How to find the best centre on an irregular lump?

[rsser]

While Melb forumites are having fun down at 's and I deal with NZ's revenge on old pharts trying to ski, I've been wondering.

Jefferson posted on this recently, in respect of a burl cap, and I've had the same puzzle with an off-centre nat edge lump.

You have a flat face on the cut side. On the other hand you have to take your ref from the top and you can't see both at the same time. You want in the case of nat edge to get both your high points and low points at the same height.

I've dealt with this by playing around with a rule and making some rough guesses, or by mounting btwn centres and wasting wood to see what are the best centre points.

But maybe there's a better way and the feeble imagination has run along these lines:

Mount a lazy susan bearing (which I think has a hole in the centre) on an MDF square, also with a hole in the centre and with eg fishing line cross hairs.

To the MDF square you attach a U shaped arm, the bottom and the top of the arms || to the square face. The arm slides laterally on the MDF base.

To the top arm at the end you have a drop-down rod. The rod might simply be a flexible bit of plastic.

Drop this 'jig' across a pair of saw horses or similar.

Plonk your irregular lump on the lazy susan bearing, and then by trial and error move the arm in and out laterally, and the drop-down rod vertically while repositioning the lump til you have the optimal centre.

Then mark the centre underneath through the holes, using the cross hairs if you've gone that way, or otherwise with an appropriate size dowel with a point in the centre.

What do you think?

[Robomanic]

That sounds like a good balance between functionality and simplicity. I like the lazy Susan idea.

Pity Susan is remembered for being lazy and not for being a good engineer :;

[joe greiner]

Sounds like an excellent idea, Ern.

The Lazy Susan could be a turntable, usually plastic, sold in housewares for spice racks and such; doesn't have to be fully engineered; they seem to have more load capacity than needed. Easy as pie to locate the centre of a circle for a new hole.

I'm not sure you'd need the U-shaped contraption, although I'm at a loss to sort out exactly why. Spinning at a slow speed should identify problem areas, such as the high spot to engage the tailstock for initial turning between centres, or interrupted irregular natural edge, or bizarre interrupted profile from top to bottom. I think you're on the right track, though. Thanks for the notion.

Joe

[robutacion]

Well rsser, I'm not 100% sure if I understood your question, but if the question is to find the centres (top and bottom) of an uneven surface/object, I normally do the following;
If I'm looking for the connecting points of an invisible line (as if was through the object), I use one of those flexible arm desk lamps (old style) with a laser dot (normally red, cheaper) taped (affixed) to the lamp's top, pointing vertically down. On the bottom surface I use a piece of MDF/ply (anything solid about 20"x20" or so) fixed temporary into 2 work horses, with a small hole (3mm) in the centre. I clamp the desk lamp on the bottom board, turn the laser on and line-up the laser perfectly vertical with the hole on the board, using a small piece of string with pointed weight at the bottom. Normally, these lamps allow hight adjustments of up 1 meter (some of the better old style ones), so the lump of wood can be of some considerable size. By laying the lump of wood of the centre of the board, where the red laser dot is pointing, you know that the other end of the laser is pointing where the small hole is in the bottom board. After making the necessary adjustments, when is time to mark the centres, 2 ways it can be done, one you simply mark it with a marker pen, or if you want to have a more "visible" markings, use a 2" to 3" nails with small head to nail "slightly" one through the bottom hole, and the other on the red laser mark at the top. This way when you lift the lump off the board, you get the nail through the board, allowing you to have a good idea where and how the "invisible" line runs through the lump of wood.

If an optimum circle (best use of the lump of wood) is necessary, I use the same technique but this time with a small hand/pencil torch fixed to the desk lamp, the same way as the laser. You need one of those torches that have adjustable beam, lots of those pencil torches do. By having an adjustable beam, you can easily open or close the circle to get the most out of the lump of wood, needing only to mark the outside beam line with a marker pen or a pencil.

You can also set-up all this in the work bench top, if a clearance is possible to swing a hammer from under the bench top to drive that bottom nail up slightly on the lump of wood. The only problem I found using the work bench top is that, sets everything up too hight for a clear view from the top, compared with the low work-horses, and that clear top view is very helpful and important for this job.

If this answer your question, I'm pleased, if not, I'm sorry!
Hopefully, this will help someone!

Cheers:2tsup:
RBTCO

[pitbull]

Great tip on using the torch robutacion, i'll keep that one filed. :2tsup::D

[Gil Jones]

Hi Ern,
Excellent idea!! Now I finally have a reasonable use for that 6-rpm geared motor contraption I bought 'cause it seemed a good thing to have at the time. Plus, I have just enlarged my 10" (254mm) Donut Chuck disk out to 20" (508mm), and it will provide the perfect turntable when needed.

My only problem with all this "best center finding stuff" is that we have this one flat surface as a reference plane, while it may well be cut at the wrong angle to let us find the opposing center point, AND still make the best use of ALL that marvelous lump of wood. I guess I could alternately place wedges under various points of the flat surface to shift the reference plane angle, and find a “better” opposing center point.
I am thinking of inserting a long, slim, live center in the center of the rotating platform as an easily shiftable bottom center point for the flat surface.

This can become more complicated than it needs to be, and that may be when a beer is in order.
THANKS :)

[rsser]

Good points Gil and Joe.

The point of the U - arm with drop point Joe is really to measure where you otherwise only guess by eye whether your two valleys and two peaks on a nat edge piece will be level with the lump at a given centre. The drop point is essentially tracing where the rim will end up.

Not sure I'm explaining this well.

[Manuka Jock]

:worthless:


bring out ya prototypes , me minds eye is on leave :p

[Cliff Rogers]

All depends I reckon....

It depends on what you & that lump of wood agree on.

If you have a small/light/mobile lathe, then you have to aim for the best balance & if that doesn't suit the shape you want, then you have to hack bits off the lump to get it closer to balance.

If you have a monster/strong lathe that doesn't care if the piece is out of balance, then you can be more 'artistic' with the final shape.

Finding a balance can be done roughly by sticking the lump between two freewheeling centres & pushing it around by hand & moving one or both of the centres til it balances.

A sheet of ply/MDF with a mob of concentric circles & a hole in the middle for a marker will let you find the middle of the top face in a hurry for a maximum width bowl/platter.

A square sheet of ply/MDF with parallel horizontal lines across it stood on edge behind the lump lying on a bench will let you 'eyeball' the highest point if you want the deepest bowl available out of the piece & if you just push the lump around on a sheet of ply/MDF with a couple of lines drawn across it that intersect at the hole in the middle & line up the high point with the intersecting lines & mark the centre through the hole, that will give you the deepest bowl available out of the lump.

[Ed Reiss]

Guys...good thoughts on all the variations for finding a "good" center line, they have promise.

However, since I'm old school, I always start irregular stuff between centers just using a live center in both the headstock and tailstock with a 12.7mm dowel cut to 6.35mm long to act as a pad between the live center's point and the wood....don't want to put holes in the wood yet.

Then by trying different positions it is possible get damn close to the centerline or offset if you want to include a particular feature of the piece in the final product. Once the centerline has been established, I spray around the dowel with spray paint to mark the location, loose the piece from the lathe, put the dowel pads aside, put a drive center in the headstock then remount the piece using the round paint marks as a bullseye. Then it's ready for a tenon at one end for the chuck.
Sounds complicated, but really only takes a few minutes...piece of cake!

'Ern...be looking forward to some pics of your jig.

[Frank&Earnest]

I wrote a 300 words answer to the wrong question, then I read your post again. :-
Sorry I can't contribute, I do not do nat edges. Too kitsch for my conservative taste...:D

[joe greiner]

A combination of Ed's technique and the "bookend" dead centres I used here: http://www.woodworkforums.ubeaut.com.au/showthread.php?t=57412 might be made adjustable, I think, by making it all into a jam chuck of sorts. Add pieces of plywood or MDF to prevent penetration into the workpiece itself until satisfied. When all criteria of form and balance are good, transfer the preferred centres into the blank. The piece could be turned by hand slowly, and let "drift" identify optimum balance.

In fact, I wish I'd thought of this a few weeks ago, when I drilled an odd-shaped blank for a Cliff Rogers socket. Balance is now hopeless. I'll be carving with the Lancelot for a day or two or three. If it works, I'll post a new thread entitled "Odd-shaped Blank" in honour of a similar title from Skew a while back.

For a more orderly blank, where the top is better defined, the Lazy Susan still might be easier.

Joe

[rsser]

Essentially the drop rod is to measure two valleys and two high points, so would need to be marked and retractable.

A nat edge lump isn't hard to centre by eye if it's regular (ie the bark surface is a regular radius and height 'tunnel' ) but often interesting lumps aren't. And nothing looks worse IMO than a 'hipped' rim.

Mounting btwn centres with a movable marked rod mounted || to the bed and height adjustable would do the same job but many of my lumps are too big for easy repositioning. And mounting btwn centres would be hard to adjust with Gil's point in mind, ie. you need to change the cut side plane.

Sorry MJ - dunno how to use Sketchup .. will see what can be done with pencil and paper ;-}

[wheelinround]

rssr easy as pie a frame U shaped with extendible diameter and height mounted on one side a nail which is the side you can mark easy. The other a laser beam lined up with the nail point. :2tsup:

[Robomanic]

Essentially the drop rod is to measure two valleys and two high points, so would need to be marked and retractable.

A nat edge lump isn't hard to centre by eye if it's regular (ie the bark surface is a regular radius and height 'tunnel' ) but often interesting lumps aren't. And nothing looks worse IMO than a 'hipped' rim.

Mounting btwn centres with a movable marked rod mounted || to the bed and height adjustable would do the same job but many of my lumps are too big for easy repositioning. And mounting btwn centres would be hard to adjust with Gil's point in mind, ie. you need to change the cut side plane.

Sorry MJ - dunno how to use Sketchup .. will see what can be done with pencil and paper ;-}

What about rods which are held in a keeper which lets the rod swing? You could slide the rod through the keeper to get depth, but the swing limit could be adjustable so it can only swing in so far. You could place the rod near your point on one side, swing it away (maintaining depth) spin the piece around then let the rod swing back in to its stop. The stop would make it fine tunable and repeatable on each side. My next couple of bowls will be nat' edge and i like your idea a lot Ern, I can seriously see myself making one.

[rsser]

You got the idea Shannon.

Swing rod will be patent no. 2 ;-}

Damn good idea.

[rsser]

Here's a related string of thoughts re irregular lumps and processing them:

You've got a half round log you want to cut into spindle blanks on the bandsaw.

The edge of the cut face is of course irregular.

You need a straight edge cos you want a consistent width of first cut.

I've been producing the straight edge by getting out the electric planer. Takes time; makes a mess; wears the blades.

How would you go instead by projecting a series of in-line points onto the log by means of a rack of laser pointers mounted under the bandy top-wheel housing? The rack runs || to the side edge of the table.

Pointers are cheap. The spots tell your eye roughly where the cut should go. May only need say 3 on the infeed side and one on the far side of outfeed.

What d'ya reckon?

[RETIRED]

I just make a cut down the centre of the log and use that as the edge.

[rsser]

Cut with the bandy?

You have steadier hands than mine; though maybe the bandy is working better than mine as well :rolleyes:

Time to extract the digit and mount the Henry Bros 3/8 bimetal blade and then readjust every friggin thing on the saw.

[RETIRED]

Yes.

[Cliff Rogers]

Stick your irregular log/lump on a sled, just a bit of ply or MDF with a straight edge.
If it is light, you may have to secure it to the sled with a couple of counter sunk screws through the sled or, if you don't want to have screw holes in the blank, use hot melt glue.

[rsser]

Ta Cliff.

Yep; grind a bit off the right end of the fence rail to get a straight line. Keep the outfeed roller inside the straight edge. Maybe double sided tape between sled and log, or router mat or some points.

[Robomanic]

You got the idea Shannon.

Swing rod will be patent no. 2 ;-}

Damn good idea.

Thanks. When it comes to good ideas - I'm full of it :;

[rsser]

Just came across a tip in American Woodturner for this task.

Tack a length of cardboard on top of the log to guide the cut.

And if a round log, screw a jointed bit of timber to the log parallel with the blade to help stop it twisting while cutting.

Sigh, damn simple.