Re: Milling Machines


soolinehistory <destorzek@...>
 

--- In STMFC@..., "steve_wintner" <steve_wintner@> wrote:

one concern is that to use very small mills you need a very high rpm, which most mills cannot do. the proxxon does. and for the tasks you describe a 3/8 that bit will work, sure, but is overkill. think of milling off molded on grabs...
Steve, I suspect that Bill is thinking of smaller HSS (High Speed Steel) milling cutters with 3/8 shanks, and of course, the speed of the tool is calculated on the actual cutting diameter, not the shank size. When Bill gets into this further, he will find that there are all sorts of "miniature" end mills available with 3/16" shanks, and even smaller carbide end mills with 1/8" shanks.

To view a selection, go to http://www.mcmaster.com/ and search on page 2478 for the HSS, 2485 for carbide.

--- In STMFC@..., "lnbill" <fgexbill@...> wrote:

Thank you Jack & Steve:

I did look at the Proxxon Machine and it looks like another good candidate. One thing did concern and that is its speed (5000 rpm the slowest) when using it with styrene. I am worried it will melt the styrene.

Bill Welch

Bill,

Melting the plastic is not an issue when milling. Melting is cause by the friction of a tool rubbing against the work. Since a milling machine keeps the tool properly oriented, it cuts rather than rubs, so the chips come off as fine shavings. I typically do CNC work on plastic, mostly ABS, which cuts similar to styrene, with the spindle running balls-to-the-wall at 5000 RPM, and wish I had more speed. So long as the chips don't wrap around the tool and rub the work, they don't melt. Having a source of compressed air to continually blow the chips away is helpful, but messy. Rigging a shop-vac to blow the chips into might work.

The down side of running too slow a spindle speed is you can't feed at a decent rate, and will break cutters because you get bored by the need for sloooow feeds. Not a problem with CNC work :-)

I'm going to suggest a couple things others haven't touched on yet:

Make sure if you buy a metric machine that it comes with collets in at least 1/8" and 3/16" imperial sizes, or you will be severely limited in the selection of end mills you can find with metric shanks. They certainly exist, in Europe, but not here. Yes, you can grip any size shank in a drill chuck, but it isn't good practice. First off, drill chucks aren't designed to withstand side loads, so you may knock it off its tapered mandrel when milling. Secondly, inexpensive hobby grade chucks have a lot of "runout": lack of concentrically. This not only affects the surface finish of the cut, but tends to break little drills and cutters. Collets are better for precision work.

Also, I wouldn't buy a machine with metric lead screws, as you will forevermore have to convert all your dimensions to mm. Not a problem if the machine has a digital readout, but I'm not aware of any of the little machines so equipped. Even a cheap two axis DRO is worth about $1200, if it would even fit these small mills.

Also, get a machine with the biggest table you can. Our models tend to be made from relatively soft materials, and it often takes some creative fixturing to hold it to work on it. Very few things can just be held in a vise. Many years ago I had one of the original Edstal Unimats, the little 3" x 4" milling table was just about worthless for everything.

Dennis

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