Tap and die trouble

Hi all,

today I was trying to thread some thin bar about 10mm thick top and bottom about half inch deep and it was bloody hard work. I was using 3.8 die.

no matter how tight I had the bar in my vice it kept moving round and made it a nightmare to do, only managed to do one end.

Is it because I’m using a die too small for the bar.

I haven’t done anything like this for many years.

what am I doing wrong.

thanks for your help.

chris m….

What is a 3.8 die? If that is 3/8", then your 10mm bar is too big. Turn it down to a bit below 3/8", about .370" should do. And put a good chamfer on the end.

We await more details.

Tony

Beat me to it Hopper.

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.

‘About” is not close enough when it comes to threading bar.

The quality and condition of the die is also something to consider, just as the material it is made from.

Likewise the bar material and state of hardness can have a considerable bearing on how easy it is to work.

Were you using cutting compound? That helps.

Clearly the bar is not too small for the die used, but may turn out to be far too much over-sized, as stated above.

Was it a split or solid die?

Edited By not done it yet on 06/07/2022 18:05:20

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

Surely 9.3mm dia.?

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

Have you bought a copy of Zeus Charts, or Tubal Cain's "Model Engineers Handbook"?

There are lots of places that tell you the dimensions of threads, external and internal so that you can get it right rather than struggling.

You can learn a lot by studying books, and avoid struggling, when doing things correctly makes life so much simpler.

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.

Good advice above Chris but let me add another thought - it is very hard to hold larger bar for tapping in the vice (or 3 jaw) chuck without marking it or it slipping.

So I generally use an ER collet block to grip the material and then hold the collet block in the vice soft jaws. It will not overcome oversized material but does make tapping very much easier. Love my collet blocks!

Regards,

IanT

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.

It goes without saying that Trefolex, or Rocol RTD should be used as a lubricant.

Making the material the right size helps a great deal, hence the popularity of tables of Tapping Sizes, such Zeus Charts or table provided by suppliers..

Howard

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

If they want it, (Or even know how to use it )my grandchildren will not be short of RTD, spirit blue, or Micrometer Blue

Howard