How can I repeatedly drill a precise offset hole into round stock/part?

Hello

I am new here but I'll get straight into it :-)

Someone pursuaded me to make some bobbins for an antique sewing machine. These have an offset hole in the one side of the spool where a springloaded pin goes into when the bobbin is wound on the winder.

I am now stumped as to how I can repeatedly drill this hole accurately. The OD of the spool is only 9.5mm / 3/8" and the holes sice needed is 1.8mm / 70mils.

I want to drill that hole after machining and parting off the bobbin so that the remaining round stock can remain in the lathe for the next one.

My guess is that I need to make some sort of jig (of course!) but I am just not sure how it should work / look.

The bobbins are about 1 1/4" long with the ends being about 45mil thick discs.

Any suggestions will be greatly appreciated.

You want to drill the hole in one of the spool end faces/disks, yes/no?

If you have fewer than 100 of these to do (quite likely), I'd drill them on the drill press. Find a V-block you can set on your drill press table with the V axis vertical. Block height in this orientation should be just over your 1.25" length. Make a setting tool with the OD of your spool ends. Chuck the setting tool in the drill press, and locate the inside faces of the V on the setting tool. Your V-block is now set to drill a spool on-center. Using a spacer of the desired offset distance, move the V-block over by the desired distance. Your V-block is now set to drill offset from center. Clamp the V-block to the table.

Clamp a spool to the V-block. Drill and unclamp. Repeat.

sfriedberg said:

You want to drill the hole in one of the spool end faces/disks, yes/no?

Click to expand...

If the end isn't flat he may have to plunge a small flat with an end mill first ... some bobbins have a taper on the shoulders and the drill will walk if that's the case.

Yes, drill through the end/face of one spool side

I actually to supply these in small batches as needed but I am not sure that I can do it repeatedly with enough accuracy.

Excellent idea with the V block and drill press but how would I clamp the spool to the block?

The spool end will probably also need to be supported on the inner surface because the whole thing is quite flimsy. The shaft is only 100mils diameter

What is a setting tool?


And yes, the ends are indeed rounded on the outside.

For a setting tool, just turn a piece of rod with two concentric diameters: one convenient to put in your drill chuck, and the other the same as your spool OD.

To support your 0.045" flanges under drilling pressure, turn a cylinder to just under your spool flange OD, and just 1 or 2 thousands shorter than the distance between flanges. Drill a generous clearance hole for the spool axle, then cut this piece in two through the centerline. Deburr. To use, make a sandwich of "half spacer, spool, half spacer" and clamp that in the V-block.

To deal with curved spool ends: Either ignore the problem for the bottom flange as it's not seeing drilling pressure, or turn a small chamfered ring or depression in a scrap less than your spool flange OD to help support the bottom flange. Place your "half spacer, spool, half spacer" sandwich on top of the ring or depression.

For the top flange, you can ignore the problem if the slope of the flange is small, or you can spot drill, or you can face a small flat, or you can use a drill bushing. Spot drilling and end-mill facing would require a tool change. Unless you have an old Burgmaster or a gang drill, you won't find tool changes on most drill presses. However, you might find a quick-change chuck system to use with your drill press.

If the slope of the flange is too steep, and tool changes per-part are too time-consuming, do what people have done for 150 years. Buy a drill bushing of the appropriate drill size, mount it in a piece of bar stock, then clamp/screw the bar stock to the top surface of the V-block. Note: this requires locating two things. Locate the V-block as previously described. Use your drill as a 2nd setting tool, and use this tool to locate the drill bushing before clamping the barstock+bushing to the top surface of the V-block. Note: consult any decent machining textbook for more info on drill bushing best practices.

To clamp the spool to the V-block: You do not need massive force for drilling a thread spool, especially since you are drilling an eccentric hole. Once the flanges are supported by the spacer sandwich, you can press on either the OD of the flanges or the outside of your spacer sandwich. A more robust solution would be to press on the top spool flange, with something similar to the ring or depression mentioned in the 2nd paragraph, but if you are using a drill bushing that will certainly be in the way. Note: Offset the V-block such that the rotation of the drill will turn the spool into tighter contact with the V-block. This will reduce the change of spinning the part while drilling.

At the cost of slightly more involved one-time work, you can make the OD of your spacer slightly larger than the OD of your spool flange, and the ID exactly the same as the OD of the spool axle. Then use a setting tool with OD equal to your spacer OD, not the spool OD, to set the V-block. Clamp the "half spacer, spool, half spacer" sandwich directly to the V-block.

Make a fixture to hold the part firm including a cover that has a drill guide bushing.

All goes into any handy drill press as the fixture is what makes it accurate and repeatable.

Seems that simply boring a hole into a chunk of scrap that the bobbin simply drops into.

Then the cover clamps the bobbin in place and guides the drill.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk

I make a lot of small items with offset features that need to be, if not precise, at least absolutely the same geometry from part to part.
These are only a few examples/one family of parts for a woodworking plane adjuster.

If the part is small enough to offset on a 5c E collet, that is the easiest. larger parts can be offset on larger face clutch collets with closers. Or what I do out of being a cheapskate - i have a couple extra depth clutch collets with closers bored to 2" & 1-3/4", and make inserts off of standard steel and Al bars that size as needed.



The nice thing about using an offset collet and a turret lathe instead of a DP is the facility to say: C-drill or make a flat or counter bore, drill, and chamfer/deburr or quick abrasive polish. There are even tools that will deburr the inside/breakthrough side at the same set up, though i've never used one. Maybe not as tiny as the holes in your project.

smt

Steve Thomas's idea is one I like also. Step bore a hole on a soft collet and you will get the same offset every time you put it in the lathe. If your lathe does not have a collet closer it could be done with a split bushing in a three jaw chuck. the bushing could be anchored to something in the spindle by a threaded rod or have a shoulder to locate against the jaws. The offset bushing in a 3 jaw chuck will not last as long as a collet but will be repeatable till it wears out. Photo of an offset collet in a turret lathe, could have been easily step bored as well.

As an alternate to the drill bushing, make a jacket for the drill that leaves just the end of the drill exposed. This will minimize drill walking or breaking.

Tom

Some nice work there Stephen.

Regards Tyrone.

Another method not yet mentioned is the combination 3- or 4-jaw chuck. The chuck has jaws that are scroll operated for clamping and removing the part, but each jaw is independently adjustable for accurate centering or controlled offset. Once the part is centered or offset, you can change parts and the center will repeat. I use one for turning small batches of eccentric parts, not so many that I want to offset bore a 5C emergency collet.

Here are pictures of a Swiss 4-jaw and a German 3-jaw. The inner jaw engages a scroll like any common universal chuck and the outer jaw is like any common independent chuck. The outer jaws can be reversed. The chuck body is a little thicker than normal, but otherwise the chuck looks normal.

Larry

QT:[I want to drill that hole after machining and parting off the bobbin so that the remaining round stock can remain in the lathe for the next one.]

A simple box bushing can be made for finish off-set hole on drill press.. with having a surface grinder parting wheel split so goes tight in drill press vise..

One that is set so the part goes in from bottom and the Drill from the top can be best.. with a flush feel to see the part is not set on a burr or bug.

Sams can be made for the 4jaw lathe chuck..so one jaw is loosened to release part..and other three retain position.

Perhaps a closing screw set to pressure would not over tighten on the part.

TDegenhart said:

As an alternate to the drill bushing, make a jacket for the drill that leaves just the end of the drill exposed. This will minimize drill walking or breaking.

Click to expand...

Or buy a stub or screw-machine length drill, instead of a normal jobber's length drill.

Here's a different thing to try.
Look up "tool post drill" on youtube - it's basically a lathe tool holder which holds a shaft, one end of which has a chuck, the other end of which is grabbed by a drill (cordless likely easiest.)

Face the part.
Use the compound to locate the hole.
Drill the hole, deep enough to be sure of clearing the inside of the flange.
Now, turn the part as before.
Expect to have to do some deburring on the hole when done.

Thanks everyone.
There are lots of great ideas here and I now need to see how /if I can utilise any of them in a feasable way.

The main problem I see with the collet methods is trying to hold / clamp the finished bobbin and to get the even into a collet.

I suppose I could try and get some sort of collect holder that I can hold in the 4 jaw chuck at the correct offset.

The drill press idea is also great as I could use my Sherline milling machine instead.
Still, the holding and repeatable clamping needs more thought.

Not sure yet but all your posts have helped me a lot.


Regards
Bernt

I'm no where near as good a machinist as most of the people above, but, I'd suggest drilling your hole first while there's more steel to hold onto. A 5C collet block would be best to hold it in your sherline and would be highly repeatable. If possible drill both sides of a 6" bar and flip and part off as you go.