Thread Cutting insert choice | Model Engineer & Workshop Magazine

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.

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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

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.

I hunted the internet and found a radius for the size 11 inserts I have for general use and then calculated the distance from a true point to the actual tip based on that radius. This gives distances of 0.107mm for the 60° inserts and 0.124mm for the 55° insert. You can then look at the calculated figures from a true point to the outer surface of the thread that are a given proportion of the pitch and from these calculations I created a spreadsheet for the required depth to cut to give the correct thread. Tried it out and it worked in that I cut the “normal” depth and it was too shallow, went to the calculated depth and the nut I had made to check the thread then went on with what felt like the correct amount of clearance.

Martin C

These days I tend to use inserts because they give very nice threads compared with hand ground HSS bits. I like to use full form inserts, especially with Whitworth, BA and ACME threads, because the complete thread-form is a significant part of those standards. I use full form metric inserts where I’ve got them because it makes the calculations simpler and the bolts stronger.

I have got some generic 55° and 60° inserts, but I only use them in desperation because I have known threads to fail with low cycle fatigue due to having a sharp root radius.

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Usually, with inserts, the thread angle/helix of typical threads is handled with acceptable accuracy (i.e. you ain’t going to measure the form errors) with a flat anvil in the tool holder and the clearance inherent in the inserts and their relation to the work (a smaller diameter leads to more clearance than a larger one, which mitigates the clearance problems of the larger angle that comes with a smaller diameter for that pitch) .

If you can get them, different anvils can be very useful when cutting coarse left hand threads, especially ACME or other coarse threads. If you need them and you can’t get (or can’t afford