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View Full Version : Any reason I can't cut threads with a TCGT insert?



beanbag
08-17-2009, 07:01 AM
Both internal and external threads. I couldn't find that many references to people doing it this way.

Glenn Wegman
08-17-2009, 07:59 AM
Depends on the pitch due to the available nose radius.

If you need 4 tpi, it would probably work.

smiller6912
08-17-2009, 10:21 AM
Yes, way too much nose radius on the insert. A thread cutting tool needs to have a sharp nose. Usually threads are cut slow , shallow, and with a lot of lube, inserts are usually not the right answer for this operation.
A thread cutting HSS toll is one of the simplest tools to make and will make a far better tool for threading than carbide insert tooling. (unless you get a specific "profile" cutting insert).

Glenn Wegman
08-17-2009, 11:28 AM
Not a sharp point! At least for proper UN anyway.

Tool tip should have a small flat or radius on it.

DR
08-17-2009, 12:42 PM
Both internal and external threads. I couldn't find that many references to people doing it this way.

No reason why you couldn't. As others have mentioned, be sure to buy inserts with a small corner radius.

I think I have even seen a commercially made holder to do this, can't remember where though.

Glenn Wegman
08-17-2009, 12:59 PM
http://en.wikipedia.org/wiki/Unified_Thread_Standard

Jim Shaper
08-17-2009, 01:39 PM
Glenn, the side clearance is going to be a big factor as well. The cutter will rub in the thread if it's not ground such as to clear it. That's why there's a steep side relief on threading tools. If you started hogging into the holder to create that clearance, you're likely to weaken it's performance on regular stock removal, so it doesn't make much sense to do it.

You can cut threads with a parting tool if you grind the end such as to allow for the clearance and get the 60 degree angle right.

I picked up a whole vardex internal/external carbide insert threading set on fleabay for $100. Two holders and a whole slew of geometries.

Glenn Wegman
08-17-2009, 03:09 PM
Jim,

Good point

Never tried it or even considered using such tooling so I never really thought about all of the consequences!

Thanks,

Glenn

Carld
08-17-2009, 04:27 PM
Yep, side clearance is an issue. Just buy the right tool holder and the right threading inserts. Cutting corners like that is not a good idea.

lazlo
08-17-2009, 04:59 PM
Yep, side clearance is an issue. Just buy the right tool holder and the right threading inserts.

I got a toolholder in a package deal that had a positive rake triangular insert that had been kludged for threading: they ground the flanks in for about 1/8" near the tip for clearance, and sharpened the tip to get the UTS crest . It looks like hell :)

Black_Moons
08-17-2009, 05:36 PM
Iv threaded with those cheapo triangle TCCT inserts (or whatever those UBER cheap chinese 'my first insert' kits are) its ok for REALLY course big OD threads (like 1/2" and bigger) if you get the fine nose inserts and have like 7~14 degrees of releif on your inserts.

I also have a weird tool that appears to use a carbide square 'shaft' thats tiped with a carbide shaped threading profile, the tool itself looks much like some of those cutoff blade holders but it holds a square tool insted of a rectanglar tool.

But yes. HSS bits are soo easy to grind, I made one with an angle grinder and diamond coated files to hone it and to manualy grind off the last few degrees to make it a perfict fit to my thread gage.

Idealy you want to try against a real bolt at that point and check/fix the nose radius too (for best bolt strength anyway)

lane
08-17-2009, 08:03 PM
I Have done it with TPG inserts with no nose radius and that worked . Not sure just what insert you have with out looking it up.

beanbag
08-18-2009, 03:45 AM
Sorry, I should have provided more info upfront.

I'll be cutting internal and external threads approx 10-12 tpi on something that has a 3.5" diameter. So I think the clearance will be ok using a TCGT insert. The nose radius is 0.008, so that should be sharp enough as well. However, I think as a final pass, I will have to shift the insert over sideways to get the proper depth vs length of crests and troughs.

Also, this insert was designed for aluminum (the material I am working on), so it has a big ol ramp at the cutting edge. However, since the diameter of the piece is large, this shouldn't matter.

This insert cost me $11, whereas otherwise I'd have to get separate internal and external thread cutting inserts at $16 each. Such a cheapskate, I know.

DR
08-18-2009, 06:57 AM
I'd have to look to verify, but as I recall the laydown style triangular threading inserts don't have a great deal of relief ground on the front edge. They tilt the insert down radially and axially to achieve the relief.

The higher end threading holder insert combinations use replaceable angular seats under the insert to achieve correct front relief. The amount of angle on the seat varies with the pitch and diameter of the thread being cut. Of course, this is the ideal situation, but double the cost, insert plus seat needed. These are usually only found on external holders since there isn't enough room on the tip of smaller internal holders for the seat + insert.

There's also the issue of how close to the bottom of a blind hole you can get using the TPGT style insert. Not a problem for through hole threading though.

beanbag
08-18-2009, 07:06 AM
There's also the issue of how close to the bottom of a blind hole you can get using the TPGT style insert.

Ohhhh, crap. Right....

Glenn Wegman
08-18-2009, 07:25 AM
The radius on your proposed insert is also too large for that pitch range.

You need a .010" flat for 12 tpi and about .0125" for 10 tpi internal.

That's why they make specific inserts for threading!

oldtiffie
08-18-2009, 07:38 AM
Beanbag, Glenn Wegman and others.

The OD (using that instead of pitch diameter - it's easier) is ~3.5" x 3.1416 = circumference ~ 11.00"

Using 10 tpi the lead is ~ 1/10 = 0.100"

Helix angle = atan 0.100/11.000 ~ 0.5 degrees

The helix angle will have no real effect - if any - on the tool or the screw.

For all practical purposes the tool can be ground as if it is cutting a groove (with no helix).

Here are the standards for UNC:
http://i200.photobucket.com/albums/aa294/oldtiffie/Screw_threads/Screw-thread_form7.jpg

and here:
http://i200.photobucket.com/albums/aa294/oldtiffie/Screw_threads/Screw_threads2.jpg

In practice the radii - if used - cannot/must not impinge on the straight flanks/sides.

That assumes a "100%" thread where-as the actual &-age figure can be anywhere between 70% and 80% but normally 75%.

So its a wise move to check the helix angle to see if the rake and clearance angles are critical.

In this case they are not and can be ignored.

Here are some TC UN inserts with a range of TPI - internal and external each with a range of TPI - and with the index insert holders:
https://www.machineryhouse.com.au/Products?stockCode=L056

https://www.machineryhouse.com.au/Products?stockCode=L058

Simple - isn't it? The radius is not so important after all provided that its the (better job and longer tool life) and needs only be no larger than that required for the largest TPI (smallest pitch/lead) in the cutters range/s.

DR
08-18-2009, 07:55 AM
.................................................. .....................

Helix angle = atan 0.100/11.000 ~ 0.5 degrees

The helix angle will have no real effect - if any - on the tool or the screw.

For all practical purposes the tool can be ground as if it is cutting a groove (with no helix).

.................................................. .........




Nope, absolutely wrong!!

The tool has to be ground with a bit of relief to account for the helix, or if an insert, it has to be mounted with a slight tilt to account for the needed relief.

And, grooving tools should have a bit of side relief too. Ever tried to groove or part off using a tool with no side relief?

oldtiffie
08-18-2009, 08:06 AM
Nope DR - I don't think so.

I have an open mind and am quite prepared to be convinced otherwise.

Even a grooving tool has side clearances on both sides.

The OP was concerned with using TC threading inserts - which as I've shown in my links all have more than adequate side clearances and side rakes to cater for a wide range or TPI's on probably smaller OD's than than the OP's 3 3/8" and pretty well certainly with a larger helix angles than the OP's.

Each of those tool-holders that have a "tilt forward" built in will only have one to cater for the whole range of thread OD's and TPI - and helix angles - within the range of the cutter and tool-holder combined.

Glenn Wegman
08-18-2009, 08:39 AM
oldtiffie,

Please explain how you get a proper internal thread form (PD) for 12 tpi, in accordance with the chart that you posted, using a cutter with a tangent
.008" nose radius.

Thanks,

Glenn

lazlo
08-18-2009, 09:53 AM
This insert cost me $11, whereas otherwise I'd have to get separate internal and external thread cutting inserts at $16 each. Such a cheapskate, I know.

Ouch! IMHO, HSM'ers shouldn't be using $16 inserts. It'll make you cry when you chip one :)

I get my inserts off Ebay. Even modern inserts like APKT are around $2 - $3 each. That includes the Top Notch and laydown threading inserts.

But if you don't want to do Ebay, and especially especially since you say you're just starting out in machining, what about grinding a HSS threading tool?

oldtiffie
08-18-2009, 11:51 AM
oldtiffie,

Please explain how you get a proper internal thread form (PD) for 12 tpi, in accordance with the chart that you posted, using a cutter with a tangent
.008" nose radius.

Thanks,

Glenn

Thanks Glenn.

You do ask some buggers of questions!!!

Easy.

Draw the standard thread form in CAD and over-lay the tool with the 0.008" radius on it. Pick ("Snap" to) the end/centre of the radius then the distance perpendicular to the thread centre-line.

Double it and there's the diameter at which the cutter with the 0.008" radius will have cut a true Pitch Diameter.

Bore a short referencing hole at that diameter and cease thread-cutting when the cutter just scrapes the reference diameter.

Test against the mating part and cut thread further for "feel" of "fit" to suit - as you would do if using a thread guage.

The external thread can be cut using the same principles and process/es.

beanbag
08-18-2009, 11:55 AM
Ahhh, that's right... The 0.008" radius is too big for the troughs on the internal threads. :( I'm going to return the previous insert (one of the benefits of buying locally) and exchange it for one with 0.004 radius.

Lazlo: I know that just using HSS bit would have been easier, but HSS is old skool, and I am trying to move into the 21st century ;) Also, there's no way I would have gotten the nose radius right.

oldtiffie
08-18-2009, 12:14 PM
Beanbag.

I'm pretty sure too that the 0.008" radius won't do it.

From a previous post here you say that you envisage cutting 10 to 12 TPI.

As 12 TPI is the "finest"/least I will use that.

Now refer to this pic that I posted previously (UN threads as used in the USA are on the right):

http://i200.photobucket.com/albums/aa294/oldtiffie/Screw_threads/Screw_threads2.jpg

P = 1/12" = 0.0833"

the width of the flat on which the radius in imposed is = P/8 = 0.0833/8 = 0.0104"

The maximum radius will be about 0.005" so the 0.004" radius tool should be OK.

Glenn Wegman
08-18-2009, 12:37 PM
Beanbag.
I'm pretty sure too that the 0.008" radius won't do it.


That's the answer I was looking for with the above question!

Now we're on the same page again.:)

oldtiffie
08-19-2009, 02:14 AM
Thanks Glenn.

You ask some excellent questions that make me ask them of myself and wonder why I didn't ask them of myself without prompting from you and others.

So, if I didn't ask them of myself, I'd have never known if I knew or could find the answer.

Quite some years ago, I went to a multi-day seminar about getting and passing on "knowledge" (it was the "In Thing" at the time). It was to do with Instructors and instructing in a Defence training environment.

The course Instructor was an ex-Army Colonel who was more than a bit full of himself (ie he was "up himself").

The "students" were both Service and Civilian employees of Defence and all had quite a few "years in" and we soon got sick of the instructor and his attitude and antics.

We students had all been down the "boozer" for a "liquid lunch" (as you do) and we were "primed and ready" for the afternoon session.

First thing after lunch "He" asked what "knowledge" and "motivation" were in our fields of endeavour.

No reply.

So, he wrote "KITA" on the black (chalk??) board and said that "KITA" was the answer and asked if any of us knew what it meant.

No answer was the stern reply.

So he was getting a bit upset by this and hollers at us (wrong thing to do) and said: "KITA" stands for Knowledge Is The Answer" - "Got it?" "Any comment?"

Another wrong thing to do.

From the audience (us) came the comment:

"In my experience KITA works and works well, but KITA means "Kick In The Ar$e".

The instructor almost dissolved into tears and fled.

Another wrong thing to do.

So we all de-camped back to the boozer, made a "day" of it and went back to our Instructing the following day.

It turned out to be exactly the right thing to do as unbeknown-st to us, the Instructor we had was "On Notice" (ie "Under Review") for a succession of past inadequacies - and resigned (before he was "re-assigned" or "fired").

So, in this thread, as in the instance above - I "got the message".

KITA!!!

beanbag
08-19-2009, 06:05 AM
So it turned out that the store didn't have the 4 thou radius insert after all. Oh well, I was still able to cut the external threads.

Using an insert designed for Aluminum worked really well, as it cut smooth and clean. I dread to think what a ground HSS with no top rake would have been like. The first piece I did completely by hand (by turning the chuck) as I didn't have a good feel for the forces involved and was scared to crash the tool into the chuck. Near the end of the cut, the chip coming out was about 0.080 x 0.003-0.004. It made a kkkkkkkk slight crackling sound as the long chip was coming off. I didn't want to go thicker because I couldn't clamp the workpiece too tightly.

The next three I used the machine on, and overall, it was uneventful (which is good). I used a formula from a book, which probably assumed something about the cutter shape, and thus I ended up with crests that were wider than the troughs. So I just advanced the compound some more while backing off the cross slide, thus resulting in a net movement sideways of the cutter. In the end, I got slightly thinner crests than troughs, so close enough :)

Glenn Wegman
08-19-2009, 08:05 AM
I dread to think what a ground HSS with no top rake would have been like.

Threading tools, as well as form tools, usually have no top rake as the rake angle affects the actual form cut in relation to the form of the tool.

You may already have this, but it may be helpful to you.

http://www.metalartspress.com/PDFs/60_degree_threads.pdf

http://www.wswells.com/data/htral/1913_htral/1913_htral.pdf

Paul Alciatore
08-19-2009, 11:56 AM
I have to fully agree that you need a smaller radius tool to make a perfect thread, but on the practical side, IN A HOME SHOP where you are the only inspector, you could cheat and just cut a few thousanths deeper with the larger radius tool. The fit will be a bit loose, but who will know after assembly. And at 10 to 12 TPI, you will still have sufficient strength for all but the most critical uses. Of course, if you just like spending money, .....

On the other hand, I don't see anything horrible about a HSS tool. For internal threads, I use a boring bar with a 3/16" HSS tool in it. I did use a bit of top rake. And I can just dress the tool for the proper radius for the job. When it gets too short and I have to replace it, I will buy several so I can keep the various radaii. The boring bar has a 30 or 45 degree hole at one end so reaching the bottom of a blind hole is no problem.

beanbag
08-19-2009, 06:49 PM
OK, next up I need to make some matching internal threads. How much tolerance or gap should I put in for a "reasonable" fit? Not too tight and not too loose :)

oldtiffie
08-19-2009, 07:23 PM
You are doing very well beanbag.

The entirely wrong theoretical approach but never-the-less very successful practical one - which I'd use - is to bore your internal diameter about 0.005/0.010" over-size and then bore a short (say 1/8">1/4" deep) "starter" at the thread outside diameter.

Start thread-cutting and keep going until your threading tool just scrapes the "starter".

You are are just about there.

Try the external thread for "fit" and "feel" - ie use it as a "thread guage".

Keep cutting light cuts until it "feels right".

Note the depth of the cutter for future/next thread.

Job done.

oldtiffie
08-19-2009, 10:18 PM
Beanbag.

An after-thought - that I should have included earlier.

If I had a cutter which had too large a radius - as yours may have been - I'd use it but I'd "break" the corners of the external thread (ie a small radius) and that should solve any problems.

The key - but unstated - requirement is to get sufficient "mating/matching" on the flats/sides/flanks of the thread. The little bit you might "lose" will not matter - at all.

Never mind the "purists" in this instance - or the "worry warts" either. If you can make it work satisfactorily with what you have you have done a good job.

beanbag
10-14-2009, 08:38 PM
I'd have to look to verify, but as I recall the laydown style triangular threading inserts don't have a great deal of relief ground on the front edge. They tilt the insert down radially and axially to achieve the relief.



Thanks for mentioning this. Yesterday, I was about to make a tool holder for one of those triangular inserts with the cutting points sticking out sideways. I needed this because I was going to thread a bottoming internal hole. I spent some time looking at the insert wondering why it didn't seem to have any relief under the cutting edge. A quick google search brought me back to this thread.