Round holes with flat bottoms

It's more that there's really no way to cut the center of the circle -- the best any drill bit or center cut mill can do is sort of mash the metal out to where it can be cut, and to minimize the amount of material that has to be so "mashed".

I'm no machinist, so take these with a grain of salt. But here are some solutions that I can think of:

1: get a more rigid machine to work with. I can do what you're talking about just fine in aluminum on a Smithy "3 in 1" (lathe, drill, lousy excuse for mill), but I doubt it'd like me trying in steel. 2: Check your alloy -- are you using one of the absurdly easy-to-cut alloys like 11L14? Can you? 3: Pre "drill" with a smaller mill, let it walk around, then finish with a larger mill. 4: Put the part in a rotary table, and come down in the 'z' direction while rotating the table -- this will keep your cut moving around, and should make it easier on the machine. 5: I assume you'd be doing this already if you could, but consider drilling a 1/16" pilot hole, and milling down over that. It'll leave a hole or a dimple in the middle of your big hole, but even that small of a pilot hole should span what your mill really can't easily get through.

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In addition to Tim's suggestions, use a quality cutting lubricant, a quality endmill, and recheck tightness of head-to-column and column-to-base screws.

Forces like those you're encountering can cause the material around the tips of setscrews to be displaced, effectively allowing them to walk.

If the drill point/web is fed to the desired final depth, the endmill will then be cutting everything but the center of the hole, until it stops without using more downward Z force. Retract the endmill immediately to prevent it from just rubbing, which will dull a good endmill fairly rapidly.

The tendancy to go around in circles hints that there might be something slightly asymmetric in the mill. In my minimill, the collets are well centered on the rotation, but the rotation axis angle is slightly off from the z-axis, but not enough to cause problems with the coarse stuff I have done so far. The perpendicularity of the Z-axis to the stage was way off, but at least that was adjustable.

Are you going full depth in one peck?

Thanks, Rich

The harsh reality is that end mills do not make good drills.

A study of the geometry of an end mill quickly discloses the fact that there is little chip relief between the flute and the centerline of the cutter. All it takes is a slight chip load, one side greater than the other, for the end mill to begin oscillating as has been described. The problem can be addressed by relieving the end mill towards center, creating greater chip space, but that comes at the cost of losing considerable strength in the end mill, possibly resulting in a broken lip.

Do note that an end mill does not normally create a flat bottom, but leaves a high spot in the center.. They are ground with a slight taper towards center to insure that the periphery is the lowest point in contact, creating a flat cut when the cutter is advanced on material.

Part of the problem with using an end mill for drilling is the fact that the periphery is relieved with a primary and secondary land. They are intended to cut on the periphery. Drills, by sharp contrast, are circular ground, so they pilot. The circular portion is generally relieved to reduce the contact area in an effort to lower friction.

If one desires to create spot faces or shallow counterbores, the option to use a counterbore with a pilot is always open. That yields not only a flat cut, but prevents the oscillation that is so troublesome. Negative aspect is that it's easy to break the pilots.

Harold

All one eighth of an inch? If it were a deeper hole, I'd be wondering if it wasn't material being caught up between the bit and the bore and suggest a vacuum to suck it away or a stream of coolant to wash it away.

-- Small opportunities are often the beginning of great enterprises. -- Demosthenes

Tim, if you haven't had the mill head column off of your machine to improve the metal-metal contact area to the headstock top surface, you might consider doing this procedure to gain some rigidity when milling.

The one I worked on was a 12x20 made in 1999 maybe.

While you're milling something fairly aggressively, put your left hand fingertips at the seam of the contact surfaces at te base of the column. If you feel movement, this procedure may increase milling rigidity considerably.

The mating surface of the column flange is likely fairly accurately flat, as it was probably turned (faced) on a lathe. The top surface of the headstock may be one of those areas where a factory worker ran a power scraper on it, just to smooth it somewhat, followed by filling the seam gap with filler putty.

I managed to get the headstock surface flat, by rough filing, and checking for contact area with plain printer paper laying on the headstock surface, and rubbing a (machined flat with a large shell mill on a large machine) flat thick slab of aluminum on the paper to make a carbon copy (although aluminum oxide) of the contact area.

I put cardboard and shop rags in the opening to catch the filing swarf.

When I started, there was probably only about 15% contact between the two surfaces.. when I had finally had enough exercise, the contact area was probably closer to about 75%, and the rigidity of the mill was increased greatly as a result.

My biggest complaint about rigidity on that thing is the quill, which I can somewhat deal with by adjusting the quill lock.

My biggest complaint about the use of the thing as a mill at all is that the quill drive (which is the only adjustment in the z plane) is much coarser than the x and y feeds and an odd number in both english and metric, being something like 0.43 inches. To top off the pain, it's got an itty bitty knob. So even when I've got the rigidity issues dealt with, it's very difficult to get the height difference between features accurate.

So even if the thing were made out of tungsten and worked as smooth as silk, I'd still be wishing for a mill that had a decent vertical feed mechanism.

Can I interest you in a Gorton MasterMill?

Needs a bit of work, but nothing major and would be a fun winter project for folks

Gunner

"Confiscating wealth from those who have earned it, inherited it, or got lucky is never going to help 'the poor.' Poverty isn't caused by some people having more money than others, just as obesity isn't caused by McDonald's serving super-sized orders of French fries Poverty, like obesity, is caused by the life choices that dictate results." - John Tucci,

Yep, I knew the downfeed had to be changed as soon as I saw it. I ended up making a different knob and added a dial indicator for Z adjustments.

There are several other methods of changing the downfeed.. one includes a small gearbelt, two pulleys and a handweel/crank. I think I saw this modification in the Chaski 3-in-1 Machines forum.

There are lots of refinements that most of the Chinese machines need. They aren't particularly difficult modifications, but they can involve significant amounts of time.

A big knob and a dial indicator (or a clever mount for a digital caliper) would go a long way to making it more usable.

If you were closer, maybe -- if you factor in lost time I think I can get a decent mill locally for as much as the trip down & back would cost. But if I'm ever going to be in your area with a trailer I'll probably contact you.

Ideally I'd get a smaller mill -- the biggest things that I seem to end up making are model airplane parts, and those just don't get big.

The problem is almost certainly one of machine rigidity. Yesterday I milled several 1/2" holes through 1/4" HRS (3" sq tube) with a three flute center cutting end mill with no pre-drilling and no issues at all on my Bridgeport. The holes milled smooth as butter with a few drops of cutting oil, and finished with the usual thin disk of metal hanging by a thread that knocked off with no effort.

That's how it worked with my manual Bridgeport too. :)

i

My ~700 Lb Clausing is marginal for boring steel cleanly with an endmill. It's OK at 1/2" diameter but not much larger. It has some trouble fishmouthing 3/4" and 1" pipe with large taper-shank endmills, not that chatter marks hurt a stick welded joint.

I think it's a little less rigid than a mill-drill. I didn't cut much steel on the RF-31 and have bored only plastic and sheet aluminum on a Bridgeport.

jsw

With my old Bridgeport, I once drilled/endmilled a 5/16" hole in a 51200 hard chrome steel bearing ball. It was painful (lots of heat), but it worked.

I still have it, I put that ball on a chuck key handle. i

I figure the Gorton Ive got is worth tops...$800 at most. It needs some cleanup and painting and the quill is stuck in the head..but its not a big deal to get it fixed up. B&S#9 ..which Wells Index will grind to R8 for $250

If a guy wants a mill to fix up and use..Id make someone a hell of a deal on it. Trade for guns, cash, other Stuff...shrug

Im not in love with it and Ive got other things to do this winter than fix it up. I took a year rebuilding my other Gorton, doing an hour here and there at a buddies shop and it meets my needs very nicely.

Anyone interested...its up for grabs

Gunner

"Confiscating wealth from those who have earned it, inherited it, or got lucky is never going to help 'the poor.' Poverty isn't caused by some people having more money than others, just as obesity isn't caused by McDonald's serving super-sized orders of French fries Poverty, like obesity, is caused by the life choices that dictate results." - John Tucci,

[...]

5: I assume you'd be doing this already if you could, but consider drilling a 1/16" pilot hole, and milling down over that. It'll leave a hole or a dimple in the middle of your big hole, but even that small of a pilot hole should span what your mill really can't easily get through.

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I am not sure that I understand that: As it is now, for a, say, 1/2" hole I would pre-drill with a 15/32" drill. This will leave a hole with vertical sides and a bottom that is a cone with 118 degrees included angle (give or take). As I then come down with the 1/2" mill I am OK till I start hitting the cone. I have stoped the procedure when the machine begins to labor and move around. I pull the end-mill out and there is still a conical hole in the centre which is quite a bit bigger than 1/16". I am not clear how a

1/16" pilot hole would help. Even if I could drill one which I cannot :-)

I take all the other points. I shall try with a 2-flute mill next time when I am doing a hole of the size I have available.

No question that the mini-mill is lacking in rigidity. My main concern was that the behaviour of the mill may be something abnormal. From what I am gathering here it is not.