Author Topic: Grip/finger knob  (Read 3202 times)

Offline smfr

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Grip/finger knob
« on: January 13, 2013, 06:34:53 PM »
The reversing gear on my Stuart has a little grip knob that tightens down to hold the level position, and I was pretty unhappy with the first version of this knob that I made by knurling. You can see it on the right here:



Not long after making it I noticed a really nice little wheel on a shop-made depth gauge that I acquired, shown on the left. It has a nice, distinct grip with a shoulder on each side, and a slightly domed top. This was the ideal model.

However it also puzzled me, as I couldn't figure out how that grip was made. The grooves have a slight angle, and the roots of the grooves follow an arc. Was this made on a  gear hobber, or with a special knurl?

I experimented with a home-made tool, like an engraver, to cut the grooves on the rotary table. I took a broken tap with about a 1/8" diameter, and chucked it in the Dremel to grind the end into a cone against the grinding wheel. Then I ground half of the end off, and added a slight relief behind the cutting edge:



This was then tested on some scrap:



It took a few tries to get the point on-center; an off-center point resulted in a flat-bottomed groove. I ran at hight speed (2000RPM) while advancing the part into the point with the carriage. DOC was around 15 thou. This seemed to work pretty well, but was nowhere near as fine as the sample knob.

In addition to the engraver, another tool was required; a profiling tool to cut the radius that will be grooved. That started out as some oil-hardening stock with holes drilled on the scribed line:



The part is at a slight angle to add relief to the cutting edge. After this the end of the part was ground down to that line. I've made two holes of different diameters for different profiles.

Here it is after heat hardening and honing:



Now the profile of the part is roughed out in brass; the radius cutter makes it easy to do the internal profile:



Some clearance was added on the left so the profile cutter would only cut the appropriate section, then we apply the profile cutter:



Well, that cutter worked terribly! The straight edge part just didn't' want to cut, either with zero rake, or a bit of negative rake, and on-center and a bit under center. I did manage to force it into the material, but that profile hole is shot afterwards. Maybe the hardening failed?

Also, at this point, I realized that a half-circle profile is not what I needed to match the model knob; it's a more gentle curve. Oh well, soldier on!

Onto the rotary table to cut the grips:



I turned 8? between cuts (45 cuts in all) and DOC was about 15 thou.

Well, the result is prettier than my original knob, but nothing like the model:



The grooves cleaned up a bit with a file, and it looks quite good in situ:



I think with a bit of filing to smooth the surface, it'll do. It was a fun experiment.

Simon



Online Jo

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Re: Grip/finger knob
« Reply #1 on: January 13, 2013, 06:40:34 PM »

However it also puzzled me, as I couldn't figure out how that grip was made. The grooves have a slight angle, and the roots of the grooves follow an arc. Was this made on a  gear hobber, or with a special knurl?

Simon, it looks much nicer than your original one :ThumbsUp:

It might be worth considering making a concave knurl if you want to continue experimenting ;).

Jo
Usus est optimum magister

Offline zeeprogrammer

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Re: Grip/finger knob
« Reply #2 on: January 13, 2013, 06:53:50 PM »
Pretty interesting.
I did a little googling...

Try 'concave knurl' or 'rope knurl'.

I think that's what you're after.
Also found an article that talked about making your own knurls.
Carl (aka Zee) Will sometimes respond to 'hey' but never 'hey you'.
"To work. To work."
Zee-Another Thread Trasher.

Offline smfr

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Re: Grip/finger knob
« Reply #3 on: January 13, 2013, 07:35:16 PM »
Aha, rope knurls, that's the ticket! I found a few useful pages:
http://www.frets.com/HomeShopTech/Projects/RopeKnurl/ropeknurl.html
http://www.ronayling.dsl.pipex.com/knurling/knurling.html

You learn something new every day!
Simon

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Re: Grip/finger knob
« Reply #4 on: January 13, 2013, 09:17:23 PM »
I'm surprised some enterprising supplier has'nt thought to have Rope Knurls mass produced in the Far East. I looked for them ages ago with no luck.