finally built one

[looseplucker]

Spine testing jig. Had a few patterns to choose from but opted for a simple version of Jim Hill's - and the darned thing works - well I figure it does given that it gives me the range on the 5/16" VA shafts I got off Forro a while back.

Still have to make a couple of mods and will then purty-fy it and post pics.

[Gringa Bows]

good mate,keen to have a look.

[looseplucker]

Cheers - a couple of bits are still rough - but it is very functional. Making the 2lb weight was fun - length of pipe with snapper sinkers in it!

[Stickbow Hunter]

Waiting for the photos.

Jeff

[Gringa Bows]

Does'nt matter what it looks like,it works thats the main thing.

[looseplucker]

Jeff/Rod - doing some mods this weekend and will then post pics as a retro buildalong.

[Stickbow Hunter]

Sounds good mate.

Jeff

[yeoman]

Good work John.

Not to derail your thread, but his seemed like a good discussion to enter the thing I'm about to introduce.

If you know the modulus of elasticity of your wood, which you can fairly easily measure, you can calculate the required diameter of the shaft for any given poundage. It is more accurate to measure the MoE of the piece of wood you are about to use, rather than rely on Bootle or CSIRO. They are excellent guides, but sample-to-sample data can vary by up to 20% either side of a published figure.

In the attached spreadsheet, I've put all the required formulas in. Knowing the poundage of your bow, type the corresponding required deflection into the blue cell. The green cell will give you the required diameter of the shaft.

Theoretically. Hopefully it works!

John, I'd be intensely interested to see if arrows made to the measurements given in my spreadsheet are anywhere close to the spine as measured in your spine testing jig.

Or anyone else, for that matter.

Cheers,

Dave

[looseplucker]

Dave

The proof of the pudding is in the eating. Email me off forum and we'll make a time for you to come around and we'll test the theory.

As for piccies, I am doing the modifications this weekend and will do the pix.

John

[rodlonq]

Gidday Looseplucker,

Looking forward to seeing pics of your new spine tester. I had a dekko at Jim Hill's design and it certainly is simple, to the point and should give accurate readings. I have always been intrigued as to the formula for converting the shaft deflection reading to spine - and there it was. Thanks for the lead. I hope your trials go well. I reckon you could use your theory and Dave's spreadsheet to perfectly match a group of shafts to the spine of the lowest spine shaft.

Dave, your spreadsheet is a real gem mate - good work there. It could save a lot of trial and error for those making shafts from square blanks. Once you get it round you can calculate the required diameter by measuring the deflection the rounded shaft in your spine jig. Given the deflection and the diameter of the roughed out shaft you can determine very closely the MoE for that shaft. I hope you don't mind but I have taken the time to extend on your excellent work . It includes input data for the roughed out shaft to calculate the final diameter to meet a required spine. Please check over it and make sure I haven't stuffed it up. I also added imperial measurements for the folk with old school measuring instruments.

Cheers, Rod

[yeoman]

Hi Rod,

Good work on the spreadsheet. Great additions! I don't mind at all. They're only numbers after all. I can't copyright them.

The only alteration I would make is to fill in cells C9 and D9 green, to let the user know for sure that they don't need to put a value in there. Had my simple mind confused for a second!

I had one more idea for it. This colaboration is great.

I put a second tab into the spreadsheet, and into that tab I put my original sheet, so that the file as a whole can be used whether you do, or don't, already accurately know the MoE.

Hi John, will do.

Dave

[rodlonq]

John,

Sorry to have taken over your thread, no malice is intended. In fact your post has me thinking my pivot style spine tester could be modified to use a dial gauge and result in more accurate measurement. It wont help me until I put in a lot more practice, but starting with a matched set of arrows has to help reach the end goal a little quicker. Thanks again.


Dave,

You have it nailed, I should have thought of copying the original into a new tab. Sorry about not changing the MoE to green. That could cause someone to overwrite the formula and lose it forever. I am happy to collaborate for the common good anytime.

Cheers, Rod

[rodlonq]

John and Dave,

Calculator updated as per Dave's comments attached. Thanks Dave.

Cheers, Rod