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Heavy Mod - 1/18 21st Century Toys P-47D Razorback


JayW

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I really like the jig.  I can't count the number of tires I've modelled and been disappointed with the result.

 

RE:  not getting the symmetry you're desiring:  When you are using it, are there small movements in the alignment pins, or does your saw blade flex?  Or is it in the spacing in your geared cog?  Any one or all could be the problem.

 

I can imagine what kind of hand movements you would have to make to get the saw from one side to the other, and over the arch.  I think if you completely eliminate wiggle, spacing in the cog, and human error you would eliminate symmetry issues.

 

It's a fantastic idea!

 

Gaz

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Gaz says:


 


"RE:  not getting the symmetry you're desiring:  When you are using it, are there small movements in the alignment pins, or does your saw blade flex?  Or is it in the spacing in your geared cog?  Any one or all could be the problem.


 


I can imagine what kind of hand movements you would have to make to get the saw from one side to the other, and over the arch.  I think if you completely eliminate wiggle, spacing in the cog, and human error you would eliminate symmetry issues."


 


 


EXACTLY!  The blade flexes.  I abandoned my original plan to use a reinforced saw blade because it complicated the slot it slides in.  A taller saw would help.  I got the spacing between upright rods just right nominally, but they flex as well.   The gear teeth are pretty exact, but truth be told I could do without it.  It's better to take pi times the wheel diameter, make 28 equal spaced little marks on a piece of tape, wrap the tape around the tire, and create little drill starts at each mark.  That gives a feature to aim for with the saw blade.


 


I cannot imagine doing this in a smaller scale, BTW.


 


Craig - 3D printer.  That would be awesome.  But I no longer have my awesome CAD program and am too lazy to learn a new one.  I will especially be wanting it as I do the second tire! 


Edited by JayW
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TwoHands - pull saw.  That must mean the blade shape is reversed?   What advantage do you think such a saw provides?

 

So yes, the saw is a big key here.  The blade cannot be thicker than .025 inch.  That gives a good scale gap between diamonds.  Actually the blade ought to be thinner still, because repeated passes tend to widen the cut.  Any blade that thin will be flexy, unless it is re-inforced with thicker metal on the opposite side of the saw from the teeth (like most modeling saws - I have a few of them).  So while the saw is much less flexy, the total thickness of the saw is much thicker.  This would require a wider slot to guide the saw motion, and the saw teeth could wander, introducing as much error as a flexier saw.

 

I am beginning to think machining accuracy is required to get a diamond pattern just right, as opposed to hand cutting.  I have not thought of a way to do that yet.

 

I am also considering a new improved version of the jig, now that I have some experience under my belt.  I would turn the upright posts to be perpendicular to the saw cut direction, to reduce prying between two posts.  Also, the brass cross pin that engages the slot in the special wheel has a bit of unavoidable slop, allowing the tire to rotate freely a degree or two at each of the 28 detents where I have to make a cut.  That just adds to the tolerance.  Maybe the tire can be clamped up once positioned, eliminating that slop.

 

Then again, I might just use the existing one and take advantage of learning curve!!

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The diamond tread jig had some problems with the first tire, which resulted in a less than precise tread pattern.  So I made a few improvements:

 

1.  increase the thickness of the plastic strips bonded to the two sides of the saw blade - this decreased the clearance between the saw and the upright posts.

2.  Create equidistant drill starts on the circumference of the tire to create a more precise target for the cut.

3.  Clamp up the tire prior to cutting, to avoid shifting due to slop between the axle cross pin and the wheel slot that it engages. 

 

This pic shows the clamping arrangement, and the saw blade with the plastic shims on either side:

 

9thRVf1l.jpg

 

And the equidistant drill starts:

 

uZNs2Dbl.jpg

 

Once the tread patterns were done, I converted my lathe to an end mill and machined flats onto the tires:

 

mA02eL6l.jpg

 

Later I will apply some putty for a bulge, and apply the black top coat.

 

Anyway, after the tread cutting, this is what I got:

 

VFJSlUll.jpg

 

You see both new tires with diamond tread and flats, the beginnings of the first brass wheel rim, and the existing toy's wheel/tire combo (what a contrast).  The trusty well-worn jig is in the background.  I am very happy with the second tire - my improvements really worked!  It is just about all I expected.  But now I am in a bit of a quandary.  The first tire looks a bit ragged by comparison.  I can make a third tire - but......   It takes the better part of a day to do one (machining only - the tread cutting takes a few hours too), it's alot of extra machine time on a tired old lathe that has a finite lifetime, and most importantly the jig is getting tired too.  It gets a good bit of force applied to it during the cutting process (56 cuts), and I am not sure I can maintain the new-found precision.

 

I value opinions - do you think there is a large difference between the two tires?  I will not say which is the first and which is the second, but it should be evident by the picture.  Would you do a third tire?

 

Next post you will see a completed wheel assembly - ought to be pretty cool!

Edited by JayW
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Jay, Japanese saws cut on the pull stroke. I have a couple different ones I have nought for modeling. the basic concept is since your pulling rather than pushing so the blade stays straight. This is is the general type I'm taking about but there are many.  https://www.amazon.com/Vaughan-BS240P-Pull-Stroke-Handsaw/dp/B00004Z2WO

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I would support you making another one. The tires are one of the best part of a P-47 and key to this upgrade (as well as being very visible). You'll stare at the mistakes forever but it'll just take a day to correct. Fantastic work, Jay. Rare to get some tricky scratchbuild objects right the first time. Too much to learn.

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Good honest advice all.  And TwoHands - what a marvelous looking saw.  I ought to get it; it might just fit inside the uprights!  That is should I bite the bullet and make a third one.  That requires a night or two of sleeping on it.

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"Can't you make a resin copy of #2 ? "

 

Hubert - recall I made that resin prop blade!  Didn't even think about making the tire from resin.  The part requires machine tolerances on the ID so that the wheel rims fit properly; not sure I'd get that with resin.  I will think on this.

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