In my opinion, it's virtually impossible to create significant CNC programs without first spending a good deal of time working with cutting tools, fixturing, machines, and materials. Coding, as with application programming, is often the easiest part of getting the job done.
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I spent twenty-five years doing cnc programming and process planning in a variety of machining environments. I always aimed for the middle of the tolerance, be it unilateral or bilateral. I don't recall ever getting any negative feedback from customers that had to do with my interpretation of...
I worked for several years for a major manufacturer of large electric motors. Their manufacturing documents specified a 55% thread for cast iron and low-carbon steel. I don't recall hearing of any thread failures.
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My calculator comes up with 70 SFM at .0086 IPR for HSS drills on a similar material. That's about the same cycle time as you're doing now - the question would be is tool life any better.
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You would benefit greatly from thru-coolant tooling, particularly if you're using a vertical machining center. Peck drilling with carbide is generally not recommended, as it can lead to thermal shocking and/or recutting of chips.
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What's the width of the groove? If really small, you might have to go around two or three times, which could get expensive.
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http://closetolerancesoftware.com
As others have said, single-pointing (lathe) or thread milling (machining center) would be better approaches. If lot size was small enough, you could even threadmill with the single-point tool (slow).
The tap would be an expensive, single-purpose tool.
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This free program is designed to convert G-Code to dxf. Judging by the user comments, it works pretty well.
http://www.cnczone.com/forums/opensource_software/8814-g-code_dxf.html
Software For Metalworking
http://closetolerancesoftware.com
If the tip of the threading tool has chipped or broken down, or if a tool with the wrong radius has been selected, you'll never know it using thread wires as your only check. The pitch diameter and pitch can be good while the root geometry isn't.
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