How could I make this curved conrod?

Yes. They were very successful in sports car racing in the early thirties.

Russell

I've made several of these over the years and have always used a method similar to that laid out above by Phil H1.

Quickest and simplest way.

Roger

Brilliant answers covering a wide range of possibilities!

Very helpful to know that the form I'm asking about is 'Fish belly' or 'Barrel Shaped'.

Apologies, if I missed anything but in order of accuracy the techniques you've described are:

1. Approximate the shape by blending tapers. I accept that this is plenty 'good enough' in most cases. (Phil, Jeff)
2. Calculate the diameters at a series of points along the rod. Cut and blend them. This would be much closer to the specification. (Jason & Chris)
3. Bend the rod to the required radius. May not work on a short stubby rod. (Russell. ega & Jason)
4. Free the cross-slide and follow a template or linkage (Clive). I like the template approach because (as shown in Nobby's pictures), it has many applications - different shapes can be made and copied with it, but making one is quite an investment (Jeff, Nobby, Raglan, Mike)
5. A ball turner variant. (Hopper)
6. A CNC or hydraulic lathe programmed to cut curves. If only I had one! (Andrew, Jeff)
7. Grinding. This, I guess, is how such shapes are mass-produced. (Sam)

Neil highlighted a problem with perfectionism. Inspector Meticulous would not be satisfied with just an R300 fish belly: he would insist on sharp ends as well! The more I look at problems like this the more I appreciate what goes into exhibition standard work.

Special thanks to Jason who can remember advice he posted in 2014 (I can't remember what I did yesterday) and also to Harold Hall for the link to his website where he provides much helpful detail on 'barrel shaping', including a table of coordinates.

I have to cut grass next but will report back after I've made a real one.

Many, many thanks,

Dave

Edits, typos...

Edited By SillyOldDuffer on 08/10/2017 11:36:23

Edited By SillyOldDuffer on 08/10/2017 11:38:15

Any Inspector Meticulous worth his salt would sack the designer for creating a poorly engineered and difficult to make part with no fillets.

Andrew

It does make me wonder how many designers fail to think about difficulty of making the thing. Or of maintaining it! Most of the cars I've owned had difficult to reach fasteners, some much worse than others. After failing to get a water pump off I took it to a garage. 'How will you do that?' I asked. 'It's impossible - you have to take the engine out' he replied. Most of the bill was labour costs plus VAT...

Another picture of Nobby's method. The cross slide stays engaged, just follow the template with the dti using the crosslide handle. The only work involved is cutting out the template and mounting it in the tailstock chuck.

Hi Raglan
Thank you for posting this photo of my method . I found works quite well
Nobby

Hi Nobby,

Thanks for that. I thought that picture might help to show how quick it is to set up.

For the benefit of those of us who don't know how a hydraulic copy attachment works (apart from it copies a profile by some magic means), how about someone writing an article for Neil?

I thought Andrew had already done an article on the copy attachment.

Phew, saves me blowing my own trumpet.

MEW 167 September 2010 - Fitting and Using a Hydraulic Copying Unit

MEW 256 July 2017 - Making a Valve Rod Coupler (uses the hydraulic copying unit)

Andrew

Thanks from me too - I'd jumped to the conclusion that Nobby's method was complicated because the cross-screw has to be disconnected. Wrong again! Actually, now I've finally 'got it', following a template in the tail-stock should be very easy to do. I particularly like that templates can be used to create copies, useful for example when building a multi-cylinder engine. I shall have a serious play with Nobby's method tomorrow.

Cheers,

Dave

Useful things Spreadsheets.

Whilst having no relevance to machining a barrel shaped conrod some may be interested in the use I eventually put the spread sheet to that I developed for the task.

I had decided to make a dining table and felt that a slightly curved edge, lengthwise, would add some character to the outcome. The usual method of a piece of string anchored at one end and a pencil at the other was not practical as the string needed to be far too long. I then realised that the spreadsheet I had produced for the conrod could be used. From the result I produced a template for using with a hand held router. Having developed it for a radius of around 400mm it was then used to provide the coordinates for a curvature of about 25Mtrs, values from memory.

Should anyone like to see the end result its on my website here. **LINK**

Harold

Cannot imagine any of my joiners using a spreadsheet normally just 2 nails & a bent batten did the job

That's an interesting idea Sam. but then, I have not been a cabinet maker as a trade. Will though consider it should I have the need again, though unlikely. In any case, the spreadsheet already existed so it came to my rescue. Incidentally, neither was I a metalworker, but working as an electrical systems design engineer in large power, multi motor projects. In any case, for me, working out a method to achieve something is part of the pleasure in making something, even if its not the best method.

Harold

Hear, hear!

Harold

There is absolutely nothing wrong with doing it the way you did it & you have clearly achieved your aims with perfection. However, if one imagines an absolute beginner following your comment & then wondering how to set up & work the spreadsheet then setting it out do you not feel that he/she would be put off at the first hurdle.

if he knew that he could do the job in 90 seconds with greater accuracy with a simple tool do you not feel that he would be more likely to proceed.

I sometimes wonder about some of the things I read on this forum. As an example I read about forumites using Fusion to draw stuff first then make it & I get lost at just trying to get a free copy of fusion let alone using it. Then I begin to loose interest in the project before I even start.

I know that is a wild example but can you not see what I am saying. As a beginner I want to know the easiest way not the most complicated way known to mankind.

Once I can do it then i will look at ways of refining the process & getting perfection. For now I just want to make something that at least works !!!

so 2 pins & a bit of batten it would be- worked for my father & grandfather

Now I wait to get slaughtered !!!!!!!

Edited By Sam Longley 1 on 09/10/2017 08:29:20

.

What you describe is, I believe, dignified by the term 'spline' ... and it provides a geometrically elegant solution, requiring no hard sums !! ... That's good engineering in my book.

MichaelG.

.

P.S. ... It does not produce a segment of a circle; but something much more sophisticated [and also very appropriate to the con-rod situatin]

Edit: For reference https://en.m.wikipedia.org/wiki/Spline_(mathematics)

... Skip foorward to the 'History' paragraph.

Edited By Michael Gilligan on 09/10/2017 09:45:55

Edited By Michael Gilligan on 09/10/2017 10:11:20

From Ernest Joyce's Funiture Making:

"For the drawing of long sinuous or continuous curves a spline made from thin plywood, cut so that the grain of the outer skin lies across the width of the spline, is indispensable. It is a two-man job to use it."

Perhaps father and grandfather did it together!

This addresses the point that the "batten" needs to bend evenly if geometric accuracy is needed.

I tend to keep a couple of thin strips cut from the edge of a sheet of MDF as it is consistant so you get an even bend. I manage OK on my own, bit of hot melt glue will tack the batten in place so you can run a shallow groove with a laminate trimmer, then bandsaw down the middle of the groove and finally cut to the edge of the groove with a flush trimming bit.

I also use CAD for getting a set of co-ordinates, did an oval table top that way recently so there is a good reason to draw it out first. Plus the fact that most joinery works of a reasonable size these days will have a good size overhead CNC router which will cut any radius be it constant or variable.

If you have a decent DRO on your lathe you can use that to generate the cuts for a 300mm radius, just tell it the centre position if about a foot away from the lathe axis when you use the ARC function.

Edited By JasonB on 09/10/2017 10:26:04