Last week, our forum delved into the nuances of machinist work, with members sharing insights on specialized training and design courses relevant to defense contracts and mold creation. There was also a lively debate about thread types, and practical discussions on optimizing shift schedules and achieving consistent finishes in machining processes. Members also explored terminology quirks in die design, rounding out a week of diverse, technical conversations.
This Week’s Hot Topics
Worthwhile training for cleared defense work
Members are discussing the best training programs for machinists working on defense projects. This is a crucial topic for those looking to enhance their qualifications and meet specific industry standards. Read more here
Courses that improved your mold design
This thread gathers recommendations on courses that have significantly improved mold design skills. It’s a valuable resource for anyone in the mold-making sector seeking to refine their expertise. Read more here
Second shift reshuffle to hit Friday ship
There’s a practical exchange happening about how to best organize second shift work to ensure shipments go out on Fridays. This is key for operations looking to optimize productivity. Read more here
UNJ vs MJ thread nerd-out
A technical deep dive into the differences between UNJ and MJ threads, with members weighing in on their preferences and applications. It’s a must-read for thread enthusiasts. Read more here
Proper term for a ‘ballizer’ in a die
The community debates the correct terminology for a ‘ballizer’ used in die design. This discussion is a great example of how terminology can vary across the field. Read more here
Consistent RA on torque-plate hones
An important conversation on achieving a consistent roughness average (RA) when honing with torque plates. This topic is essential for improving finish quality. Read more here
Thanks for keeping up with the discussions and sharing your knowledge. Looking forward to more engaging exchanges in the week ahead.
I’ve had better luck thread milling UNJ in 17–4 H900 with an AS8879-profile cutter and two light spring passes — it keeps the root radius honest and the Go/No-Go stops arguing after passivation. If you switch to MJ, trim radial DOC and watch crest truncation; anodize will steal your green light faster than you think. @CNC_Lisa is right on shifts — we front-loaded tool touch-offs to hour one and our 6061 finish stopped drifting like someone finally cleaned the lenses.
Quick tip from last week’s UNJ vs MJ rabbit hole: remember ‘MJ isn’t just metric UNJ’ — the internal tap drill needs to be a hair bigger, and that extra few thou saves me the No-Go fights after hard anodize, which drives me nuts. On 17–4 and Ti for defense work I’ll roll the external and single-point the internal to keep the root radius honest; the standard M/UNF charts will burn you here — use ISO 5855/AS8879 data (https://www.gewinde-normen.de/en/mj-thread.html). @AeroChris, are your gauges happier when you open the minor a touch?
I kept failing MJ external inspections on crest truncation even though the gage was fine until I ditched the generic 60° insert for an ISO 5855/MJ-profile tool. On 17–4 H900, a single light spring pass (+0.0001" comp) and a 0.015" × 45° lead-in chamfer made the Go slide consistently; pushing more comp just swelled PD. Ref: https://www.gewinde-normen.de/en/mj-thread.html — @AeroChris, have you seen off-the-shelf inserts bite you here?
On the MJ side, switching to thread rolling for externals cut our crest rejects to near zero; the trick was using the proper blank diameter (this calculator helps: https://cjwinter.com/resources/thread-rolling-blank-diameter-calculator/) and a tiny lead-in undercut so the dies don’t bruise the start. It’s not always practical if you’re tight on shoulder clearance or the part’s too hard, but it sure beats arguing with the gage, @GageGuy.
On defense work, our MJ internals kept failing at the runout until we added a small relief groove per ISO 5855 — the GO gage stopped hanging and the finish stabilized… In gummy 316L, form tapping MJ keeps the root radius consistent and size stable, but watch torque in blind holes and bump the drill about +0.05 mm if there’s post-coat. @nathan_bell34 is right on the drill sizing; that tiny extra clearance has saved me scrapping parts more than once.
On UNJ externals, swapping to a true J-radius laydown insert and finishing with a 29° flank infeed cleaned up the root and the GO ring quit sticking. For defense jobs that get dry-film lube, we offset PD -0.00025" so post-coat still clears. @thompson30 if your internals still feel grabby, try a shallow 0.5P entry chamfer instead of a big undercut.
Quick example: on MJ internals in 17–4 H900, a single ‘one spring pass’ at 50% rpm with zero infeed plus a 45° x 0.15 mm entry chamfer made the GO ring slide clean by knocking down the entrance feather. Small caveat: in Ti‑6Al‑4V, slow the last two passes about 20% and bump coolant to about 11% or the J-root smears and feels tight even with PD in spec.
Found a bunch of UNJ/MJ ‘feels grabby then fine’ issues were just temperature — parts cut warm on a long shift gauge tight, but at 68°F they’re okay; we park them on the granite for 10 minutes pre-inspection and the drama vanished. If you can’t stabilize temp at the machine, bump wear comp to sit mid-PD and note it on the traveler, @nora39.
Small tip that saved me last week on UNJ internals per AS8879: bump the pilot drill +0.002–0.004 over the standard UN value so the thread mill isn’t bulldozing the minor radius — finish got better and gaging stopped being fussy. @taylor_h93’s temp note is real, but even cold, Inconel 718 still needed a tiny undercut at the runout or the pullout burr made it feel ‘tight then fine’.