Designed by Touch: a Dual Polymer Knife Handle

I've always enjoyed making custom knife handles, and have used a few I found on Instructables as a guide (shown here, e.g. LINK). However, many are crafted in wood, and almost all are solid. I was curious to not only create a custom profile - but a custom hardness also. This required me to work in a polymer, Sugru to get the desired result. I think this is quite an unconventional REMIX, and hope you'll vote if you are inspired to do other personalised projects out of polymers!

WHY BESPOKE?

Many consumer products are designed for the '95th percentile' - i.e. the majority of people, but although this works pretty well for things like toothbrushes, which are unlikely to justify bespoke design/manufacture, (being disposable and cheap), but for tools and other items which become personal possessions for many years, perhaps there is an exception to be made?

One option would be to to make a 100% bespoke knife, but few of us have the cash or even ability to design an entire knife from scratch. So I wanted to make a bespoke grip to my hand.

ADVENTURES IN SHORE-A HARDNESS

This project is aimed at people who like custom gear and materials. It's a bit niche, so not for everyone, but I've shared a lot of these tips and for those who need to sculpt and shape things, so thought it might be useful to share here at Instructables, perhaps as an exercise in learning how to prototype in different grades of rubber/polymeric materials. If you take a company such as JosephJoseph, (who uses silicone rubbers extensively) one can quickly appreciate that being able to prototype - and specify technical design details like this, is effective in terms of cost and time, and of course allows for more user testing without having to produce expensive moulding tools.

CREATIVE LINKS

To be clear, I did work for Sugru, but I did this sort of exploratory work in my free time, examples include (Oyster Card Hacks (LINK), Wireless Charging SatNavs, etc. in my other Instructables). My views are my own, and you attempt anything here at your own risk.

Handle-less Knives

I was lucky to be able to source an 'un-finished' knife from knife makers TOG*, which had a tiny defect (they are perfectionists - love it!). However, as this was luck on my part and TOG does not commonly do this, the suggested alternative, would be to:

a. Buy a blade from a company such as F.Dick http://www.dick.de/en/special-tools/catalogues/

b. Buy a finished knife and remove the handle by inserting a saw blade between the 'Tang' (see diagram) and the wooden/plastic handle, and saw it from the surface, before grinding the adhesive away. Then you'd start from the same point as myself.

*Blade kindly supplied by TOG Knives.

Safe Working

The "Swivel-Vice" is a valuable item if you have it (typically around $15-$20), as it will allow you to work at a multitude of angles - while keeping the blade not pointing towards you (or in the direction you are working). Please cover the blade with a cover at all times when working, it is all too easy to injure oneself when focused on a detail of the handle, and then absent-mindedly knock into the blade. You have been warned.

Variable Shore-A Hardness of Handles

Sugru Softener is an additive which reduces the Shore-A Hardness of Sugru from 70 (bike type) as low as 45 (elastic band). It is not widely available, as is in beta-testing. Similar results can be achieved by either using different modelling putties, (Green Stuff, Fimo, etc) or by adding Blu-Tac to Sugru. Some experimentation is required here, but it's interesting if you are into material science.

BUY: Sugru Family Safe and Sugru Original are now available and have different softnesses, and can be blended.

Tools/Items not shown:
Dremel/Proxxon Rotary tool., IPA, Wire Wood,

1. Keeping the blade totally covered, either with a guard as shown, or else making one out of card.

2. & 3. Take handle, place on Styrofoam (cut to roughly 1/2 inch or 12mm thick sheet)

4. Draw around - both Left & Right sides.

5. Cut out & back with double-sided tape (or loop the masking tape if you don't have DS Tape)

6. Place Left/Right halves on the Tang.

7. Clamp the knife. Keep the blade covered.

8. Trim the foam with the scalpels. You might find that 'curved' blades work best with foam, rather than 'straight' blades. More on this at Design Modelling, if you are interested.

9. Cover the Bolster with masking tape, to keep sanding scratches to a minimum.

10. & 11. Using the Sanding Boards/Paper work the handle to the desired shape.

While performing various chopping action with your knife, occasionally 'freeze' your motion, and look at the grip you have - draw around your fingers, taking note of the pressure you are exerting with different fingers - this will be informative for modifying the bespoke grip later on.

Some sketches of various thoughts and iterations around this. Considering different densities of the grips of the handles.

Static vs Dynamic Grips: You might see custom-grips made from Sugru, 'polymorph' or the like. I personally think that making a grip by squishing your hand into a moulable material (and letting it set) is not a good appropriate for a Chef's Knife: This is a 'Static' grip, and it becomes apparent when using it for something like this - that a 'Dynamic' grip is what is needed. This is why multiple grips are shown here and I propose softer/harder 'zones', (light and dark blue) rather than a fixed profile which locks your hand in place.

1. Detach the Styrofoam handles.
2. Clean off any tackiness, with IPA (rubbing alcohol, or detergent of you don't have this).
3. Taking note of the essential areas you wish to develop in the grip, you can transfer these onto the Tang, while even adding some minor aesthetic embellishment as you see fit.
4. Rough-up the surface with the files.
5. Using a Rotary Tool, grind in the drawn-on details.
6. Clean off the Pen with IPA, so that you have a clean surface to stick the sugru to.

If you are intending to work with 2 Shore-A Hardnesses of material in the grip, you may choose to accentuate these with a different colour scheme. I find that making a quick sketch in Photoshop helps remove some of the guesswork.

A great tip for those not great at picking colours schemes is to use Adobe's "Kuler" programme.

https://kuler.adobe.com/

The Hardness of Rubber is measure using a Durometer. The following compositions of Sugru + Softener are shown in the next steps, but I thought I'd include this to show how to put numbers next to tactile preferences. Naturally, Wiki has more info.

After some trial an error on my part, I found that the preferred 'soft grip' for me was around 40 Shore A (dark blue) and the more dense/harder grip was around 60 Shore A. This step is not essential, other than it helps you evaluate your work and the industrial standard would allow you to commission rubbers, etc, which would feel 'right'.

Colour mixing with Sugru requires some practice. I've found that 'stacking' colours together and 'twisting' them together is a good was to get uniform mixing.

I then applied the mixed Sugru to the surface of the Tang, and moulded it into the approximate profile I had found for my grips, noting the left/right sides carefully! I used cocktail sticks/kebab skewers to help get finer details.

LEAVE TO DRY FOR AT LEAST 72 HOURS*. Ideally a bit longer, as you may have considerable thickness of Sugru, so I left mine for the week, in an airing cupboard, and picked it up again the following weekend. As always - consider a safe place to leave it.

Once dry, I trimmed the Sugru back to the profile I had engraved with the rotary tool. Now it is ready for the second colour and/or material...

*As a rule of thumb, Sugru cures 3mm in 24 hours. So this was about 7-8mm thick.

Following similar steps to the previous, I added the Sugru Softener to the colour mix and matched this to the scheme I picked in Kuler.

I trimmed back the excess while uncured, and left for another week to be sure it was cured. (This is not to say, that it would not be dry sooner, but I was happy to wait while doing other projects in tandem). Naturally, if you have a faster drying mouldable rubber adhesive in mind - do let me know!

After trimming down the excess Sugru on the handle, I used Wire Wool (Grade 000, Extra Fine) firstly to work the Sugru down to a fine finish, then also to buff-out any scratches on the Bolster of the knife.

Various Metal Polishes are available and may also help finish the prototype.

Give the final piece a wash in soapy water and rinse thoroughly, as you don't want any debris in your food.

Having used the knife for some time now, it's a pleasure to use, but I'm bias of course.

As to the next steps of this/similar projects, thoughts include:

PROTOTYPING:

- Creating a sub-structure on the handle, which the Sugru would stick to. This would reduce drying time and also use less Sugru. This could be 3D Printed in ABS, which Sugru Bonds well to.

- Considering how Sugru mixes with other adhesives, like epoxy to make extra-hard.

PRODUCT:

- Does this prototype require considerable work and refinement to be fit for tooling? Yes, but as a way to explore the extent to which considers can realise that 'last 5%' of the design customisation for themselves, I believe it's an interesting first step. Given that custom earbuds are available, perhaps more consumer good will be in time.

The techniques used here were also the same used to create prosthetic fingertips for Nigel, and amputee, who wanted to have better custom grips to his Prosthetic Arm. Instructable.

Indeed, this too is somewhat of a remix again.

Hope this inspires you to experiment! Please vote if it does =)