Prod mortice angle. Why?

Hi,
Totally new to crossbows and I am in the middle of my first project.
Can members please advise why a prod has to placed in an angled mortice
and as I am unsure about the angle for my bow what info I would need to determine
the angle?
I have read some posts re mortices but as a beginnner they a bit above me.

Thanks in antcipation.

Micky hi and welcome to the forum, the prod needs to be set in the tiller, stock, at just the right angle so as to reduce the friction of the string on the deck, im not all that good at explaining things, so look for all Rizzars posts, in there is one where he explains how to use google sketchup to determine the angle , and you will see why the angle must be there, im sure that some of the more experienced builders will jump in and explain as well, im relatively new in comparison to some of the builders here.

Hey there

The key with mortise angle is to find a geometrical compromise between string friction and the line the string moves during the shot.

If you draw your prod the string moves usually in a direction 90° to the back of the prod up to its maximum draw lenght.

With a simple bow this line of draw is normally aligned at the prods center of height.

A more experienced bow will have that line lifted to minimise friction during the shot, since the Angle must be chosen to have the string at the butt of the bolt when spanned.
Otherwise the chance of failure (spring hop or underwalking) will be potentized.

As rule of a thumb, the higher the string line is lifted (optimum would be string diameter to travel on tiller deck without friction but difficult to realise with common length prods) the more benefit to bolt speed.


So getting back to simple: the string should lay on the bolts butt level when spanned without being forced either up or downwards. This is why an correct angle -which is strongly dependent of the prod itself- should be provided.

Greetings Rizz

The elegant way the ancient try to solve this problem was to give a cant to bow tips , shape the prod to upper the string, and to give a light angle to prod . But they could shape the prod an cant the bow tips with steel prod only, wood and composites  bow could not withstood this treatment and supported only a light angle. In modern time the best solution is centrer shot, and with fibreglass materiel, canting and giving angle is not a problem.

Based upon the Alchem design, isn't the proper angle 6 degrees?

Wouldn't that work for an Alchem prod or a Slobow's prod (as Gary supplies Alchem)

My .02.

I may have mentioned before, I am lousy with maths. To the frustration of Geezer, who can talk angles and drawlength, and FPS, and more.. while I stare at him, eyes glazed over.

When cutting this angle, in the prod socket, I ran into problems.

I learned (which, will not help  you all!) that when I help the square 'just so', and that there was 'just that much gap' along the top deck, that that was the correct angle to scribe, and cut the socket out.

I also cheat.

I swiped an aluminum prod from Geezer, (as we mostly do aluminum for clients) I strung it up, so that I had just the prod, strung.

Now, I cut the prod socket as close as I can, (using my 'hold the square just so' method) but before I tie the prod on, and discover that the string flies, or presses too hard, I place the strung prod in the prod socket, and press on it. I can then tell, if the string flies, or presses too hard. Also, I can check for tilt, right or left. 
And, adjust the socket, if needs be, so that the string just kisses the deck..

Since I have done this, I have not (yet, knock on wood, now that I SAID this, I am likely in trouble) tied a prod on, and discovered my angles were too incorrect, and needed to cut it off and re-tie.

And that is how a person with no math skills does a go-around on the socket angles.

Cheating.

Best!

Lightly

Concerning prods from alcheminc/slobows you can possibly trust in the mentioned 6°.
They shpuld all be designed do have the same draw length and design.
As long, as you have the correct string diameter and length.
By alternating diameter you can get to an slightly different level when drawn.
Alternating length can change everything since the whole drawlength is influenced.
Another influence on the mounting angle comes from the thickness of the upper prod mounting, the thicker it is the higher the angle must be to get into the same position.

Unluckily I have a prod from alcheminc but I cannot remember its mounting angle anymore (suppose it was lower).
But I can remember measuring it individually when I received the bow since my drawweight was much lower than I ordered and the bending itself was irregular.


Lightly, you are confusing cheating with earned experience on your prods^^
As long as you do not suddenly change them I see no problems with your technique.
Past successes apparently prove it works for you!

Though measuring and calculating could give some little more fps.


By the way I wish canting the limbs was as easy as on the pictures above!
I´d really like to do this more progressive on some of my future prods (have to build a hardening oven before) but I apply my bendings with a selfmade press which is guided on the upper prodline.
Unfortunately this would not work when there is no straight line and I would not want to sacrifice a good and even design for some more elevation.
A pity I am not that experienced with my forge to get this done by hand.

Rizzar

Rizzar
 
I have 2 Slobow supplied prods of the same basic length and width but one is 3/16”thk. and the other 7/32”thk. The 3/16”(SB102/2006) has a string rise (vertical offset) of 5/16” measured from top center of prod to a line connecting nock centers. The other one (SB112/2012)has an offset of ½”. The latter is also deflected at about an inch whereas the former has a 2” deflect. In this case the actual braced prod on hand comes before mortise or slot mount.
 
Tom

Thanks-a-million to all who replied. I found the info really useful.
Cheers for now

Good morning

Concerning your different prods I´d suggest you to measure it by hand without stiffening on those 6°.

Alternating offset at two prods of one manufacturer seems to me like an indication not to have the same mounting angle as formerly supposed.

The deflex itself only inflicts the angle by the draw length the prod has which can haave with 2 different thicknesses and deflexes the same amount.


Hope I understood you as it was meant to be as I am not a native english speaker and had some difficulties concerning the last sentence.


Greetings Rizzar

Rizzar
 
I am describing the condition of 2 unbraced and uninstalled prods. My last sentence is a little bit unclear. Obviously the angle of installation and distance from tiller top can be determined from the actual braced prod.
 
Tom

I think I got it now.

For determining the prod angle you should use a braced prod.
But the measured angle will only be valid for that prod. As you described the other prod has got another offset, so it will be different, as long as the draw length is not compensating the offset, what is kind of doubtful to me.

I wrote a thread about determining installation angle with google scetchup somewhere here (just have a look) but the same thing will work with pen and paper, too. Use an 90° angle at the back of your prod and the string center to determine the string movement line by longening it to the draw length calculated or given by the manufacturer. (look at the following steps in the other thread please)

The distance from tiller top has a big effect to the mounting angle.
In theory, if you make the upper mortise tiller so narrow to get it between your prod and string with an 90° angle your system will run with very little or no friction losses ad as long as optimised bolt weight high projectile speed.
But consider the top to be strong enough to withstand the force when the prod pushes it upwards.

So generally a high offset leads to improved prod performance.


Rizzar

the 6 degree angle would put the string in the air at the nut with the offset prods from slowbow and Alchem. They claim the ends are offset 1/2 inch but actual measurement to the string is 3/8. This requires an offset of only 36 seconds of a degree to put the string at the rail level at the nut and pulled .090 of an inch at rest.  If you make the notch 90 degrees square to the rail, you can just put a .010 inch shim in behind the prod at the top of it and have your angle. In fact, with only 1/8 inch pressure you could leave the prod square with little wearing. The sine of 6 degrees is over .1 so flat bows requiring the 6 degrees equals almost 7/8 of an inch down pressure from the 8 inch travel from the nut to rest position. No wonder they go thru so many strings! The calculations are based on the Alchem design with the prod 1/2 inch down frm the top. Cad drawing are available.

Thanks for the info!  I never thought about shimming the top of the mortise....

Even though I have cut the mortise at 6* already...I will just shim the bottom!

Not a good idea, shimming concentrates pressure to points rather than a large area that can easier distribute forces.


Just do not stick to numbers in plans, measure everything yourself and be sure it will work out.

Rizzar

2 shims,each with an angled face on each side of the prod, and a slightly enlarged prod mortice will save the day.Much more preferable than scrapping a just about finished tiller because the prod angle is wrong.
          Hermit.

Hey there!

I do not mean to scrap a finished tiller. but a correction of a couple of degrees normally should not be a problem.
Just enlarge the mortise to the needed angle.
losing a couple of milimeters draw usually does not hurt that much, and if necessary put in some leather spacers as replacements for the lost material.
That should compensate the loss.
But do it on the complete area and secure it with some kind of glue.


Creating a different angle by shiming some smaller areas is too risky compared that above method will do the job very good without being too much efford and avoiding patchwork (I do those spacers on every bow).

Rizzar

You can find the angle for yourself Micky.If you are using a commercially made prod,the  top of the prod will likely be upswept at the ends.you need to measure this distance.I did it on mine by pressing the ends of the bow onto a flat surface(the kitchen table)and measuring the distance from the table top to the edge of the prod closest to the table top.Make a note of the measurement.All crossbows have a groove down the top of the tiller,this is for guidance,and clearance for the cock flight on a 3 fletched bolt.Groove depth is usually 5/8ths of an inch.With this arrangement,it should be obvious the the top of the prod should be 5/8ths of an inch,minus the upsweep below the top of the tiller,if not,the bow will foul the cock flight.......not desirable.Subtract the upsweep measurement from the groove depth measurement,record the difference.This measurement will be the distance from the top of the tiller to the top of the prod mortise hole.
                  On a piece of scrap ply,with a straight edge, make a mark an inch or so from the end,then with a square,drop a line off the mark on the straight edge.Measure the width of the prod,then halve it,and record.Add the half  prod width to the measurement for the top of the prod mortice.On the line down that you first made,make a mark at this last measurement.Add together the brace height,and the draw length you are going to use.Make a mark at this measurement on the straight edge.Draw a line joining these 2 marks.You should now have a very shallow triangle.Draw a line at 90 degrees to the line that joins the two points(the hypoteneuse) bisecting the 90 degree angle formed by straight edge,and the first line you drew.Flip your square,and on the underside of the line,continue the line for 3 or 4 ins.Setting a carpenters adjustable bevel off the straight edge,with the blade following this line will give you the required angle for the mortice.If you don't have a carpenters bevel,you can make a template by cutting and filing a thin piece of aluminium so that one edge is level with the tiller top,and the other follows the line that gives the prod mortice angle.This method worked for me.
                              Hermit.

Te upsweep measurement is made at the centre of the prod,apologies for forgetting this important piece of info.

ve3sqb wrote:the 6 degree angle would put the string in the air at the nut with the offset prods from slowbow and Alchem. They claim the ends are offset 1/2 inch but actual measurement to the string is 3/8. This requires an offset of only 36 seconds of a degree to put the string at the rail level at the nut and pulled .090 of an inch at rest.  If you make the notch 90 degrees square to the rail, you can just put a .010 inch shim in behind the prod at the top of it and have your angle. In fact, with only 1/8 inch pressure you could leave the prod square with little wearing. The sine of 6 degrees is over .1 so flat bows requiring the 6 degrees equals almost 7/8 of an inch down pressure from the 8 inch travel from the nut to rest position. No wonder they go thru so many strings! The calculations are based on the Alchem design with the prod 1/2 inch down frm the top. Cad drawing are available.

Uh... what? Granted I barely passed my math classes, but I can't follow this at all. I'm not at all sure what that business of "the string in the air at the nut" is about.
I don't think its really that complicated. You angle your mortice so that your string just touches the bolt track at brace height. When spanned, the string is at whatever height your nut holds it and travels straight to brace position when released.
The Alchem plan is a perfect place to start, but it is not the end- there will always be fitting and measuring and tweaking to get it just right.
Or am I missing something here?
Gnome

I could'nt agree more Gnome,there are so many other factors involved,that I don't believe it is possible to give anything other than a ball park figure for prod mortise angle.Using high level geometry,and quoting angles accurate to minutes,and possibly seconds,is giving the impression that a relatively simple operation that can be done simply, is not.All this is going to do for a first time builder,is confuse and discourage him.
                     Hermit.