Hornet gyro measurement question....

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I've had the Hornet plans,version 14,for quite some time now and decided to really take a close look at them today........I was both impressed and baffled at the details in the plans,the plans are very detailed.
The majority of the component dimensions were pretty straightforward and I figured the big picture out fairly easily.
However,there are several dimensions given that I could not interpret.
I'm very versed in converting decimal points into fractions and vice-versa,but many of the measurements given in decimal form I couldn't convert over to a fraction even with the help of getting online and using a converter/calculator.
Now,I'm a very good fabricator but I am not a machinist by any stretch of the imagination.....
Is the issue of not being able to understand the measurements due to the fact that I think like a fabricator and not a machinist ?

Kevin

wanttobuild

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Very interesting Kevin.
I was just looking at the Bensen plans. I will look at the Hornet.
You can find a fraction decimal equilivent chart online. I am a carpenter and I convert back and fourth all the time, it's the mm to inch conversion that's really crap.

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Here is a quick example.....but there are many throughout the plans.

.373= 3/8
.250= 1/4
1.125= 1 1/8
Etc......
No problem....this is just basic math conversions....

What is .058 ????
It's not a 64th and it's not a 16th.
I've even tried converting it online and haven't had much luck.
There are many measurements in the plans that I can't convert,just like this....

I'm wondering if some of these measurements are more suited to a machinist rather than a fabricator in his shop.....

Kevin

Turd Ferguson

Well-Known Member
Decimals are often used in aircraft dimensions. Rutan dimensioned his planes with decimal measurements. You can buy a decimal rule and tape measure if you want to go that route. Any decimal can be converted into a fraction but it may not be in the normal 8ths, 16ths or 32nds increments we normally use for measuring. Rounding off a decimal measurement to the nearest 1/16" on an airframe will be more than adequate 99% of the time.

Last edited:

Turd Ferguson

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What is .058 ????
It's 58 thousandths of an inch. It is 5 thousandths shy of 1/16" and I doubt your pencil will mark that accurately.

Turd Ferguson

Well-Known Member
I'm wondering if some of these measurements are more suited to a machinist rather than a fabricator in his shop.....
For the bellcrank, yes because when a 1" axle is fitted inside that bushing, it will have .009" clearance which would make for a good operating fit between the two parts.

TFF

Well-Known Member
.058 is the material you will buy. All the dimensions are fine; what is missing is what and why in those drawings. You don’t care about .058 as a dimension, you care as it’s the material you need to buy. All the dimensions do stack together, but all are not going to be manipulated by machining or cutting or whatever. If it is going to be machined, the plans need to be clear after you buy .058 tubing. Normally most legacy plans will have everything dimensioned in fractions but the materials in metal will be in decimals as that is how you are going to buy it. If you just want a fraction, convert it to the closest 1/64. Now if you go buy .058, they sell a tolerance; it’s not going to be perfect.058. It will just be .058 material.

Aerowerx

Well-Known Member
To clarify what TFF just said...

0.058 is a standard "off the shelf" nominal wall thickness for 1.125" outer diameter tubing. In many cases the wall thickness is such that the next smaller tubing size is a sliding fit. In this case, a 1 inch tube can slide into the bellcrank with 0.0045" clearance on all sides (when centered). IIRC, this is defined as a sliding fit.

BJC

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HBA Supporter
Decimals are often used in aircraft dimensions. You can buy a decimal rule and tape measure if you want to go that route.
That is a good option. I find that working in multiple systems of measurement leads to errors when converting to a different system. Just buy a decimal scale and use it when the dimensions are decimal. Use a 1/16, 1/32, or 1/64 scale when the dimensions are given in typical fractions.

The same concept applies to SI verses the US system of measures.

The only time it is beneficial to convert is when the raw material is not available in the dimensions used on the drawing.

BJC

wanttobuild

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Good ole HBA.
Kevin, you got a lot of answers and I agree with them.
ACS has info on 4130. Sizes and sheet thickness.
Ben

bmcj

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I’m glad you guys came up with the correct answer... I was afraid that I would have to throw out the answer of 297/512ths.

Turd Ferguson

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0.058 is a standard "off the shelf" nominal wall thickness for 1.125" outer diameter tubing. In many cases the wall thickness is such that the next smaller tubing size is a sliding fit. In this case, a 1 inch tube can slide into the bellcrank with 0.0045" clearance on all sides (when centered). IIRC, this is defined as a sliding fit.
"Assuming" the material is steel tube and not bronze, aluminum or some other bushing material.

TFF

Well-Known Member
I worked for a steel distributor. The way they sell is they look for material at the thinnest you can call the size but sell at the heaviest it can be. That’s the wiggle room for pricing. If they get a bunch of thick stuff they loose money. The more costly the material the tighter that becomes. No one gives away gold.

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Ok....so I just googled decimal tape measure.
I had never heard of it before but now I know.....who would've thunk it...lol
But I'm still confused......sigh.

I understand the whole concept....so let me just say that.
What I dont understand are measurements in the plans such as :

Measurements that don't convert....
6.099 what is the .099 ?
3.974 what is the .974 ?
And so on and so on through a lot of the given measurements in the plans.
Are the charts that I'm looking at just not detailed enough to break down every thousandth of an inch or am I just missing something in my translation ?

I'm assuming the .313 & .257 listed on the drawing are the drill bit sizes ....not sure what number bit they would be though.

Kevin

Turd Ferguson

Well-Known Member
I'm going to guess the CAD program used to make the drawing automatically assigned some of the less practical dimensions.

.312" would be 5/16". Maybe they expect the hole to be reamed slightly for clearance. The 1/4" holes apparently need more clearance for fit up or else you'll
have to order a bunch of specialty bolts like this:

Aerowerx

Well-Known Member
All I can say at this point is get yourself a digital caliper and don't worry about fractions!

lakeracer69

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For the love of God, this ^

BJC

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HBA Supporter
But I'm still confused......sigh
6.099 what is the .099 ?
The part, as shown in the drawing, is 6 and 99/1000 inch long. Note that the dimension at the top is 0, so the distances going down are the distance from there. It would be easier to understand the shape of the part if there were breaks in the extended lines that show the 30 degree angle. It is good that an isometric view, to the right in yellow, shows the overall shape of the finished part.
3.974 what is the .974 ?
Same thing, 3 and 974/1000 inches. Also 26/1000 inch, about a spark plug gap, short of 4 inches. That is not a homebuilder-friendly design (it probably could be 4 inches plus or minus 1/16 inch), but if you stick with it a bit, you will get it done.
And so on and so on through a lot of the given measurements in the plans.
Are the charts that I'm looking at just not detailed enough to break down every thousandth of an inch or am I just missing something in my translation ?
If you are hand shaping the part, it is not possible to build it to the given dimensions, because you can neither read a scale to 1/1000 inch nor accurately place a center punch, drill bit, saw blade, etc., to that accuracy.

Try to understand the function / purpose of the part, and how it mates or interacts with other parts, and build it to mate, and as accurately as you can.

And recall the thread that discusses the presence of errors in almost every set of plans.

BJC

mcrae0104

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All I can say at this point is get yourself a digital caliper and don't worry about fractions!
For the love of God, this ^
For the love of Bernie Pietenpol, forget about digital calipers and use your sense of engineering proportion!

The difference between 0.058" and 1/16" is about one sheet of typing paper. Does it matter for what you're doing?

I have heard, although I can't remember where, that John Monnett reminds folks that they're building a homebuilt airplane and not a Swiss watch. 1/8" tolerance is generally not life-threatening.

(Typing paper, for those too young to remember, is like that funny paper they used to run through printers with holes all along the edge and perforations you could tear off.)