Threading inserts – why different sizes?

[simon Hewitt 1Participant @simonhewitt1]

Why do indexable thread inserts come in different pitch sizes? Surely the geometry is always the same? Its just the thread depth? Or is it a question of strength? If I only want to buy one or two, to cut from say M4 to M12, what should I get?

[simon Hewitt 1Participant @simonhewitt1]

[simon Hewitt 1Participant @simonhewitt1]

Looking again at the inserts (eg here https://www.shop-apt.co.uk/metric-iso-external-threading-inserts/16er-175-iso-t300-external-threading-insert.html – usual no connection, they are quick and reasonably priced). They have what looks like a 'parallel' cutting edge, would I be right in assuming that it will start cutting the OD if I try to cut beyond the thread depth?

Which suggests a coarser pitch insert will cut finer threads no problem?

[pgk pgkParticipant @pgkpgk17461]

The helix angle will be different for coarser/finer threads and different pitches.. why many inserts claim a limited range of pitches.

[Nick_GParticipant @nick_g]

.

I would 'imagine' that the rounding at the very tip of the tool would increase in size relative to the pitch desired to be cut.

Nick

[JasonBModerator @jasonb]

"Full Form" inserts have the correct rounding for the crests and valleys so are size specific, "partial form" are not rounded so will cover a range of pitches per insert but you may need to knock the corners off the crests with a fine file

Have a look at Cutwell, they show the range of pitches covered by partial form inserts here

Edited By JasonB on 20/11/2016 12:39:54

[Anonymous]

There are two basic types of indexible threading inserts; those specific to one pitch, and those that cover a range of pitches.

Inserts that cover a range of pitches have the correct thread angle, but the root dimensions are set for the finest pitch, and will therefore be incorrect for coarser pitches. In addition they do not form the outer diameter or crest.

Those inserts designed for a specific thread pitch will correctly form both the root and crest for the particular thread pitch.

I screwcut a lot of imperial, mainly Whitworth form threads, and I prefer to use the pitch specific inserts as they give the correct root and crest radii.

Note that external and internal threading inserts for the same nominal thread are different, to account for the differences in internal and external thread forms. As in this picture of external and internal full profile inserts for the same thread form, in this case 2mm pitch ISO:

Andrew

[MuzzerParticipant @muzzer]

You want a "partial profile" insert (aka "agnostic" by the Mercans). This has the 60 degree angle but doesn't have the rounded tip which is particular to a specific thread pitch. You should be able to make do with 1 or 2 such inserts to cover the range you need.

Cutwel keep them too.

Edited By Muzzer on 20/11/2016 12:43:15

[Clive FosterParticipant @clivefoster55965]

One gotcha with partial profiles is that they need a bit of extra infeed over book values for all except the smallest pitch. Due to the point dimensions being set for the smallest thread they are a little more pointed, and so longer, than book size for the other threads in the range.

One of the many reasons I like the Zero-2-Zero threading method is that its a lot easier to sort out what's going on and make corrections for such things.

Clive.

[JonParticipant @jon]

If your doing threading that don't require or can get away without the proper form and tolerance, choose a universal angle as mentioned above which purely cut a V. These will come either 55 or 60 degree designated by angle after the ER or IR number ie IR16 internal or ER16 then A55 A60 etc. Unless Sandvik that use special coding irrelevant to to what all the rest abide by.

Full form will be to the specific pitch ie ER16 1.25 which will be external 1.25mm pitch with correct radius for the root.

Personally do a lot of high tolerance stuff that require every morsel of strength, no play. Full form way to go as each cut takes down the root and o/d.
Major reason I like the set to zero and plunge in 90 degrees, no messing about way is you have a direct measurement of depth going in which will change with every pitch and tolerance.

Best inserts found that cut well and last longer than all others are Iscar and Seco. Do have cheapies from various places and they don't last in comparison by as much as 4 times.

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