Boring blind hole

[David Morrow]

I'm boring the hot end cylinder for a Moriya Stirling fan. It has an I.D. of about 1" and a depth of about 2". I drilled the hole to the approximate depth and diameter and then carried on with a boring bar to final dimensions.

The problem is with the bottom of the cylinder which is a blind hole. Due to the depth, I had to use a larger boring bar. But I could not get the bottom of the hole perfectly flat. It actually became a bit convex. To solve the problem, I put a 3/8" end mill in my drill chuck / tail stock and then slowly taking the bottom to final depth but only for the 3/8" diameter of the end mill. I then had a little divot into which my boring bar could penetrate and then work towards the outside of the bore. ( I hope that makes sense ).

Is there a better or easier way to do this ?

 

[kvom]

That's the approach I would take, using the largest endmill I have.

 

[rake60]

I don't know if there is a better way, but it is how I do it as well.

Rick

 

[joeby]

Using an end mill to flatten the hole bottom does the job just fine, and I do it that way too sometimes. The only problem I have with it is that my tailstock ram is well worn and therefore doesn't care to be used this way.

I have numerous boring bars made up that have the hole for a HSS tool put in from the end, and several others that have the hole put in at a 45 deg. angle but terminating more on the end than the diameter. Does that make any sense at all, I wonder?

I also just use commercial boring bars for boring heads on smaller size holes.

Many times you will have to have the bar almost rubbing on the far side of the hole to allow the tool tip to cut from center but you can face the bottom just before finishing the id so any rub marks will be removed on the finish pass.

Kevin

 

[Mainer]

Making a truly flat-bottom hole is, as you have discovered, hard. Luckily, "almost flat" is generally good enough, and I would probably do it about the way you did.

The only reasonably sane way I know of to get a truly flat-bottom hole is to bore a through-hole and put on a flat end cap, but circumstances don't always permit that.

 

[David Morrow]

The only reasonably sane way I know of to get a truly flat-bottom hole is to bore a through-hole and put on a flat end cap, but circumstances don't always permit that.

I was really tempted to do just that but I've seen so many Stirlings with blind hole cylinders I couldn't blink when faced with a challenge that others had overcome. Too stubborn although a flat end cap screwed in place would have been a nice variation.

 

[tel]

T'other solution would be a big 'D' bit, or something akin to it made out of a bit of broken file mounted on a shaft.

 

[rake60]

The perfect way to finish a blind bore is shown in the animation
on the Virtual Machine Shop Page HERE. That works great when
the bore is large enough OR the boring bar is small enough.

One of the parts we make regularly at work called for a 1-5/8" ID blind bore.
It was done on a CNC machine that made it a bit easier but the process
would be no different in a manual mode.

First a 1-9/16" spade bit was called up to drill the hole to it's finished
depth with the Z "0" taught to the point of the bit.
Next a 1-1/16" end mill would call up to peck to the finish depth again.
Now the bottom of the bore was flat out to a 1-1/16" diameter.
The next tool up was a 3/4" boring bar. It would take the bore to 1.595
and face across the bottom face as far as the width of the bar would allow
without touching the back side of the bore.
Finally a 1" boring bar would come up to take the finish cut and generate
a .010" radius in the corner.

All of that happened in about 2 minutes time.
Then I would take that piece out, stick another one in the chuck,
teach the Z axis and press the Cycle Start button again.
Hey, it pays the bills.

Anyway, that IS how we do it.

Rick