Thread: Finish Quality versus DOC

are you milling or turning?

this maybe wrong but with milling i think you need to "load" the tips up, when you have 4 or 5 tips working at a low feed there isnt enough "load" on each tip so 1 or 2 tips will "skip" over the cut so the next tip will have enough "load" to make the cut but it looks like dogs balls...

sorry missed the pic.... again i think "load" is important what do you mean by DOC?

DOC = depth of cut .


Why I hardly ever use carbide tips on the lathe.
Could be the tip radius on the insert ,they have different radii for different applications.

Kev.

The mild is a bugger to get a good finish at the best of times with carbide,more so on a shallow cut.
As woodlee has said nose radious will or can also paly a big part in surface finish.
Most of the smaller centree or bench lathes would be pushing it if the NR was much bigger than .4,some if they had the HP may show reasonable results with .8.
The carbide in general prefers a reasonable doc,say .5mm as a minimum.
If you wanted to do another test with the same cutting conditions I would sugest reduceing the Nr of your carbide,just put it on the grinder if you hacve a suitable wheel and see how that goes.
I would take a guess and think that you would more than likely have a .4 or .2 nr at the moment,try taking some off it and see if it makes a difference.
Also from your picture it appears that there is a bit of vibration present.

Hi Pipeclay, Kev, new_guy

Thanks for the comments.

Thanks Kev, I think I might have to go back to grinding HSS when I get down to final dimensions..

When the DOC is shallow compared to the sharpness of the cutter edge, it just doesn't seem to actually cut, it sort of ploughs up the steel and bounces off. I seem to get reasonable results when the DOC is more than 0.5 mm (as Pipeclay suggested).

Originally Posted by Pipeclay

If you wanted to do another test with the same cutting conditions I would sugest reduceing the Nr of your carbide,just put it on the grinder if you hacve a suitable wheel and see how that goes.
I would take a guess and think that you would more than likely have a .4 or .2 nr at the moment,try taking some off it and see if it makes a difference.

Don't you mean increasing the Nose Radius?

The inserts I'm using are 0.4 nr, but... I'm a little confused by the term nose radius, I take that to be the radius of the tip of the cutter as seen from above, not the sharpness of the cutting edge. So a higher nose radius (and or lower feed rates, since they both work together ) would generally produce a better finish (have I got that right?)

The inserts I'm using are Kyocera TCMT 16T304HQ (re 0.4)

Originally Posted by Pipeclay

Also from your picture it appears that there is a bit of vibration present.

You are spot on with the observation about vibration, I get some high frequency vibration at some combinations of speed/feed rates. More often with the higher speeds I think.

Regards
Ray

I mean make the nose radious smaller,in the past I have needed to make the radious smaller to achieve finish and reduce vibration.

I have just touched the cutting edges on both sides of the Tip (side and front clearence) to reduce the radious,I have not touched the actual point of the tool.

This method will render that particular cutting edge of the insert useless for other machineing work.

I usually just rotate to the next cutting edge or remove that particular tip and keep it for future use.

As you are all aware, I only have small lathe ..a toy compared to some others on here

When I first started "playing" I was using carbide....got a brilliant finish on SS and some hardened stuff and some pump shafting...but on MS bar from the local steel merchant..it was absolutely awful

Got my self one of those Eccentric tool holders' and with a sharp cutter I get a really good finish on MS.

I will try to remember to take a heavy DOC on my toy and see if the carbide works any better than it did before...worthwhile experiment/test I reckon

Originally Posted by pipeclay

I mean make the nose radious smaller,in the past I have needed to make the radious smaller to achieve finish and reduce vibration.

I have just touched the cutting edges on both sides of the Tip (side and front clearence) to reduce the radious,I have not touched the actual point of the tool.

This method will render that particular cutting edge of the insert useless for other machineing work.

I usually just rotate to the next cutting edge or remove that particular tip and keep it for future use.

Thanks Pipeclay,

Ok, I think I've got the idea. I guess it's to make it "bite" a bit more.

I'll play around a bit and see what works.

Regards
Ray

If I understand what your thinking that would be correct.

If not by reducing the nose radious to virtually a point you in essence have the same result as useing a HSS tool sharpened to a point.

You are reducing the amount of area incontact with your material being machined,most indexable tooling likes to have a reasonable amount of load on the cutting edge,it is (ripping) or (tearing ) the material rather than (cutting) or (sliceing),as HSS does.

If you have a cemented carbide tool you may find this easier to reduce the nose radious the the indexable type.

If you try it with the indexable type touch the tip up while still held in the holder if at all possable.

I have noticed the exact same thing as you, i.e a heavy cut = great finish and a light cut much worse.

The only way around it I have found is to make sure the last cut is around .5mm (.25mm DOC). Anything less than that and the finish is pretty bad, although max RPM (2000) and finest feed (.04mm per rev) seems to help.

Yep and the HSS benefits with a larger tip radius.

I usually use a fine slip stone on hand ground HSS tool bits to get it razor sharp ,tip radius is done by holding the HSS steel between my thumb and fingers and using the corner of the grind stone to apply a radius to the very tip of the tool .You need to hold the tool so that the radius goes down the whole corner at the same angle as the clearance angle Then I polish all faces and the radius with a slip stone.

Hope my explanation gets across ok , harder to describe in words than to actually do it.

Kev.

Originally Posted by RayG

So, here's my question, why do I get a better finish with heavier cuts than I get with a lighter cut.

Ray, to answer your question, the answer is because with a deeper DOC the tool is cutting, and with a shallower DOC it is rubbing to a greater or lesser extent.

As to what to do about it, the most obvious answer is simply to take a deeper DOC. These inserts were developed for industry where a large amount of material needs to be removed quickly to final size. They found their way into the home market and seemed an ideal choice for a tool in our disposable society; no more sharpening! However the way we use our tools is very different to industry, and we tend to have much less rigid machines, remove far less material at a time, and tend to "sneak up" on the final dimensions. Personally I find myself using insert tools less, and HSS tools far more often, as I don't generally require the hardness of the carbide for machining hard materials or high speed stock removal. HSS can be easily made to be much sharper in the home shop than an insert tool, and that sharpness will allow for finer finishes. Rather than regrinding, I touch up the tools with a small diamond hone and they will last a long time without need to re-centre the tool in the holder.

I have a small Hercus lathe and had been using 1/4" HSS as that was simply the first I bought and so I just kept buying it. The size is also easy to grind. I recently tried 1/2" HSS and was impressed at the increased rigidity. A sharp 1/2" HSS tool will give a beautiful finish on mild steel.

Pete

Edit: By pure coincidence HSS inserts were being discussed on another BB. I'm not sure if they are available in Australia. I had heard of them previously but have never personally seen any. Here is a link to the manufacturer's site. http://www.arwarnerco.com/warner_catalog_inserts.html

Hi All,

I tried Pipeclay's suggestion of reducing the nose radius, and after a couple of goes at it, finally got the hang of it (well sort of) just a tiniest bit off the sides to reduce the radius of the tip and works much better on light cuts. Still not quite as good as the deeper cuts, but heading in the right direction. I ended up mounting a diamond cup wheel on the side of the grinder, (an idea I pinched off krisfarm's grinder deluxe cart) and that gives enough control freehand to just lightly touch the sides.

PeteF,

That's exactly what I found, light cuts on mild steel with carbide, just don't bite enough to cut. Pipeclay's method seems to noticably improve things.

Thanks for posting that link to HSS inserts. Might be an option if they were cheap enough.

One of the factors driving industry towards insert tooling, is probably the move to cnc. it's more accurate and consistent to just replace the inserts on a tool

I've been moving more and more towards carbide inserts, and love em. Just a few areas where they don't perform. I have no problem taking 5mm cuts on ms at 1500 rpm and getting nice finish. Although it's not a real heavy machine (Shaoxing CQ6230 12x36, with a 2hp motor & VFD) it does the job well enough.

The area where HSS still does better is sneaking up on a final dimension in mild steel.

Regards
Ray

I'd agree 100% with that Ray; carbide likes it thick 'n' fast

Changing the nose radius should definitely help, however you're possibly also making the insert somewhat sharper than it was as standard. It would be interesting to isolate that factor by simply sharpening without changing the radius (if you haven't already done so) and compare the results.

I must admit the insert tooling I use is cheap stuff and I understand (like most things) there can be quite a difference between cheap and quality brands with regards performance.

I've heard those HSS inserts aren't cheap, but it would be interesting to get a better idea of detail of that. As someone pointed out, they have the advantage that you can rough out the part with the carbide, then simply change inserts and carry on with the finishing passes without having to change anything else in the setup. Maybe not such a factor in this day of cheap QCTP holders (for those using the AXA/BXA style anyway), but just the same an advantage.

Pete

Hello Ray,
We need to examine the tool geometry and its effect on metal cutting. (Not DOC vs Surface finish at this stage)

The Kyocera insert you are using has a HQ chip groove geometry:

Primary rake: 5 degrees 0.2mm wide
Secondary rake: 15 degrees
In addition it is a coated insert which has "edge rounding" of possibly 5-10 microns to assist the coating process.

If your feed rate was below 0.2mm per rev then your effective cutting rake is only 5 degrees and if below 0.05mm (0.016 in your case) then the edge rounding comes into play giving a negative cutting rake, as the feed/rev is less than the edge rounding.and shows the ploughing effect you mentioned. Instead of a continuous or C shaped chip it compresses due to the negative rake and builds up like a deck of cards between the workpiece and to insert surface.
Also it increases the tendency for vibration. Evident on your photos.

If you can find Kyocera uncoated inserts with an XP chip groove geometry this will give you a primary rake of 20 degrees and minimal edge rounding as it is uncoated.
A much sharper cutting edge for sticky materials.

As Pipeclay suggested, take your current insert and use a suitable diamond wheel to reduce the radius.
I would also grind a small chipgroove across the insert point at 15+ degrees cutting rake about 2-3mm wide and use only for the light finishing cuts.

Coated inserts and their associated geometries are ideal for high powered rigid CNC machines with a controlled environment and produce excellent metal removal rates and surface finishes.

For the hobbyist, uncoated inserts produce better results and can be modified without losing capability as the high wear resistance is lost once you grind a coated insert.

Col