# How to bore a large blind hole?



## JPMacG (Dec 28, 2017)

My really basic dumb question of the day....    I need to make a 1" diameter blind bore on the center of a 4.5 inch diameter round of 6061T6.  The bore needs to be 0.750" deep and flat at the bottom.   I have the work piece mounted and centered in a 4-jaw chuck on my lathe.

What are my options?   Should I start by drilling a 3/4 inch hole, maybe 5/8 inch deep and finish with a boring bar?  Do I even need to use a drill - could I start and finish the hole with a boring bar?  Is there a better way?


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## BGHansen (Dec 28, 2017)

I'd do it exactly as you described.  Drill a clearance hole first staying short of depth and diameter.  Then creep up on the diameter and depth with your boring bar.  I've used an end mill also to get a flat bottomed hole, but it'll be oversized in diameter.  Naturally needs to be a center-cutting end mill.

Bruce


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## Tozguy (Dec 28, 2017)

Drilling short like you mentioned and then boring out to final dimensions sounds good to me too.


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## 4GSR (Dec 28, 2017)

Used to have a tool ground for doing short flat bottom holes.  Start out with a pilot hole near the depth needed and finish out the flat bottom hole with this tool.  I'll see if I can find it and post pictures of it.



Tool is ground to use in a 4-way tool post or a Aloris style holder.  I did not put an back rake or top rake on the tool.  Take cuts up to about .100 deep and face outward to near the diameter you want.  Once it's rough out go back and finish to size.  The one shown in my picture was ground to go in about 13/16" deep.
Ken


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## kvt (Dec 28, 2017)

Drill short of depth.   Then bore out to size and depending on your boring tool.  You may be ale to get the hole worked to depth with it.   If not flatten the bottom with a small ground HSS tool   I have used long HSS to do the bottom of a couple of holes before.


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## EmilioG (Dec 28, 2017)

If you have large telescoping gages and a  4-5" micrometer, that will help you too.


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## f350ca (Dec 28, 2017)

Mount a 1/2 inch end mill in your tool holder with the flutes horizontal. Centre it up, plunge to depth then use it as a boring bar to get you 3/4 dia.

Greg


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## mikey (Dec 28, 2017)

Jon, ideally you want to use a positive lead bar. Drill with the largest drill you have and stop the tip of the drill just shy of final depth. Use your boring bar that is set to final depth (use a carriage stop to set this limit) and skim cut the sides of the bore to straighten the walls and then feed toward the center to flatten the bottom of the bore. This initial cut toward center is to get rid of the tapered material left by the point of the drill. Once you get the bottom flat with this initial cut the rest is just boring to final ID and a cut toward the center after the final pass to clean up the bottom of the bore.

Flat bottomed bores like this are simple routine jobs you will do time and time again. You need to use the right bar geometry for this; in this case, a positive lead bar. Zero lead bars tend to rub and cause chatter and finish issues in blind bores. Positive lead bars typically have a 5 degree lead angle so that only the tip cuts, not the entire leading edge. This geometry is commonly found with inserted bars like an SCLCR bar. If you intend to use a solid bar of HSS, cobalt or a brazed carbide bar then you need a positive lead bar or you must grind a clearance angle up front. 

If you do not have a carriage stop then rig some kind of solid stop so the saddle stops at a precise point.


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## Billh50 (Dec 28, 2017)

I have done those with just the boring bar after drilling to within .010 of depth. I bring the diameter in to size and set my dial to zero at that point. I then start at center with the boring bar tip and face the bottom out to the zero on the dial.


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## JPMacG (Dec 28, 2017)

Thanks everyone.  This place is great!


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