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Drilling 17-4 H900 with Cobalt?

mhajicek

mhajicek

Diamond
Joined
May 11, 2017
Location
Maple Grove, MN, USA
I need to drill about a dozen .067" diameter holes through 1.75" of 17-4 H900, HRC about 43. I usually drill this material with carbide, but I have a tool rep recommending a cobalt drill. Would you have confidence in that?

Thanks
 
Absolutely. Even a cheap HSS drill will work just fine.

At this small diameter cobalt would be better though.

What machine are you using? it matters.
 
That's a tough one. Mill or lathe? With enough pecks cobalt is fine. At that depth any wandering of the carbide drill can be a problem. Getting it started on center and straight will help. But carbide will run so much faster, do you have enough RPM to take advantage?? I'm going to check back and see what others say.
 
I believe the answer is in the quantity. You only have a dozen holes. Wrong move with the carbide drill this project will get pricey. So I vote cobalt for this job.

If you had a lot more holes and thru spindle coolant I might suggest a Very Pricey drill. The Mikron Crazy Drill Cool SST-Inox (<--click). The 1.7mm 30xD will get you a coolant through drill ideal for S/S capable of 2" drill depth. This series has kindof an egg shaped coolant holes instead of round for more coolant flow.
 
5 year old Haas VF-3SS with 15K RPM and 1000PSI TSC. Based on this advise I think I'll try the cobalt drill, pecking, with TSC blasting through an ER-11 collet. Maybe helical plunge a little starter hole first.

Thanks
 
1- Make sure to spot. .067" in 43HRC will probably want to wander.

2- Make sure it's a nice quality drill, with a split-point. Much less thrust required with a split-point, which means less prone to breakage, especially nearer the shank where the flutes are ground larger, and the web is the thinnest.
 
mhajicek said:
5 year old Haas VF-3SS with 15K RPM and 1000PSI TSC. Based on this advise I think I'll try the cobalt drill, pecking, with TSC blasting through an ER-11 collet. Maybe helical plunge a little starter hole first.

Thanks
Click to expand...

I have been wondering lately if 300psi coolant will break a chip in SS or if you need 1000psi to break chips. Definitely let us know how these holes drill for you. I am curious.

Sounds like a good idea for a setup. I always spot drill first in the small medical screw machine SS. I understand it is a must even with split point drills.

Now the question I have is...What kind of drills do people use to Spot drill. I have used center drills that are ground back far enough that it is just like a spot drill. I know you can buy Spot drills pre made already but I hear people tend to use centering drills as is for spotting.

I suggest a really sharp "Spotting Drill". Not a centering drill. Do not Dwell on the Spot.
 
DanASM said:
I have been wondering lately if 300psi coolant will break a chip in SS or if you need 1000psi to break chips. Definitely let us know how these holes drill for you. I am curious.

Sounds like a good idea for a setup. I always spot drill first in the small medical screw machine SS. I understand it is a must even with split point drills.

Now the question I have is...What kind of drills do people use to Spot drill. I have used center drills that are ground back far enough that it is just like a spot drill. I know you can buy Spot drills pre made already but I hear people tend to use centering drills as is for spotting.

I suggest a really sharp "Spotting Drill". Not a centering drill. Do not Dwell on the Spot.
Click to expand...


You use a spot drill to make a spot. You use a center drill to make a center hole.

These are really nice spot drills: MAGAFOR
 
I interpolated some .05" deep starting bores, then used a short cobalt drill to peck .875" deep. That worked great. Now I want to use the long cobalt drill to peck the rest of the way, retracting above the stock with each peck and between holes, but not redrilling the first .875" of depth. What's the best way to do this?

I tried this but it wants to start pecking at the top of the hole:

T39 M06 (#51 .067 COBALT DRILL)
T4
G59 G17 G90
G00 A0. B0.
M88
G01 G94 X.0984 Y0. S1255 M03 F833.
G43 H39 Z4.3465
G99 G83 Z3.3715 R5.2215 Q.025 F.05

Thanks

Edit:
I'm trying this:
G99 G83 Z3.3715 I.875 J.85 K.025 R5.2215 F.05

It's closer, but still using the feedrate to enter the hole.
 
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