Rob Cosman Offset Method of Cutting Dovetail Pins

[Thomas McCurnin]

I did a search on Google and on this site and did not find an answer or any opinions.

Rob Cosman developed a technique during the pandemic to cut dovetail pins (he is a tail first kinda guy) in which he cuts the pins with an offset for a saw kerf. I admit its a little complicated for me but is based on the assumption that the tails and pins will line up better if one offsets the pins by the thickness of the saw kerf, moving one side over about 20 thous and the other side of the pin the other direction by about 20 thous. He uses a wheel type marking gauge or a brass set up block (called a Shawn Shim) to accomplish this. https://www.youtube.com/watch?v=E8SrH6HbDdQ at 35:40.

My first reaction was this was making a simple operation more complicated to sell more tools (his saw with a graduated tpi, his special saw type marking knife ground to the thickness of his saw, and these set up blocks, also gauged to his saw). It seems awfully fussy at first glance.

Then again, I spent the better part of this afternoon trying to fix a poor sawing technique, and then realized what my instructors at Homestead Heritage kept telling me that 90% of dovetail success is sawing, not chiseling, and if you are spending more time chiseling and paring tails and pins, then the blame is your sawing technique, as it is easier to saw than chisel. It seems to me that he lines up the kerfs for the pins and tails nearly dead on accurate with this technique. This might make the difference, as I hate fixing dovetails with a chisel.

Has anyone here ordered Cosman's saw, the saw type marking gauge, the shim or used that offset method?

[Jim Koepke]

It seems Glenn Drake was demonstrating the offset method before Rob Cosman was known to many woodworkers. At least a decade before the pandemic hit. My memory may be off but my recollection is of seeing him at a Tool Event in Oakland, California which means it was before moving to Washington. That was in 2008.

The method is here > https://www.popularwoodworking.com/wp-content/uploads/Glen-Drake-Technique.pdf

When marking the pin board from tail board (or even the tail board from the pin board) with the boards held aligned, the line scribed with a pencil is not in the waste are but in the area needed by the pin to make a snug joint.

Glenn Drake was demonstrating his Kerf Starter Tool. They were (may still be) available in widths to match various dovetail saws. To make the mark work for accurate sawing the tail needs to be offset so the mark is made fully inside the waste area. Glenn Drake used a set of feeler gauges to set the offset.

This didn't convince me to purchase a Kerf Starter. It did give me a revelation that helped me to improve my dovetails.

One of my marking knives is made from a saw blade:

483048

It is my favorite for marking dovetails. The handle is rosewood and turned on my lathe. The brass is a compression ring from a Parker 1/4" tubing fitting. There are also small ferrules available in copper.

jtk

[steven c newman]

Being a Tails first guy...and prefer to do things simple...I'll just hang out for a while...and see just how complicated some people can make this simple joint

[Derek Cohen]

I did a search on Google and on this site and did not find an answer or any opinions.

Rob Cosman developed a technique during the pandemic to cut dovetail pins (he is a tail first kinda guy) in which he cuts the pins with an offset for a saw kerf. I admit its a little complicated for me but is based on the assumption that the tails and pins will line up better if one offsets the pins by the thickness of the saw kerf, moving one side over about 20 thous and the other side of the pin the other direction by about 20 thous. He uses a wheel type marking gauge or a brass set up block (called a Shawn Shim) to accomplish this. https://www.youtube.com/watch?v=E8SrH6HbDdQ at 35:40.

My first reaction was this was making a simple operation more complicated to sell more tools (his saw with a graduated tpi, his special saw type marking knife ground to the thickness of his saw, and these set up blocks, also gauged to his saw). It seems awfully fussy at first glance.

Then again, I spent the better part of this afternoon trying to fix a poor sawing technique, and then realized what my instructors at Homestead Heritage kept telling me that 90% of dovetail success is sawing, not chiseling, and if you are spending more time chiseling and paring tails and pins, then the blame is your sawing technique, as it is easier to saw than chisel. It seems to me that he lines up the kerfs for the pins and tails nearly dead on accurate with this technique. This might make the difference, as I hate fixing dovetails with a chisel.

Has anyone here ordered Cosman's saw, the saw type marking gauge, the shim or used that offset method?

Rob is a curious mixture of being an excellent teacher - certainly in the same category as David Charlesworth (whom he greatly admires) - and along with David, is precision-orientated, with techniques to facilitate this. At the same time, he is a salesman, has many tools and implements to sell, and manages to find a way to incorporate these into his demonstrations. Can you blame him being a salesman? That is is job and income.

When I first came across his dovetail videos, about 20 years ago, they focussed on sawing technique. This was done without aids and encouraged hand skills. Today, he offers aids to minimise the risk of errors. It is not simply about transferring marks to saw out the pin sockets, it is to ensure success at all costs. There is no "workmanship of risk", as David Pye might describe.

By contrast, I introduced the use of blue tape to aid in the transfer of marks. I also introduced a tape method (which he uses) to replace the #140 rebate trick. This involves the positioning of the tail board. You still need to saw, and judge where to saw to the line. Rob's "saw knife" creates the kerf into which the saw blade is placed. There is no longer any decision where to saw, and therefore no risk. He simply wants his students to succeed every time, as this sells his equipment. No different from using a honing guide.

Regards from Perth

Derek

[Scott Clausen]

I played around with the technique but balked at his kerf knife price and bought a no name version on amazon. At the end of the day I was adding steps and slowing down. I also had the Kats Moses magnetic guides. I later decided that perfect should not be the enemy of good. I jerked the training wheels off and focused on my saw technique. I still need more practice but I can fake my way through it now.

[Tom M King]

You hear "take the line" and "leave the line" all the time in carpentry, but I've never heard it by woodworkers. The reason I don't like knives for marking is that it's not readily apparent how you "leave the line". They work fine when you take the line. If you don't leave the line, they are cut too narrow. The offset just allows for that.

I use a Really sharp pencil. I'm particular about the pencil. I can leave the line, and then swipe it off with a block plane before assembly. If you do leave the knife line, there is a little indentation where the scribed line is.

Derek's blue tape method works by "leaving the tape".

For really narrow, tiny tails, I like to cut the pins first, and mark the tails by them. I can't get my pencil down to the bottom of tails cut with the tiny opening for tiny tails. Then I leave the line, and they fit. The offset is just another step making something so simple more difficult.

[steven c newman]

Been a carpenter long enough...remember there are 3 ways....take out the line, split the line, and leave the line.....I tend to leave the lines, cutting on the waste side only. I also find I get better results using the pins to lay out the tails...I use two types of "squares"....one to make a knife wall as the baseline...the other is a bevel gauge to do the lat-outs with..

I also tend to chisel out the waste....as adding a fret/coping saw to the mix...just one MORE tool to mess with....besides, one still has to pare with a chisel, anyway....let the chisel do all the work...

[Tom M King]

I left out "split the line" because it doesn't really have a place with dovetails. I take the line cutting pins so I don't have to get rid of the pencil marks another way.

[steven c newman]

I usually will split the lines when cross cutting with a handsaw....
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However, with a rip cut, I just follow the kerf..
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[Graham Haydon]

The offset method is at least 120 years old. It features is George Ellis' work on Joinery. You don't need a specialist tool, the saw will do it.

[Edward Weber]

The offset method is at least 120 years old. It features is George Ellis' work on Joinery. You don't need a specialist tool, the saw will do it.

It's probably older than anyone knows.
It's one of those techniques that craftsmen of a certain era developed through trial and error until they settled on the method of a slight off-set. It didn't need to be written down or marketed in any way, it just made a better finished product.
Many woodworkers of today are simply "re-discovering" many of the methods and techniques that have been employed for decades if not centuries and presenting them as new.

[Clifford McGuire]

I have good luck with the blue tape method. (Thanks Derek).

And since I 'leave the tape' (similar to 'leave the line'), I'm getting the needed offset. I just don't have the kerf to set the saw into.

[Charles Guest]

Nudge the tail board a little past the end of the pin board or the lap line, make a nice, deep mark then saw IN the mark not next to it. Technique has to be as old as the joint itself. You're marking the pin board a little fat, and using the mark as a ditch for the saw to run in.

I'm not sure if this is what Cosman is proposing, but if it is you don't need any special tools, tape, or anything else. Try a practice corner or two.

[Peter Mich]

I enjoy using the Rob Cosman tools techniques and tools because my dovetails are far more attractive and precise when I do so. Kudos to Glenn Drake, Derek Cohen and George Ellis for advancing methods that lead to improved outcomes and making my time in the shop ever more productive and satisfying.

[Jim Koepke]

For me, it is just trying to remember which side of my knifed line is the waste side.

Making a vee notch on the waste side used to help, but it takes extra time.

Different methods have been tried. Many of them work.

What seems to have worked best for my dovetail work is marking clearly and sawing appropriately in relation to the lines.

jtk

[J. Greg Jones]

I don’t know who first discovered this technique, nor do I think it really matters. But Rob first started discussing his results with this technique not during the pandemic, but over ten years ago. He played with it on his subscription channel at least 2-3 years before releasing the commercial DVD in 2014.

[Jim Koepke]

I don’t know who first discovered this technique, nor do I think it really matters. [edited]

It was probably discovered by the first person to cut dovetails and realized the mark was not in the waste but in the area of the pin that was to be retained.

Many likely understood this and cut to the waste side of the line. Then along came those who figured a few tools and a bit of salesmanship could make them some money.

jtk

[Edward Weber]

It was probably discovered by the first person to cut dovetails and realized the mark was not in the waste but in the area of the pin that was to be retained.

Many likely understood this and cut to the waste side of the line. Then along came those who figured a few tools and a bit of salesmanship could make them some money.

jtk


+1
Just as I said earlier. Who came up with it and who marketed it are two different things.
There are many "routine" techniques I've used over the years that I don't really give much thought to. It's funny to see them in a magazine as some sort of "tip" or "trick" or a'"a new way to blank". It's just nonsense. Everyone has their own methods of work, not all of them need to be marketed IMO
Not everything warrants the title of "new technique" or needs to be over explained as if it were some sort of revelation.

Even if RC released his method in 2014, that's still over 100 years later than George Ellis put it in his book. How long was it in use before GE wrote it down, I wonder?
A dovetail is a simple joint that has been in continuous use for a couple of thousand years, there really isn't much new, other than finding new ways to argue about them.

[Thomas McCurnin]

I have George Ellis's book, and the dovetail section is surprisingly quite short, and he only mentions half of Rob's technique, cutting the tails without removing waste, then placing the tail piece on the pin board to mark the pins using the kerf. There is no offset mentioned, but I only read a few pages staring at page 67 and it could be buried elsewhere.