Getting rid of the pip -

I used 9mm slot drill to bore a flat bottomed hole in a piece of 10mm diameter EN1. Only its not a flat bottom - I've got a pip.

Whats the best way of getting rid of the pip (all walls are 0.5mm) to leave a nice finish? and how come a slot drill has left a pip aynway?

TIA

Steve

Reply to
Steve W
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Sounds as though the longer of the two cutting edges on the bottom of your slot drill doesn't extend past the centre as it should. However, since the ends of most cutters are ground slightly concave, you won't get a perfectly flat bottomed hole even with a correct geometry cutter. One solution would be to finish off with a flat bottomed D bit (easily made from silver steel).

Mike

Reply to
mike.crossfield

Reply to
Bill

If it's not too deep try a dremel. If it's really deep and you can't get a centre cutting slot drill/end mill in there, then consider drilling or boring it through and turn a plug on the lathe to fill the end.

Peter

Reply to
Peter Neill

Reply to
kenneth

Cos it's a crappy slot drill that's not properly ground?

Either use a smaller slot drill offset to one side a bit or grind up a small boring tool and do the job on the lathe.

I think you'll find that most slot drills won't leave a perfectly flat bottomed hole anyway because the cutting edges are ground at a slight angle to the horizontal. From time to time I have to make up 'lash caps' which sit on the top of race engine valve stems to set the valve clearance. Hole diameter last time I did this was 8mm so there's not much room to work in and modern engines can have much smaller valve stems. 6mm isn't unusual. You need a proper flat bottom to the hole or it'll pound either itself or the valve tip out of shape until they match and alter the clearance. I came up with a number of solutions.

1) Rough the hole out with a slot drill leaving a bit of stock in place and then finish with a tiny boring bar ground out of a bit of HSS. 2) Bore from scratch with said tiny home made boring bar. 3) Bore in one pass with home made D bit. 4) Drill a tiny hole first slightly deeper than the base of the hole is going to end up so the slot drill has nowhere to leave a pip. However still not a perfect solution if the slot drill doesn't have a perfectly flat profile to its cutting edges anyway. You could also do this in combination with 2 or 3.

In the end I went with option 2 because the holes in lash caps are only about 2mm deep so not much material to remove. For a much deeper hole I'd have roughed out with a slot drill in the tailstock first to get fast stock removal.

Reply to
Dave Baker

Thanks Dave, this is a cap to sit on top of a valve stem and I used a brand spanking new 9mm slot drill (Interstate I bought from J&L) in the tailstock to bore the hole - 8.5mm deep.

Sounds like I need to clean up with the little boring bar I made the previous evening to bore out some bearing housings.

The first one I cut produced a really big pip and the slot drill got a strop on and destroyed the workpiece, which I didn't expect. Taking more care with the second did give a good result barring the "pip".

Reply to
Steve W

Well, there's a coincidence then :)

and I used a brand

That's hell of a long for a lash cap unless it also has some other function. You should only need a couple of mm of engagement on the valve stem. It can't fly off once the valve clearance is set unless you rev the engine to the point of such massive valve bounce that a gap opens up.

You might also need to consider the hardness required which should be at least the same as the valve tip itself and those will either be heat treated if made of hardenable steel or stellite tipped if not. I make my lash caps out of either silver steel or old high tensile bolts, heat to cherry red, dunk in oil and then give them a rough and ready 'file test' afterwards to make sure they're not still soft.

Reply to
Dave Baker

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