Screws fit, plugs don’t, and every offset change seems to make it worse. Here is what the gages are telling you on small 3B threads.
Category: Threading • Reading Time: ~8 minutes • Audience: Intermediate to Advanced Machinists
Screws go in and out just fine. The Go plug won’t enter. The No-Go won’t enter either. You chase it with a tap and suddenly both plugs go. Or you keep adding compensation until the Go finally threads in, and the No-Go falls through like there’s no thread left.
It’s a frustrating failure on small, tight 3B internal threads, and #10-32 UNJF shows why, because the tolerance leaves so little room. The machine usually isn’t the problem. A screw, a Go plug and a No-Go plug each check different things, and a worn or deflecting thread mill can satisfy one while failing another.
QUICK ANSWER
If the Go hangs while the No-Go starts to enter, stop adding compensation. Pitch diameter is already oversize and the thread is short at the major diameter, usually from a worn thread mill tip. Replace the tool, split the cut into several radial passes with a spring pass, and check the UNJ minor separately with pin gages, because the thread plugs don’t check it.
What UNJ Actually Changes
UNJ (SAE AS8879 and ASME B1.15) is a Unified thread form with a controlled radius at the root of the external thread. That radius is what improves fatigue life, which is why UNJ shows up in aerospace and other high-stress work.
Internal threads don’t need a root radius under those standards. They need a larger minor diameter, so the crest of the internal thread clears the rounded root of the screw. Pitch and major diameter stay the same as standard UNF 3B. Read the print anyway, because some aerospace customer specs add their own requirements, including a radius on the internal thread.
| Feature | #10-32 UNJF 3B | #10-32 UNF 3B (reference) |
| Minor diameter | 0.1596″ to 0.1675″ | 0.1562″ to 0.1641″ |
| Pitch diameter | 0.1697″ to 0.1726″ | 0.1697″ to 0.1726″ |
| Major diameter (min) | 0.1900″ | 0.1900″ |
The minor moves up 0.0034″ on a 0.0079″ tolerance band, so more than 40 percent of the band shifts. The usual UNF tap drill, a #21 at 0.1590″, starts under the UNJ minimum of 0.1596″.
Why the Screws Work and the Plugs Don’t
A screw that fits doesn’t prove much. A commercial 2A #10-32 screw runs 0.1658″ to 0.1688″ at the pitch diameter and 0.1831″ to 0.1891″ at the major. The Go plug starts at 0.1697″ pitch and 0.1900″ major, so the screw has room the gage doesn’t. A UNJ 3A screw has less room, but it still isn’t a gage.
Each gage has a specific job:
- Go plug: a cumulative check of every thread element except minor diameter, per ASME B1.2. Its major represents the minimum major of the internal thread. On an internal thread, the major is the root, the deepest part of the cut.
- No-Go plug: checks maximum pitch diameter and nothing else. Its major and minor are cut back for clearance, so a short root can’t stop it. A No-Go that doesn’t enter is a pass, and B1.2 lets it enter up to three turns on inch threads as long as it isn’t forced.
- Pin gages: the only check here on minor diameter. Neither thread plug will catch a bad one.
So the only real failure in that opening scenario is the Go. The No-Go refusing to enter is what it’s supposed to do.
What Produces the Pattern
A Worn or Wrong Tool Tip
Start here. Mike Oddy of Vermont Gage wrote in Quality Magazine that major diameter interference covers the largest percentage of these problems. A worn tip, or one with too large a radius, leaves the root of the thread short. The Go plug’s major hits it and stops, even with pitch diameter in tolerance.
Add radial compensation to get the Go in and you open the pitch diameter along with the major. On a #10-32 3B the pitch diameter band is only 0.0029″, and a radial offset moves pitch diameter twice as far, so 0.001″ of comp uses 0.002″ of that band. The No-Go doesn’t care about the root, so it drops in as soon as pitch diameter passes the limit.
When the Go Hangs and the No-Go Enters
The No-Go has the larger pitch diameter. The Go has the larger major. If the No-Go enters and the Go doesn’t, pitch diameter can’t be what’s stopping the Go. Look at the root of the thread first, then lead and roundness. More compensation won’t fix any of them.
Deflection and Taper
Thread mills take radial load on a slender body, and the tool bends most at the tip. On a multi-form tool, that usually leaves the thread larger near the top of the hole and tight toward the bottom. A screw with clearance to spare may still run full depth. A Go plug starts, then stops partway. Stickout hurts here the same way it does on an end mill, with deflection rising by the cube of length.
Too Much Stock in One Pass
A single heavy pass maximizes side load and leaves a poorer form than several lighter ones. An undersize pre-drill makes it worse. It won’t stop the Go plug directly, because the Go’s minor is only a clearance feature, but it gives the thread mill more stock to push against. The minor is still out of spec, and neither plug will tell you.
Chasing With a Tap
A tap can clear a short root, which is why the Go suddenly enters. Its pitch diameter is set by its H-limit, so if the No-Go now goes too, pitch diameter was probably already over from compensation, or the tap is cutting large. Either way the chase hides the cause, the next part comes off the same, and you’ve added a tap to the list of things that can break.
What the Plugs Are Telling You
| Go plug | No-Go plug | What it means | What to do |
| Enters full depth | Stops within 3 turns | Functional thread is good | Check the minor with pin gages and run |
| Won’t start | Won’t enter | Pitch diameter is small or the major is short | Inspect the tip, then open up in small steps |
| Starts, stops partway | Won’t enter | Likely taper from deflection | Add radial passes and a spring pass, reduce stickout |
| Hangs | Enters past 3 turns | Pitch diameter is oversize and the major is short | Change the tool and back the offset down. This part is likely scrap |
| Enters full depth | Enters past 3 turns | Pitch diameter is oversize | Back the offset down. This part is likely scrap |
How to Control It
1. Set the Minor Before You Thread Mill
Drill or bore to the UNJ minor, not the UNF tap drill size. For #10-32 UNJF, a #20 (0.1610″) or a 4.20 mm (0.1654″) is a common starting point. Both sit inside the 0.1596″ to 0.1675″ limits, but the 4.20 mm leaves only 0.0021″ to the top, so check what the drill actually cuts. Don’t count on the thread mill to create the minor.
2. Use the Right Tool and Check the Tip
Internal UNJ threads don’t need a root radius on the cutter, so a thread mill for the pitch will do the job. Thread mills ground with the J radius are for external threads. What matters is the tip. Look at it under magnification and replace it when it rounds off, instead of compensating around it.
Multi-form tools cut the whole thread in one helical revolution, which is fast, but more teeth in the cut means more side load. Single-profile tools run with less side pressure and more control on small, tight threads, at the cost of a longer cycle. Our Inconel threading guide covers when thread milling is the right call in the first place. Either way, keep stickout short and hold the tool in something rigid.
3. Split the Cut and Add a Spring Pass
Take several radial passes, each lighter than the last, because engagement climbs as the cut gets deeper. Finish with a spring pass, which repeats the final pass with no added offset. This is usually the cheapest fix for deflection on small 3B threads. Program the spring pass before you dial in size, since it can change the size.
4. Gage Each Feature Separately
- Thread plugs for the functional check. UNJF and UNF 3B plugs share the same pitch and major limits. If your customer or auditor calls for J-marked gages, use J-marked gages.
- Pin gages for the minor. On a #10-32 UNJF, 0.1596″ goes and 0.1675″ doesn’t. Check after threading, not just after drilling.
- Clean the hole and the gage first, and never force either plug. A plug is not a chaser.
5. Break the Compensation Habit
Don’t keep adding radial compensation until the Go goes and hope the No-Go is still okay. Check both plugs after every offset change. Once the No-Go passes three turns, pitch diameter is over. If the Go is still hanging at that point, the problem is form, and the next move is a tool change.
A Practical Checklist Before You Run
- Minor drilled or bored to UNJ limits and confirmed with pin gages
- Thread mill pitch confirmed and tip inspected under magnification
- Stickout as short as the part allows, in a rigid holder
- Multiple radial passes and at least one spring pass programmed
- Go and No-Go checked on the first article, plus the minor after threading
- Both plugs rechecked after every offset change
- Coolant and chip evacuation adequate for the depth and material
UNJ Thread Milling FAQ
Why does the No-Go enter when the Go won’t?
Pitch diameter is oversize, which lets the No-Go in, and the major is short, which stops the Go. A worn thread mill tip plus added compensation is the usual cause.
How far can a No-Go plug enter?
ASME B1.2 allows up to three turns on Unified inch threads, without forcing it. Exceptions apply for thin or ductile material and short threads, and your customer spec can be stricter.
Can I use a UNF 3B plug on a UNJF internal thread?
Functionally, yes, because pitch and major limits match. Your customer or auditor may still require gages marked UNJ. Either way, check the minor separately.
Do I need a thread mill with a root radius for internal UNJ?
Not under AS8879 or ASME B1.15, which only put the radius on the external thread. Some customer specs add one, so check the print.
What drill should I use for #10-32 UNJF?
A #20 (0.1610″) or 4.20 mm (0.1654″) is a common starting point. The UNF standard #21 (0.1590″) starts under the UNJ minimum.
Chapman Can Help
Next time the Go hangs, look at the tool tip before you touch the offset. If that doesn’t settle it, call us. Chapman carries thread mills from Helical and Emuge-Franken, along with thread and pin gages from Vermont Gage and Meyer Gage. Tell us the size, the material and what the gages are doing, and we’ll go through the process with you and point you at the right tooling or setup change.
Browse threading tools at shop.wcchapman.com, or call us at 410.686.6860.