A convex hole?

[ehamady]

I want to create a hole in steel, aluminum, and brass like those in the center of the screws in the attached picture. I've tried grinding an HSS tool to bore such a hole on my lathe but I can't seem to get close. I wonder if there is a bit on the market that will leave a convex bottom in a hole like this. Any direction will be appreciated.

 

[chips&more]

Need more details please. Is it for cosmetics only? Surface finish requirement? Would you like a two tone finish? How about inserting a steel ball in the center…drill a hole and press fit the ball into the hole.

Edit: or you could make a concave punch and use it like a die and hammer the shape in with the formed punch.

 

[DMS]

I think the path you are on is the right one. Clearances will be tricky though. Can you post a picture of the original tool grind?

 

[Don B]

If the tool you ground was taking it all in one cut that might have been your trouble, in two stages that profile should be possible, if you make the first tool like a parting tool to take out the meat, and the second to finish the profile, but make the second tool thinner than the finished size and work it from side to side to widen it out.

Edit: sorry I should have been clearer, the reason I'm suggesting 2 tool is that a tool like that will take time to grind and hone so its not the one you'd be wanting to remove the meat with and causing the need for premature regrinding, and by making the finish tool the proper profile but thinner will allow the tool to be cutting on the tip and one profile at a time reducing the stress on the tool and the possibility for chatter, if you grind one of those things your making in half you'll have a good pattern to grind the tool from with a better sight line to get the form right.)

 

[JimDawson]

I have never seen a commercially available tool that would cut that shape. Grinding your own tool bit or having one ground is probably your only option. I assume these are pretty small, like a 10-24 or there abouts and the hole is not very deep. I'm afraid I'm going to state the obvious here, but you just need to mirror the geometry of the hole in the tool bit, provide proper cutting clearance on the cutting edges, and plunge straight in with it.

This is a relatively complex shape, with three separate cuts that could be done simultaneously. I would grind a tool that would do all of the geometry in one cut.

I'm sure someone else will have a better idea, there is a wealth of knowledge on this forum.

 

[Andre]

[video=youtube;cQPtc5pKH7Y]https://www.youtube.com/watch?v=cQPtc5pKH7Y[/video]
Maybe that special radius cutter will work.

 

[ehamady]

Need more details please. Is it for cosmetics only? Surface finish requirement? Would you like a two tone finish? How about inserting a steel ball in the center…drill a hole and press fit the ball into the hole.Edit: or you could make a concave punch and use it like a die and hammer the shape in with the formed punch.

I really like the idea of press fitting a ball. That would be easier and look great. I'll give that a try.- - - Updated - - -

If the tool you ground was taking it all in one cut that might have been your trouble, in two stages that profile should be possible, if you make the first tool like a parting tool to take out the meat, and the second to finish the profile, but make the second tool thinner than the finished size and work it from side to side to widen it out.Edit: sorry I should have been clearer, the reason I'm suggesting 2 tool is that a tool like that will take time to grind and hone so its not the one you'd be wanting to remove the meat with and causing the need for premature regrinding, and by making the finish tool the proper profile but thinner will allow the tool to be cutting on the tip and one profile at a time reducing the stress on the tool and the possibility for chatter, if you grind one of those things your making in half you'll have a good pattern to grind the tool from with a better sight line to get the form right.)

I was trying to do it in one shot. Never thought about doing it in 2 stages. I'll make 2 tools and see how it works. Thanks.

 

[ehamady]

I think the path you are on is the right one. Clearances will be tricky though. Can you post a picture of the original tool grind?

 

[Don B]

Sorry to be a pain but could you do an angle thats showing the other side of the tool, that is the side you'll have clearance issues with, I misinterpreted the profile it looked like a taper to me on the bore, I'd try that tool again but do you have an end mill that you could relive the center with before you try the tool (i.e. An end mill a little smaller than the finished diameter and plunge it in first as close to the depth of the ball shape in the middle)

 

[chips&more]

When you plunge a formed tool like that you will get a phonograph finish/tooling marks. And the more you get towards the center the harder it will be to get a good finish because of the lack of cutting speed. Your pictured example does not show any tooling marks. If you plunge cut, you will have tooling mark issues. Is this important?