Thread Inserts

[Pete F]

Bob sent me a link that I tried to reply to but I cannot get my reply posted due to the age of the thread. There is no box to tick to allow the reply to be posted that I can see. The moderators can stitch these two threads together if they wish.

Here is the original thread and below it my reply.


https://www.woodworkforums.com/f65/an...nserts-153347/

Bob just provided me with a link to this thread as I asked him what type of thread insert he was using. There seems to be a little confusion here with regards to the shims and what they do.

The shims are not merely there to support the insert, that could easily be designed into the pocket of the holder as per normal turning tools. In fact they're there to incline the insert and provide clearance due to the helix angle of the thread (determined by the pitch and the workpiece diameter. Obviously small diameter work and coarse threads will be the worst case scenario in that regard). For those of us who grind our own threading tools, the required clearance angles can be ground in to the HSS tool. However the inserts are too brittle to enable those clearance angles, if they tried to design them in, the edge would have insufficient support and would probably fracture.

To counteract the possibility of the edges of the insert rubbing on the flanks of the thread with high helix angles, a shim is provided under the insert to rotate the insert anti-clockwise (if viewed from the operator) for a right hand thread, typically 1-2 degrees. Bob has shown a RH tool above and he should be able to confirm this if he looks at the insert carefully in the holder. For most of us cutting standard V profile threads the standard ~1 degree shim should provide sufficient clearance. I've never had a need to cut a multi-start thread, but if you did, especially on small diameter work, you'd possibly need a higher value shim.

The issue then becomes left handed threads. Bob's tool is a right handed tool, but the same holder and insert can be used to cut left handed threads. I don't know if some manufacturers allow for their shims to simply be flipped, probably not. Remember these things aren't designed for home use, they're designed to be used by some imbecile in a factory who couldn't give a toss. Invariably some moron would load the shims wrong way up and cause a run of parts to be rejected. Dunno. Anyway, the solution for LH threads is to use a negative value shim, that will rotate the insert the opposite way and ensure adequate clearance. The shims are all designed to be interchanged and yet keep the centre height of the insert the same.

With regards the shim screw, as Bob said, it seems to be common amongst different manufacturers, but I have no idea why it's required. Not all holders have them and I would have thought the clamping pressure of the screw would be sufficient. The holder I'm looking at ordering now for example doesn't have one. However they wouldn't put it in for a laugh and a giggle, so there must be a good reason for it that I'm not aware of.

Hopefully that's all of some interest.

Pete

[Anorak Bob]

I will have a close look and a measure tonight Pete.

Bob.

[Anorak Bob]

Pete,

I'm looking at the toolholder with a 4X loupe and I'm a bit puzzled. There does not appear to be any inclination of the shim from horizontal around the long axis of the toolholder. The top of the insert looks flat, no top rake. I will attempt to photograph the tool in detail to share my observation and puzzlement. Measurement with the tool insitu on the lathe will provide the most accurate results. Probably be Friday during the day before I can press the Nikon into action.

BT

[Pete F]

No it may not be your eyes Bob. 0 degree shims are available too and maybe that's what is fitted. It's possible that with a zero degree insert and that brand of insert the clearance is sufficient for most threads and diameters likely to be seen in a home shop. If you go to an insert manufacturer's web site there should be a graph (at least that's all I have ever seen anyway, maybe some use tables or a formula?) of the work diameter and ... ah pitch IIRC. I think there's TPI and metric pitch on the same axis. Sorry I'm just trying to recall but am off to bed so don't want to try to chase one down now. Anyway, you enter with those 2 variables and it will tell you the shim angle to use. Most are around the 0-2 degree mark IIRC. I thought most shipped with these values, but maybe not, at least for the home market???

If you mic the pocket and shim you should be able to see if either are tapered. If not, and it's working ok, then it's a good thing as it means you can cut LH threads without changing anything. The point anyway was simply just what it is and why it's there. I'm guessing most of us won't need to worry about it in practice as the helix angles aren't that great, but I could definitely see a potential problem if you has a 1-2 degree shim and then tried to use it for a LH thread. That may get a bit tight on the flank.

Pete

[Michael G]

Pete, sorry to digress slightly, but what sort of speeds do you need to run threading inserts? I normally thread at around 80rpm, which is way down on what is recommended for HSS let alone carbide. Do you get a decent finish from the inserts or is it the normal 'torn' finish that carbide gives when used at slow speed?
Now that I've experimented enough with carbide to get a reasonable finish (the picture is part way through making up a collet chuck)
P1010634 (Medium).JPG

I'd love to get my thread cutting up to a similar standard of smooth (functionally it's fine although it feels a little rough), but my reflexes aren't good enough to go faster

Michael

[Anorak Bob]

No it may not be your eyes Bob. 0 degree shims are available too and maybe that's what is fitted. It's possible that with a zero degree insert and that brand of insert the clearance is sufficient for most threads and diameters likely to be seen in a home shop. If you go to an insert manufacturer's web site there should be a graph (at least that's all I have ever seen anyway, maybe some use tables or a formula?) of the work diameter and ... ah pitch IIRC. I think there's TPI and metric pitch on the same axis. Sorry I'm just trying to recall but am off to bed so don't want to try to chase one down now. Anyway, you enter with those 2 variables and it will tell you the shim angle to use. Most are around the 0-2 degree mark IIRC. I thought most shipped with these values, but maybe not, at least for the home market???

If you mic the pocket and shim you should be able to see if either are tapered. If not, and it's working ok, then it's a good thing as it means you can cut LH threads without changing anything. The point anyway was simply just what it is and why it's there. I'm guessing most of us won't need to worry about it in practice as the helix angles aren't that great, but I could definitely see a potential problem if you has a 1-2 degree shim and then tried to use it for a LH thread. That may get a bit tight on the flank.

Pete

Hi Pete,

I had a look at my Kennametal lathe tooling catalogue this morning. 26 pages devoted to laydown triangular inserts. A lot of information pertaining to shim angles. I will photograph and post some of the pages.

Michael raises a pertinent question. These inserts do tend to tear at low speed especially on tough steel like 4140, often in my case necessitating a clean up with a die. I wonder how others get on? I can attain a nice finish on 4140 at say 1200ish rpm. Can't thread too much at that speed especially when a shoulder is in the way.

BT

[simonl]

This is all very interesting. I have always wondered, my indexable threading tool holder has a side screw. It looks like an adjustment screw. Now im thinking its to adjust side rake for such clearance?Cheers.Simon

[Pete F]

Pete, sorry to digress slightly, but what sort of speeds do you need to run threading inserts?

Michael I just consulted both my Machiney's Handbook and the new Machinist Calculator for a definitive answer.

There is a specific formula that is directly related to the size of your goolies. But the bottom line is "As fast as your pucker allows!"*

Pete

*Why I like LH threading! ... well unless it's to a centre like the other day

[Michael G]

I can understand that insert threading tips would be great in a CNC lathe where you can avoid collisions by fractions of mm but on manual machines do they offer that much of an advantage? The only advantage that I can think of off hand would be negating the need to sharpen tools. On the down side, they are expensive and as Bob and I are suggesting, can not easily be run anywhere near their optimum speed giving a poor finish.
Its one reason why I like the look of the disc style thread cutting tools - the right tip angle is built into the tool but they can be run slower (= less pucker ). I think the Eccentric diamond style tool holder claims to be able to grind 55 or 60 degrees without drama too - perhaps that is a better way to go for manual threading.

Michael

[Pete F]

Michael I've had considerable success with honing carbide with a hand held diamond stone to sharpen them up a little. Of course all the experts will say it can't be done, but that's not my own experience. I find it helps lower the speed at which it provides a good finish.

The new lathe has a traverse stop and I've done some dummy runs with that, though haven't currently got it wired up (it's interconnected with the covers and I decided to put in a bipass switch so took it out of the circuit). I'm able to stop the spindle within a few degrees repeatably, even at relatively high speed. It should prove very handy to thread at quite high speed and not need to be concerned about having quick reflexes. I can't recall if you've got a VFD on your CVA, but if so it wouldn't be difficult to make up a stop with a microswitch or similar on moveable stop.

Pete

[Anorak Bob]

"The new lathe has a traverse stop"

Now I'm really interested. Any chance of a bit more information and I know I'm pushing it, maybe a photo or two?

Thanks Pete

BT

[PDW]

There is a specific formula that is directly related to the size of your goolies. But the bottom line is "As fast as your pucker allows!"

I *cough* can thread up to a shoulder or into a blind hole at 300 rpm and never crash a tool.

PDW

[Pete F]

"The new lathe has a traverse stop"

Now I'm really interested. Any chance of a bit more information and I know I'm pushing it, maybe a photo or two?

Thanks Pete

BT

Here you go Bob. I thought this was bag standard 260?



That arm is adjustable so it you can stop the saddle at different points. I had it wired in the e-stop line, but I had the covers open often at first, so it was a PIA as it's interconnected with their safety cutouts. The intent now is to move my speed control pot on the front panel and put a bipass switch in. That was if I need to run the machine with the covers open I can. With threading to a hard shoulder or small runout groove the spindle and obviously carriage will stop at the appropriate point, you go through the usual motions to withdraw the tool etc, and as the carriage is traversed back the motor will start again automatically and away you go as per usual. As I say, I've only done dummy runs when I was setting up the machine but it seemed to be working ok. I've just been too busy using it to fit the switch!



On an older 9" you could probably adapt something like this Bob. This is my dial indicator and stop (the stop is removed in this case). The dial indicator could be replaced by a microswitch to provide exactly the same function as the 260 above.

Pete

[Anorak Bob]

Thanks Pete,

That is standard 260 fare. The adjustable stop and casting would fit on my 9's saddle but the fly in the savlon would probably be my single phase motor. It does not stop abruptly. I imagine it would require venturing down the 3 phase / VFD path for more controlled motor stoppage. Problem there is that I'm fond of the 1 horse plant's appearance on my lathe. Some squared off ASEA is not going to cut it.

Bob.

[Pete F]

You can probably find a used 3 phase motor of appropriate appearance easy enough Bob, I think people are swapping them out all the time. Yes I agree, the VFD actually brakes the motor, and it doesn't just coast to a stop. Once you've had the luxury of variable speed drive however you'll wonder why you took so long. I am constantly adjusting my speed on the lathe depending on what I'm doing. There's no way in heck I would be changing belts to do the same thing.

Pete