View Full Version : Parting tool question(s)

09-04-2011, 02:08 AM
From what I can recall, two types of parting tool (not counting inserts though that is moving up the list rapidly), sorry about proper names..."taper" and "T" (?), anyway after yesterdays slight misadventure when parting I took a closer look at the tool bit/blade.

I was under the incorrect assumption the "taper" version was ground in a "V" shape at the factory and then the user removed more by grinding top clearance and angling it back (from top to bottom) to provide all the necessary clearances.
What I found on closer inspection was it is not a "V" shape (and thinking about it that makes perfect sense) but rather a modified "v" shape in that one side is flat and the other is half of the "v", slightly complicating to my noob brain is this means the top, instead of being horizontal is actually not.

Question being, so which way does this go? since in effect either side could go against the tool holder but means the top is angled to be lower on either the headstock side or the tailstock side...given what this is doing, plunging in, for lack of better term, why is this not level?

I thought at first maybe it was just the weird way a previous owner had ground the tool but in looking at brand new ones, they are the same shape...

09-04-2011, 03:03 AM
On further thinking...one way (direction of angle top) it will tend to clamp down and hold the blade, the other way it will tend to push the blade away from the holder ?

09-04-2011, 09:09 AM
I know what you mean about confusion on this. I'm somewhat familiar with all types of tooling, but I also get misty-eyed when people start referring to "T" and now "P" type of HSS and HSS-Co parting blades.

I don't have my Machinery's Handbook here at home, it's in my toolbox at dad's shop. Perhaps somewhere in there it shows the different profiles.

A lot of the the old "plain" blades I used years ago in rocker tool posts seem to be pretty flat. Others seemed to have top-to-bottom taper, but never enough of it. I often battled with rubbing on one side or the other, depending upon the blade and holder. Getting a blade so it didn't rub when using an off-hand holder in a rocker post was often an exercise in futility.

Then we get the "T" style that has the dual ridges along either side at the top, and you need a relief groove in the holder for those. Those also seem to have a little bit of top-to-bottom taper within the "T" section, for side angle clearance. You can't even put those (better) blades in some holders without modifying the holder.

Now I see people referring to "P" blades. What the heck is that? Is that the shape or a reference to a holder system or blade material or ??

I gave it all up years ago and went right to the insert style cutoff tools that I'd been using at work with such great success. And I have never looked back.

I recently "upgraded" my system from blade-holder blocks mounted into regular QCTP tool holder blocks. By that I mean I sold off the VGTB blocks and got No.7-71C and No.7-71-32 integral-style blade holders. This is what I sold off, and don't get me wrong, they work well.


Notice the clearance at the top, just under the clamp bar, for the thick part of a "T" blade. This is good, but the dovetailed block with this type of blade clamp integral to the block is a little more rigid, the most important asset in parting. These also put the blade up to 1" closer to the the QCTP block for increased rigidity. On most CNC machines, the separate block can be the only way to get a blade on the machine at all. This is what my new blocks look like, and I wish I'd just done it this way to start as it's less money overall.


If you currently have a QCTP on your lathe but do not have a blade holder, this is the way to go. What most people have is the No.7 style block that holds a HSS blade at an upward-to-center angle, as that's what came with the QCTP kit. The blocks I now have are along the same idea, but hold blades parallel to the block. There are some non-angled No.7 blocks out there that use a single screw and block to clamp blades. I prefer the long-bar clamp to the single short block that runs down an angle to clamp.

I'm every bit as confused as you Russ, when it comes to how a HSS blade and holder are supposed to work together to get clearance on both sides of the part-off groove. I suppose the only way to know if there's any angle on the holder, as set up in the machine, would be to mount an indicator on the chuck and sweep it across the surface of the parting blade holder to see how parallel it is or isn't.

Rich Carlstedt
09-04-2011, 09:49 AM
You grind the top on a tapered cutoff toolbit to match your mounting method. It dosen't matter which side you use against the holder
Some of the old Latern style holders allowed the blades to be vertical or cocked (slighty)
In short, if you mount the tapered side next to a vertical holder, you grind the top to match. If you center the taper, the top is ground to it.
The reason for the taper on one side is due to a manufacturing method used since the 1800's
The more modern 'T" shape will use holders that have a line, or step in the side of the holder to accomodate the T shape, and hold it vertical and balanced.

When using a tapered cutoff blade, be sure you grind the top flat and parrallel to the bottom. There is a bad habit that some machinists have where they grind a positive rake on the top. When you do this, it may cut well for you, but as soon as you reach the gullet of the blade, the blade becomes thicker than the cut, and results in a crash. Trying to remove this "fix" means a sizable loss of material on the blade

09-04-2011, 10:24 AM
I use the "P" blades which seem to be the "T" style. In my Phase II AXA holder these sit cocked because the top section is wider so I add a shim on the side to get the blade vertical. I have another similar parting holder where I used a slitting saw to add a shallow groove so the wide section of the "T" doesn't touch the side of the holder, eliminating the need to shim.

These holders clamp the blade at a small upward angle so the top of the blade need not be ground. The front relief should be 7 degrees plus this upward angle in order to get 7 degrees effective relief.

I add a small groove in the center of the top on parting tools as suggested by Geo. Thomas and this helps considerably by making the swarf narrower and weak so it breaks into chips, allowing it to exit the slot more easily.

One big help when parting difficult material is to raise the tool until it doesn't cut, then lower it slightly until it cuts again. By doing this, if you have 7 degree front relief then it limits the infeed by rubbing just below the cutting point if you attempt excessive feed rate or the tool nods. This generally reduces/eliminates chatter and greatly reduces the chance of a hog-in. Diameter diminishes rapidly when parting so you'll need to lower the tool occasionally to keep the tool cutting - increasing force needed to cut lets you know when this is necessary. The rule of thumb is the tool tip should be about 1% of the cutting diameter above center to minimize chatter.

The 7x12 is a limber little machine so with a more rigid machine these methods may not be needed.

09-04-2011, 06:55 PM
The rule of thumb is the tool tip should be about 1% of the cutting diameter above center to minimize chatter.

Where did you find this or how did you determine this particular value? I've been putting my parting tools right on-center for years. On rare occasions I might run an insert cutoff a "couple thousandths" above center to counter the downward pressure of heavy-feed parting and leave no pip at the center, but I've never used a formula. To have a parting tool .010" above center on a 1" or .025" above on a 2-1/2" diameter workpiece seem a bit much.

I kill chatter by simply increasing the feed. Tools cut best when they're on-center, in my experience.

There's many theories around the internet, including running a parting tool below center. I setup as described above, and here's the result:


09-04-2011, 07:17 PM
A quote:

The cutting-off tool should always be set exactly on center, as shown in fig 81 This type of tool may be sharpened by grinding the end of the cutter blade to an angle of 5* as shown in fig 82 [side view, 5* from top to bottom] the sides of the blade have sufficient taper to provide clearance, so do not need to be ground. when cutting off steel, always keep the work flooded with oil No oil is necessary when cutting off cast iron
that's from HTRAL 42nd Ed

and the only reference I can find in either HTRAL, Lathe Op (Atlas) or Machinery Handbook

I have since found references to "A" which has only side clearance, "C" which has side clearance and chamfers (metric), "L" which has clearance on length and sides (?) and "S" which has side clearance and chamfer (standard/Imperial).

Pixman: still looking for similar blocks suitable to the AXA size, so far the smallest attachment dimension appears to be 5/8" which is just that bit too big (if putting it "piggy back" into an AXA holder "slot")...got a couple of good photos so making one is not entirely out of the question...the model Dorian has, I believe is #7-71C and those start at BXA (?) [Edit: oops, saw but did not recall in my looking you had mentioned that :o ]

09-04-2011, 07:59 PM
The "1% of work diameter above center" rule of thumb is from "Design and Use of Cutting Tools" by Leo St Clair, see pg 36.

I've found St. Clair's book to be a reliable reference. But I believe only half of what I read so I've verified that his method works well - at least for me. My cheapo machine is fairly limber so it lets me know when my technique isn't good; your machine may be less fussy.

09-04-2011, 08:43 PM
OK, I get it. On little machines using AXA size toolposts, using carbide insert blades is an issue.

Dorian does in fact make a D25AXA 7-71C, and it accepts a 3/4" or 7/8" tall blade. Industry Depot has them list for $145.44, plus shipping. Then you'd need to find a quality blade and insert package. While that's something I'm very good at finding, it's still quite expensive for a home shop.

What you might do instead is get a monoblock holder that simply fits into one of those $20 AXA tool blocks from Tools4Cheap.net (http://www.tools4cheap.net). You get one of these kits from LatheInserts.com (http://www.latheinserts.com/product.sc?productId=46&categoryId=82) and you're done, for a very reasonable price.

09-04-2011, 09:27 PM
The "1% of work diameter above center" rule of thumb is from "Design and Use of Cutting Tools" by Leo St Clair, see pg 36.

I've found St. Clair's book to be a reliable reference. But I believe only half of what I read so I've verified that his method works well - at least for me. My cheapo machine is fairly limber so it lets me know when my technique isn't good; your machine may be less fussy.

I've never seen nor heard of the book before. When was it published?

There might be a chance that theory was put forth in the days of those lantern post holders that held a blade out at an angle and had a big cut in it to make it "spring-loaded". Perhaps your machine may have the "loose spindle equivalent" of the spring-loaded holder, and that helps it work better. ;)

You've got a near-neighbor over in Oxford CT who I helped with this whole issue on his Harrison M300 lathe. "Fullautomike" had put two clapped-out M300's together to make one decent one, and it worked. After him coming up to my dad's shop for me to show him how to make the new cross-slide screw & nut, I helped him get all tooled up with insert tooling. A monoblock style carbide insert cutoff tool holder and inserts was part of that. He also has some YouTube videos out there, one of which is parting on his machine. Trust me, even after building his machine out of the best parts of two, it's still no cream puff. But it works and he loves the parting tool(s).

09-04-2011, 10:00 PM
"Design and Use of Cutting Tools" was published in 1952 by McGraw-Hill. It has near 400 pages devoted to single point cutting tools. It's out of print so it's difficult to find but is the best book I've seen on the subject of lathe bits.

09-04-2011, 10:43 PM
I wish there were more copies of that book floating around. I've been diligently looking for one for a good time now. None around. It does sound like an excellent reference book that is unlike any single other source currently in print.

09-04-2011, 11:12 PM
You might try this:

There seem to be several libraries near Chicago with copies so an inter-library loan may work for you.

A friend has a copy he bought several years ago for $80 so it has been in short supply for a while.

09-04-2011, 11:30 PM
Would you happen to know anything about University libraries and public access?? I have seen that, but they are not in my public inter-library loan system. I AM a graduate of the U of Illinois, though ;) Maybe that could work in my favor even though Champaign/Urbana is ~3hrs away. :)

09-05-2011, 01:10 AM
Not sure how it works in the US but inter-library loaning here in Canada can be a bit fraught with minefields [not connected at all to the more general public library system, strictly inter-university].
Claims that it makes no difference if you are a current student, or not, abound...starts with "as long as you are a card carrying 'member'" soon turns into a quick search for someone you know who IS a current student...not sure where alum fall in those groupings and even then the "inter" part often is not fulfilled as its way down the priority list...it works but chances are sometimes quite slim [unless you want the entire book it can be faster and cheaper to just get pages copied...not that I am encouraging that sort of thing...]

Back to cutting blades...found a few listed as replacements for HSS or brazed carbide, pics/drawings make it appear as if they are just longer, modified versions of HHS blade holders...DGFS, TGFS, SGHS, not sure as some of the inserts are tough to find, more common can cost more but are two ended so price is not that much different if you count edges...many more are available but with limited diameters to full depth...as even just starting out I have already needed to part about 1.25" diameter...man, the variations on a theme...solids, materials, thin wall tube etc. etc. etc.
Slightly surprised no "off shore" copies of those #77 and #71 holders seem to exist...yet

Given how quite a bit of the Coromant line appears to be blades cut in half and fasten directly to holders (small screws/bolts as opposed to clamping system), is there any reason one could not just make up some solid blocks, put some horizontal holes and matching ones in a full length double ended blade and have at it?

Edit: forgot to add, all this searching improves knowledge...going by what Sandvik states (I am assuming the same thoughts apply to inserts or other tooling) I now "get" why the R and L...the pip remaining can end up on different parts (the stock remaining or the part parted) and according to them, the larger angles reduce the pip size but do not produce a straight cut and can result in poorer finish as well as shorter tool life

09-05-2011, 02:44 AM

and different yet somehow the "same"


09-05-2011, 09:04 AM
I've had mixed success with university libraries that aren't part of the system.

In one case our local librarian called them and arranged for loan of a book for me, worked out well.

I'm a UConn graduate but couldn't get a book from their library, had to go there and copy the short section I needed from a book that likely hadn't been taken out for years.

09-05-2011, 09:25 AM

I agree, it is strange that the off-shore makers are still copying the #7 style holders and have ignored the taller and parallel blade orientation of the #7-71C or #77 holders made by Dorian and Aloris. Someday they'll catch on.

The idea of drilling holes in blades and mounting them on a block is not without merit. It just seems a lot of work as when a comparable expense of a 1/2" shank tool holder that does the same thing with no work required exists. The blades can be up around 40Rc to get some spring to them, and that can be tough to drill.

09-05-2011, 05:37 PM
Pixman: another PM about to be sent...I tell you, I'm going buggy from all the data...