Making a center height gage to set lathe tooling

[wquiles]

Well, as it turns out, although I still consider it a great buy, the gage I got is "not" giving me proper center when the bubble is in the middle (target). The way I know? Do a facing operation. Only when the cutting edge is truly on center, you will get a flat face with no "nub". When I tried it using the target position, it ended up being a tad below center, and I of course ended up with a nub. I just adjusted the cutting bit until it got me a perfect facing operation, and then used this "true center" position to "calibrate" the center gauge. Now I know that I need the bubble in between "target" and the outermost line - so from now on re-centering tools will be easy and quick

Piece of Al I used to do the test:

Even here where the bubble is a tad off-center, it still was a tad too low:

Will

 

[Mick]

If you are using an insert with a large positive rake and a large tip radius you will measure the tip but actually be cutting at a lower point on the insert and get a nub.

 

[wquiles]

On positive or negative inserts, the first part of the edge to touch metal starts the cut (really a shear action), and depending on the depth of the cut, the amount of edge engaged in the cutting/shear varies for each type of insert - that is part of the design of any insert.

With the inserts that I am using, the topmost edge "does" start the cut in turning/facing operations, and even in the lightest of cuts it would leave no nib, but only when perfectly on center. So I still stand by my earlier comment, in that when I set the gage to that leading edge, the gage is not properly giving me a "center" reading - it is off center.

Will

 

[precisionworks]

an insert with a large positive rake and a large tip radius you will measure the tip but actually be cutting at a lower point on the insert and get a nub.

All inserts start the cut at the pointy edge, but more of the edge beside or behind the point is engaged as both the DOC and the feed are increased. Inserts are always brought to center using the tip, whether that tip points up (pos. rake), down (neg. rake), or straight (neutral rake).

I still stand by my earlier comment, in that when I set the gage to that leading edge, the gage is not properly giving me a "center" reading - it is off center.

You are correct :thumbsup:

 

[Mick]

On Korloy CCGT32.52 inserts the rake of the tip extends through the nose radius. This insert will not cut on the highest point of the tip until the insert is engaged ~.015 into the work. If you make a .005 finishing cut you will leave a small nub if the tip is on perfect center. I just looked very carefully at the insert and the nose radius is not flat, it is sloped to the rear.

If you look closely at Will's photos of his TMX inserts you can see they are made the same way as the Korloy.

https://www.candlepowerforums.com/threads/246303

 

[precisionworks]

The insert above is Korloy CCGT 32.51 Hi Carb (Travers Item #22-286-702). I realize that the .51 nose radius is smaller (sharper) than the .52, but the photo surely makes it appear that the pointed tip is the first part of the insert to contact the work, as the side of the insert is curved inward starting just below the tip.

If you make a .005 finishing cut you will leave a small nub if the tip is on perfect center.

How do you determine when the tip is on center? Of the dozen or so different inserts that I run on a regular basis, each one leaves no nub when set on the lower step of my center height gage. The boring bars are all set .010" above center, but don't get used for facing or O.D. turning.

 

[wquiles]

(snip pic)
The insert above is Korloy CCGT 32.51 Hi Carb (Travers Item #22-286-702). I realize that the .51 nose radius is smaller (sharper) than the .52, but the photo surely makes it appear that the pointed tip is the first part of the insert to contact the work, as the side of the insert is curved inward starting just below the tip.

+1

All of my positive inserts are the same way. The highest point "does" contact the work first.

I must admit that I still can't quite "see" an insert that has an edge "higher" than the rest of the insert that would not cut/touch/shear the work first.

 

[Mick]

If you set your tool post at 90 degrees so you can take advantage of the 80 degree diamond insert's ability to turn and face without rotating the tool then these inserts will contact on the side below the tip. The situation is worst for a larger radius but I suspect the 1/16 radius inserts will leave a nub also.

The highest point on the insert will contact first if you set your tool at 45 degrees.

 

[wquiles]

That is exactly how I set my 80 deg inserts, and my tool post is always at 90 deg (Mirage_Man will joke that mine is welded at 90Deg as I even thread straight-in!), and I don't have my insert contact the work below the tip, so I guess we will have to agree to disagree on this one.

 

[Mick]

Take a very light face cut and watch the chip come off the insert.

Better yet, blue the insert and look at the contact point.

 

[precisionworks]

If you set your tool post at 90 degrees

AFAIK all insert tooling is designed so that the insert holder is set 90 degrees to the work, as each insert shape has a designed "lead angle" (usually from 0 degrees to 45 degrees). Setting the tool at any angle other than 90 can be done for special situations, like getting the point into a corner, but normal set up for inserts puts the holder at 90 degrees to the work.

Some info from Lindsay: http://www.lindsaycuttingtools.com/pdf-sections/tech.pdf