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Fine Finish on Crankshaft

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JOHN BRIDGE 111/02/2012 10:52:29
104 forum posts
11 photos
I am trying to finish the Crankshaft on my Stuart Twin I have all sufaces down to about 10 Thou of finished size but the finish is not how I would like it is not nice and smooth but a little rough, I am using tools with inserts, should I be using a different type of tool or is there some other method of obtaining the final finish.
John
colin hawes11/02/2012 15:56:41
570 forum posts
18 photos
I agree that a sharpened HSS tool will produce a better finish in this instance provided it is ground with a good rake in all cutting directions.Hard-tip tools are ,in my opinion, very much overrated for use on most materials used in our hobby.Cast iron is the most likely metal give you the benefits of carbide tips .HSS tools are easy to sharpen and that is most important for a good finish.
Colin
Roderick Jenkins11/02/2012 17:08:29
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2376 forum posts
800 photos
Tubal Cain argued in "Hardening,Tempering and Heat Treatment" that silver steel can be given a harder edge with finer grain (therefore sharper) than HSS. With fine cuts that don't overheat the tool this may be the way to go.
 
Just a thought.
 
Rod
Steve Withnell11/02/2012 18:08:52
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858 forum posts
215 photos
On my hobby lathe I usually use carbide tip tools for roughing and finishing cuts with HSS/Silver Steel hand ground for the job at hand. Cutting charts for speeds and feeds are only a rough guide. I was struggling to get a fine finish in a 25mm cast iron bore, solved by running at 50rpm with silver steel tool (as advised by a professional. Compare that to a standard speeds/feeds chart.
 

 
 
Russell Eberhardt11/02/2012 21:28:49
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2785 forum posts
87 photos
As others have said, sharp tools are essential for a fine finish. It is also important to ensure that there is no vibration anywhere. Put your hand on the headstock while the lathe is running. Do you feel any vibration? If so it will be transfered to the work. It is also important to make sure that the tool and slides are rigid with no play. Lock any slides that are not beig used and make sure the tool is rigid enough.
 
Russell.
mgj11/02/2012 22:30:33
1017 forum posts
14 photos
The way that got me a very nice highly polished surface on a TE crank was as follows.
 
1. Make spacers or braces so that you can stiffen or block any bits that are not being machined. Tubal Cains book Workholding in the lathe shows this very well. Rigidity is key, especially as one is likely to be turning between centres.
 
2. Then with a nice deep and narrow parting tool, ground with a radius on each side and with a face truly flat, I machined each section down to about .002 oversize, but a little bigger wouldn't harm.
 
3. Make up a dummy split bearing, and attach to a dummy conrod. coat each journal in turn with valve grinding paste, and jsut lap in with the lathe at low speed. You just keep tightening the "bearing" until it is exactly to size.You don't even need to hold the end of the dummy conrod. I rested it on a bit of wood on the top of the compound slide. Better than holding it.,
 
4. Finish/polish with fine emery and oil.
 
Lovely and shiny, dead parallel.
JOHN BRIDGE 115/02/2012 15:22:03
104 forum posts
11 photos
Decided on this tool in the end, quite pleased with it, my first attempt at grinding my own Hss, I will improve.
 

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