There are two issues with partial form threading inserts. One the root radius is set for the finest pitch. So for coarser pitches the depth of cut needs to be increased above the theoretical value. It can be calculated, or done by trial and error. Two the crest of the thread is not finished so will need to trimmed with a fine file to get rid of the inevitable extrusion of metal during the cutting process.

I always use full form threading inserts. Most of my screwcutting is Whitworth form. A full form insert forms the correct radius on the crest as well as the root. No messing with files, just a quick brush to get rid of swarf before testing for fit. Experience shows that thread depth, as put on via the cross slide, is usually within a couple of thou of the theoretical value.

To summarise I use full form inserts for all thread forms; Whitworth, Unified and metric. Just makes life easier. I tend to cut a lot of same pitches/tpi so don’t have anything like the full range of inserts; just buy as needed.

Julie

Unless you’re working on a high stress / cyclic load part or need to conform to a certain standard? Nah, you’re usually good as long as the nose radius isn’t over spec.

The biggest benefit of a full profile insert is simplicity, they will produce thread that is on-spec, burr free and can be checked with a single micrometer measurement.

Partial form satisfy my needs. I don’t have a lot of call for screwcutting and what I do need is usually fine pitch 0.5 to 1.0mm but of larger diameters upto 50mm. I can live with running a needle file over the crests or simply starting with a reduced diameter.

If it is a standard thread like Metric coarse or BSP then I can part screwcut and finish with a die so that takes care of teh final profile.

As with everything what the threads are intended for, how many you are going to cut and budget will often be the deciding factor and only you can answer that.

I use both partial form for general screw cutting which may be finished with a die but for some parts i.e. a lathe spindle I use a full form cutting insert as the hollow spindle could be compromised by the root depth being too deep on both the chuck holding and the rear bearing tensioning threads.

David

Bill, My 16er holders are 10mm tall by 12mm wide!!!. They came from Jenny at JB Cutting tools.

On 23 December 2025 at 15:41 larry phelan 1 Said:

Pity me ! I,m still using bits of HSS ground by eye and mounted as near centre height as I can see !  Still, the bits go together OK.

works for me too

Tracy Tools sell holders for Coventry die chasers. These are designed for both hand-chasing if mounted in a suitable handle, or holding in the tool-post.