Anodizing and engraving questions

[Needham]

I'm going to be engraving and anodizing a couple of 80% lowers that I completed and built into rifles with BBs/evil features last year. I am thinking of getting them type III anodized.

I'll be doing the engraving myself on my CNC mill. Spindle speed is limited to 4000 RPMs.

I have a few questions that I'd appreciate some help on.

1. Does type III anodizing cause fit problems? Is it correct to assume that the type III anodizing is about 0.005" thick, with a about 0.0025" growth in dimensions?

2. Should I open up the FCG pocket, and the pocket around the front takedown pin by a similar amount? Same for the hammer and trigger pins and selector hole?

3. Any recommendations for an engraving bit in terms of tip size, included angle, and depth? Trigonometry would suggest that the narrower the tip size and included angle, the deeper I'd have to go, specifically, that the depth prior to anodizing would be 0.003 + growth/sine(included angle/2). Any practical suggestions?

Thanks,
Bob

[kcstott]

Type III is not that thick. It's actually about .001"-.002"

The original ordinance prints call out all dimensions to apply after finishing with the expectation of anodizing being apart of the process. i.e. every small hole is reamed .0015" over size and the selector .002" over minimum.
It's a guess really but a good plating shop can honor a request to a mil spec anodize and that is also thickness if the correct mil spec is used.

As for engraving. a 45˚ tool will show up twice as wide as you are deep. and a 30˚ tool will be 1:1 ratio of depth to width. Single lip cutters are the norm

If you are using conventional end mills and just milling to depth, .002" depth of cut per pass max to avoid tool breakage. At 4000 RPM max you have a hell of a chip load on a tiny tool and and can snap a it quick.

[eaglemike]

Since you are using CNC, it should be easy. Run a practice piece first, of course. Just run .001/rev, or 4.0" /minute. easy peasy in one pass. These numbers are conservative. I often engrave at a higher feed rate/rev, and deeper than .003 - in fact, I would suggest planning for .004 or .005 depth, just to be sure. Dab a little oil on there if you are not running coolant. Proper engraving bits are easy to come by.

Type III is typically .0005" penetration, and .0005" build up. So figure .001 smaller on hole diameter (at least). 7075 can build faster than 6061, depending on tank settings. A good ano house knows all this stuff and can keep things under control. For threaded holes, you'd need to allow for more build up, due to the angle on the flanks of the thread. If you are tapping, this is easily done by using a tap that leaves a larger hole, like a H5.

[67Cuda]

I like using a 0.020/0.030 ball nose end mill for engraving.

Are you just engraving standard information or are you going to do graphics too?

[Needham]

Thanks for the input.

The lower is 7075 (which I would say machines even nicer than 6061 -- less gummy) . I'm running an MQL (Accu-Lube) -- the compressed air takes a surprising amount of heat out of the cut. Finishes are really nice.

No graphics planned at this time. Just standard info, so far. If someone posted G-code for a picture of Hillary or her husband, engaged with, say, a farm animal, I might reconsider.

Bob

[Nathan Krynn]

Anodize per milspec is .0008" - .0012". I get mine done at the place that anodizes for the US military all non-black (and a lot of black). I see more PO's for this then I care to admit. The exact wording is below.



You never open it up for anodize. The print is pre anodized so if you open it up then you are oversizing everything. We use these engraving bits from 2L inc.

T2-125x2.0-60-.010

[kcstott]

Anodize per milspec is .0008" - .0012". I get mine done at the place that anodizes for the US military all non-black (and a lot of black). I see more PO's for this then I care to admit. The exact wording is below.



You never open it up for anodize. The print is pre anodized so if you open it up then you are oversizing everything. We use these engraving bits from 2L inc.

T2-125x2.0-60-.010

Well My 1970's print calls out dimensions "After finish" and the finish is speced at Type III mil spec Anodize. So the new(er) prints are corrected so the machinist doesn't have to guess