Breaking Tools

Breaking things and failing is definitely going to be a part of this project…

Over the weekend when I was making my first mold, my really nice ball-end carbide endmill tried to go through the bolt that was holding my workpiece to the table. This ruined the endmill, made the stepper motors lose steps, and detached the workpiece from the table. The whole job went very well until this happened. It made a few gouges in the surface of the workpiece, but overall I could see that the toolpath worked well, so it wasn’t at all a complete loss.

On Monday night I decided to try milling some steel. I had some 1/8″ steel plate left over that was the perfect size to try out this design on: http://www.thingiverse.com/thing:16694 Then I could have some steel Rayban frames! Fun!

Unfortunately milling steel is much more complicated than milling aluminum. So I ruined another endmill in that job. This time, luckily, I was watching it when it happened. I programmed the toolpath to take more than 1/16″ deep passes. It turns out that my little 1/4″ endmill couldn’t evacuate chips fast enough when it plunged down that deep. This was a monumental problem because I now had an endmill filled with steel that wasn’t going anywhere. As the endmill continued to try cutting, it was only creating friction because the steel welded itself to the cutters on the endmill. So within a few seconds, the endmill and the metal surrounding it turned bright red hot. I quickly hit the software E-Stop button (speaking of which, I need a proper, physical E-Stop button now). When I looked at the endmill, it was filled with steel that didn’t want to leave. I got most of the steel out of the flutes, but there was still some steel welded to the cutter tips and the cutter tips were chipped, so this endmill is now useless. I think this happened it part because I was running it at 2500rpm even though my software said I should be running it at ~1600rpm, which I didn’t realize when I wrote the toolpath. This meant that the cutter didn’t have time to cool down and release the chips. I may have also been plunging too slowly, which made the endmill rub and heat up. But I think the thing that made this mess up the most was the fact that I was using junky steel of indeterminate alloy. This is “weldable steel” from OSH. That means it is probably some form of low carbon (mild) steel, which doesn’t machine very well.

So because I ruined 2 endmills in 2 days, I decided to take some pictures.

Here are the two endmills that I ruined:

Here are some pictures of the fancy ballmill (right in the above picture) under a microscope. These show that the whole surface of the cutting edge is ruined and chipped.

All of these should look like smooth perfect edges, much like the edge of a knife.

This is what an endmill is supposed to look like (I broke this one too awhile ago):

Close up of a 1/32" solid carbide UltraTool endmill

Close up of a 1/32" solid carbide UltraTool endmill

Of course I also ruined the regular endmill I used with the steel. You can see that there are little bits of steel welded to the corners of the cutting edges of the endmill which means this endmill will never cut again:

Overall this wasn’t a huge deal and it definitely taught me a lot, most importantly that machining is a science and it requires a lot of experience to get right. The fancy endmill cost about $20 and the regular one was part of a set of cheap ones, so replacing it would probably cost $5.

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