Multi centre / eccentric chuck

**Oldgreybeard** · 4th January 2015, 10:55 AM

I have always told my photography students "If you can't take decent photos with the equipment you have now, spending hundreds of dollars on new equipment will not make a difference"

And now I am about to do the same, only this time with lathe chucks

I woke early this morning and quietly contemplated my problem and the answer became uncomfortably clear - there was nothing wrong with the equipment I was using. The reality was that I was obviously not doing something right and spending $500+ was not going to help - the same problem would still exist. There are other reasons for considering buying the new chuck, but to get any real advantage from doing so, I need to resolve this issue.

I will use the 'Commissioaire's hat box' to illustrate the problem I have.

50designs040b.jpg50designs040c.jpg50designs040e.jpg_DSC2993.jpg _DSC2998.jpg _DSC2999.jpg
The lid and the base are each turned on 2 axis and these are the same for both. Chris's recommendation is to turn a small spigot to mount the blank in your standard chuck, then hollow out the base, turn the spigot for the lid and the half cove on the outside of the box. That is straight forward - see image 2. The base is then removed from the chuck and turned off centre to produce the profile of the hat - Looking at Image 1 the small finial is the new centre point - offset from the original centre by 12mm (the base is the same profile without the finial)When the profile is completed, the high point of the rim is 4mm higher than the low point on the opposite side of the base. (Image 3).

I won't embarrass myself further by showing my original 'gerry built' offset chuck setup. But I am now working with the Teknatool 150mm face plate and taking advantage of the sliding screw slots to achieve the appropriate degree of offset and turning a recess in the sacrificial block to jam chuck the base using the lid spigot. Up to this point everything works well and I can use an additional screw in one of the single screw holes to ensure that I have a level of repeatability. Remember the lid must be turned to the same offset profile. Only 2 srews have been used for these images - I would use 2 each side to affix the sacrificial block for turning.

Now the problem. Even with a very tight fit of the spigot in the recess i.e. it has to be knocked in, it seems there is always some rotational 'creep' due to the intermittent cutting of wood and air. The wall tickness of the spigot is only 2 mm, so I don't think that double sided tape would have sufficient contact to be effective - I haven't fully tested it yet as I only have carpet tape on hand and that did not work. Maybe I should be using that strawberry jam!

The problem manifests itself when turning the lid. Any rotational creep will mean that the grain and colour will not line up when the lid is fitted on the base - or if the grain lines up, the shape of the lid and base will be out of alignment.

I know that I am the problem, so don't feel shy about pointing out my faults. Your help is desparately needed.

Thanks
Bob

Edit - Should have added that i am using an 8mm P & N detail gouge. Cut is predominately a push cut from outside rim to tailstock centre. Tailstock is up except for final cuts to remove T/S dimple.

**Skew ChiDAMN!!** · 4th January 2015, 03:08 PM

Have I got this right?

You're hollowing (turning the inside of) each half, then reverse mounting and using a jam chuck that fits inside each half to turn the outside(s)?

Is the 'length" of the jam chuck sufficient to press up against the flat at the bottom of each recess? Shortening it a mm or so, then using double-sided tape on this flat may mitigate any rotational creep.

I also salvage old VCR heads for use with hot melt glue. I glue it into a recess of some scrap, (leaving a mm or two of the head protruding, so I can later mount it in my chuck jaws) turn the scrap down to a suitable diameter for use as a jam chuck and then flatten the end until the metal face is exposed. ie. I make a wooden tyre around the metal VCR head.

The timber supplies the correct size for a jam fit, while I can also apply a smidgin of hot melt glue to the metal flat. This holds very well... and dismounting simply involves heating the metal to loosen the glue. I place it on a hotplate for a min or so, but others I know use a hot air gun. Horses for courses. I do suggest not trying to extract it from the piece with bare fingers though. Hot, hot, hot!

Some people freeze it instead (throwing the whole job in the freezer!) to make the glue brittle and easy to crack... but I find that in a jam chuck this really doesn't work too well! 'Tis OK for faceplate work, though.

**Oldgreybeard** · 4th January 2015, 05:26 PM

Originally Posted by Skew ChiDAMN!!

Have I got this right?

You're hollowing (turning the inside of) each half, then reverse mounting and using a jam chuck that fits inside each half to turn the outside(s)?No. I have been turning a recess (see image 6) The recess is esentially the same size as the recess in the lid and fits over the spigot on the base. In many cases a hemisphere is turned when hollowing the lid.
Is the 'length" of the jam chuck sufficient to press up against the flat at the bottom of each recess? Shortening it a mm or so, then using double-sided tape on this flat may mitigate any rotational creep. Is it possible to jam chuck in this manner with curved sides? Is there any brand of double sided tape which is better that I should buy?

I also salvage old VCR heads for use with hot melt glue. I glue it into a recess of some scrap, (leaving a mm or two of the head protruding, so I can later mount it in my chuck jaws) turn the scrap down to a suitable diameter for use as a jam chuck and then flatten the end until the metal face is exposed. ie. I make a wooden tyre around the metal VCR head. Unfortunately I don't have any old VCR heads

The timber supplies the correct size for a jam fit, while I can also apply a smidgin of hot melt glue to the metal flat. This holds very well... and dismounting simply involves heating the metal to loosen the glue. I place it on a hotplate for a min or so, but others I know use a hot air gun. Horses for courses. I do suggest not trying to extract it from the piece with bare fingers though. Hot, hot, hot!

Some people freeze it instead (throwing the whole job in the freezer!) to make the glue brittle and easy to crack... but I find that in a jam chuck this really doesn't work too well! 'Tis OK for faceplate work, though.

Thanks for the comments . Is creating a jamb chuck which fits on the outside of the spigot rather than inside as you suggested the cause of my problem?

Bob

**DaveTTC** · 4th January 2015, 07:01 PM

Bob you really have my grey matter ticking away on this one.

I may have to have a go at something like and see what I come up with

Dave the turning cowboy

turning wood into art

**Oldgreybeard** · 4th January 2015, 07:23 PM

Originally Posted by DaveTTC

Bob you really have my grey matter ticking away on this one.

I may have to have a go at something like and see what I come up with

Dave the turning cowboy

turning wood into art

Thanks Dave - look forward to what you come up with.
Bob

**steamingbill** · 4th January 2015, 08:09 PM

Hello,

Not really sure that I understand the whole process but decided to toss is a suggestion. Might inspire further ideas.

Can you glue together the bits that are slipping ? and then when you have finished, carefully cut/sand/grind away the sacrificial bit that you dont want, maybe do it carefully by hand ? Then tidy it up. Very tedious and time consuming. There is also that glue chuck trick of putting a piece of paper in the glue between the two wooden pieces ? Still need to tidy the mess afterwards.

Can you somehow use some threaded bar to tighten up the bits that are slipping - i.e. make a slit with a saw to crate some slack then drill and install the threaded bar across it and putnuts in to increase the holding power ? Could apply internal or external pressure ? Need to watch out you do.nt end up damaging the workpiece.

Somehow use some sandpaper between the slipping bits to increase the coefficient of friction and tidy up any dings and scratches by hand afterwards ?

Some sort of expanding mandrel to grip the piece from the inside ?

Bill

**wm460** · 5th September 2015, 02:32 PM

Originally Posted by mjl09

Also see the projects book http://www.robert-sorby.co.uk/pdf/ec...ckprojects.pdf
.

I tried this link and When I click on it I get "Sorry, but this page doesn't exist..."

Is there any where else I can get a copy from?

**Big Shed** · 5th September 2015, 02:39 PM

Mr Google is your friend

https://www.google.com.au/url?sa=t&r...kOXfW7uKh-zFVg

There also a couple of youtube videos

https://youtu.be/p16qbVqDfTs

https://youtu.be/3a4sj_txPTo

**wm460** · 5th September 2015, 03:03 PM

Thanks for this Fred.

For some reason google let me down.

Originally Posted by Big Shed

Mr Google is your friend

https://www.google.com.au/url?sa=t&r...kOXfW7uKh-zFVg

There also a couple of youtube videos

https://youtu.be/p16qbVqDfTs

https://youtu.be/3a4sj_txPTo

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