PDA

View Full Version : Drawbore size for Roubo bench build



Tyler Keniston
03-22-2016, 1:41 PM
So I've been toiling away on a Roubo build for some time. I am about at the point of drawboring the short stretchers and I was hoping for a bit of advice. Most every where I look it seems that 3/8" is the size pin being used for this type of operation. Yet I do recall Chris Schwarz mentioning using 1/2 inch (and then later 5/8" because the box store was out of 1/2" dowels). Though he may have been using a single pin whereas I am using two per a joint.

So I could easily just say, 'hey, why not 3/8", shouldn't be a problem.' But I just kind of like the idea of 1/2 for some reason. Looking at timber framing, they obviously use much larger, such as 1" pins, and this bench feels border line TF to me.

I fully expect responses stating I am over thinking this. And that's fair enough. But for any that have experience with drawboring, is there a reason to choose a specific size such as 3/8 vs 1/2 inch? I have also considered 7/16" since I will be making the pins. I might actually be leaning to this size now.

Of course you need the details:

It hard maple, with 2.5" deep x 1" thick x 3.5" wide tenons. (actually those are the larger ones, the smaller upper stretchers are 2.5" deep 3/4" thick by 3" wide)

As far as offset, I am thinking 1/16 seems reasonable. Hard maple is hard (tough though?) and the joints are large, but I don't want to blow anything up or make a sub optimal joint.

Jim Koepke
03-22-2016, 1:52 PM
The only answer that comes to mind is more philosophical than practical.

The pin should not big enough to over power the other material in the joint. It also shouldn't be so small as to have people look at it and comment about how such a big hulking bench is held together with tooth picks. For draw boring the pin is supposed to have some flexibility.

Since you will be making the pins, you can make them with heads larger than the working part for the visual aspect. While inside it is sized to do the job without splitting things.

For the sizes mentioned a 7/16 - 1/2" pin should work.

jtk

Roger Green
03-22-2016, 2:49 PM
Tyler,
I'd use 5/8" pins with 1/16" offsets. I'd also drill the holes clear through the legs. This let's you put a long taper on your pins, to help you get them threaded through the hole before it starts to do it's work.
I have recently finished my French oak Roubo bench. I used two 5/8" pins in each of the rail joints. My opinion is these are robust benches and the rail mortise & tenon joints are robust and therefore need robust pins.
I live in southwest Washington State & one of the local Guild members swears that when the big earthquake comes, if I can get under my bench, I'll be OK. Of course, this is woodworking and there are many ways to everything!
Random ROG.
PS: As Jim often offers if you are in the Portland, Oregon area your welcome to come by for a visit or help in assembling your bench!

Robert Hazelwood
03-22-2016, 2:57 PM
I think you will be fine with 1/2". I just did this step last week on my douglas-fir bench in progress. You can see details on my blog here: https://roberthazelwoodwoodworking.wordpress.com/2016/03/16/roubo-build-7-finishing-up-the-base/

I used red oak dowels from lowe's, carefully selected for minimal grain runout. My tenon dimensions are similar to yours, albeit of softer stuff. I was aiming for a 1/32 offset, which is probably too conservative. However I ended up with about 1/16" offset since the transfer punch I used made a giant crater on the tenons instead of a pin point, making it pretty much impossible to mark a smaller offset than 1/16". My pins are also unusual in that they are blind pins - I didn't feel like buying a longer bit and drilling from both sides was not likely to turn out well with a cheap drill press that struggles to drill straight holes in thick material. The upshot is that I had to use a very abrupt taper on the end of my pins. If you are going all the way through the leg with your pins, you can use a much more gradual taper which should ease the process.

I was worried about it up until assembly, where it worked like a charm.

The only issue I've had with drawboring was where the mortise walls were very thin. This was the case on some food pantry shelves I made, each having 40 drawbored M&Ts. For 20 of them, the mortise was 1/4" centered on 3/4" soft maple, leaving a 1/4" wall. On a couple of these joints, as the pin was driven through and bending through the offset, the point at the end of the taper caught in the hole on the back side, and took a chunk of the workpeice with it. I didn't notice until I flipped it over. Even then, the M&T connection still had integrity, and I just had to glue the missing chunk back on. This was a 1/4" pin. I think I could have solved this by not making the taper so pointy, instead terminating in more of a ball end. In any case, I don't think this could happen if there's a lot of material on the far mortise wall - so no need to worry about this happening on your bench.

To answer your more general question, I don't think there is an established guide to refer to for drawbore pin sizing. But the sizes you refer to, 7/16 or 1/2 @ 1/16 offset, seem very typical for stretchers on a roubo.

Warren Mickley
03-22-2016, 4:54 PM
I used 5/8 pins for my bench many years ago. I would recommend you use 5/8 or 1/2 and make them out of hard maple like the rest of the joint. 3/8 is too skimpy. Also you want to have the pins go all the way through the legs. In Roubo's design the stretchers are a little thinner than the legs so you can knock out the pins from behind for disassembly.

Malcolm Schweizer
03-22-2016, 5:50 PM
I was going to use 3/8", but then I saw a build where the guy used 1/2", and it looked proper for the size of joint, causing me to rethink just like yourself. My choice of 3/8" was based on a very scientific and mathematical evaluation... you ready?.... That's the biggest drawbore pin Veritas sells. Confession complete.

Funny thing- I just happen to have my Benchcrafted plans out working out my cut list, and before hitting the post button I checked; it says on the plans 3/8" dowels, so we would not be wrong using 3/8".

Phil Mueller
03-22-2016, 5:52 PM
I used 3/8 on mine...4 1/2" leg, 2" tenon. Monterey pine leg with oak dowel. I did apply a little glue as well. My bench has only been used for 5-6 months, but have seen no issues so far. With that said, larger certainly can't hurt...I think Schwarz is going larger these days as well.

Brian Holcombe
03-22-2016, 8:15 PM
General guideline is to leave 3x the diameter of the dowel between the side of the dowel and the end of the tenon. So 1/2" means you need about 1.5". All-in you need about 2-1/4"~. Sounds like you are good but should not go larger than 1/2".

Tyler Keniston
03-22-2016, 9:10 PM
Thanks for the thoughts. Helps me feel not crazy for thinking about using the larger pins. I've already put so much effort building this thing like a tank, I wanted to make sure to give it the appropriate bomb proof pins.

My biggest concern was that as the pins grew larger, their bending resistance (modulus of elasticity?) would grow exponentially given the nature of a cylinder, and that perhaps going much above 3/8 on dry hard woods was simply pushing it no matter how tank-like the build. On the other hand, I didn't want something too small that would take a set and not provide that wedging action... But it sounds like 1/2 should do the trick. I'm planning on using white oak.


I'd also drill the holes clear through the legs. This let's you put a long taper on your pins, to help you get them threaded through the hole before it starts to do it's work.

I'm glad you mentioned this. It is something I intuitively thought would be wise, to use a gentle taper so that the pin is already through the tenon, providing more of a wedging action than a bending around a corner action. Most write-ups and videos (that I've come across) don't seem to really talk about that or hint at doing anything but the most minimal taper point.

When you drill through the leg, are you driving the pin any certain distance through? Obviously you want to drive it until the tapered part is through the joint, but given the tapers you've used, does this typically mean out through the opposite side, just flush with it, not quite out...? I don't want to make super long pins unless there is a reason to drive it all the way flush with or proud of the opposite side.

Jim Koepke
03-22-2016, 9:52 PM
I don't want to make super long pins unless there is a reason to drive it all the way flush with or proud of the opposite side.

Just a thought, if it goes all the way through and is a bit proud, at least you know it is doing its magic on the tenon. It also makes it easier to knock the pin out in the future if there is any need.

jtk

Warren Mickley
03-22-2016, 10:22 PM
We usually make a tapered pin where the head is somewhat larger than will fit in the hole. The small end will fit in easily. Then we drive it in until it is somewhat snug. There is a little bending, but the main action is the pin wedging the joint up snug and a little crushing (compression) in both the hole and the pin to make a snug fit. It is best if the pin and the tenon are the same or woods of similar hardness so there is compression in both.

In the back you can leave the pin as is, but usually we saw it near to flush, or in the case of a cabinet door flush it off with a chisel. As mentioned, you want to be able to remove the pin by hitting from the rear.

My bench has 5/8 pins and they are 5/8 from the shoulder. My barn has 1" pins about 1" from the shoulder etc. You want the hole a lot closer to the shoulder than the end of the tenon because one way that tenons fail is that the tenon breaks out along the grain from the pin to the end.

Glen Canaday
03-22-2016, 10:43 PM
I drawbored mine as well. I made it out of construction pine. It's a Nicholson with a top of laminated 2x3s.

The dowels go through. I used 1/2" oak from the home center, carefully selected to be kinda round ;)

I used a #8 auger bit and put the holes in first, then put the tenons in to full depth. Marked the hole centers. Pulled 'em out and marked the tenon 1/16 up. I don't ever intend to take this apart again, so I chamfered the dowel end and glued the tenon and dowel when I pounded them in. Ain't goin' nowhar never.

Tom M King
03-22-2016, 11:03 PM
There is a house not to far from where I live that was built in 1770. It was documented that it was moved about 1/4 mile in the early 1790's. There has been much speculation about how they were able to move the house back then, including theories about rolling it on logs. I was called to fix a spreading problem in the floor. The floor joists did not have pegs in the holes through the tenons mortised into the center beam under the house. I told them there was no longer a mystery about how the house was "moved".

Jim Koepke
03-23-2016, 2:46 AM
I told them there was no longer a mystery about how the house was "moved".

Okay, I'll bite, how was it moved?

jtk

Malcolm Schweizer
03-23-2016, 4:19 AM
Okay, I'll bite, how was it moved?

jtk

Apparently they took it apart and put it back together, but someone forgot a peg or two.

Jim Koepke
03-23-2016, 12:51 PM
Apparently they took it apart and put it back together, but someone forgot a peg or two.

Thanks Malcolm, my dot connector is kind of on the blink... Since as long as I can remember.

jtk

Tom M King
03-23-2016, 6:53 PM
Pegs had been in those holes. I expect they tore them up somehow getting them out, and for whatever reason didn't make any more. Most other pegs I've seen in frames where through pins, so they could just knock them out, and put them back in. I've seen some 1-1/2" diameter pegs with more than a 1/2" bend in them from drawboring. Those empty holes were straight down through the tenon into stopped holes in the beam the mortises were in.

Also, the largest contributor to failed old window sash joints I see is too aggressive draw boring in quartersawn tenons. It pops the grain loose regardless of how long the tenon is in relation to the hole. Just had a few thoughts about drawboring that I've seen in old stuff.

Tom M King
03-25-2016, 9:35 PM
334534I found the peg with the 1/2" drawbore bend under my truck seat today. It's 1-1/4" diameter. It was holding a large tenon on a 10x13 beam mortised into a 14x14 Oak sill. It's Pine. Termites had eaten the Pine beam, and we hewed a replacement. That house was built sometime between 1777 and 1780.