Pen mandrel support

[midnightlunchbox]

Hi folks,

I've been turning pens for years now and no matter how hard I try my mandrel always has a slight wobble on it, presumably due to the shaft being bent. It's not severe or, to the layman, noticeable, but I know it's there and it bugs me. Even a brand new shaft has the slightest wobble and it's not the alignment of the head/tail stock. I have tried giving the morse taper end a clean in case some dirt was knocking it off centre but that doesn't affect it. At the moment I shorten the shaft so I turn only one barrel at a time but I'd prefer to see both parts spinning to get a consistent shape.

So, I have been looking at a pen mandrel support which is essentially a hollow live centre for the tail stock which puts pressure directly on the bushes and prevents warping of the shaft (or so it's claimed). Does anyone have one of these? Are they any good?

Cheers
Craig

 

[chipmunk]

Hi Craig,
Your gripes cry out for putting the mandrel under tension rather than compression.

If you hold the headstock end in a collet chuck and the other end in a rolling tailstock chuck like this... (http://www.arceurotrade.co.uk/Catalogue/Chucks/Drill-Chucks - you will also need a draw-bar in the tailstock), it will prevent any tendency for the mandrel to bow.

Jon

 

[midnightlunchbox]

Thanks Jon, I was hoping not to have to spend as much as all that would cost, I reckon over £100. What do you mean by a draw bar in the tail stock?

Cheers
Craig

 

[Inspector]

How do you square the ends of your blanks? If not square when you tighten the mandrel they will put a little bend in the mandrel. This can even happen when using normal bushings only while turning between centres. We gave up on pen mandrels early on because of out of round issues and went to turning between centres only. Unfortunately that restricts you to one barrel at a time, which you said dislike.
Second thought I have is if there is a slight misalignment between headstock and tailstock all mandrels will be bent when you use them. Have you tried to put centres in the head and tailstock and see it the points are aligned? Opps! I just reread that there isn't a misalignment issue. Sorry.

Pete

 

[CHJ]

Where is this discernable 'wobble'?

Do you have a dial gauge to check the actual run-out?

If so where is it apparent, immediately in front of the headstock spindle or further down the mandrel rod.

Does the run-out amount change when you bring up the tailstock centre ?

When you say headstock tailstock alignment is fine, how do you check, just because a centre point kiss test looks good does not mean the tailstock is in perfect alignment further down the lathe bed. The tailstock revolving support as apposed to centre will not help unless tailstock is perfectly aligned in all planes.

 

[Dalboy]

How are you supporting the mandrel at the tailsock end if with the tailstock and a revolving centre be aware the not a lot of pressure is applied, the other solution is to buy a mandrel saver . I have a Robert Sorbey Revolving centre which I turned a new end to do the same thing this is a better method as it pushes directly on the blanks and not on the shaft which can cause wobble.

 

[midnightlunchbox]

I square the blanks with a simple pen blank trimmer and sleeves of whatever diameter I need. But, I had never considered that being a factor so it's something I will look into, it might be the issue.

As far as alignment, I don't have a dial gauge to check, I just have to use a drive centre in the head and tail and match them up. The wobble is apparent on the bushes but is very slight, I'm just trying to get the blanks to run as true as I can.

The mandrel saver is exactly the gizmo I fancied trying. Do you find it makes a difference?

 

[CHJ]

How good a fit are your sleeves on the mandrel shaft?

I have two mandrels, one home made and one commercial.
My home made is 0.247" diameter, started off at 0.250" but some 7mm pen sleeves were too tight, brass tube wall thickness varies.
The commercial one is 0.243" diameter.

I make my own bushes to suit the mandrels as good a fit as possible so that they run true.

I have in the past had to modify sleeves for folks because they were not matched to their mandrels so different brand mandrels and kit/sleeve suppliers obviously vary, perhaps because of metric/imperial standards being used.

 

[midnightlunchbox]

They are fairly snug, not too tight but very little play in them. I have also found some bushes/shafts differ in diameter, some bushes differ too. It's rather frustrating at times.

 

[Dalboy]

The mandrel saver is exactly the gizmo I fancied trying. Do you find it makes a difference?

My one I find great has reduced any wobble. I still however turn the blank on the mandrel 180deg just to make sure and also measure the final diameter of the pen even though I use bushes. It may be over the top but it works for me. See my latest pen HERE and judge for yourself.

 

[midnightlunchbox]

Okay, thanks for that, I think I will give the mandrel saver a go.

 

[chipmunk]

A draw-bar in its simplest form is just a length of threaded rod with a washer and nut on the end to stop the morse taper pulling out under tension.

If you use a mandrel like the Axminster Deluxe mandrel you can double up the draw-bar in the headstock in place of a collet chuck.

Axminster also do a mandrel saver but these still could result in distortion of the mandrel rod IMHO....
http://www.axminster.co.uk/axminster-evolution-series-quick-change-pen-centre

Jon

 

[CHJ]

I too like Dalboy use a home made version of the compression sleeve saver to fit my hollow centre system, it does take out all tendency for the compression forces of a standard centre to bend the shaft.

As my mandrels are held in the chuck and do not have a rod length adjusting collet at the headstock it allows the support rod length to be kept to a minimum.

One point I had to sort out was that not all my chuck jaws and accessory jaws were good enough on their very central bores to be perfectly concentric. Not a problem for holding wood on the intended dovetails etc. but significant when holding a steel shaft or twist drill.

 

[Dalboy]

A draw-bar in its simplest form is just a length of threaded rod with a washer and nut on the end to stop the morse taper pulling out under tension.

If you use a mandrel like the Axminster Deluxe mandrel you can double up the draw-bar in the headstock in place of a collet chuck.

Axminster also do a mandrel saver but these still could result in distortion of the mandrel rod IMHO....
http://www.axminster.co.uk/axminster-evolution-series-quick-change-pen-centre

Jon

They can't distort the rod as there is no pressure put on it the pressure is transmitted directly through the blanks, Remember the saver is hollow right the way through. The saver also eliminates the screw on the mandrel holding the blanks in place

 

[midnightlunchbox]

Yes I understand a drawbar now, there is a good YouTube video on it.

I turned a couple of pens this morning and took some time to check if the blank ends were square, alignment was correct (or as correct as I can make it), there wasn't too much pressure on the mandrel etc. Everything seemed as spot on as I could get it. However the mandrel rod is definitely bent slightly, when up to speed it is more like a vibration than a wobble as I previously described.

To let you understand how small the vibration is, I use a CA finish on my pens and once I have finished the barrel there is a small smear of CA on the bushes about a quarter of the way round, this is the area the sandpaper is missing. It might be the case I'm just being overly fussy.

In the meantime I think I'll order a mandrel saver and see if there is an improvement.

Thanks for all the help lads.
Craig

 

[chipmunk]

They can't distort the rod as there is no pressure put on it the pressure is transmitted directly through the blanks, Remember the saver is hollow right the way through. The saver also eliminates the screw on the mandrel holding the blanks in place

You are right if the spacers are perfectly square and the ends of the pen blanks/barrels are true as well...

...but I'd suggest that errors in any of these and the mandrel could still be pushed out of true just the same.
- It comes down to the precision of your barrel trimmer and the cleanliness of your spacers.

My point is that if you can put the mandrel under tension then it can run straight and true almost independent of the state of the blank ends and spacers, within reason of course.

It's proposed in this article... http://www.davidreedsmith.com/Articles/MakeYourPoints/MakeYourPoints.htm
and Stuart Mortimer has suggested its use for turning long thin stemmed goblets.

Jon

 

[Normancb]

I'd be inclined to investigate a bit more deeply before spending too much money. I'd be surprised if your mandrel shaft is bent. Out of curiosity I took my oldest pen mandrel, which is at least 20 years old and has been well (ab)used. Laid on a good surface plate I can't get an 0.0015" feeler gauge under it anywhere at any rotation. I put it in my engineering lathe and supported the outer end lightly with a centre just holding the end, with no real pressure, as I would use it. The maximum runout, measured with a good DTI, is less than 0.0015" anywhere. I doubt it's possible to do a lot better. I tried the surface plate test with two other mandrels and both are straight to better than 0.0015". May be worth checking tailstock alignment? Wood lathes are not built to engineering precision.

 

[Lons]

I'd be inclined to investigate a bit more deeply before spending too much money. I'd be surprised if your mandrel shaft is bent. Out of curiosity I took my oldest pen mandrel, which is at least 20 years old and has been well (ab)used. Laid on a good surface plate I can't get an 0.0015" feeler gauge under it anywhere at any rotation. I put it in my engineering lathe and supported the outer end lightly with a centre just holding the end, with no real pressure, as I would use it. The maximum runout, measured with a good DTI, is less than 0.0015" anywhere. I doubt it's possible to do a lot better. I tried the surface plate test with two other mandrels and both are straight to better than 0.0015". May be worth checking tailstock alignment? Wood lathes are not built to engineering precision.

I actually wouldn't be at all surprised if it was bent. I managed to bend one of mine very easily and the quality of the shafts seem to vary.

That was 18 months ago and I haven't had a single problem since buying a mandrel saver, so much so that I recently bought a second one for my new Axi lathe. This mandrel system in fact.. http://www.axminster.co.uk/axminster-ev ... en-mandrel

cheers
Bob

 

[midnightlunchbox]

And how do you find that mandrel Bob? How true does it turn? It looks a very well thought out piece of kit, might get one myself.

 

[Lons]

And how do you find that mandrel Bob? How true does it turn? It looks a very well thought out piece of kit, might get one myself.

I like it and it's well engineered though I don't think it's any better than the cheap mandrel saver I already have. I probably wouldn't have bought it but I had a gift card to use up and liked the look of it.
I think you might be surprised at how the tubes and bushes vary as well. The tubes can be slack from one kit and tight on another which can show when turning.

The problems with mandrels are usually over tightening, which the saver solves, blunt tools and too much pressure / aggressive toolwork.

Here's a brass sedona I made using the cheap saver and light cuts with a carbide tool.

cheers
Bob