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December 10, 2019, 08:25:41 am


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Author Topic: 20X Deep Drilling  (Read 165 times)
Bob La Londe
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« on: December 01, 2019, 01:09:41 am »

I'm making some molds that will require inserts drilled through 80 times with a 1/16inch drill.  I need to make 4 of these inserts, and the drill depth is 1.29 inches.  (Drilling to 1.375 for clean holes)  That's 20X deep drilling.  I looked at my usual sources and didn't find any carbide drills that long (I should have looked elsewhere).  I did find some 6 inch split point cobalt aircraft extension drills from one of my regular vendors.  I looked at them under magnification.  They are legitimate split point drills.  

I tackled the job.  First I drilled a pilot holes with a stub length carbide drill.  It has a flute length of only .4 inches, so I drilled .375 pilots.  Then I chucked up the aircraft bit in a 1/16th ER16 collet and spun it up to speed in the machine.  The tip turned into a blur that was easily 50% diameter runout.  That was no good.  I shortened the drill until it was just long enough to do the job.  I thought maybe ringing on the long section of the drill sticking through the collet might be causing it to vibrate.  It helped.  Maybe 1/3 diameter runout after that.  Still a lot, but small enough that it could pickup the pilot holes that had been predrilled with the carbide drill.

I mounted the work piece on the vise over top of a sacrificial backer to prevent problems on breakout.  It took right at 2 hrs to run the part, but amazingly the back side looked almost exactly like the front side.  I was quite pleased with myself.  

I ran the second work piece and I could see the holes had started to drift a little.  For this application it was ok as long as one hole did not break through into another.  It was of course a real risk since the through holes are only a few hundredths of an inch apart.  It was still ok.  

On the third work piece the holes drill had drifted so that holes broke through into other holes several times 3 or 4 atleast from memory maybe more.  That was absolutely not ok.  

I thought about it for a while as I roughed out a couple more work pieces on the mill to make those inserts.  

When I was done I decided to look at what I had.  The collet had been a bit loose on the shank of the drill so I measured them all.  0.062 plus a couple tenths.  That's what they should be.  I didn't have another 1/16th collet so I tried loosening and tightening it a few times, but the run out was the same.  Pretty bad.  To be fair I didn't expect an HSS/cobalt 1/16in drill with a 1.5 inches of stickout to be perfect.  I was thinking my only option would be to chuck up a fresh drill and give it another go, but it bothered me.  

Then totally on a whim I cut a new drill to length and threw it into a drill chuck on the same type tool shank.  When I stuck it in the machine and spun it up to full speed (5120 RPM) it still looked like a 1/16 drill bit.  The tip didn't turn into a blur.  I'm running it now for the next 80 holes on the third work piece.  I'll know soon enough how it worked out.  I certainly didn't expect a Jacobs style drill chuck to be better than a collet.  I guess it was a crappy collet.  

Just for the heck of it I looked at the used drill under magnification and could see that the corner of one flute was rounded or chipped.  The other was still pretty sharp.  Either the runout caused the drill to wear unevenly, or a couple hundred 20X deep holes is just to much for a 1/16 drill with that much stick out.  

I did find a few alternative sources that do have carbide drill that diameter and long enough for the job just in case I have to cut these inserts again. 
 
« Last Edit: December 01, 2019, 01:11:39 am by Bob La Londe » Logged

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Bob La Londe
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« Reply #1 on: December 01, 2019, 01:33:34 am »

The first work piece using the drill chuck for the long drill has come off the machine and it looks quite good.  Here is hoping the next one looks just as good.  I am piloting the holes with stub length carbide drill right now. 
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Getting started on CNC?  In or passing through my area?
If I have the time I'll be glad to show you a little in my shop. 

Some Stuff I Make with CamBam
http://www.CNCMOLDS.com
Bob La Londe
CNC Jedi
*****
Offline Offline

Posts: 3807


^ 8.5 pounds on my own hand poured bait.


View Profile WWW
« Reply #2 on: December 01, 2019, 03:38:16 am »

OoooooKaaaay. 

The second piece with drill chuck looks worse than the third piece using the collet chuck.  I think this may just be the limit of life for the tip on this drill in this application. 

Carbide drills enroute. 
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Getting started on CNC?  In or passing through my area?
If I have the time I'll be glad to show you a little in my shop. 

Some Stuff I Make with CamBam
http://www.CNCMOLDS.com
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