Timber shaping

[Chris J Dixon]

I am about to construct a pergola. SWMBO rather likes the look of shaped timbers as seen here on rose arch kits:

The effect is as if a circular cutter has been used to take a quadrant slice from the end of each crosspiece.

I guess that a deep enough holesaw of at least 100 mm diameter might make such a cut. If I can fit it into my pillar drill, and clamp the timber in place, might I actually be able to complete the job successfully?

The other shape I am looking to make is 4 off like the larger of the two corner braces shown here

I don't have a bandsaw, and haven't room for one. I guess a good jigsaw might have the capacity, but I don't anticipate further need for one. Should I just grit my teeth and buy them ready made?

Chris

[newshound]

Chamfer offers almost the same visual effect. I find "external" curves fairly easy to carve using a coarse abrasive disk on a 125 mm angle grinder. Internal are more difficult, but can still be done. Another (not ideal) option would be a router.

[alan_m]

What about hand tools such as a coping saw for the curved bits?

[Thomas Prufer]

Pilot hole all the way through, hole saw, drill from either side using the pilot hole as a guide. Clean up using flap sanding disc in an angle grinder. Timber that size won't need much clamping, but clamp a block as a stop, so that they all come out the same.

Beware that hole saws tend to burn and scorch the wood, and its difficult to sand out.

50 mm isn't much as a radius -- and larger hole saws cost and require plenty of power.

Make a mockup of cardboard, waste ply, whatever, to check the silhouette.

Thomas Prufer

[Roger Mills]

Also, I suspect that the curves on the ready-made ones are not arcs of a circle. They look more elliptical to me - which you can't achieve with a holesaw.

[John Rumm]

Yup, BTDTGTTS:

At £13 a pop, probably...

Personally I would have made up a MDF template of the shape required and then fettled that until its a nice smooth representation of the finished shape. Then draw round that onto the timber, and cut out close to the line with a jigsaw or on the bandsaw. Finally, stick the template on the wood, and route round it using a pattern following cutter.

(alternatively, just cut accurately with a jibsaw and sand out the cut marks)

[tabbypurr]

and if the OP has no jigsaw, any saw could get close enough to complete the job with a chisel & sander, even if it means sawing at 90 degrees to the e xpected direction.

FWIW it's quite possible to use a handheld circular saw by swinging it left & right rather than forwards, just be a bit careful doing so, don't let it try to bite off too much at once.

NT

[Chris J Dixon]

That might just work for the corner braces, but is still quite a lot of cutting.

Chris

[Chris J Dixon]

That was a slight worry.

That is also a concern.

That would be part of the plan.

They do appear a bit like that in the photo, but when I got to see them the cut did look pretty circular, maybe with the centre slightly offset away from the edge of the timber.

Chris

[Chris J Dixon]

That is the kind of thing. The timber I would be cutting is nominally 50 mm thick.

I guess you used an offcut or backing piece to locate the pilot drill?

Right now I don't have jigsaw, bandsaw or router.

If I was to get a jigsaw (1) what would be the chance of my being able to follow a curve with a blade suitable for 50 mm material? I can see I might have more chance with thinner material for the corner braces.

(1) Recommendations appreciated here.

Chris

[Chris J Dixon]

My initial option was to suggest that, but the domestic authorities were not keen.

The crosspieces are nominally 50 mm thick, so hat sounds like a lot of chewing.

Chris

[tabbypurr]

2" and a jigsaw aren't a good combination IMLE. You've been very secretive about what saws or other cutting kit you do have.

NT

[Andy Burns]

I found a 65mm holesaw was on the limit for my HR2450 drill through 50mm thick oak, you'd probably get away with a bigger diameter through softwood, 100mm probably ok ...

[Chris J Dixon]

That was what I thought.

Simply because, despite plenty of history of DIY, I'm not in the habit of machining significant bits of wood, other than to get them flat, smooth or square.

Right now, in terms of cutting a curve, a coping saw is about the only tool that could do it, but I see that as impractical for repetitive shapes in 50 mm softwood.

I have no jigsaw, router or bandsaw. The circular saw is effectively irrelevant. I have no drum sander.

The pillar drill (which I have) was my thought in connection with a suitable holesaw (which I don't have), but was seeking advice from anyone who may have already tried it.

Chris

[John Rumm]

Within the capability of a hole saw, even if you have to flip the piece and cut from both sides.

I did not use the pilot at all. I just made a quick "fence" with a off cut of MDF as a base and a couple of battens glued to it:

Clamped that to the drill table, and offered the wood into the "corner" and clamped it. The position set such that it bit a perfect quarter circle out of the corner.

Problem is there are jigsaws and jigsaws... Most peoples experience is with offerings on the budget end of the market. Based on those they usually end up with a fairly low expectation of what the tool will do well, and also wonder at the sanity of people who spend £100+ on a jigsaw, since it "obviously" can't be that much better.

However if you have used both categories of jigsaw you understand what the difference is - and it is quite marked.

To be fair, you would probably get a reasonable result with a budget one for this application - use a good quality sharp blade, and some pendulum action if the tool supports it, but you *will* get a decent result and need to do less sanding with a "pro" level machine. (they are also much nicer to use IME)

Hence I would recommend something like:

I appreciate that for a one off job its probably excessive. However once you find how well they cut, you may find other jobs for it.

General notes on jigsaws:

[Thomas Prufer]

Maybe jigsaw/hole saw/chain saw to hack the wood to dimensions roughly, and the a sanding drum to get it true and/or unscorched?

The sanding drum I mean could be used in a pillar drill.

Random hit:

These can be diy-ed from wood, and also made to take sandpaper sheets...

Thomas Prufer

[John Rumm]

My (good) jigsaw[1] handles 2" thick softwood with ease. I have used it often for things like birds mouth joints on rafters. You can even stick a 5" cut depth blade in it if you want...

(my old B&D jigsaw, less so!)

[1] 15 year old Makita 4340CT

[John Rumm]

Its much easier on the drill when only taking a quarter of the circle in a cut - its also easy for it to clear the sawdust from the cut (since it falls out of the open end of the cut).

Doing the full circle (especially if drilling down), its much easier to choke the saw in the hole. The clutch on the HR2450 would probably let go with larger saws into hard wood when cutting like that. (I have cut

107mm diamond cores with mine - but you have to keep the cut very straight and take it very gently to keep the clutch engaged)

(My drill press is only something like 1/2" HP, and that managed ok with a relatively low gear selected)

[Tim Lamb]

A bench saw with a small diameter blade could work for the crosspieces. Set the cut depth with the blade stationary and fix a stop such that the saw teeth form the radius you want. Now adjust the width of cut such that the kerf just breaks the surface.

Go through all your crosspieces on those settings and then adjust the cut width for the next slice. Spokeshave to tidy up.

[Chris J Dixon]

Thanks, that's really useful information.

Chris