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Blowing up Gun Drills???

Chris P

Plastic
Joined
Oct 23, 2003
Location
Thorold Ontario, Can.
Hey all.

Wondering if anyone might have some experience with Gun Drills on a Manual Lathe. Speeds, feeds, and set-up? They keep blowing up inside the parts I am working. I'm thinking that coolant pressure is most of the problem. The drill is set up true and I'm useing a pilot hole. The material I'm cutting is D2 and H13. I've done some research and it looks like what they call a Ventec Drill will provide better chip flow. Recomended speeds and feeds seem to be extremely high, reason being? Or are these speeds and feeds unobtanable with my low pressure coolant? 5 to 10 psi.
Thanks in advance.
smile.gif
 
Are the drills the conventional gun type, carbide tip, hollow oil shaft with a V- oil return groove? Talk to the drill maker, they will give you tips for speeds & feeds and coolant flow.

The rifle barrel makers use 500 to 1000 psi, splash hoods, adjustable pressure relief to tailor the pressure for good chip removal without making a huge mess. They mostly use 4140 CrMo and 416 SS for barrels.

And speeds are way up there. Over 1000 rpm, some small bores at 3000.

Takes a good bit of experience to figure out the correct speed, feed , coolant type and flow,etc.

What is the pilot hole you mention? Does it go all the way thorugh? A gun drill does not need a pilot hole, just a starting center. Once the gun drill starts, the configuration of the nose grind self centers it in the stock. If you have a pilot hole all the way through, the coolant will go through the pilot hole and will not back flush the chips out and the gun drill will not self center and will run off. Gun drills are designed to flush the chips out back along the v-groove in the hollow oil tube. My hunch is you are jamming the tube groove with chips and twisting it off.

Years ago I got the hots to buy a barrel shop that was for sale. After playing with the Pratt & Whitney twin spindle gun drilling machines and reaming machines, sine bar rifling machines, and broach pullers, I quickly figured out that I did not have enough lifespan left to learn all the tricks to running the machinery and graduate to actually making high quality barrels and making a profit.

[This message has been edited by John Ricks (edited 10-23-2003).]
 
keep trying.

the manufacturers recommendation can be two things, blown out of proportion for sales or actually calculated for success.

you should make a pump though.
 
We do a fair amount of gundrilling, but it is all done either on our gundrill or a horizontal machining center.

Two things that you must have:
1. good guide bushings. This is tough to get on a lathe. If the drill is wobbling, it's going to break.
2. lots of coolant flow. 5-10psi isn't going to make it on a gundrill. On our horizontals, we have 500psi. Coolant flow is the only way that you have to clear chips. Gundrills don't work real good with a peck cycle.

You don't say what size hole or how deep, so it's a little hard to give specific help.

JR
 
Chris
You did't mention the size of your gundrill, but the following applies to all drills.
The smaller the drill, the higher the pressure. Gundrills need pressure and volume ! They are very flimsey if you look at their construction, which is basically sheet metal formed into a 3 quadant shape (last quadant for chip/oil extraction)
The hole in the tip is a restrictor of sorts. By restricting the oil flow, pressure builds up in the drill, to the point where it has extreme rigidity (like a beer can before opening!)If the pump cannot meet the pressure / volume for the drill, rigidity suffers and that means less axial load capability.couple this with a feed rate or chip load that is too big and "Crash !!"
So be sure you are not over .001 feed rate(.0005 to .0007 ideal ?)and have enough pressure as specked by the drill maker.
FYI, for a .25 hole, 3000 PSI is not unusual, and a .75 hole needs about 1000PSI plus (and 20 GPM)
The guide bushings are important as mentioned above.

Hint: if the drill is blowing out when you are deep in the hole, you need more pressure !
If pressure falls because the oil is hot, a neat trick is to drop in a large chunk of dry ice in the oil resevoir,
 
I would also agree, with everyone higher preasure is probably needed to keep pushing the chips out of the way of the drill. If the chips build up on the tip of the drill they will always break the drill bit. I run a techni drill deephole machine that requires up to 50 gallons a minute flow rate to keep ejecting chips out of the drill tube when the cips build up or get stuck in the thru holes on the drill it busts everytime.
 
Like John asked, aware you doing a pilot hole or just center drilling? Gun drills dint require a pilot hole, and that could be the problem. They are meant to drill without pilot holes because it reduces the chance for the drill to "catch" on the pilot hole while deep in the cut. This would be my theory, I don't have any experience except for watching people with gun drill.
 
The cuttings must be flushed out,drill needs sharpened regulary a close inspection is needed as if it goes half way in a job you cant see it-and bang-the drills need resharpened on a grinding attachment that comes with the machine.Why do the drills keep breaking?Some say when oil pressure drops,some say drill needed resharpened,reps will say do this /that- but they still keep breaking-normally when your least expecting it-even with the nylon guide bushing in-
 
A pilot hole is not neccesary,we normally drill Nimonics which is maybe why the drills break but it pays to note the average number obtainable before a regrind if its say 6jobs@ 10ft long 1.250ins hole,dont try and push your luck as if your doing them on a deephole driller it takes a minute to sharpen the drill-on ordinary materials stainless etc,we dont encounter the problems that we do with the Nimonics.Also you get to know what the cuttings look like off a drill cutting well and the cuttings off a drill thats saying sharpen me,its really down to the operator.Beware what reps tell you.they are trying to sell you their product and the speeds/feeds are often way too much,and down to condition of machine
 
We always bored a starting hole to pilot the carbide. We bored them +.0002/.0005 over the drill size and used that instead of bushings. We only bored them as deep as the carbide is long. I am guessing that's what the OP is doing. Then we would position the drill inside the hole (with the spindle off of course), start spindle, hit cycle start - and away she goes. Close eyes and hold breath.

That being said, coolant pressure is too low - as has already been stated. We never had one NOT work as long as we followed maf'rs specs and had plenty of coolant pressure.
 
Last edited:
Hey all.

Wondering if anyone might have some experience with Gun Drills on a Manual Lathe. Speeds, feeds, and set-up? They keep blowing up inside the parts I am working. I'm thinking that coolant pressure is most of the problem. The drill is set up true and I'm useing a pilot hole. The material I'm cutting is D2 and H13. I've done some research and it looks like what they call a Ventec Drill will provide better chip flow. Recomended speeds and feeds seem to be extremely high, reason being? Or are these speeds and feeds unobtanable with my low pressure coolant? 5 to 10 psi.
Thanks in advance.
smile.gif
.
feeds and speeds often need to be reduced and some material are just plain abrasive and tool life is shorter than expected if used to drilling 1018 steel
.
even spade drills often need rpm reduced to 50% if there is no high pressure through the drill coolant. usually they do not say in big letters not recommended if you do not have the proper coolant system / equipment
 








 
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