can you take small cuts with carbide?

[Runner4404spd]

i've got a 9x20 lathe and i'm using idexable carbide tools. the tool bits are 5/16 so they are fairly small. i am trying to take .0005 to .0015 cuts and it seems that the tool just doesn't cut but goes along the surface. i am turning hot rolled steel, but i seem to have this problem in aluminum too. in steel i can take a .015 cut with no problems and nice surface finish at 600 RPM. what am i doing wrong? this became a problem when boring a hole for a bearing interference fit. i don't want to change tools at the end of the cut as i don't want to lose my position either. i find it difficult to regain my end point after switching tools when boring. all tips are greatly appreciated.

thanks
Jerry

[Geof]

Check whether your tool is exactly on center or a very tiny bit below. If the tool is above center slightly a decent cut can deflect the tool and work so it comes onto center and does cut but a very small cut causes no deflection and the tool just rubs.

[clarkea1]

Generally, carbide tools like to be worked hard to get the best results in terms of finish - so high feedrates, bigger cuts and higher spindle speeds are typical when using carbide tools. I've also had trouble with carbide when trying to dust the last few thou off a part. Best to use a HSS toolbit for the very fine cuts, as on a relatively small/light machine this will give the better results, and keep the carbide for the heavier cuts.

Hope this helps

Alastair

[Runner4404spd]

my tool bit is definitely set just above center. i will reset to put it on center or just below. at least for my boring bar. thanks for the tip.

[Geof]

Alastair's comments are quite correct; carbide and small machine sometimes is not a good combination. Part of the reason is that carbide inserts are not really sharp.If you look at most inserts with a decent microscope you can see the cutting edge is rounded or rough; actually with high power magnification you can see the powder particles size that the carbide was sintered from. The chip size taken by the tool has to be larger than the extent of the edge roughness so the chip as it peels off is riding on the top surface of the tool behind. A very small cut doesn't do this as effectively. This is a large part of the reason that carbide inserts are not as effective on plastics sometimes, particularly a flexible and tough plastic like nylon. Micrograin carbides have a much smaller initial particle size so they can have a much sharper edge, approaching razor sharp as the scar on one of my fingers can attest.

[DareBee]

For boring the cutter should be set slightly above center to keep from gouging oversize when cutter deflects.
What you need is to get inserts with a high positive rake that are specifically designed for light cuts.
For instance - when you check out Korloy for boring inserts in a CNMG - CCGT style you will find that there are more than 10 grind styles of that insert ranging from ultra fine finishing - to heavy duty cutting to specific aluminum cutting.
You need to pick the right tool for the job.

Wherever possible for bearing fitting I hand polish the last .0015 to exact size, much more accurate and perfect surface finish.

[justin_sane420]

Another thing is that with cutting hot rolled, that you have to take a heavier initial cut to break the scale as well or the cutting action will not be very nice for you and will most likely dull out the carbides quickly and will also produce more chatter and vibration. and in a small machine a small amount of vibration can have a huge impact on the effectiveness of your machine tool

[Adobe Machine]

I've had good luck with alloy steels and heat treated steels using CNMP 80 degree diamond shaped inserts with a positive rake and chip breaker. They are real sharp and tend to lessen the cutting forces,as Geof stated a lot of the carbide inserts are not sharp, depending on brute force to machine.
Just my experiance the last couple of years

Adobe (old as dirt)

[HuFlungDung]

For manual type operations in my shop, we take the old slightly worn out inserts, and grind a very high rake hook nose at the front of the insert.

For this purpose we have a narrow natural diamond wheel permanently ready to go on a 6 inch bench grinder. The wheel is 1/16 radius, 1/8 thickness. Been using the same one for 20 years now.

The razor sharp edge is delicate, but you can shave off a half thousandth with it, and get an excellent surface, providing you use a bit of oil or WD40 on it.

If I went to work anywhere else, I'd personally buy my own grinder and diamond wheel before I'd buy my own mics

[Adobe Machine]

Hu: Thanks from a lot of us..( went out to the shop and started looking for inserts I had tossed..have small diamond wheel, will save inserts..)

Adobe (old as dirt )