Correcting dowel hole runout

Finding and correcting the cause of dowel hole runout in CNC-machined parts. November 7, 2000

We are using a new Rover 23 to drill and size melamine panels for kitchen cabinet boxes. When we run the panels we size them using the router and then drill for dowel holes. The hole pattern seems to run out when going back so the floors seem to run out on the gables, being flush on the front and ending up projecting on the back. We have noticed a small amount of warpage on the panels, but this problem seems to be greater than that. Has anyone else seen this kind of problem?

Are you using single or multiple spindles for either of your drilling operations? Do you know which holes are off, vertical or horizontal? Do you have your vaccum pods DIRECTLY under where you are doing your horizontal boring? Does the floor protrude below the end or vice versa?

Brian Personett, forum technical advisor

Heck yes, I have scratched my head about this for the last year and still do not have a definite answer--only speculation. I think more than anything, our pods do not consistently suck the board down the same every time. Let me make sure we're talking about the same thing. You're saying that the floor lines up flush in the front but runs out at the back of the case, right?

A second message was posted by the author of the original post:
I am finding the horizontal holes to be the biggest problem. I drill these with one drill as it is the only way to parametrically run the floors. The problem is the worst on small upper floors which will only fit on one pad. Yes, they protrude below the end gable at the back and are mostly flush at the front. There is no exact symptom, panel to panel--only general patterns which typically have the floor flush at the front and running down by about 1mm at the back.

To solve your problem you will need a dial indicator, a set of calipers, some steel dowels, and a fair chunk of time. You might be able to get your equipment supplier to come up with some or all of the above, but I wouldn't hold my breath.

First, identify what specifically is wrong with the boring (which holes are out, which way, and how much). Inserting steel pins in the dowel holes gives you a better reference than trying to measure to the edge of the hole. From this you can come up with a list of likely suspects.

For example, if you find that the horizontally drilled holes are out in the Z direction, it would probably be good to start by checking if your horizontally bored panels are pulling down flat. I'm not sure how your Rover operates, but you need to pull a panel down and then run around the top surface of the panel with a dial indicator mounted to the boring head. If you don't change the Z position, but your indicator reading changes significantly, you have discovered a fixturing error. This may be corrected by machining pods or rails, buying flatter panels, or just being more careful positioning your pods.

It's going to be a detective thing. You may even find more than one cause for your problem.

Thanks, that makes some sense. I have the same exact problem, but the only time it becomes a quality concern is on upper cabinet bottoms. Otherwise it never shows. I have taken a panel and sucked it down multiple times on the same pod in the same location and measured it with a digital capliper and come up with multiple height measurements, sometimes varing as much as .5mm.
Any thoughts?

If all of these factors are staying the same (pod configuration, programmed boring location, and which spindle is used), but the holes are still wandering around, I can think of three possibilities.

1) The bits are wandering. A bad grind job and bits that are longer than necessary make this situation worse. Also, the smaller diameters tend to wander more.

2) Some pods are pulling down sometimes and not pulling down other times. Check out pod levels, gasketing, and plumbing.

3) There is a mechanical problem in the machine. Use a dial indicator to check Z axis backlash and tool carrier tracking. Also use the dial indicator and strike off of the bottom of the horizontal drill while leaving the drives on. Lift, pull, and twist on the tool carrier to see if there is slop in the mechanism. Also jiggle the horizontal drill unit itself to see if it is loose.

It could certainly turn out to be none of the above, but this is a start.

The information about machining the pods is right. If you're using a bp 80 then it's not a lot different from the bp 140 I used. We had one pod that got gnarled pretty good. That one pod got bondo-ed and re-machined. When the pod was re-surfaced, it was re-surfaced 1.18mm more than the other pods. This was a very noticeable discrepancy when attempting to hold down panels larger than 2' X 4'. The result was that I had to re-machine each of the pods to match the one defective pod. The other factor was the gasketing in that one pod was also not up to par and that contributed to the lack of adequate suction.

I was having the same problem and thought it was the ptp. I checked into the situation and found that the dowel holes comming off the ptp were where they were supposed to be but the sides of the cabinets were not cut square. It looked like the dowel holes were out of wack, but that was not the case. Good reason to buy a new beam saw. I did so and now the parts are nice and square. That solved the problem.

The BP 80 has individual pods with a rubber boot on them. I believe the 140 has a hard surface pod with a removable strip type gasket and they run in the rails.

A couple more items to check:

Verify the level of the machine. A twisted frame can cause this. Slow down your horizontal drilling and see if things improve. If they do improve, the part may be moving some, or the drill is drifting on the banding before penetrating the panel edge.

You might also try manually dropping the horizontal block and measuring the distance between it and the drilling unit. Run the spindle up and down and measure it multiple times to see if it varies any. I have seen spindles that were sticking just a little give inconsistant measurements.