Any machinists out there? Help with countersink

[gumba]

I'm trying to countersink a 7/8" diameter hole in the back of this wheel hub in the slanted section. It will be approximately .200 deep from the high spot. I used a 2 flute hss center cutting end mill. I tried plunging first, not much luck. Then coming in from the side taking a .025 pass. Though at this point I believe I already ruined the end mill. I've got 10 counter bores to do. I'm brushing on cutting oil as I go.
My questions before I order another end mill.
What is the best approach, plunging or from the side?
Should I be using a low or high speed.
Would a colbalt end mill be better than the hss?
thanks,
Harold

[SMV911]

Harold,

It's not exactly clear to me what you're trying to do but perhaps I can help.

1) First, a 'countersink' implies creating a tapered form such as for a 90° bolt head. If you're plunging using a square endmill and creating a flat bottom, you'd be creating a 'counterbore'. Lastly, if you're coming in from the side you're simply milling. Which is definitely what I would recommend if you can. The primary function of an endmill is to side-mill. Plunging is a secondary function.

2) A 2-flute HSS cutter is good for something soft like aluminum. If you're cutting something hard such as a high strength steel I'd recommend a 4-flute carbide cutter.

3) A 7/8" cutter is huge. Not knowing the best speeds/feeds off the top of my head, I'd recommend very low rpms. Perhaps 120-200rpm. Unless you need to achieve a 7/8" counterbore, I would definitely go smaller with the cutter size. Perhaps 1/4" or 3/8" assuming you can side-mill. Then run 600rpm or 300rpm, respectively. The smaller cutter will also be MUCH more affordable.

4) Perhaps most importantly: Secure that part the best you can! This will give you the best cut and longest tool life. If you can, turn those jaws around on your chuck and grab on the larger diameter of the part - I think you'll have much better results. Also, be sure to choke up on your tool as much as possible. Whenever you have chatter, the first thing to consider is the rigidity of your setup.

5) You're depth-of-cut of .025" is fine. Hopefully with a more rigid setup you could go deeper, up to half the diameter of the tool. Using oil is good.

Steve

[dw1]

Imho:

Cutting from the side is better than plunging, as there is more space for the chips & oil and amount of material cut can be a bit better controlled. "Plunge cutting" is best done with drill bits. My preference is for a 3-flute or 4-flute cutters, and much smaller diameter than the diameter you started with.

Be sure you are using a good cutting oil and enough of it to keep the area from getting too hot. From the sound of it, your original cutter overheated and dulled quickly.

By all means use the cobalt milling cutter, as you will probably notice a difference between it and HSS when cutting harder materials. Be careful, though, as cobalt cutting tools are more brittle.

The next step would be carbide. I have cut 17-4 PH using hss cutters and generous amounts of cutting oil, but using carbide cutters made it able to be cut quite a bit easier & faster.

I also wholeheartedly agree with the comments in the previous post.

[DUK]

I too agree with everything these guys have told you. I would add some air cooling for the cutter would help. Looks like the hub is fairly hard. Plunge cutting is not the way to go.

[gumba]

Thanks for the replies. I'm gong for a counter bore. I figured I already wiped out the end mill when I tried plunging. The 7/8" diameter is slightly larger than the diameter of the Porsche stud head. I'm going just deep enough to get a flat surface for the head to sit into.

I bolted the hub to the table for a more solid mount. I was using the 3 jaw chuck on a rotary table for the location. Also drilling the 9/16" stud hole first.

If 7/8" is the final diameter do I start smaller end mill and work up to that, or just start with the 7/8" and make shallow passes?

[r_towle]

Side mill the "counterbore"
On that angle, you will end up milling a u shape anyways.
There is no need to go deep enough just so you can remove more material.
It should be as little material removed as possible.

Rich

[Seabear]

Gumba
Like the guys above have stated, clamp the hub as firmly as possible and use lots of cutting oil to remove the chips. One of the biggest issues in machining is work hardening the part being machined causing early tool failure. This is accomplished by vibration, heat, dwelling at the end of a cut up against a shoulder, or taking too shallow a cut where the heat is not dispersed with the chip.
Use a cobalt end mill, and since you will be side milling there is no need for the more expensive center cut end mill. Keep your feed constant until you reach the end of the cut, then ease up to the shoulder but immediately back off so as not to work harden the shoulder. You should be able to get a decent depth of cut, .050 or so depending on how hard the base metal is, and use appx 200 RPM or so.
Remember that vibration is not your friend so clamp the hub so you feel no vibration during cutting or you will have early tool failure.
Don't throw a worn out end mill away, they can be resharpened for future use.
Good luck with your project, and don't forget your safety glasses!

[dad911]

Are you trying to mill through welds?

[gumba]

Not trying to mill through the welds. I cut off the old studs and welded them in. The new holes are going into the angled area between the old studs.

Looks like I made pretty much every newbie mistake on the first hub, not clamped down properly causing vibration and the hub moving, plunge instead of side mill, wrong end mill, not enough coolant, paused through the cut giving the metal a proper chance to harden, wrong spindle speed. The new end mills are on order and should be in by the early next week.

There should be a book, "Dummies guide to basic milling".

[Nine9six]

I assume your hub is chucked up in the 3 jaw indexing head, with the indexing capability to support your intended 5-hole pattern; and that the indexing head is mounted to your mill table.

Once your hole circumference and location have been established, I would start by center drilling all 5 hole locations, with a center drill shank size, larger than your next sequential drill size. In other words, center drill with a .500 shank center drill, and follow by, through hole drilling with a .375 drill. You do this so that your .375 drill flutes will not be affected by the angled top surface, when making initial contact with the hub material, causing the initial thru hole drill to be pushed or "walk" off center.

P.S. You cannot plunge cut with an end mill with 3 or more flutes; only a 2 flute end mill will support plunge cutting

Now that you have thru holes in all 5 locations, you can use a 3 or 4 flute end mill.

Depending on your part rigidity, you can either bring the holes to finish sizing prior to threading; or sequentially increase the bore of the thru holes to the final size prior to threading.

Once finish sized on the thru holes, I would counter bore using a 4 flute .875 end mill.

Follow with a 60 deg. chamfer on the thru hole, to prepare for tapping to size.

Tap to size.

P.S. All that welding on your hub could have heat hardened the parent material, which may be why you initially sacrificed a 2 flute HSS end mill. If you eat up a HSS center drill on the first hole, all machining will need to be done with carbide, in lieu of HSS.

There are many ways in which to arrive at your desired end (You could side mill the hub until you have a flat surface)...I personnally dont like making multiple set-ups, as long I can achieve the ridgity to support the machining operations.

Hope this helps,

[gumba]

The 3 jaw chuck is mounted on a rotary table. I used a Porsche hub to locate the pattern. I made the aluminum shaft to support the hub since there isn't enough material on the hub for the chuck to grab. The hub is held in place with the 14mm nut, washer and lock washer. This works fine to drill the 5 holes, but slips when I tried to counterbore by plunging. Probably would hold if I used a left hand thread. For counterboring I'm clamping the hub to the table. Since this is new to me, and I'm only doing 2, I'm having to do multiple set ups. Also having to do some lathe work, which hasn't been a problem. At this point I'm not tapping the holes but press fitting the studs in.

[Nine9six]

Harold,
Based upon your statement, "...slips when I tried to counterbore by plunging", indicates that your set-up lacks the necessary ridigity to accomplish the desired machining operations.

How is your 3 jaw chuck attached to your indexing table? Your set-up would be significantly more robust if you could mount your hub directly to the indexing table; however your setup would then need to include dialing in the bore of the hub, to assure concentricity to the bolt hole pattern that you will establish with your subsequent machining operations.

Part and cutter ridigity are always paramount for proper machining operations. I had to chuckle at the previous statements, regarding vibration being your enemy...Its actually an indicator of an insufficient setup, that lacks the necessary ridigity, and is an unsafe maching situation that can destroy parts, eat up cutting tools, shatter cutting tools; or worse, having your doctor separating cutter remnance from body parts (eyes). Whatever the outcome, the net result is not pretty.
Please be careful...

[gumba]

The hub spun about 1/4" on the aluminum shaft. I don't how to avoid that unless I index the shaft to the hub somehow. The chuck to the rotary table wasn't the issue, nor did the aluminum shaft turn in the chuck.
The chuck is held in place with 4 T nuts & 6mm bolts. The rotary table has slots for the T nuts. No problems with the hub moving when I mounted the hub to the mill table.

[Nine9six]

Harold,
I understand...After looking at your setup I knew that it was the hub moviing on the alum shaft. To prevent slipage, what about drill and tapping a 1/4-20 hole, thru one side of the bottom of the hub and into the alum shaft. Insert 1/4-20 bolt to prevent slippage.
Just a thought...

[Seabear]

Will the chuck accomodate the hub if you turn the jaws around and crank it out to its fullest effective diameter?

[redx999]

Harold:
With all the good advice that you have already received, I'll throw in my $0.02 worth.

It seems to me that you are doing the equivalent of a spotfacing operation. As stated above, rigidity is paramount. If you have a set of reverse jaws for your 3-jaw chuck, you could mount the hub directly onto the chuck.
I would also side mill the pockets and only remove what was necessary. This will minimize the integrity (strength) losses of the hub.

-JHG

[gumba]

Thanks for all the help. I tried mounting the hub in the chuck first and it didn't work. The hub has a slight cone shape so the chuck doesn't get a good bite . Also tried the reverse jaws.
Since I'm waiting for new cutters I may try either notching the side or top of the aluminum shaft and putting some type of key to to engage one or two of the splines inside the hub.

[Scott Douglas]

If you have a second hub are the holes already welded up?
If not, machine yourself a flange to mount it to using the old holes to bolt it on the flange. Do the machine work before you do the welding.

[Nine9six]

Originally Posted by Scott Douglas

If you have a second hub are the holes already welded up?
If not, machine yourself a flange to mount it to using the old holes to bolt it on the flange. Do the machine work before you do the welding.

Not a bad idea, but the heat from welding usually has a tendency to distort or warp any subsequent machining. This is why you usually weld, stress relieve, and machine; in that order.

The further away from center your machining operations are from your mount point (in this case, the alum shaft) the less ridigity to your setup.

To do this properly, a four point setup on rite-height adjustable stands and clamped to the rotary table, are in order. The bore of the hub will have to be dialed in concentric at both the bottom and top of the id bore for concentricity to the new stud holes and counterbores that will be established by the subsequent machining operations you are about to perform.