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6th January 2019, 03:01 PM #1Novice
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How strong does the apron have to be?
Hi All
I have taken on a job to build a BIG blackwood table for some friends. And I'm starting to hit a few challenges. The top itself is to be 2.7m x 1.1m. All the timber for legs/apron/t.top is 30mm thick dressed blackwood.
The legs are just three strips laminated and then taper-turned, top 90mm diam down to 70mm. They will be epoxy glued and hang below the apron at each corner.
The apron is the tricky bit, its made up of 80mm tall, 30mm thick boards....which end up in a 2580 x 580mm rectangular config. The apron has bridle joins in the 4 end corners and two ladder-like cross pieces [30 x 80mm] located at one third and two thirds along the apron. I've also installed short 45 degree braces across each corner and on the joins of middle cross-pieces. The resulting ladder-like creation is quite stiff end-end, but if you pick up and raise one of the corners, the corresponding corner down at the other end doesn't lift until the corner bit you raised is more then 2 or 3 inches off the ground. There seems to be a lot of twist or play in the frame in the lateral axis, but the joints are all quite tight glued and screwed etc.....its just a long way down to the other end....and 30mm blackwood is pretty flexible???
I'm anticipating that when the table top is connected to the table apron using plenty of blocks [or 'buttons'], the whole thing will stiffen up significantly.
However, I've noticed that the top boards, ie the dressed planks sitting in the shed waiting to be joined together; are starting to 'cup' a little. I'm worried that the apron as implemented won't provide much resistance to possibly quite cranky top boards, if/when they start to twist.
I'd appreciate any thoughts from more experienced builders on what I can do to stiffen things up up bit...or if in fact its as good as it gets.. I'm more used to working with hardwoods like river red gum. I'll try and post some pictures in a few minutes.
W
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6th January 2019, 04:55 PM #2Taking a break
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Don't worry about the base twisting without the top, that's normal and the top will hold it once it's fixed down.
The cupping boards are going to give you trouble; the edges are no longer square to the face and it's not going to join up flat without dressing it all again
Does the 580mm width include the legs? If not, the biggest issue I think you have is that the base is much too narrow for the top; I'd be going for overall 8-900 wide for an 1100 top. If the 580 includes the legs, it is going to be unstable and, IMO, look quite silly.
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6th January 2019, 06:20 PM #3
I'm also worried about your proposed leg to apron joint.
Based on what I understand is your design, the leg to apron joint will be very fragile and subject to catastrophic failure should someone kick a leg or attempt to drag the table.
I'm also not sure what you mean by the corners of the apron are joined with "bridle joints" -- do you mean finger joints or half lap joints?
regards from Alberta, Canada
ian
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6th January 2019, 06:57 PM #4Novice
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Hi.
Thanks for the thoughtful replies Ian and Elan Jacobs. Firstly, that was a typo the apron size is 2580 x 980, so the planned overhang of the table to apron is 60mm. I'm glad you are not worried about the base twisting with the top. The top boards are monsters 275mm wide and I intend getting a local joinery shop to
a) check that the boards are square, probably remachine them and biscuit join. b) If its not considered too difficult or expensive; I may get all the boards ripped into something like 135mm wide
I'm also worried about the legs. Epoxy will make the leg to apron joint strong, but they could still break the leg at the neck when moving it. This could be a caveat emptor deal, with yellow safety warnings on the legs ). The table top boards themselves weigh at least 50-60kg. The thing is wide enough to have some sort of support coming from the centre of the apron to each leg, but its supposed to seat 10-12 people.
If you know of any designs that get around this weak turned leg issue, I'd be very interested.
And a bridle joints is I think a sort of mongrel mortice and tenon joint, for corners.
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6th January 2019, 08:10 PM #5Taking a break
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Ah, 980 makes much more sense. Absolutely rip them in half and shuffle them around to combat movement.
Epoxy will make the leg to apron joint strong
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6th January 2019, 08:49 PM #6Novice
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Cheers Elan thanks for the info. Lo My 'joints' need all the epoxy they can get.
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7th January 2019, 04:30 AM #7
Can you post a photo of your "mongrel mortice and tenon joint" ?
and a sketch of what you propose for the leg to apron connection.
There is almost certainly a type of joint that get's around the "weak turned leg issue" but first I need to see what you have.
Lastly, have you thought about how the table will get from your shed to the customer? 2.7 x 1.1 x whatever the height is can be nearly impossible to get into an apartment.regards from Alberta, Canada
ian
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7th January 2019, 09:00 AM #8Novice
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Hi Ian
Below url depicts bridle joint as I've done it.
Https://en.m.wikipedia.org/wiki/Bridle_joint
I can't seem to reduce jpegs to a duitable forum size and my sketchin sucks.
I just glued all 4 legs to the apron, they have vertical channels that match the apron and just dop over the complette joint,
Anyway all the corner/leg joints are now swimming in titebond 3 and will be left undisturbed for 3 days.
The owner lives in a house close by and I propose taking the ttop as one seperate cargo, and the apron legs frame as load 2. Assemble on site.
Thanks for you interest and hopefully it'll be reasonably sturdy. Maybe they can bolt the legs to the floor )
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7th January 2019, 09:28 AM #9
The top will be fine, but I am not comfortable with the aprons being 8cm high. It is not enough even for a smaller table.
Visit my website at www.myFineWoodWork.com
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7th January 2019, 09:36 AM #10Taking a break
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7th January 2019, 11:37 AM #11GOLD MEMBER
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If there is timber available, I would seriously consider adding a 30mm strip to the bottom edge of the apron pieces. It could be set proud at the outer edge to form a mould.
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7th January 2019, 01:08 PM #12
Where does the taper start on the legs?
If they've been left square up top, they don't start tapering 'til after the apron, then you may be able to simply affix extra crosspieces between the long rails, rebated to the appropriate inside face of the legs, to effectively "double" the leg joint and increase racking resistance and load support across the short end of the table.
An apron inside the apron, as it were.
Of course, this depends on how you've fixed the diagonal corner braces and whether they can be removed/shifted.
- Andy Mc
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7th January 2019, 02:38 PM #13
now you have me really worried.
Does your use of the term "bridle joint" mean that the apron is 80 mm wide and 30 mm high?
Or are you using "bridle" to mean a finger joint?
I just glued all 4 legs to the apron, they have vertical channels that match the apron and just drop over the complete joint,
Anyway all the corner/leg joints are now swimming in titebond 3 and will be left undisturbed for 3 days.
regards from Alberta, Canada
ian
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10th January 2019, 08:12 AM #14Novice
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Thanks for all your consideration. And you are absolutely correct. The apron is 30mm wide and 80mm tall/vertical. Despite 45 degree bracing in every corner and on the middle cross bits....the whole thing flapped/twisted like twig in a gale..ie when I glued the heavy legs onto each corner. But...illumination struck, I ran up a couple of Oregan boards 40 x 80mm that fit exactly between the cross braces at each end. Than I clamped it really hard, to the existing blackwood 30 x 80mm apron...and hey presto it was a 70mm wide apron piece and the rigidity of the end improved by ? 20%. I am going to completely go over the whole apron and laminate a second 30 x 80 blackwood board onto the current flimsy structure. Then I think it'll have a chance of a decent life.
I also spent a lot of time shaping 80mm tapered plugs to squeeze in between the top of the table leg and the 45 degree corner brace in the apron corners. Once the apron is beefed up I think I can move forward.
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10th January 2019, 03:06 PM #15
I must admit to being less concerned with how much the apron frame wobbles from side to side -- as once attached to the top this movement will be irrelevant -- but more worried about the apron to leg joint. It is the depth of this joint that determines how strong the connection is, and thus how sturdy the whole table is. The width of the apron members is not that important.
In the picture I posted you can see how a deep apron is morticed into a square section of the table legs creating a rigid and strong joint.regards from Alberta, Canada
ian
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