Bending form irregularities

[Robin Cruz]

I made a form to bend some laminated oak.
1/8" strips, 4" wide about 19" long. The shape is a gradual arc for a chair back stretchers.

I cut the form on the band saw (3/4" blade) in 4" thick glued up MDF and there are irregularities in the arc shape where I didn't turn the wood evenly as I pushed it through the bandsaw blade. I sanded with a drum sander to even out the arc but its not perfect. Maybe a 1/8" off the perfect arc line at a couple of spots.

Is this form good enough for chair back stretchers? there are 4 of them. I thought maybe the stiffness of the wood would tend to even out the irregularities especially with some spring back. Would adding a layer of cork to the inside of the shape help also?

Thanks for any tips

.

[Frank Drew]

Robin

The few times I laminated I lined the inner faces of the forms with a rubbery tape, maybe a heavy 1/16" thick and as wide as the form; that evened out minor irregularities. If the forms are fine except for short little spans where you went off course the rest of the form will probably bridge the off spots just fine. Anything more than that you might consider recutting the pattern to new layout lines if the forms have sufficient material left.

[Todd Bin]

line the form with cork (both the positive and negative curve). You can get 1/8" cork rolls at most craft stores and probably Wal Mart. You can use double stick tape to stick it to the surface. Then using the clear packing tape to cover the cork and a couple inches in on the edges of the form. That way no glue will stick to the form.

The cork does an excellent job of smoothing out little imperfections in the form.

Good Luck and Post some picutres of the chair when you are done.

[Peter Quinn]

Might be easier or at least more consistent to cut your forms in a single 3/4" layer, sand one layer perfect, glue and nail the next on, flush trim, etc. to make your width. Its a lot easier to push 3/4" of MDF through a band saw then 4". Once you get the first set of positive and negative templates made you are assured a nearly perfect pattern from there, though lining with cork is still a good call in any event.

A router/trammel set up is a pretty quick way to make the original pattern for segments of an exact radius but wont let you follow curves with a little english to them so to speak.

[Chris Padilla]

David Marks does this all the time and does exactly as Peter described above...or is it that Peter does exactly as Mr. Marks does! ;)

[Robin Cruz]

Those are all great tips!
Im going to make several of these 4" wide bent stretchers, so I might try to improve the shape I already made just to be safe.

The flush trim idea is a good one. Id have to trim each 3/4 thickness one at a time and then assemble as I dont have a 4" long flush bit. I have another thinner shape to make and I will try that approach.


There is plenty of material left in the mold so I think you were suggesting to clamp them together and then bandsaw down the seam to remove the irregularities? I can try that and if I fail just make new one with the flush trim approach.

The shipping tape tip is a great one. I was going to use wax paper which would have been a hassle.

Thank you all again for the fresh perspective

[Frank Drew]

Ideally, a bending form should be cut from a full-sized drawing laid over your form material, cutting on two lines (outside and inside) with the middle waste piece discarded. If the shape you want has any width to it at all, the two sides of an accurate form won't exactly nest because the inside curve of your furniture piece's finished shape is from a circle with a smaller circumference than the outside curve, so the curves will be slightly different. In other words, cutting a solid form piece apart on one line will give you two very closely mating pieces but maybe not matching both sides of your desired shape.

Just a thought.

[With all the enthusiasm here for resawing I'm surprised at the reluctance to cut 4" of MDF in one go.]

[Michael Schwartz]

You should start out with a full scale drawing of the template and attach it to the MDF with spray adhesive

Start by cutting a single layer out of 1/4" or thicker MDF, then refine the shape of the template. Disk sander works well, and I often use a spokeshave for this as it is more controled and less mess. Yes it dulls the blade but it works nicely. Wrasps leve a coarse cut but also work out ok. If a flat section needs to be refined I keep a jointer plane arround for this task.

Then trace out the template on 3/4 mdf or partical board and rough cut each lamination on the bandsaw or with a handheld jigsaw. I prefer using a handheld jigsaw for this. Using a flush trim bit in the router table simply brad nail or screw each layer on and trim it flush with the previous layer.

Occasionaly forms and templates get dammaged and there are a number of ways to deal with this. One is to use a 2 part wood filler or Bondo as it can be built up a bit and sanded nicely.

I have built forms for use in the vacuum press where I have decided the radius was too tight after building the form and taking a dry run in the bag, and fixed this by adding a few layers of bending ply to the form to decrease the radius.

[Robin Cruz]

Good points. The thickness is 3/4 so I can make two different cuts for arcs I have from a full size drawing. I can try glueing the first try back together and maybe using a narrower bandsaw blade blade with fewer teeth to do better at guiding the cut and then take another crack at the big block cut. That failing I can go the template and trimming bit approach.
thanks again

[Peter Quinn]

Chris, I got that method from somebody else but I've seen David Marks use it extensively.

Robin, I think your seeing this a bit bass akwards. You need a 1" bottom bearing flush trim bit. You use your full size template to make the first pair of 3/4" forms adjusting the two arcs to accommodate the thickness of your glue up plus any cork lining. Cut the first template layer close to the line on a band saw or with a router trammel if appropriate, sand it with a flexible strip or OSS or even sandpaper wrapped around a dowel. Trace the next layer from the first, cut the second layer on the band saw staying a bit off the line (1/16"). Glue and nail (I use 1 1/4" 18 ga brads or staples) the second layer to the first, then use a router with a 1" bottom bearing bit to flush trim the second layer with the bearing referencing the first. The router sits on top of the second layer with the first (master template) below. Cut and nail the proceeding layers similarly and you end up with a perfect sandwich.

Pickle and mayonnaise are optional!:D:D

[David DeCristoforo]

Do what Michael suggests. But instead of MDF, try 1/4" "Masonite" (AKA tempered hardboard) for your template. This will be very easy to refine and is also hard enough to provide a solid bearing surface for a flush trim bit. Use this template to make your first form layer. Then stack additional layers one at a time and flush trim to the one below until you have the thickness you need. Instead of trying to repair a messed up form with bondo or cork or rubber "cauls" (which may make it difficult to get even pressure leaving you with gaps in your lamination) you might just want to view this as a "do over" (MDF is cheap) and get it right. Bending is hard enough without having to fight poorly made forms.

[Tony Joyce]

Try using scrap plastic laminate between the cauls or forms and the wood strips(both sides of the wood). This will smooth out minor imperfections some.

Tony

[Robin Cruz]

Ok. Ill start over with the trim method. thank you for the help

[Doug Shepard]

If you've taken off too much, you could use bondo to fill the sanding divots then re-sand to blend it smooth.

[Robin Cruz]

I see. Ive used a table router for so long I was visualizing its use when the template was 1st mentioned. I see now that I can use my smaller handheld router to trim each layer until built up to thickness.

Going back to the steam bending approach for a whole 3/4 thick piece, I see there is an advantage in that tenons can be cut prior to the bend making that part of the job a lot easier for the chair stretchers.


Chris, I got that method from somebody else but I've seen David Marks use it extensively.

Robin, I think your seeing this a bit bass akwards. You need a 1" bottom bearing flush trim bit. You use your full size template to make the first pair of 3/4" forms adjusting the two arcs to accommodate the thickness of your glue up plus any cork lining. Cut the first template layer close to the line on a band saw or with a router trammel if appropriate, sand it with a flexible strip or OSS or even sandpaper wrapped around a dowel. Trace the next layer from the first, cut the second layer on the band saw staying a bit off the line (1/16"). Glue and nail (I use 1 1/4" 18 ga brads or staples) the second layer to the first, then use a router with a 1" bottom bearing bit to flush trim the second layer with the bearing referencing the first. The router sits on top of the second layer with the first (master template) below. Cut and nail the proceeding layers similarly and you end up with a perfect sandwich.

Pickle and mayonnaise are optional!:D:D