Tips for cutting mortice into green oak posts

[zeroseven]

Pergola build. Green oak, 4 posts 150mm x 150mm x 3m

I need to cur a mortice in each post to accept the tenon from the horizontal 'connectors'. The tenons I'm ok with, i think. The mortices i'm going to drill rough with a Forstner bit and then clran up / tune with a chisel.

Problem.....I'm not a chippy and am really doing this to learn, hopefully not by making expensive mistakes. I've read about mortice chisels, registered chisels but as I'm really just cleaning up could I make do with a normal bench chisel. And how do I keep the edges square? I'm assuming (!) that if the tenon is 50mm the chisel for the mortice should be 50mm to use as a constant width guide?

Any advice, tips would be very gratefully taken on board.

 

[Bm101]

From the little I know, my one tip would be take time to mark and cut the edges of your mortice as cleanly and accurately as you can first. Makes a big difference in my (very limited experience). Hope it helps. Good luck!

 

[marcros]

i would just get the biggest pig sticker that you can- ideally 3/4" and a big mallet- just go for it by hand. You could rout out some waste or drill it out but I dont think that it will save that much time. Go half from each side, nobody will see if it doesnt meet up perfectly in the middle.

Mark out, knife the lines. move in a fraction from each and hog out the waste. then clean up.

The pig sticker makes the job much more efficient in my limited experience, and easier to keep it square. You may want to clean up with a bevel edged one.

 

[Phil Pascoe]

Make sure you bore your first holes touching the end lines of the mortice so that anything you can't drill easily is between two holes rather than an odd 10mm or so at the end to chisel out. It's a lot easier to get it out of the middle. A two inch bit will cost you an arm and a leg, so you'll probably be better boring four one inch holes in the four corners to start with. If they're through, drill from both sides. As far as two inch chisels and bits are concerned, what you do for a one off job and what you'd do if you were doing it every day is a lot different. An ordinary auger is cheaper and easier than a (decent) Forstner.

 

[AJB Temple]

If you can hire a chain mortiser use that. This is what I use and it is very fast and perfectly square. I have my own though.
If you can't do that then try to get your hands on a framing chisel. People have wised up to these now though and they are usually pricey on eBay.
Failing that old one and a half inch or two inch chisels quite often come up on eBay really cheaply. I prefer this over pig stickers which are quite narrow.
Green oak can be surprising buttery - take care with the drilling as the wood can drag the drill all over the place. Make a real effort to keep the drill square on. I agree with Phil re auger bit. But use whatever works for you - all you are doing is getting rid of waste.
Think about joint placement - not all framers put the joints in the middle and they will consider the vertical alignment (or otherwise) of mortises for main joints and braces.
If your pergola has a roof on it braces (sometimes called knee braces etc) may well be a good idea to deal with wind loading.

 

[custard]

As has been pointed out green Oak is softer than dry Oak. Way, way softer in fact. It's like the difference between cutting your toe nails after a bath or before a bath!

So don't over think this, drill out the bulk of the waste then just get stuck in with whatever big old chisel you happen to have to hand and you'll be fine.

Good luck!

 

[Bm101]

Wash your tools too after. Green oak is pretty acidic I think.

 

[Woody2Shoes]

Hi - I did some of these quite recently. My advice is pretty much in line with what's already been said:

- Careful marking out (and then cutting/chiselling to the line) against a reference face pays large dividends.
- Rather than use a Forstner (not really the right tool for this job), I used a drill bit that pulls itself into the hole and cuts more agressively e.g. http://www.irwin.co.uk/tools/drill-bits ... oring-bits and a battery drill/driver.
- I used a wide bench chisel (one I don't mind getting beaten up or hit with a steel hammer) e.g. http://www.dm-tools.co.uk/product.php/s ... /STA016266

To keep my drilling/chiselling square to the surface I used an offcut with a square corner cut out of it - hold it on the workpiece as a 2-dimensional visual guide. I used masking tape on the drill bit and an adjustable square to gauge depth. This sort of idea: http://pop.h-cdn.co/assets/cm/15/05/54c ... 514-de.jpg

It's pretty quick and easy (and fun) to do - cleaning the black tannin stains off hands and tools afterwards is probably the hardest part!

Cheers, W2S

 

[Stiggy]

Just remember - if you're going right through with the mortice by hand, mark out and cut out from both sides so you meet up in the centre, then you should be perfect.

 

[Shrubby]

I would use a 25mm auger bit or beam drill ( just a big brad point bit) It's a useful size to have for other jobs and wont strain your drill
You can clean your hands with lemon juice
Matt

 

[Bm101]

Just a thought. In case you haven't seen it done and have circular saw... then just use a hammer and the wackitabit method. Very satisfying and fast. Cut the last one by hand. Clean up the cheeks. Job done.

 

[zeroseven]

Making a right pigs ear of this, the tenons were easy enough but the mortices are proving to be a nightmare. I've tried to size them to the tenons but they seem a little oversized, the problem being that the 'green oak, rather than 'buttery' is just downright wet......but to my question....the bottoms of the mortices are rough as a bears buttocks, but presumably this isn't important as I'm not going to be gluing but securing with oak dowels.

If I do need to smooth the bottom of the mortice, or should I need to in the future, whats the best way to do it? the mortice is approx 50mm wide, 125mm long and 125mm deep.....too long for a router.

Thanks

 

[ColeyS1]

If you really wanted the bottom of the mortice cleaner (can't see why it really matters though) you could use a forstener bit with an extension. That should leave them nice and flat.

Coley

 

[Fitzroy]

Zero,

I had to do some large (for me) mortices for the legs on my garden table, made out of greenoak 150mmx80mm, mortices were c. 30mm wide and 100mm deep, I drilled them out with a spade bit on the drill press (to get them as square as possible) then removed the rest with a chisel.

After making the first one I learnt that the biggest issue I had was with the marking out. I cut the tenons first and I was using these as a template to mark around however once the mortice was cut it came up large and bit sloppy. I changed from using a pencil to mark out to a stanley knife, I then cut out the tenon slightly inside the knife line prior to test fitting the piece and adjusting the mortice as required.

The joints I did were wedged through tenons so I came at the mortices from both sides, but despite best endeavours they didn't meet up that well and needed some real effort to get to the point that the tenons would slide through ok without being sloppy. At the end I couldn't say I was that happy with the joints, but once wedged they were solid as the shoulder on the tenon will be square and tight against the mortice. I expect your pegs will do the same, especially if the frames are drawbored with offset holes in the tenons and mortices, is this done on green oak frames?

However having said all of that about my lack of satisfaction with the joints, a year later the table has moved a bit through the seasons and the errors I thought were there are lost in time, the joints are still totally solid and square enough for a garden table! I'm sure your frame will be just the same.

Fitz.

 

[custard]

After making the first one I learnt that the biggest issue I had was with the marking out. I cut the tenons first and I was using these as a template to mark around however once the mortice was cut it came up large and bit sloppy.

One of the basic rules of woodwork is first cut the hole, then cut the bit that goes into the hole.

It's puzzling that when considering mortice and tenon joints it's the mortice that usually gets the lion's share of the attention. But in my experience about 20% of the skill in a well fitting M&T joint is in cutting the mortice, and 80% of the skill is in cutting the tenon. Mortices are easy, tenons less so.

 

[Bm101]

With green oak you should be leaving some space at the bottom of the mortice. If you allow for shrinkage of both parts of the timber in the joint and you dont leave some recess behind the tenon as the oak dries it may shear your pegs. . Other thing is that drawboring is not the same as pegging the joint. Again, it's a difference between green and dried timber. If you can spare the money Rupert Newmans book Oak framed Buildings is good and explains it well. All oak will shrink by a large amount as it dries, the drawboring is key to this and will lock the timber tight as it dries but you need to factor in those tolerances before you joint up a little bit I think. I'll pm you a picture that explains it well.
Cheers
Chris

 

[AJB Temple]

For those who are interested in this subject, I have every book on making timber framed buildings that is currently on the market in the UK, America and Germany. (At some stage there will be an academic paper emerging). I keep meaning to do a review as a stickie, but as ever time is an issue. If you only buy one book on this subject, if you REALLY want to know how to cut joints, lay out, etc by far and away the best book currently available is A Timber Framer's Workshop by Steve Chappell. This deals with tools, methods, joints, design and all relevant structural calculations for most structures we are likely to encounter. It is a US book but the principles work fine in the UK.

Agree 100% with previous advice, cut the tenons first, be super careful to get them square on large timbers, and then use these measurements to cut the holes. You can easily make the hole smaller but it is a right pain to wedge tenons to make them bigger. Also don't obsess over it I had a walk around Great Dixter yesterday, which has about 15 timber framed buildings from about 1600 to recently constructed. Some of the joints are exceptionally rudimentary, and timber dimensions are virtually random at times - but the buildings stay up fine.