Hi Dom
I am a mould designer/maker based in Scotland so also spell mould with a 'u' !
We usually drill and ream our ejector pins holes from the front before any core or cavity detail has been cut. This means on a flat plate you can centre drill all your holes first as they are all on the same plane. This is probably the most accurate way of positioning them relative to the mould cavity.
Most of the pins we use are below 12mm dia. and we drill first with a drill 0.2mm below the reaming size. We then ream to the nominal size of the pin. Ejector pins from DMS and Hasco are usually 0.025mm below the nominal size so reaming should give a good sliding fit and the clearance is usually sufficient for venting.
The length of ream fit is typically 5 x pin dia. for up to 4 or 5mm and maybe 2-3 times for larger dia. Too long a reamed fit will prevent venting.
We then turn the plate over and drill a clearance hole (0,5mm dia. greater) up to meet the reamed hole from the front. We also countersink the clearance hole at the back to provide a 'lead in' for the pin.
Doing all the operations from the back means less set up but we usually find that we cannot get long enough reamers to go through the plates. Also drill wandering can be a problem for accurate position relative to the cavity/core.
Machine reaming in aluminium can be tricky and we find sometimes holes cut oversize - if you only have a few holes hand reaming may be more accurate.
To size the small ejector pin holes we use pin gauges.
I've just noticed that you are using very small milling cutters and ejector pin holes may cause problems with cutters wandering into holes and breaking. It may be safer for you to drill and then hand ream from the back of the plate.
We use split 2 plate ejector plates as supplied in standard mould kits. We drill a clearance hole 0.5mm larger than the pin and a cbore for the head 0.5mm larger. The depth of the cbore should be 0.05mm deeper than the head thickness. The pin should be able to rotate when the plates are assembled. We sometimes ream the push back pins in the ejector plate but for alumunium moulds we often use ejector guides to prevent 'firing up' of the ejecting assembly.
If you have a pin which needs keying there are a number of options. Grind a flat on the head and locate it a matched D pocket in the plate or mill a slot and lay in a keysteel pin to locate against the flat ground on the head. Alternatively, drill a small hole in the side of the head and fit a timing keysteel pin which locates in a slot.
With regards cutting the form on the pin you can buy soft pins which can be milled but we usually spark erode the form or grind it by hand using a die grinder.
To cut the pins to length we assemble the mould and determine the overall length from the back of the front ejector plate to the split line. We then determine the length of each pin and first rough cut them to length (0.5mm long) using a 'cut off' wheel on the surface grinder. We then finish grind them to length. Be careful not to grind too much off as the small surface area of the pin can heat up and the pin length expand. Let the pin cool down between cuts - use a piece of copper to sink the heat out of the pin between cuts.
To hold the pins on the grinder you can clamp a 'v block' to an angle plate and lock the pin in position for grinding the front face. Place a slip block on the mag table and sit the pin head down onto this surface as a stop. You can then alter the size of the slip block to suit the different pin lengths required.
Hope this helps
Davy  |