Bending 1/4" thick banjo rim

[Brian Glass]

I am completely new to this forum and hope someone has experience with bending 1/4" material for a banjo rim. I have tried steaming them but seem to over cook them and they warp like mad as they dry or, they crack as I bend them. I clamp them on to a solid form during this process. I have been wondering if I should be using a bending iron instead of a steam box? Any thoughts?

[Charlie Schultz]

Hello and welcome to the forum! Please note that we require the use of real names (first and last) here, please PM me or one of the mods to get your registration updated.

I think it's going to be tough to bend 1/4" wood to that tight a radius- what kind of wood are you using? Are you using a bending strap on the outside?

[Dave Stewart]

No idea. I've made lots of windsor chairs & things so I've steam bent heavy oak / ash.
I recall seeing a "How it's Made" segment on solid drum kit rims & remember it was brutal .... lots of heavy equipment to backup, bend & hold in position, then scarf & join the ends, lathe cut to perfect diameter etc etc. Me? I think I'd laminate strips of bendable 1/16" ply around a form, then veneer.

[Andy Birko]

How long are you steaming? As I recall, it's supposed to be 1 hour/inch so 1/4" should only be 15 min.

I haven't bent 1/4" but I have bent 1/8" and that just required hot and wet (poured boiling water into a tube holding the stock). It seems like it shouldn't be that bad.

[Brian Glass]

I am bending to a 12" or 13" radius. Have just started so haven't gotten as far as getting a bending strap yet other than a 1/4" piece of plastic I had on hand. wondering where I can get a 3 1/2" by 48 long piece of spring steel to use for this? Used a pice of 1/4" thick maple on the last bend and got it around the form ok but It went haywire a few weeks later when I took it out of the jig and let it sit in ambient air. Looks like I have to learn a lot on this one. Wasn't sure how long it should steam for and put it in the steam box as it was heating up and left it for a good hour plus after that. I think I should be getting the steam box up to temp and then add the wood for the required 15 minutes or so. Any thoughts?? So is that a no to using a bending iron instead? Is this too thick for an iron??

Brian Glass

[Michael Lewis]

What temperature is your steam box getting to? Steam is super heated water, so you need to be well above 212F (100C). What sort of boiler and pressure system are you using? Roger Siminoff heats his wood with steam and it gets floppy while it is still hot but cools and stiffens quickly. You might contact him for some pointers here, as steam can be very dangerous if it gets out of control. Roger is full of information.

A bending iron or heat blanket can deal with 1/8" (3mm) thickness wood, so use more layers and build up the thickness. It would be much easier, unless you have a good steam set up.

[Greg Robinson]

No actual experience here myself, but an idea. Would giving it another heat cycle on the form help? Like in an oven or something? I know a lot of the acoustic builders here with bending jigs do one or more temperature cycles still on the form for less springback.

[Brian Glass]

Michael, I am using a homemade steam box made from plywood with an interior size of 8x8x 50 inches. I use a Home Depot bought wallpaper steamer to generate the heat and the wood does come out of there ready to bend. I can get about 30 seconds to work it before it stiffens up. I never thought that I could over steam it, but now I wonder if that is what I have been doing. I have generally been throwing the wood in the box and plugging in the heater and leaving for an hour and a half to heat up and then bending it up. I will try bring the box up to temp today and then put the wood in for 15-20 minutes and see if that helps. Now, how do I find Roger Siminoff.......

[Michael Lewis]

Find Roger with Google!

When bending wood for guitar sides we generally use MUCH LESS water and more heat. Since your wood is thicker you may need more time getting it heated through but I don't think you need to soak the wood.

Steam is invisible, when you see the clouds of "steam" it is cooled and condensed, and no longer steam. You need more heat, more than 300F, and water boils at 212F unless contained in a pressure vessel.

For safety sake I suggest you use two layers of wood to make your 1/4" (6mm) thickness, as working with live steam is fairly dangerous, it makes the worst type of burns on human flesh. Talk with Roger.

[Bryan Bear]

My guess is that you are under cooking rather than over cooking. I doubt that a wallpaper steamer is able to make enough steam/heat for an 8x8x50 inch box. You may be getting it hot enough to make the outer surfaces pliable but the inside of the stock is not really at bending temperature. You are able to muscle it onto the form, but when it cools the outer surfaces wants to stay bent but the inner part of the wood is still under tension. Releasing the clamps will make it crack or go wonky. Just a guess though. . .
I’ll pile on and say laminate thinner stock. By the time you figure out how to beef up your set-up enough to do ¼ inch material, you could have been finished with laminates and it will be a whole lot safer.

[Jon Whitney]

I got here late, but here's my 2 cents worth and it comes from actual experience:

I've bent 3/8" white oak to a 12 1/2 diameter single ply rim for two different minstrel banjos. I haven't done it with maple but I've heard that maple steam bends well.

1) Sorry Michael but you're way off on the temperature. If you can get it above 200 you're OK, you don't need to go to 300 and there is no way to do that with a wooden steam box anyway.

2) a bending strap is absolutely necessary. It is not necessary that it be stainless spring steel, a pieced of galvanized sheet metal (such as is sold as flashing for roofing) works fine. That is usually wider and longer so you'll have to cut it to size with tin snips. Beware of sharp edges.

3) 1 hour per inch has worked fine for me - in other words, 15 minutes for a quarter inch.

4) as you have experienced, you have precious few seconds to get the wood bent around a form and clamped into place. Plan your work carefully. Use the princple of leverage.

5) It doesn't take long to cool. Once it is cooled, you need to glue the joint to keep it together or clamp it. I've didn't have any problem with warping on my two rims. I did have problems with springback once I weakened an area of the first rim by cutting a hole for the banjo's dowel stick (it made the rim slightly egg-shaped). I think the less tension you have to induce back into the rim by cold bending to get the scarf joint together, the better. So it might be best to bend it tighter to allow for springback.

6) I've thought about reheating to relax the wood more (once it's in some kind of circle clamp), and about cutting the scarf joint before bending, and about pre-soaking the wood with water, either cold or hot - but I've never done any of these things yet so I can't tell how effective they would be.

[Brian Glass]

Thanks for the input Jon. My perforated stainless show up so i am going to make a drying form out of it and try reheating in the oven to see if that a) helps dry it more evenly and b) allows tge wood to take the shape better. Will let you know how that goes in a few weeks. I am looking for some spring steel for a backer but am not sure what guage I should be using. Thinking 20 guage but is that too heavy or not heavy enough. I am going to make a smaller steam box and wrap it in polyisocyanurate insulation and seal it up tighter and monitor the temp better as well. So much to do, so little time....

[Michael Lewis]

The glass transition temperature of maple is in the vicinity of 340F - 360F. This is where the wood easily yields to to take new shape. If you are bending the wood at lower temperatures you are forcing it and risk fractures and for sure spring back.

When bending sides for guitars and mandolins it often scorches as it becomes pliable and relaxes as you direct it into the new shape.

[Jon Whitney]

The glass transition temperature of maple is in the vicinity of 340F - 360F. This is where the wood easily yields to to take new shape. If you are bending the wood at lower temperatures you are forcing it and risk fractures and for sure spring back.

When bending sides for guitars and mandolins it often scorches as it becomes pliable and relaxes as you direct it into the new shape.

No doubt it would bend a whole lot easier at 340-360F, but that is impractical for the home builder. I really doubt you could heat 1/4" thick wood to that temperature with a bending iron or blanket, since wood simply does not transfer heat readily. It's a whole different ball game from bending wood half or a third of that thickness. We just have to use compression straps to minimize the fractures and try to deal with the spring back another way.

I'm looking forward to seeing the results on the re-heating to control the spring back.

[Brian Glass]

Jon, I have yet to really see a compression strap in use. What are they made from and how are they attached? BTW I got ahold of Roger Simonoff and had a great chat with him about what I am doing and have decided that I likely need a lot more steam in my unit. Thinking about adding a heating element to my next steam box but not sure where I am going to get it. Maybe a dishwasher heating unit.......

[Steve Senseney]

If you go to Omega silicone heating blankets, you could get a heater that would be ideal (but a little more expensive perhaps).

On Ebay, you can search for flexible heater or silicone heater and perhaps find something less expensive.

Cartridge heater are not very expensive and are available on Ebay. If you could adapt this to your design, it might be the least expensive choice.

[Jon Whitney]

Jon, I have yet to really see a compression strap in use. What are they made from and how are they attached? BTW I got ahold of Roger Simonoff and had a great chat with him about what I am doing and have decided that I likely need a lot more steam in my unit. Thinking about adding a heating element to my next steam box but not sure where I am going to get it. Maybe a dishwasher heating unit.......

I don't have any photos of the setup I used, and it is cumbersome to describe with words alone. But I did find this page on the net:http://www.sagmoenbanjos.com/shoptalkbend.htm- that looks about like what I did.

Stainless spring steel would be ideal for the bending strap but I just used some galvanized steel left over from roof flashing. My biggest problem is controlling the length of the strap. I was using a block screwed on the end of the strap butted up tight against the end of the slat, and it needs more slack than that. Sometimes the slat would start bending the reverse way towards the end because the strap was too tight - like a bowstring. I think maybe just clamping the strap to the end of the slat with a C-clamp or two would work as the strap would slip a little and adjust itself. One of these days, when I get back to bending single ply rims, I'll figure this out better.

One more piece of advice - leave your slat at least 12" or more longer than your rim circumference and scarf joint overlap, so you can use the extra length as leverage and so you can get it completely around your form, and so the last little bit, which is impossible to bend, can be cut off as waste.

[Brian Glass]

Good news to report! I rebuilt my steam box and brought it down to a 6x6x60" size and siliconed the joints up tight, wrapped it in isocyanurate insulation and fired it up. Got up to 220 degrees after 45 minutes run time. Looks like some great performance from the wallpaper steamer. Will try bending up some fresh stock tomorrow with all the new infirmation I have received. Will report back after.

[Bill Rickard]

Brian:

Jon Whitney's suggestions in regards to heat requirements and using a compression strap is right on. I'm presently in the process of building up a rim bending system that hopefully will be capable of rolling approx. 80 to 90 rims per week. What you'd call a rim bending production line. Up until now I've probably steam bent a couple of hundred banjo rims and finally think I've got it sorted out. I also bend all my rims from a wheelchair as I lost my left leg at the hip and left arm at the shoulder as a result of a real serious motorcycle accident awhile back. Everything that I've set-up can be accomplished one handed if the need be. Hopefully you won't ever have the need, but once you get the hang of how wood bends you'll find it a breeze, especially with two hands!
What I've been doing lately to give me more time from the steam box to the rolling machine, is to by pass the steaming and boil the wood ply in a vertical stainless steel tube that sits on a large propane cooker. Once the water comes to a boil I leave the wood submerged in the boiling water for approx. 15 minutes. By the way steam works just as good but the boiling process is something that I've been experimenting with for the past 6 months with great results. When you put the wood in the rolling device you must have a compression strap. If not 50% of what you roll will end up in the fireplace! You also need to move real fast as the steamed wood will start to dry out and loose it flexibility in less than 45 seconds. After that forget it. You also need to be sure the wood is being fed 100% in line with the rolls as you don't want it to spiral up the rolls. Spiraling will cause problems down the line when you glue the rim plies up. When you insert the steamed ply in the rolls you will need to clamp the ply to the compression strap in approx 3 locations along the length. Just remove each clamp just before it enters the rolls. By doing this you'll actually compress the outside diameter cells of the wood, and therefore force the ply to roll smoothly plus not crack as it's being rolled. Without the compression strap the out cells of the wood pull apart and this causes breakage of the ply. Example is if you bend a 1/4" thick ply into an 11" outside diameter rim. The inside length or circumstance of the ply is 33.00 inches long and the outside ply circumference or length is 34.571 inches long. This means that if you don't compress the outside ply it has to stretch approx. 1.571 inches to roll around the circle. You'll never compress it a full 1.57 inches but aim for approx. 50% compression. After the ply is fully rolled I leave it for a short time being heated by a heat blanket and then move the ply into a drying form. The above is greatly simplified but don't get overwhelmed, as you can also keep the process real simple if you only want to roll a couple of rims at a time.

I'll try to post some photos of the whole process later this evening and hopefully it will make some sense.

Bill Rickard
http://www.rickardbanjos.com

[Bill Rickard]

Here's some photos of my second rim rolling machine that I built from scrap iron, laying around my metal fabrication shop. Definitely an over kill but it rolled many rims before being retired for the system that I'm in the process of building. You can basically roll 1/4" thick plies with a much simpler wood mandrel. Again the secret is the compression strap. You must compress the wood to get good results.