Well, I was doing great with milling aluminum until one of my hold down

Well, I was doing great with milling aluminum until one of my hold down clamps let go. Needless to say the workpiece and the carriage went flying, but nothing is damaged save for my fingers. This poses the question: what are you using for hold down clamps, and what do you recommend? I need ideas, I’ve been having this problem since day one with my home built machine

Please excuse the poor photo, G+ is not cooperating today…

If carpet tape doesn’t hold I drill holes and screw it to the mdf.

When I mill metal that won’t go in a vise I tend to use a lot of hold-down clamps, or if possible drill a couple of holes and put bolts down into the mill table t-slots. I’ve also had to build table clamps – big chunks of metal on either side of a piece that was particularly unhappy, to act like a large vise, in addition to clamps to hold it down. Double-sided/carpet tape does help a lot if the piece is tending to slide out from under hold-downs.

@charlie_wallace I tried the carpet tape, but it was not working out very well for metals.
@John_Bump Good to know, I’ve been thinking of making a sort of woodworking fixture that would secure the piece by being bolted into the table.

When I’m cutting wood, I have a piece of waste pine bolted to the table, and put multiple screws through the workpiece down into that. Aluminum needs more, in my experience. (And brass is just terrifying because upcut mills yank it out of just about any fixture.)

@John_Bump I haven’t had any trouble with wood, thankfully. Generous amounts of hold downs tend to do the trick, but unfortunately, my table simply doesn’t have enough holes in it for sufficient hold downs for aluminum. Brass I have only done once, but again had no trouble, then again I do not know if I have an upcut mill.

It may be worth making a hold-down plate: 2’x2’ chunk of aluminum in which you’ve cnc-drilled #7 holes every 2 inches in all directions and then tapped 1/4-20. That way you can bolt stuff down to it with a ton of bolts, and it’ll have the mass and surface area to hold to the main table’s hold-downs better.
Drilling brass can be routine or it can suddenly get extremely exciting. Kind of random, and seems routine right up until it snaps a hold-down bolt and throws a milling vise across the shop.

@John_Bump Ironically, that is exactly what I have. It’s still not enough, mostly because this piece is very small. My table is not nearly 2’x2’ however, it’s 9"x12".

yeah i had to rough up my mdf for the carpet tape , the coolant tends to attack it. thats why i use the screws, i generally only use two though for 3/8" plate.

Personally i would check your tool and depth of cut. Speed your tool up and lower your feed rate and depth of cut, x, y, & z. If you have all these right you wont need such heavy clamping

It looks like you was forcing a butter knife though the aluminum.

@Aaron_Helmick the step down was one half millimeter, feedrate 50 mm/min. I certainly thought that was slow enough, especially for a 3.175 mm tungsten carbide bit… What do you recommend?

What’s your rpm?

@charlie_wallace to be honest, I can’t say for sure. My machine is uh… not good, to say the least, and it has been upgraded many times, but the spindle is unfortunately a generic, no-namer with no labels, and I don’t have a tachometer to check with. Chances are it’s lower than I think it is.

ok, does the metal coming off look like chips or dust, do the edges look like they’re melted or clean? do you know which spindle it is?

@charlie_wallace I have no idea what spindle it is, there are absolutely no markings on it. The metal coming off is chips and dust, which I assume is bad, but I’ve never seen anything except drilling make clean cuttings, so I also assumed that may not happen.

so if your RPM is too high for the feed rate you’ll basically be grinding/melting the aluminium versus cutting it, the edges won’t be clean and it’ll be dust. the bits will over heat, the metal will heat up and the tool life will shorten. you might do better with a single flute cutter if the RPM is high, like 8K+ … i run my 3.175 mm at about 30-50IPM which is 1270mm/min, but it depends on your rigidity and so on, unfortunately it’ll be some trial and error if you don’t know some of the parameters to calculate the feed and speeds. https://goo.gl/photos/RDtW5fs4oihftZGH7 compare to my chip size. and look at the nearest hole in this video, and you can see an extreme example of it melting/grinding instead of cutting. https://goo.gl/photos/ufnxHTeB59uN12rg7 the raised edges are melty looking vs a burr. a little coolant/cutting fluid like a9 won’t hurt either.

@charlie_wallace thank you so much! I probably run much higher than 8K RPM, and I have a 4 flute cutter. I see what you mean about melting, but that’s not a problem - your cutter could probably cut my spindle in half without breaking a sweat. I’m using “tap magic” for my cutting fluid, but I’ll check out a9 as well!

@Andy_Meyer yeah probably too slow of a cut http://zero-divide.net/?page=fswizard punch the numbers in here see what you get for speeds.