Basic Milling Question

[Nelson_2008]

Well I finally bought a mini-mill.

I was looking at the Micromark #84630 for $742, but at that price, I thought, I might as well spend a little more and get the LMS #3960 (for $819), but at that price, I might as well spend a little more and get the G0704 from Grizzly, but the G0704 is just too big and heavy for my cramped situation...so the only solution was to wait for a better price on the 84630, which finally happened (in the form of a 25% off sale).

I know these SX2 mills have some shortcomings, but for $556, I bought it with the mindset that I was buying a "fancy drill press", which might give me the ability to do some light milling.

Now I have to teach myself how to use it...just like I had to do with the Micromark lathe.

Anyway, to get to the question: I'm wondering, what would be the technique to mill a slot in something at a controlled depth (i.e., the slot doesn't go all the way through), where the slot doesn't extend to any edge.

In other words, suppose you had a 4" square slab of aluminum, 0.5" thick, and you wanted to mill a 0.125" wide x 2" long slot in the center of the piece, parallel to an axis, it, at a depth of 0.125".

I assume you'd put a 0.125" end mill into a holder or collet and you'd use the top of the work piece as a zero reference, right?

Would you then use the Z axis fine-feeding wheel to plunge the end mill 0.125 into the work piece? And then just move the piece 2" along one of the axes?

[hoss2006]

Basically that's it, though if you want an accurate .125 slot you'll need to use a smaller endmill to cut either side of the slot,
a .125 endmill alone would make a slot probably .005+ over size due to runout and/or tool flex.
Also you'd want to take that in a number of passes at say .025-.05 deep.
You need to take some time and read some books or watch some movies on machining so that you go into this using proper techniques.
There are several books and vids linked here that could help you out.
Attention Newbies! - CNC - The Hossmachine Forum
Hoss

[109jb]

Depending on the material, I would not just plunge a 1/8" diameter end mill in 1/8" deep. I generally use 1/2 the diameter as the max depth of cut if the cut is full width (slot milling). You may have to take more than one pass to get the full desired depth. When milling a blind slot you will also have to contend with the chips getting sucked back around into the cutter. Flood coolant works best, but a cutting oil combined with enough air from a blow gun works too. WD-40 works OK as a cutting oil and is easy to find locally. As mentioned a 1/8" cutter will cut a slot a bit wider than 1/8", but it depends on the tolerance you want to hold whether you can use the 1/8" cutter. I like to use a smaller cutter and make a final climb milling cut along both sides.

You can get machining manuals online. Do a search for the "US Army machining manual". It is online and has some good stuff to get you started.

[Nelson_2008]

Hello Hoss2006,

Thanks very much for the informative reply. Yes, I need to get a book to read and maybe look at some video tutorials.

Anyway, that fine feeding wheel seems somewhat problematic to me. It's ok when moving the headstock up, but when going down, I have to put my hand on the headstock and apply downward force, otherwise, the headstock seems to get hung-up...it seems like there's lots of backlash or something. I can see myself getting into trouble with it.

Maybe I have the headstock gib tightened too much, or maybe I didn't lube the surfaces well enough?

BTW, with an end mill, are the cutting forces directed downward, i.e., tending to pull the tool out of the collet?

[hoss2006]

Yes the fine feeds on the x2's are notorious for having lots of backlash.
If the gibs are too tight you can get stiction where it binds up and needs greater force to
get it to move and it tends to jump instead of moving smoothly.
With too much lash an endmill can definitely be pulled into the work.
The way the spirals are on a typical endmill is similar to screw threads so you can see why they can be pulled into the material.
There are upcutting endmills out there but it's simple enough when manual milling
to use the locks to hold the head in place when making a cut.
Hoss

[RPCElectronics]

I watched this whole series of MIT Machine Shop videos when I first started with manual milling and they helped me really get a jump start!

This is the first video in the series, but you'll see in the bottom right, links to the others. They are numbered, so it's easy to go in order:

MIT TechTV – Machine Shop 1

[Nelson_2008]

@109jb: Thanks for the info. I just downloaded that manual.

@RPCElectronics: Thanks for that link. I'm going to check it out later.

@Hoss: It's "jumping" just like you mentioned. And that's very annoying. When the headstock is up near the top of the column, the fine-feeding wheel works smoothly, but as it moves down, the motion becomes "jerky" when it gets to about the half-way point. If I remove the torsion spring arm so that it's just gravity pushing it down with no counterforce, it goes smoothly all the way down.

I don't particularly like that arrangement they've got there.

[photomankc]

You need to fiddle a bit with the gib on the Z axis then and find a compromise where the movement at your cutting hieght is smooth and not binding even if the top travel is a little loose. The gibs on the X2 are notorious for being a pain to adjust properly. Make sure all the packing goop is really out of there and then oil the ways well and get start working on the adjustment. Way oil can help. My PM-25 (G0704-like) was sticking like crazy when I was using motor oil to lube the ways. I switched to Mobile Vactra #2 way oil and the stick-slip routine quit on my head. I could make 0.001 adjustements using the Z hand-crank where that was not possible before. I had to order it though. Not something you find at the local discount store.

That fine feed knob is not much fun but it's workable if it's moving smoothly on the ways.

[Nelson_2008]

I feel like just getting rid of that torsion spring (and associated hardware).

Other than that it will make raising the head a little more difficult, is there any reason to not remove it?

[hoss2006]

For manual use you need to have some sort of help, with that spring removed the head will drop like a rock.
Many convert to the LMS air spring to give more travel and more even tension.
LittleMachineShop.com - Air Spring Conversion Kit
Hoss