rib bending question

[samb]

I was really excited to borrowed a cheemaun form. To you guys in the US & Canada, it might be common to be able to borrow a form, but in the UK it is not. This is the first I've seen and only know of a couple of others.
Along with the scarcity of forms is a scarcity of white cedar so I am having to use red. The home grown red cedar is quite knotty so I am having to use imported.

I managed bending the ash inwales and stem with no problem. I used steam and compression straps. This morning I tried a dummy run on a rib and although it certainly wasn't a failure, I'm not too happy. I decided not to use a compression strap because I'd be limited to how many ribs I could do at a time. The rib was slightly thicker than I intend to use. I soaked it over night, steamed it for 15 minutes and bent it using the technique described by Rollin (well it is his design after all). Everything went well, I thought, but I've ended up with what look almost like small creases on the inside of the curve.

Did I soak for too long / not long enough?
Did I steam for too long / not long enough?
Did I bend too quick / too slow?
Should I have used a compression strap?

I'm sure there must be an explanation. Can anyone help or are these marks expected? Would I be able to sand them out - or is it sign that there is now a weakness at that point?

Any ideas would be welcome. You obviously have the knowledge and it's probably on the forum some where but I just can't find it.
Thanks.

Sam

 

[Greg Nolan]

Red cedar does not bend as readily as the white cedars. Thinning the rib may well help. How much is "slightly" thicker on the sample you bent? The small creases are at the point of the greatest bending on the inside of the bent curve (at the turn of the bilge), they are likely the result of the wood collapsing from too much compression at that point -- I think that if you take your sample when dry and bend it further, you will find that it will break at one or more of the creases. I don't think that sanding can cure the problem -- it is not a cosmetic issue -- the structure of the wood has failed. A thinner piece of wood will not generate so much compression on the inside of the curve and so will bend more easily.

Fifteen minutes or a little bit more should be enough time for a 1/4" rib -- if the steam box temperature is high enough. You need the temperature as near to 212 F as possible for the entire time the bending stock is in the steam box -- it certainly should be over 200 F. If you had no problem with bending ash, your temperature is probably ok. But 1/4" rib stock will cool quite quickly -- you need to get it from the steam box to the form as quickly as possible

 

[samb]

Thanks.
The rib was nearer 3/8th than 1/4 - I haven't got access to a planer just at present so had to use what was available.
That's a good sign then - the thinner rib should be more successful. I'll give a 1/4" one a go when I can get in the machine shop.

I reckon I had it from out the steam box to clamped in place in about a minute. If this is too long, I'll have to get another pair of hands as I didn't hang about!

Sam

 

[Greg Nolan]

I sounds like you are getting it on the form and bent quickly enough. But a 3/8" rib is half again as thick as a 1/4" rib -- you probably could have used another 5+ minutes in the steamer. And just a little extra time in the steamer is better than too little time -- the wood has to be heated through for a successful bend.

 

[Fitz]

I would leave it in the steam box longer. Try half an hour.

 

[Jan Bloom]

Steaming: Rule of Thumb 1 hour per inch of thickness, 1/4 inch is 15 minutes, 3/8ths is 22.5 minutes. All times are + or - a bit depending on the wood. Soaking prior to steaming allows for better transfer of heat. Extra hands are always welcome when bending ribs. But 3/8 ribs are heavy, get your stock planed down to 1/4".

 

[divedog]

I tried bending some 3/8" red cedar ribs for my Morris and got the results you describe. I bought white cedar ribs from Rollin and had no problems. It was like night and day.