Stuart V10 engine Crankshaft manufacture

I am following the excellent instructions by Harold Hall for this project. I am a bit confused with this part

"Mount the length of material on the top slide, packed up to centre height, and with a centre drill in the drill chuck position it such that its larger diameter just touches the materials rear edge. Traverse the saddle so that the workpiece is clear of the centre drill and then, using the cross slide, move the part 5/16” plus half the centre drills diameter and centre drill the end of the workpiece. Traverse the cross slide by a further 3/16” and centre drill once more"

Surely the size of the centre drill will move the centre point or am I missing something ?

Edited By JasonB on 30/01/2015 20:22:42

Basically you touch the edge of teh casting against the edge of the ctr drill. If you then move the topslid eby half the drill dia this will mean the centre of the drill is at the edge of the work. Now move a further 5/16" and the hole will be drilled 5/16" from the edge of the work.

You are in effect using the shank of the drill as an edge finder where once the finder touches the edge you are tehn 1/2 the end dia from the edge of teh work.

J

Thanks Jason, I now understand. I am experimenting manufacturing a crank just for experience. I found that by measuring and marking the ends and drilling on vertical drill a lot easier than squinting into the lathe. I assume I am achieving the same result.

This is going to be a long, long project as I am very much a novice although long in the tooth so more questions to follow.

The reason for doing it on the lathe (or mill for that matter) is that you can accurately get both ends the same distance apart by using the handwheel dial. No everybody can mark out and punch holes to 0.001" accuracy.

Here is one I did last week on the mill, the marking out was only toi get the approx position of the first hole the rest were done using the mill to get the spacing

J

Edited By JasonB on 30/01/2015 20:29:55

Thanks again. Not sure I can get to within 1 thou on my lathe but have been experimenting with tyre depth DRO so you never know. If you don't try the you don't know

Hi Hamish,
The method that Harold describes avoids having to line the centre drill up with a line. It just relies on the lead screw on the cross slide for measuring. I use a slight variation of this method. I insert a feeler gauge between the drill and the edge of the work piece. that way I can judge when the items are just touching better. You then need to move the cross slide by half the diameter of the drill plus the thickness of the feeler gauge.

Les.

Thanks Les, I understand.   Made a classic mistake today on my first attempt.  Having machined the shaft I then cut out the wrong waste part between the webs.  Making from scrap  flat plate I have handy so back to square one.

Edited By Hamish McNab on 30/01/2015 20:38:52

oFurther to yesterday's disaster I started from square one. I had an eurica moment overnight and thought I would use my clock gauge to measure the movement across for the centre drill positions rather than rely on the graduations on the feed handle as I had a lot of play (backlash) which at that time I did not understand. Anyway dry runs looked promising so on with the power and drill the actual metal. Although I achieved a result what I found that due to the chuck being off centre (another story) my cross slide rattle about due to what I now understand to be due to backlash. Using the clock gauge I found that I had 20 thou back lash. I reversed the screw nut and this is now about 13 thou.

The question I have now is should I get a new nut, about £48, or just work around making allowance for this. The assumption, possibly incorrect, is that this would reduce the backlash but then if I am allowing for a smaller backlash what's the difference other than a tighter machine, and £48 less out of pocket.

Any advise much appreciated

Hi Hamish,

Don't assume all the backlash is a loose/worn nut. Check for any looseness in the thrust bearings first.

Neil

Hi Hamish,

I would not bother about getting a new nut. Most lathe cross slides have backlash unless you have a precision ball screw with back lash nuts. Always back off the cross slide and then advance the cross slide to the new position. This ensures that the nut and screw are in full contact while measuring and cutting.

Paul.

Thanks Neil. Will look again tomorrow as at the moment the backlash will have reduced considerably due to contraction as its cold enough to freeze the b.... off a brass monkey

Thanks Paul. Now speaking with a higher pitch as lathe in garage. Had another look and could see no further adjustments. Turned the nut back to where it was as it will be easier to remember 20 thou compensation.

I theory Harolds method will not be affected by backlash as the work is always moving in one direction.

When drilling lock all the slides and that will help stop things moving about.

I would not spend the money on a new nut at the moment, just know the backlash is there and take it into account.

J