Boring Machine Tolerances

      Boring different door parts on different machines has led to some alignment problems. The solution: bore holes slightly oversize. October 2, 2007

I've inherited a small interior door making operation. Currently this operation produces about 200 raised panel interior doors annually in small batches. Doors are eastern white pine with a few in poplar. The first few batches of doors were bored using a Ritter single head, 2 spindle machine with good success but poor efficiency. Recently I brought the Ritter 3 head (6 spindle) machine on line to bore the stiles. The rails and mid stiles are still bored on the single head machine. This has not been without some problems. There are slight differences between all the holes when bored and quite a few of the doors have split when put in the door clamp. Differences of a few 1000's of an inch in spacing, angle, etc. of the holes seem to be the cause. The design of these machines does not allow for anything other than gross adjustments of the heads. Any fine adjustment would appear to have to be done with shims. Should it be possible to get all 4 heads into exact alignment? Or should I be using oversize bits and/or undersize dowels? Or bury the old Ritters?

I am currently using 1/2" bits and 1/2" X 5" grooved dowels and TiteBond I for glue. Dowels are very snug fitting. Tenons are cut on a single ended tenoner (no cope/profile), and grooves cut on a shaper. Dowels are centered in the 1 1/8" stock and all operations are referenced to same surface (with the exception of 1 stile which must be flipped in the 3 head machine). As an experiment, a small sample of doors were made with all of the rails double bored (flipped for second pass) and results were better with the slightly larger holes, but still unacceptable.

The plan is to increase volume and add offerings in hardwoods and cope and stick profiles. Any thoughts on how to cost effectively get this operation back on track? Different bits? Different dowels? New machines (absolute last resort)?

Forum Responses
(Architectural Woodworking Forum)
From contributor G:
We have a similar process, 2 single borers, one for stiles, the other for rails, and have no problems with alignment. Sounds like you need to take the time to set up the equipment correctly, maybe do some part repair, replacement and review, retrain the operators. To increase your volume, sounds like you need to add more machines and employees, which leads to more headaches.

From contributor L:
All the machines I have used for doweling have the bits oversized by 1/64". This will allow enough play to make your tolerances acceptable. It shouldn't affect the strength of the joint and it will give you a very slight wiggle room for minor adjustments in your part spacing.

From contributor M:
Contributor L is right; 1/2" boring bit and order your dowels undersized.

From the original questioner:
I went over the machines again and there is absolutely no way to make adjustments to the boring heads. The only adjustments are horizontal position of the head and table height. I did shim the worst head about 3 thous. It looks like I will have to work with what I have here. I am going to order a set of 33/64 bits and hopefully I will be off and running.

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