Lathe carbide tool issue

I’ve machined a couple of different grades of Titanium with HSS and it cut fine - most of the time. I let the tool rub on two occasions that I remember, and it set fire to the Titanium scarf below! One of the joys of having a hobby lathe without coolant I suppose.

So I’ve taken a cut of 0.2mm each time with 3 different tools. The carbide insert, brazed carbide and HSS tools. The brazed carbide amd HSS took off the material exactly when measured with no strain on the lathe and the carbide insert took off only 1/2 of the cut as initially described in my original post with a bit of effort to cut. I’ve added a picture of each tool with the titanium and some swarf(if it helps!)

Looking at your insert, and assuming the tool shank is 10mm, then the insert is DCMT 07. any numbers after the first two, such as 02 or 04 will be the radius of the tip in mm.

The last two numbers should be the radius, I believe it’s thickness then radius, but could be wrong.

My mistake, Baz, the common DCMT 07 02 (02 or 04) would have the second pair of numbers being the thickness, followed by the tip radius, commonly 02 or 04 in mm.

So the commonest inserts of this size would be DCMT 07 02 02 (2mm tip rad) or DCMT 07 02 04 (4mm tip rad).

The aluminium grades are usually designated H01 and can be called DCGT---- or DCMT----, depending on the source.

Edited By old mart on 12/09/2022 20:07:38

In a post higher up, he gives the Sherline part number. If you look it up, you find this:

https://www.sherline.com/product/7605b-replacement-55-carbide-insert-10pack/#specifications

The data there might help with your deliberations.

Which is what I linked to earlier at 14.55

It's actually a DPMT 11degree clearance not the 7deg of *C** but a DCMT or DCGT will fit and if you look up the ANSI code 21.51 that is for a 070204

Given teh lighter lathe I suggested the DCGT 070202 as the smaller radius will not require so much power, be better suite dto light cuts and the 7deg clearance makes it a bit more durable.

Jason, you must have delved deep into the Sherline part numbers to find out they were selling the DPMT type which is as rare as hens teeth. I hope the DCMT will fit in the toolholder as the bottom of the DPMT insert will be slightly smaller than a DCMT one.

Yes hoping it will as the Ps are not common as you say, being a small size it should have the least effect

Probably cheaper to get a new holder than track down the P inserts if they won't fit

To cut titanium, you need a sharp tool or a lot of power and rigidity. Grinded( HSS or Widia) tools are usually sharp and will cut titanium nice but not for long.

A carbide tool insert is normally a moulded one (DCMT070204, Diamont type, 7 mm long, 2 mm thick, 0.4 mm nose radius). The DCGT0700204 is the same size insert but its dimensions (accuracy letter G) are more accurate. To get that accuracy the moulded inserts are grinded. This grinding gives them sharp edges.
Sharp edges are good for plastics, aluminium and also titanium. Because the cutting forces are lower, grinded inserts are also an option to cut difficult materials on hobby machines. If you use a smaller nose radius (DCMT070202 0.2 mm nose radius), the radial cutting forces will be a bit lower.

G5 Ti is no big deal to turn really. I use the Sharp inserts for Ti, are specially coated do hold their sharp edge longer than uncoated inserts for Ali. Not sure on your rpm limits etc, but with the 0.2 radius it will work fine. The ccgt range and the DCGT range will work well. Be careful, as Ti can catch fire, a bit like Mg can. So always pay to have a fire extinguisher handy. Me personally have never had a fire with either of the mag or Ti. I use a soluble coolant that is rated for Ti, or I use cutting oils. I very rarely cut Ti dry but you can for light cuts. I treat it like 316 stainless steel , in that I use the slower surface speeds and higher feedrates for roughing. Typically for Ti with an R0.2 insert , I aim for around the 0.15 to 0.2mm per rev feedrate. Finishing I aim for the 0.05mm per rev feedrate with a R0.2 insert. Kyocera do a great insert for Ti, CCGT060202MP-CK I think the coating grade is PR1425, another that is great is in the DCGX070202 AG CX10 is a generic ground and polished insert for AL, but work well for finishing in Ti as well as other materials like plastic, aluminium etc. HSS does cut the Ti, but does dull quite quickly and is show through the swarf no longer chipping and the part getting hot again. For drilling I tend to use split point style drills and start with a coated spotting drill. You can tell the drill is going off when the energy to feed it starts to rise. Then just resharpen at that point and carry on. Some say not to peck drill, but generally I peck drill by micro stopping the feed to break up the chip if it is coming out in long streams. Adding a tiny break or curl just behind the inside edge of the drill will force it to break into little curls, but on drills under 4mm diameter, that is very hard to do.

The carbide insert in your picture looks like a molded insert which is blunt and is designed to be used on powerful machines taking deep cut. Switching to ground inserts will solve your the problem. I use carbide tools only on my hobby-grade lathe ( Emco Compact 8 ) to avoid grinding HSS tools. Have encountered the same trouble until I learned about ground inserts.

I did find some of the P style inserts on ebay, most were prohibitively expensive, over £10 per insert, and there was a much smaller choice of grades than the common DCMT or DCGT style.

I was concerned about their interchangability, as the tops would be the same size, but the base of the 11 degree angle type would be marginally smaller, and a lot depends on how accurately the pocket in the toolholder is made.

Edited By old mart on 13/09/2022 14:51:54

Surely its the top of the insert that does the locating?

A good quality holder will have sloping sides to match the intended insert so it will be the whole side that provides the location. Cheap one may have vertical sides in which case it will be the top edge of the insert that makes contact.

Is there anything on the side of the holder such as SDLPR? It's not on Sherline's site and the text says it is 5deg approach angle but drawing shows it as 3degrees in which case it may be SDJPR

I've found that titanium cuts nicely with dead-sharp HSS tools with 5-8° rake, much as you'd use for EN1a.

Then you can toast it carefully with a blowlamp till it goes blue...

Sorry, took a while to go through and digest all that information. What I’ve gathered this far is as folllows:

1. Corner radius is important and should be 0.2mm ideally as opposed to the insert I have which is 0.4mm.

2. It should be a ground insert not a moulded one and that means it’s a lot sharper.

3. The type of the insert should be one that is capable of cutting titanium!

4. HSS do the job too, but are going to get worn down quicker.

I always have the fire extinguisher handy anyway and I didn’t realise titanium reacts to heat like steel with the colours!

So I’m going to try with a DCGT 070202 insert, If I needed to get another holder would it be specific to this type of insert?

Couple pictures of the holder below, can’t see any numbers on it anywhere. The holder is just over 9.5mm high too.

If you do need a new holder then it will take any DC** 07 02## insert so it could also tale 0.4mm and 0.8mm tip radius inserts of the same overalls size and shape with the 7degree ( code C ) clearance angle. It will take the DCMT 0702## inserts too but I think you will find the DCGT will work a lot better on all materials on the smaller machine.

You could also go down to a 6 or 8mm square holder but not upto a 10mm which I think wlll put the cutting edge too high on the Sherline

The DCMT 07 and DCGT 07 are a very good size for smaller machines and there are boring bars which also use them.

I use a Taig which is the same size as the Sherline. I took Jason's advice a few years ago and opted for APT's CCGT 060202 inserts in a set of ARC's SCLC 6mm x 6mm holders. On my little lathe they cut brilliantly for anything ( alu, brass or steel) that I turn. The holders need very little shimming in the standard Taig toolposts and the tips will take very fine cuts if needed.

Edited By Peter Cook 6 on 15/09/2022 11:48:20