Stainless Steel Milling

[electrosteam]

I am finally getting to my CNC Kondia ( 1 year late ) and currently making new slide covers for the knee in 1.5 mm stainless.
The slides require a large rectangular hole to accommodate the Y-axis ball screw nut assembly.
I have hacksawed close to the line and using an end mill to trim to the line.

The cut surface finish comes out very rough, almost cratered.

I am using a new 10 mm solid carbide bit and I have tried 680 RPM and 1100 RPM dry with 0.5 mm cut and 1.0 mm cut.
Swarf looks OK - nice slivers.

Any suggestions on how to improve the finish ?

John

[RayG]

Hi Electrosteam,

A couple of clarifications,

How are you holding the 1.5mm sheet? Are you milling on the side of the cutter, or the end?

Regards
Ray

[electrosteam]

Ray,
The sheet is clamped on its side about 2 mm from the line with continuous support by a jig plate with 4 separate top plates each with two clamping bolts.
The end mill is held in an ER16 chuck in a small vertical mill and cutting on the side.
At this point I am about 3 mm from the line.
The unsupported width would be no more than 5 mm.

The next cut (tomorrow) I will monitor sheet flex with a hand-held probe (screw driver).

John

[Stustoys]

Hi John,
You wouldnt be climb milling would you?

Next time I have a problem with thin material and helix angle I'm going to try some cheap router bits

Stuart

[electrosteam]

Stuart,
No, I am not climb milling.

The interesting thing is, the pressure on the handle varies, just like it does when climb milling.
I think the sheet flex thing is worth pursuing in the morning.
Problem is, the jig support was manufactured for this cut, it would need a new jig to get the clamp support any closer to the cut.
Perhaps the surface finish will get better as I get closer to the line.

John

[Stustoys]

Hi John,
Didnt think so, but thought I'd ask to be sure.

"The interesting thing is, the pressure on the handle varies, just like it does when climb milling."
That sounds just like what was happening to me.

Not that it would be much help in your case, but would tilting the head over to the helix angle fix the problem?(unless you feel like tilting it four times)

Stuart

[Michael G]

That pressure variation is probably because you only have one tooth in the cut. I sometimes wipe things with a wax candle to lubricate (mainly to prevent a build up on the edge). Perhaps try that - it might be a friction thing.
Depending on the grade of stainless, it might be just the way it is. Stainless is remarkable in the variations of properties that you will get under the generic banner of stainless. Some of them you have to abrasive machine (grind/ sand) just because that's what they are like.
The flex won't help. Can you sandwich the sheet between a couple of strips of steel? It will reduce flex but also smooth the cut a little as the edge will be in contact with material longer.

Michael

[electrosteam]

This morning tried a cut of 1.0 mm at 370 and 200 RPM dry - same problems.
Then 1.0 mm 370 RPM with coolant - 1: soluble oil / 2: turps, same general result but not quite so severe.

Now so close to the line that I think I will make a filing jig and finish it on the bench.
If careful, I may be able to simply clamp the jig plate in the vice.

I have 4 slide plates to machine, each with 4 edges - this milling approach is not going to work.
It will kill the tool, and frustrate the hell out of me.

There must be a better way.
I could try a slitting saw approach, but with the mill head rotated to the horizontal I think I would run out of table travel.
With a vertical head, I would have to hold the slide plate vertically clamped to an angle plate.
My existing jig plate would allow about 6 mm depth for the saw - could work.
The jig plate started as 12 mm black bar, so it would have the stiffness if held by two angle plates.

No more machining today, I have a date with some termites in the annex of our caravan on site.

John.

[Kody]

Make sure there are no "loose" spots along the clamping plates. With only about 3 to 5 mm overhang from the milling table, you should be getting a super finish. The first thing I would check is the direction of the endmill. Is it rotating the correct way? It should be rotating clockwise when viewed from above. Next, I would up the speed to at least 1500 rpm when using a carbide cutter.
Run a light cut along the edge and then take a cut of about 0.2mm to clean it up but take this last cut by climb milling. Adjust the slide clamps to prevent a "dig-in" and use a slow feed by hand. Use coolant if you can, it will make a mess but will improve the finish a lot.
If there are any loose spots under the clamping plates, nothing you do will make a difference in the finish.
Stainless Steel is quite soft but is also tough. It's a wierd metal to machine because of these properties but climb milling works a treat with a light cut. Do make sure the cutter is rotating the correct way. A solid carbide endmill will cut far better than an inserted tip style. Use either a two fluted cutter or a four fluted one, even a three fluted cutter will produce what you want.

:2tsup:

Kody

[electrosteam]

Kody,
The mill will go to 2000 RPM, and the job is still set up.
I will try a light cut (I think I have enough metal outside the line) with coolant as suggested.

I was staying away from the high speeds because of numerous warnings on the risk of heat hardening.
All the cuts reported so far leave the work and tool barely warm to touch.

I have broken a 6 mm ball nose milling bit on this machine in the past while climb milling.
Makes one a bit wary of attempting it again - but I may try.

John

[Kody]

Hi John
A 10mm end mill will run quite happy at 1500 rpm. I can fully understand your anxiety about climb milling but a ball nose cutter is a far cry from a normal solid end mill. The secret to success when climb milling is to adjust the slide a little tighter/firmer. With such a small cut (0.02mm), high revs and firm slides, you will find it is a "piece of cake".

:2tsup:

Kody

[PDW]

Kody,
The mill will go to 2000 RPM, and the job is still set up.
I will try a light cut (I think I have enough metal outside the line) with coolant as suggested.

I was staying away from the high speeds because of numerous warnings on the risk of heat hardening.
All the cuts reported so far leave the work and tool barely warm to touch.


It's a speed/rigidity/horsepower tradeoff.

You can go fast but you need to keep the tool from rubbing so you also need to up the feed as well. To run high speeds & feeds you need a lot of HP.

In the case of a 10mm carbide end mill, I'd say you need around 0.004" cut per tooth so work out what you can do in the way of speed & feed. The Kondia I used years ago would run out of feeds way before HP on a cutter that size.

Stainless will work harden for sure if the tool rubs which is why using too fine a feed is bad news. Climb milling may well be better if you can stop the table/job being sucked into the cutter.

I hate machining or drilling stainless (except 303).

PDW