Thread inserts or screwcutting inserts?

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

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.

Essentially with a partial form and small radius if you cut to theoretical depth you wind up with a groove which is too narrow to accept the mating part of the thread. Assuming the depth has been reached by the cross slide using a straight in cut the thread groove can be widened out incrementally using the top slide to shift the tool sideways a little and shave the flank. This is continued until the thread is symmetrical and a good fit is obtained. You wind up with a flat bottomed root of the correct diameter and tidy crests. The crests can be cleaned up ( for whitworth form using a chaser, a die or a file) I prefer a die.

regards Martin

You can get away with general 60 degree inserts for quite a range of sizes both metric and Unified. Only get specific sizes if the ultimate precision demands it. Same goes for Whitworth form threads. You would not get away with a great difference, say a 12mm 1.75 pitch insert would be too blunt to cut a 6mm thread, but it would work with a 10mm 1.5 pitch. If you buy generic sizes, they usually give the range they will cover. I have bought a lot on ebay over the years, and also from APT who sell threading inserts singly to private customers as well as the trade.

As for a good size to start with, er16 covers from about 0.5 mm pitch right up to 8 ACME, but er11 might be better for model makers.

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.

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 🙂 ) special anvils, then it’s just as easy to tip over the tool holder with a tapered shim made on the mill, shaper or what ever you have. I’ve used that when making replacement LH ACME feedscrews for a BCA jig borer. Helps to use a set of punches, or similar, to mark them up so you can remember what they’re for next time…

Edit to add:-

If you can set the top slide over to a bit more than the flank angle of the thread (e.g a bit more than 60° from parallel or less than 30° from radial) and adjust the tool back to ‘straight in’ and feed in with the topslide, then the tool will mostly be cutting on the leading edge and only taking a skim cut on the trailing edge. This makes the chip much more flexible and drastically reduces the number of chipped tools.

If you do that, the depth you need on the topslide is (the thread depth)/(sine of the angle you’ve turned the topslide). That gives you a guide until you start offering the nut up to the thread to see if it’ll actually fit. Calculators may be used in this exam 😀 .

It should also be noted that the internal threading inserts which are generally available are of limited use producing standard threads. They need larger bore sizes than the threads that match their pitch. One can get some toolholders and inserts that’ll make standard internal threads, but double checking is vital to ensure that tool and insert will fit the inside diameter needed. For standard threads, I tend to use taps and only single point larger diameters.

Sorry for waffling, but hope there’s some valid information in the ramble…

I’ve just checked and mine are Posithread.

I guess you could get an 11BA left hand internal insert specially made. Might need an electron microscope to see the screw that’d be needed to mount it to the custom made toolholder 😀

Failing that, the smallest internal inserts and toolholders available are 6mm inserts and holders that can just manage a 6.8mm ID. So no good even for 0BA…