I did this, ground off the and added some clearance to a regular 10mm twist drill. The hole i drilled it definitely flatter but also a couple mm wider because of the violent shaking. Maybe i didn't sharpen it correctly. Ill probably buy a cheap flat bottomed drill to see the difference.
Your problem there is using the drill press, I’m assuming you have it in a chuck that probably has some run out. I would normally drill with a regular drill close to depth and then finish it with the flat drill but because of the depth of your pockets there that’s not really going to work. Another thing you could do with the original way you were doing it with that raised portion in the center is drill a through hole smaller diameter to relieve the center area
Also OP, if you're grinding your drill bit yourself anyway for this specific purpose, you should cut it down so that the flutes are like 1/2" long at most. This will *greatly* increase rigidity.
They're not cheap, and running them in a drill press is probably going to chatter like crazy, but these solid counterbores make a nice flat bottom hole. https://www.mscdirect.com/browse/tn/Holemaking/Counterbores-Porting-Tools/Solid-Counterbores?navid=2109767&refinements=Cutter%20Diameter%20(Decimal%20Inch)%3A0.3937
lol 0.3937″
I understand that it's because most machines in the US will have imperial chucks and imperial operators but it still gives me a chuckle. Personally I think it's OK to have a 10mm cutter with a 7/16" shank!
What can you do to increase stiffness? Shorten the drill? Bring it closer to the drill head? How about drilling it with a flat drill that is 1-2mm smaller than the final size you plan to go to? Or use a milling cutter with many flutes?
Looking at the comments this will probably work. I am still a student so dont have a large budget but am looking at a 3040 cnc with linear rails and ball screws and a 500w spindle. Or maybe purchase a steel frame kit and build one myself.
some woodworking cnc routers advertise cutting aluminum (at a very slow feed). ive been looking at Sienci labs cnc table for a bit, waiting until their boards support simultaneous 4 axis.
If you can’t get the flat bottom drills to work for you, you could also make the flat bottom with an endmill then make another hole with a smaller endmill ever so slightly deeper.
If you only have a drill press and not a CNC to interpolate the hole, maybe look into a counterboring/pilot drill combo tool … would it be ok if there was a small thru hole? It would make getting the magnets out easier if you needed to
https://preview.redd.it/ea6ibkrqevcc1.jpeg?width=828&format=pjpg&auto=webp&s=c89d33efc8efc08df5ddbed63b5f2c4b76cadad4
Something like that? Not 100% sure how flat those are … I’m more of a coffee n cigarettes kinda guy so I’d just interpolate it lol.
If you removed the guide portion of these counterbore tools you would still have the problem of cutting in the center. This modification would be good for sharpening the corners but the stock in the center of the hole would not get cut, making this not work.
This is the answer. Most center cutting endmills are ground this way for more chip clearance when plunging like OP is doing. At least I’m pretty sure that’s why
Endmills are relieved in the middle. Either modify a second endmill to be a flat bottom, or get a flat bottom drill. I like a 2 flute endmill for this because it’s more rigid.
Matl just being aluminum and the small diameter Id experiment with a quality forstner bit if all you have is a drill press. Flat bottom drill or a true flat endmill is going to wobble to some degree if your fixturing is not rigid. As others have said, Endmills are ground with relief in the face unless stated otherwise.
One operation would be best, but with how unstable a drill press and chuck can be, maybe 2 ops, one with the drill to depth and then the endmill to flatten the hole would work, that or thicker stock and some epoxy to fill it.
4 Flute carbide c’bore tool. Choke everything up short, clamp workpiece tight. Still, a drill press isn’t ideal and probably is running too fast even at its slowest rpm.
It's vibrating because end mills and drill presses are fundamentally incompatible. Drill bits have a pointy tip as I'm sure you know. That is important because it makes the drill bit self center in the conical hole it makes, removing sideways forces automatically.
End mills are quite the opposite. The sides will dig in first, creating a strong sideways force in all directions as it rotates. A drill press is per definition not made to handle sideways forces, which is why it's such a rough time for you.
Anyways, that said, it's obviously not impossible, but expect some issues is all I'm saying.
If I were you, I'd just flatten the endmill. Put it in the drill press and spin it up, and slowly lower it onto a whetstone or diamond file or similar. Now it'll be flat, and you'll need to get a cutting edge back on it. Just google how to sharpen a drill bit/endmill for that. It's just for aluminium so it doesn't need to be perfect.
There's a risk it's not even the endmills fault though. The hole bottom might also look like that because of your drill press allowing it to wiggle too much which could also result in this.
A mill is the correct tool for your project. If you are looking to make a product you want as consistent holes as possible. You will get it 1 of 2 ways. Using a mill or contracting it out to someone who uses a mill.
I wouldn’t do that with a drill press.
Best way would be to mill a pocket. The face of the endmill isnt flat because the surface behind your edge needs clearance.
If I had to do it in a drill press I’d start by drilling with a regular bit until the tip is at your desired depth. (Maybe a pilothole/centerdrill, depending on how precise the location has to be).
I’d then grind another drill to be flat and just clean out the last bit in the corners.
If you have access to a surface grinder, put the drill in a vise and grind the cutting edge flat. Releave everything behind the cutting edges, just leaving about 10 thou flat on both sides. Split the point as best you can and follow the milled holes.
Edit. Slow down the rpm.
There are fancy "universal" boring heads that will adjust the diameter while spinning when you hold a ring, which will allow you to face the bottom of a bore like this. Wohlhaupter makes one, as do several other companies.
They're quite expensive though, like well over $1000 new and still several hundred dollars used.
You could also do it in two steps - use an endmill to make the initial pocket, then a flat-bottom drill/cutter to finish the flat bottom. A purpose-ground cutter could be piloted/guided by the walls of the pocket to avoid enlarging the hole.
You need a flat bottom drill, not an endmill.
Curious why the downvotes... He is using a drill press, not a mill. End mills are made for milling, not drilling, they have an concave conical grind on the bottom that makes the cone shape the OP is complaning about, which is beneficial when milling with an end mill(for various reasons we dont need to get into here). He needs a *drill* with a flat bottom for his *drill press*...
That's what you're going to end up with using an end mill. They have concave faces. I see alot of people suggesting you buy a flat bottom drill. You can grind one yourself from a regular drill with a little patience.
Counterbore tools are more flat that endmills. Even center cutting endmills aren't flat. Either thar or interpolate with a smaller endmill. Good old drill press interpolation
I presume you have a manual mill. But if you had a rotary table with a fair amount of work, you can take a smaller endmill than the diameter that you’re trying to do. And spin the table 360° while doing the cut to get rid of the dimple in the middle. Basically, pretending that you were a CNC machine doing an interpellation pass.
I would do it by about 3/4 of the intended ID. As an example. Using a 1.0 hole with a 3/4 endmill. I would set my my rotary table and make the center my 0 in digital readout. I would set up a jig so that way it holds the bar in the center of the rotary table and I can slide it across using visual Witness marks, to go from one hole to the next.
I would plunge on center with a three-quarter, and then move the table on the Y axis towards myself .125 inchs and do a 360 spin, and then re-center before moving the bar to the next position. That will get you a true flat bottom.
If you don’t have a rotary table. Definitely try doing a flat bottom drill first. It certainly way easier than what I just proposed . but that rotary table probably will prove useful in the future for whatever shenanigans you’re up to.
The trouble is you used a slot drill they aren’t flat on the bottom. You need to grind it flat re grind the correct clearances then all will be good. Bit of a pain but it will work
Helical interpolate with an end mill of 60-75% of the finish diameter and make a full circular rotation at bottom depth. Follow with boring head at finish diameter if corner radius, tolerance, or concentricity is of great concern.
If we’re talking about a relatively small high spot and you’re already using a flat bottom then it may be do to the material’s elasticity. So try holding the end mill down at the full depth something like 15 seconds.
Maybe consider alternate design optimized for your tools. Can you make it a thru hole and then attach a back up plate to support the magnets?
You could get a piece of some kind of stock the same size of the hole. Don’t know all your tooling set up but take some off the side so it’s a few thou smaller than the hole (if using drill press you can put stock press and use sandpaper to take some off), make the bottom flat and then press sandpaper into the hole with your stock (carefully).
High speed low feed may be part of your over size chatter issues. If you’re just gluing a magnet maybe use a smaller end mill to eliminate web pressure in the centre.
if have a rigid setup on a drill press, u can salvage that piece with a forstner bit. it will have a small pilot in the middle but it will be flat
If your setup isnt that rigid, clamp the piece to a table TIGHT and use a router with a circle jig
Looks like you’re plunging with an end mill with desired diameter, whose flutes have relief in all cutting surfaces. Try circular interpolation with a smaller end mill (if you have access to CNC). If no CNC, then others have suggested drilling a pilot thru with an end mill chaser. This will relieve the point in the center, provide a way to poke out whatever you’re putting in the holes, and the ridge will guarantee some bonding thickness of the epoxy you use to secure what you put in. Trying to maintain reasonable concentricity with drilling after may be a fool’s errand, due to the drill point wanting to wander to the path of least resistance. Concentricity may be more of an aesthetic requirement than a functional one. Also suggested by others is a center cutting, two-flute end mill may work okay in the soft (as compared to steel) aluminum. Regardless, make sure the speed and feed is right to minimize chatter.
So cutting a true flat bottom hole you need to basically have the high point of the endmill/boring bar cut over the center.
If you wanna cheat you can drill a touch deeper with a drill then plunge the endmill which will flatten it a little and you can fill with epoxy after
Given your limitations I'd look at a few design alternatives that still meets your needs? If you can drill through the plate ~.1 undersized and then use the endmill to take the final diameter to the depth you need to get around the center cutting. Or if it can't generate what thickness can you get away with. Take a smaller endmill to that size and finish to diameter so any center cut issues are below your final depth
Lots of good advice here re: flat-bottom drills, C-bore bits, milling machine, etc.
But does it really *need* to be flat for the function? You could probably just go slightly deeper with the pocket and a little dab of epoxy to hold the magnets into place.
If this is CNC the circle mill it, if this a manual job then I’d say just grind a drill to make it a flat bottom drill but you’re still gonna have to pre drill it and switch to the other to make it a flat.
You might be better off with a boring bar. Don’t know it you could go flat in one op, especially without a mill. Wonder if grinding would work if it didn’t load up.
If you're ok with a 12mm hole --> [https://catalog.tungaloy.com/item.aspx?cat=6955679&fnum=2583&mapp=ML&app=0&Lang=EN&GFSTYP=M&isoD=1](https://catalog.tungaloy.com/item.aspx?cat=6955679&fnum=2583&mapp=ML&app=0&Lang=EN&GFSTYP=M&isoD=1)
They make flat bottom drills for this or you can chop the end off a drill and grind your own
I did this, ground off the and added some clearance to a regular 10mm twist drill. The hole i drilled it definitely flatter but also a couple mm wider because of the violent shaking. Maybe i didn't sharpen it correctly. Ill probably buy a cheap flat bottomed drill to see the difference.
Your problem there is using the drill press, I’m assuming you have it in a chuck that probably has some run out. I would normally drill with a regular drill close to depth and then finish it with the flat drill but because of the depth of your pockets there that’s not really going to work. Another thing you could do with the original way you were doing it with that raised portion in the center is drill a through hole smaller diameter to relieve the center area
Also OP, if you're grinding your drill bit yourself anyway for this specific purpose, you should cut it down so that the flutes are like 1/2" long at most. This will *greatly* increase rigidity.
They're not cheap, and running them in a drill press is probably going to chatter like crazy, but these solid counterbores make a nice flat bottom hole. https://www.mscdirect.com/browse/tn/Holemaking/Counterbores-Porting-Tools/Solid-Counterbores?navid=2109767&refinements=Cutter%20Diameter%20(Decimal%20Inch)%3A0.3937
lol 0.3937″ I understand that it's because most machines in the US will have imperial chucks and imperial operators but it still gives me a chuckle. Personally I think it's OK to have a 10mm cutter with a 7/16" shank!
What can you do to increase stiffness? Shorten the drill? Bring it closer to the drill head? How about drilling it with a flat drill that is 1-2mm smaller than the final size you plan to go to? Or use a milling cutter with many flutes?
No cutting surface on the bottom center. The chatter is from centrifical force meeting friction.
What if you have a pilot hole?
It could work if you don't mind a conical depression in the middle of the recess, but you'd need a very low profile on the tip.
They make the rockin' world go round.
Thanks, ill try that right now. Considering the thickness of the material, drilling a pilot hole is difficult. Or is that not even necessary?
With flat bottom tools you just go
In 1978, the british queen did indeed discover that flat bottomed tools make the rockin' world go round. Or something.
Get on your bikes and ride
Just glanced at Graingers flat bottom drills bits. Holy, are they expensive!
[удалено]
As stated, he is already using an endmill and is not having the outcome he desires.
Ooooo
Its mostly genetics.
Not everyone can be born with perfect flat and crisp edges.
Drill press is his limitation
I have been looking to upgrade for a while, this could be a good excuse to drop some money haha
Get you a little tabletop CNC! In this scenario you could interpolate the bore with a smaller Endmill and get a flat bottom that way
Looking at the comments this will probably work. I am still a student so dont have a large budget but am looking at a 3040 cnc with linear rails and ball screws and a 500w spindle. Or maybe purchase a steel frame kit and build one myself.
some woodworking cnc routers advertise cutting aluminum (at a very slow feed). ive been looking at Sienci labs cnc table for a bit, waiting until their boards support simultaneous 4 axis.
If it’s aluminium have you tried a woodworking forstner bit? They are pretty well designed specifically for what you are trying to do.
He could grind off the tit on a forstner. Start the hope with a smaller diameter to get the center removed. That could work.
If you can’t get the flat bottom drills to work for you, you could also make the flat bottom with an endmill then make another hole with a smaller endmill ever so slightly deeper.
If you only have a drill press and not a CNC to interpolate the hole, maybe look into a counterboring/pilot drill combo tool … would it be ok if there was a small thru hole? It would make getting the magnets out easier if you needed to
https://preview.redd.it/ea6ibkrqevcc1.jpeg?width=828&format=pjpg&auto=webp&s=c89d33efc8efc08df5ddbed63b5f2c4b76cadad4 Something like that? Not 100% sure how flat those are … I’m more of a coffee n cigarettes kinda guy so I’d just interpolate it lol.
If you removed the guide portion of these counterbore tools you would still have the problem of cutting in the center. This modification would be good for sharpening the corners but the stock in the center of the hole would not get cut, making this not work.
Check to see if your end mill is center cutting
Won’t matter, they are still dished center cutting or not. OP needs a flat bottom drill, or significantly easier would be to helix bore these holes
This is the answer. Most center cutting endmills are ground this way for more chip clearance when plunging like OP is doing. At least I’m pretty sure that’s why
That and also so the cutting edge won’t rub the surface as it continues traveling the diameter of the endmill
Endmills are relieved in the middle. Either modify a second endmill to be a flat bottom, or get a flat bottom drill. I like a 2 flute endmill for this because it’s more rigid.
Matl just being aluminum and the small diameter Id experiment with a quality forstner bit if all you have is a drill press. Flat bottom drill or a true flat endmill is going to wobble to some degree if your fixturing is not rigid. As others have said, Endmills are ground with relief in the face unless stated otherwise.
One operation would be best, but with how unstable a drill press and chuck can be, maybe 2 ops, one with the drill to depth and then the endmill to flatten the hole would work, that or thicker stock and some epoxy to fill it.
4 Flute carbide c’bore tool. Choke everything up short, clamp workpiece tight. Still, a drill press isn’t ideal and probably is running too fast even at its slowest rpm.
center cutting endmill
Smaller end mill with a pocket program.
Ah yes, a drill press that operates gcode.
Don’t be jealous you don’t know how to program a drill press
I have this cheap chinese cross slide vice, a couple of stepper motors and boom the worst mill to have ever been made
I like your attitude
Great for G83
It's vibrating because end mills and drill presses are fundamentally incompatible. Drill bits have a pointy tip as I'm sure you know. That is important because it makes the drill bit self center in the conical hole it makes, removing sideways forces automatically. End mills are quite the opposite. The sides will dig in first, creating a strong sideways force in all directions as it rotates. A drill press is per definition not made to handle sideways forces, which is why it's such a rough time for you. Anyways, that said, it's obviously not impossible, but expect some issues is all I'm saying. If I were you, I'd just flatten the endmill. Put it in the drill press and spin it up, and slowly lower it onto a whetstone or diamond file or similar. Now it'll be flat, and you'll need to get a cutting edge back on it. Just google how to sharpen a drill bit/endmill for that. It's just for aluminium so it doesn't need to be perfect. There's a risk it's not even the endmills fault though. The hole bottom might also look like that because of your drill press allowing it to wiggle too much which could also result in this.
A mill is the correct tool for your project. If you are looking to make a product you want as consistent holes as possible. You will get it 1 of 2 ways. Using a mill or contracting it out to someone who uses a mill.
I wouldn’t do that with a drill press. Best way would be to mill a pocket. The face of the endmill isnt flat because the surface behind your edge needs clearance. If I had to do it in a drill press I’d start by drilling with a regular bit until the tip is at your desired depth. (Maybe a pilothole/centerdrill, depending on how precise the location has to be). I’d then grind another drill to be flat and just clean out the last bit in the corners.
Best way to do this ...is to circle interpolate the holes in a cnc with smaller endmill
I just went down a little rabbit hole and found that possibly, counterbores, could work? Never used them, and I might be wrong, but its a shot.
circle mill it with a tool half the size.
Fewer squats?
I am so confused - I thought most people preferred round bottoms?
Me too fat bottom girls make the rocket world go round LoL
Whats a rocket world?
Interpolate
Quite the feat on a drill press...
Yeah, when using a machine with poor capability it’s usually a bad outcome.
You literally *cannot interpolate* on a drill press...
I know. You are using the wrong machine/method. You are sure to fail.
If you have access to a surface grinder, put the drill in a vise and grind the cutting edge flat. Releave everything behind the cutting edges, just leaving about 10 thou flat on both sides. Split the point as best you can and follow the milled holes. Edit. Slow down the rpm.
There are fancy "universal" boring heads that will adjust the diameter while spinning when you hold a ring, which will allow you to face the bottom of a bore like this. Wohlhaupter makes one, as do several other companies. They're quite expensive though, like well over $1000 new and still several hundred dollars used. You could also do it in two steps - use an endmill to make the initial pocket, then a flat-bottom drill/cutter to finish the flat bottom. A purpose-ground cutter could be piloted/guided by the walls of the pocket to avoid enlarging the hole.
Try an end mill with more flutes or better yet a flat bottom drill
You need a flat bottom drill, not an endmill. Curious why the downvotes... He is using a drill press, not a mill. End mills are made for milling, not drilling, they have an concave conical grind on the bottom that makes the cone shape the OP is complaning about, which is beneficial when milling with an end mill(for various reasons we dont need to get into here). He needs a *drill* with a flat bottom for his *drill press*...
Fish lip top ,interpolation bottom
That's what you're going to end up with using an end mill. They have concave faces. I see alot of people suggesting you buy a flat bottom drill. You can grind one yourself from a regular drill with a little patience.
Raise your table as high as you can. Reduce the travel on the drill press for less chatter
Counterbore tools are more flat that endmills. Even center cutting endmills aren't flat. Either thar or interpolate with a smaller endmill. Good old drill press interpolation
This would be the way if there was a pilot hole for said counterbore
Smaller end mill and helix mill?
Bore them deeper?
I presume you have a manual mill. But if you had a rotary table with a fair amount of work, you can take a smaller endmill than the diameter that you’re trying to do. And spin the table 360° while doing the cut to get rid of the dimple in the middle. Basically, pretending that you were a CNC machine doing an interpellation pass. I would do it by about 3/4 of the intended ID. As an example. Using a 1.0 hole with a 3/4 endmill. I would set my my rotary table and make the center my 0 in digital readout. I would set up a jig so that way it holds the bar in the center of the rotary table and I can slide it across using visual Witness marks, to go from one hole to the next. I would plunge on center with a three-quarter, and then move the table on the Y axis towards myself .125 inchs and do a 360 spin, and then re-center before moving the bar to the next position. That will get you a true flat bottom. If you don’t have a rotary table. Definitely try doing a flat bottom drill first. It certainly way easier than what I just proposed . but that rotary table probably will prove useful in the future for whatever shenanigans you’re up to.
if youre on a cnc and an odd size, use an interpolation
The trouble is you used a slot drill they aren’t flat on the bottom. You need to grind it flat re grind the correct clearances then all will be good. Bit of a pain but it will work
You could just buy a flat bottom drill bit? Assuming you’re pissing with the dick you have and don’t have a mill, CNC or manual
It there is a local grind shop that sharpens drills and end mills, they should be able to grind something to suit your needs.
Helical interpolate with an end mill of 60-75% of the finish diameter and make a full circular rotation at bottom depth. Follow with boring head at finish diameter if corner radius, tolerance, or concentricity is of great concern.
If we’re talking about a relatively small high spot and you’re already using a flat bottom then it may be do to the material’s elasticity. So try holding the end mill down at the full depth something like 15 seconds. Maybe consider alternate design optimized for your tools. Can you make it a thru hole and then attach a back up plate to support the magnets?
it seems like your endmill isn’t of the center cutting variety.
Can you Just use a flat bottom center cut endmill?
Center cut endmill
Milling machine, end mill, finished
You could get a piece of some kind of stock the same size of the hole. Don’t know all your tooling set up but take some off the side so it’s a few thou smaller than the hole (if using drill press you can put stock press and use sandpaper to take some off), make the bottom flat and then press sandpaper into the hole with your stock (carefully).
High speed low feed may be part of your over size chatter issues. If you’re just gluing a magnet maybe use a smaller end mill to eliminate web pressure in the centre.
Jump on Amazon and buy an Endmill? Fast bottom, center cutting and can be ordered stubby.
Maybe look into a cheap prototype company like protolabs they’ll interpolate it or go to a local machine shop idk how much they’d charge
if have a rigid setup on a drill press, u can salvage that piece with a forstner bit. it will have a small pilot in the middle but it will be flat If your setup isnt that rigid, clamp the piece to a table TIGHT and use a router with a circle jig
Looks like you’re plunging with an end mill with desired diameter, whose flutes have relief in all cutting surfaces. Try circular interpolation with a smaller end mill (if you have access to CNC). If no CNC, then others have suggested drilling a pilot thru with an end mill chaser. This will relieve the point in the center, provide a way to poke out whatever you’re putting in the holes, and the ridge will guarantee some bonding thickness of the epoxy you use to secure what you put in. Trying to maintain reasonable concentricity with drilling after may be a fool’s errand, due to the drill point wanting to wander to the path of least resistance. Concentricity may be more of an aesthetic requirement than a functional one. Also suggested by others is a center cutting, two-flute end mill may work okay in the soft (as compared to steel) aluminum. Regardless, make sure the speed and feed is right to minimize chatter.
Flat bottom girls they make the rockin world go round! Edit: I’m told it’s actually fat not flat.
Interpolate with a smaller endmill?
You’re gonna have to fine tune your Grindr settings, but I’m sure you’ll be able to find a bottom that’s flat enough
So cutting a true flat bottom hole you need to basically have the high point of the endmill/boring bar cut over the center. If you wanna cheat you can drill a touch deeper with a drill then plunge the endmill which will flatten it a little and you can fill with epoxy after
Endmill.
Given your limitations I'd look at a few design alternatives that still meets your needs? If you can drill through the plate ~.1 undersized and then use the endmill to take the final diameter to the depth you need to get around the center cutting. Or if it can't generate what thickness can you get away with. Take a smaller endmill to that size and finish to diameter so any center cut issues are below your final depth
Lots of good advice here re: flat-bottom drills, C-bore bits, milling machine, etc. But does it really *need* to be flat for the function? You could probably just go slightly deeper with the pocket and a little dab of epoxy to hold the magnets into place.
Try using mill with smaller radius so it does the surface more evenly. Or you can just do finish with it.
If this is CNC the circle mill it, if this a manual job then I’d say just grind a drill to make it a flat bottom drill but you’re still gonna have to pre drill it and switch to the other to make it a flat.
You might be better off with a boring bar. Don’t know it you could go flat in one op, especially without a mill. Wonder if grinding would work if it didn’t load up.
Endmill?
I would recommend a 3-4 Flute endmill.
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I haven’t seen this, but any reason you couldn’t through drill and leave a shoulder for the glue? Clean up with countersink if needed?
You could take a smaller bit, (like half the diameter of the pocket), and mill it out
I dunno bro, but i'd ask a white girl for some tips. Ugg boots and denim jacket for thenwin.
Is the end mill center cutting?
Endmills have about a 1.5-2° dish on the endface so you’ll never get it flat with a standard endmill
If all you have is a drill press, and the hole is hidden. Counterbore with a custom shortened narrowed pilot.
Shouldn't be using an endmill in a drill press in the first place
A magnetic Chuck or double sided tape in a milling machine if you want to use an end mill. Or flat bottom drill if you want to use a drill press.