INDEXING DRILLING FIXTURE
The basic indexing tool in article No 1 (Basic Indexing - No. 1) shown on the HISG website used 16DP gears. This method of indexing replaces the normal plates as used on a proper dividing head. Nobody "officially" seems to know who actually invented the method in the first place but there have been academic arguments as to who did. All I know is that it is a very old idea passed down by generations of fitter/machinists as it was to me when I was training.
The plastic Meccano gears (Binns Road) are 7DP, which is not a standard pitch in the engineering trade. I developed this tool primarily to make spoked wheel centres for the driving wheels on locomotives and in particular a model of Crampton’s "Liverpool" which had 8 ft x18 spoked driving wheels and it can be used to make the crankcase parts for radial or rotary engines for world war one aeroplane models. The part shown on the fixture is an 18 spoke Loco driving wheel centre.
I noticed that the Meccano axle fitted the roots of the teeth on the two large plastic Meccano gears (18T & 24T with bosses removed on both faces) and that I could use both wheels on just one base plate by making the clamping flange 6mm thick.
The base is made from a piece of scrap Ø 100mm Aluminium Alloy Round bar approximately 20mm thick, machined down to Ø 3.625" to take the detent pin housing so that it just clears the largest Meccano plastic gear. The detent pin block can be a piece of any suitable square or Hexagonal material 1.25" long, drilled and tapped as shown to allow fixing to the base and for locking the pin, which must line up with the centre of the gear teeth.
I used a 2B.A. x 1" Allen bolt for fixing the block to the base flange and the pin locking bolt is Meccano size. I decided to make a square headed brass bolt for mine to save chewing up the 1.5" Meccano axle used as the detent pin. Mine has been threaded and fitted with a knurled nut for convenience, and the base flange is drilled Ø 6.5mm for two bolts so it can be bolted to an angle plate for radial hole drilling.
The gear is removed and replaced as the hole is drilled which is a rapid process for drilling holes on any PCD.
The workpiece is bolted directly to a Ø 625" threaded boss permanently fitted (peened) in the gears and the chosen gear is controlled by the detent pin which is fixed in position according to which gear is fitted.
These two gears are sufficient to provide
several of the most common indices used in a workshop. My only disappointment
is that Meccano never made any gears with 14 and 32 teeth in this plastic
Indices - 24T gear: 2, 3, 4, 6, 8, 12,
Indices - 18T gear: 3, 6, 9, 18