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HOWTO: Draw an approximate ear trim line with no gaps


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Those darned ears.  I've overtrimmed my share and had to start with new ones a number of times.  Cutting along the mold line how the ears were trimmed originally may not be the correct line.  A beginner's error is just to trim a fixed amount relative to the outside of the ear for a consistent thickness.  This often results in overtrimming. (The outside of the ear actually plays no part in determining the appropriate trim line except for determining how much ear you have to work with.)  And once you've overtrimmed in just one section, you either live with the gap or buy new ears.

 

As I thought more about what makes a proper ear trim line I realized it's not a trait of the ear, it's a trait of the helmet.  It's the width contour of the helmet along the points where the edges of the ear touch the helmet that defines the ear trim curve for no gaps.  The trick is how to copy this contour onto the ear.

 

Here I show a simple method for creating what should be a gap-free trim line, using a pencil, a toothpick, and a rubber band.  In my trials with this method, it usually gets within +/- 2mm or so around front and back of the desired trim line; it's not perfectly accurate.  Some additional fine cutting or sanding is also needed to finish the trim.

 

As a minimum, this technique can be used to get a general idea of what the trim line should look like and to show where the various inflection points are, and where the sections are such that if you trim too aggressively there, you may run out of ear elsewhere.

 

I show an example where I'm using an ear already on a helmet for which I may want to reduce its thickness, but the same technique can be done a new untrimmed ear.  The prerequisites are that the helmet's faceplate and cap-and-back have been assembled together and that you've decided where the ear will be placed on the helmet to establish the location where the ear's bottom edges will meet the helmet.  The untrimmed ear then needs to be held or secured level in that location on the helmet as the front and back contour lines are traced.

 

While I only show the front half of the ear in these pictures, it's necessary to trace the helmet contour on the entire ear, front and back, using the same vertical offset throughout.

 

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Edited by Sly11
Edited to restore brocken image link by Sly11 2020
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Nice trick to modify the thickness of an ear, but IMO what most people are looking for is how to get the first trimming line, even if it's to thick. However, i'm sure it will help some people around ;)

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Yeah it looks too easy when the ear is already trimmed... But the technique works for untrimmed also. I have one set of untrimmed ears. I can add pictures showing how the ear trim line would be drawn for those; I agree that would be more relevant.

 

 

Sent from my iPhone using Tapatalk

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Here's a quick example with an untrimmed ear.  The ear should be placed level with how it will lay on the helmet.  For that I used some non-stick putty.  The offset between pencil and toothpick will be greater since the ear is higher, but otherwise the procedure is the same.

 

And again, this technique is not a "one and done" method.  It gives an approximate trim line only.  It's always a good idea to go with a thicker ear cut initially.  The helmet contour will change a bit when the ear is installed with screws, and precision cutting and/or sanding will still be needed.

 

I traced this several times, and as you can see there is plenty of variance.  

 

Lots of sources of error including the toothpick moving, not keeping it directly under the pencil, and my hand changing angle.  It would be great to see suggestions how to tighten this up and reduce these correctable sources of error (for example, place the pencil and toothpick on a clamp to ensure the offset remains fixed).

 

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I may have to try this sometime with may spare set of ATA ears. Very cool.

 

I wonder if you could use an old fashioned compass that is utilized for drawing circles for this, as long as you put some tape over the needle point to avoid scratching the helmet.

Edited by DroidHunter
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I wonder if you could use an old fashioned compass that is utilized for drawing circles for this, as long as you put some tape over the needle point to avoid scratching the helmet.

 

That's exactly how I did my ears.

 

I have an old drafting set and you can remove the needle from each compass.

There are three different sizes, so I removed the needle and used the smallest.

 

Nice post!

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  • 2 months later...
On 5/23/2014 at 10:40 AM, bpoodoo said:

Good call on using a compass I had to give it a try. I found my old engineering drawing set which also had a compass with removable pin. When I used it the trim line traces tightened up nicely.

 

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Did that end up working? Maybe it's the angle, but it looks like the ears would be almost flush with the helmet if the compass stays at that width. Not to doubt your skills, I intend to copy the technique if it's effective. Seems like it would be hard to get the ear to lie at the same level at which it'll end up resting when you install it, even with putty!

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The trimline in the above picture was what I traced with the compass at that angle all the way around.

 

What you might do is use this technique to draw very conservative trimline that would result in a much thicker ear than you ultimately want so you can first be confident that the technique works.

 

Pay close attention to the section just above the tube with a sharp turn and also at the very bottom following the undercut. It's easy if you don't follow the contour exactly to draw trimline a few millimeters too high which will cause you to have a gap there.

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I've been tempted to buy this item "FastCap Accuscribe Scribing Tool $13.99" to see if it would work any better than a compass: http://amzn.com/B0001GUDT6. The same basic concept as using a compass, but the articulating arm might provide a bit more flexibility in tracing the the trim line.

 

UPDATE: I did buy the Accuscribe to try it out, but I can't recommend it. For our purpose it really needs articulation for the arm that is tracing the contour, not just the arm with the pencil.

 

I've read of some using a contour gauge such as this http://amzn.com/B00004T7RA. But I'm not sure how well that would work because it only picks up a contour along a straight line, when you really need to pick up the contour along the perimeter of the ear.

 

So a good quality compass seems to be the best bet so far.

Edited by bpoodoo
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Scott - I think it was just the angle of the picture. At the top there was about 5 mm ear thickness as I drew the line though it looks like it was diminishing to no ear thickness!

 

Which is exactly how I think this method can be truly useful - particularly for beginners - to see where the thin spots will be and adjust the trimline accordingly BEFORE CUTTING. My heart sinks when I see that someone's overtrimmed an ear on the first try, though our fine members and armorers are quick to assist.

 

And you're right until you install the ears with tightened screws you won't know for sure how it will lie exactly. Tightening the screws in itself will change the profile of the helmet, so the detailed fine-tuning will be needed.

 

I have used this technique successfully with a new untrimmed ear with good results following conservative, incremental steps:

 

1) Starting point is you have the front and back of the helmet assembled with at least 2 rivets or screws installed at the ear location.

2) Using an untrimmed ear, secure it level in some way (e.g. putty, tape, etc.) level above the helmet.

3) Set the compass to an offset that provides a very conservative, thick trim line; perhaps 10 mm thicker than you'd ultimately want.

4) Trace all around the ear with this constant offset. Do not press the ear against the helmet as this can result in a incorrect trim line, especially near the bottom of the ear.

5) Remove the ear and trim it along the drawn trim line.

6) Install the ear onto the helmet, including drilling and tightening of all screws attaching ear to helmet. Now you should have a true helmet profile.

7) Set the compass with an offset that would result in a trim line for an ear perhaps 3-5 mm thicker than what you'll ultimately want.

8) Trace all around the ear with this constant offset.

9) Remove the ear and trim it along the drawn trim line.

10) Install the ear. Now the ear should be flush against the helmet (or "close enough"), just 3-5 mm thicker than you want. But you have confidence in the method (and some practice). If it didn't work, you should still have a viable ear you can complete trimming using other methods.

11) If it worked, draw a final trim line to the desired ear thickness; remove ear; trim ear; cross fingers; install ear.

12) Manual fine-tuning / sanding

Edited by bpoodoo
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  • 3 weeks later...

one of the most challenging aspects of this is the proper placement and alignment of the helmet in the first place, and

the second aspect is the amount of tension that will shape the ear when it is mounted!

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  • 5 months later...
  • 8 months later...
  • 3 months later...

I have one of those flexible molding curves.  It can be useful for some applications to capture and transfer a 3D contour, but only when the curves change gradually.  The sharpest bend you can get from them is limited (approximately 1 cm minimum radius of curvature), and not good enough to capture the transitions near the tubes of the helmet.

Craft wire is also an option.  16 gauge (1/16" diameter) worked pretty well for me.  But it can be frustrating and difficult to get the wire to lay down perfectly on the helmet all the way around.  To capture the shape accurately, you need to bend the wire slightly more than the curve you're trying to follow.  Otherwise, it tends to spring back.  And as you're bending in one spot, you might affect the trace you've already established elsewhere.  It can be done, but it's tedious.  And even after you're done, if you accidentally bend it, you've got to recheck and probably redo.

What would be ideal is to capture the 3D contour with something that is moldable and lays down on the helmet easily but then firms up and is solid when you're done.

Instamorph is the best option I've found.  It's moldable after it's been heated in water (140-150 deg F).  While it's warm, it's moldable for a few minutes (when it's clear), and hardens to opaque white and retains its shape.

Instamorph when warm and moldable will stick to many plastics (including ABS) and rubber , so you MUST cover the ear/helmet with cellophane wrap as you're shaping it.  After it's cooled and opaque white, it won't stick at all.

One of the big advantages of using a removable 3D contour whether from craft wire or instamorph, etc. is being able to see a variety of potential ear trim traces by placing it over the ear.  In particular, you can see how the trim can change with different angles (top-to-bottom and back-to-front).  Below I show how if you have too thick of an ear trim at the bottom, using the top as a pivot, you can change the angle top-to-bottom of the contour and cause greater deflection at the bottom curve of the ear and not change the thickness at the top of the ear very much.

I still use the compass as a tool for fine-tuning all the way around the ear after it's been attached to the helmet, and of course refer to the ear trimming tutorial by gazmosis.

David

 

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  • 4 years later...

For this week's offering of PDF resources I've now compiled this thread into printable/downloadable PDF form for builders who may want to either print it or store it on an iPad. The loathed photobucket overlays have even been removed by accessing the original images directly! This thread compliments the other Helmet Assembly tutorials I've compiled on my All-In-One Resource Post, and many thanks to David (OP) for this excellent how-to!

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  • 4 months later...

Another thank you for writing this up, I'm starting my first build and very confident in my ability to measure once, cut three times.  

Bookmarked for use, and just bought a compass on Amazon.   :duim:

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