View Full Version : It just doesn't cut it

11-23-2001, 09:02 PM
Hey everyone,

I am having trouble making my carbide bit cut. The HSS is no problem. When ever i put the carbide tiped holder on the toolpost i am garantying a rough finish. I do double the speed but that doesn't seem to help.


11-24-2001, 05:25 AM

First off, make sure the bit is properly adjusted to center height of the lathe.

Common occurance carbide cuts rough is a damaged insert. The Insert can be damaged from fracturing, notching, or a BUE or "built up edge" which is an accumulation of the material to be cut welded to the insert thus permanently damaging it. The surface finish from a damaged insert is very rough (looks like a bad filing job). Only fix is a new insert edge. If it is a brazed bit it is time to resharpen or replace it.

If you want to play with carbide, by all means get Sandvik Coromant's "Modern Metal Cutting" it will save you many busted inserts - trust me, I found that out the hard way.


[This message has been edited by Thrud (edited 11-24-2001).]

11-24-2001, 08:16 AM
From some of the other reads on the BBS i have gotten the impression that i need a carbide tip tool to cut welds. Dos this make sense? It seems that when ever i try to cut down a piece of metal after i have welded it, my HSS gets dull.

I have been doing my best to make sure my bit is at the center height. I bought a depth guage and things are going better now. That has realy helped with the HSS. So i figure i am on the right track. I tried the carbide again and crap came out.

I was woundering that maybe my bits aren't that good? I have the type that are held on with an allen head srew. 3 sided inserts that you turn when they get worn. I have a pretty small toolpost so i don't have alot of options availabel to me. I am thinking of buying a welded on tip and milling down the holder to fit in my toolpost.

What do you think?

C. Tate
11-24-2001, 12:16 PM
Are you taking interrupted cuts with your tool? This can very easily cause damage to the wrong grade of carbide. What is the material, size and your cutting data. This information will help us understand your problem better.

11-25-2001, 04:15 AM
If your weld is less than beautiful & perfect, grind the excess off with an angle grinder or belt sander first this will help smooth things out for the cutting tool (whatever you use). Just remember a weld has hard inclusions and it will dull HSS fast. As Mr. Tate says, some carbides do not take well to interupted cuts.

The triangular insert you are using probably cannot handle the rough duty you are asking it to. Try a brazed tool or T-15 tool steel.

11-25-2001, 10:16 PM
The material was metal sleeves that are about 3/4 inch in diameter. They are sleeves that go between bearings and a shaft. I originaly bronze welded them up so it would be easier to cut down. Well that would just wear down my HSS realy fast. I thought about using the grinder to get things a little closer but i thought the cutting tools might handle it.

I hasn't used carbide on this yet because i had had no success with carbide tips until yesterday. I am not quite sure but things are going better. Maybe i am now on the center of the work, where as before i was using my eye

I am thinking about buying a bronzed on carbide tool. What is t-15??


11-26-2001, 01:35 AM
T-15 is a tool steel super alloy. It has very high "hot" hardness (ability to retain its edge as it heats up under machining conditions) and far outlasts HSS in the same job. It can cut stuff like Stainless, Iconel, Hastaloy (High temp refactory metals) and works quite well with most Titanium. It is pretty good with interupted cuts as well. Aluminum is no problem.

It has one drawback - it is a bitch to grind unless you use CBN, Ceramic (synthetic Ruby - I am told they work pretty good), or SiC. Diamond cuts it like butter, but is not recommended for steels as it affects carbon content (Diamond is crystaline Carbon) and greatly reduces the life of the Diamond wheel.


11-26-2001, 06:26 PM
So T-15 isn't probaly for the hobyist. It can't be much good if you can't sharpen it. Man there is so much to learn about metals...

Is there any trick to identify which metals are which. Like a drill rod from a regular rod etc.


11-26-2001, 08:49 PM
spark test....put it to grinder in subdued light...hi carbon non alloy has carbon bursts all over like 4 july sparkler....white..,.if alloy in it ...yellow ...mild steel has very few carbon bursts...hi speed has orange/red streaks....hard to describe ..when u get some known steel ,spark it & keep a piece around (MARKED) SO U CAN RECLIBRATE UR EYES..there are some texts w/ pics but colors????? .
best wishes

11-26-2001, 11:55 PM

I am afraid you have mistaken my ramblings as a put down of T-15. I was just trying to give both sides of the picture. Its good properties far outweigh the "hard to grind" thing and it is perfect for the home shop - tough and cuts a long time! Sharpening does take longer, but if you need the performance, the extra effort is worth it. Label it the "ultimate best choice" for steel tool bits.

Most guys have a SiC (green or sometimes black) wheel, they work just as well on carbide as T-15.

CBN is the best wheel for hardened steels as it is the next hardest thing to Diamond. CBN should never be used on soft steel as it will wear the wheel out in no time (a strange quirk of "Cubic Boron Nitride") Hardness of the steel should be above 38 "C" Rockwell scale. CBN wheels are $50-100 more than a good Aluminum Oxide wheel, but is great for sharpening bits & drills. It will last longer than regular wheels and should never need to be dressed if not abused.


Doc, that is not a bad idea for the new guys to consider (and us old farts too).

C. Tate
11-27-2001, 09:38 PM
Try taking very light cuts at high rpm you can dress the insert on a silicone carbide wheel and keep using it to rough. Once the weld is clean you will be able to index the insert to a good edge and make a finish pass.
Who cares if the edge is chipped or worn a little on the rough passes as long as everything comes out on the finish cut.

kap pullen
11-28-2001, 08:02 AM
Carbide needs to run in the 300+ surface speed per minute range and with a .030 depth of cut for a good finish on steel.
To get a good finish with a couple thou depth of cut crank it up to 700+ surface feet.
This is not possible with most modelmakers lathes.
To get thru weld slow it down, up the feed, stone the edge, and expect to regrind or index often.
Carbide's most useful to the model engineer cuting through scale on castings.
A "knife" tool(HSS), with positive rake and small radius is most useful on light duty machines on steel, stainless, and, alum.
A zero rake tool is best for bronze alloys
and cast iron.
Happy machining
kap pullen

11-30-2001, 09:05 PM
Check the clearance at cutting edge of the carbide insert by setting the insert on center and look where this brings the cutting, if you are using a rocker tool post you will need a toolholder for carbide tools with different clearance, if you are using insertable carbide you should use inserts with a positive edge to prevent rubbing the insert instead of cutting.