Repivot Tapered shank for center drilling in lathe

[cspinner]

I would like to use the repivoting attachments in my lathe to center drill a hole to accept the new pivot. See attached reference diagram for what I plan to do. I have to repivot a pallet fork arbor with a <0.15mm pivot that I cannot find a replacement for.

However, I am missing a suitable drill bit that would sit within the hollow spindle/runner from the tailstock. My 8mm Boley has a tailstock that accepts tubes around 6.5+mm OD. The smaller spindle that would then run within that tube has an ID of just over 3mm. There are tungsten PCB drill bits that have a 3.17mm shank. 1) is this shank large enough to attempt to taper? If so, how do I achieve the correct angle? OR 2) can I instead just part it down to fit within the spindle? I’ve read that the drill bit for this application need not be extremely tight.

My lathe did not come with any tailstock attachments for holding collets nor a drill chuck. It is also my understanding that these attachments, especially if purchased aftermarket, wouldn’t be suitable for center drilling when considering the tolerances. I’m at a loss as to how I would shop for one given the strange sizing of my tailstock.

 

[gmorse]

Hi cspinner,

This old thread by Jerry Kieffer might interest you. For the drills, you would be better off looking at industrial tool suppliers; the drills are usually more expensive than the ex PCB types, but they're better suited and more accurate for the job you need to do and because of that the breakage rate is far lower. Notice especially Jerry's comments about the importance of the drill tips being ground accurately to a centre.

Regards,

Graham

 

[karlmansson]

I would like to use the repivoting attachments in my lathe to center drill a hole to accept the new pivot. See attached reference diagram for what I plan to do. I have to repivot a pallet fork arbor with a <0.15mm pivot that I cannot find a replacement for.

However, I am missing a suitable drill bit that would sit within the hollow spindle/runner from the tailstock. My 8mm Boley has a tailstock that accepts tubes around 6.5+mm OD. The smaller spindle that would then run within that tube has an ID of just over 3mm. There are tungsten PCB drill bits that have a 3.17mm shank. 1) is this shank large enough to attempt to taper? If so, how do I achieve the correct angle? OR 2) can I instead just part it down to fit within the spindle? I’ve read that the drill bit for this application need not be extremely tight.

My lathe did not come with any tailstock attachments for holding collets nor a drill chuck. It is also my understanding that these attachments, especially if purchased aftermarket, wouldn’t be suitable for center drilling when considering the tolerances. I’m at a loss as to how I would shop for one given the strange sizing of my tailstock.

View attachment 742602

I made a new runner for my 6mm lathe. It's pretty straighforward as you have a lathe. It doesn't really need to be hardened either as the forces are small when drilling. Just be careful to clean it before use an use a little oil on in to prevent wear in the bore.

Then there is the matter of the drills. You can either use some sort of very small set screw, such as on watchmakers screwdrivers, or modify your drills to have a tapered flat at the shank end. This can then be registered against a cross drilled pin in the runner. Some boring bar systems use this for orientation and registration. As you will always have axial load along with the rotational force this might be enough. You would just have to be careful when retracting the drill.

If it was me, I would go for the set screw and make a runner to fit the small, straight shank drills that I had access to.

Regards
Karl

 

[Jerry Kieffer]

I would like to use the repivoting attachments in my lathe to center drill a hole to accept the new pivot. See attached reference diagram for what I plan to do. I have to repivot a pallet fork arbor with a <0.15mm pivot that I cannot find a replacement for.

However, I am missing a suitable drill bit that would sit within the hollow spindle/runner from the tailstock. My 8mm Boley has a tailstock that accepts tubes around 6.5+mm OD. The smaller spindle that would then run within that tube has an ID of just over 3mm. There are tungsten PCB drill bits that have a 3.17mm shank. 1) is this shank large enough to attempt to taper? If so, how do I achieve the correct angle? OR 2) can I instead just part it down to fit within the spindle? I’ve read that the drill bit for this application need not be extremely tight.

My lathe did not come with any tailstock attachments for holding collets nor a drill chuck. It is also my understanding that these attachments, especially if purchased aftermarket, wouldn’t be suitable for center drilling when considering the tolerances. I’m at a loss as to how I would shop for one given the strange sizing of my tailstock.

View attachment 742602

From your description, I take it that you have not purchased this attachment yet. I have a similar one in my tool collection that is impressive to look at , but not very practical to use in comparison to normal metal working drilling procedures.

While I think I follow Karls suggestion, I suspect mine will be very similar.

Actually you can use your Lathe as it is and current runner for a one time setup as follows.

(1) Mark your runner per the red arrow first photo. This will need to be kept in the same position for all drilling.

(2) File a slight taper on a piece of stock while rotating in the lathe spindle to get a rough fit to the runner hole. Tap into the hole until you get a snug fit such as the blue arrow first photo.

(3) Spot drill and Drill a Drill shank size hole in the runner using the lathe spindle. Commercial drills designed to drill steel will have standard size shank sizes in the drills you will need for this project.

(4) The drill can now be inserted in in the runner and assure center accuracy. To prevent the drill from turning in the hole, grind the a chisel angle on the back side per second photo as Karl has mentioned.

I would suggest the purchase of drills designed to drill steel from Machine tool supply houses also per Graham. Third photo shows such drills with the red arrow showing a spotting drill. Google your needs and they will show up.

I would not suggest the use of circuit board drills for this type of work. While the carbide in these drills will cut steel, the flutes are very thin to allow large amounts of debris to be removed. The fourth photo shows the difference between circuit board drills and drills designed to cut steel. The heavy flute drill is designed to cut steel with a far less chance of breakage than a light flute. If you break off carbide in your work piece, your in deep trouble.

Jerry Kieffer

 

[cspinner]

All great advice. Much appreciated. I do have the tailstock attachment with the graduated center holes for the arbor mounted in the headstock. I also verified alignment (as good as I can see from the razor blade test) that the lathe is capable of center hole drilling. One difference of my plan relative to the diagram is I won’t be using the dog since I won’t be working between two centers. I simply plan to use a wire chuck to hold the arbor as I drill, or is that not a good idea?
I’ll have to research the drill bits you suggested. You definitely give me pause over the possibility of losing a carbide bit inside the work piece. Still waiting on the new motor to arrive then I’ll be ready to give this a shot.

 

[karlmansson]

Not sure what you mean by "wire chuck", not familiar with the term. Since you are repivoting a wheel that will need to run as true and upright as possible I would suggest drilling between centers and using a lathe dog or similar to drive the work. The distal end of the arbos will effectively be held in a center regardless of approach used, your center discs of the "flag method" with sacrificial stock held in a T-rest, but the chuck or headstock end can move around if held in a fixture that runs out. That would give you a crooked pivot that would need to either be straightened or burnished straight and flat. Which kind of brings you back to square one.

 

[gmorse]

Hi Karl,

Not sure what you mean by "wire chuck", not familiar with the term.

It's another term for a split collet, as used in Geneva pattern lathes. You're quite right, if you use them they must run true, or at least true enough for the work in hand. I've had no trouble mounting the arbors in this way, but I do check very carefully for runout.

If the arbor is a friction fit, it's often just as easy to make a complete arbor, rather than re-pivot the old one.

Regards,

Graham

 

[cspinner]

Hi Karl,


It's another term for a split collet, as used in Geneva pattern lathes. You're quite right, if you use them they must run true, or at least true enough for the work in hand. I've had no trouble mounting the arbors in this way, but I do check very carefully for runout.

If the arbor is a friction fit, it's often just as easy to make a complete arbor, rather than re-pivot the old one.

Regards,

Graham

I came across another thread on this forum discussing the alternative of making a new arbor rather than repivoting the existing one. The one I have appears to be friction fit and perhaps that approach will be easier. However, I would like to attempt the repivot as I’ve been eager to learn center drilling on the lathe. Worst case, I mess things up and I have to turn a new one anyway.

 

[gmorse]

Hi cspinner,

It's always good to have a Plan B . . .

Regards,

Graham

 

[Jerry Kieffer]

the diagram is I won’t be using the dog since I won’t be working between two centers. I simply plan to use a wire chuck to hold the arbor as I drill, or is that not a good idea?
I

Actually holding an arbor in a quality collet will be the most secure and accurate method but may not work well with your drilling devise.
I can explain if you have issues.

Jerry Kieffer