Need experienced advice for thicknessing laminations with a belt sander

[jaybirdjones]

Anyone have any tips for the specific grain sanding belt to use, or method. ive done about 10 already, two were throw away because they got notched from pushing too hard, and two others broke at the thinnest spots after glue up and tillering. They were maple tapering from a hair or so over 1/4 in. down to under 1/8 in. at the tips. I do have some hand planes of the rusty garage sale variety and im too afraid to get after the limbs with those. The next batch will probably be bamboo, and before anyone says anything yes...the grain on the wood was MAGNIFECENTLY straight. Which was why i was so bummed out about losing them

[Stick Bender]

I have used  grits from  36-60 grit currently I use 50 grit it's aggressive enough but gives a smooth taper & the 50 grit surface gives you a nice gluing surface , if your using a stationary belt sander & you have a band saw  you can make a guide or sled to precut your lams prior to grinding in other words make a sled to match the taper you want , just start slightly thicker then you want & grind to the butt thickness you want after cutting them on the band saw , you can usually get more lams that way to , hope that helped !

[Bayou Ben]

So are you free hand grinding on a the belt sander?  And 2 broke at the thinnest spots, so these thin spots were caused by grinding before you glued the bow? 
Sorry, I'm trying to understand exactly how you are grinding to offer some suggestions.   

[DC]

Are you using a hand held belt sander?

[jaybirdjones]

So i was using my tabletop belt sander, freehand. Yes i when i tapered them down with the sander i pushed too hard and made a concave groove like an idiot with Frankenstein hands, last night before i went to bed i third person thought about my question and an obvious answer was never work the cylinder end of the sander in the first place. And yes after glue up and about a half hour of shooting it broke at the mistake. Ill tell ya right now, reading Stick Benders answer i was being too cautious with my grain iwas using 120 and 80 grit

[Bayou Ben]

Yeah I use 36 grit on my belt sander.  I have a cheap combo sander where it has the round disc on the side.  That side disc is much better for sanding to a marked line.  It's really hard to be accurate free-handing a belt sander, and you will have dips if you use the cylinder end.

The way I would do it is mark your taper lines as precisely as possible on both sides. Using a band-saw or table-saw (properly equipped to make it safe) with a sled of sorts to cut the bulk material away will save you a bunch of time.  Then with the board at an angle, sand to the draw line on both sides. Take your time and be precise with this part.  Then use the flat part of the belt sander to bring down the thicker middle section even with the sides. 
With maple, it's probably easier to do this same method with a rasp, and it will probably be more accurate. 
The thickness taper needs to be accurate before you attempt to glue it up.  The more accurate the easier your tillering will be.  You'll be chasing your tail if you have a known thin spot and you glue it up assuming you will tiller it out.  That never works out. 

   

[Stick Bender]

There was a guy on another thread that made a real simple sled by stair steping masking tape on a flat board he made the board longer then a 36 " standard lam that way he coud put the butt end at what ever taper rate he wanted along the board you could make one of those for probably $5 for your band or table saw to get the taper you want ! The taper really needs to be a smooth countinious transition lam bows for the most part 90% of the tiller is done at glue up !

[jaybirdjones]

Thanks guys! Im a bit more motivated now