Is the tailstock aligned with the headstock ? The end mill could be cutting on one side only and working like a boring tool.

Paul

If you don't like boring small blind holes or your bar is long and you don't like fixed steadies then.

1. Turn a 10mm spigot on the end of the bar say 5mm long.

2. Turn up a collar 9mm long with a drilled and reamed hole up the middle. OD a tad bigger than the bar.

3. Loctite the collar to the spigot.

4. True up the bar in the four jaw and turn the outside of the collar to match the OD of the bar.

regards Martin

I'm surprised here,

You can grind your own flat bottoming drill out of a regular twist drill. The geometry isn't that complicated to do on your own bench grinder.

Bear in mind that the drill is not suitable for anything other than bottoming at that point. But what's the cost of a 10mm twist drill? not much I wager.

Not the best image, one with angles would be better. But it demonstrates the basic cutting shape you're after..

So it's basically flat or 180 degree kind of shape, but with relief angles towards the edges, so that it cuts. Theres another couple of angles on the sides, that's just to make sure the cutting point goes all the way to the centre and doesn't leave a pip or dimple.

You can buy flat bottomed drills commercially in some places, but I think they cost considerably more than a regular drill, simply due to their uncommon usage.

So to use it, drill your normal 10mm hole, to depth, including the point of the drill. Then run this one down the hole, make a note of how deep the angles of the point are on the regular drill, then that's your number for your flat drills working depth. Drill to that depth, and you should have an accurate flat depth.

My experience for this comes from college, where I had a 15mm thick piece of steel where we had to drill a variety of tapped, reamed, spot facing and flat bottomed holes!

Michael W

Edited By Michael-w on 08/02/2017 10:22:34

Michael there is not much difference between regrinding a drill and using a milling cutter like the OP did but getting the hole spot on dia is not easy with either method.

I'd also use a small boring bar, if you do't have one a slot drill of say 6mm dia held in the toolpost makes a reasonable boring tool.

End mills are usually ground to nominal size with better accuracy than twist drills. The're also shorter/stiffer than your average twist drill.

If your hole came out 10.35mm suggests to me around 0.175mm misalignment in the tailstock/chuck combo ?

As others have said bore to size is best for accuracy.

If however you're lazy / don't like boring small holes ( like me) ~try drilling with a 9.8mm/9.9mm or 25/64" drill, then "ream" out & flatten bottom with 10mm EM.

Or maybe drill 3/8" if you think above drills will still result in oversize hole before "reaming" ?

just a suggestion.

I've had good results with EM's 'boring' holes in the tailstock, wasn't looking for super precision though.

Very interesting!

How did you set the tool height?

Paul

Um, should have been clearer, I meant how high was the cutting flute set… at centre height as usual? Not having a tool to hand I can't visualise the fall off/relief to think if it would cut properly set like that or whether it would need to be set lower and rotated a little.

EDIT But on reflection it's obviously no different to normal milling just in a different orientation and with the material curving in toward the cutting edge. I must try this later!

Edited By PaulR on 14/02/2017 14:19:37

Just imagine that the other flutes are not there and set it as you would any HSS boring bar with the corner on ctr height.

You do neet to set the tool at a very slight angle with the blunt end a fraction further behind the lathe axis to stop the risk of rubbing and this equals out the slight conical end you get when plunging with a lilling cutter.

Even better, you can use that cutter with all but one point knocked off. Never throw away old HSS tools, taps, end mills, center drills with the tip broken, they all make emergency lathe tools.

Ian S C