offset problem with pens

[mu]

Hi, I'm new here and I'm a beginner with the spinning wood.

I bought an adjustable pen mandrel from Ax, and try to make some pens, but have this problem: the wood is not in axis with the brass tube...

Tried clean the lathe morse hole and reduce the pressure of the tailstock, my fear that the mandrel may not be accurate because the bar has a little movement in the bore of the morse taper, is that normal?

Any suggest?

Thanks :lol:

 

[Blister]

Have you made sure the lathe lines up ?

By putting in a drive center in the head stock and a revolving center in the tail stock , move them together and make sure that both points line up exactly

 

[Paul.J]

You aren't over tightening the tailstock onto the mandrel are you :?:
This can cause the mandrel to flex slightly,giving the problem shown.

 

[John. B]

Mu,

After ascertaining the head-stock and tail-stock line up, As you have an adjustable mandrel, try shortening the bar to single tubes,

tighten the tail-stock just enough to support the bar, turn the cap and finish. Then turn the barrel and finish.

That way, the bar will have the least chance to flex.

John. B

 

[Spindle]

Hi

Not sure how mis-allignment of the centres could cause this effect - tapering yes, but off centre?

My first guess would be a mismatch between the mandrel and tube.

Pressure on the mandrel could result in this effect but it is normal practice to mount the blanks as they will be when finished - in which case the greatest error would be at the centre of the pen.

Regards Mick

 

[nev]

another thing to consider - We have had a discussion or two in past threads about Axminster's bushes being fractionally too small to fit on other make mandrels. So i would guess that the axminster mandrel is fractionally smaller than others, and therefore if using a non Axminster bushing set on an Axminster mandrel, it may leave a little room for movement or off-centreredness? (if that's not a word, it should be)

 

[jumps]

Hi Mu,

Would try all the above, and also try turning the headstock by hand to see if the mandrel is actually straight in the first place too (yes the tailstock will hold it but it's another force, and flex, you don't want)

However, I agree with Mick that fundamentally the tube locates on bushes that locate on the mandrel, or direct on the mandrel, and there shouldn't be any play at all. If there is play and you lock it up with the mandrel thread, unless you are very lucky and manage to lock it dead centre it will simply end up turned slightly off centre and create the problem you have here. If there's play and it's not locked tight you will end up with a chattering tool and no finish!

let's here how you get on

D

 

[deserter]

Have you drilled the blanks with correct hole size? I ask as when I started I was drilling all my blanks on a pillar drill and had a similar problem after investigating everything I found that the drill chuck wasn't spinning straight making the hole it drilled half a mill out, when I tightened the mandrel the blank was sitting on the bar a not central on it, obviously after turning this resulted in one side being slightly thinner than the other and the aforementioned step. I solved the problem by getting a drill chuck for the lathe.

 

[Neil Farrer]

How are you squaring the blanks after you have inserted the blanks? I have had this problem in the past. I make a lot of pens and dont use the barrell trimmer but a jig on a disc sander. After I had replaced the paper on the sander once the table was out of square by a fraction, and it was a tiny fraction. The result was that the axis was not an absolute perfect right angle with the end of the blank, it was out by a fraction, and I mean a fraction, of a degree. When the mandrel nut was tightened to stop the blanks from stopping dead in their tracks when being turned, the mandrel was being distorted by an equally small amount, regardless of how much the tailstock was or was not tightened. The result was that the entire blank rotates but the centre of the blank is not the enter of the mandrel axis , it is off set, hence the offset blank on the completed pen. A small distance but a dramatic effect.

This can happen if you use a barrel trimmer and the wrong size central shaft or, as in my case, the disc sander was not properly set up.

Would bet that this is the problem!

 

[myturn]

To determine if it's the mandrel or bushes at fault.....

Place the bare mandrel between head and tailstcok and run the lathe. There should be no discernible wobble of the mandrel and the shaft should turn true.
If there is wobble the indent in the end of the mandrel may not be located centrally.

Then make some scrap pen blanks, drill them to fit over the mandrel and place them on the mandrel with the bushes located as they would be when making a pen.
Tighten the knurled nut and run the lathe. Check for any wobble of the bushes. I've known bushes to be drilled or turned inaccurately and off-centre and they have a visible up and down movement.
Overtightening the knurled nut is unlikely to cause any distortion of the mandrel as the force applied stretches the mandrel rather than compresses it.

Overtightening the tailstock and compressing the mandrel can cause a wobble most prominent in the middle of the shaft.

Locating the right-hand end bush over the threaded part of the mandrel can also cause the bush to runout as the bush is not as tight a fit over the thread as it is over the shaft.

If using stepped bushes (where the brass pen tubes fit over the end of the bush) are the tubes a snug fit? There should be no play.

 

[mu]

Thankyou very much for all the answers, I've checked some of them, not all (sincerely I'm not sure if I understand everythink for my bad english ).

The tailstock to my eyes seems aligned with the drive center. Here I have a picture of the mandrel and shaft and a video, just to try if you can evaluate if the movement is too much for a correct work.

http://www.youtube.com/watch?v=Ykh_srHjDSw&feature=player_embedded

 

[L2wis]

that does look like a lot of play to me, which parts of the mandrel are the bits in the video? Is the bit your wobbling the end of the shaft coming out of the back of the mandrel?

 

[jumps]

Thankyou very much for all the answers, I've checked some of them, not all (sincerely I'm not sure if I understand everythink for my bad english ).

The tailstock to my eyes seems aligned with the drive center. Here I have a picture of the mandrel and shaft and a video, just to try if you can evaluate if the movement is too much for a correct work.

it looks like you have play between the bush and the mandrel.....this shouldn't be there.

the tollerances should be small - will measure in a moment.

edit - mandrel was 6.17mm and most bushes were 6.25mm, this difference was barely discernable in terms of slop. However, one set were 6.30mm and these did have an element of play in them.

 

[nev]

I may be looking at the wrong thing but if it is the deluxe adjustable mandrel like this...

then the play looks like it is between the shaft and collet? (what's holding it) as opposed to shaft and bushes.

I have not seen one in the flesh but i guess there is a way of tightening it? or using a smaller collet? or should that be larger?

 

[jumps]

see your point Nev!

it would be smaller collet, but then again I beleive the collet is in the headstock end of the mandrel and holds the mandrel stem in position - if that was loose then you wouldn't be able to tighten and hold anything on the actual mandrel at all. Whilst they do do different collets they are to use it as a collet chuck with things other than the mandrel stem.

I thought what Mu was showing us was the stem in the tapered bush - but accept that I was making assumptions!

It would certainly be useful to see a more complete image of how Mu's locating the tubed blank on the mandrel.....

 

[mu]

The video is the end of the tapered cono morse. Tomorrow I'll try to make more pictures, many thanks for now :lol:

 

[Shrek]

Hi,
I had a similar problem and was advised just before the last cut to stop the lathe, loosen the mandrel locking wheel and turn each piece of pen in the bushings, by holding each bushing and turning the pen around half a turn, then re-tighten and finish. This stopped it for me. But it may be as suggested earlier.

Regards

Pete

 

[mu]

Sorry for the late of reply

Yesterday an expert woodturner visited my shop and in five minutes has find the problem: the shaft was not straight. Verified putting the tool rest really near the shaft and turning by hand you can see the differences. With a few strokes of a rubber mallet he solved the trouble and the next pen was ok

Again many thanks for your help :wink: