Adventures in Milling
I have been working with my youngest son to make a radio-controlled tracked crawler. The electronics are working just fine; but, that sort of thing is what I do for a living. The trouble starts when I work on the mechanical pieces. The objective was to make two sheet metal plates to be used to mate the top and bottom halves of the fiberglass body. The strategy was based on the idea that .001” accuracy wasn’t really necessary, so to simplify construction we used an old 2-D drawing program to print cutting layout guides onto full sheet Avery stick-on labels, which were affixed to the raw metal plates. These plates are cold-rolled 1008 #20 gauge steel.
This a view of the general milling setup. It would have been better to have used longer clamp pads to prevent the sheet from warping under clamping stress. The idea was to move the table until the tooth could be aligned with the cut line carried by the Avery stick-on. Tramming the plate was accomplished by tapping the alignment until a scriber's point would follow the printed line.
I have no idea why the images paste into the document so large; however, I cannot seem to be able to resize them.
I chose a ¼ inch four flute end mill. My research tells me that this was my first mistake. At any rate, the procedures involved aligning the tooth of the cutter with the cut lines, and then take .020” from the path with each successive cut. The waste ranged from about 1/16 to a bit wider than the end mill.
As you can see, the cutting job looked like it was made by a troop of metal-eating termites on an orienteering course. It seems that the more I know the worse my machining becomes.
Those burrs are the biggest I have ever seen. It was a brand new milling cutter made of HSS with a TiN coating.
Here those termites have made it about halfway to the next corner. This is the stretch between the two clamps. The full depth was achieved in three or four passes with the cutter.
The excrement really hit the blower one I rounded this corner, as if things weren’t bad enough already. It seems that I plunged down twice for this pass. The end mill got really hot and wouldn’t cut anymore.
I finished the trimming job with a different end mill. The original was pretty nasty looking.
Lessons Learned
I went to my books and the web in order to discover the source of my trouble. First, some general information. Speed and feeds were originally determined by consulting the Machinery’s Handbook, so I wasn’t a total boob when I set out. Well, not totally, I guess.
As I mentioned before, my post-mortem reading seems to indicate the following:
A Startling Confession
This cutting problem isn’t the worst of my experience yesterday, as unbelievable as that might seem. In order to turn the milling cutter to align a tooth with the cutting mark, I stuck an Allen wrench into the R-8 tool arbor as a handle. Yep, you know what happened next: I forgot to take it out. When I turned on the machine, the Allen wrench immediately struck the knob that operates the quill-stop, stalling the motor. I didn’t get my fully-deserved face carving, but I did get a severely bent Allen wrench and a messed up set screw in the tool holder.
I don’t know what the run-out was on my Grizzly G1006 mill before this happened, but it now appears to be about .0005”. Maybe I bent the shaft, maybe I didn’t. Anyone know what the typical run-out of those machines is supposed to be?
OK, Chop Me Up
Now comes the important part. Please flood me with appropriate pointers, information, scolding, and kicks in the ass. I have learned quite a lot with this “Miller’s Waltz”, but I think the opportunity should be taken to make this a prime outlet for guiding the rest of the newbies in “what you should not do.
This is not what I hoped my first post would be like, but I do need some words of wisdom so that I don’t have to do so much milling with the file. I did less filing when I did my cutting with a hacksaw and a jeweler’s saw.
I have been working with my youngest son to make a radio-controlled tracked crawler. The electronics are working just fine; but, that sort of thing is what I do for a living. The trouble starts when I work on the mechanical pieces. The objective was to make two sheet metal plates to be used to mate the top and bottom halves of the fiberglass body. The strategy was based on the idea that .001” accuracy wasn’t really necessary, so to simplify construction we used an old 2-D drawing program to print cutting layout guides onto full sheet Avery stick-on labels, which were affixed to the raw metal plates. These plates are cold-rolled 1008 #20 gauge steel.
This a view of the general milling setup. It would have been better to have used longer clamp pads to prevent the sheet from warping under clamping stress. The idea was to move the table until the tooth could be aligned with the cut line carried by the Avery stick-on. Tramming the plate was accomplished by tapping the alignment until a scriber's point would follow the printed line.
I have no idea why the images paste into the document so large; however, I cannot seem to be able to resize them.
I chose a ¼ inch four flute end mill. My research tells me that this was my first mistake. At any rate, the procedures involved aligning the tooth of the cutter with the cut lines, and then take .020” from the path with each successive cut. The waste ranged from about 1/16 to a bit wider than the end mill.
As you can see, the cutting job looked like it was made by a troop of metal-eating termites on an orienteering course. It seems that the more I know the worse my machining becomes.
Those burrs are the biggest I have ever seen. It was a brand new milling cutter made of HSS with a TiN coating.
Here those termites have made it about halfway to the next corner. This is the stretch between the two clamps. The full depth was achieved in three or four passes with the cutter.
The excrement really hit the blower one I rounded this corner, as if things weren’t bad enough already. It seems that I plunged down twice for this pass. The end mill got really hot and wouldn’t cut anymore.
I finished the trimming job with a different end mill. The original was pretty nasty looking.
Lessons Learned
I went to my books and the web in order to discover the source of my trouble. First, some general information. Speed and feeds were originally determined by consulting the Machinery’s Handbook, so I wasn’t a total boob when I set out. Well, not totally, I guess.
- Work was #20 gauge 1008 cold-rolled steel sheet.
- Cutter was ¼ inch HSS with a TiN coating.
- I brushed Tri-Cool MD7 cutting fluid onto the “in-going” teeth of the cutter.
- The spindle speed was 1200 RPM. Computed speed was 1376.
- I was feeding about .1” per second on the hand wheel; and, the computer feed was .54” per second.
As I mentioned before, my post-mortem reading seems to indicate the following:
- Use a larger end mill that ¼ inch if such would do the job. A ¼ end mill flexes too much to get a smooth cut.
- If one gets a crappy looking cut, slow down the feed in spite of what the computed speed turned out to be. This is particularly true when small cutters are used.
- Small end mills clog up with chips when the feed rate is too high for prevailing conditions. Use a two-flute cutter, especially when slotting.
- Bring the head closer to the table so that a long quill travel isn’t required to bring the cutter into contact with the work.
- The base problem in this work was “chip packing”. The chips can’t get out of the way of the next flute in time, so following cutting passes just chop up the chips when the chips get caught between the cutting edge and the work. Progress through the cut is little more than a bludgeoning process.
A Startling Confession
This cutting problem isn’t the worst of my experience yesterday, as unbelievable as that might seem. In order to turn the milling cutter to align a tooth with the cutting mark, I stuck an Allen wrench into the R-8 tool arbor as a handle. Yep, you know what happened next: I forgot to take it out. When I turned on the machine, the Allen wrench immediately struck the knob that operates the quill-stop, stalling the motor. I didn’t get my fully-deserved face carving, but I did get a severely bent Allen wrench and a messed up set screw in the tool holder.
I don’t know what the run-out was on my Grizzly G1006 mill before this happened, but it now appears to be about .0005”. Maybe I bent the shaft, maybe I didn’t. Anyone know what the typical run-out of those machines is supposed to be?
OK, Chop Me Up
Now comes the important part. Please flood me with appropriate pointers, information, scolding, and kicks in the ass. I have learned quite a lot with this “Miller’s Waltz”, but I think the opportunity should be taken to make this a prime outlet for guiding the rest of the newbies in “what you should not do.
This is not what I hoped my first post would be like, but I do need some words of wisdom so that I don’t have to do so much milling with the file. I did less filing when I did my cutting with a hacksaw and a jeweler’s saw.
