# Perplexed - A Little Help Needed



## Nyala (Aug 1, 2019)

A friend who has made a couple of AR15 rifles from 80% lowers encouraged me to give it try since I have a milling machine - PM-25MV.  I decided that since I would not be using the drill press/router method that he uses, I would first buy some stock and make some practice cuts, rather than destroy a lower receiver.  I've done 6 practice cuts so far and they all suck.  I learned some things along the way and things were improving and I expected number 6 to be great but it wasn't.

First I blue the stock and use a height gauge to make layout lines for a guide.  I have made sketches with all the dimensions on how far I need to move the part.  I have double checked the math and it works.  My mill has a DRO.  I also am using a brand new American made 2-flute 7/16 carbide end mill with AITIN coating.

I make a long cut downtime center of the part to a depth of 1.248" and is 2.562" long.  This takes a while since no cut is more than .025".  Once the cut reaches .630" in depth the length of the cut is shorter by .491".  Each dimension that I work with is written on my vise so that I can always refer to it if necessary.  When the dimensions change I remove to old ones and write down the new ones. 

Once the centerline cut reaches 1.248" deep, I need to move on the Y axis .126" each way from center to make the width of the slot .690".

Every time I do this my finished parts do not measure out.

The width of my slot should be .690" but routinely measures about .683"

For the depth, which should be 1.248".  When I took it out of the machine I looked at the readout and it said 1.249".  Something I could live with.  However, using a caliper I measured the depth and and it was reading about 1.265".  Perplexed I put the part back into the machine and zeroed out the z axis on the top of the part then put the cutter on the bottom of the slot and the machine showed that it was 1.270".  Now I had just taken this out of that same machine and the reading said 1.249".

The short cut that is .491" long should be at a depth of .630" but it was about .650".

The long dimensions, like the 2.562" length is fine.  But depth and width dimensions are off.

The head is locked, and each axis is locked, as needed.  So much so that once I make a cut for length and return to the zero position, I lock the x axis again - over and over and over.

I'm beginning to think that the fine adjust mechanism for the z axis is bad, but that wouldn't account for the y axis dimension errors.

Any ideas?  Am I doing something wrong?

Thanks for the help.

Denny


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## Janderso (Aug 1, 2019)

I'm certainly no expert. 
Just curious, why carbide? Aluminum likes sharp HSS end mills in my experience.
Accuracy is not an accident. Good for you to make practice cuts on a blank.
Someone will be along with more than worthless information.
I tried cutting one of those 80% lowers on my wore out Bridgeport. Accuracy was not part of my experience due to the slop in the table, gibs, quill, spindle etc.
Good luck.


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## JimDawson (Aug 1, 2019)

What is the actual diameter of the end mill?  Sometimes carbide endmills run a bit small, not a big deal, but you have to compensate.  I always measure endmills before use.

As far as the depth, it could be an issue with the DRO or could be your caliper.  You need to check both against a standard because right now you don't know what is correct.  Getting an accurate depth measurement with a caliper is somewhat difficult and requires proper technique, so some practice might be in order.


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## Nyala (Aug 1, 2019)

I used carbide because the HSS end mill that I was using was giving me the same problems.  Not knowing if it was the end mill, I opted for a dollar carbide, hoping that it would help.

The diameter of the end mill is .437".


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## JimDawson (Aug 1, 2019)

Have you checked calibration on your DRO, calipers?


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## Nyala (Aug 1, 2019)

Jim, You're over my head.  Don't know how to even begin to do that.

Denny


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## fradish (Aug 1, 2019)

For the DRO, a simple way is to clamp down a 1-2-3 block (make sure it is square to the table), then put another 1-2-3 block right up against it and then use a test indicator to sweep along the second block.  Zero out your DRO, remove the second 1-2-3 block and move your table (let's say we're working in the X axis) until your test indicator reads the same as it did when you had the second 1-2-3 block in there.  If you had the block oriented with the 3" side along the x-axis, your DRO should read 3".  I'll look for the YouTube video for TouchDRO.  The basic idea works even if you have a different make of DRO.


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## JimDawson (Aug 1, 2019)

Sorry. You need to check the calibration against a known standard.  I normally use a precision 123 block or micrometer standard, but anything that is a known size would work.  For internal measurements, a reamed hole will be pretty close, good enough for the home shop in most cases.  You could bore a hole if you have a boring head or a lathe, and just try to get a comparative measurement between the DRO and your calipers.  Without some standard to measure to it's pretty hard to get accurate.


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## fradish (Aug 1, 2019)

Starting around 2:34 you can see how you can verify your DRO:


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## shooter123456 (Aug 1, 2019)

Is your end mill held securely?  It is possible it is pulling out and cutting deeper, which would account for the depth that was too deep, the DRO reading that was correct, then rezeroed and measured incorrectly.  

The Z axis has a quill and a DRO on it right?  The DRO on the quill and the Z axis should account for any error in the adjustment mechanism.  It doesn't matter how inaccurate it is, the DROs measure actual movement.  

.007" isn't a terrible number to be getting, especially if you are new.  I would make sure you are locking your gibs when you cut, make sure the workpiece is supported so it isn't being pushed away when you try to cut it.  Also, you are using a larger diameter end mill than is really needed for an 80% lower.  A smaller one will take longer to cut, but will induce less load on the machine.  That could help you get it closer.

Don't worry if you don't get it perfect at first.  The machine is very capable and you can get great results as you learn to use it.  This was made on the same machine, so once you get it down, 80% lowers are a walk in the park.


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## Flyinfool (Aug 1, 2019)

A couple more thoughts.
My calipers is off by .009 when measuring depth. I always have to adjust the zero before each depth measurement and then adjust it back for "normal" OD measurements. Check yours and also double check your DRO with something of a known dimension.

The slot being to narrow could be the caliper reading incorrectly for inside dimensions, or it could be flex of the machine and or the cutter. 1.248 deep x .126 side cut is a pretty big cut, something is bending. Are you climb milling or conventional milling?
Use the DRO and the calipers to measure the slot width. if they do not agree then you need to get something with a known inside dimension to find out which is correct.


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## benmychree (Aug 1, 2019)

Janderso said:


> I'm certainly no expert.
> Just curious, why carbide? Aluminum likes sharp HSS end mills in my experience.
> Accuracy is not an accident. Good for you to make practice cuts on a blank.
> Someone will be along with more than worthless information.
> ...


I hope that was before they were outlawed in Ca., Jeff ------- I had one that had to be "destroyed" because of being in an illegal form, did the 80% lower thing and re registered it with mods to make it legal, then came the new law, presently in force; I had a licensed dealer (class 10) friend sell it out of state; lost money, seems the bottom dropped out of the AR market.  Besides the law, it was not to my taste, I like vintage big bores.


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## Cadillac (Aug 1, 2019)

Did California make a law after this last earthquake that makes it illegal for people to work on their own cars at home? I heard that somewhere and couldn't believe it.


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## zjtr10 (Aug 1, 2019)

Google offered me the This









						"Sacramento County Says It's Illegal to Work on Your Own Car in Your Own Garage" - Overlawyered
					

It’s common for communities to use zoning codes to exclude commercial and industrial uses from residential areas, but Sacramento County, California, seems extra-zealous about making sure that residents don’t try to operate auto repair businesses amid homes. While it concedes to residents the...



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## Latinrascalrg1 (Aug 1, 2019)

I understand wanting to keep your neighbors from going full on Fred Sandford and all but that is Ridiculous!


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## markba633csi (Aug 1, 2019)

I've been telling my friends that eventually personal internal combustion will be so expensive as to be essentially impossible for the non-wealthy
In fact, if the government rescinds the smog exemption for pre-'75 cars there will be riots in the streets, especially in LA
This has been a public service message.


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## Bob Korves (Aug 1, 2019)

I have lived in Sacramento for almost 60 years, work on my car regularly in the garage, quietly with the door closed, don't use my automotive air tools, and am not concerned in the slightest about this legislation.  It is a non issue.  I am in an ordinary neighborhood and my neighbors and I all get along well.  I have always gone out of my way to not make annoying noise when people are sleeping or on early weekend and holiday mornings.  I get annoyed myself when someone is making too much noise during quiet hours, even if it is from their lawnmower.  This is mostly about people doing things that are well beyond normal working on their own stuff.  I have zero concerns about this legislation, though it does reek of silly ninnies.  'Nuff said...


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## C-Bag (Aug 2, 2019)

Bob Korves said:


> I have lived in Sacramento for almost 60 years, work on my car regularly in the garage, quietly with the door closed, don't use my automotive air tools, and am not concerned in the slightest about this legislation.  It is a non issue.  I am in an ordinary neighborhood and my neighbors and I all get along well.  I have always gone out of my way to not make annoying noise when people are sleeping or on early weekend and holiday mornings.  I get annoyed myself when someone is making too much noise during quiet hours, even if it is from their lawnmower.  This is mostly about people doing things that are well beyond normal working on their own stuff.  I have zero concerns about this legislation, though it does reek of silly ninnies.  'Nuff said...


+1, you go Bob! If you read the reply's there is some interesting stuff, mostly nattering ninnies menthinks.. I do next to no auto repair anymore, thankfully. But I do all the service work on our cars with no problem. I guess it depends on the area but I don't have any problems and I have neighbors who's wood working equipment and yard equipment is way louder than my metal working stuff. I'm very carefull to work during the day when 90%of the neighborhood is gone. I have one neighbor who seems to be a collector of marginally operational transportation with 4 cars(two barely functional) two motorcycles, a scooter, gas powered rubber raft, and the latest a gas powered golfcart. Nice guy but firmly in the what we called the "rev to repair" camp. The golfcart is just worn out. Smokes so bad, stinks and will barely start. And when it does they spend all their time trying to rev it up over and over. The only reason I don't ask him to stop is the smoke blows right into his house.  Thankfully it's parked. My other neighbors car died over 6mo ago and is just sitting in their driveway. And as long as it's in the driveway and not an eyesore seems nobody cares.


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## rogerl (Aug 2, 2019)

This is all well and good but what does it have to do with the original posters machining problem. Remember this is a machining forum we should be talking about machining and solving this guys machining problem.

Just my 2 cents
Roger L


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## Janderso (Aug 2, 2019)

Nyala,
Have you ever checked the accuracy of your equipment?
The reason I ask, you may need to make adjustments to improve accuracy or the ability to repeat cuts.
I haven't done this yet on my newer equipment but out of curiosity I drew a plan and tried to produce the part with my old Bridgeport.
I found if I locked the table and quill whenever I could the accuracy improved.
The reason I say this is you seem to be struggling with under and over cuts.
A DRO is a great tool as it measures movement regardless of backlash. As I was making an X cut I noticed the Y was drifting = not good.
Food for thought.
The beauty of the AR platform is it's versatility and endless ways of adapting or personalizing your firearm.
Don't own any since the new laws. Not worth it if you can't even go to a range without worrying about getting arrested or having your investment seized.


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## higgite (Aug 2, 2019)

Nyala,

You may be having the same problem that I ran into with my mill before I added a quill stop. Mine isn’t a PM, but I think the same principle may apply. There is backlash in the rack and pinion that drives the quill up and down when you use the handwheel or the fine adjust knob. When I plunge milled with the fine adjust engaged and without a quill stop, I would run the the end mill down to my target depth and stop. The end mill would continue going deeper for a few thousandths until it took up the backlash. It drove me crazy until I noticed the Z-axis DRO changing after I stopped plunging. Before I installed a quill stop, I learned to sneak up on my target depth once I got close. Try plunging to a target depth with your fine adjust and then stop and watch the DRO to see if yours does the same as mine.

Tom


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## Bob Korves (Aug 2, 2019)

Janderso said:


> I found if I locked the table and quill whenever I could the accuracy improved.


Yes, Jeff!  Lock every axis that is not moving for every cut.  It is easy to get lazy about this when working on soft materials, but it will come back to haunt you, and is a major source for doing inaccurate work.  It needs to be an ingrained habit.


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## Nyala (Aug 2, 2019)

I did a little testing this morning and here is what I found out.

Using a dial indicator I checked all three axis in .010" increments through .060".  My cuts are 99.9% of the time no more than .025" so that covers more than anything I would ever do.  All three axis were spot on or off no more than .0005" at any time.  I attribute the .0005" to me not the machine.  I used the fine feed adjustment for the z axis.  I first moved the DRO .010" then checked the dial indicator.  After reaching .060" I reversed the process, moving the dial indicator .010" then checking the movement against the DRO.

The calipers were giving me a reading for the width of my slot at .683" using my digital calipers and it should have been .690".  Using a Brown & Sharpe inside measuring gauge I measured the slot at .686" and using Starrett adjustable parallels I measured the width at .683".  I measured the inside gauge using a Brown & Sharpe and Starrett micrometers that were NOT digital and a new Accusize digital micrometer.  The digital micrometer gave me the .686" reading while the manual micrometers gave me .683".

As for the depth of cut, I do not have any reliable method to measure that.  When I took the part out of the machine yesterday the depth of cut showed 1.249" and I was very happy.  Two minutes later when I measured the depth with calipers it read about 1.265".  Today I went to the old fashion method of a 1" wide rule divided into 64th of an inch.  The depth of cut showed at 1-17/64 inches or possibly 1-9/32", which is within a couple thousandths of what my caliper was showing.  However, all of this has not bolstered my confidence in calipers.

I'm thinking of cutting a slot in the part so that I can insert an inside gauge and measure against a the bottom of the cut and a parallel used to form a cap on top.

My process for making a cut is to plunge to .025" with X & Y locked.   The head is always locked.  I let the mill and cutter settle out so I can get an accurate reading of depth.  Once it settles out I lock the Z, unlock the X and make the cut to length using conventional milling.  Once I reach length I reverse the cut, using climbing milling for a better finish but do not change the original depth of cut.  It simply cleans up the surface.  Once I return to zero, I again lock the X axis, unlock the Z axis and begin the plunge milling for the next cut, etc., etc., etc.

It appears to be something that I'm doing and not the tools.  I'll cut some more stock and try again, measuring more frequently along the way, and double checking everything that I'm doing.

Somewhere I have some Stainless steel dowel pins that I can get a accurate reading on diameter and check that against the mics and calipers.  I just couldn't find them this morning.

I appreciate everyone's help with this.  As a newbie I lack the skills and/or knowledge to perform some of tasks that you suggested but did learn how to do them following your advice.  You have been most helpful.  Thank you.


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## JimDawson (Aug 2, 2019)

Since you are trying to do precision work, I think you need to get set up with a standard in your shop.  I would recommend a gauge block set, doesn't have to be an expensive grade AAA set, but just something to set all of your measuring devices to.  https://www.amazon.com/Accusize-Ste...s=gage+blocks&qid=1564765462&s=gateway&sr=8-3  Close enough for the shop floor.

The slot width issue might just be ''spring'' in your machine, and you just need to compensate. But first you need to be able to measure to some standard.


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## pontiac428 (Aug 2, 2019)

I would like to point out that using your DRO and punching big cuts to the nominal dimension by cranking the handwheels in one go will rarely get you to within 0.001 of your target.  Even with CNC, which "can" do that, you would first hog out an undersized pocket and then skim your final cut to dimension in a finishing pass.  

I suggest roughing your features and expanding the pocket in steps, then finish cut the final pass to blueprint specs.  Also stick with conventional milling for bulk removal.  For an AR lower, I step my depth in increments and don't cut final depth until my finish pass.  Same on edges, I cut about 1/4 of a mill diameter or less on the final pass.  Any irregularities in the cut, such as bowing, wowies, ridges, parralelism are your machine and fixtures.  Any overcuts, dropped tool cuts, and dimensional issues are operator techniques.  Some pictures would go a long way with helping you here.


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## Nyala (Aug 2, 2019)

Pontiac428,

My cuts are no more than .025".  It takes me over 50 passes to get to a depth of 1.248".  Cuts to widen the slot are done in .020" or less, sometimes half that, so it takes 6-12 passes to remove .126" at about 1/2" deep.  Then repeating the process 3 times until I get to the desired width and depth.  Then repeat that for the other side.  No hogging big amounts here.  My machine does NOT like big cuts.  I'm going to try reducing the .025" to about .015" to see if that helps.


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## MikeInOr (Aug 2, 2019)

Why does everyone make fun of California?  It is still legal to to take a deep breath of fresh air in CA... (of course you had better buy some carbon credits before you plan on exhaling!)


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## higgite (Aug 2, 2019)

MikeInOr said:


> Why does everyone make fun of California?


Because there's so much to make fun of and so little time.   

Tom


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## JimDawson (Aug 2, 2019)

Let's keep this thread on point guys


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## brino (Aug 2, 2019)

Nyala said:


> My cuts are no more than .025". It takes me over 50 passes to get to a depth of 1.248".



Have you tried a "roughing" cutter for bulk removal and a finishing cuter for final dimension?
It might be worth the cost.

Don't give up!
You will get there and we will always try to help!

-brino


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## pontiac428 (Aug 2, 2019)

Nyala said:


> Pontiac428,
> 
> My cuts are no more than .025".  It takes me over 50 passes to get to a depth of 1.248".  Cuts to widen the slot are done in .020" or less, sometimes half that, so it takes 6-12 passes to remove .126" at about 1/2" deep.  Then repeating the process 3 times until I get to the desired width and depth.  Then repeat that for the other side.  No hogging big amounts here.  My machine does NOT like big cuts.  I'm going to try reducing the .025" to about .015" to see if that helps.



Okay.  You should be able to take cuts approaching 0.100" deep and half a cutter diameter wide.  How are you managing backlash?  On a manual machine, you should only turn the lead screw one direction into your dimension.  Avoid overshooting and pulling back, unless you back way out and re-approach from the feed direction again.  Whatever the DRO reads is where your spindle centerline is, but if you're in the dead band of your lead screw's backlash, the spindle will wander.  On some machines a lot!  Not trying to insult; gotta cover the basics first.


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## wrmiller (Aug 2, 2019)

When I had my PM25 3/8" roughers (coarse and fine) were my friends!


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## markba633csi (Aug 2, 2019)

Nyala: do some tests making heavy cuts vs light cuts, I'm sure you will see a difference due to the flex or springiness of the machine
Smaller hobby machines have more flex than big industrial ones
Mark


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## Nyala (Oct 15, 2019)

I'm bringing this discussion back up again only to relay some information I found out.  

After being unable to resolve the depth of cut problem I contacted Precision Matthews, told them of the problem, and they were as helpful as ever.

I was advised to removed the cover plate from the fine feed digital screen where I would find a small vertical DRO.  I was to check the screws on the DRO to insure that they were tight.  They were not.  I tightened all four screws (all four were loose) and checked the depth of cut against a height gauge that I adapted to check depth.  The depth of cut on the height gauge and the fine feed digital screen matched.

Looks like I'm good to go.  Other PM-25 users might want to check these screws as preventative maintenance.

Many thanks to PM for their help.


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