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Parabolic drills cutting oversized holes

tgford58

Plastic
Joined
Nov 10, 2010
Location
Indiana USA
I’ve hit a pretty consistent problem. Drilling 6061-T6 with parabolic drills I get oversized holes if I’m anywhere close to manufacturers speed and feed. Today is an example; .3438(11/32) Guhring parabolic, bright. Drilling .75 deep. Small spot using 3/8 140deg spot. Manufacturers data was .01 per rev SFM 260. First hole was .361 and tapered to the bottom. Second hole ran at .005 per rev was .354. Changed spot to full depth. At .005 feed hole was .348. At .002 per rev hole was right on with .343 slip loose. .344 no-go.

i have this issue with most parabolic drills over .25. What am I missing?

Thank you for any help.
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
Sounds like your drill is nutating/wobbling on entry. The spot must not be doing a good job for that particular drill. How long is the drill? Perhaps try a screw machine length drill instead.
 

dian

Titanium
Joined
Feb 22, 2010
Location
ch
interessting, so regular drills dont have the problem on the same machine?

generally the size of the hole is a funny bussiness when drilling. most people will say they get oversized holes while in my experience they are undersized (where the drill shank barely goes in and a nominal pin is a press fit) most of the time even when not spotting.

what effect were you hoping to get with parabolic drills? my impresssion is they are for deep holes.
 
Last edited:

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
Drill point pays a pretty big role in this. Parabolics generally have a big thick central web, with a split point. In ny experience if the geometry on that isn't just right and/or the hole isn't started in a way that stabilizes the drill it will tend to wobble at the start of a hole, especially with longer drills.

I think we've discussed this before, but drilling is a roughing operation. It's generally a rarity to get a hole that's dead on the money - they almost always drill oversize, and are rarely close to perfectly cylindrical. If a hole needs to be right on size, a drill is not the proper tool for the job. A high quality (read: expensive) drill, especially those that are solid carbide, has a better chance of getting closer to making right on size and cylindrical holes.
 

dian

Titanium
Joined
Feb 22, 2010
Location
ch
right. many people will say they have a thin web and will break but they hae a thicker web, at least the ones iv seen. the 140° spot seems just right. too small maybe?
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
If you can't use a shorter stiffer drill, I would try a spot drill to full drill diameter, start the drill at the lower feed until it's 1/8" deep then run it at the .010" feed for the balance of the hole.
 

tgford58

Plastic
Joined
Nov 10, 2010
Location
Indiana USA
interessting, so regular drills dont have the problem on the same machine?

generally the size of the hole is a funny bussiness when drilling. most people will say they get oversized holes while in my experience they are undersized (where the drill shank barely goes in and a nominal pin is a press fit) most of the time even when not spotting.

what effect were you hoping to get with parabolic drills? my impresssion is they are for deep holes.
Here are my thoughts on parabolics, perhaps you can help correct any incorrect assumptions. Parabolic by design have a thicker web and are therefore more rigid. I like the chip removal characteristics much better than a standard twist drill, the drill seems to manufacture chips instead of the strings I'm used to in standard drills. Finally, because of the design they excel on deeper holes, 3Xd initial push is better than what I've been able to achieve with HSS twist drills.

The drawback is, if it is one, I have a drawer full of parabolic drills so that's what I use for a <2Xd job that I'm doing now.
 

tgford58

Plastic
Joined
Nov 10, 2010
Location
Indiana USA
Drill point pays a pretty big role in this. Parabolics generally have a big thick central web, with a split point. In ny experience if the geometry on that isn't just right and/or the hole isn't started in a way that stabilizes the drill it will tend to wobble at the start of a hole, especially with longer drills.

I think we've discussed this before, but drilling is a roughing operation. It's generally a rarity to get a hole that's dead on the money - they almost always drill oversize, and are rarely close to perfectly cylindrical. If a hole needs to be right on size, a drill is not the proper tool for the job. A high quality (read: expensive) drill, especially those that are solid carbide, has a better chance of getting closer to making right on size and cylindrical holes.
I agree that drilling isn't the way to get a precise hole but I do expect plus/minus .005. Within my tolerance. I'm getting +.018 and unpredictable. My drill is running true, I indicated the cutting surface to assure a center grind.

Can this be from too hard of an initial push? I start at model top so the drill impacts(my visual) the surface already at feed. I'm wondering if I slow down the initial entry until the point is buried, then go to full feed, if that will have an effect. I'm trying this morning.
 

boslab

Titanium
Joined
Jan 6, 2007
Location
wales.uk
Have you equal land lengths, I was showed how to deliberately oversize a hole by grinding one land longer than the other displacing the neutral axis to make essentially an eccentric drill, standard length not so consistent with stub ovality can appear too, blue the pin and stuff it in 2 lines is probably oval, do you actually need a hyperbolic for such a shallow hole?, there not somthing I’ve ever had to use much ( ever asked for a 3” hyperbolic drill in the tooling stored, piss off is the most likely response
)
Mark
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
I agree that drilling isn't the way to get a precise hole but I do expect plus/minus .005. Within my tolerance. I'm getting +.018 and unpredictable. My drill is running true, I indicated the cutting surface to assure a center grind.

Can this be from too hard of an initial push? I start at model top so the drill impacts(my visual) the surface already at feed. I'm wondering if I slow down the initial entry until the point is buried, then go to full feed, if that will have an effect. I'm trying this morning.

I wouldn't say "too hard of an initial push." The drill is still doing its job and cutting a hole, and that hole is likely on location. But changing to the light feed on hole entry then shifting back to the heavier feed may very well solve the problem you're having... Already suggested that. I believe that will get your hole a lot closer to the drill size.

The failure here I think is in using too long of a drill for the hole starting procedure you're using. When starting a hole with a long drill, in my experience you need to create the correct conditions for the drill to succeed. Often that means using a stub drill to create a full hole for a certain distance, then switching to the longer drill. The same type of procedure is necessary for gundrilling due to the long flexible drills - they're not great for starting holes. You may find success using a short, stout, stubby drill as your spotting drill.
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
Have you equal land lengths, I was showed how to deliberately oversize a hole by grinding one land longer than the other displacing the neutral axis to make essentially an eccentric drill, standard length not so consistent with stub ovality can appear too, blue the pin and stuff it in 2 lines is probably oval, do you actually need a hyperbolic for such a shallow hole?, there not somthing I’ve ever had to use much ( ever asked for a 3” hyperbolic drill in the tooling stored, piss off is the most likely response
)
Mark

I think he must, or the hole wouldn't have tapered back down to size, nor drilled on size with lighter feed rate. But a good thing to check nonetheless.
 

tgford58

Plastic
Joined
Nov 10, 2010
Location
Indiana USA
I think he must, or the hole wouldn't have tapered back down to size, nor drilled on size with lighter feed rate. But a good thing to check nonetheless.
I checked the run-out and lands first. Both were fine.
So I ran a few more trials. I can't say I am, any closer to an AH HA moment but here's the latest.

First I ran 4 holes all spot with 140deg down .05. 2 holes ran at .002 feed - good holes, .343 pin. Second two ran pre-drill with 11/32 down .125. Both at .008. Both holes had taper but were good with a .343-.344 pins.

I took out the drill to look at the edge and promptly dropped it chipping a edge ---- off the the drill doctor. I eyeball adjusted the DD to try and reproduce the 130 deg drill angle, ground minimally to remove my oops and remounted.

Drilled 8 holes - all spot as before. 4 were ran .002, the remaining 4 were pre-drilled to .1 deep, just to get the drill below the material, 2 - were ran at .006, the remaining 2 were ran at .01. All holes either pinned at .342 or .343 with the 2 ran at .006 being the best overall, pinning smooth with .342. The 4 at .002 seemed rougher and the 2 at .010, though acceptable, still showed some taper.

Since I buggered up my drill I guess I won't know if grinding had an effect. It did make the exits rougher leaving more material to clear.

I've ordered a new drill along with a 135 deg screw machine, Jobber twist drill and another brand of parabolic. I think I have enough information to run this job but not enough for me to comfortable predict feeds and speeds for future sizes. .
 

tgford58

Plastic
Joined
Nov 10, 2010
Location
Indiana USA
I think he must, or the hole wouldn't have tapered back down to size, nor drilled on size with lighter feed rate. But a good thing to check nonetheless.
boslab - I always check the lands. I'm old enough to remember grinding drills to get the size I didn't have. Then there was a feel involved since holes were made on a drill press or using the quill on a Bridgeport. CNC takes away that feel. Great stuff at times, a pain in others.
 

boslab

Titanium
Joined
Jan 6, 2007
Location
wales.uk
I don’t have cnc experience, closest was a Colchester alpha 400 lathe, it was hybrid but I was starting to feel comfortable with the thing but tried to get a post-mortem during work time and retired, figure button pusher on a robodrill but not program
Mark
 








 
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