Thread Cutting insert choice

[Speedy Builder5Participant @speedybuilder5]

A lot of info in thread inserts BUT how does one choose a suitable insert.

I assume each insert is for a particular thread size, Pitch/TPI/Ba or other, then there is the material selection let alone chip breaker etc.  That’s before we start on internal, external, left or right handed.  Obviously thread angles for those 3 are fixed, but the thread radius changes for each diameter. Do you go out and buy a “special” each time you cut a thread, or what?

Bob

[bernard towersParticipant @bernardtowers37738]

Thread inserts or screwcutting inserts?

[JasonBModerator @jasonb]

Never seen external helicoils so assume it is cutting inserts.

You can go out each time and buy a “full form” insert which will have the correct geometry for that particular thread and pitch where the tip radius and crest rounding are exactly right.

Or you can buy “partial form” where the basic shape is 55deg whit, 60deg metric/UN, etc but the point of the tool will have the smallest radius for it’s given pitch range. This works fine for most hobby uses but you will have to play about a bit with depth of cut (typically a bit deeper) and may need to dress the crests of the resulting thread with a file run against the rotating thread.

[Speedy Builder5Participant @speedybuilder5]

Thanks. So if you choose “Partial Form” screw cutting inserts, as a hobby turner cutting mild and low carbon steel, and brass/bronze, are there a “general purpose” inserts. Ie if using plain old turning tools, we alter the rake angle for certain jobs – quick touch on the grinder and off we go, but not so for inserts.

[JasonBModerator @jasonb]

Not really, in partial you may get some intended for stainless but otherwise the “general use” will do for all metals etc.

The full form ones do have a few more material specific options like aluminium.

[David SeniorParticipant @davidsenior29320]

In an ideal world you would have the correct tip for every pitch. I ignore material and chipbreaker variations but I do prefer the tip that gives the correct geometry for the pitch. It is possible to buy tips that cover a range of pitches, which can be useful in the absence of the correct one.

The thing that gets missed in most catalogues is the angled shim that the tip sits on. I don’t know whether changing them is no longer required, but I always believed the tip should be angled to suit the helix angle of the thread. This is especially so if cutting a left hand thread (i.e. the tip needs to be angled in the opposite direction). I have never been able to find out what side clearance angle the tips have – maybe that has increased on modern tips hence the requirement for changing the angle is minimised.

So for me I would have the holders (external and internal), a set of shims, and then the correct tip for every thread  I am cutting (and probably the ‘universal’ tips for 55 and 60 degrees). I am lucky in that I have been able to acquire several tips from industrial clear-outs, which keeps the cost down

Of course there are different sizes of tips, but most of the threads we will need use the size 16.

Dave

[mgnbukParticipant @mgnbuk]

Easy enough to determine if you want inside or outside, LH or RH and the thread standard / included angle ?

Then two options – partial profile (cuts a range of pitches, 55 degree or 60 degree) or full profile (cuts one pitchof a particular standard  only).

Partial profile inserts will be too “pointy” for the larger pitches, as the tip radius will be to suit the finest pitch the insert will cut. This also means the thread will have to be cut deeper than it theoretically would be if the tip radius was sized for the pitch being cut. A quick look at a Vardex catalogue shows that one of their external 60 degree partial profile inserts Size 11 will cut from 0.5 to 3mm pitch or 48-8 TPI.

Full profile inserts have the full thread form including the root and crest radius (or flat). Cutting depth should be according to thread data, as the tip radius / flat will be correct for the pitch. But you need a specific insert for each thread pitch / standard you want to cut. But a, say, 1.5mm pitch ISO insert will cut that pitch on any diameter component, you don’t need a different insert to cut M10x1.5 or M50x1.5 as all the details of the thread are referenced to the pitch, not the OD.

Cutwel have their range of threading inserts on one page. Select one and on the page for a particular insert is a link to the Vardex thread cutting tool catalogue.

At ex-work all the CNC lathes were equipped with an external threading tool holder in the turret, usually populated with a 0.5-3mm pitch partial profile insert that did for most general purpose metric & Unified threads. The cutting depth for the thread was adjusted in the program until a thread gauge fitted correctly, not to thread depth taken from tables. Threads outside that range usually had a specific full profile insert bought in to do that job, as did internal threads.

Nigel B.

 

[Speedy Builder5Participant @speedybuilder5]

Thanks for the info, time to look at the catalogues here in Europe.

Bob

[Martin KyteParticipant @martinkyte99762]

Does everyone cut deeper with partial form threading tips. I cut to depth and then increase the width of the thread.

 

[Martin of WickParticipant @martinofwick]

Need to consider what size of threading. Common internal insert sizes IR/ER 11 and 16. For internal threads you can get the 11 inserts on tool holders down to 8mm. For 16 inserts I think the smallest holder is 13mm (someone is bound to find otherwise!). Generally the full form cutters that are readily available are 16 size (or larger).

Beware some of the partial form cutters can come as designated A or AG ie from .5 to 1.5 pitch and .5 to 3mm pitch respectively. Why? who knows, perhaps it is useful to have a smaller cutter projection for tight spaces. Just sayin’ because if you wanted to cut a 2.0mm  pitch thread and only had an IR11A60 you might be disappointed.

 

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