Milling 101, sanity check...

[Davek0974]

Hi all,

so i need to mill a slot through a block of mild steel 4.5" x 1.5" x 1.25", the slot runs the length of the bar and is 1.125" deep and 0.875" wide.

Into the vice it goes, clamped to half-depth and I throw in a 3/4" two-flute slot-drill with an R8 collet so its right up in the quill (Bridgeport). Speed was 375rpm.

Anyways, first cut i take it lightly, say 1/3rd of a turn on the knee lift and things look good so i thinks to myself, slot-drills can go to half-width in depth (so my chart says) and i crank it a fat half-turn on the knee and throw in the power feed (about 3.5"/min). It appears i have some bad slack in my bed that i hadnt noticed before!! The whole table was moving side to side as it went along, the head was bucking a bit and it looked like the whole machine had turned to jelly!

I clenched them up :-) and crossed my fingers and nothing serious happened, the cut finished, no tools broke, the work was not torn from the vice and so on. The swarf was coming off in nice thick curls.

Now, what i want to know is, what happened?

1 - Was it just too much of a cut for an ageing BP?
2 - Too big a cutter?
3 - Knackered leadscrew (wouldnt stop the head bucking though)?
4 - Wrong tool, should i have used a three flute cutter or other?
5 - Am i an idiot :-) ?
6 - Something else?

Anyway, the next cut was back to 1/3rd of a turn lift for DOC and the job progressed at a steady pace with no more excitement. I know i will be tramming in again soon!

BTW this was a low accuracy job so i thought to open the slot with the 3/4" then go in with a smaller 4-flute to each side.

Thanks in advance for any tips.
Dave

[aboard_epsilon]

it was your vice closing the slot up .

all the best.mark

[tattoomike68]

When roughing out a big deep slot I like to plunge cut the bulk of the metal out then go to milling the slot.

If you lock the Knee , X, Y you should be able to hog the bulk out fairly quick by plunge cutting.

Just an Idea..

[Davek0974]

it was your vice closing the slot up .

all the best.mark

Sorry, good point but the cut at that time was above the jaws.

Dave

[Davek0974]

When roughing out a big deep slot I like to plunge cut the bulk of the metal out then go to milling the slot.

If you lock the Knee , X, Y you should be able to hog the bulk out fairly quick by plunge cutting.

Just an Idea..

Sounds workable, by locking the knee, are you proposing to use the quill?
Plunge cutting would need the two-flute cutter, am i right?

Thanks
Dave

[tattoomike68]

Sounds workable, by locking the knee, are you proposing to use the quill?
Plunge cutting would need the two-flute cutter, am i right?

Thanks
Dave

Yes the quill is fast, you will need a center cutting endmill. you can get 4 flute cutters that center cut.

Use a large step over with a 3/4 endmill, like 5/8" that way you are not loading the forces one way a whole bunch.

kind of like drilling a row a holes to rough out a slot.

[Davek0974]

Ok, i have one more to cut, i'll give it a go.

Dave

[mechanicalmagic]

Surface feet per minute looks like 35, a bit low, if you have Kool Mist or coolant, I'd run closer to 100 sfpm, with a mister.

3.5"/min at 2 teeth 375 rpm = .0046 per tooth, OK given you are only 50-75 deep, should be OK.

I suspect a dull cutter.

[Carld]

What you got is what will happen with that setup. If the metal had parallel sides such as cold roll the work was not closing in the vise. If hot roll it may have but I doubt it. What you saw was the reaction of the two flute endmill, the speed and the feed. What you should have done is slow down the feed and let the endmill work. Your not looking for big chips on a turret mill and any time the head and table starts shaking your working it to hard. Your lucky it didn't suck the endmill out of the collet.

By slot drill I assume you mean endmill. But to assume is to make an ass-u-me.

I hope you were using cutting oil with the endmill, if not it's trash now.

If you use a roughing endmill and cutting oil you could cut the full depth in one pass but your feed rate has to be right and that means it ain't knocking everything around.

Yes, you knocked the head out of square and it will have to be dialed in.

You were abusing your mill and if you keep doing that you will be sorry. A full time shop can do that because they are making enough money that machines ar expendable and some shops treat them as such.

[Rustybolt]

When making slots in thick material relative to the cutter diameter I do it this way;(2or3 fluke em)
After setting the axis to where the slot is going to start I lock the table and the qill and and crank the knee up-slowly.After I've gone all the way through the part.I lower the knee. With only one axis unlocked i nibble at the slot by moving the table a quarter turn(+/_ depending on the diameter of the em) and lower the quill. like drilling a hole.At the end I unlock the axis and move the table over .010 on one side and then the other. I do this mainly for screw clearance. Your application might be require something else.