Indexable threading insert for hardened steel

[IanHParticipant @ianh]

Hi,

I have a hardened steel shaft something like 60 hrc and I want to cut a 20tpi external thread on it. I am well used to using full profile threading inserts but thought something a bit special may be required for this job. I wondered if anyone had a recommendation for an insert manufacturer, grade (and supplier) that will do the job? Any hints on cutting conditions would be helpful as well, I am assuming slow and dry.

Ian

[DC31kParticipant @dc31k]

I like the idea of an indelible insert. Same thinking a a permanent marker I guess. In addition to being indelible, most of my inserts are also inedible.

For this application, look in the technical literature of the insert manufacturers. Sandvik make inserts in what they call CB7015 grade, which are claimed to be good for up to 62 hrc. If they make them, their industry peers will also make them.

[DiogenesParticipant @diogenes]

Yes, you are into CBN territory – threading inserts seem to be a bit thin on the ground amongst the places I’d normally look for that kind of stuff..

What is the shaft / is it a ‘critical’ component – would it be possible say, to use a normal CBN insert to turn off the hardened skin and cut a thread on less-hard material beneath?

[IanHParticipant @ianh]

This is the input shaft for the Morgan Three Wheeler gearbox. The main shaft is captive in the gearbox shell, the coaxial input shaft though can float in it bearing. The spigot on the end of the main shaft runs inside a bronze bush in the input shaft and I have too much clearance between the bronze bush and the shoulder on the main shaft spigot. I could make a custom bronze bush to take up the clearance or introduce a spacer between the input shaft and it’s bearing. I was considering the option of locating the input shaft in its bearing with a sleeve on the outside of the shaft and a screwed collar to pull it all together.

The input shaft is relatively thin walled with key ways in the bore so not enough meat on the bone to machine through the case. A custom bronze bush is probably the easiest way to go….

Ian

[DC31kParticipant @dc31k]

CBN (and PCD) ER16 and IR16 inserts in AG60 and AG55 are available from eBay sellers direct from China. The one I saw sells two inserts for £26. Note that in this form, each insert has only one cutting edge, so your cost is £13 per edge.

20tpi coincides with a Clarkson thread, hence the Whitworth form ones above might be useful in that context. Would thread grinding, using a toolpost mounted device to spin the abrasive, be an option?

Following on from above, if the item is through hardened, or the thread has to be the same diameter as it is currently, turn it down with a CBN cutting insert (a lot cheaper than a CBN threading insert), shrink or loctite on a sleeve of softer material and cut the thread as normal in that.

[DiogenesParticipant @diogenes]

Ian, yes, I’d probably just replace the bush – are the shafts unusual sizes? – if they are, say fractional imp. dimensions you might still get something off the shelf..

[David George 1Participant @davidgeorge1]

have a look at the APT web site.    https://www.shop-apt.co.uk/metric-iso-external-threading-inserts/16er-050-iso-t250-external-threading-insert-premium-grade.html

They have inserts for hardened materials at a resonable price and if you call them they are very helpful.

David

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