Milling off center problem

Varga Knives

Varga Knives

Joined
Aug 12, 2006
Messages
1,302
Hey everyone!

I’m no machinist, my father was, not me. I’ve been using a HF mini mill for many years and ran into a problem that’s stopped me dead.

The problem occurs when switching tooling from a drill to an end mill. Example operations are drilling a pivot hole then switching to counterbore a pivot collar - pocket for the pivot collar is off center. Or, drilling a hole to be threaded and then creating a pocket for a lockbar stabilizer - stabilizer is off center.

I’m using an Albrecht chuck for the drilling with the x and y locked. Switch to an R8 collet then go right into the milling.

The problem doesn’t always happen but it’s a 50/50. The pivot collars and LBS are not out of round and I believe the off center bias is toward the same direction.

I trammed it up with decent run-out tolerance but that was a while ago.

I can not feel any play in the head/spindle and it happens with brand new end mills.

My apologies if I’ve screwed up all my terminology but I’d be happy to explain better.

Thanks as always!
Mark
 
Have you checked that your head/column is square to your table in both x and y? If you're going from a drill chuck to an end mill in an R8 collet then you'll be dropping the head down quite a lot farther with the end mill. If the head/column isn't square to the table then you'll have problems. If you chuck an indicator in the spindle and run the head up and down measuring against a reference square on the table that should tell you if you have an issue to address. A way to mitigate this issue is to hold a new (smooth shank, no bite marks) twist drill in an R8 collet instead of a chuck so you have the head at a similar height for both operations.
 
Ah, ok. I did adjust the runout as best as I could but only at one height. I will look I totally that.

Thanks
 
If I were you, I'd just buy a piloted counterbore and save myself a headache.

An alternative option would be to verify that you're still on center after you're tooling change, by placing a center finder into your collet before you commit to plunging the endmill. If you don't have a center finder that will fit the collet, just make your own "test pilot" with a piece of round stock the same size as your end mill. Turn the end to whatever your pivot hole diameter is, and that can be your test gauge after swapping tooling.
If you don't have a lathe, you can just as easily turn it with your mill. Use your vise to hold a tool bit, and your collet to hold the stock to be turned.
 
All good ideas except I’ve never had much luck getting piloted counterbores to last and one for an undersized 2-56…

I have a wiggler but no center finder. That sounds like a no-brainer to pick up.

Still doesn’t explain the problem, though.

Thanks!
 
I’d double check your fixture and wether or not your x/y stops are actually holding. Maybe put a dial indicator to measure any table movement.
Plunging will put a lot of torque on there when the tool enters the workpiece. I know the locks on my minimill don’t hold all that well.
 
As far as explaining the cause of your problem, CallumRD1 has a good idea about checking your column for square.

I do have some additional questions though:
Is the counterbore cut by your endmill staying round and within tolerance diameter wise?
How far off center are we talking?
Have you indicated the runout on your drill chuck as well as your collet and endmill?
Are you spot/center drilling before you drill your initial hole? It's possible that your drill bit is walking over slightly and drilling off center from the spindle to begin with.
 
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