TNMG insert

**LINK**

Anyone tried these inserts in a hobby lathe? Offering six cutting edges they seem to be pretty good value.

Sadly JB tools don't seem to have a holder for them. Anyone seen one at a good price?

I've found these holders with 16mm shanks:

these

or

these

What's the difference and do I need to add a shim?

Edited By JasonB on 02/07/2017 12:17:12

Perhaps a moderator can edit my last post to reduce the text size!? For some reason copying and pasting increases the font size on this website. Never had a problem elsewhere.

I use TNMG 16 inserts but not the polished ones you linked to. They are economical in having six cutting edges but I have a feeling the toolholders are less easily available than holders for single sided inserts held in place with a central screw.

Holders for TNMG have the tip angled downwards to create front clearance for the insert (as it has a square edge).

Ian P

Thanks Ian. I think I'll buy a cheap holder and give it a try. I'll need to machine a couple of mm off the holder as they don't seem to come small enough to fit my lathe. I've machined a tough Sandvik holder for a mate before using a carbide mill so I'm fairly confident of doing it again.

Thanks also to the mod that edited my second post!

No, I can't change the font size!

You need to paste as plain text otherwise your formatting gets carried across, but it uses the website's style sheet (e.g. if you use Word's 'heading 2' the pasted text will use the website's style for 'heading 2', not the original style.

Neil

These inserts are negative rake which may well be a problem with low power hobby lathes, but always worth a try and with six cutting edges will prove very inexpensive. The simple solution to the problem of finding holders is to make your own, it is not difficult. The Torx screws are readily available from ebay and mild steel is all you need for the body.

Doug

There is only a "paste" option on an iPad. Many other forums have a font size option for text, this site only seems to have bold, italic and strike through?

I've found a holder on the auction site for just over seven quid Doug so not worth making my own. It should arrive next week. Once I'm sure I can trim it to size I'll order a couple of inserts. As a point of interest the holder uses a clamp rather than a Torx screw for holding the insert.

Yep,

Although Jason seems to have made the text bigger, I don't know how?

(I'm sure they tell him things they won't let me in on).

Neil

Neil the font was about 30pt on the screen before I altered it and shortened the links. Funny enough it was only showing as 17pt and I just changed it to 12pt, biggest pain was that the font size was in there about 10 times throughout the post.

For future ref go to edit the post and then click "source" top left to get all the code to show up.

There are TNMG holders with a 12mm shank are available. TNMG16040? is a good choice as there are so many choices of inserts available, with the sharpest being around 0.1mm radius out to 1.2mm radius.

Neil

In the current and last MEW there are articles by a bloke making some quite nifty looking tool holders/system for which these TNMG bits would seem to be ideal.

Many toolholders just have the insert held in place with a Torx screw but for some reason the tool holders for TNMG inserts are quite different and use a shim underneath the insert and a post over which the insert is placed, the insert is then held down with a clamping piece. Maybe the reason for this will become clear when the tool holder and inserts arrive. I enjoy making my own tooling but at £7.45 for a tool holder it's not really worth it.

As they have cutting edges top and bottom they do not sit down flat like the single sided ones so can't be held in teh same way.

I'm not sure why TNMG are different in that respect to single sided inserts. They do have a relatively small centre hole which is not a convenient size to take a screw and all the holders I have seen just have a close fitting peg. The tip on the holder I have is held down by sideways pressure only and does not have a clamp over the top.

All the 'shims' I have ever seen are flat so the TNMG ends up sitting on a flat surface, the cutting edges must be set back slightly. I suppose the purpose of the shim is be a washer to provide a harder and less deformable surface than the toolholder shank material.

Ian P

When the 1604 TNMG inserts turned up yesterday I was not that hopeful they would work very well on my lathe. The tool holder arrived today though so I trimmed it down by a couple of mm on the mill. The steel was quite tough but not hardened. I managed to cut it with a carbide mill and the resultant burrs were easily removed with a file. Fitted to the lathe the first insert I tried cut quite well. The finish was not quite as good as some of my other tools but I expect the inserts to be a lot more durable and last somewhat longer than the "hobby" inserts I've been using. I'm glad I took a chance as I'm sure they'll come in handy. The chip breaker in particular works very well, no "stringy" swarf! As a point of interest I have a BV20 8" x 14" lathe.

Considering that they are about the same price as many other inserts, the TNMG offer far better value giving six rather than just two cutting edges.

The insert sits flat on the shim which is slightly smaller in size therefore protecting both the tips and the edges from damage when clamped. Replacement shims are commonly available.

I have some TNMP (double sided) and TNMS (single sided) inserts that perform better than TNMG in smaller lathes. They have a molded in positive cutting geometry. TNMP is readily available but I suspect TNMS is out of production.

I have a lifetime supply of TNMG inserts I use for most work and will save the TNMP and TNMS for finnicky work.

\

I use TNMG negative inserts and find them to be effective, unless they are chipped by a hard bump against them or the spindle is stopped without withdrawing the insert from the work.

Much higher spindle speeds produce better finishes. Also, better results occur if minimum depth of cut is .005" The inserts have a built-in chip-breaker groove which produces a virtual positive rake IF the cut-depth of .005" is met.

This has generally increased the speed I run my small lathe, including parting-off at 300 RPM!

The toolholder must hold the insert with negative rake to provide relief. If you view the insert edge it lacks any relief as it is molded at 90 deg.

Paul

Quality TNMG inserts are able to take moderate interrupted cuts without chipping. Higher RPM seems to fix  problems with carbide inserts (and this was a difficult transition away from slow and careful). The quality holder I purchased from PlasTools 10 years ago is a "turn and face" style. You set it to face without a pip and then you begin turning.

I made this drawing to illustrate the differences of TNMG vs TNMP (the TNMS is 20° compared to the TNMP's 10°. Note that these inserts must be mounted in a tool seat at a negative rake. In the illustration, the seat is approximately 7° compared to the phantom green base line. TNMP do not like interrupted cuts.

The width of the land determines the minimum cut. If you take cuts less than .005", the insert will not be cutting in the positive ramp area behind the LAND, so it will exhibit tendencies of other negative rake tools (higher pressures, not-as-nice finishes). The TNMG inserts I have used have a very narrow land and I can remove .001". Eventually this land wears from heavy use and you then won't be able to take fine cuts. But, the price per insert tip (6 tips) is attractive and if you follow the guidelines, the name brand TNMG inserts last a long time.

At higher rpm, the chips turn blue, the insert stays cool, the workpiece stays cool and the finish is very nice. Try .025" (radius) cuts in mild steel at 300 sfm and see what TNMG will do on your machine.

Edited By Paul Fallert on 21/07/2017 17:57:36

Edited By Paul Fallert on 21/07/2017 17:59:24