I have over 35 years experience as an amateur in designing and making single pieces of furniture, and am a long time reader of Fine Woodworking, but as you will see, I am not an “expert”.
I just acquired a doweling jig that made me rethink the general problem of how tight dowels should fit in their holes. I have used standard 3/8″ dowels for decades, and I used the old Stanley Doweling jig with a 3/8″ drill bit. Because of the difficulty in getting good accuracy, I have rarely used more than 2 dowels in joining a rail and a stile. I have never had a failure. I know how hard I need to press the single dowel into a single hole, and how hard to press the rail and stile together with two dowels. Now here is my problem.
I just acquired a jig which will let me use many 3/8″ dowels in a single joint, with great accuracy with placing the holes. The jig allows me to drill five dowel holes in a single placement of the jig. The more dowels you try to use at a time, the more pressure it takes to clamp the two pieces together when glue is applied. So the jig came with a manual which said you should use use a 9.7 mm (.382″) drill with the 3/8″ dowels (.375″) if you are using “multiple dowels at a time”. It also says you can use a 9.6mm (.378″) bit for a tighter fit, or a V bit (.377″) or a 3/8″ (.375″) bit for increasingly tighter fits.
I found that when using the 9.7mm bit, the dowels were a bit “sloppy” when I put them in the dry holes. The holes were nominally .007 larger than the dowels. However, when I put glue in the holes and on the dowels, the fit was tighter. I used five dowels in joining two pieces of 3/4″ thick wood that are 5″ wide. I was able to press the two pieces of wood together with my hands. Is this “too loose”?
I then tried a 3/8″ drill with the 3/8″ dowels. The dry fit was nice and tight but they could be pressed in by hand. When I added glue to the holes and the dowels, the fit was very tight. Again I used five dowels to join a 5″ wide board to another 5″ wide board (as in joining the front and the side of a drawer). I needed to use a Bessey K Clamp to pull the two pieces together.
I realize that there are no easy general answers to my question. I have seen that the holes are “larger” in pine than in hardwood. The larger the number of dowels used, the more difficult to pull the two pieces together. Etc.
I could have just as easily made these drawers using dovetails, but I just bought the new doweling jig and wanted to see what it could do. Maybe you might have some rules of thumb on how tight dowel holes should be (drill size versus dowel size)? Maybe you have some rules of thumb on how tightly a dry dowel should fit into a hole, given that it will be harder to get in when you put the glue (white) on.
I have done a lot of research on the web to find the answer to this question, and have found many scholarly articles on single dowel holes, and on the effects of moisture, etc, but I have found nothing on how tight a fit one should try for when using multiple dowel holes. On the Fine Woodworking site, I found Bruce Hoadley’s article on “The Dowel Joint” and James Krenov’s “Doweling”, but they didn’t really address my issue of dowel tightness when using multiple dowels.
A last note: I tried the “compressed” 3/8″ dowels which are supposed to expand by .005″ when waterbased glues are added, as well as the old standard 3/8″ dowels with the spiral groove. I am sure that complicates the issue. Any advice on compressed versus regular dowels?
Any advice you give will be greatly appreciated.
I apologize for the length of my “question”.
Thank you.
Mel
Replies
There are several factors that affect the tightness of dowel fits.
The first obviously is friction between the dowel and the sides of the hole.
A second is the hydraulic pressure required to move the excess glue past the dowel through the relief grooves if there is excess glue in the hole and not enough additional depth in the hole to accommodate the surplus glue. The approach to minimize this problem is to be sparing in the use of the glue and to leave some space in the dowel holes to accommodate the excess.
A third force that will resist a dowel entering a hole is air pressure if the fit is so tight and the glue sufficient enough to prevent the air from escaping through the grooves in the dowel. The fix for this is again to be sparing with the glue and to use dowels with at least one large and straight pressure relief groove, shallow spiral grooves being too easily blocked by glue.
A fourth factor is the glue setting up and grabbing before the joint is fully seated. This could be affecting your assembling the larger joints with multiple dowels since the glue will be sitting open a lot longer in the joint elements as you apply glue to the several dowels and/or holes before you mate the two halves. Many glues, including the common yellow glues, can thicken considerably in just a minute or two once applied to wood. The fix for this is to use only fresh glue and to dampen the insides of the holes with a water dampened brush before applying the glue.
A last factor when using multiple dowels is the overall alignment between the multiple holes. This is a very common source of problems when assembling dowel and finger type joints. If the hole spacings are off even slightly then the misaligned dowels will need considerably more force to be pushed into the mating hole.
My personal preference when assembling a joint is that there should be some resistance, showing that wood is clearly touching wood, but not so much pressure that the glue is being scraped completely off the mating surfaces and/or wood fibers are being torn. In practice this translates into a joint that can be assembled with either blows from your fist or light tapping with a mallet.
If there is so much resistance to mating that very firm pressure from clamps is required for assembly, then I would try to loosen the fit slightly. From your experiments it would seem that a 9.6 mm bit, midway between the too tight and too loose joints you made, might be about right. If you wanted to fine tune the process you could start out with a small bit for all of the holes and then enlarge just one or two holes to get the best fit.
John White
John,
Thank you for your analysis and advice. I will take all of your advice. I really appreciate it.What would you think about using liquid hide glue (from out of the bottle). I haven't gotten to the point of using hot hide glue. I have used the liquid hide glue in antique repairs, and have gotten very solid joints. It has a very long open time. Any thoughts?
Thank you.
Mel
Dowls Too Tight For Holes
I know I'm replying to an old post, but I found this post while trying to solve the same problem; Maybe this will help someone else. I had the same problem; Dowl pins were so tight that I had to tap them in with a hammer. I solved the tightness by puting the dowls in my wifes food dehydrator for two hours. This removed the moisture and shrunk the dowls enough, that the dowls just slid into the holes without any force. I assume they will re-swell when they absorb the moisture from the glue. Give it a try; It worked for me.
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