Identifying the pitch of threading inserts

This topic has 6 replies, 5 voices, and was last updated 26 June 2026 at 06:47 by JasonB.

Viewing 7 posts - 1 through 7 (of 7 total)

Author

Posts
25 June 2026 at 21:49 #853436
Bill Phinn
Participant
@billphinn90025
I was sent five randomly selected external threading inserts a while ago when I bought an indexable threading tool from a well known ebay seller of lathe tooling.

The seller didn’t tell me what inserts were in the package. That didn’t matter for two of the inserts, because their markings identify them, but the markings on the other three aren’t quite enough of a help.

I gather from the appearance and markings that are there that all three are partial profile inserts and two of them are 60 and one 55 degrees. The 55 degree one has Q788 on it, one of the 60s has M412 on it, and the other 60 degree one has no markings on it at all.

Ed. Sorry, I’ve somewhat booboed here, haven’t I? These are partial profile inserts, not full form, therefore the pitch isn’t fixed.But how do I know what the biggest pitch they can do is?
Advert

25 June 2026 at 22:30 #853441
Andrew Crow
Participant
@andrewcrow91475
Hi Bill, looking at the radius of the tip of the tool I would suggest that the minimum pitch would be more the question as they all seem to have quite a large tip radius. The maximum pitch would be governed by the depth that it would cut.

Alternatively perhaps you could contact the supplier to see if they can provide more information.

Andy.
25 June 2026 at 22:35 #853442
Mark Rand
Participant
@markrand96270
AG55 and AG60 are, as you realised, partial profile ‘general purpose’ inserts. assume .5 to 3mm pitch or 48 to 8 tpi. US and ISO threads for the AG60. BSW and BSF for the AG55

Being old before my time (less less the case as the years progress!), a bit of a pedant, and having a preference for Whitworth and BA threads, I’d say that the AG inserts are OK for stuff that really doesn’t matter or in an emergency.

Full form inserts produce a far nicer thread form especially if you are interested in root and crest geometry.
25 June 2026 at 23:07 #853443
Julie Ann
Participant
@julieann
On 25 June 2026 at 22:35 Mark Rand Said:

Full form inserts produce a far nicer thread form especially if you are interested in root and crest geometry.

+1

Probably 99% of my screwcutting is done with full form inserts. Especially good for Whitworth threads when the proper rounded roots and crests are formed.

Julie
25 June 2026 at 23:21 #853444
Bill Phinn
Participant
@billphinn90025
Thanks for your replies, Andrew and Mark.

I wasn’t fully aware there might be a minimum as well as a maximum pitch, Andrew. That’s important to know.

Mark, the two other inserts I can identify are of course identifiable because they’re full-form inserts.

Two threads I hoped to be able to cut at some point soon are 1” BSW and M24 coarse. I can single-point cut a 3mm pitch on my lathe, but not an 8 tpi unfortunately. I tried tapping a piece of 36mm af mild steel hex bar the other day with an M24 tap, having bored out to 21.2mm.

I did it, but it was very hard going, and I think the nut moved in the chuck causing a slight skew in the finished thread.

While we’re on thread cutting, what’s the consensus on which is the better of the two lathe options for reduced laboriousness in single point threading: an ELS or a gearhead lathe?
25 June 2026 at 23:29 #853445
Bill Phinn
Participant
@billphinn90025
The full form inserts aren’t exactly unaffordable, so I will bear your and Mark’s advice in mind, Julie Ann.
26 June 2026 at 06:47 #853450
JasonB
Moderator
@jasonb
What about the third option of a couple more change gears to be able to do 8tpi or 3.175mm pitch. It really comes down to how often you thread cut and what you may want that is outside what can be done with change gears. But if you are otherwise happy with your lathe then go with the ELS conversion but bear in mind that just like you a small lathe may find 8tpi hard going.

Myself I don’t thread cut much so am happy to use change gears on the banjo and 100% of threads cut with inserts are partial form, againjust don’t do enough to justify having inserts for each.

Also be aware that partial form inserts need feeding in further than full form

The other question is why did you tap a hole but buy external inserts presumably for the same job? or is it the mating male thread you now want to cut?
Author

Posts