How Do I Bore A Shallow Hole?

sanddan

sanddan

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I want to make a .200" deep X .905" diameter pocket on the end of a 1.5" dia bar. I am not sure just how to start this. The pocket is flat bottomed so doing a pilot hole and then enlarging it with a boring bar doesn't work. I could set it up on the mill and use a 3/4" end mill to get the depth and hole roughed out before boring it on the lathe but I'd like to do the whole operation on the lathe as I have to cut a taper also. I have enclosed a drawing showing a section view. This is for a set of machinist jacks I'm making. The set will include a 1" and 2" base, the counter bore mates to a feature on the main body of the jack so they stack for different heights.
 

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Unless you have a collet setup for your tailstock, and a very snug quill, put the endmill in your toolpost. Orient the flutes horizontally so you can adjust the carriage towards you to enlarge the bore, only cutting on one flute of course.

Endmills won't get you a flat bottomed hole, they have a 2* inwards taper so the center of the tip doesn't rub.
 
I get part drawings weekly with such features, flat bottomed counterbores with no provision for drill bit angles at the bottom of the hole, the cost of doing such a thing should be included in your quote. If a hobby project do as mentioned above, bang a center cutting endmill in there from the tool post or tail stock just short of finished depth then use a boring bar to finish the ID and Z axis depth that you desire.

I ran a 150 part job last year, 1.50" aluminum rounds, flat bottomed counterbore 1.375" Dia. X 1" Depth, I ran a 1 1/4" 2 flute endmill .990" deep at .020 inches per revolution held in a QCTP holder at 750 RPM's, finish boring, parting and OD turning at 1500 RPM's. This lathe has a 2 speed motor and the simple twist of a switch doubles or halves the spindle speed. After several dozen parts I failed to reduce the speed for the endmill operation and banged that tool in there at 1500 RPM's, it worked fine but made a terrible sound. I ran the rest of them that way(-:
Talk about chips, over 75% of the solid bar was reduced to chips, 18" turned into chips by the 1/8" parting tool.

We did a job last Winter, 11000 parts, first step was cut the blanks with a largish bandsaw that uses 1/16" wide blades, .06" X 11000 cuts = 660" or 55 Ft. lost to the saw alone, we are only a small shop I can not possibly imagine what a company capable of making 1,000,000 of these parts would look like.
 
If it were mine I'd mount a carbide boring bit from a mill in my tool post and run a center drill to start the bore and finish with the boring bar. Since your stacking the parts the center drill hole wont matter to the finish if it's a little deep or set your depth on the tail stock .
 
Silverbullet said:
a center drill to start the bore and finish with the boring bar. Since your stacking the parts the center drill hole wont matter to the finish if it's a little deep or set your depth on the tail stock .
Click to expand...
If you would like an opinion from someone that does this sort of thing 40+ hours per week 50 weeks per year then do yourself a favor and only use center drills for putting centers in lathe parts, buy a couple of spotting drills for starting holes in a mill or lathe and I suspect that that you will not be disappointed with the results. No disrespect intended, center drills do what they are called, drilling centers for lathe work, spotting drills do exactly what they are named for which is making an accurate starting point for a finish drill and reamer if needed
 
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sanddan said:
I want to make a .200" deep X .905" diameter pocket on the end of a 1.5" dia bar. I am not sure just how to start this. The pocket is flat bottomed so doing a pilot hole and then enlarging it with a boring bar doesn't work. I could set it up on the mill and use a 3/4" end mill to get the depth and hole roughed out before boring it on the lathe but I'd like to do the whole operation on the lathe as I have to cut a taper also. I have enclosed a drawing showing a section view. This is for a set of machinist jacks I'm making. The set will include a 1" and 2" base, the counter bore mates to a feature on the main body of the jack so they stack for different heights.
Click to expand...

As several others have said - use an end mill in the tool post. You can also bore the flat bottom with a small boring bar. I have done so numerous times - with just a conventional orientation boring bar. What I did to get as much as I could with a normal set up (noting my depth with a mag dial keeping track of the carriage position), then pull out, change to reverse rotation and come from near the outside towards centre (obviously from the other side of centre), using the same carriage position. The main thing is that the bar must be small, relative to the hole (as in a bit less than 1/4 of the hole diameter).

One technique, often overlooked by folks new in this game - though pointed out here a number of times (in fact it took me about 20 years to clue into this trick) is that end mills will work fine as boring bars (held in the tool post, light cuts, not very deep, but good for some pretty small holes).
 
Andre said:
Endmills won't get you a flat bottomed hole, they have a 2* inwards taper so the center of the tip doesn't rub.
Click to expand...

I may be wrong but I was under the impression that this is not the case with centre cutting end mills?


Sent from my GT-I9505 using Tapatalk
 
Wreck™Wreck said:
If you would like an opinion from someone that does this sort of thing 40+ hours per week 50 weeks per year then do yourself a favor and only use center drills for putting centers in lathe parts, buy a couple of spotting drills for starting holes in a mill or lathe and I suspect that that you will not be disappointed with the results. No disrespect intended, center drills do what they are called, drilling centers for lathe work, spotting drills do exactly what they are named for which is making an accurate starting point for a finish drill and reamer if needed
Click to expand...

Is a spotting drill an ordinary average everyday drill bit that I've got heaps of in the shed or is it something a little bit special ?

Off to do some googling.

Found it : http://workshopshed.com/2014/01/spotting-drills-vs-centre-drills/

Bill
 
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