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Deep drilling 1/8 inch


Oct 11, 2016
Sorry for flogging a tired horse - I did read some other threads about deep drilling, but still not confident.
I need to make a punch/die for a pattern of holes a little over 1/8 inch. Just to punch a few hundred prototypes.
I will finish the holes with number drills to get the exact diameter, but obviously twist drills will not be straight enough.
To drill through the block, stripper, and die, is 1.5 inches, 12:1 depth ratio. I was going to pre-drill with an available 1/8 carbide endmill, but I can see that thin flat-end mills start to wander right from initial contact (surprise!).
To improve the outlook, note that the hole only has to be accurate at the stripper/die interface. Also the die can be thinned out from the back beforehand.
Also the punches will be 1/4in A2 drill blanks, ground to size at the end. That means the upper block (and maybe the stripper) can be pre drilled at 1/4 inch, almost down to the die block interface. So the current plan is:
1) Pocket the die block underneath the working area
2) Clamp the stack face up (die block on the bottom)
3) Drill the guide pin holes (4 x 3/8in) & insert guide pins before proceeding (with carbide tool)
4) Drill 1/4 inch through the top block & halfway through the stripper (carbide)
5) Drill 1/8 inch through the rest of the stripper and die block down to the pocket (carbide)
6) Number drill to size (HSS)

So the actual DOC can be about 1/2 inch and the critical interface only about 1/8 deep.
Unfortunately the material is 4140 half-hard rectangular bar (which has a crust) and will be fully hardened. That's all I have to work with. All work done on a BP with Accu-rite DRO, no coolant (just air nozzle).
I'm mainly wondering what to drill with. I'm leaning toward drill-pointed mills, but don't know if 2- or 4-flute is more rigid?
Can pointed endmills plunge cut? They don't look like the tip is good for that. I have some carbide "spot drills" with utterly useless points (well, they're eBay)
For the 1/8 inch hole x 1/2 deep, maybe a spade blade? Seems like the lack of side cutters would be an advantage (if I peck & blow the chips out)? Even though the DOC is limited, it still has to stick-out about an inch to reach the hole.

I do expect the 4140 die to last long enough punching 16-ga mild steel for the qty needed, and we have a surface grinder.
(We used to have scrap D2, S7 etc. but they threw it away!!!) We do have a press with enough force & can stagger the punches.
Everything has to be out-of-pocket, including the die springs, product material, etc. & I already spend $100's a month on tools, so I'm broke... nothing fancy!

I wish I had enough expertise to give back to this forum as I have received, but unfortunately I am only an electrical engineer. Any electrical questions I would be happy to help, but please, no more "free energy" projects (yes, I have been asked)!

BT Fabrication

Nov 3, 2019
Ontario Canada
ive drilled many holes 1/8" wide and 4" deep. drill wasn't long enough on hand so it was from both ends and met precisely in the middle.

easy to put in a mill vice with a hard stop and flip if the drill isn't long enough.

Just go fast enough to make a chip, but full retract so it does't bind up in the hole.

drill straight is easy, just make a tiny spot drill to begin.

hole alignment with a DRO is pretty easy, just make sure your head is trammed within a tenth on the BP.

and 4140, keep it cool or it will work harden on you.

and any carbide split point drill will work. no need for an endmill. they don't work as well as a drill does.


Jun 7, 2012
Unfortunately the material is 4140 half-hard rectangular bar (which has a crust) and will be fully hardened.

Half hard? What is that?
And what is the crust? Is it a flame cut edge? Or is it the hot rolled edge?
I assume you're referring to 4140PHT which is 28-32 Rc. Cuts like butter.

Since you don't have coolant, are you able to hook up a mister?
If not, then I'd grab a coke bottle, fill it with coolant and start drilling.
Center drill first because of the crust to maintain location.
The depth you're wanting isn't anything special.


Jun 23, 2002
Vancouver BC Canada
Hi rabtrfld:
If you want to do a decent job, you should consider predrilling undersize, poking down a 1/8" endmill to get the holes round and on location, and then reaming to final size.
You can buy reamers in 0.001" increments so just get what you need.
It's going to cost you maybe 50 bucks, but at least the holes will be round, on size, smooth and in the proper spot.
The punches will cut better, you can get the die clearance you actually need, and the slugs won't hang up so badly.


Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining


Jun 15, 2006
Sacramento, California
Assuming you don't want to use a drill guide/busing and a template.

Consider thinking about what I do all the time when needing precision relatively deep holes in various steels including 4140 HT. Start by using a screw machine drill bit (stubby bit) so it won't flex and is 1/64" under the desired size. Drill about 0.4-0.5" deep. Next ream with the desired finished size of the hole 0.4-0.5" deep. You now have a nice drill guide in your part. Drill with the final size twist drill (same as the previous reamer size) and you will be spot on every time.

If its a really deep hole say 4" or greater consider using the above approach halfway from either side.

If your goal is to have a finished reamed hole then do the following: Screw machine drill 1/32 under size 0.4" deep, ream 1/64 under size 0.4" deep, drill 1/64 under size all the way, ream to finished dimension.

I realize it sounds like a lot of tool changes and it is, but if you need spot on deep holes its well worth the extra tool changes.

The approach described above assumes you have sharp, symmetrically ground bits to begin with.

This is what works for me.

BTW, a few weeks ago I needed a 1/4" hole about 10" deep in some 1018. I noticed that my 1/4" long bit (~7") was sharp but had a slight orbit to it. I used the above approach on both ends and the two holes lined up with no ridge in the middle. Basically for most of the work I do, if I get the bit started straight and held with a guide it's not going to wander much if its ground correctly.

I'm sure if you were to drill really super deep holes this approach would not work and the bit would eventually start to wander as it went deeper. Gun barrels are drilled using a gun-drill which is a single lip cutter that tends to self center and prevent wandering.


May 31, 2017
I will finish the holes with number drills to get the exact diameter, but obviously twist drills will not be straight enough.

To drill through the block, stripper, and die, is 1.5 inches, 12:1 depth ratio.

not straight enough? Says who?
What sort of tolerances you are after?

IMO two most important basics if you want straight holes: spotting precisely and continuing with longer drill exactly same location. At least you have DRO so hitting the same location is easy after spotting. If your spotting drifts off 0.001" from the spindle rotational axis everything is on downhill after that.


Aug 5, 2005
Here is a fool proof way (pretty much but don't test it) of getting deep holes very straight. 1/8" though 4" and it was within a thou was my personal best 32x. If not lathe work, start with a boring head for the first step

Ideas - Metallum

Its not too tedious for a couple, but obviously not a production technique...but it will create very straight holes


Jan 6, 2007
Tedious way I did it was drill undersize, ream to size then use a longer drill, seemed to work as the reamed bit acted like a drill bush to guide the next bit, in my case I didn’t need to meet anything it was a thermocouple hole but plenty deep, worst part was it was in a 16 ton cast beryllium copper mould, and I got to repeat the process about 150 times, not funny I’m no edm expert but I’d guess that edm was the way to go, the second mould was sent out for thermocouple holes by Ed, I wished that was done the first time myself,
Corrodrill 865 comes to mind I saw recently