locating holes in mill, what am I doing wrong?

[gellfex]

I've decided to use the mill more for repetitive small fastener holes, rather than laying out, center punching, pilot drilling, then main drilling.

So I put a stop on the mill vise jaw, and use an edgefinder to locate the back jaw and the stop, then set the XY for my hole and drill it in delrin using a screw machine length #30. It's off by 3 or 4 thou in both axes. I then layout the hole on the work, prick it, and use the sharp ended edge finder to double check my location, it's dead on. I switch to a 5/16 center drill, figuring maybe the stubby drill is wandering, and still get the error.

The drill is in a 3/8 Jacobs with a 1/2 straight shank in an ER32 collet chuck. I have about a .001 runout at 1" from the Jacobs, but even that should just give me an oversized hole, at least with the center drill. (question:actual runout is half the measured variation on the indicator, right?)

So any ideas on where I'm going wrong? On these pieces it's not critical, but I'd sure like to get my methodology right, rather than having to zero in by trial and error.

[KiddZimaHater]

Do you lock the table before drilling?
Is there a backlash problem?
Are your gibs loose?
There can be ALOT of variables on a milling machine.
Even a good, new mill.
Trying to hold tight tolerances on a manual machine can be tricky.

[Virgil Johnson]

Is the head of the mill perpendicular to the table? (trammed in) You mention edge finding the jaw and stop. Depending on the type of part I'm running I sometimes clamp a 123 block in place of the part and edge find off of the 123 block. Are you compensating for backlash when edge finding (if necessary)?

[gellfex]

Yes, I lock the table and account for backlash. My backlash is a lot more than 4 thou!

Remember, the pricked layout jibed with the edgefinder method. Maybe locking off the quill gives me the error? I'll try with an unlocked quill.

Edit: yup, that's it. It jumps 3-4 thou when I lock off. This is a Jet 16 mill-drill, so I can't use a knee instead, thought that would be a pain anyway. I don't think there's any way to tighten the quill.

If my shop wasn't in a basement with tough entries I would have upgraded to a knee mill a while ago. When I moved in I fantasizes about breaking out the steps from the outside hatch and making it a hoistway.

[juergenwt]

Lock your knee. "Snug up" your table lock screws. Pick up your position and than lock your table.

[gellfex]

So I did the edge finding with the quill unlocked, still had a couple of thou error on the X, compensated and was dead on for the part run. I noted the position of the X-Y corner "home", and we'll see if it's repeatable next time (since I never move my vise), or I need to trial and error again. Trial and error wroks nice on delrin since a drill can cut to it's new position easily, on steel I guess I'd need to be using scrap.

[aboard_epsilon]

locking the knee

if you do this ..decide weather you are going to do operations with the knee locked ...or knee un-locked ...and tram the mill for either knee locked or unlocked ..

cause there may be a lot of difference between these positions.

same with locking the table this can push it over .....

so like i said .when you tram ...think about these locking functions and tram locked or unlocked ..for the most accurate work.

all the best.markj