Actual ply thickness?

[Chris152]

I need a piece of 4mm and a piece of 6mm ply to drop into a jig I've made for thicknessing small stock.

(The total depth of the pieces the plane will run along is 12.5mm, so allowing a tiny bit for the depth of the blade and slight tightness of fit into the grooves that the planed pieces will fit, inserting a 4mm strip of ply will leave a depth of just above 8mm for the stock, and 6mm ply will leave slightly above 6mm for the stock.)

The problem is that as I go through my scraps of ply, none are the standard sizes they're sold at.
So - have I just got some duff scraps of ply or are they all different from the stated thickness, in the same way that planed wood bought off the shelf can be?

Thanks.

 

[MikeG.]

I still find most ply I've got is imperial, not metric. What I thought was 3mm ply is in fact about 3.2mm......an eighth of an inch as near as damnit, and 6mm stuff is generally 6.3mm. You might have to plane up some solid wood if you want really accurate dimensions.

 

[Chris152]

That's my fear, Mike - without the assistance of a jig to get it even thickness... :?

 

[Chris152]

Apologies, I should have searched more carefully before asking.
Looking at this (scroll down a bit):
http://www.wisaplywood.com/Products/pro ... fault.aspx
it seems ply simply isn't consistent, that there's an accepted tolerance that's not so small?
And here it says that actual is typically 1/32" less than nominal:
http://theplywood.com/thickness

What I can't find is if mdf is the same - nominal is different from actual? Not that I want to buy two 8x4 sheets of mdf to make inserts for the jig, but i might be able to find offcuts.

 

[AES]

I've found the same thing, and in a country that's supposed to be all Metric.

As above, I've found the only way to get an exact thickness (for a jig or something really important) is to plane down a piece of hardwood, OR maintain a stock of really thin ply (in hobby shops, etc, you should be able to find some about 1 mm thick or less) then build up your own ply from a piece of nominal thickness that you need, plus a piece of the very thin ply (in fact, using it like shim in metal work).

Otherwise, if you're marking out fresh, use the thickness of the ply you actually hold in your hand to mark out a trough or a rebate or whatever you need to rout or plane out.

 

[Chris152]

Yep, if I'd checked the ply thickness before I'd started I could have made the runners for the plane the right height to work with the ply I already have.
Making up ply the right thickness is a good idea if I can find a source. That said, I have access to a decent thicknesser just up the road - is 4mm thick (x 50mm) thick enough to thickness down to on a machine or does it get tricky being thin? I've only ever used it on larger stock and no idea how it works with thinner pieces.

 

[AES]

Personally mate, I have no idea, I don't have a planer/thicknesser. But there's plenty of members here who have, and no doubt one or more of 'em will be along in a mo.

 

[MikeG.]

I'd be tempted to make a pair of very long very thin folding wedges, one of which you could fix permanently to the base, and the other would slide in and out depending on the depth required. Even better if this was done on the underside of the jig, pushing the flat base up parallel to the top of the sides. Too late for the latter now.

 

[MikeG.]

Yep, if I'd checked the ply thickness before I'd started I could have made the runners for the plane the right height to work with the ply I already have.
Making up ply the right thickness is a good idea if I can find a source. That said, I have access to a decent thicknesser just up the road - is 4mm thick (x 50mm) thick enough to thickness down to on a machine or does it get tricky being thin? I've only ever used it on larger stock and no idea how it works with thinner pieces.

Yes, you'll be fine, but you'll need a false bed sitting on the bed of the machine. No machine will let the bed get that close otherwise. If you make one of these, you'll need a lip on the trailing edge to prevent the bed being pulled through the thicknesser when the work is fed in.

 

[thetyreman]

the last bunch of 6mm ply I got was for some reason 5.5mm which was quite annoying because I wanted a tight fit on a drawer bottom, I'm not sure if it was a mistake or you can't actually get 6mm plywood.

 

[Woody2Shoes]

Hi - I'd remove the need to put packing underneath the workpiece by making the height of the sides adjustable - instead of screws holding the sides in firmly, I'd enlarge the holes in the sides into vertical slots and then use threaded inserts, bolts and washers.

Just like the guy does in this video: https://www.youtube.com/watch?v=zBTo3s3OB-s

This way you avoid trying to find strips of packing material of the exact thickness you want - which will always be problematic.

I think the reason that we often get ply in imperial sizes is particularly where we import from South America, or perhaps South East Asia where their primary market is probably the US. As someone's pointed out upthread there are surprisingly wide tolerances on ply thickness anyway.

Cheers, W2S

 

[Noel]

Ply is simply inconsistent. I don't think it has anything to do with metric/imperial but maybe it is. As mentioned 6mm ply at the moment is 5.2mm (at least from my local yards). 12mm seems to be 11.3mm or something. I have other bits that are as described, 6mm/12mm. Could well just be the usual supermarket ploy- same price, less product or likely bigger price, less product.
Regarding thicknessing, yes, use a board. Not only to raise the height but also in case the blades do come into contact with the table accidentially, some thicknessers will going down to zero.

 

[custard]

The stated thickness of ply is "nominal", ie there's a tolerance. Even on top quality Birch ply you'll find variation, and what's more you can sometimes find thickness variation even within a single sheet! It's inevitable really, there will be variation within the veneer slicing and the glue thickness will also be subject to variation.

If you need accurate thickness then use good quality MDF or solid timber, stated ply thickness will always be just an approximation.

 

[Inspector]

Would it be out of line to suggest you make a jig to suit each thickness you need? Otherwise I suggest getting slightly thinner plywood and build up the thickness with tape or glue on layers of cardboard. Or look to other materials like plastic, aluminium, steel, whatever you can find.

Pete

 

[profchris]

Acrylic sheet? This comes in 4mm and 6mm thickness and is pretty cheap (loads of suppliers on eBay). I'd guess the thickness is pretty accurate, though I've never measured any.

 

[Chris152]

Thanks everyone. Turns out my cheap digital caliper is useless - the digital readout is all over the place, so I've been using the traditional markings it has instead to measure the ply. I tried it on some scraps of perspex which to my joy registered only slightly over 2mm and 4mm, which might do the trick. Then I realised that set to no gap, it registers less than 0mm by about 1mm, so the scraps I have are actually 3 and 5mm. The caliper's now in the bin.

Do you think a couple/ few layers of masking tape to increase the depth of the perspex by 1mm work, or will that soon compress?
edit - nope, it'll need loads of layers to build up 1mm. I'll look into some of the other suggestions above!

 

[MikeG.]

I might have a little play with something like this at the weekend. I can see a gentle inclined plane cut into the side pieces, and a screw adjustment, with a scale on one of the sides.......giving a universal range and no need for packing pieces.

 

[custard]

Turns out my cheap digital caliper is useless....Do you think a couple/ few layers of masking tape to increase the depth of the perspex by 1mm work, or will that soon compress?
edit - nope, it'll need loads of layers to build up 1mm. I'll look into some of the other suggestions above!

My rule of thumb for callipers is either dig deep and get expensive digital. Or save a load of money and use the fibreglass bodied, dial callipers from Rabone or DialMax (make sure you get the Swiss made versions, not the Chinese copies). These dial callipers measure accurately and repeatably to 0.1mm, need no batteries, are easy to read, are unaffected by temperature, and never rust!

For 1mm shims you can use the plastic spacer blocks that glaziers and joiners often use. Another good spacer tip are decent quality playing cards which are generally an accurate 0.5mm. If you're stuck just PM me and I'll post you a bunch of both.

 

[custard]

Chris, if this is for thicknessing purposes then the workpiece should really be fully supported, rather than just at a few individual points. The downward pressure from the feed rollers is pretty extreme and will flex the workpiece, therefore ruling any accuracy and consistency, if there are any voids below the workpiece.

 

[AES]

Chris, I also find that provided they're good quality, old business/visiting cards are good for shim stock (provided there's NO sliding movement in your jig, JUST downwards pressure). Also old credit cards, hotel key cards, and suchlike are good, but with old credit cards you do of course need to cut off the embossed bits (sharp knife).

Personally I wouldn't be so sure about "Perspex" (there are various types of course - I usually cannot tell the difference) as some types at least are a bit brittle, and are especially prone to cracking around holes or sharp cut outs if any hole drilling or right angle cut outs are involved