Perplexed - A Little Help Needed

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
 
Janderso said:
I found if I locked the table and quill whenever I could the accuracy improved.
Click to expand...
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.
 
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.
 
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.
 
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.
 
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.
 
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!)
 
Let's keep this thread on point guys
 
Nyala said:
My cuts are no more than .025". It takes me over 50 passes to get to a depth of 1.248".
Click to expand...

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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