In looking at this question:

What is crankshaft grinding and why is it done?

@Pᴀᴜʟsᴛᴇʀ2 gave an excellent answer that got me thinking. First, lets include the same picture from that answer so we're on the same page:

Crankshaft and it's parts

When reconditioning the crankshaft, it is spun on a lathe to either remove enough material from the journals (both main and crankpin journals) to make then round again, or just to polish them. [Refer to the diagram] The crankshaft is designed to be balanced when spun along it's central axis, which is the center of the circle made by the main journals. So machining the main journals is just a question of the usual machining techniques, and polishing is as described in the referenced answer.

But, what about when trying to machine or polish the crankpin journals? These are not concentric with the main journals and there doesn't seem to be any way to mount the crankshaft so that it is spun along the central axis of each crankpin journal. How are the crankpin journals machined or polished?

  • What repair do you need help with? – cory Jan 11 '17 at 18:25
  • 1
    The general approach to how to repair a worn crankshaft – cdunn Jan 11 '17 at 18:57
  • Take it to a machine shop. They'll take it from there. Most people don't have the specialized machines necessary to complete the job in their garage. – cory Jan 11 '17 at 18:58
  • What's the year, make, and model of the vehicle? – cory Jan 11 '17 at 19:01
  • 1
    And now that the question has been well answered, I and anyone searching for this kind of information knows that it's not a DIY kind of project. – cdunn Jan 11 '17 at 20:09
up vote 6 down vote accepted

First, it's not spun on a lathe to make it round again, at least not the typical machine lathe you'd think of. It's a machine which has a huge grinding wheel, thus the reason we call it grinding a crank. The setup looks something like this:

enter image description here

(Sorry for the small image.)

In the image, you can see the crankshaft is setup on a machine. In the back, there is the large grinding wheel. You can also see up front the dial gauge. The gauge shows the machinist how much material is taken from the journal or the difference between what the journal size was and what the size is at any given time during the process.

The crankshaft is placed on the machine and centered on the journal which is being machined. It is noted how much material must be taken from the journal to get it to its new size. The dial gauge is set against the journal surface and is zeroed. The crankshaft is then set to spinning at a very slow speed. The grinding stone in the back is then spun up to speed and is then brought into contact with the journal face. As material is removed through the grinding process, the machinist pays attention to how much is removed, then stops the process once just shy of the finished dimensions. Each journal in turn is ground in the same manner. The main and rod journals can be ground independently, but are usually matched to sizes between themselves (rod size = rod size; main size = main size).

Please see this answer for a better understanding of the different parts of a crankshaft and why it may be ground in the first place.

But, what about when trying to machine or polish the crankpin journals? These are not concentric with the main journals and there doesn't seem to be any way to mount the crankshaft so that it is spun along the central axis of each crankpin journal

Actually, yes there is. You can install the crankshaft off-center on the grinding machine:

Crankpin journal grinding

In this image, one of the crankpin journals is aligned with the machine centerline, so you can grind this pin with a stationary grinding wheel. You'd have to adjust the crankshaft for each crankpin journal.

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