Problem drilling aligned holes

[dcsleep]

I recently purchased a mill and am doing a simple project to get acquainted with it - a vise jaw stop.

I successfully milled the jaws to size within .001” in all dimensions which I thought was pretty good. Each jaw has three holes - one for the screw and two for the guide rods. To locate the center hole for the screw I used an edge finder to locate each edge and divided in half using my DRO. The numbers on the DRO during this process matched my expectations. I drilled the center hole for the screw and then used the incremental mode to place the holes for the guide rods equally on either side. I used a center drill to spot each hole before drilling. The same process was used for each jaw with the inside face of the jaw facing up.

Here’s the problem: When the jaws are assembled with the screws and rods they are slightly offset by about .010” - easily noticed with your finger. However if I assemble them in the same orientation that they were drilled (inside faces in the same orientation rather than facing) then everything matches up perfectly.

So obviously the holes are not properly spaced in the jaws which I confirmed by measuring each jaw - they are offset by a few thousandths. However I’m confused since I thought the process I used should have ensured that the holes were centered. I could also understand some “newbie inaccuracy” but since both jaws are identical in the orientation they were in the machine seems to indicate that something else may be going on.

I doubled checked my DRO with a dial indicator and it seems to be reading correctly. I also practiced edge finding with a 1-2-3 block and was able to get the results I expected on the DRO.

I’ve made two vise stops and have had a small offset both times. Any thoughts on what I might be doing wrong? Quite possibly an operator error...

 

[Tinkertoy1941]

Put your holes from the datum side.
If you just drill them from the top side the drill could drift.
How much drift depends on how the drill cuts and how long the part is

 

[JohnG]

Drills do drift, and holes that are just drilled are not usually considered to be accurate enough for precision alignments. Boring and reaming are needed to turn drilled holes into precision holes. One method, of several that would work, is to drill the guide holes undersized and the screw hole full size, assemble the jaws with the screw holes aligned and then bore and ream both guide holes in a single setup so they are aligned and parallel. Do you have a machinist's textbook? I return to Machine Shop Practice regularly for its excellent descriptions of technique.

 

[dcsleep]

I thought about drift. The parts are rather small - 3/8" thick - so not much room for drift through the material. To check this I measured the holes from both the top and bottom of the part. Looks like they are within .0005" from top to bottom as best I can measure them. But still good practice to drill from the mating faces. (BTW I assume that incorrect head tramming could also introduce this error.)

When I measure the holes from the datum side they are slightly offset from the center. On the first stop I made I didn't center drill the holes and figured the drill drifted a bit off center. So on the second one I center drilled the holes to prevent this.

Anything else wrong with my approach? I went slow and tried to be very careful on the second attempt to correct the error in the first one but got the same result. If it was just a thou or two I wouldn't worry but it's off enough to notice easily with your finger. Doesn't matter for this project but could for future projects so I want to figure it out. Not sure what I'm missing except to be even more careful on the next one.

Thanks, Tinkertoy1941!

 

[dcsleep]

Do you have a machinist's textbook? I return to Machine Shop Practice regularly for its excellent descriptions of technique.

Thanks, JohnG. I don't have a machinist's textbook. I'll have to look for one. If others have suggestions I'd love to hear 'em!

 

[RJSakowski]

When drilling small precisely located holes on a mill, I will drill a small pilot hole followed by an end mill that is slightly undersized and followed by a ream. Like boring, the end mill will cut a hole true to the spindle axis. The ream gives the proper hole diameter. I made tooling plates for my CNC mill and the holes are placed accurately enough that I can use six dowel pins for mating the plates to the mill table. Note that this is over-constrained to the extreme.

If using a DRO, the machined position should be as good as the DRO reading. If using the mill dials, the position should always be approached from the same direction to avoid backlash issues. Another factor when using the dial is lead screw wear which can result in varying backlash.

Long before I had a mill, I would make matching holes by using one part as the drill guide. Drill an undersized hole for a dowel pin to fix the parts and pin them together. Continue drilling and pinning the parts until all the holes have been drilled. A variation when one hole is tapped is to drill both for the tap diameter then drill the top plate for clearance and tap the bottom hole. Insert a screw to secure the two parts and go on to the next hole and repeat. This will insure that all holes will be aligned in the finished assembly.

 

[kd4gij]

When making parts like that I will clamp them together the way the go and drill all holes through both at once.

 

[Dan_S]

Check and make sure your vice is square to the table. If it's off, you can get inaccuracies like this.

questions:

1. are you using a spot drill followed by a regular drill?
2. what type of drills are you using?

As others have said drills can wonder. If you need an accurately located hole, the standard process it to drill undersized and then bore.

 

[dcsleep]

Thanks for all the thoughts!

@Dan_S: Yes, I spot drilled and am using a standard twist drills. The full set cost about $85. The holes are small (.125") so boring isn't an option. I intended to drill undersize and ream them but my reamer appears to be undersized (cheap set).

I don't think this is drill drift as when I stack the pieces in the orientation in which they were drilled they line up perfectly. If there was drift I would expect some misalignment, right? As I said above, it looks like the holes are slightly offset from center despite my being careful. I originally suspected my DRO but when I test it with an indicator or use it to measure a 1-2-3 block it looks fine. I'd be interested in approaches to verifying the DRO is accurate.

The conclusion I'm coming to is that I just need to be more careful (if that's possible!). I'm going to give it another shot this weekend.

 

[P. Waller]

If the holes are in the correct place at entry and are off on the far side the only reasonable cause is that the spindle is not square to the table, nothing else will cause this to happen.