Steamboat Crankshafts – Lessons & Manufacture Pt#2

This post continues/concludes the story of manufacturing a new crank for Befur from the last post.

Here we can see the re-assembled engine with new crank. We are still to install all the ancillaries (reversing gear, lubrication, condenser and feed/air pump & alternator drive.)

It took 6-man days from receiving the crank back from the grinders to reach this stage.

Once we have tested it on air, we will reinstall it on the boat and undertake this year’s boiler test and check all is as it should be.

Machining Crank Pins

We need to rough machine the crank pins prior to sending the crank off for grinding. Given that my Harrison L5 is not large enough to swing the crank with the end pieces in place, I decided to mill the journals on the Rambaudi V3 mill.

We set up the crank on the centres in the bold-on end pieces. It took a while to decide how to determine the correct position for these, so that the crank pin journals finished in the centres of the webs and at 90-degrees to each other.

The solution was to measure the height of the rough pins above the mill’s table at two positions 180-degrees apart. Once these measurements were (approximately) the same we could reasonably expect the pins to be in the centre of the cast crank pins. Doing this again with the finished pin, allowed us to set the end-pieces correctly for the other pin.

The two end pieces had been made making sure that the sides were parallel to a line between the crankshaft centre-line and the off-set centres for the crank pins. This meant that we could also check that the two end pieces were parallel to one another with a square on the table- it has been vastly quicker to write these two paragraphs than it was to do the setting up, with much head scratching and trial and error of other solutions.

Once set up, the large 25mm diameter end mill could be used to rough out the pins. One has to take care that as one machines the ends of the crank pin, and the faces of the web, you ensure you are always turning the blank to avoid “climb-milling”. This means that for one face the crank needs to be turned clockwise and fir the opposite face, anticlockwise.

The other point which needs to be remembered is that it is not possible to set the pin exactly beneath the centre-line of the end-mill, as it is an end-mill not a slot-drill, so it cannot cut straight down. So, the pin must be offset either in front of, or behind, the cutter to allow it to cut on the edge. Again, the side needs to be swapped depending on the direction the crank is being turned.

Lastly, this offset means that the mill will not cut a flat surface but a dished one – this is not a big deal, as the final grinding will clean this up – but one needs to be sure that you are measuring at the minimum diameter, so as not to cut too deeply.

Stress-induced bending

As is well known when you cut steel (or cast iron) there is a chance that the internal stresses in the casting/material will cause it to bend. This may be immediate or occur over a period of hours.

I had decided to leave 20-thou on the diameters of all the journals for grinding.

In the event this was not enough, as between leaving the workshop and arriving at the crank grinders the centre main bearing had become about 15-thou off-centre, with corresponding error’s elsewhere.

If I was doing this again, I would increase the amount left for grinding to 30 or even 50 thou, to allow for this warping.

In Praise of Crank Grinders

The finish ground crank

I took the crank to Lancaster Engines for grinding. I must complement them on an excellent and reasonably priced job (Under £100). Also I must compliment their crank man Malcolm, as with considerable skill he was able to rescue my warped casting, and produce a finished article with journals correct to about 0.0002″ on diameter and run-out at the ends of the crank of about 0.002″ and 0.005″.

A massive save!!!

As can be seen at the left, the journals were ground with a good radius at each end – as this helps to relieve stress at the ends of the journals.

This necessitated matching radii to be machined into the main and crank pin bearings, but is the “right way” to do things.

Fitting the Bearings

It actually took 4 days to finish fit the bearings to the crank. I had asked Lancaster Engines to grind the journals 1-thou less than the notional diameter of the bearings – this is the ideal finished clearance, but allows for no errors. As it happened the line-boring of the bearings had resulted in the HP end bearings being about 2-3 thou smaller than the LP one (which were bang-on size). So the need to machine radii in the bearings, machine the ends of the bearings to get the correct end-float (and position of crank pins in the correct horizontal position) and to ease the centre and hp-end bearings meant quite a deal of time – much assembly and disassembly and a lot of patience.

Re-machining the Crank-pin bearings

During the initial assembly of the engine (2-years ago) I had realised that the crank-pin and piston-rod bearings were not entirely parallel. My conclusion was that this was because the crank pin bronze bearings were machined on the lathe face-plate and then fitted to the connecting rods.

So, given that these bearings had to be re-machined to fit the ground crank pins, I wanted to find a better set-up to ensure parallel results this time.

My solution was to fit vertical a bar matching the piston rod bearings into a 3-jaw-chuck clamped to the milling table, then mount the rod and bearing assembly on this, and then bore the bearing using a boring head on the mill. The two pictures in this section show this.

I first clocked the “pin” to ensure it was truly vertical, then put a jacking screw under the end of the bearing/rod to ensure it was true, then clamped the bearing/rod to the table- ensuring it was firmly located.

I then centred the mill over the existing bore (using a touch-sensor) in X and Y planes, then fitted the boring head.

I found I got the best results running the mill on the lowest speed, at the presence of the oil-feed holes in the bore, set up a vibration in the cutter if I ran faster.

Lastly, I made a plug gauge (matched to the ground crank-pin) to check the dimensions and fit.

Lots of small cuts here, as there is no tool for putting metal back on!!!  But all went well, and when fitting the connecting rods the improvement in parallel alignment was noticeable.

Final Stages

The last stages were firstly to machine the slots for the eccentric and drive pulley keys.

The angular position of these slots was measured from the failed crank.

This was done by mounting the crank between centres on the mill, arranging the crank pin at the 3 and 9-o’clock positions using a height gauge (as before), noting the reading on the rotary table, then turning the crank until the slot was the the 12-o’clock position (using a digital angle gauge to check for horizontal position of an inserted key) and then reading off the angle from the vertical from the rotary table. I repeated the measurement 2 or 3 times and averaged the result to get the best accuracy possible,

Milling the slots was then a matter of mounting the new crank, finding the 9-o’clock position of the crank-pin with the height gauge and then turning the crank the required number of degrees on the rotary table – I got scarily close to machining the first slot 180-degrees out – measure twice, cut once again!!!!

The last job was to paint the crank. So I masked up the journals, returned the crank to the lathe between centres, and (having covered the exposed bits of the lathe) sprayed and air brushed it to the proscribed GNR Buffer-Beam Red !!!! (My spraying is a bit rubbish, but you CAN’T get runs if the job is continually rotating at the runs can’t decide which way is down!!!! 🙂 )


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