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WarBlock Receiver Height AR15 Gas Block CNC Machining (13): double down and fixture fixing

After I machined fixture plate 1 to have the correct sized bosses I decided to jump right in and machine two blanks simultaneously, the first time I’ve done more than one on the fixture. The only problem was the the gastube hole was drilled slightly off location in reference to the main 14mm bore hole (slightly too far away). This caused the blanks to require way too much force to seat to the fixture, much more than usual and ended up raising a small ridge of deformation on the back of the blank ultimately preventing it from seating flat to the plate. After I fixed this by filing the ridge down and chamfering the holes deeper I realized two problems. One; I think I changed the origin point of the spot drill operation and this resulted in it not drilling deep enough so there was nearly no chamfer around the hole. The chamfer is important because it relieves the contact area around the boss and facilitates the blank seating flush to the plate. It also lessens the necessity of perfect alignment since if the hole engagement is only near the top of the boss then some amount of flex is possible to ensure fully mating to the surface of the fixture. Lastly if there is some deformation of the blank caused by excessive seating force it will not affect the ability of the blank seating flush to the plate. Secondly; when I was re-indicating the blanks after I flipped them to drill the gastube hole I noticed the Y was off 0.001″ from where it was before. Consequently this is also where the error was. In the future I need to simply indicated the Y off of the fixed vice jaw and keep that value after the flip. Doing it this way will also require that the blanks either be perfectly square (which these weren’t) or I need to add a strip of tape as Brad at Tactical Keychains does or perhaps some aluminum welding wire as Tom Lipton at OxtoolCo suggests in his book Sink or Swim to the movable vise jaw to allow the side against the fixed jaw to sit flush and square. Other than that small issue everything cut perfectly.

S4080004

Double Trouble

There was a slight code problem when cutting the bayonet with the 0.1875″ endmill. Somehow I managed to change the tool in the CAM to a 0.375″ so I had to go back and fix that.

The spiral cuts with the 0.5″ rougher went fine and I was pleased that it cut the two blocks simultaneously just fine. I had never tried it before it was all just theory in my head. The cuts did sound a little rougher than usual because that endmill is getting dull and has a few chipped teeth. I may retire that one rather than risk breaking something. Hell, maybe I’ll just break it. We’re testing here right?

The finish pass and two spring passes cut just fine and reduced the top to bottom draft from 0.003″ to 0.001″. The only issue here was the long thin chips building up between the parts like steel wool. I blew them out with air but perhaps will one day search for one of those serrated finishing endmills. Those leave a fine finish but break up the chips much smaller so that they can be more easily washed away by coolant. In fact I might reposition the nozzles a little so they blast more coolant between the parts without one part occluding coolant flow to the other.

The 0.109″ roundover worked great after I increased the birth to 0.006″ and this left no ledge. Preventing a ledge is tricky with these cutters and seemed to just require trial and error. The chamfering worked great too but I had to go and do it twice because for some reason the Logo didn’t cut deep enough. I remeasured the tool and determined it was slightly off a couple of thousandths, I then set the new offset and ran the op again. The logo turned out great at 0.002″ deep. It is subtle but boldly emblazoned in the most conspicuous place on the gasblock, the butress, just forward of the flight deck. I am fiercely proud of my logo, my work, and my reputation. At the same time I don’t believe a serious part like this needs to have a billboard plastered all over it. Plus it only takes 16 seconds per part to engrave. In the future I may add more text somewhere with a laser engraver but until then the logo will suffice and will probably always be machined in as a matter of syle.

0.002" deep War Machine logo

0.002″ deep War Machine logo

I made some minor changes to minimize unnecessary machining on the recoil grooves of the Picatinney rail. Other than that the top side ops all went perfectly. I’ll probably go back and increase the plunge feed to rapid to further speed things up now that I feel more comfortable with it.

The main changes I made to rectify prior issues in the bottom ops was to widen the 0.047″ roundover to take a 0.005″ wider pass to eliminate the ledge it was always leaving behind. This worked perfectly and left no ledge at all. I also changed the entry point of the Woodruff cutter to initially engage in a thicker area as well as to raise the RPM slightly to take less of a chipload and have less deflection. The most significant change I made to eliminate the bump it was pushing out on the side walls was to leave 0.003″ of material per side up from 0.001″. I actually tried 0.004″ at first but the sling socket swivel button did not fully retract after being depressed so I bumped it to 0.003″ which worked great and left no bump at all. A new path I added and you probably noticed it from the last video is widening the amount of the material the 0.375″ rougher takes from the bottom. I did this when I realized the side gutting entry would partially engage the cutter on the conventional milling side and I wanted to avoid that for the sake of tool life.

Speaking of tool life I improved the entries of the rougher so it doesn’t abruptly enter the material over and over again. I did this by introducing radial engagements, cut rounding, and mixing climb and conventional milling to some of these toolpaths. One of of these radial entries I accidentally left as a rapid. This claimed my favorite endmill, the mighty 0.375″ rougher. I would like to give this tool a 21 gun salute and taps. Its death was a violent as its life and it will not soon be forgotten.

maritool 0375 rougher

Maritool 0.375 rougher

Its successor is the “Fireplug” from Lakeshore Carbide that I’ve talked about before. I’ve not had a chance to use it much at all yet but what little that I have done with it suggests it will give be even better performance. The verdict is still out but I do have some concerns about tool life with the fireplug. I used it to machine down the face of fixture plate 2, an operations that I have done twice before with the Maritool rougher not to mention all of the other cuts I’ve made with it and it seemed to appear like new for a long long time. In fact it wasn’t until very recently I even noticed any damage to this endmill. When using the Fireplug I dialed down the aggressiveness of the cut a little bit to better extend tool life because speed was not an issue here. Disappointingly and quite surprisingly that one op left several small chips in the flutes of the endmill. I’ll just continue to use it and gauge the total life of the tool before making any determination about which one to stick with.

Coatedrougherstubendmillcarbide2

LakeShore Carbide 0.375 “fireplug” stub rougher

The problem. So after I machined the two blanks I put them on fixture 2 and started taking some measurements. Curiously it appeared that one side was still higher than the other side despite fixing the side to side play issue of the bosses on fixture plate 1. In fact it was a very consistent measurement even between the two different in-process gasblocks, one side was always 0.004″ higher that the other. I first thought that it was caused by the index point being slightly off from the first boss. After investigating this I discovered that this was partially the case. The numbers I was using for the offset from the sides of the plate where I probe and the first boss was off in the X by about 0.002″, I could even barely see the misalignment with my eye in the spot where the large roundover intersects with the bore. Great, simply change the offset. Wait, this didn’t account for the other 0.002″. I took both of my fixtures to the surface plate and then to the mill for some very accurate and precise final measurements. What I found was that the bosses on either fixture plate were not parallel to the sides where I probe from and thus were not centered as they should be to the body of the plate in the case of fixture 2 resulting in Z height errors. In the case of fixture 1 it would result in increasing Y axis errors going from left to right in the X, which would ultimately result in all manner of location errors when in-process gasblocks are loaded onto fixture 2. This error was 0.002″ at its maximum deviation. Bingo there it is.

The cause. How could this be. I always carefully align the vise when mounting it to the table but deductively this was the only way this error could have occurred. I had recently removed the vise and disassembled it for a thorough cleaning so I had oiled it, reassembled it, and adjusted the movable jaw lift set screw. I dressed both the vise bottom and the table surface with a file to make sure everything was as perfect as it could be before mounting it. First I aligned it as I usually do, with the Haimer 3D Taster across the fixed vice jaw. Having a suspicion about the validity of this measurement given the backlash discoveries I made about the Haimer I re-indicated the fixed jaw with the dial test indicator. Whoala! it was off 0.001″ over the 6 inches of the jaw. This translates into an error of 0.002″ over 12 inches, the length of the fixtures, and consequently the exact amount of maximum error I was measuring on the plates. The bottom line is that the amount of backlash in the Haimer makes it unsuitable to indicate and square a vise to less than 0.001″. Unacceptable. I mentioned before that I had contacted Haimer USA to see about having my 3D Taster inspected and perhaps repaired if it was indeed found to have a backlash problem outside of normal specs. I was told the inspection would be as high as the cost of a brand new unit. Ok that is seriously effed up right there. Scratch that idea. I guess I will abandon using the very expensive Haimer for aligning anything accurately in the XY plane and instead use the more sensitive and much cheaper Mitutoyo DTI instead.

The fix. Well fixture 1 was a relatively easy fix. I simply aligned the bosses instead of the sides in the Z axis using a 0.001″ shim, adjusting the shim until the bosses were perfectly parallel with the X travel in the Z axis. and re-faced all of the sides square. I then very carefully measured the offsets from the new side surfaces to the bosses using the dial test indicator and Noga magnetic base attached to the spindle. I did this several times and it was always within a few tenths. Easy Peasy.

Fixture 2 was more tricky. I first repeated the procedure as before with fixture 1 to square up the two sides relative to the bosses. This was somewhat more complicated because unlike with fixture 1 I had to machine a precise amount of material from each side to ensure that the bosses were perfectly centered between these two sides. Once I did that and verified it I then had to buzz down the bosses on the other side and re-machine the whole face again using the new offset numbers from the other side. I then verified that everything was right with the world on all sides in every possible way to the extent of the precision of my meteorology tools to verify everything so that it could no longer being a lingering variable in my mind.

The aftermath. I had several stiff drinks and fell asleep.

 

 

 

 

 

 

 

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James Dressman
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