More Chairs! - this time serious production

[Alan_c]

Here are some chairs that we produce for the local pre school and primary school market. Previously these were made with conventional solid pine timbers, mostly butt jointed and screwed - not meant as a high standard furniture item - very utalitarian but they got the job done for reasonable cost. Since we have had a few return issues I decided to re-look at the design taking into consideration what is now within my scope having a MechMate at my disposal. They are super strong and should last well in a demanding environment - I can sit on them and rock on the back legs with no flexing that I am aware of and I think I weigh a bit more than the average 5 to 7 year old!

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These are made from 18mm shutterply (I soooooo wish we could get decent quality reasonably affordable ply in this country - its either hight cost Marine grade or really crappy shutterply :mad:) The smaller size (320mm seat height) is the standard size we make and I am able to get 5 complete units from one 8'x4' sheet (2440 x 1220) Cutting time with three passes using an 8mm slot drill at 2000mm/min is about 1hr 35min per sheet.

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The larger size (400mm seat height) takes about 1.25 sheets to produce 4 with a bit more waste, not really an issue as these are only ever custom made for special order so I can charge a bit more!

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Of course having the CNC enables me to brand all my products easily.

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The first batch had very tight clearances on the slots and tabs and held together very well on their own, but the screws are added as we are talking about kids here and if anybody can test items to destruction, its kids! The second batch had larger clearances, but this time a little too big so the third batch was just right - a bit like Goldilocks :D

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The ply is a very nominal 18mm so on some of the chairs I have had to trim the protruding tabs with a bearing guided cutter in a router - this can be a bit frustrating - see point about ply quality above.

[MetalHead]

Cool Chairs. Do you leave them raw wood? Or do you paint them?

[sailfl]

Allan,

Have you thought of creating cut files for different thickness and measuring the wood before you start?

[Gerald D]

Good job Alan. That pic with the text engraving; is the ply above the text really that bad? The thicknesses of the layers appear to be badly uneven?

[CAM Craft]

all pine ply in SA varies from sheet to sheet or within the sheet the ply varies +-0.5mm. we have had "18mm" ply that ranges from 16.5mm-17.5mm. fun and games for interlocking components

[Alan_c]

The chairs, in fact all our products get 2 coats of gloss clear polyurethane, cost effective and durable.

The varience within one sheet can be so bad that when I run the engraving, depending on where exactly I zero the cutter, in some places it will cut too deep and in others the tip will barely scratch the surface. I now have to run the engraving as 5 seperate files, one for each area of the board with a local Z-zero in each area. :eek:

[CAM Craft]

what a mission, why not just brand it after cnc. therefore there is no tool changes per sheet of ply. or use a 2mm str bit set at 2mm deep. ie doesn't really matter on ply thickness

[Greg J]

Nice work Alan.

[PEU]

very nice, edges are rounded? can't tell from photos.

[Alan_c]

All edges are rounded with 3.2mm radius round over bit, see 5th image - joint assembly.

[Kobus_Joubert]

Nice simple and strong...Would like to see those oakies trying to break them.:p

[hennie]

Nice Alan, I have the issue with ply also real crap that we get.I ordered 6 mm ply and after measured it was 8.5 mm thick.You must see what some cabinet makers do with shutter ply but it takes a lot of sanding to get it right.

[lumberjack_jeff]

very nice chairs. Would the cost of 18mm marine ply make the project cost too much?

[Kobus_Joubert]

Marine ply in SA is VERY expensive...I just came back from town. Paid R 150.00 for pine ply 6mm thick 1.2m x 2.4m.

Marine ply quoted at the same time was R 450.00... 3 x the price

[lumberjack_jeff]

Marine ply in SA is VERY expensive...I just came back from town. Paid R 150.00 for pine ply 6mm thick 1.2m x 2.4m.

Marine ply quoted at the same time was R 450.00... 3 x the price

I guess everyone's economic constraints are different. Here it would probably be worth it to spend somewhat more for materials for each chair if it meant not having to do the second operation to mitigate the unpredictable thickness - or scrapping parts because of core defects, or fielding complaints because an undetected one led to product failure and complaints.

Perhaps the seat surface could be made from shutterply and the legs (since that's where all the stresses will be) made from BS1088.

Those prices are comparable to what I pay here in the states.

[KenC]

Plywood here has similar problem, I usually buy exterior grade plywood, so call water proofed plywood mainly used in cement casting form work. When cut, I get voids too...
I eventually resort to glueing up the thinnest available sheets of 3mm & build up the overall thickness. here, 3mm plywood are made out of 3 plies, 2 out of 3 of the plies are visible so we can control of 2/3 of the plies are in good shape. Rather than 18mm plies with 4 out of 6 plies that we can not see. This way the thickness variation is a lot less, +/- 0.5mm thickness variation is possible.
Don't do it very often as the only way to apply pressure for glueing is by placing many 4 gallon water filled buckets all over... but when my vacuum table is operational, I will surely be doing more of this...
Even with this, I'll still have to measure each & reconfigurate the joint dimension for each sheet to ensure good fit.
Just my $0.02

[hennie]

I am looking into using bamboo ply the thing with the sheets are that the thickness is the same all over the board.It is a bit pricey though.I got a price from a supplier to bring it in,in a container and that brings it down.Ken maybe if you have access since it comes from your side of the world give us some more detail on using bambo vs ply.Also it is a green product

[KenC]

Bamboo ply is a rare here.
We export a lot of very high quality plywood but the quality for local market isn't as good as those for export, the local wholesaler told me that the local market can't pay for the quality...
If you are looking for environmantal friendly stuff, I personally love to use rubberwood (http://en.wikipedia.org/wiki/Rubberwood), they are easier to cut too.

[Gerald D]

. . . used in cement casting form work. . . .

That casting form work is also called "shuttering" and that is where the name "shutterply" comes from . . . . plywood made for concrete shuttering construction . . . . often used once for a few days and then thrown away.

[KenC]

There is always something to learn
Thanks Gerald