Perplexed - A Little Help Needed

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Nyala

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

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

 
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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