Today is just one of those days, why does everything always seem to happen all at once you think to yourself. Last week things were a bit quiet, today you’re under an avalanche of quotes and jobs that need to go out the door. The phone rings and its Mark, he needs those parts desperate, “where are they?” he asks, they’re on the machine and you’ll have them over to him this afternoon you tell him.
Just to be sure you decide to go over to Chris who’s machining them, you explain that Mark has been on the phone chasing them and he’s desperate. Chris is’nt surprised as most jobs for Mark usually are and gets back into it.
Anyway, you’ve barely got back to your pile of quotes when there’s a commotion coming from Chris’s machine so you decide to investigate half dreading what you’ll discover. By the time you get there, Chris has already stopped the machine and is staring at the nylon billet in the chuck and there’s a dirty great crack either side of the hole in the middle.
“How did that happen?” you ask, Chris replies that he reckoned there must have been a fault in the material. You check the drills he’s been using and discover that Chris has typically taken the time as usual to sharpen them all and it was obvious that he’d also been stepping up through drill sizes, he’d not just gone straight from the centre drill right into the 30mm without a pilot hole or working his way up through increasing drill sizes at all. You’re also certain he was “pecking” with the drill and wasn’t just winding it in there.
Mark isn’t going to be over the moon but you easily get Direct Plastics to send another billet over on an A.M delivery for you and you do your best to assure Mark he’ll have most of his parts this afternoon as promised and as for the missing item, that will be with him as soon as you can tomorrow. Actually Mark is fine with the solution, realising that these things happen and emails over a new enquiry for good measure.
The Science
Nylon is particularly susceptible to cracking and by pushing a large blunt drill up the centre of nylon rod has a good potential to crack it. The localized heat and vibration cause a shock, and particularly the pressure point caused by the tip of the drill has the potential for nylon to fail, almost like splitting a log,
4 Tips to prevent cracking;
- Only use freshly sharpened drills
- Always start with small drill and open-out with bigger drills
- Use plenty of coolant
- Use a gentle “pecking” action to release swarf and heat build up
