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Any Tips For Machining A Small Cast Crankshaft?

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tomw

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#1
Dear All,

I am building the PMR #5 steam engine. It comes with a cast iron crankshaft. I am now trying to machine the crankshaft, and running into some chattering problems. He just keeps talking and talking about his difficulties with fairer sex. I feel like slapping him, but he obviously has not responded to that in the past.

This the cast piece as I received it:
Crank blank and drawing.jpg
Also note the drawing.

I am trying to machine the main shaft. As I said above, I am getting a hell of a lot of chatter, chitter and whining. This is really annoying.

This is a photo of my setup. The steady rest was put in to try and the reduce chatter. It kinda helped.
Crankshaft set up.jpg

This is a photo of the machined surface of the crankshaft. It's very ugly. It's ugly like that dame's ex, the one that shot me in that dark alley.
Crankshaft close up.jpg

I have tried carbide insert tools and HSS tools ground for CI. The HSS tools were ground with 10-12 degree angles for side relief and front angle. The back rake was about 8 degrees. The front cutting edge angle varied from 80 to 60 degrees (the point as you look down on it).

I am spinning the piece between 200 - 450 rpm. Any faster and it starts to wobble like my drunk uncle that had ADHD.

All operations are being done on a Sherline 3.5" lathe.

So, my questions are:

1) What can I do to reduce chatter?
2) See question 1.

If your answer is "buy a bigger lathe" I will have to ask for your credit card number.

Seriously, what about trying different tool angles, or a different set up, or making a virginal sacrifice to Hephaestus? I'm assuming the sacrifice would involve virgin metal.

As I said, seriously, what can I do with the equipment I have to reduce chatter and make a nice looking crankshaft.

Thanks tons,

Tom, your humble beggar of advice.
 

ARKnack

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#3
I really don't know the answer. It may need a follower rest. My self, I think I would continue to rough it out with a hss bit. When close I would grind it to finish size. You may be able to rig up a Dremel tool for that. The Sherline lathe is very popular and I'm sure you can find someone that did by Google or YouTube.
 

Wreck™Wreck

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#4
This may seem silly but, use a piece of wood or plastic stock and hold it by hand against the turned surface opposite or above the tool whilst turning, the more massive the better, this works well towards dampening vibration.
Old School
 

francist

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#5
This is a complete shot in the dark as I've never turned a crankshaft before, but I wonder if part of the problem is the offset nature of the crank? It's not a continuous straight length (duh), and does that contribute to the flex in the part? If so, would tapping a small packing block into the gap between the cheeks while you're turning it gain some rigidity back?

Like I said, total shot in the dark.

-frank

Ignore above. RCDon shows the support pillar still in place while turning and I'm guessing yours is too. I just couldn't make it out in the photo. -f
 
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Sandia

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#6
Tom, take a look at this website. This is a build on a Redwing hit and miss engine. Click on the Experimental Projects tab and then the Redwing engine. There is some quality info that will probably help you out, including turning the crank shaft.
rcdon.com

Bob
 

Paul in OKC

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#7
Couple of thoughts. The length of the tail stock extending is quite a bit, and I know much can't be done with that. Do you lock the tail stock once set? try unlocking it. Also how hard are you setting it in the part. If pretty hard, might be able to back off a bit. Sometimes we can make things too rigid for our own good. Maybe turn your tool post 90* so you can mount the tool on the 'back' side and move the carriage closer and allow the shortening up of the tail stock. Just some thoughts.
 

timmeh

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#8
You may not be able to, but, slow the spindle down, 1/2 or even a 1/4 of what you've been using.
 

TommyD

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#9
I think that is the nature of beast when turning cast, milling too. I can remember rubbibg a flat on a carbide tipped tool bit trying to figger feeds and speeds. I think I had the best results with a decent radius on the cutting edge of the brazed carbide toolbit, diamond ground. This was AFTER the skin was removed. Try the follower, as mentioned. I found that, sometimes, hanging the heahy tool wrenct on the nut head of the toolpost (we ALWAYS had the old style, single tool holder on our lathes) seemed to stop vibration JUST enough.

I think grinding would give the best finish. Another reason to buy more toys......I mean tools.:congratulate:
 

tomw

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#11
I had a thought, which is always dangerous. My thought was that if I can get the shaft machined to within, say, 15 thou of the final dimension I could use a file to get the rest of the way.

Unfortunately, this material is quite resistant to filing! I have filed CI before, and this does not behave like I would expect (given my limited experience).

The photo below shows my attempt at smoothing the surface with a file. I used about 30 strokes of the file to get this.

Crankshaft after filing.jpg

The surface has not changed in appearance.

I then tried some medium grit 3M emory cloth. That did nothing to surface (no photo, sorry).

The set up is certainly stout enough to get something of a result from filing and/or abrasive cloth.

The lathe filing and sanding were done at around 300 rpm. The file was sharp, and the emory cloth was new.

Thank you for your help.

Cheers,

Tom
 

mattthemuppet

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#12
sounds like it's seriously hard if a file won't make much of a mark on it. You probably have too much flex and vibration to use carbide at the speed it wants, and HSS will probably just rub. What does HSS sound like in the work - squeaking and howling? Or producing chips? I'd agree that the live center tip is a long long way out from the tailstock - it certainly doesn't look like a normal live center. Perhaps try a dead center with the tail stock quill retracted as far as possible? How much material do you have left to take off? Is it a small enough amount to use a dremel rigged up in the toolpost?

Also, even if you do get all that done, how are you going to turn the center journal?
 

Kenny G

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#14
Ok if I am way out of line please tell me as I am very green, but why are you trying to do this between
centers why can't you chuck one end in a 3 jaw as close to the cam as possible then turn and do the other half? Seems to me that would be a much more stable.
 

T Bredehoft

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#15
The shaft finishes at .375, sticking out four inches needs some sort of support. A dead center would supply a far better right end than that long a live center. Just grease it (lead pencil graphite would do) and turn it reasonably slow. After all, you got all day.
 

Steve Shannon

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#16
Ok if I am way out of line please tell me as I am very green, but why are you trying to do this between
centers why can't you chuck one end in a 3 jaw as close to the cam as possible then turn and do the other half? Seems to me that would be a much more stable.
Kenny,
3 jaw scroll chucks are usually not perfectly centered for the entire range that they grasp. Independent jaw chucks, like many 4 jaw chucks, can be centered pretty accurately but it might take a little while, which must be redone every time it's removed and replaced. In fact I'm not sure you could ever remove and perfectly replace something from a chuck.
Working between centers it's very easy to remove and replace and return to exactly the same center.


[emoji1010] Steve Shannon [emoji1010]
 

f350ca

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#17
The casting they supplied you looks very rough, if the file won't cut it probably was chilled and hardened. I built a #1 years ago and the crank machined nicely without the bridge between the mains. Didn't have a small lathe at the time that would hold decent tolerance, so it was done on the 17 inch Summit with a 12 inch 3 jaw holding a centre.
IMGP1200.jpg

Greg
 

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Billh50

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#18
Is that tape you have wrapped around the shaft?
 

Kenny G

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#19
Kenny,
3 jaw scroll chucks are usually not perfectly centered for the entire range that they grasp. Independent jaw chucks, like many 4 jaw chucks, can be centered pretty accurately but it might take a little while, which must be redone every time it's removed and replaced. In fact I'm not sure you could ever remove and perfectly replace something from a chuck.
Working between centers it's very easy to remove and replace and return to exactly the same center.


[emoji1010] Steve Shannon [emoji1010]
Steve thanks I think I understand, but it only raises more questions in my mind but will try to ask in a different thread so as not to hijack this one
 

tertiaryjim

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#20
Could it need to be annealed prior to turning to relieve internal stress.
This might reduce vibration and let it machine cleaner.
 

ogberi

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#21
Try vertical shear tool, ground from HSS. Low cutting pressure, excellent finish, but low metal removal rate. Easy to grind and resharpen, though.
 

stupoty

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#22
Ok if I am way out of line please tell me as I am very green, but why are you trying to do this between
centers why can't you chuck one end in a 3 jaw as close to the cam as possible then turn and do the other half? Seems to me that would be a much more stable.
Centers should be one of the most secure holding meathods as it's possitively locked in every direction.

The amount of flex in the work piece is somthing else though.

Stuart.
 

Billh50

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#23
I would do what Billy G suggested and use a follower steady rest. But you might also try something that sometimes works. The reason I asked if that was tape is because I have used rubber strips in the past to absorb vibrations. I wrapped the part tightly where I was not machining and secured it with a staple. It did make a better finish.
 

tomw

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#24
Overhanging the tool 1/2 of its length is probably not beneficial, but what do I know?
I noticed that as well. I was mainly pointing out the rubber wrapping he used as a vibration damper.

What does HSS sound like in the work - squeaking and howling?
High pitched squealing. I get chips, but it certainly doesn't cut very deep. If I try a 5 thou cut, I might remove 2 thou, and not evenly.

Perhaps try a dead center with the tail stock quill retracted as far as possible? How much material do you have left to take off? Is it a small enough amount to use a dremel rigged up in the toolpost?
I have tried a dead center, and I have tried to have less stick out. The odd looking live center is Sherline's way of making an adjustable tail stock. The non-adjustable tailstock is the one part of this lathe I don't like.

I need to reduce the diameter 60 thou total on both ends and the central rod journal. That is a lot of grinding.

The casting they supplied you looks very rough, if the file won't cut it probably was chilled and hardened. I built a #1 years ago and the crank machined nicely without the bridge between the mains.
Wow, that looks like a really nice crankshaft! Your set up doesn't really look much different from mine, except for the larger machine.

If the casting has been chilled and hardened, can I heat it up to anneal it? Will that work on CI?

Thank you all for your help.

Regards,

Tom
 

TommyD

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#25
Personnaly, I think Matt the muppet and a few others identified the problem, vibration caused by an off-center weight on a thin, long shafh. You have to dampen the vibration and the follower rest or a steady rest would probably go a long ways in curing it. Once that is solved you might try carbide again.
 

tomw

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#26
I just spoke with Matt at PM Research. He said that it sounds like the piece never got annealed. He said that sometimes happens. He will be sending me a new crankshaft in the mail.

Thank you tons for your help.

Tom
 

f350ca

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#27
I think that will solve your problem Tom. If another one is on the way you could try annealing the old one. Heat it up in a charcoal barbecue and let it cool in the ashes. Nothing to loose.

Greg
 

mattthemuppet

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#28
neat, it'll be good to hear how the new one turns. High pitched squeaking and squealing is a sure sign of the tool rubbing and not cutting. Probably there was enough flex that every so often the work and tool flexed down, it bit off a piece and then everything sprung back up to keep rubbing.
 

34_40

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#29
We used to turn brake drums & rotors at the shop I worked for. There was a wide thick band of rubber for the drums and a narrow thick band for the rotors.

Without wrapping the part to be cut, the sounds and finish would be awful. Make your ears bleed bad!! and the finish would be erratic almost "jumpy"..
 

Sandia

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#30
Tom, glad to see you got PM to send you a new casting, I was going to suggest that. When I turned the crank on my Redwing engine, I used carbide and I also set the tool height a little bit below center, which helped a lot. The crank journal is the part you need to get really smooth. I also wrapped my casting with tape and it helped some with the chatter but not a great deal.
Keep us posted on the new casting they are sending you, hopefully that will help you solve your problem.
 
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