Attn. engineers! what is causing this wobble?

[irtuk]

Hey all.

In my quest for "thinness" today I tried turning a blank with some new dimensions.

Normally I thread the barrel to M16 on the outside, tapped to M12 on the inside, cap to M16 to attach to the barrel, the section has to be drilled 7.3mm to take the proprietary bock tap with an M12 threaded tenon on the back to fit into the M12 inside the barrel.

Ok, so this is working fine for me, but I wanted to try stepping everything down a bit to get a thinner, finer pen. So, I started with the section to see if I could get an M10 tenon and then drill the 7.3mm hole for the bock tap. I used a very short shank starter bit to get a guide hole and having done this it looks feasible, just about, but not when your drill bit is doing this:

so, question for the engineers, what is causing that wobble? is it an off-centre starter hole? the shank on the drill being too long?

The drill and the chuck in the tailstock are not moving, its the tip of the drill that's moving and I can't cope with that much movement. I don't have the tolerance or thickness in the section, it's got to be dead straight or it will just blow the tenon apart.

Any ideas?

 

[pittswood]

You will have to drill a centre hole first. They are like a short "jobber" drill. This will give you a true centre drilled point to then drill with your larger hole.
Cromwell Tools - Experts in Hand Tools, Power Tools and PPE
Hope this helps.
Kelvin

 

[ValleyBoy]

Hi Ed

Could be a couple of things. I’d start by checking your lathe headstock and tailstock are aligned. Put a centre in each and being then right up to each other. They should meet and be aligned both horizontally and vertically.

I’d start there.

Cheers
Ash

 

[Terry Q]

Your drill bit point is not on centre. Kelvin's advice is spot on.

 

[irtuk]

great thanks all. I will take a look at the lathe now.

 

[Curly]

There are a number of reasons for that.

As mentioned alignment of head and tailstocks being out.

There was a pip or little stub left when you faced the blank and that pushed the bit to the side initially. The pip is from the lathe tool being just a touch below centre. Use a short stiff drill to get a start. Most people use a centre drill but a better one is a spotting drill with a 120º tip to match your normal drills. Okay close enough to the 118º they come with. Pulls the drill bit better to the centre than the 60º a centre drill makes. Or adjust the lathe tool to cut a shallow V or conical depression in the end before using a drill.

The bit is bent a little so it starts off a little.

The bit is not sharpened properly and one edge of the point is longer than the other. Cheap or possibly a dull bit.

There are more but my brain has stalled for now.

Pete

 

[Dalboy]

Some of these may have already been mentioned.

The chuck seems to be running off of true you can see it wobbling.
The tailstock also seems to be wound out try starting with it fully retracted(except for the self eject if it has it)
The item that needs drilling need to be drilled with a short centre drill which has a bore size slightly larger than the web on the drill
Drill bit needs sharpening correctly so the web is central, test with a drill gauge.
Tailstock not lined up.

Not in any particular order

 

[wm460]

Here is a link comparing sot drill and centre drills

Access Denied



McMaster-Carr

 

[monophoto]

What kind of lathe are you using? I sometimes see that kind of wobble when I drill on my Chinese-made midi wood lathe. In my case, the problem is that the tailstock can rotate a fraction of a degree when it is not locked down - there is a metal protrusion on the bottom of the tailstock casting that aligns it between the bedways that is very slightly more narrow than the opening between the bedways, thereby allowing the tailstock to twist. The amount of error is very small - less than one degree of angular rotation, but that means that it is possible to start drilling the hole with the axis of the drill bit very slightly misaligned with the axis of rotation of the headstock spindle. To prevent the wobble, I always find the exact center of the workpiece by cutting a dimple with a skew chisel, and then I align the drill bit to that exact center before locking down the tailstock.

 

[irtuk]

Lots of good input here. It's this axminster craft lathe and yes the tailstock has quite a range of movement, a good few mm in all directions. I will have to look at ways of getting that aligned perfectly as suggested above.

Something else that struck me this morning, the M10 dies I have are from the sealey set off amazon. I think these are more suited to putting threads on something that does not need to be drilled, they are M10x 2, M10x1.5 and M10x1.25.

This is a significantly bigger thread pitch than the M10x08 sets, so I am thinking that these dies I have way too large a thread pitch, which, along with all the other issues I might have going on, is simply going to cut too much material off the tenon to make drilling it viable.

What's the thinking here? should I be springing for the M10x0.8 set? it's surely going to be of a higher quality....

Ed.

 

[Penpal]

It all in the the entry on,perfect this rfirst.

Peter.

 

[21William]

I always use a short spotting drill to make a start for the hole and then drill with the shortest bit that will do the job. I keep a selection of spotting drills and stub drills.

This one is carbide but HSS is fine.

Osborn 120o Cobalt Spotting & Centre Drill By Europa | R.D. Barrett

Stub drills.

Cromwell Tools - Experts in Hand Tools, Power Tools and PPE

I also use a collet chuck for accurate work and if the hole has to be as good as possible it’s drilled first then finished off with a boring tool.

 

[Ratty]

Looks like you are battling all the same issues I have had over the last 12 months trying to get going with the kit less pens.

My lathe is a cheap Midi wood lathe lathe with plenty of slop in the tailstock. I have learned to align it correctly through trial and error(mainly error).

I always add a small dimple as said before to allow the drill to centre, along with a short centre drill to start.
I use a cheap M10 x 1 tap and die from eBay which works fine for the section for now. Making sure to do things in the right order to ensure everything stays square and true.
It is taking lots of time and practise is what I am finding, it is a whole new skill set to acquire.

 

[21William]

Also worth saying the jaws used aren’t really suitable for accurate work. Engineering type jaws like these would be better.

Axminster Internal & External Safety Jaws | Axminster Tools

 

[Phil Dart]

I sent you a PM about this Ed, so you already know what I'm about to say, but I think it's worth putting it out there for everyone else too. I don't know if the problem is entirely attributable to this, but certainly it will be a major contributing factor.

You are better off getting your drilling out of the way before you start sizing the outside of the tenon.

If you size the tenon first you are effectively creating a very thin wall which is unable to support the hole whilst being drilled. Assuming that you have mitigated against other possible causes (centre spot, central point on the bit, accurate jaws, aligned tail and headstock etc,) a lot (if not all) of the problem you are seeing will be because the workpiece itself is moving and taking the drill bit with it.