# Force on press fingers



## redoaks (Oct 12, 2011)

why do they have to be aluminum? Wouldnt steel flex less and need less material to accomplish the same goal?


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## cncjerry (Jun 28, 2011)

I have a lot of .5625 aluminum.


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## Todzilla (Feb 27, 2005)

What grade of aluminum will you be using ? I made mine out of 6160 and no problems what so ever.


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## ymurf (Feb 4, 2010)

We did several testes with my fingers I made.Actually made a test with a 100# scale.We bottomed it out at 100 # with one finger made from 6061 aluminum.No way you would ever break one pressing a bow.Not gona happen.We even went farther and still no give with this finger.


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## cncjerry (Jun 28, 2011)

ymurf, how thick are the fingers?


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## ymurf (Feb 4, 2010)

cncjerry said:


> ymurf, how thick are the fingers?


1/2" thick.


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## robm8806 (Jan 11, 2012)

Wouldnt it just be like 20lbs pressure on each finger for an 80lb bow? I'm no engineer or math teacher so I'm.probabaly way wrong


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## TJK (Dec 2, 2010)

robm8806 said:


> Wouldnt it just be like 20lbs pressure on each finger for an 80lb bow? I'm no engineer or math teacher so I'm.probabaly way wrong


I was thinking the same thing


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## ymurf (Feb 4, 2010)

Your on the right track,Not sure how much pressure but not much.


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## bcarchery (Jan 29, 2011)

The amount of cam movement to remove the string works out to 20-30lbs when pulling it down on the scale. I've checked it a lot when I was milling my fingers. 

Pointer for those fingers above; Thin down the top so it can fit between the limb tip and a draw stop. With those fingers you will have to remove or loosen every draw stop. Good job they look nice.


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## dwagoner (Sep 27, 2007)

i would bet a 70lb bow has at most 30-40lbs on the fingers to press them towards each other, theres actually a guy that made his own press out of wood, YES birch hardwood and he swares there more than strong enough. theres not hundreds of lbs on the fingers, you can use 3/8" AL but the width of 1/2" helps to get the bow in there solid.


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## SeeMont (Jan 18, 2011)

I have compressed my bow with 68 lbs with 3/4" oak fingers on my pipe press with out a problem. With the linear press, You have to account for possible unequal pressure so lets say one fingers presses the bow then the finger would be required to hold 70 to 80 lbs. I can easily hang 100lbs of my 1/2" steel fingers with no bending at all in fact I can hang my body weight on my 5" finger clamped in my bench vise with out bending.


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## jrdrees (Jun 12, 2010)

For everybody's safety....keep in mind, if you're talking about compound bows, that there is a "compounding" reduction in effort to pull a 70 pound bow via cables and pulleys. I'm sure a lot of guys can do dumbbell "flys" with a 35lb weight in each hand, but these guys cannot use the same motion to "squeeze" a 70lb bow's limbs even a 1/4". I've heard (don't know the exact figures, probably varies between bow designs) that it's more like 300lbs or so to compress a 70lb bow. So if you're going to rely on wood to press your bow, wear safety goggles, gloves, and flak-jackets. At the least you want to engineer your press to withstand triple the typical forces so you can use it more than a few times before you've stressed it to its max.


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## McDragon (Oct 30, 2011)

And our resident Engeineer Nuts&Bolts has what to say on this topic. I have read quite a bit of what he has to say, and he is usually spot on. Just my 2 cents.


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## McDragon (Oct 30, 2011)

And our resident Engineer Nuts&Bolts has what to say on this topic. I have read quite a bit of what he has to say, and he is usually spot on. Just my 2 cents.


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## cncjerry (Jun 28, 2011)

I set my 70lb z7x on the floor and pressed down. I was able to loosen the string pretty easily. I doubt there was 100lbs on the limbs.


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## dwagoner (Sep 27, 2007)

cncjerry said:


> I set my 70lb z7x on the floor and pressed down. I was able to loosen the string pretty easily. I doubt there was 100lbs on the limbs.


exactly, there is no where near 300lb of force needed to compress a bow, that guy has never obviously tried that, buy i have like you put bow on my foot and pushed and made cables slack


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## TJK (Dec 2, 2010)

I would have to agree there is probably more than 20-30 #s but I don’t think there would be any were near 300 #s. I’m no engineer but I would think that every bow would be different. Also you are not moving the limbs very far, I would guess that you move them less than a ½” to get the strings and cables to slack. If someone had a portable bow press I would think you could put a inline scale on it and test how much overall force is needed to slack the string and cables.


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## bcarchery (Jan 29, 2011)

TJK said:


> I would have to agree there is probably more than 20-30 #s but I don’t think there would be any were near 300 #s. I’m no engineer but I would think that every bow would be different. Also you are not moving the limbs very far, I would guess that you move them less than a ½” to get the strings and cables to slack. If someone had a portable bow press I would think you could put a inline scale on it and test how much overall force is needed to slack the string and cables.


x2 and

300 lbs of pressure would bankrupt you in Plasti-dip.


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## Mich.bowhunter (Jun 29, 2007)

Mine are 6061 aluminum.No problems.


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## SilentElk (Oct 6, 2004)

Will be much more than 20-30 per finger but less than 300 for sure. 4 fingers x 300lbs each = 1200 lbs tension between the limbs. With that much tension, your riser would be a taco or pretzel- what ever your food choice is. 

We can easily determine the pressure required on press a bow. Different bows will be different dependant on how 'tight' they are. I use the word tight to express how prestressed the limbs are. For now lets use my Bear Carnage because, well I have it sitting here and so it is easy to do.

Step one, measure Axel to Axel undrawn. This number equals roughly 32"
#2 measure Axel to axel fully drawn. I came up with roughly 28.75"
#3 Determine total limb movement: 32"-28.75" = 3.25" limb movement
#4 Calculate the mechincal advantage. Bow is 7.25" brace height and length is currently at 29". Total draw stroke of 21.75". 21.75/3.25 = 6.69 mech advantage.
#5 Determine AVERAGE poundage. This much harder to determine because for the first few inches and last few inches you arent even close to your max poundage. We could calculate this very accurate but isnt worth the effort for this exercise. Mine is set at 70lbs, but an realistic average is closer to 50-55 lbs. Probably closer to 50ish as an ave but we will use 55. We want to know a higher number as minimum for safety.
#6 Determine limb tension. Mech advantage x force (lbs). 6.69 x 55lbs equals 368.1 lbs of tension between the limbs.
#7 Divide by contact points to determine the individual force required. Simply: 368.1lbs/4 fingers = 92.0lbs per finger.

On my carnage current setup, its basically 92 lbs per finger. a 80 or 90lb bow with very pre-stressed limbs could be closer to that 120lbs range. In my opinion, I would design for 200lbs per finger for safety. I show your design using 1/2" thick material 6061-T6 to have a factor of safety of 2.12081 using 200 lbs. You could slim the tips up a wee bit but I wouldnt mess with it. If you really want thinner, use steel. At 200lbs I see deformation of about 1/40th. If I set it to 500lbs it probably would be closer to the 1/8th your seeing. Your design is solid. I restrained the part using just the base as you did. If I would do anything, it would be to beef up the pretty little red spot on picture 2. I think you used a fillet of 0.25 there. I would try 0.75 give or take you preference.

BTW: I see you are using Catia. Never used but I hear it is very good.


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## cncjerry (Jun 28, 2011)

Silentelk, nice work. Not familiar with the software you are using but it looks like the output is almost the same. 

I've been using Catia since the mid 80's. I have the full suite of products so I can go from design thru manufacturing all within Catia. Make a change, enter the manufacturing module and the gcode for the mill or lathe in my basement is ready to go. Took me years to get to this point. I love it when all the hobbies come together. With archery though, because of all the risk involved, I tend to over analyze things which prompted the question above. I've seen all these presses people make and though they are clearly well made, they look like you could press thousands of pounds with them. I saw one with a hydraulic jack on it. I haven't had the time to analyze the mechanics of a compound so was worried about using the aluminum I have on hand. I am now pretty sure that you can make the fingers out of 1/4 aluminum plate except for the bearing hole where it would wobble. It would need collars on each side.

Nice work.


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## SilentElk (Oct 6, 2004)

Thanks Jery. Solidworks 2011. I cheated on the design. I copied your picture, scaled it until my sketch of the hole was 1/2". Then drew lines over the outside, filleted them to likely numbers used and so it is probably pretty darn close. You 'Could' use 1/4" plate. I sued 200lbs and that had a margin of safety of 2.1 so 1/4" plate should be safety margin 1.05 on 200lbs and 200lbs is already a 2ish safety margin. Still, I dont like leaving things to chance that could twist my finger off it failed.

Ok, back to real work for an hour or so.


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## cncjerry (Jun 28, 2011)

SilentElk, check for a PM.


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## thwackaddict (Sep 1, 2006)

I squeezed a 40 lb bow by hand while a buddy put the string back in the groove. He put his release strap around the string. He forgot it was on there and it rolled the string off of the bottom cam when he pulled his bow and then let it back down. I have used a cheap ratchet strap to press my 60 lb bow


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## ymurf (Feb 4, 2010)

I water jeted some out of .375 material.It was 6061 and I am sure would be strong enough but I think over time the hole would ware. But you could press a bushing in it.


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## Ricky 2feathers (Jan 12, 2012)

I believe the fingers have no problem built out of 1/2 Alum. or Metal holding up under the pressure But what about the Adjusting threads and adjusting Bolts? How much force can they take before Stripping out?? Just a thought???


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## SilentElk (Oct 6, 2004)

cncjerry said:


> SilentElk, check for a PM.


PM recieved. 

I came up with: Safety rating 1.596 with 200lbs force and fixed bottom.

Just finished up some work. Spent 20 minutes and here's what I came up with for easy to do strength increases but minimal material change.

Changed 3 of the fillets and got a FOS (Factor of Safety) of 2.44 Amazing how a quick radius change helps. 1" on one near top of the base and 2in on the others. Picture Finger Stressed 3.

Now, I changed the fixed surfaces with the modified file. I selected the 1/2" hole and the veritical surface at the end of the base. FOS 2.44387 Basically identical. Main stresses are in the arm. Picture Finger Stressed 4.

Now I increased the fillets. Adds very minimal material allowing good access the Cams, but increased it to 3.214 FOS. Same fixed points as above. Finger Stressed 5. *See dimension picture Finger Stressed 5-3.

Finally, going to try using just the top filleted area just above my other force points. This is the highest point the Bow would contact increasing the leverage. The FOS dropped to 2.278. Big change! Finger Stressed 5-1. 

Picture Finger stressed 5-2 shows the orignal shape and deformed shape. Appears to be to about 2/3 of the 0.5 width or 0.33. Def scaling is 12.2 so likely bending is only 0.027in. Hardly at all.

Basically, adding a few fillets strengthened this alot and also takes into account the a higher leverage point. 1/4" plate would work fine. Personally, I would just use 3/8" or 1/2" depending on what is laying around. If I was going to produce these with other people using them and the chance of the press falling on them when not in use, etc. I would NOT go any lower thinner than 1/2" for liability. Cheers!~


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## cncjerry (Jun 28, 2011)

SE, hey, thanks for doing the work for me. Are you able to define the .5 hole as a rotary bearing surface? so the force vector would be on the lower right edge of the "L" and the lower surface of the bearing where it is thinnest. And your fingers are starting to look pretty sexy!


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## SilentElk (Oct 6, 2004)

Not with my version of Solidworks. Need my fatherinlaw to do that on his version. I cant test full assemblies using different materials. I can do flow analysis on the unit and check the airflow around it. Just incase you ever want to strap the press on the top of your car or truck and see if it may case any whistling sounds. However, unless you going 100's of miles and hour your pr bably fine.

Actually, I just had a thought. I might about to check it to some degree. give me a minute.

EDIT: SE. That used be my nickname when I used to online game. Seems like that was a lifetime ago. Nostalgic


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## SilentElk (Oct 6, 2004)

I just added extruded a 1/2" AL part through the hole. Extends 1" beyond each side. The extra length is so that part would twist similiar to how the real model would react. You can now see stresses occuring on bottom and down the top. The bottom is minimal as this is just under tension for the most part and the top compression. I noticed the stresses on the top are pretty darn even which is good!


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## SilentElk (Oct 6, 2004)

Here ya. 465.9 mph and still serious turbulance. Ok, I have to start working and get a few things done i nthe next few hours. Took me a while to remember how to do this testing.


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## SilentElk (Oct 6, 2004)

EBit on the Above: Still *NO* serious turbulance


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## jrdrees (Jun 12, 2010)

So at 300 lbs...75lbs per finger, just to press it. That's why I "overbuild" my press so I can operate it many times...whoever said that guy don't know...build your wooden presses!


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## ruttnwapati (Sep 22, 2008)

*Wood Fingers on Bar clamp presses*



SilentElk said:


> I just added extruded a 1/2" AL part through the hole. Extends 1" beyond each side. The extra length is so that part would twist similiar to how the real model would react. You can now see stresses occuring on bottom and down the top. The bottom is minimal as this is just under tension for the most part and the top compression. I noticed the stresses on the top are pretty darn even which is good!


Can someone model a piece of red oak with that modeling program to show how much stress is really applied at
loaded conditions to these wooden bar clamp presses being made?


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## cncjerry (Jun 28, 2011)

I can do it, but the issue will be the grain which is unpredictable. Bottom line is that the way the "L" bracket is designed, it won't hold up as is. The one i saw out of wood had a different design. 

Instead if wood, I suggest nylon cutting board for the "L" bracket. Stronger in some ways, self protecting, easy to cut. I'm sure 1/2" cboard would more than handle it using SE's radiused fingers. Stay away from polycarbonate and the other hard plastics.


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## SilentElk (Oct 6, 2004)

ruttnwapati said:


> Can someone model a piece of red oak with that modeling program to show how much stress is really applied at
> loaded conditions to these wooden bar clamp presses being made?


Do you have a link to picture of a wood press? wouldnt mind seeing one

As Jerry Stated, that wood grain is gonna be a killer. Basically between piece to piece the wood will vary in strength. Now a laminated wood wouldnt be too bad and be more consistant. I ran the same finger design but used Nylon. It gave a FOS of 0.858. Basically it failed at about 172lbs. I didnt save the picture but the unit deflected ALOT. Basically it moved about 2/3 of 1/2in or .333" but the deformation scale is 0.395. This means: 1/0.395 = 2.53 MORE deformation than indicated. So the the part moves about 2.53 * .333 = 0.842" which is ALOT on short finger like that.

In truth, if you have a bandsaw, you can cut AL. Table saw too but use lots a WD40 with a table saw or it will seize and do very bad things lets just say


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## ruttnwapati (Sep 22, 2008)

Here's a picture of one. I personally have never made one. Just curious how close they are pushing the envelope with this device ?


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## SilentElk (Oct 6, 2004)

Hard to determine but honestly my gut tells me its safe. Assuming other stuff we talked about, you can assume 185ish lbs of force on each end. My gut tells me with the ends bolted how they are it will easily hold it. I bet it you removed the bow, hung one end on a rafter and had a 300lb guy hang on the end of the wood on the other end, I bet he could bounce on it and it would hold.

The real issue arises after a year or 2 and the wood gets very dry and brittle. I wouldnt hessitate to use it unless it was old. Then, its harder to guage how sae it is. But I bet you could remove and old set of wood and make a new one, then bolt it back in an hour.


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## jrdrees (Jun 12, 2010)

ruttnwapati said:


> Here's a picture of one. I personally have never made one. Just curious how close they are pushing the envelope with this device ?


I'm sure the screws holding the wood would hold for a while. But if they're screwed into the wood, and not into anchor nuts, eventually they will give. Snap, strip the wood, something. I would rather have 1/2" metal (aluminum or steel) with bolts, lockwashers, and nuts... and I would trust this design all day.


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## cncjerry (Jun 28, 2011)

I set my z7x, roughly 65lbs draw weight on a scale, braced the limb with a rolled up towel, and pressed down. The string went slack at 108.6lbs. So now we know. Well, actually we don't, because I didn't press it down far enough to slip the string from the cam which would have been a little farther but no more than 120lbs total.


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