How to mill incredibly small slots in a milling machine

[mmagliaro]

In pondering something about a future project, I find myself again wondering how one uses
micro-sides cutters in a milling machine (like .010" diameter) to cut tiny slots into something like
a plate of brass or steel without the cutter just instantly breaking off.

You will find that there are plenty of end mills made in sizes like that, but they are all carbide from what I see.
My experience using carbide bits in a little pin vise is that it's pretty much hopeless.  They are razor sharp,
but if you breathe on them, they snap right off. 

So I cannot imagine loading a .010" carbide end mill into my milling machine, and trying to cut a .010" wide slot in
a piece of brass or steel.  I imagine that the moment I nudge the handwheels to move that work piece under
the bit, that bit is going to snap right off no matter how gently or slow I go.

So... to those of you who are machining pros, what's the secret to milling super-thin cavities like this?

[VonRyan]

I'm no expert, but I've picked up a few things, and as long as you're approaching the material to do a normal cut you should be ok as long as you go slow. You might also want to put a drop od 3-in-1 oil on the surface of the material where you're looking to cut. And be sure to feather the bit to the surface rather than trying to start the cut right away.

I once had to drill some holes into a cast-metal faceplate for an old Powermatic wood-lathe in my High-school woodshop and the only correct sized bit we had was an old cheap-o one. Without the 3-in-1 oil it would barely cut, but with the 3-in-1 I was able to get a steady flow of chips and get the holes drilled.

[JoeD]

Machining 101...the smaller the bit, the higher the RPM, the shallower the pass and the slower the movement xyz.  Use lots and lots of cutting oil the whole time to cool your bit and cut back on chattering on a dry surface.  That flexing will weaken and and then break the bit.  We make our own cutters even smaller and generally use air drives on the mills to get to even higher RPMS. 

Hope this helps

Joe

[garethashenden]

I used a 1/64th end mill once in a manual Bridgeport. I was cutting acrylic if I remember correctly. Crank it up to full speed and go slowly, lots of oil too. The problem I had wasn't that the bit would break immediately but that the chips wouldn't clear and would then melt and melt the part a little and then the bit would break.

In the end I redesigned the part so that it had 1/64th holes rather than slots. Did that with the endmill and it worked fine.

But in short, speed is essential. Going slowly is good but the bit has to really be flying for it to have any chance of surviving.

[mmagliaro]

Okay....
I would have expected to use a steady supply of oil, keep the speed high, feed slow, and only plunge in a little on each pass.

Ya think that will make it possible to cut a .010" slot in steel?   

How high is high speed?  I'm just using one of those little Micromark mills.  I think the top RPM it can do is 2500.
Is that too slow?  Too fast?    With normal sized mills (like 1/8" or more), I usually cut brass at around 500 rpm and it worked nicely.

I haven't cut any steel in a while, so I don't know where I decided a good speed was for that.

And another big problem is that tiny sized end mills like that cost a lot of money, like 20 bucks or more.
So I can't burn through 20 bits trying to figure out how to do this.

[garethashenden]

That's pretty slow. CNC mills get up to 14K and I'd expect them to use that sort of speed for this sort of thing.

Give it a go of course but I'm not convinced that it will work at that speed.

[victor miranda]

if you are doing just a linear slot.
consider using a disk with teeth cut in it.

the very fine milling bits break if your table jumps.


I have read the high speeds help, as has been said.
use air to clear chips and cool?
I also read CNC helps to keep the cut even.

the air powered spindle has my curiosity.
how do the bearings hold up?

victor

[robert3985]

The "incredibly high speed" isn't always the answer as the tool temperature without constant liquid cooling (as in a CNC machine) will cook your fine tool almost instantaneously.

Micro Mark offers a liquid cooling setup that isn't very expensive if my memory is correct, which is also a lube...which is normal.

Also, depending on the slot's location and design of the part you're talking about, you would probably be better served by using a slot cutter that's .010" thick, but is a wheel with teeth on it basically.  The larger metal surface allows it to cool more efficiently as well as be exponentially stronger than a teeny end mill poking down from your collet.

The slot cutter isn't going to be okay for slots that are very short, as the ends of the cut will be circular.  If you need the ends of the slot to be vertical, you can use the .010" end mill for dressing the rounded ends of the cut the slot cutter is going to give you.

If you're stuck with using the end mills, then start out with the highest speed you've got, lube the hell out of the tool as you're cutting and make a very shallow cut at first of maybe 2 to 4 mils with a very slow, even feed.  It's all going to be visual as you're not going to be able to feel it with your fingers on the wheels of your table, so wear a good Optivisor and make sure your cutter is well illuminated.

Buy cutting tools that are just an RCH longer than the depth of the slot you'll be making.  Shorter = stronger.

Plan on breaking at least one...which will be used to establish your feed speed/cut depth parameters.

Successfully using tiny machine tooling for the home hobbyist is a matter of taking your time...for horizontal cuts you do high RPM, very shallow cuts, very slow feed with lots of cutting fluid specific to the metal you're using...or use something like the Micro Mark liquid cooling setup.  For drilling ultra small holes, the trick is to keep the RPM's low...like around 300 RPM, drill slowly, removing the bit or end mill often to remove the chips and use the proper cutting lube for the material you're drilling.

A visit to your local machine tool shop with a list of questions is what I'd recommend as those guys usually have an experienced machinist in house. Ask about slot cutters as well as end mills, RPM's, feed speed, cut depth, direction of cut, coolants and lubes for your material.

[peteski]

Max,
if the bit is hard enough and the steel soft enough, it will machine it.  I have a high speed attachment on my Sherline mill which gets me up to 10,000 RPMs. But that is barely enough for small diameter cutters like the one you want to use.  Milling machines are designed tokeep the piece being machined steady and stable so I don't think it can jump and break the bit (if you secured it well enough).

But the slot you want to mill will nto be very deep. Milling cutters have cutting depth not all that much more than about 2 times the diameter.  So, your 0.010" bit and only plunge in about 0.020". That is not a very deep slot.  I don't know if there are extra long cutters available, but if they were, I think they would be very expensive.

[Andrew Hutchinson]

Hi Max,

I'm in  a bit of a hurry and Bob said a lot of what I was going to but it would be good to know what it was that you are wanting to make.

A couple of things stand out...

Mini-mills (X2s)are bad at what they do in their intended envelop and you are working below that.  I own one. They are endlessly frustrating.

If you are going to be working small going forward save up and buy a microscope if you don't already have one. They make life better all round. I have a modified Lomo and for the price I can recommend it.

If you can, use a slitting saw with a home made arbor that supports the teeth very close to full cut depth. If you can't, you're going to be stuck using a slot drill and as Bob said, you'll break a few along the way. A lot of parts/cavities require them but if you can get away with it you'll be better off with a small saw.

Finally, go to GEARS this weekend. It's in Portland at Doyle McCormack's (sp) place. You'll get better input there than on a model choo choo site.

http://gears.cbnetsolutions.net/

Tom Hammond would be a good guy to look for at the show. He's a long time 2mm association guy. He usually has a table where he's working a home made hobber and a pantograph. Real helpful guy.


Andrew Hutchinson
Surrey BC Canada

[mmagliaro]

Good Heavens!  Look at that GEARS Show!  I had no idea... and it's right in my backyard.
You BET I'm a-goin'.  THANK YOU.

Hi Max,

I'm in  a bit of a hurry and Bob said a lot of what I was going to but it would be good to know what it was that you are wanting to make.

A couple of things stand out...

Mini-mills (X2s)are bad at what they do in their intended envelop and you are working below that.  I own one. They are endlessly frustrating.

If you are going to be working small going forward save up and buy a microscope if you don't already have one. They make life better all round. I have a modified Lomo and for the price I can recommend it.

If you can, use a slitting saw with a home made arbor that supports the teeth very close to full cut depth. If you can't, you're going to be stuck using a slot drill and as Bob said, you'll break a few along the way. A lot of parts/cavities require them but if you can get away with it you'll be better off with a small saw.

Finally, go to GEARS this weekend. It's in Portland at Doyle McCormack's (sp) place. You'll get better input there than on a model choo choo site.

http://gears.cbnetsolutions.net/

Tom Hammond would be a good guy to look for at the show. He's a long time 2mm association guy. He usually has a table where he's working a home made hobber and a pantograph. Real helpful guy.


Andrew Hutchinson
Surrey BC Canada

[bbussey]

Max, there's a formula to determine what the feed rate should be based on the metal being milled, bit size, number of flutes and RPM.  I wrote a program to write G-code and included the formula in that.  I can find it, or reconstruct the formula from the program code later tonight if someone doesn't produce it in the thread before then.

I have a Sherline.  I've never had problems milling brass at 7500 RPM, and I've milled as small as .005".

On the basics:
— the depth of each pass should be no more than ¼ the diameter of the milling bit.
— you don't need a coolant if your milling brass, and you can use standard end milling bits.  If the RPM, feed rate and depth are within parameters, you can mill a .001" slot in brass with standard bits.  If this is for modeling purposes, I would think that half-hard brass should satisfy the need rather than using steel.
— good .010" diameter end milling bits cost more than $20.  If you require long-reach or long-length, they cost more.  If you're cutting deeper than .030", then you need long-reach.  Two good economical/quality sources are Harvey Tool Company, and Bits & Bits Company.

[mmagliaro]

Thanks Bryan.

And all...

This all started as I pondered a future loco project (which will probably be a small SP&S steam switcher, like an 0-6-0).
I have always wondered about making a 100% scratch-built N Scale steam engine, including the drivers

That means no wheels from somewhere else, and no "cheating" by casting wheels from another model, or drawing them
and sending the CAD file off to Shapeways.   I would want to make them myself.

So I wondered how feasible it would be to actually mill the hub and spoke pattern into a metal plate as a mold, and then use
some sort of casting plastic (urethane, whatever) to pour into it to make the wheel center. 

Sounds like it's a tall order.

[mmagliaro]

Hmmmm.... so there are "machinable media" boards, such as "Renshape".
They are intended to be machined on a mill to make molds.

Has anyone ever worked with material like this?  I assume it is much easier to machine than
steel or brass, but can it preserve detail down to, say, .002"?  The spokes on a steam driver are
not much more than .010 - .015" thick, so there can't be much surface error in a mold to allow them to come out well.

[up1950s]

Buy spares , and if it's a manual feed take some Nodoze. The shallower the cut the less chance you will break a mill cutter by herky jerky cranking . Much better to make loads of passes than to go so slow that you fall asleep , nod forward , deviate your septum with the spinny shinny thingy .