Some People Should Not be Allowed to Have Machine Tools
May 14th, 2010No Reloading for Me
I am a machining genius. I’m so grateful for the humility which keeps me from getting conceited about it.
I wanted to machine a slot in a Hornady shell plate which was supposedly hardened to about Rockwell 33. I put my rotary table on my mill and tried to mount the plate in the 3-jaw chuck. The plate wouldn’t fit. It has five pockets around the edge, and the pointed inner faces of the chuck jaws wouldn’t line up with them in a way that would make the plate concentric with the chuck.
I posted a message on a machining forum, and I thought about possible solutions, including machining a round piece of scrap to hold it, with clamps attached. Then I tried to put it in my lathe, which has an 8″ 4-jaw chuck. And it fit. I wondered why it was so easy. Then I noticed…the 4-jaw chuck’s jaws had long concave surfaces facing inward. And guess what? The jaws on my rotab chuck were removable. They’re two-piece jaws. When you turn them around, they have concave surfaces, too. I switched the jaws, and the plate fit perfectly.
Anyone else would have figured it out in about thirty seconds, but I managed to do it over the course of several days.
Now I was all excited. I had a cutting tool which should have been at least Rockwell 62. I had oil. I had the workpiece mounted. It should work, right?
I found a seemingly applicable chart in Machinery’s Handbook, with BRINELL hardness figures. Okay, fine. This is what the Internet is for. I found an online calculator, determined that I was dealing with Brinell 311, and came up with a figure of 30 FPM. I found another online calculator, put the data in, and came up with about 600 RPM. I reduced that to 450 out of caution, and I went back to the garage.
I applied Ridgid pipe cutting oil and tried to take a cut. When the cutter went up onto the workpiece, it did nothing whatsoever, as far as I can tell. It rubbed the metal in a way I can only describe as friendly. I assume the $2 cutter is ruined. There is no way this thing will cut the metal.
Now I have grave doubts about my highly reliable Internet-derived hardness figure.
Anyway, nothing is going to happen until I get carbide. Maybe not even then. But I solved all the other problems, and it was fun until the moment of FAIL.
I could try a carbide lathe tool, but with the interrupted cuts I would expect a busted insert in about a hundredth of a second.
At least now I know my chuck has two-piece jaws.
May 14th, 2010 at 9:13 PM
The shell plate might have a surface hardness approaching file hard if the tool just skipped on the surface.
450 rpm is going to be too slow.
Using a formula based on a general alloy steel with a high speed cutter of 1/4″ diameter I came up with this formula using the Machinist Handbook charts.
12X50(feed per minute)=600
3.14(Pi) X .25(Diameter of tool)=.785
A feed per minute between 40 and 60 is general for high speed steel tools in most carbon and alloy steels.
Adjust to the actual tool diameter multiplied by Pi if you are using a larger/ smaller end mill.
A speed/ feed formula can be further refined when a base RPM is determined by using the formula;
Feed= inch per minute X number of teeth on cutting tool X chip loadX RPM
600/.785=764.33rpm 750-800rpm actual depending on the amount of variable spindle speed adjustment capability your machine has available.
I know too fast a spindle speed causes excessive wear on the tool but so does running a tool too slow.
Tool slow can cause the tool to catch and snap if it does manage to break through the surface hardness shell.
The material you are trying to break into may be a metal injected mold item with a very hard shell and very soft interior but because Hornady says they can alter the plate I doubt this to be the case.HTH
May 15th, 2010 at 11:56 PM
The alloy hardness of the part might be as you describe it, but I’m pretty sure they hardchrome these things.
I believe they are MIM parts that are then hardchromed.
Surface hardness higher than most any cutter you’d have handy; and once you did cut it, the exposed metal would be so soft as to be nearly useless.
There’s a reason they cal MIM things like “marshmallow immitating metal”.
May 17th, 2010 at 12:06 AM
Consider, if you will, the cheap diamond blades sold at such nefarious outlets as Harbor Freight. Making a holder for them is a simple exercise, with a non obvious catch. If this sounds useful, let me know and I will tell you everything I know, just takes a minute.