the take down pin bores CTC distance within +/- .002" in both directions you will be doing well. BTW: What is the tolerance on the 6.375" spec?
cmjohnson
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Posted: 5/17/2012 9:55:33 PM
[Last Edit: 5/17/2012 10:01:09 PM by cmjohnson]
I've made some test cuts and had them measured via a Faro arm. I then use the scale compensation feature in my Acu-Rite DRO 300M,
and make additional test cuts and have them measured.

I do the measurement from same side cut surfaces. Left side of cut to left side of another cut, spaced exactly X inches away. This way
I don't have to worry about diametric cutter tolerances throwing the results off.

When I first got this setup, the scale compensation was set to a ridiculous number and the work was way off, several thou per inch.
I guess that's understandable as I got the DRO from one source, and the scales from another.

I'm now zeroing in on as close to zero error as can be reliably measured.

I made a compensation adjustment and then found out I'd gone in the wrong direction. The manual for the DRO really wasn't clear
if adding points to the compensation adjustment would shorten or increase the actual length of travel. I was wrong the first time.

I'm not saying I can hold tolerances of .0001 or better. I'm saying I want to iron out the scale compensation factor so that there's as
little error as I can get. If I make an error, I want to be able to only blame myself, not the scales or the machine.

The nice thing about an error of .0001 per inch is that it's really easy to calculate the error in PPM. It's 100 PPM. The compensation
factor is entered in PPM.

i don't even bother to read the dials anymore. It wouldn't do me any good anyway, as they're graduated for inch measurements and
the ball screws in the mill (Yes, it has ball screws instead of lead screws on the X and Y axes) are metric. Relying on the dials would
guarantee that you'd always be totally off by insane amounts.

I got the mill with those ball screws already installed. Given that they really make the dials useless, I'm not so sure that I wouldn't
rather just install fresh lead screws instead if I had to do it all over again.

But now, I'm so used to using the DRO that doesn't matter.

CJ
"Now they will know why they are afraid of the dark.
Now they will learn why they fear the night."....Thulsa Doom

"How are your eyes adjusting to light? It must have been dark under that rock. "..Mosin_nagant
Schleprock
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Posted: 5/17/2012 10:06:13 PM

Originally Posted By cmjohnson:


I use a modified 1.5" diameter keyway cutter for this job. Its modification consisted of sending it to the local tool grinder
and having the flat face of it turned into a facing cutter. This allows me to plunge cut the corner of the deck in a single operation,
leaving a very smooth finish and it easily handles this in ONE pass.

CJ



I just want to see pictures of this 1.5" "end cutting" woodruff key cutter.


cmjohnson
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Posted: 5/17/2012 10:12:09 PM
[Last Edit: 5/17/2012 10:18:59 PM by cmjohnson]


I had the local tool grinder/custom tool maker put an end mill type face on it. He does all end mill resharpening, and makes
customized cutters, for machine shops all around the area. I've had him make two other custom cutters for me as well,
which I use for certain parts of guitar construction.

He did a nice job modifying the keyway cutter, and it works well. Of course, it can't take a big cut but I'm not asking it to do that.

CJ
"Now they will know why they are afraid of the dark.
Now they will learn why they fear the night."....Thulsa Doom

"How are your eyes adjusting to light? It must have been dark under that rock. "..Mosin_nagant
Motor1
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Posted: 5/17/2012 10:40:03 PM
Originally Posted By cd johnson:
I've made some test cuts and had them measured via a Faro arm. I then use the scale compensation feature in my Acu-Rite DRO 300M,
and make additional test cuts and have them measured.

I do the measurement from same side cut surfaces. Left side of cut to left side of another cut, spaced exactly X inches away. This way
I don't have to worry about diametric cutter tolerances throwing the results off.

When I first got this setup, the scale compensation was set to a ridiculous number and the work was way off, several thou per inch.
I guess that's understandable as I got the DRO from one source, and the scales from another.

I'm now zeroing in on as close to zero error as can be reliably measured.

I made a compensation adjustment and then found out I'd gone in the wrong direction. The manual for the DRO really wasn't clear
if adding points to the compensation adjustment would shorten or increase the actual length of travel. I was wrong the first time.

I'm not saying I can hold tolerances of .0001 or better. I'm saying I want to iron out the scale compensation factor so that there's as
little error as I can get. If I make an error, I want to be able to only blame myself, not the scales or the machine.

The nice thing about an error of .0001 per inch is that it's really easy to calculate the error in PPM. It's 100 PPM. The compensation
factor is entered in PPM.

i don't even bother to read the dials anymore. It wouldn't do me any good anyway, as they're graduated for inch measurements and
the ball screws in the mill (Yes, it has ball screws instead of lead screws on the X and Y axes) are metric. Relying on the dials would
guarantee that you'd always be totally off by insane amounts.

I got the mill with those ball screws already installed. Given that they really make the dials useless, I'm not so sure that I wouldn't
rather just install fresh lead screws instead if I had to do it all over again.

But now, I'm so used to using the DRO that doesn't matter.

CJ


I'm not crazy about Faro arm measurements but in my case it was measuring things that were 6 feet or more apart. Even the CMM has it's flaws but usually a good operator can work them out. With 25 years behind me you can bet I never want to have to use the dials on a mill ever again. Mostly because I have been spoiled with digital read outs. I can't even imagine trying to make something with a lead screw machine and dials.
I have a friend who also has a tool grinding shop. That is one area I haven't worked in yet. He invited me to come out and see if I like it or not. So hopefully by next year I will have some tool grinding experience too. May be a good part time job after I retire.
Good luck with your project and thanks for not going ballistic on me.

cmjohnson
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Posted: 5/17/2012 11:00:42 PM
I'm not the type to go ballistic on people who know more than I do about the topic in question, and are just asking honest questions.


I'm an amateur machinist, at best, but I understand the basics well enough. And one thing about a manual Bridgeport style mill is that it's actually
kind of hard to make the cutter do the wrong thing. Most modern cutters are designed for speeds and feeds that I can't even approach by hand.

Sure, I could break a small diameter end mill, but with a 1/2" carbide end mill, it'll take more than my 2 HP machine has to give. At least in aluminum.


CJ
"Now they will know why they are afraid of the dark.
Now they will learn why they fear the night."....Thulsa Doom

"How are your eyes adjusting to light? It must have been dark under that rock. "..Mosin_nagant
Schleprock
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Posted: 5/17/2012 11:04:35 PM

Originally Posted By cmjohnson:

He did a nice job modifying the keyway cutter, and it works well. Of course, it can't take a big cut but I'm not asking it to do that.

CJ

And now for the real question...... How much???

cmjohnson
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Posted: 5/17/2012 11:09:18 PM
[Last Edit: 5/17/2012 11:10:12 PM by cmjohnson]
It was a while ago but I think he charged me 10 bucks. Seems fair to me.


It sure was a lot cheaper than buying a 1.5" diameter end mill of any kind.
"Now they will know why they are afraid of the dark.
Now they will learn why they fear the night."....Thulsa Doom

"How are your eyes adjusting to light? It must have been dark under that rock. "..Mosin_nagant
jodachma
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Posted: 5/18/2012 10:44:12 AM
If you are still in need of .312 diameter end mill you can get them from MSC. They have dozens of options from $20-$35.
cmjohnson
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Posted: 5/18/2012 12:43:55 PM
Not too long ago I bought a USA made 3/4" carbide end mill. It has been used to remove no more than 20 cubic inches of 6061,
taking moderate cuts only, and it's already due for resharpening. It's dull and the tips are gone.

I don't buy budget cutters anymore. I just won't waste the money. I buy G.W. Schultz, Ultratool, or, in a pinch, Mastercut.

Bargain cutters are no bargain.


CJ
"Now they will know why they are afraid of the dark.
Now they will learn why they fear the night."....Thulsa Doom

"How are your eyes adjusting to light? It must have been dark under that rock. "..Mosin_nagant
Motor1
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Posted: 5/20/2012 6:19:00 PM
[Last Edit: 5/20/2012 6:19:38 PM by Motor1]
CJ,
Have you ever consider using a fly cutter? I worked in aerospace and machined a lot of aircraft aluminum. I have fly cut surfaces on the Bridgeport that were over 4 inches wide with a single pass and they come out beautiful.
cmjohnson
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Posted: 5/20/2012 8:29:22 PM
I don't have a fly cutter that's small enough.

In fact, I don't own a fly cutter. I did borrow one recently for facing some aluminum plate, about 4" wide, but I gave it back.

It's also a pain in the ass to set a fly cutter to exact cut dimensions. I'd rather use a good end mill that's the right diameter.

"Now they will know why they are afraid of the dark.
Now they will learn why they fear the night."....Thulsa Doom

"How are your eyes adjusting to light? It must have been dark under that rock. "..Mosin_nagant
Cole2534
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Posted: 5/23/2012 1:44:09 PM
Dude, use an offset boring head.

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