Drilling 4140 Annealed. Feeds/speeds/drill selection

[fastline12]

We are drilling about 100 holes in some 4140A, .500 deep,.470 diam. No biggy right? We previously made these parts from 4140HT but were having problems pushing parts from the high cut force in milling and selected 4140A and will heat treat after.

Anyway, we previously drilled with HSS in a pinch and though slow, it worked decent but drills were paying the price. We decided to go for some carbide tipped drills and two toasted in 6 holes. Tips were chipped and one flute was cracked.

Here are the previous parameters. S1300, F6, Flood coolant, peck .200. We decided to go get some cobalt screw lengths to get it done but seems the decision on feeds/speeds varies greatly from SUPER ass slow, to slow with high feed.

I am not sure if we are really running into a work harden condition here or not but even at full hard, those carb drills should have hammered it. I guess I am not second guessing my plan. I was going to run the cobalts in the 60-80sf range and about .006/rev for starters. These are Morse split points, 135*, nothing special. Faster is always better but changing tools every 3rd part is not going to work either..

 

[mrainey]

You would benefit greatly from thru-coolant tooling, particularly if you're using a vertical machining center. Peck drilling with carbide is generally not recommended, as it can lead to thermal shocking and/or recutting of chips.

Software For Metalworking

http://closetolerancesoftware.com

 

[berkshire]

If you can, run a low speed with High feed.
If you use cobalt drills run the first hole on a new drill at a lower feed to condition the drill first.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[fastline12]

Right now I am running 700 rpm and 5 ipm which works out to about 86sf and .0035 per flute. Seems to be working but not sure how many holes I will get yet. If this does not work, can you recommend a safe feed/speed? Is the .220 peck too much for this job?

I have TSC but having issues with it and do not have any TSC drills anyway. Might be something to consider on next run but for no more time than it spends in the hole, it may not pay off. Whole operation for 3 holes takes maybe 30sec and a lot of 100 parts.

 

[mrainey]

My calculator comes up with 70 SFM at .0086 IPR for HSS drills on a similar material. That's about the same cycle time as you're doing now - the question would be is tool life any better.


Software For Metalworking

http://closetolerancesoftware.com

 

[fastline12]

I am fighting another issue at the moment but the drill has lasted 10 holes without any concerns yet. The carb tipped drill surprisingly recommended faster speed but only .004/rev feed. I am pushing well over that with cobalt and really not losing much time. We have pecked carb drills in the past running much hotter but never ran a carb drill this size. I did not expect an issue but I was also thinking about that thermal shock since the drill would come out hot and get douched with coolant on the peck. There is also that shock that comes with the re-entry.

 

[berkshire]

If you are only going in .5" do you need the peck? It would appear to me that the thermal shock of starting and stopping the cut do more harm.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[ornerynorsk]

What BE said. No need for peck on that L/D. You could try a gundrill or an inserted drill for chip management, though. A twist drill is only give you a machine cabinet full of "razor wire".

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[fastline12]

I agree with the peck argument. Just habit I guess from bad experiences and the work harden concerns in 4140. I would otherwise enter with a 2:1 L/D and peck at maybe 1xD.