[nfrees114]

Hey all- So I have done this job for a customer multiple times and just have never really gotten it right. Thought i would throw this on here and see if i could get some suggestions.

I'm drilling a series (300 holes) #36 in a long (6') SS tube 1 1/2" OD 1/8" wall. 3 Tubes actually. I am currently using some machine length cobalt steel #36 drill bits. I've tried using a center drill, i've tried no center drill. I've tried different depths on the center drill. I've tried different feeds and speeds. sometimes I'll get 150 holes no problem. Sometimes i'll get 300 holes no problem. sometimes i'll get 10 holes and my drill bit will break. I just can't seems to get any sort of consistency. It seems like when I center drill then the drill bit chatters really bad getting past the center drilled hole. if i don't center drill the bit will walk sometimes. I am just curious what you guys would recommend? Center drill or no? Speeds and feeds? anything to help.... I don't do a lot of stainless steel so whenever i make changes i'm just taking a shot in the dark.

What I am around for S & F I got from "Machinst Toolbox" and is roughly 1300rpm @ 3.5ipm

Thanks for the help!!
-Nate

[txcncman]

Are you peck drilling? Peck at about 0.010"-0.015". I think your RPM is too slow. 2500 sounds better. Feed is about right. With the center drill, there is no need to drill through. Just spot about 0.025" deep so that the #36 drill has a starting point. Don't use cobalt in stainless. Go with either HSS or carbide. Should be able to get a couple of thousand holes per drill.

[nfrees114]

Are you peck drilling? Peck at about 0.010"-0.015". I think your RPM is too slow. 2500 sounds better. Feed is about right. With the center drill, there is no need to drill through. Just spot about 0.025" deep so that the #36 drill has a starting point. Don't use cobalt in stainless. Go with either HSS or carbide. Should be able to get a couple of thousand holes per drill.

Thanks for the quick reply! TX!!!! I'll try this tonight when I start. One other question that just popped into my head, is it crucial to have my center drill and drill bit be the same drill angle? Just wondering because I believe (I'll have to check for sure) my center drills are 118 degree and one of the shops I bought the drill bits from said I should use a 135 degree drill bit. could that be one of my issues also?

[txcncman]

135 degree will work slightly better on stainless. But for your application, probably not enough difference to matter really.

[fizzissist]

txcncman is on the right track.
I'd just add 135deg cobalt split point screw machine drills... but use a 90deg spotter rather than a center drill, and if the material is clamped well and doesn't chatter on the spotting op, use a carbide spotter and make the spot about .110 dia...
Assuming you're on the part's centerline.

I don't think you should need to peck on a 1/8"wall

[Joe S.]

Hi Nate, Just my 2 cents worth, unless you are using flood cooling I would not increase the speed, you heat that SS and you heat treat it, then drills will break, likely taps too. I would probably decrease the feed in that scenario. With flood then I think you should be Ok up to about 1800 rpm. Are you sure it's 304 ? Which is to ask if you sourced the material or did the customer? Big also; when drilling stainless, do not try to save money with drills made in China.

It seems like when I center drill then the drill bit chatters really bad getting past the center drilled hole.

Might be you are getting things too hot with the center drill, try slowing it down. NO BLUE or BROWN chips allowed in SS.

[nfrees114]

Hi Nate, Just my 2 cents worth, unless you are using flood cooling I would not increase the speed, you heat that SS and you heat treat it, then drills will break, likely taps too. I would probably decrease the feed in that scenario. With flood then I think you should be Ok up to about 1800 rpm. Are you sure it's 304 ? Which is to ask if you sourced the material or did the customer? Big also; when drilling stainless, do not try to save money with drills made in China.

Might be you are getting things too hot with the center drill, try slowing it down. NO BLUE or BROWN chips allowed in SS.

First off, Thanks for all the great responses!!! 2nd, seems we have some difference of opinions. All is good info though, gives different ideas since i'm sure there is no "right answer". So yes, i am using flood coolant. I'm running these in a Haas VF-2 with the piece clamped in 3 vices. I do have some good drill bits, can't remember which brand right of the bat, I think cleaveland. The material was sourced from the customer and it has 304/304L on it. So I drilled 2 of these tubes last night using TX's recommendation and all seemed to work great! Other than it took a lot longer than when I was doing it. Now I wasn't breaking bits like i was before so that was a huge plus, but it still took more time. I think it was all the peck drilling. so then according to Fizz if peck drilling isn't needed i could probably save some time there. I guess I'll try all the other options tonight and see how those work. Also, the chips looked good, no discoloration.

[fizzissist]

The reason I wouldn't peck is that you should have a pretty good chip load on the drill, and you're in the realm of 1x dia depth. Should be able to run right on through.

Curious why txcncman doesn't recommend cobalt? Don't they take the heat a bit better than plain HSS? ... just curious.

Personally, using HSS on 304L, I'll generally keep the SFM in the 60-70SFM range for the sake of tool life. 25-35SFM for taps. The 3.5IPM feed seems high to me, I'd start out with 1.5-2IPM... (my conservative nature) ... More chips to clear, yes, which is why I'd prefer the 3.5..but...

Whenever you drill into a curved or angled surface, it'll chatter on you on entry, because the drill's dynamics of loading are changing every 90deg of rotation.... they don't like that, which is why I'd suggested spotting the holes to just slightly over the drill dia for a clean start.... and why I said you don't need to peck, since most of the material for this hole is already gone when it's the drill's turn.

Going to the screw machine length drill with a split point will really help chatter too. Rigidity... Boo Ya!

[txcncman]

I used to think the same about cobalt. What I found after a dozen times or so of trying small diameter cobalt drills in stainless is that they seem to be more brittle that HSS and tend to snap off at inopportune times, like when you leave the machine running while you go pee. No so much of a problem with HSS.

Another reason I would choose pecking over continuous feed is the burr you will probably get on the inside. In your application it might not be a problem.

[fizzissist]

I used to think the same about cobalt. What I found after a dozen times or so of trying small diameter cobalt drills in stainless is that they seem to be more brittle that HSS and tend to snap off at inopportune times, like when you leave the machine running while you go pee. No so much of a problem with HSS.

Another reason I would choose pecking over continuous feed is the burr you will probably get on the inside. In you application it might not be a problem.

A lot of times I'll gently stone a small radius on the corners of the drill, and this reduces the burr a lot.

Hadn't noticed the brittleness problem with cobalt.. I'll watch for that now..thanks!

[DareBee]

I would use a really short carbide "die drill" (like a Hi Roc).
No deflection or chatter and speeds/ feeds will be greatly increased.
No peck for 1/8 wall either.