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Taper on connecting rod

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Richard Brickwood30/03/2020 20:16:42
5 forum posts

Hello - I want to reduce the diameter of a connecting rod from 1/4" to 3/16" over a distance of 1".

I've tried setting ther topslide over but it still cuts almost parallel even at 10 deg. I've tried from 2.5 deg upwards.

What am I doing wrong?

Nigel Bennett30/03/2020 20:22:56
337 forum posts
11 photos

Are you actually twiddling the topslide handle rather than moving the carriage?

Andrew Johnston30/03/2020 20:25:40
5425 forum posts
629 photos

The angle needed on the topslide is about 1.8 degrees. So I suspect a set up problem. A picture would be helpful. Less likely is that the work is deflecting. Is the turning exactly parallel or just close?


Richard Brickwood30/03/2020 20:38:33
5 forum posts

Hi Nigel - nope - moving the carriage.

Hi Andrew - things to do when not gliding! -, I thought it was about 1.8 degrees offset but got very little change in diameter over the 1". Luckily using scrap BMS as test pieces rather than castings! 'Almost' parallel is about 5 thou.

roy entwistle30/03/2020 21:01:11
1154 forum posts

Moving the carriage is your problem

old mart30/03/2020 21:14:20
1557 forum posts
136 photos

Turn the topslide 1.8 degrees anticlockwise from zero as Andrew suggests. Set the tip of the tool right next to the end of the work. The work should project the minimum possible from the chuck, say 1 1/8". Lock the carriage (saddle) if you can. Then adjust the cross slide to allow small cuts using the top slide. Each cut should reach a little closer to the chuck. Don't try to cut too deeply, or the work will probably bend with the force. Test on scrap metal until you get good at it. Good luck.

Mick B130/03/2020 21:51:32
1557 forum posts
83 photos

As said above, you certainly need to run the tapered cut with the topslide, not the carriage.

Consider whether you could put a hole in the little end to use with a running centre for tailstock support. If you can, it might still be a fiddle to create clearance to turn the topslide handwheel, but tailstock support would make turning a taper on the conrod a good deal safer.

Andrew Johnston30/03/2020 22:56:16
5425 forum posts
629 photos
Posted by Richard Brickwood on 30/03/2020 20:38:33:

Hi Andrew - things to do when not gliding!

Sounds like you've got the taper issue sorted. Could be a lack of currency? smile

I'm probably the most current pilot at Gransden as I flew my DG200 the Thursday before last (20th) at Milfield where I was retrieving it, fortunately just before the lockdown kicked in.


Richard Brickwood31/03/2020 09:37:12
5 forum posts

..............this all makes sense - thank you: will spend a happy afternoon turning BMS into swarf - and let you know the results.


John Hinkley31/03/2020 09:49:24
853 forum posts
287 photos


Glad you've got your taper turning sorted. However, I'd be inclined to investigate why your lathe is turning an (albeit slight-ish) taper of 5 thou over a distance of one inch when just using the carriage? Unless I've gotten the wrong end of that particular stick.


Richard Brickwood01/04/2020 12:24:47
5 forum posts

..thank you all for responding - the problem was definately feeding using the carriageto feed instead of the top slide.

Made one from scrap yesterday - perfect (well amost) - more practice today.

John - re your comment on the slight taper - it was my measuring error and poor recollection when writing: the real error is negligible - something like 0.0001 - but thanks for the suggestion.

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