Any Tips For Machining A Small Cast Crankshaft?

[tomw]

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:

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.


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.


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.

 

[Bill Gruby]

I would use the follow rest as well as the steady rest. The follow rest will greatly reduce the chatter at the cutting edge.

"Billy G"

 

[ARKnack]

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]

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]

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

 

[Sandia]

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]

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]

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]

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.

 

[tomw]

Thanks folks the ideas! I thought this idea from radon.com was quite interesting:



Lots of ideas to try. I will report back!

Cheers,

Tom