Just so you know, I am a stamp collector as well as machining, welder, all round King of Fools (KoF) plus a foo other things too.I never thought of dog washing. My fall back on "I'm going to start a less demanding hobby" was always stamp collecting. There are only two steps:
Paste in book.
Neither can be done to even .001"
Dog washing sounds easier, although probably messier. Probably depends on the dog.
Just so you know, I am a stamp collector as well as machining, welder, all round King of Fools (KoF) plus a foo other things too.
I looked at this part for hours yesterday. Yes, it might well actually be usable. Yes, it's going into an engine where it will never be seen so "ugly is OK." I just know I can do better, so I'm going to do that.
Go Bob Go......
One thing I do to insure the hold down screws are in the exact center, is to drill the holes with the CNC mill set to 0,0, then tap them in the vice. It is a little akward, but insures the center of the conrod holes are exactly where your CNC program thinks they are in both the X and Y axis.
I have a good feeling about this one.
It is looking really good. What is your concern with changing end mills? The X and Y should remain dead on and the Z height needs to be changed to account for any difference in height between the two end mills. I will machine with the first size end mill, then bring the end mill down and touch off on the top of the vise, record the Z value displayed on the DRO. Load the second end mill, again drop down and touch off Z at the same spot on the vise, set the Z value to the one recorded. Then machine with the next tool path.
That's what I do. Basically, any time I change the setup, I get concerned. Work holding is everything. I haven't adjusted the Y axis setting in the mill since I started working on version 1. A full month? I have reset X with an edge finder when I turned the first one over, and then again with this one. Tedious but I trust it to under 1 thou.
Yesterday, after I cut the circular areas and I switched to the 1/4" dia. EM, I made a mistake that I spotted soon enough to not damage anything. I went to the flat area between the raised bosses and set the Z to 0.000. Only it's not. It's -0.062. I noticed doing the fine contour pass (taking off 1/8" all the way around the part) that the heights looked wrong. That's when it hit me what I did. Stopped Mach3, reset that zero to what it should be, stripped the last pass around the part out as a file and ran the last pass by itself.
It wasn't a big deal, it just would have cut the tooling piece deeper than I want.
Keep going Bob, I will chuck-in ideas as I think of them, but you have made what seems to be a very good con-rod!
- Just odd ideas (from what I remember from when I worked on Engine design years ago!):
The con-rod-bolts. What material are they? They "look OK", but as an engineer, that is where I always see a big "CAUTION" sign! The tensile strength of the con-rod bolts are severely stressed, and fatigued, so need to be a VERY robust design (usually) to cope with the stresses and resist fatigue. Typically, the endurance limit is less than half the tensile strength of a bolt (the single cycle failure point). EVERY corner of the bolts is usually radiused with quite a large radius, unlike simple "bought" bolts. Never having design the bolts, I cannot advise much, but on car engines it is common for the bolts to be 120ton steel, often forgings, heat treated, and very complex shapes. At 2000rpm (a pretty slow model?) the rods perform 120,000 cycles in 1 hour of running, or 1 million cycles in little more than 8 hours running. Of course a car engine/industrial engine can be expected to perform thousands of hours running, and typically up to 6000rpm or more. (Motorcycle engine s easily double that). So lifetimes are designed for hundreds of millions of cycles.... for reliability. Can you get some high tensile hex. socket cap screws with solid shanks through the most part of the rod? (I can only see the heads, not shanks, of your bolts) - preferably installed in "sized" holes (reamed?). (I guess you have done so?). And another idea... the washers under the bolt heads are normally precision ground so they do not create a skew face for the bolt heads to sit upon. The stress raiser of even the tiniest "out-of-square" between the axis of the bolt and the face under the bolt-head can dramatically reduce the life of con-rod bolts, and is OFTEN a cause of failure of the bolt-heads. (Ask guys that race engines!). Many engine makers specify replacing big-end bolts with brand-new parts EVERY time that the rod-end is dismantled, and re-assembled. It is that critical. Models mostly survive because they have very short running periods, at slow speeds, and low loads. Except aero-engines.
I guess there will be some simple maths somewhere on the web to determine bolt sizes against your design? I'll have a look when I get some time. (DIY at the Daughter's house today!). AND often the threads are special rolled threads, as machined threads are simply too prone to early life failure at the "pre-engineered cracks" in machined thread roots. Nuts also need to be as good as the bolts.
Bolted Joint_R1.doc (live.com) - may be of interest/relevance?
Also, based on root diameter of thread, the #6 x 40 is about 13% "stronger" in tension than the comparable #5 x 40. (88% of tensile stress), because of the increased thread root area... Bolts threads should be as "perfect" as you can get them, lubricated and torqued accurately to the pre-load so they are never slack in the dynamic loading on the rod. (allowing of the coldest start-up - differential expansion). Best Way to Improve Fatigue Resistance of a Bolted Joint - Nord-Lock Group may be of interest?
Well done so far with the rod! I'm sure your expertise (and CAD-CAM) has set datums that work, (as otherwise you would have a lot of scrap!).
Most interesting from my chair.
Thanks for posting your process and thinking for making the rod.
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