Hi All,
Thanks Terry for your comment; if it helps someone, maybe even provoking a thought, then it makes it worthwhile.... I must have learned something worthwhile in over 60 years in the workshop !!
As a little aside I thought I'd show a couple of workshop 'machines' that help with accuracy as well as in production rates..
When it comes to items such as cylinder covers, glands and flanges etc., that have an array of bolts/studs one of the basic methods to gain accurate pitch circles/divisions is to have some form of indexing system. Rotary tables/ dividing heads/ and using digital read-out are amongst other methods.
A far simpler, easily accessible means is using an indexer attached directly to the main machine in the workshop, your lathe.
Some years ago I 'standardised' my chuck threads used on the lathe, dividing head, rotary table chuck, and indexer to that of the lathe... 1.5" dia x 8tpi w/f....thought that a logical step.
I took a little time one afternoon to drill the periphery of all my chuck backplates with 48 holes with a BS4 centre drill using the dividing head. This will allow common useful factors such as 2, 3, 4,6,8,12 .
A swing clear arm attached to the headstock of the lathe can position the spring-loaded detent over the chuck. Thought...when the detent is in place DON'T SWITCH ON THE SPINDLE MOTOR !!
I've done it several times !!!! It also pays to mark the indexing steps on the hole circle using a felt-tip pen to aid repetition mistakes. 6 hole steps gives 8 holes positions on pcd. etc..
Using the toolpost drilling spindle, the spindle bore of which was drilled/reamed/ bored
in situthus ensuring accuracy. The centre/start-point is easily attained by placing a centre in the t/stock and parallel centre in the chuck to line up . The pitch circle radius can then be set by using the cross-slide feedscrew, a rule, or digital read out whathaveyou.
The spindle is driven by a 80watt sewing machine motor and I put a domestic dimmer switch on the supply to give speed control.....I only hope I can get the motor back on domestic managements sewing machine before she decides to use it next!!
I usually put a BS1 centre drill in the small Jacobs chuck and 'spot' the locations....drilling for tapping size/clearance as appropriate on the drilling machine. All the cover, flanges and glands were done like this.
Although I have an Archer No2MT tapper , it is often easier to tap the holes using part of my George Thomas (of hallowed memory) tapping attachment...it gives vertical guidance and very good 'feel' for small taps such as 8BA. I've attached a pic of the 'tapping' machine and next to it one of the developments George made for closing small rivets.
I hope the attachments help explain my ramblings
Cheers for now, Stuart