Offset turning query

**cultana** · 29th March 2010, 02:00 AM

I have spent some time tinkering with various versions of some arms for a chair. So far most have hit snags or turned out not that effective. These range from some square to round ending with a square end, looked good but still missed something; a long oval taper, almost made it but the point of linking to the back just was not there. Hard to say but looked wrong.

Some of these exercises have ended up as odd bored pieces.

Well I have finally hit on a solution but it has a few little catches. It is two ovals at 90 degrees. One will flow onto the rear leg/back and the other will end up as flat arm rest, tapered or not, have not decided.
Sorry can visualise it but not easy to draw.

Some details: The legs, front and back are 40 mm wide where the armrest will meet them. I am looking at making the armrest out of 70 x70mm block of laminated timber, 3-5 different colours mood, (mine), dependent.

Now my real big query is what sort of off set should I be looking at or in reality how does one determine the needed offset to turn these things.

I am looking at a finished oval of about 60 - 70 mm, (error handling), wide and a thickness of 40 mm.

In the thread https://www.woodworkforums.com/f8/i-got-bored-115356/ the piece, THING, has the two ovals at 90 degrees was about 80mm in Diam and the offset for turning was in the range 10 - 11mm. The end result was something what I want but that sort of offset hammers the hand holding the chisel a bit.
If I make the offset it too small I will end up with a fatter oval and that is not what I want.

So simple query how do I determine the correct off set to do this turn, without making many test pieces?

OK I admit it I am sucked into this wood turning thing...

**RETIRED** · 29th March 2010, 07:12 AM

Draw it out full size (Ken will love that) using a compass to determine the centres and the amount of ovality or as someone here once said ovulation.

It gives you an idea of where to start without destroying a heap of timber and you always have the reference to go back to each time you do another.

This may help: Parameters for Drawing Ellipses or Ovals with Pin and String with Calculator

**hughie** · 29th March 2010, 08:37 AM

This may help: Parameters for Drawing Ellipses or Ovals with Pin and String with Calculator

Now that brings back memories of Tech drawing classes of yesteryear

**RETIRED** · 29th March 2010, 10:13 AM

Originally Posted by hughie

Now that brings back memories of Tech drawing classes of yesteryear

Oh yeah.

**cultana** · 29th March 2010, 11:15 AM

Originally Posted by hughie

Now that brings back memories of Tech drawing classes of yesteryear

Now I gather you will have nightmares about T Squares, set squares, using hard and soft pencils properly and all them little nasty things about tech drawing of yesteryear.

**cultana** · 29th March 2010, 11:22 AM

, Ok done the drawing bit but deciding where the offsets goes is still a bit confusing.

Attached PDF is 1:1. The dimension text did not come out but the big diam is 60 mm small diam is 40 mm.

The pin points are marked out for one of the ovals, short red CL mark.

I have not shown the transition between the vertical oval to the horizontal one as this is a gray area on how it will finally end up

**RETIRED** · 29th March 2010, 12:47 PM

Originally Posted by cultana

, Ok done the drawing bit but deciding where the offsets goes is still a bit confusing.

Attached PDF is 1:1. The dimension text did not come out but the big diam is 60 mm small diam is 40 mm.

The pin points are marked out for one of the ovals, short red CL mark.

I have not shown the transition between the vertical oval to the horizontal one as this is a gray area on how it will finally end up

I would firstly cut the timber to length plus about 20mm (to cut off centre marks).

Mark out the centres for both turning a cylinder and your offsets as per your drawing. Don't forget that centres will vary for the 60mm and 40mm so make sure you mark both ends for each and number them.

Mount in the lathe using the true centres, mark the centre of the length and turn the ends to your diameter (60mm and 40mm) plus a little bit for sanding.

Using the tool rest as a straight edge mark the centre line down the length of each "high" spot in the oval.

Using the previous marks (offset) turn the ovals a bit short of the middle so that the centre remains circular.

Remount on true centre and blend the 2 ovals into each other.

A little bit (all right, a lot) of hand sanding should finish the job.

**cultana** · 29th March 2010, 01:23 PM

Thanks for that.
So I am looking at an offset from centre for the 60 mm oval of 22-23 mm

Looks like a serious case of a test piece. Was going to do that anyway.

Blending is going to be fun

Only lots of hand sanding?

Big lots!

**RETIRED** · 29th March 2010, 01:55 PM

Only lots of hand sanding?

Big lots!

Shheee you want everything. You didn't think it was going to be easy didja?

**cultana** · 29th March 2010, 02:35 PM

Originally Posted by

Shheee you want everything. You didn't think it was going to be easy didja?

Never occurred to me it was going to be easy. I don't have, like many here and yourself, huge experience to fall back on.

The sanding was not a concern. It was your understatement on sanding, lots is an understatement!

**cultana** · 30th March 2010, 12:33 PM

I just finished my first test run to see how messy this is going to be. The main part is the transition from one oval to the one perpendicular to the first.

The short section oval goes onto the back leg and turned a peg to fit into the leg. I could remove it and just run with a normal dowel but may effect the connection strength vs a fully integrated pin.

The biggest problem is getting the centers to go in on the correct point. In this test one centre was out a small amount and did cause a slight off line with the oval that would butt onto the back chair leg. As this section is small it may not be a big issue but like all things there is that BUT!

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