Time for new collets?

I need to make a piece of tooling involving some odd sized small holes. I could bore one of them, but the other two are a bit small, so I decided to head off down the home made D bit route.

I carefully made 2 of the three D bits required on opposite ends of a piece of silver steel, then hardened and tempered them. I set the silver steel up in the 4 jaw and clocked it up before turning the required diameters so all nice and concentric.

Transferring the finished tool to the Bridgeport, I mounted it in an R8 collet and drilled a test hole. It drilled fine, but I was disappointed to find the resulting hole oversize!

I had a think and decided to measure the run out of the tool in the collet - TIR 5 thou. I took the collet out and measured the spindle run out - bob on as it should be. I cleaned everything up and put the collet and tool back in the spindle and same result TIR 5 thou. My conclusion is that it is the run out that is causing the oversize holes.

These are typical R8 collets and not the rather more expensive high precision collets that can be bought.

I have got into the habit of running milling cutters in these R8 collets rather than the Osborne collet chuck, partly for reasons of rigidity and partly because not all my cutters are of the screwed shank type.

So.....should I invest in the high precision R8 collets, or should I go for one of the ER types that seem popular with folk on this forum. If ER, which one and what source to get decent precision?

Ian

I don't think you will find the usual ER collets any better unless you buy the expensive ones such as Regofix, with the chuck to match. Maybe just buy one expensive R8 for the size you want?

Maybe the steel distorted on hardening?

Hi Ian

If you want to hold a range of sizes you need a good quality R8 to ER chuck to ensure utmost accuracy, any run-out within the chuck will never be corrected by high class collets.
If you are always using the same size tooling clearly a top quality R8 collet will suit your needs.

Emgee

I've bought a couple of replacement R8 collets from Gloster tooling on ebay (but they do have a website) to replace the most eccentric of the ones I got in an Axminster set with my mill. Very impressed with them...

5 thou. run-out seems a lot even for the type of stuff I favour buying.

Are you measuring the 5 thou runout at the shank of the tool or the cutting edge, quite possible it distorted when quenching.

My "typical R8 collets from ARC give half a thou or less.

Ah ha! Swapped out my D bit for a commercial milling cutter and clocked that - spot on near as makes no difference.

The problem is my D bit. Sighting down the length of it (approx 2" with a straight edge, it is clearly not straight. Could be heat treatment distortion or maybe it was just not straight to start with? It came from my store of silver steel bits and pieces so no idea what it's history was.

Time to start again I think. Any hints on keeping it straight during heat treatment?

Ian

quench it vertically.

Seen as you are making it from silver steel, it needs to be annealed before the final shaping and hardening and grinding..

I have found it best to harden and temper while the blank is still symmetrical, and quench vertically as suggested by Dave, then check it is hard and straight.

Then rough grind the flat on the off-hand grinder if nothing better is available

Then finish the flat with a diamond file while supporting the blank in a rough vee groove in a piece of hardwood held in the vice. The blank rotates slightly in the groove under the pressure of the file and the flat comes out better than one might think. Check progress with a mic.

Rod

Add : could use a flat oilstone instead of a diamond file.

Rod

Edited By Rod Renshaw on 31/07/2020 11:30:35

When you file or mill away the steel to produce the flat, make sure to finish it a few thou over half the diameter, or the "side" will have a cutting edge, and will remove material fro the side of the hole.

Maurice Cox