Crankshaft truing jig.

[Cato]

Truing a multi cylinder crankshaft once in a while is OK on the lathe, but I'm getting a few requests now so I need/want to set up something dedicated. Given the breadth and depth of the engineering knowledge on here I thought it worth asking for opinions and maybe sources for the bits.

It's nothing hugely complicated given what goes on here with some of you. Essentially it's a pair of tall narrow 'V' blocks sitting on a flat surface so that you can measure the run out at each end of the crank and adjust the webs accordingly. The usual tolerance to aim for is two thou, with a maximum of four.

A surface plate for the base would provide the flat surface and also allow positioning of two mag base DTIs. I can't see any suitable 'V' blocks available, so I'd need to get some accurately cut plate to make stands for the 'V'. Plain cut rather than with bearings for accuracy as I'm looking at run out rather than balance. Thoughts?

The next thing would be, is anyone on here able/prepared to cut some plate and make up the stands?

Ideally I'd buy a set up but they just don't come up and the balancing/truing machines are hideously expensive.

 

[Pete.]

You could make it all from RSJ I should think

 

[Cato]

You could make it all from RSJ I should think

Possibly, my concern with that would be ensuring it was flat enough.

The other similar thought I had was to use a lathe bed as even a moderately worn one would be accurate enough. If I could get one with two tail stocks then 90% of the work is done.

 

[Pete.]

Possibly, my concern with that would be ensuring it was flat enough.

The other similar thought I had was to use a lathe bed as even a moderately worn one would be accurate enough. If I could get one with two tail stocks then 90% of the work is done.

I doubt flatness is critical, not to that extent. You're suspending the crank on two points, all you need is something to hold those two supports steady. If you cut two slices off an RSJ you could up-turn them and add edge guide then you have two ready-made pedestals that can slide up and down to adjust centres. Fabricate the vee pads and fix them to your pedestals.

 

[awemawson]

Bolt standard Vee blocks to 1-2-3 blocks and you can accurately make a Vee block as tall as you like - then sit them on a surface plate - job done

 

[Cato]

I doubt flatness is critical, not to that extent. You're suspending the crank on two points, all you need is something to hold those two supports steady. If you cut two slices off an RSJ you could up-turn them and add edge guide then you have two ready-made pedestals that can slide up and down to adjust centres. Fabricate the vee pads and fix them to your pedestals.

My thinking on the flatness was moving the crank to adjust it or the dti to take another reading. If the base/stand part is flat then it'd save setting up again. I agree that it's not strictly necessary but it seems like a useful benefit if one has to be made.

 

[Cato]

Bolt standard Vee blocks to 1-2-3 blocks and you can accurately make a Vee block as tall as you like - then sit them on a surface plate - job done

Standard 'V' blocks are about four or five times wider than would fit so I can't use them.

 

[awemawson]

Then use the 'ball bearing trick', supporting a shaft on a pair of bearing balls to give an effective Vee block of virtually any narrowness that you want. Balls sit in holes on top of turned pillars of desired height that are easily made to good accuracy on even a simple lathe.

Many ways of killing a cat !

 

[graffian]

Then use the 'ball bearing trick', supporting a shaft on a pair of bearing balls to give an effective Vee block of virtually any narrowness that you want. Balls sit in holes on top of turned pillars of desired height that are easily made to good accuracy on even a simple lathe.

Many ways of killing a cat !

Couple of bits of wood with Vs cut into them?

 

[roofman]

can you gauge any ideas from this...wheel balancer

 

[roofman]

 

[Fiji]

Watching with interest as I have a Matchless crank to rebuild, and yes I DO know for that I COULD probably use a couple of bricks and a piece of string !!

 

[Cato]

Watching with interest as I have a Matchless crank to rebuild, and yes I DO know for that I COULD probably use a couple of bricks and a piece of string !!

The more you look into it the greater the number of options are. I'm probably making things a bit more difficult for myself by aiming for something which can be used for a variety of crank lengths/cylinder numbers and having it as accurate as possible. As others have mentioned for a one off or small number of jobs it's overkill. However overkill is a forum speciality.

At the moment in terms of ease in construction, accuracy, adaptability and cost an old lathe bed and a pair of tail stocks looks to be the best starting point. Then make/adapt accessories as required, things like a travelling platform for a dti for example. A set up like that can not only be used for cranks, but also checking run out on gear shafts, rotating things like fork clamp spindles for welding without risking spatter on the actual lathe, etc.

 

[Seadog]

Watching with interest as I have a Matchless crank to rebuild, and yes I DO know for that I COULD probably use a couple of bricks and a piece of string !!

Scribe a couple of lines at 90° the crank pin using a decent quality square. That goes a long way to helping you true up.