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Machining Crankshaft

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Ady106/02/2012 10:52:43
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The idea of milling away as much as possible has the same advantage as grinding, the cutting loads are less than for turning

That does seem to be the best system for hogging out most of the material, the shaper would be more fiddly.
 
The other problem is stiffness between the centres, you need to squash the workpiece between centres, which becomes a big issue as you hog out more material.
 
Perhaps holding the workpiece between two four jaw chucks would be better, certainly it would be less stressful
If you only move a single jaw on the 4 jaw when revolving the workpiece for the next offset crankpin, so you reposition the WORKPIECE, and don't move 3 jaws, then you will get...well moderate concentricity
The original setup between 4 jaws will need some dial indicator work.
 
I made a revolving centre for my chucks to fit on the tailstock(great for hogging down wonky bars of metal), a smaller one would be more sensible, you can see the general thinking.
Thats a three jaw in the tailstock example, obviously I would use a good small 4 jaw...if I could afford one.
So with a good quality revolving centre in the tailstock you have stiffness and security, but very little crushing force on the workpiece.
 
Final high accuracy finishing work with light cuts and a carbide quality parting tool can still be done between the centres
 
 
2cents

Edited By Ady1 on 06/02/2012 11:14:00

Ady106/02/2012 12:06:52
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For positioning the workpiece a centre which protudes only slightly from the middle of the 4 jaw body would make setting and resetting the job miles easier.
 
This photo is obviously not what I mean, each 4 jaw would have a specially made centre where the workpiece could be positioned easily between these centres, and then the 4 jaw chuck jaws secure the workpiece in position
Saves messing about checking stuff once the initial work for the holes for the offset centres have been done.
 


To get the holes set accurately at each end I would do one end, then make a template of the first end, which I would transfer to the other end for marking out.
That would be pretty fiddly and need some thought though...The holes at the opposite ends really do need to be spot on.
Maybe better men than me could help out here, since these holes are quite critical for accuracy and a good crankshaft
 
 
Sooo
Set your holes up properly
hog out most of the material with a small milling cutter
finish off with a good parting tool
 
Seems to be the easiest system for an amateur
 

JasonB06/02/2012 12:43:08
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Or just clamp one end in a keats angle plate and move that about on your faceplate until the journal is true, this gives support right upto the web and very little overhang.
 
J
Stub Mandrel06/02/2012 19:33:41
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I've seen Jason's partiing tool before - recommended by LBSC, and I can confirm it works.
 
Neil
John Bentley06/02/2012 21:59:13
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John (Bridge) didn't actually say that his crankshaft was unusually hard and in fact mentioned that he had already removed the surplus metal at the weld. It wouldn't surprise me if he has finished the job successfully by now.
 
I have not found the Stuart crankshafts particularly flimsy while turning them normally. They cut very well without distortion in either my Peatol or my Chinese mini lathe. Similar freelance versions (up to three throws) made from solid bar have turned out equally well for me. I didn't have any experience when I started out and haven't had any trouble at all with conventional crankshaft designs like these. Success is probably the result of no more than using sharp tools at proper height with gentle cuts at sensible speed.
 
Jason's recommendation of LSBC's tool looks like a good one and I will be trying it out next time.
 
John
 
JOHN BRIDGE 106/02/2012 22:13:47
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Posted by John Bentley on 06/02/2012 21:59:13:
John (Bridge) didn't actually say that his crankshaft was unusually hard and in fact mentioned that he had already removed the surplus metal at the weld. It wouldn't surprise me if he has finished the job successfully by now.
 
I have not found the Stuart crankshafts particularly flimsy while turning them normally. They cut very well without distortion in either my Peatol or my Chinese mini lathe. Similar freelance versions (up to three throws) made from solid bar have turned out equally well for me. I didn't have any experience when I started out and haven't had any trouble at all with conventional crankshaft designs like these. Success is probably the result of no more than using sharp tools at proper height with gentle cuts at sensible speed.
 
Jason's recommendation of LSBC's tool looks like a good one and I will be trying it out next time.
 
John
This is correct John it was not unduly hard and I had removed the surplus, I have made a tool similar to the pic. and it is quite easy now to machine this journal it is now quite smooth but is still in the 3 jaw and revolving center in the tailstock there is still plenty of meat on the journal and I now intend to transfer to between centers and finish off this shaft, I am now quite confidant that I can now do the job. Like you I find the shaft quite sturdy and I do not intend to use any stengthening whilst turning.
John

JOHN BRIDGE 106/02/2012 22:31:23
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JasonB is the total width of that blade 3/32", I would like to give it a try, what sort of grinder do you use to do the fine shaping, my grinder is a clarke 6" which seems a little large for this job.
John
JasonB07/02/2012 07:52:14
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You can either do it with a parting blade like the Eclipse ones which are nice and deep but they lack side clearance so either set them up dead true with a DTI or grind from a HSS blank but where the neck is narrow is a weak point.
 
Width would depend on the journal width buy I would say something like 1/3 of the total. And as you are only taking a few thou a pass with this tool us ealternate left & right hand tools to do most of the work.
 
I only have a 6" bench grinder and a clarke belt/disc sander. a diamond stone will help with refining teh profile
 
I actually took the tool profile from Malcom Frosts (the M of MJ Engineering) traction engine articles no LBSC but guess that is where he got it from.
 
J
KWIL09/02/2012 21:06:05
3681 forum posts
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Jason has asked me to post this photo of the crank journal tool using round inserts. Setting is straight forward as a ground tool, if it needs sharpening, just rotate the inserts!
 

 
I hope you find it interesting
 
K

Edited By KWIL on 09/02/2012 21:06:31

JOHN BRIDGE 109/02/2012 21:18:01
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Thanks Kwil, It looks like a very useful tool if I ever get to do anything useful with this lathe of mine.
John
JasonB10/02/2012 18:23:41
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I came accross this thread which shows a tool ground like the LBSC shape, may give you a better idea of how its done
 
J
 
PS Thanks for adding the photo KWIL.
Stub Mandrel10/02/2012 21:29:35
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Wow Kwil, either those are very diddy inserts or that's a meaty crankshaft 3 or 4" Traction engine?
 
I like the idea of grinding crankshaft journals. A set up using my rotary table on my grinder appeals, but I'd need a different shape wheel. With a diamond wheel a mirror finish could be assured.
 
Neil
 
KWIL12/02/2012 10:53:01
3681 forum posts
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Neil,
 
They are 5mm diameter and it is a 4" Traction engine, throw is 1.5". The crankshaft was left +0.0005" for grinding/lapping.
 
K
Stub Mandrel12/02/2012 19:49:34
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Thanks Kwil,
 
Canme across a Tubal Cain crankshaft turning tool today in his Sirius series. HSS with teh double lobed end, but like your it is parallel, not tapered like a parting tool.
 
Neil
Dusty12/02/2012 20:59:20
498 forum posts
9 photos
It has long been known that carbon steel tools can be made with a much keener edge than H.S.S. tools. There is however a downside in that they do not maintain this edge very well, basically they wear, as they wear the the edge is lost and finish and dimensional stability of the workpiece is lost (it gets bigger). This is especially true of mild steel EN3( I would not attempt anything harder) and some of the forged crankshafts we use. If you do use Carbon Steel tools, speed must be reduced and the tool flooded with cutting oil. The other problem with Carbon Steel tools is they can easily loose their hardness when grinding by overheating.
I make my bifurcated tools from parting tool blades and put the groove into the front of the tool with a Dremel/Proxxon and a cut of wheel.
JOHN BRIDGE 112/02/2012 21:08:14
104 forum posts
11 photos
Dusty, Thanks for this post I was contemplating using a Dremel to cut the groove as I could see no way of doing it with my 6" clarke.
John
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