Refreshing worn prop driver
 

This is on an engine I inherited.. the driver burrs or ridges are worn more or less smooth and don't grip the prop well. I don't [want] to tighten the prop nut excessively. Is there some sort of lathe or milling tool out there to recut a grippy pattern in the front of the hub driver. Engine is actually diesel but more eyes visit the glow site...

WP, I have seen a piece of sticky sided sand paper used for that, put the sticky side toward the prop driver, the sandy side against the prop,it works well and no problem to make a new one.

Yes-its called a knurling tool-and they come in a variety of types-single, scissor, jack, pivoting head-and with straight and oblique patterns-the latter are always used in handed pairs to produce the familiar 'diamond knurl' you find on tools, and other things that need (usually) finger gripping to rotate of unscrew. This is NOT what you want to refurbish a prop driver-what you need for that is a straight face knurling tool-one that has a single wheel with teeth across the periphery of the wheel, perpendicular to the rotation. The knurls themselves are cheap enough and come in a range of pitches from very fine up to about 2mm or so pitch. The tool is held in the lathe tool post-on centre-and the prop driver gripped in the lathe chuck.....you might have to make a special fixture to hold the prop driver....and clearance can be a problem-especially if the prop driver has a centre boss on the driving face. Commercial single knurl tools are general made from a square or rectangular bar, with a slot wide enough and deep enough to take the knurling wheel milled in one end (sometimes both...you can get double ended ones) and a pivot pin hole drilled through the centre of the slot at 90 degrees to hold the wheel...the pin being a fairly tight press fit-though some use a grub screw to lock the pivot pin into the tool. About 35-40% of the wheel diameter projects beyond the end of the slot-though for a few specialist uses the wheel can be mounted on the end of the tool...[3rd pic]
The lather is run at moderate speed and the knurling tool advanced into the face of the prop driver-and the pattern on the wheel is reproduced on the surface it contacts [if the tool is not mounted on centre, a spiral pattern results from the use of a straight knurl-the spiral direction depends on whether it is above centre or below centre...

As a rough approach-if you don't want to go to the trouble of knurling-you could use a dividing head and a internal thread tool in the tool post like a shaper: wind the tool across the face of the prop driver cutting a shallow radial groove; rotate the chuck a few degrees, repeat the process until you have a fully scribed prop driver.....but the knurling is a better option.


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ChrisM
'ffkiwi'

Thanks for machining advice, I have diagonal knurling tools but forgot there are parallel ones too. BTW I have some engines of East European origin that don't have radial grip patterns; instead parallel ridges are machined across the entire driver face, cut very cleanly. So the radial solution is not the only one...

Indeed-the Russians specialise in a 'square cross hatch' pattern...whereas the Czechs (MVVS especially) seem to prefer a series of parallel V-shaped grooves across the prop driver. I suppose its driven by a combination of production time versus available tooling as to what method you use in preference....

This is a useful reference: Ron's Model Engineering and Model IC Engines Index see under 'resources' on the left hand side menu-it opens up and one of the subtopics is 'how to' ..click on that and the 7th topic on the 'How To' page is 'knurl prop drivers'

ChrisM
'ffkiwi'

what engine is this ??

you may be able to buy a new Hub here in this link,,
Model Engine Company Of America

Jim

Not too many diesel parts available on that site.....I've been waiting for twenty odd years for Elfin and Marown parts to appear in their listings...

ChrisM
'ffkiwi'