Embarrassed to ask, but haven't found an answer to this with pictures - carbide angles relative to workpiece

[rwm]

After measuring my inserts, the Inscribed Circle would seem to be based on the radius, not circumference. In my case TT321 would be 3/8" radius. I am not clear why the letters are only expressed as TT.
No takers on the rake angle question? I would really like to understand how that works with inserts. It's weird also that the HSS knife edge tool demonstrated by Mikey works so well with aluminum since it has a very high rake angle?
R

 

[Shootymacshootface]

This is a great topic, thanks op. This makes me ask something that I haven't been able to figure out on my own. I have noticed that carbide tooling does work best with heavy cuts and rather poorly with very light cuts, like when sneaking up on your final dimension or making spring passes, the surface finish goes to crap. How does one get consistent good final finishes with carbide tooling? I have been enjoying a lot of success with hss, but stopping to sharpen bits after a lot of roughing could be avoided with carbide tools.

 

[mikey]

That link that Mitch posted is a very slick site with a good description of turning theory. I like it! Everyone should review it.
I am confused about this:
"So, TCGT are a positive rake version of the TCMT " That doesn't sound right.
Position 3 is the tolerance of the insert if I understand correctly?
I do not understand how the rake angle is coded. Is it based on the tool holder or the insert? All my tool holders are zero so far. Do I need a negative rake holder?
Cutting radius is coded in position 7 in 64 ths of an inch?
Position 5 is inscribed circle. Is this based on the radius or diameter?
I am running a "TT321" insert routinely on aluminum. (1/2" tool holder) That should have a 1/64" tip radius. Does any one have a better recommendation? If so please explain why it will be better. I like to know the theory behind all this.
Robert

Debugging inserts can be really confusing when you first look at them. Every manufacturer is supposed to adhere to ANSI or ISO standards; here in the States, we usually use ANSI.

Rake is complicated because the insert and/or the tool holder can have rake, and you can have a negative rake tool holder that produces a positive rake at the cutting edge. There are tooling specialists who work in this field that can make recommendations to shops because there are so many inserts that confusion is actually not uncommon.

For my simple mind, I keep things simple. I look at the relief angle (pos. 2). If it is an N then that is a 0 degree rake insert (it has no relief angle under the cutting edge) and that means the relief MUST be provided by the tool; this is a negative rake insert that fits into a negative rake tool. Anything else is a positive rake insert but the tool holder can still give you a net negative rake when the insert is bolted down. This perspective is not entirely correct but it is useful for my limited insert use.

The chip breaker configuration and wiper configuration complicate this further because they depend on usage and material. You need to decide what is appropriate for the job at hand.

Nose radius is in 64th's but there's more to it. A 1/64" nose radius is 0.016", although many inserts are actually 0.015". This is a common radius but is on the big side for my use. I much prefer a 0.5/64", or 0.008" for most work and a 0.20/64 or 0.004" if I can get it. The reason this matters is because the nose radius determines how small a cut you can take before radial forces get too big to cut reliably. Your minimum depth of cut should be at least as large or larger than the nose radius, so a minimum accurate cut with a 0.016" nose radius is about 0.015". If you try to go shallower, it might cut but you have to try it and see if it cuts accurately enough for you. I have found that I can go down to about 1/2 the nose radius and it sort of cuts okay; you don't get what you dial in but it tends to at least be consistent. If you try to go shallower then radial forces will deflect the tool and it skates and will not cut. Your finish goes out the window, too.

I have to run some errands now but my advice to you is to choose a tool holder and download the manufacturer's info on it. They will have insert recommendations for use with that tool holder. Then figure out which insert configuration will suit your needs and then learn everything you can about that insert.

If I had to choose a single tool holder, I would choose the SCLCX class. Inserts are cheap and they work well for hobby class machines because they are a positive rake tool holder and insert. Buy the smallest nose radius you can get for finer work. You will find that the bigger the tool holder, the bigger the nose radius; and the bigger the nose radius the deeper you must cut for accuracy. Try taking a 0.001" depth of cut with a 5/8" square tool holder and you'll see what I mean. Bigger is NOT always better.

 

[Grandpop]

Those TTxxx inserts seem to not follow the standard lettering/numbering scheme. They have a 7 degree side clearance, and the molded in chipbreaker adds another 4 degrees. You often see these stated as 11 degree rake angle, which would be correct if the tool holder insert seat is parallel to bottom surface.

I use those in my big dovetail cutter in TT431 size, as TCMT431 are expensive and harder to find. While TTxxx and TCMTxxx are both 7 side angle, their center holes are different, so can't use the same screws. Not a problem for my dovetail cutter, as I built it for the TTxxx inserts.

I last bought the TT inserts from Richett on ebay, in coated grade for $20. Hold up pretty good milling in hrs and 4130.

 

[rwm]

So can I find an insert that is triangular i.e. TTxx32.5? (TTxx320.5?) I don't see to find ones with that small of a tip radius.
Robert

 

[mikey]

You may find it a challenge to find small nose radii in larger tools. You appear to be using 1/2 shanked tools; might have to go to 3/8" shanked tools to find a 0.008" NR insert. Yeah, I know that the consensus is that bigger tools are more rigid and "better" but I use 3/8" tools on my 2HP Emco lathe and they cut just fine. I also own 1/2" tools and find no significant difference in how they cut.

If you have not tried an SCLCR tool holder, this might be a good opportunity. The generic tool holders are cheap, as are the inserts. They use CCMT and CCGT inserts that are common on ebay.

I meant to mention the AK inserts in my other post but ran out of time. AK inserts are found in many different insert configurations. They are uncoated and ground to a sharp edge. They also have very positive rake angles that works well with Aluminum and plastics. They also work fine for harder materials but the edge doesn't last as long when doing so. It puts a really nice finish in most materials, though, so if you haven't tried it, you should.

The other thing I wanted to mention is that most insert tool holders are intended to be used perpendicular to the work. The tool holder geometry is set up like this and lots of folks fix or pin their tool posts to take advantage of this fact. However, many tools work better if you alter their lead angles. If you take almost any inserted tool and angle it back toward the tailstock a bit (maybe 10-15 degrees or so), you will be amazed at how much better it finishes. Play with it and you'll see what I mean.

Similarly, you can cant the tool holder towards the headstock a tiny bit so the tool cuts with the nose radius and the end cutting edge. This also improves finishes significantly but you will need to play with depth of cut to see how it affects your accuracy.

Finally, I wanted to mention that you need to determine how your insert likes to cut. Most roughing cuts are accurate when the depth of cut is slightly greater than the nose radius; a good setting is NR + 0.010". This fully supports the nose radius in the cut and reduces chatter and increases accuracy. Most inserts will cut consistently at this setting.

 

[mikey]

This is a great topic, thanks op. This makes me ask something that I haven't been able to figure out on my own. I have noticed that carbide tooling does work best with heavy cuts and rather poorly with very light cuts, like when sneaking up on your final dimension or making spring passes, the surface finish goes to crap. How does one get consistent good final finishes with carbide tooling? I have been enjoying a lot of success with hss, but stopping to sharpen bits after a lot of roughing could be avoided with carbide tools.

I meant to address this and forgot; sorry.

As in the above posts, inserts require sufficient depth of cut for the nose radius to bite. If your DOC is too small, the nose will deflect and skate on the surface. Under these conditions it will not cut consistently. What is happening is that radial forces are excessive and the tool is actually pushing away from the work surface but this is not consistent. Hence, your finish will look ratty as the tool tip moves toward and away from the surface.

The thing to note is that a carbide inserts require an adequate DOC. It would be nice if you could take a huge roughing cut and come back and take 0.001" DOC to come in on size but that just doesn't happen. What you have to do is find out the minimum DOC your tool will reliably take and rough to that point, then dial that cut in. For example, say your nose radius is 0.016". A DOC of about 0.020" or so will probably cut pretty consistently. You have to test it but if you dial in 0.020" and it reduces the OD by 0.040" consistently then you've nailed the number. Now just rough until you are 0.040" from final size and dial that 0.020" cut in and you're there.

You may be able to get the DOC smaller but there will typically not be a direct correlation between what you dial in and what you get. However, if what you get is consistent then use it. For example, say you're using that 0.016" NR insert and you find that you can take a 0.008" DOC consistently, meaning it takes off the same amount for each pass, then use it. If a 0.008" DOC produces a consistent 0.014" reduction in diameter per pass then just rough until you're 0.014" away and dial in that 0.008" DOC. You have to determine how small a cut you can take. If you drop much below that 1/2NR, radial forces will be excessive and your consistency will drop off. Get much below 1/3NR and your insert will likely skate. That's just how this stuff works so you need to sort it out and go from there.

 

[Grandpop]

The TT xxx series appear to older less common inserts, but still available. I've never seen the TT32x series in any smaller nose radius than .015 (so TT321). So that is right back where this thread started!

But yes, you can buy the TT321 inserts on ebay for $20 for 10, typically coated C5/C6 equivalents. They cut OK; much better than the really cheap ones that come with the cutter sets!

Make sure you are pretty close to being on center if you are taking light cuts; makes a big difference. As with any carbide insert, if you want to finish with light cuts, you need to index the insert just before finishing (I mark one with Dykem so I know which one to use), or keep a different toolholder just for finishing. If that insert is slightly dull at all after your roughing cuts, taking light cuts is going to be an issue (as described above). With a new cutting edge, I don't have much trouble with light cuts, including the final .001 to .002 off the diameter).

Having said that, I do not use the TT series for finish cuts when I am looking for great precision; I have other choices. I have an old TPG32x series holder. Prices of TPG321 inserts are stupid cheap these days, especially the uncoated ones. I have zero issues taking .001 cut off diameter with these, or taking several spring cuts. It may not be the worlds greatest finish, but good enough for anything I need. My 12 x 24 is like most lighter lathes, in that it typically cuts with slight taper (further away from chuck is slightly larger than close to chuck). I pretty much always sand/file to final size to get rid of that taper. Last night I was turning 1" dia O-1 tool steel down to 3/8 diameter x 2.5 inches long to fit inside a collet. This was the shank of a cutter I am making, so wanted it close to size and decent finish. I used the TPG321 with final cut of .001 on diameter (1400 rpm), and took 3 spring cuts. I ended up .0004 larger at the far end, so about 4 short strokes with good smooth mill file eliminated the taper completely.

 

[Shootymacshootface]

Thanks Mikey, you couldn't have explained that better.
Thanks!

 

[mikey]

Thanks Mikey, you couldn't have explained that better.
Thanks!

You're welcome, Sir.

I've been in situations where I had a lot of work in a piece and I'm almost there, within just a few thou of final size. When the job requires a good finish and an accurate diameter, what do you do if you only have an inserted carbide tool that you know will not make the cut? I run into this situation when fitting bearings; you know how those fits can be finicky.

The answer, for me, is to have a really good, really sharp HSS tool to make that cut. A good tool will be able to take off a thou reliably if the geometry is right.

A lot of hobby guys, especially newer hobby guys, go with inserted carbide because they cannot grind or are not interested in grinding HSS tools but they eventually learn that the tool you use in not as important as getting the job done. There is a place for both kinds of tools in every shop. I use HSS the vast majority of the time because they work better for me on my small lathe. However, there are jobs when inserts are the right tool and I sort of know enough to use them in those instances.

For me, it is not an either/or; it is a both/and.