Tried something new today, shaping on a lathe.

[Lakc]

I needed to repair an old telescope mount that was broken when it tipped over. The shape was easy enough to cast, but it needed a rather large bore with an internal keyway. having long ago read and filed away the idea of using the lathe as a shaper, it came to me that this dusty memory was finally going to get its day in the sun.

After boring the initial diameter with a small boring bit, I dug out my commercial 1/2" boring bar bit (3/16) holder and ground up an eyeball 7 degree relief on the bottom, relieved the sides gently with a diamond dresser (perhaps .0005"), and left the top (inside) of the tool at the 45 degree angle provided by the holder. I locked the headstock, which on my machine means putting both sets of belts on the drive. A fairly adequate method which is why I have not gotten around to making a proper lock for it yet.

This shot is the first setup, and after a dozen passes it became obvious the flex in the boring bar and thrust starting to twist the toolholder was just letting the tool bounce out of the cut.



Slight change of setup, and we have success. I choked up on the boring bar, leaving the end sticking out the backside of the holder. That gave me a nice lever to oppose the forces of the cut by leaning an appropriate portion of my body weight against it. Come to think of it, there are a lot of instances where I use pushing or pulling against the machine to make up for the small lathe's inherant flexibility. Best cuts with this were approx .012", as there is a definate action where the chip coming off fights to keep the bit on center.

 

[SAM in LA]

Jeff,

Good idea.

I'll have to file it away and hopefully find it when needed.

SAM

 

[Ken I]

Did something similar when stuck and desperate.

In needed to broach a 16mm wide x 8mm deep keyway in the 75mm bore through its length / thickness of 50mm - mild steel.

I considered it a long shot but being desperate an' all.

Took a piece of 16mm HSS and sharpend the diagonal on the end (as it comes only sharpened) - bolted into the toolpost on centre and angled it inwards about 2° for clearance - didn't even bother with side clearance.

I put the lathe on its lowest gearing but I did not lock the spindle in any way.

Then proceeded to peck at it with the saddle - obviously the corners went easilly but I expected to come to a complete halt once I got to the full 16mm width.
To my surprise it worked and I was able to take about 1 thou cuts - yes it was time consuming and my arms fell off afterwards - BUT IT WORKED.

Not sure how tough it was on the machine (a Colchester student) but it seemed to take it in its stride.

I recently watched a toolmaker broaching 5mm Allen key holes in EN30B using a sharpened & relieved Allen key in the quill of a Bridgeport - in one go - down a 5.1 diameter hole - no intermediate stages. (The quill was up solid and he cranked the knee up.)

This was a job for me and their sparker was down.

I'm not sure that I approve but again it worked.

Necessity is the mother of invention (and the occasional foul up).

Ken