Cutting a good crown

[rain164845]

Personally, I'd go to a solid carbide bar with minimal or no nose radius. You'll be taking minimal cuts with a very fine feed (hand feed most likely) and want minimal tool pressure. I'm guessing your insert is .008" or more radius with a chip breaker not meant for minimal DOCs.

The Warner HSS tool uses flat inserts with no chip breaker. I'll have to check on the radius.
What effect does the radius have?
I have heard that you should have a DOC deeper than the radius, is that what you mean?
Thanks!

 

[jackie schmidt]

Here is what I am working with.
I am not sure why the pictures are rotated 90 degrees to the left, but tilt your head and you can see my setup.
The first picture was the boring bar that I was initially using.

The second picture is the Warner HSS insert on their general turning tool. I am getting good results with this, but the flat on the crown doesn't look great. The actual crown is burr free though.

3rd is a lapping tool I made from brass. Since I am cutting 90 degree crowns, I chucked the brass rod in a collet, faced the end, center drilled, slit the end, then faced again. I am not sure I'll use this on the finished barrel since I am getting good results from the Warner tool.

View attachment 19172
View attachment 19173
View attachment 19174

I would skip the lapping plug thing. Trust your machine and your tools.

 

[rain164845]

Technique

I am using the finest power feed for my cuts. Any ideas on what rpm produces the best finish? I assume I should just go by recommended speed in SFM for has.

 

[jackie schmidt]

I am using the finest power feed for my cuts. Any ideas on what rpm produces the best finish? I assume I should just go by recommended speed in SFM for has.

If you notice in my video, I hand feed the little counter face, only taking a few thousandths for the final cuts. RPM is 1000.

Your HSS tool probably will not take that without loosing it's edge, even though 416R stainless is fairly free Machining. I am positive it won't On Chrome Moly.

Also, it has no positive rake. Take a good look at the tool I ground, with the positive rake chip breaker ground into it.

 

[rain164845]

Crown results

I feel really good about my work now. Lacking a way to grind a tool like yours, Jackie, I used my hss Warner bit. I ran it a little faster, maybe 500 rpm. I did a little counter ore like you did with a depth of .002 on the compound dial set at 60 degrees, then went back over the counterbore with .001 on the dial for a super fine finish. The crown looks great and didn't snag fibers.
Thanks for the help!

 

[rain164845]

This weekend, I plan on getting on my grinder and playing with some geometries on hss.

 

[rain164845]

Here it is

Finally I had some daylight and a day off at the same time and was able to grind a variation of the crowning tool in Hinnant's book. I am getting great results!
Thank you all for your help.

 

[JerrySharrett]

If you notice in my video, I hand feed the little counter face,

Also, it has no positive rake. Take a good look at the tool I ground, with the positive rake chip breaker ground into it.

Most important in cutting a burr free crown, as Jackie states, the cutting tool must have a positive rake cutting edge. It must SHAVE the material off not push it off.

.

 

[rain164845]

crowns

Most important in cutting a burr free crown, as Jackie states, the cutting tool must have a positive rake cutting edge. It must SHAVE the material off not push it off.

.

Yes, it is about a 5 degree positive rake and it is shaving. I am very happy with the results. The bit is 3/16 square. Of course it could be ground from larger stock, but the small size starts with less to grind and my bit fits neatly within the bore for a good start on the cut.