We await more details.

Tony

When cutting a thread, the stock needs to be smaller than the nominal diameter. As an example, the M5screws I've just made were turned down to about 4.9mm before I even considered getting the die out of the box.

3/8" is about 9.5mm, so your stock is about 1mm too large.

The maximum rod diameter for 3/8" is 9.5mm, which would be hard work. 10mm is way too big.

It's usual in model engineering, where full strength is rarely required, to reduce the amount of cutting done by shaving rods down below the full size a die will tackle, and by drilling tap holes a shade too big. About 10% or 12.5% less than theoretical.

A 3/8" die in good nick is much happier when the rod diameter is reduced to about 8.3mm and cutting fluid used. Although it doesn't reduce the strength much, don't abuse the principle if the fastener mustn't strip.

The 3/8" die may be blunt now it's been forced down a 10mm rod, especially if the rod is something nasty like stainless. If the die doesn't comfortably cut 8.3 mild-steel rod, scrap it. Also, don't use the same taps and dies on both steel and brass: brass likes a sharp cutter, and steel blunts them enough to make a difference in a blink. Taps and dies too worn for Brass are still useful on other metals.

Dave

Unless the female thread material is very weak, reducing the OD of the male has no effect on strength of the assembly. Threads fail either by tensile failure in the root diameter of the male, or by shearing along the cylinder of equal strength in the thread. I suppose it is just about possible to fail in compression along the helical thread face but unlikely

The Tracy Tools chart that is my usual reference gives tapping-drill diameters it tells us are calculated for 60-70% depth of thread. I have never found any need to deviate from those.

It also gives these conversions for Dave's figures (with Mick's correction!):

9.5mm = 0.374 inch.

9.3mm = 0.3661 inch, a healthy 0.009" down on nominal size, probably that given by the standard crest-rounding for the thread type you were using – though you don't state what that was.

.

Also, although trying to cut drastically over-size steel was the main problem here, the grade of steel and quality of its finish could also affect things.

Good-quality free-cutting steel, or something from …&… (other household-DIY merchants are available)?

Recently I used some of their stuff sold as 6mm diameter. It was not only roughly 1/4" dia instead so would have been no good for an M6 die had I intended to thread it that size, it also had longitudinal ridges I had to remove before it would pass through a 1/4" hole drilled in hot-rolled steel! For its purpose, draw-filing sufficed, but this material would have been no use for high-accuracy work.

You beat me to it. I had a similar problem running a die, which was a 3/8" BSW. Ended up using the collet block. Although Whitworth is very course & a swine at best of times.

Steve.

Ah, yes – the smell of hot (green) Trefolex in the workshop! Mmmnnn…

My tin is going to easily outlast me – maybe I should leave it to my eldest as part of my estate….?

Regards,

IanT