Model bolt and nut sizing

[mklotz]

One of the reasons that display quality models (such as Cherry Hill's) look so perfect is that these professionals take the time to ensure that the fasteners are to scale. This means making screws and nuts rather than using commercially available hardware.

While this is hardly necessary when making an engine for one's own satisfaction, many designs will require you to make at least a couple of custom fasteners. When I have to do this, I try to adhere to a standard that at least yields an in-scale appearance.

The rules I use for this are as follows:

D = bolt major diameter
Across flats dimension of bolt hex head and matching nut = 1.5*D
Bolt head thickness = 0.7*D
Nut thickness = 0.9*D

I would be interested in hearing from our British cousins on this point. Do these values agree with what is used over there?

 

[Bogstandard]

Hi Marv,
I think the sizes you have quoted there are almost ideal, except for the nut thickness, I sometimes like them the same thickness as the head, but it is no trouble hacking them down.
If you really wanted to go to town nearly all your nuts and bolts would have to be hand made, because in the early days of steam, the standard was to make your own assembly fixings and it wasn't until the likes of Whitworth came along they started to have standard standards.
As you most probably know, model engineering in the small scales in the UK use mainly the B.A. system for their fastening.
For scale 'looks' but still retaining the required strength of bolt, the manufacturers have come up with a very nice system whereby the bolt size is standard but you can get them with one size smaller head.

The long screws are 1" and the short 1/2". On the left are a pair of 6BA screws, the long one with standard head and the one on top with a 7BA head, the right ones are 8BA, again the bottom is standard, the top is with a 9BA head. Standard generally available screws go to 12BA, but the smaller you go the more expensive. I think, but not positive that they actually go to 18BA, but at that sort of size, sneezing could be expensive.

But Marv, aren't Cherry's models fantastic, and all scratch built. For me, the best model engineeress in the world. I once read that it takes her about six years to research and make one of her models. She was given an MBE in 2000 for services to model engineering. No-one deserved it more.

We are only playing at it.

John

 

[mklotz]

Please, whatever you do, don't put up any links to pictures of her models.

The last time I looked at some of her creations, I went out to the shop and had the twenty pound sledge up in the air over my lathe before reason kicked in.

Checking my shop notebook, I find the following re BA threads.

Full series runs from 0 to 22.
Thread angle = 47.5 deg.
Crest and root radii = 0.18p (p=pitch)
Pitches are calculated as p(mm) = 0.9^n (n=number of thread in BA series)
Diameters are calculated as D(mm) = 6p^1.2
Running:

FOR N=22 TO 0 STEP -1 : P=0.9^N : D=6*P^1.2
PRINT USING "## #.### ###.# #.###";N;D/25.4;25.4/P;P
NEXT

produces:

N OD TPI PITCH
(in) (mm)
22 0.015 257.9 0.098
21 0.017 232.1 0.109
20 0.019 208.9 0.122
19 0.021 188.0 0.135
18 0.024 169.2 0.150
17 0.028 152.3 0.167
16 0.031 137.1 0.185
15 0.035 123.4 0.206
14 0.040 111.0 0.229
13 0.046 99.9 0.254
12 0.052 89.9 0.282
11 0.059 80.9 0.314
10 0.067 72.8 0.349
9 0.076 65.6 0.387
8 0.086 59.0 0.430
7 0.097 53.1 0.478
6 0.111 47.8 0.531
5 0.126 43.0 0.590
4 0.142 38.7 0.656
3 0.162 34.8 0.729
2 0.183 31.4 0.810
1 0.208 28.2 0.900
0 0.236 25.4 1.000

 

[Hilmar]

Hi John and Marv,

On the cover of Model Engineer 16 February 1968 I wonder if the Model is
the same Cherry?
Looks just as beautiful as the models.
Hilmar

 

[Bogstandard]

Hilmar,
Don't care what she looked like, she is one of the 'lads'. Beauty is only skin deep, it is what's inside that really matters. Maybe you could post a pic so we can all see and admire.

Marv,
At 2am in the morning I really enjoy (not) looking at the charts for BA sizes. I struggle with anything smaller than 12, but honestly I didn't know they went that small, I never really needed to know, real instrument makers stuff there.
I have taps and dies to 10BA, but I don't think I have ever used the odd sizes. Up until a few years back, BA were the standard (below 3/16" whit)small sizes used by engineering electricians, and I remember my time on British produced aircraft, BA was the small standard on those.

For doing that to me I will post the only pic I could find of one of Cherry's models.

So perfect they actually use her models for reference purposes, as I said before, we are only playing at being model engineers. But we are happy just doing that.

Maybe it will inspire you to go and cut that #22 @ 257.9 TPI rather than use a sledge on your lathe.

John

 

[Hilmar]

Hi John
Here are the two pixs

Edited to display pictures correctly.

 

[Bogstandard]

Had a bit of trouble getting the pics to show.
Looks absolutely great, and Cherry Hill (nee Hinds) doesn't look too bad either. I think she has started to develop the large model engineering hands even at this early stage of her career.
It just goes to show that it isn't a totally male dominated passion, so get your ladies involved.
This is getting a bit off topic, so if we need to carry on, and not turn Marv into a raving lunatic with a big persuader, maybe onto the break room.

http://freeforums4u.com/viewtopic.php?p=4298&mforum=homemodelengine#4298


John