Having issues with runout(?), no idea how to fix it

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Skryver

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Sep 18, 2020
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Hello!

New to kitless pens and have fallen at the first hurdle. In the process of making my first section I noticed my 7.3mm drill bit was making a 7.9mm hole on entering the material, and after drilling through about 2 inches of material the exit hole is about 7.4mm.

I tried aligning my lathe (nova comet II) but the issue remains, I'm not sure what the next steps would be, and have no machining experience. Any suggestions?

I'm using a collet chuck to hold the material, and a centre drill to start the hole, if it matters.

Many thanks
 
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monophoto

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I suspect that your problem is not unique, but rather is something that is relatively common with most mini- or midi-lathes.

When you mount the blank in a collet chuck and then spin it, the blank will rotate about an axis that is determined by the lathe headstock.

As shown in the sketch below, if the drill bit is not aligned exactly to that same axis of rotation (that is, if the axis of the tailstock is at an angle to the axis of the headstock), the bit will carve a conical hole in the blank; the diameter of the entry hole will be slightly greater than the diameter of the bit, and depending on the depth of the hole, the diameter of the exit hole will also be different. The entry and exit holes will the identical only if the depth of the hole is exactly twice the dimension between the entry hole and the point inside the blank where the drill bit axis intersects the axis of rotation of the blank.
Drawing1.jpg

It's not uncommon on mini- or midi-lathes for there to be a little backlash in the mounting of tailstock, and this backlash can lead to exactly this kind of misalignment. The obviously tell is that the tailstock can 'wiggle' a little when it is not locked down. This happens because the spacing between the bedways is very slightly wider than the protrusion on the bottom of the tailstock casting that aligns the tailstock.

There is a solution - be very careful when drilling to first locate the exact center of rotation of the blank by using a skew to cut a very small dimple in the end of the blank before starting to drill. Then, align the bit to that hole before locking the tailstock down to the bedways.
 

Dehn0045

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I have had problems with a cheap bit set that the drills are not properly ground - the point is off-center. This results in a hole that is larger at the inlet than the outlet. Not saying this is the problem you're having, but something easy to check. Use a test piece and dry various drills.

 

Curly

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To add to what Louie said. Sometimes the misalignment is due to the tailstock axis being a little too high or low in relation to the headstock axis. A simple way to check is with a good set of live and dead 60º centres, bring the points together against a steel pocket rule or box cutter blade. If aligned at the points the rule will be perpendicularto the axis. If out it will tilt away. Then you can play with pushing the tailstock to one side when tightening or shimming.

Pete
 

howsitwork

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Totally agree with above comments but i do suspect the alignment of head stock to tailstock , please kiss test it. ie put sharp centres in both then bring tailstock up and check the points kiss exactly in line vertically and horizontally.

Other causes - drill bits try new quality ones and drill a smaller hole say 6 mm first . You then use a
7 mm bit to ream the hole to 7 mm and then drill again with the 7.3 mm bit. The drills will follow the initial 6 mm hole and self centre to some extent , this might cure it for you.

Oh nearly forgot check for any debris in the morse taper of the tailstock or in the collet you’re using . Any d3bris can misalign the work causing this issue.

update us as to progress, good luck
 

duncsuss

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... and do the "kiss test" twice - once with the tailstock quill fully retracted, and once with it fully extended.

The reason for testing twice is the tips of the centers can be touching perfectly but still not be on the same axis, just as you can touch your two index finger tips together at pretty much any angle you choose. Extending/retracting the quill will expose this misalignment.

Lock the quill with the handle on the side of the tailstock when doing these tests to take out any slack in the quill.
 

tomas

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Ok, all seriousness aside, I would convert everything to metric. I will still be off but at least it will be in smaller increments.

My apologies in advance.
Tomas
 

darrin1200

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I got a brand new Craftsman dill set (118 Piece). The first bit I took out, had a very slight wobble. I randomly checked a bunch more, and found two that were off. Check by rolling it on a known flat surface (tablesaw or jointer).

As for the taper holes, here is a link to a thread that discusses a few ways to clean.
 

monophoto

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As for the taper holes, here is a link to a thread that discusses a few ways to clean.

Craft Supplies sells a special tool for that - commonly called a 'green weenie'. But a possibly better solution is a brass 20ga shotgun cleaning brush mounted in a shop-made handle.
 

Skryver

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Thanks all for your comments and suggestions. After checking through all of the above I'm pretty sure it is an alignment issue.

The trouble I'm having now is that to align the headstock on the nova comet II, there are 4 bolts connecting the headstock to the lathe bed horizontally. It is proving impossible to get the rotation I need, especially after tightening the bolts. I'm considering trying to shim it somehow, I can't see any other way of doing it.
 

duncsuss

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Thanks all for your comments and suggestions. After checking through all of the above I'm pretty sure it is an alignment issue.

The trouble I'm having now is that to align the headstock on the nova comet II, there are 4 bolts connecting the headstock to the lathe bed horizontally. It is proving impossible to get the rotation I need, especially after tightening the bolts. I'm considering trying to shim it somehow, I can't see any other way of doing it.

Try contacting the nearest Teknatool/Nova technical support centre. I don't know what the set-up in Britain is, but the US office has very helpful staff who are turners and know their products.
 

howsitwork

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I’d be very tempted to check under the tailstock base for debris before trying to undo or twist that headstock.

Also is you lathe bolted down to the bench , if so are all the bolts tight or as tight as each other? Like an engine block the opposing bolts need tightening at the same time. It is possible to distort the bed slightly and that is all you need to cause issues.
 

Curly

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I'd be more wary of the bench not being flat than twisting from tightening the bolts unless the man is a graunch artist and tightens until the bench top is crushing. If the bench has a twist in it you will twist the bed of the lathe too if you cinch it down.
 

howsitwork

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I'd be more wary of the bench not being flat than twisting from tightening the bolts unless the man is a graunch artist and tightens until the bench top is crushing. If the bench has a twist in it you will twist the bed of the lathe too if you cinch it down.
fair point !
 
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