Drilling error, what to do different next time?

[J Tiers]

Couple things.

1) the angle of the center drill is wrong for you anyhow, so NO need to use it. Hole will be 82 deg almost certainly, possibly 90 deg if european. Drill has 60 deg cone.

2) Transfer punches work OK, but as mentioned, you do need to locate the drill over the punch, MUCH more error there than with the punch. Wiggler, yes, or just use the drill point on a smaller drill for a locator, moving part until drill does not deflect when lowered into the punch mark. An X-Y table on the drill press is the way to go for that.

3) When doing multiple holes, make CERTAIN the "model" and "work" do not move relative to each other. Clamp them together, and use a light touch hammering on the punch.

4) When reinforcing the punch marks, hold the center punch vertical, and tap once lightly to seat it, before hitting it to make the mark. If you don't, then any irregularity of the original punch mark may have moved the larger punch off true center. Have a look at it after tapping, to see if it is OK.

5) Those plates should locate on the large hole, the only reason for the countersink is to clear the dividing head guide arms/blades. So they actually should not need to do much except stop the plate turning and hold it down. Super accuracy is only of interest because the countersink acts as a "competing locator". In a thick plate, button head socket screws would work about as well (in a recess), and only one needs to be a good fit.

[softtail]

Probably clamping old one on top, and then indicating center on the holes and/or after first hole is drilled, 'index' it with something that fits well (maybe turned on lathe), drill second hole, 'index' that one as well, then drill third.

[Paul Alciatore]

J,

He said he is adopting the plate to a different dividing head. So, what are the odds that those two different heads have the same diameter mounts? I think there is a good chance he is going to be relying on the screws for centering. And if the plate is off center, that will introduce a once-around error when dividing with it. This error will be small, but if he is making gears it could be a problem.

Couple things.

....<snip>....

5) Those plates should locate on the large hole, the only reason for the countersink is to clear the dividing head guide arms/blades. So they actually should not need to do much except stop the plate turning and hold it down. Super accuracy is only of interest because the countersink acts as a "competing locator". In a thick plate, button head socket screws would work about as well (in a recess), and only one needs to be a good fit.

[Mcgyver]

Using it as a pump center then, not the spinning centering pin as when it's held in the spindle? Newbies might benefit from a fuller description here.

yes, indicator on tip which is seated in cross hairs or prick punch mark.......like this

[BCRider]

In practical terms I think your transfer punch marks were likely close enough provided the fit of the punch to the hole was a good one. More likely the problem started when you didn't correctly put the tip of the center drill on the mark despite your efforts. Been there and bought the shirt with the off center arm hole to show for it.

For something of this sort with the transfer punch marks I'd have drilled at least a shallow starter hole with a small and flexible drill as suggested. Something like 3/32 to 1/8 so it starts in the punch mark with little or no wandering. And drill only about as deep and the drill bit is in diameter. More than that isn't really needed. Then instead of clamping it I'd have hand held it to get a good start and only tightened the clamps once the front portion was well started. Or more likely since it's not that hard to hold I'd just have hand held the whole drill step.

You said that the drill countersink was the proper angle? That implies that the screws are a 60° flat head? That's pretty rare since most counter sunk head screws are a 45° and you want to use a 90° included angle C'sink. Assuming here that you're using regular screws you may be able to salvage this with a bit of care in how you open up the 60° C'sink's to the 90°.

[Arcane]

The simplest method is to clamp the two parts together and drill through the existing holes.

X 2. Even if the center holes were different diameters, just turn a shaft to have stepped diameters to line them up axially and then clamp and drill.

[Toolguy]

I would have used an edge finder or indicator to find the center of the big hole and called that XY zero. Then you go straight up one radius in Y and put a hole, then down Y, left X and put another hole, then right X for the last hole. Using dials or DRO you will be right on the money because you are accurately measuring rather than guesstimating. No need for layout even.

[J Tiers]

J,

He said he is adopting the plate to a different dividing head. So, what are the odds that those two different heads have the same diameter mounts? I think there is a good chance he is going to be relying on the screws for centering. And if the plate is off center, that will introduce a once-around error when dividing with it. This error will be small, but if he is making gears it could be a problem.

Then there is another error, the error of getting them lined up on-center, to go with the rest of them. Nobody said NOT to drill the holes accurately, just that in many cases they don't HAVE to be. If the hole is too small you jhave to bore it out, and may as well do it carefully.... If hoe is too BIG, then yes, it needs to be located by screws, or a bushing made.

I bought 2 extra plates for my LW that had only one with it..... they fit perfectly on the spigot, but I did have to add some holes, as the HOLE circle was different. The center was the same..... LW are not that common that I'd be likely to get a set at random.

[Forestgnome]

First off your method would have probably worked fine if you hadn't clamped the piece down. Let it float. Also one problem with center drills is that they cut on the side as well as the tip, unlike standard drills. A center drill will gladly drill offset to a predrilled pilot hole.

[BCRider]

Good point about the possible center hole. It's that center hole that needs to perform the alignment. The screws are just to keep it from turning or falling off.

[Cannonmn]

The simplest method is to clamp the two parts together and drill through the existing holes.

Sounds good to me, probably would have done that if I wasn't in love with transfer punches.

So maybe PDC = pitch diameter circle? Guess I'll look that up and see if it fits.

Btw the plate is done, had to stretch the misdrilled hole by 0.011 by side-milling, then rechamfered the 3 new holes to 82 degrees. Checked by sticking tight-fitting shafts thru the 4 holes with plates in contact as shown.

[Cannonmn]

PDC info, formula

http://math.stackexchange.com/questi...ircle-diameter

[dalee100]

Also your center drill will not have the correct countersink. They are 60 degrees and standard flat head screws are 82 degrees.

Hi,

While he most likely did use a 60deg center drill, (that's the way life seems to go). Center drills are available in 60, 82, and 90 degree and even bell shaped. All to be randomly mixed up during at the worst possible time.

Dalee

[old mart]

I would have used a spotting drill as first choice, or my smallest centre drill to start just to be able to see the drill tip on the punch mark.