Getting good finish on carbide inserts with shallow cuts

[Ischgl99]

Ok I get it.

So what makes an insert/tool bit give a good finish cut even with a low depth?

I tried grinding HSS bits and I could never get a good finish off of a HSS bit, no matter what I do, hone it, etc. it would always give a scratchy finish. Is it the chipbreaker or just how sharp the edge is?

I’m not good at grinding HSS either, so I mostly use screw style inserts for my tooling.

The angle, chipbreaker and the shape of the edge determines the depth of cut range. This file is an excerpt from the Kennametal catalog and shows the different styles and performance data. You can see that the shallow depth of cuts have sharper edge styles, and the ones for roughing have more robust edges.

 

[taiwanluthiers]

I’m not good at grinding HSS either, so I mostly use screw style inserts for my tooling.

The angle, chipbreaker and the shape of the edge determines the depth of cut range. This file is an excerpt from the Kennametal catalog and shows the different styles and performance data. You can see that the shallow depth of cuts have sharper edge styles, and the ones for roughing have more robust edges.

I tried grinding HSS blanks with the sharp angle like the FF profile in the catalog you linked, and I was still getting a scratchy finish in steel.

In fact I even tried using aluminum inserts (which is VERY sharp, has polished edges, etc.) and it would give you a horrible finish in steel but beautiful finish in aluminum. This is regardless of how shallow I tried to cut.

 

[Ischgl99]

I use my ground aluminum inserts in steel as well and get good surface finish, my steel finishing inserts tend to do well too, but not quite as good. I find different materials require different depths to get the best results with the same inserts. I haven’t turned 4140 much, but you might need to experiment with a few different styles to find something that will work well with a very shallow depth of cut. Make sure your insert is not above center, with a depth of cut that shallow, it will rub instead of cut.

Lately I have been taking deeper finishing cuts to get the better finish. I rough to about 0.5-1mm oversize, then do two finishing cuts to my final size. If I am at 0.5mm oversize for example, I would do 0.25mm and measure to see how close it is, and then do the next cut to final dimension. I am usually +/-0.05mm, and many times better then that.

 

[markba633csi]

I discovered these Kennametal high positive inserts sort of by accident (included with some other stuff I bought) CCGT3251HP series
KC730 for ferrous
KC5410 for non-ferrous
Diamond shaped- I screw them on to a homemade holder (shown)
You can use one for the other and vice-versa; not a lot of difference that I can tell- they both give a beautiful finish with light cuts
I use them in place of HSS nearly all the time now- only drawback is price- sometimes you can pick them up on Ebay cheaper

 

[homebrewed]

An HSS cutter called a Vertical Shear Bit can produce very smooth surfaces, but it's liimted to small depths of cut, maybe .002". That is about .05mm.

They're not difficult to grind, having just two facets. A more complete description of the cutter and how to make and use it can be found here.

One disadvantage of this tool is that it can't be used to cut right up to a shoulder. I used one to get a better surface finish on the tool post for my home-made QCTP. I used 4140 for that and as you have discovered it is difficult to get a decent finish with either HSS or insert style cutters with that stuff.

Ground inserts with positive rake and relatively large tip radius could be a good bet, too.

 

[Jubil]

Try this:
For taking light cuts use sharp insert (as stated above), with a small nose radius. Depth of cut should be 1/2 of nose radius. Use correct speed & feed for material being machined. I think this will work, it has for me.

Chuck

 

[taiwanluthiers]

To be honest with you, I buy my tooling and insert from China, because right now USPS is crap, and if I buy anything from ebay or whatever, it will take a month or longer to get here (if it does at all). I bought some guitar pickups from the US in the past and it took a very long time to get here, and I shipped a guitar by EMS that took over a month to get to the customer. So while Chinese quality may not be the best it's what I can get at the moment.

That said, Chinese insert makers don't publish big catalogs telling you what's what, and most inserts I see are for larger depth of cut (smallest I seen are 0.5mm) because they're all intended for CNC. So all I can go by is their chart of recommended feed/depth/speeds, and what the insert geometry looks like. They probably sell to industry who all have their preferred insert geometry depending on what they use and don't ask a million questions...

I tried that vertical shear bit thing and I had trouble making it work at all. HSS seems fine for roughing as far as I can see... only way you are getting a shiny finish off of that is if you use a lot of emery cloth.

I wish someone sells a sampler pack or something so I can try different ones out and see what works. At least Chinese sellers sell inserts by piece rather than in lots of 10.

 

[Provincial]

For those trying to understand the TCMT inserts referred to in earlier posts:

T = Triangular shape
C = Clearance or relief angle, in this case 6 degrees
M = Tolerance, meaning how precise the dimensions are held. In this case, .002-.005".
T = Hole/Chipbreaker, in this case, there is a center hole with a double countersink, and it has a formed chipbreaker

These are used in some boring tool holders and often in the inexpensive holders with square shanks that mount in regular (4-way and QCTP) mountings. The insert is held into a triangular recess with a screw vertically through the center hole of the insert. The TCMT has cutting edges formed during manufacture, and they are not ground afterwards. If you want more precision, the edges have to be ground. Conveniently, this leaves a smoother cutting edge and that can make for a smoother finish. The chipbreaker cavity is still "as-formed" so the cutting edge may have tiny scallops. The usual form for ground inserts that fit these tool holders is TCGT, and they are most often found in carbide grades suitable for aluminum. Some people have had good luck using them on steel with light cuts.

 

[taiwanluthiers]

So far my experiments have yielded the following:

I took some old inserts and ground them sharp, at least as sharp as I can make it. Result is still not quite good finish

I used a small boring bar I have that has a CCMT insert, boring bar is 10mm and the insert is quite small. I can get a glass finish as long as the depth of cut is more than 0.2mm, otherwise it's scratchy.

I used another smaller boring bar that comes with a small TCMT insert (insert is about 6mm on each side). That insert will make a smooth cut provided I spin the lathe at least 1200rpm, and it will give you a good finish even with a 0.05mm depth of cut.

I've ordered a few inserts from China. Will see how they work. I don't want to use the small boring bar because it's not very stiff at all.

I seen some Mitsubishi FY inserts on Taobao. Will order one and see how well they work (they worked great when I was in the states, when turning pre hard 4140). But then again pre hard 4140 usually looked good when you turned them.

 

[Batmanacw]

So far my experiments have yielded the following:

I took some old inserts and ground them sharp, at least as sharp as I can make it. Result is still not quite good finish

I used a small boring bar I have that has a CCMT insert, boring bar is 10mm and the insert is quite small. I can get a glass finish as long as the depth of cut is more than 0.2mm, otherwise it's scratchy.

I used another smaller boring bar that comes with a small TCMT insert (insert is about 6mm on each side). That insert will make a smooth cut provided I spin the lathe at least 1200rpm, and it will give you a good finish even with a 0.05mm depth of cut.

I've ordered a few inserts from China. Will see how they work. I don't want to use the small boring bar because it's not very stiff at all.

I seen some Mitsubishi FY inserts on Taobao. Will order one and see how well they work (they worked great when I was in the states, when turning pre hard 4140). But then again pre hard 4140 usually looked good when you turned them.

The reason your finish is ugly is because you are rubbing instead of cutting. Faster feed helps but also you must take enough cut to get past the minimum cut depth for the tool you choose.

Try to leave enough material that you can take 3 finishing passes at a depth you know the finish will be okay. Do not back out the cutter on the first two finishing passes. Just drag it back quickly.


I use a lot of CCMT to help with the poor rigidity in my small lathe. For very light cuts i use CCGT inserts with a very fine edge designed for steel but razor sharp.