Help Needed: Eriksen Lathe Thread Cutting

[pipeclay]

I would of used a drill from the start.
How are you supporting the plate , and how thick is it?

[Jekyll and Hyde]

OK I need some more help........

I have just had my 2nd go at putting the holes in my 127 hole plate. The first time I used a 2nd hand and unknown 3mm slot mill that made oversize holes then snapped on the 9th hole. I then bought a couple of carbide 3mm slot drills from General Tools. The first snapped as soon as it touched the ductile iron surface and the second snapped off in hole number 2.

I have the slot drill turning pretty fast although I do not actually know how fast. The first new one snapped I believe as I had the speed too low and was using the quill handle to 'drill' which probably gave too much downfeed. The 2nd slot mill I ran much faster and used the knee to 'drill' much slower.

Any hints why these all snapped? Speed? Feed? Material? Cheap slot mill?

I will now get a 3mm R8 collet and use a drill bit instead. I wanted to only drill halfway through the plate so that I could use the other side for other hole patterns if I ever need to but I'm over it and will just drill right through.

I'd agree with Pipeclay, but for a bit more elaboration from my point of view - some of the more experienced may consider this to be absolute rubbish, but on my slightly sloppy Bridgeport it's what I've found:

I've never been very happy even with centre cutting endmills drilling a hole with them. They always chatter and walk, and feel horrible for me. If I'm making a slot, I drill the ends (either on size, or slightly under) first, otherwise I end up with massive keyholing. If I want an accurate hole, or a flat bottom one, I always drill the centre out first. If you consider the geometry of how the end of the flutes are generally ground for an endmill, it makes sense. The end face is actually slightly concave, to provide a touch of clearance so the centre doesn't rub. Much like a facemill, the tips are the only part actually cutting on that face, so it makes sense that the endmill will kind of 'dance' on those tips when trying to drill. Not sure if centre cutting endmills are dead flat across the end, but the ones I've used don't behave like it.

Combined with that, I've broken MANY 3mm endmills machining a few small things in the past, and I think I've finally worked out that between the runout in my collet chuck, my spindle, and the amount I can deflect the spindle by hand, that the total runout exceeds the chipload of a 3mm endmill. One of the reasons I have a new set of spindle bearings to go in my mill to hopefully improve all this, until then I'm just not running small endmills (or any endmills I haven't already run in it).

No way I would have tried to drill holes with a 3mm endmill with all that in mind - others may be far more talented than I.

My guess is the first endmill was centre cutting, and the oversized holes just seem to be what happens when drilling a hole from scratch with endmills in my experience. The cutter walking on the tips gives oversized holes as it dances around and flexes, and soon enough you got a bit too keen and it broke. The next two endmills may not have been centre cutting at all, and being so small you don't get any warning that they're out of their comfort zone. You may have, like me, some runout contributing to the problem as well.

Any reason you can't only go half way through with a 3mm stub drill, to retain the ability to use the other side?

[Mk1_Oz]

The plate is 10mm thick. I guess my lack of knowledge bit me here re trying a slot mill.

Yes I could only drill past way through. I will have to measure the depth of the drill point and see if it all works. Not a big issue having a double side plate.

I have a couple of short 3mm rivet drills that may be nice and rigid.

[elanjacobs]

Just use a No 2 or 3 centre drill. Drills are for drilling, mills are for milling

[Mk1_Oz]

Nice idea. Will be able to get some of those tomorrow and continue the project