Thread cutting inserts

[steamdaveParticipant @steamdave]

[steamdaveParticipant @steamdave]

When using carbide thread cutting inserts (either the saw tooth shaped variety or the single pointed ones), do you plunge in to full depth at one go or creep up on the depth as in traditional single point thread cutting? Also, can they be used at low speeds, say in the 25 – 40 rpm range on any material. (my hand eye coordination wouldn't allow any faster speeds!)

Any advice or links for where to get info would be appreciated. Looking at Google, everything seems to be industry related or I'm using the wrong search criteria.

Dave
The Emerald Isle

[JasonBModerator @jasonb]

I recently bought a pair of holders and used them for the first time on a pair of aero engines doing a M23.5 x 1 thread. I used my slowest speed of 50rpm and took several passes to get to the depth on the ali crankcase but faster on the steel cylinder.

Rather than try and stop at the shoulder I used the internal tool at the back and ran in reverse so the tool moved left to right.

 

Edited By JasonB on 17/07/2015 07:44:45

[Roger Williams 2Participant @rogerwilliams2]

Hello, nice work there Jason !. Steamdave, I believe with inserts on a manual lathe you still have to creep up with progressively smaller cuts. I cut an 18mm, 2.5mm thread by 5 inches long at slow speed ( 70 rpm) with a partial profile insert yesterday, and the edge of the insert didnt mind !. This was a right hand thread, but I believe you can fit a negative shim under the insert and then use it for left hand threads. Someone on here might be able to verify this.

[Roderick JenkinsParticipant @roderickjenkins93242]

I've used inserts very succesfully for threads cut by turning the mandrel by hand, so slow speed isn't an issue.

I suspect that many of the doubts about the use of carbide tooling in amateur lathes stem from the early tools with negative rake – required because the tungsten carbide / binder was relatively brittle. Modern carbide tooling is made from a wide range of carbides and coatings, is more versatile and suitable for a much wider range of conditions, including relatively low speed and power. It is our good luck that e.g tips made for dry cutting aluminium at high speed turn (!) out to be rather good for cutting steel in a small home workshop machine. That's my take on it anyway.

Rod

[Graham WhartonParticipant @grahamwharton]

I use Sandvik threading inserts, but just tend to stick to about 0.1mm infeed per pass. Sometimes I may go bigger, for the first passes and then light cuts for the final pass. But as a rule of thumb, my lathe and my tooling works well with 0.1mm passes, apart from the last couple of passes where I am sneaking up on the final dimension, in which case the last couple could be 0.02 per pass.

You can get this sort of information from the insert manufacturer.

Here for example is the Sandvik Threading catalogue from 2012. **LINK**

If you skip straight to Page 71 you can see their infeed recommendations for their threading inserts. OK, these are more suited to CNC threading where you can just programme in a number of passes at the correct depths to know you are operating them correctly, but this can still apply to the home user, if you happen to be using their inserts.

The first entry on Page 71 is cutting a 0.5mm pitch metric thread. They recommend 4 passes of 0.10, 0.09, 0.08 and 0.07 to give a total infeed of 0.34mm.

For most threads the final pass is about 0.07/0.08mm with previous passes of anywhere in the region of 0.1 to 0.3mm per pass depending on the thread pitch.

I would have thought other manufacturers inserts operate in a similar window.

Hope this helps.

[EmgeeParticipant @emgee]

I like the good tip from Jason to save running to the shoulder.

When threading I always make 2 finishing passes at the same setting just to make sure of full depth cut, same as when boring.

Emgee

[KWILParticipant @kwil]

I have only use these types of inserts for threading (under power up to 100+ rpm) for the last 15 years or so, I cannot remember when I last used a single point tool.

Yes you do have to creep up to full depth in a number of passes, the manufacturer's guidance as refered to above should be followed if you want to get the maximum life out of these products.

It is very easy to break the tip of if you do not. Similarly always provide a full depth run off at the end of the thread or shoulder, (unless like JasonB you reverse cut). Even threads down to M3/6BA on S/S are straight forward.

[steamdaveParticipant @steamdave]

Thanks for all the replies. I wasn't sure whether to continue using HSS threading tools or to spend the kids inheritance on a set of carbide threading tools like:

Set of Internal & External Threading Tools 16 MM Shank (Ref: SCT400PR) from Chronos.

Not being of a generous nature, I think I will go ahead and spend!

Dave
The Emerald Isle

[Graham WhartonParticipant @grahamwharton]

For info, that tool takes standard ISO 16ER threading inserts.

Looks like that set comes with partial form inserts. Once you get to the full depth, you will then need to run over the crests of the thread to give you the appropriate flats across the tops (to get rid off the O/D burrs that you will no doubt be left with). Its basically the same process as you would go through if you cut threads using a ground HSS tool. The advantage of this type of tip is that it will cover most 60 degrees thread pitches in a single tip. A60 will cut pitches 0.5-1.5mm or 16-48TPI and the G60 will cut pitches 1.75-3.0mm or 8-14TPI.

The alternative is to use full form inserts which will apply the appropriate crest form to the thread automatically once you reach full depth. The disanvantage is that you need a different insert per pitch that you want to cut. If you did want to cut full form, as they are standard ISO 16ER toolholders, you can just buy them individually from whoever, for the pitches you need, and use your generic A60/G60 tips for the less common threads.

[MuzzerParticipant @muzzer]

As well as internal and external inserts, they also offer "left hand" and "right hand" inserts. The main difference between internal and external is that they are mirror images in order for you to be able to thread up to a shoulder, whether internal or external.

The "right hand" and "left hand" inserts are similarly mirrored. That seems fair enough. A left hand insert is for cutting from left to right of course but otherwise there is no significant difference eg helix angle clearance from what I can see.

Without the part number to help you, I suspect it would be difficult to tell the difference between an external right hand insert and an internal left hand insert (and conversely). Long story short, I reckon you can do most jobs in our workshops with an internal and an external partial insert.

BTW, I believe the Mercans sometimes call them "agnostic" rather than partial profile.

Murray

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