I've been looking at some of the projects over on http://armborst.forum24.se/ and was struck by a clever method to keep the front and back bearing blocks aligned while in-letting them into the tiller. In these pics, the two bearings have been machined out of a big piece of antler and left with a very substantial attachment at the bottom.







I have always made the front and back bearing separate, but getting everything to be coaxial and properly let into the tiller can be a lot of trouble.

last night I imagined this method....
--The two bearings are machined up using whatever method works.
--The nut is wrapped in a strip of paper to provide the proper clearance.
--The two bearings are glued temporarily to a wooden jig, with the nut captured between them.
--The whole assemble can be let into the tiller, using soot, or whatever other marking method you chose.
--When everything fits properly, the nut is removed, and the bearings are glued in place, leaving the jig to assist in retaining the orientation.
--When the glue is set, the jig is removed.

Mac
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How about this, glue antler block/wood block/antler block together with a wood strip on the bottom, then bore for nut. Then drop assembly with paper shimmed nut into motised tiller.

Todd

Tod,

I'm having some trouble following you. Do you mean that there will be another wood block on the bottom of the assembly, which gets let into the tiller?

Can you make up a sketch?

Mac

How do you machine the interior concave support, where the nut seats?
There is a need for a shaft or a string that acts as a hub for cases where the bearings are made ​​in one piece, which is introduced from above into the stock?
The nut is not trapped inside the bearings? I see to dificult to make (by machine or by chiseling) the stepped bottom inside the in-letting hole on the stock....
I know, to many questions, but i´m right now making a pile of sketches about how to do it on my crossbow.....

Juan,

I usually make the concave surfaces of the bone bearings this way...
--Make up a cylinder that is the size of the nut on a lathe.
--Wrap the cylinder in abrasive paper, and use it (still on the lathe) to machine the bearing surfaces.

I am making the bearings in two pieces because that is what was done historically. Making them in one piece is a clever idea, but it is modern.

The nut is not trapped in the bearings. It can be easily removed when it is rotated to the proper place. This is typical of crossbows. Some have threads tied through the nut to keep it from falling out. Some have a rod or pin in place of the threads, but this typically does not bear any force. It's just there to keep the nut from getting lost. The inlet bone bearings carry all the loads.

The mortise (inletting hole) is a pain in the ass to make. There is no denying that. But it is quite possible, and with practice, you get better and faster at it. It is important to work slowly and thoughtfully. If you use the soot of a candle to cover the bottom of the thing you are inletting, it will leave a black spot on the wood that you must remove. Gun makers do this when letting barrels and locks into stocks. We must borrow techniques and processes from other craftsmen.

I am sorry if I have told you things that you already know!

Mac

Mac, tanks for the answer, and don´t be sorry at all, you have been helpfully, now i´m realise the issue about the nut stuff.
Anyway, when you make the bearings, first you make a unique piece following the shape of the sand paper wraped cylindrer, and then split it in two parts?? Or still work with two pieces at all???
I´m thinking about make a brass nut, and i believe that a hard wood for the seat of the nut will be ok, because bone is hard of find out here.....
Thank you again!!!!!!!!

Juan,

I make the two bearings as separate parts right from the start. That way I get to maximize the use of my material, and chose for dense surfaces. If the bearings have porous surfaces, they are not as strong. I also worry that porous surfaces will retain dirt, and contribute to wearing out of the nut.

A brass nut will work OK. They are heavy, and will rob a bit of your stored energy. Aluminum sounds like it would be a better choice, but Geezer tells me that they make ugly black marks on the "table" of the bow.

The wood of the tiller is probably just fine for bearings. Lots of people do that, especially for light weight bows.

My wimpy little crossbows don't really need bone bearings, but I have always put them in anyways. I hope that the experience will lead me to have a fuller understanding of the bearings by the time I start playing with heavier bows.

Mac

I got it.....i´ve ben seeing on this pictures, i understand this way, but still think that you way is required of important handwork skills

http://www.flickr.com/photos/swifthoundbows/sets/72157613494516821/

Now i´m plan to do a hard wooden nut seat, and let the time do the rest. If that don´t works fine, i can allways change for other material.
Even i was thinking about make a brass plate wrapping the nut sit exactly the shape of the nut....
Again, thanks a lot for you support Mac!!!!!!!

De nada, Juan.

Mac

Todd,

Is this the sort of thing you are talking about?

http://www.flickr.com/photos/swifthoundbows/3265117114/in/set-72157613494516821

Mac

I know a gunsmith who uses cheap lipstick for his inletting markers. It is fast and easy to see on dark woods.

Well, I see somebody has posted a nice pic of a pre-cut bone-socket from Lightly's photo archive. That saves me the trouble.
We make those by gluing two pieces of bone/antler into a block of wood, then use a forstner-bitt on the drill-press to cut a 2/3 round passage between the two. Then we cut the tickler-passage in the bottom/rear. Once all that's done, we cut the finished socket down to the size we want and inlet it into the top of the stock.
If you don't want to do a fully mortised lock, just glue the blocks of bone/antler into the stock and cut the roller-passage between the two. That's how we do most of our reinforced sockets (anything over 125 lb. or alternatively, anything fitted with a bone/antler roller.
(For non-English speakers, I use the term 'antler' to differentiate from 'horn' which in English can mean hard, bony stuff like moose and deer horn, or soft, flexible lamellar stuff like cow-horn. The term 'horn' can mean either of those. 'Antler' is specifically the hard, bony stuff. Even so, there are some antlers, like American Elk, which are nearly useless for roller-nuts. I have had good success with the base of Moose horn or Axis Stag. Geezer.

How much is ussually the diameter of the roller nut that you guys make???
I find only 35mm diameter forstner-bitt, that´s about 1.3 inches......it´s enough?? Remember, for a brass nut.....

I make my nut 1 1/2" in diameter from wood and use no bearing block other than the stock itself.

That photo essay by Lightly was the one I was refering to.

Todd

Nut diameter? Payne-Gallwey suggests 1.5 inches. These days, I use mostly 1 & 3/8 inches. You see a lot of bows with rollers about that size. The original Padre Island bow has a very small roller, about one and a sixteenth inches in diameter and 15/16 inches wide. That's about as small as I have seen. One and a quarter to one and a half inches seem most common.
35 mm should work very well, though Mac is right, a Brass roller will be heavy and will eat some power. My very first medieval crossbow had a 1.5 inch by 1.25 inch brass roller. It experienced some odd teething problems that were made worse by the heavy nut, but eventually it worked well enough to encourage me to make more. Geezer.