Threading with inserts

I can't decide if this is a tip, a question or just an observation.

I have to do a fair amount of threading and often use the ER16 type inserts.

I have noticed that the threads can look quite rough and then suddenly as you get to correct depth magically get much better.

Has anyone else noticed this?

If they have do they know why?

I post this here as I have seen people complain that they don't get good results and wonder if this is why.

I should point out that I don't use angled infeed on the manual lathe but do on the CNC.

I also it doesn't seem to be a depth of cut issue because when doing batch work I am going off the DRO and am able to finish the thread with 1 spring pass.

Any thoughts?

Regards.

David.

I always use angled feed, having found that with straight in, the chips from the two faces seem to cause problems until, as you say, the final cut. Full thread or partial inserts?

I use full thread inserts 99% of the time.

I'm not cutting anything bigger than 10 tpi and mostly its 16tpi or finer.

Could simply be the second "tooth" of the full form insert is doing a fine finish cut as well as rounding off the crests which will take off any burr from earlier passes

I was looking at suggested cutting criteria the other day for a threading insert and the DOC decreased as you got closer to the final pass so may also be worth looking up what is suggested for the particular inserts you are using.

Interestingly not all insets have the second tooth. The photo shows two 1.5 mm pitch inserts...

As to the depth of cut. I suspect that I err on the side of caution. I seem to have stopped looking at manufacturers data for cutting speeds as this invariably spells disaster for me .

Perhaps next time I'm doing some threading I should push one of these inserts and see what is possible.

Another question. Has anyone tried resharpening one of these inserts? It looks like it should be possible to take some off the top of the insert and still leave the correct profile. I haven't tried this yet but I keep the dull ones in a drawer waiting for the time of desperation.

I hope I am not telling you something you already know, but are you aware that threading inserts come as full profile, suitable for only one thread pitch and as partial profile, suitable for a range of pitches?

Full profile ones look like that on the left, where the second tooth finishes the crests.

While both the inserts you show may be able to cut a 1.5mm pitch thread, only one of them is strictly a 1.5mm pitch insert.

Partial profile metric are usually labelled A60, G60 or AG60. Full profile will have a number for the pitch as part of their label.

Sometimes, on steel. Haven't seen it on cast iron or brass. I start with large depths of cut,up to 20 thou depending upon the thread pitch, and reduce DOC once I'm close to full depth. For stringy materials like steel it's possible the swarf on the first cuts rubs against the flanks as it tries to escape. With smaller DOC the swarf is smaller and thinner, so more pliable, and will have less of an effect on the flanks. For brass and cast iron where the swarf is fine particles the issue doesn't seem to occur. I do all my screwcutting by plunging straight in.

Insert profiles are rather more involved than DC31k propounds. Here are three inserts:

Which are full form and which partial?

The one on the left is marked 16ERAG60, so a partial profile for a 60 degree angle thread. The middle insert is marked 16ER-8W, a full profile insert for 8tpi Whitworth, where the partial tooth does indeed form the rounded crest, as specified in the Whitworth form. The righthand insert is marked ER16-2.0ISO, a full profile insert for 2mm pitch ISO threads. These threads have a flat crest, so the second tooth is not needed to fully form the crest, a flat is sufficient.

Andrew

Regarding sharpening this type of insert, I can think of two things. First, the height would be slightly lower, and second, they are probably tapered back slightly, so reducing the height would change the profile. The profile would reduce slightly, say from a true 28tpi to more like a 29tpi. This would only be true with full profile inserts, the partial profile ones are already a compromise for use with a range of pitches on non critical applications.

So you would think, however both of those inserts are labelled ER16 1.5 ISO and are listed as full profile.

Andrew,

Most of my work is in EN16T, which machines very well. I do reduce depth of cut as I get nearer to finished size but not in any scientific way .

The centre height issue is something I could live with given I would only want to do it in an emergency. The second issue is the more important. Perhaps I should give it a go!

By coincidence the last thread I screwcut was M16x2 in EN16T, for a cutter drawbar. The finish is ok, but since I was cutting a metric thread on an imperial lathe I couldn't use my high speed threading attachment. I was running at slow speed, 85rpm I think. Here's the thread:

The thread is better than it looks, and works fine.

Andrew,

The full profile inserts will have the correct tip radius for the thread pitch, this is an important part of the specification for threads which have to be top quality such as for aircraft fastenings. My old firm would not have ever bought partial profile inserts for this very reason. They mostly cut UNJEF threads which have a non standard root radius which is better for fatigue life. For our general purposes, the partial profile inserts will still give superior results to die cut threads, but the difference is so small that only laboratory testing could tell the difference.