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Another way for props to bite


i12fly
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I'm normally very careful.....

I was drilling out the centre of a prop today, I held the prop down on a solid wooden block and as the blades are sharp I held one of them securely with a piece of cloth just as I've done many times in the past no problem.

Unfortunately, this time the drill 'grabbed' in the boss, the blade I was holding firmly broke off and the other one came round to bite 3 fingers, the little one down to bone!

Fortunately(?) I was using a hand drill to drill so the drill/prop bounced away from my hand, but if I'd used my bench drill I dread to think what would have happened.

Lessen learnt!

How do other people hold and drill props? What is the safest way?

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The mechanical guys I am sure will fill in the details, but my dad used to "back off" drills so the tended not to snatch as much. If I remember rightly if you look at a drill bit the cutting edge has a relief behind it so it can bite into the material being cut. The problem is if the relief is too great the cutting edge bites too much material, hence it snatches.

The other trick was then cutting sheet was to fold a small piece if emery cloth (a bit bigger than the hole) and put it between the drill bit and work piece. Then drill through the cloth and metal. Useful when the material already has a hole through it

PS you may have compounded the issue as you were using a hand drill, very easy to move it off vertical and thus allowing the drill to bite more material.

PPS Has worked for me in the past, within reason increase the drill speed and decrease the rate of feed, easier on a pillar drill to control.

Anyway best wishes on a speedy recovery

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I bore props in my bench drill holding the prop in a small vice with suitable padding to stop damage. If it's only easing out a hole I'll use a prop reamer but I'm never sure about that as the reamer is tapered the hole can't be straight with parallel sides.

Hope your hand heals OK. Did you have to attend A&E in Derby? When I took my wife there back in August with a broken wrist it took ages to be seen but once seen the service was super fast.

Geoff

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Thank you all for the concern. Although the cut is deep it is only a small area so I've got it bandaged up and will see how it is tomorrow. As the hole needed to be opened from about 6mm to 8mm I didn't think a reamer appropriate -I always seem to get a 3 cornered 'hole'.

Geoff, I agree that Derby A&E is generally very good once you get seen, but if some of your (recent) past history is at Nottingham they cannot access it and causes delay for treatment, communication between them was not good.

What I'm thinking of now is to make a wooden jig that will hold the props down squarely onto a block, shielded to prevent the prop spinning then always use the bench drill. I'll give it a bit more thought, make something up and if successful post a picture. I need to get a couple of new matched props first!

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Logic sell a stepped reamer. 6, 7, 8 & 10mm. Right tool for the job. I've used one a few times - well worth it. I've tried drilling a couple of times and ended up with a paint stirrer. The reamer would have saved the wasted props.

 

Hope your hand is ok.

Edited By Nigel R on 24/11/2019 07:05:21

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Sorry to hear of your accident and get well soon.!

Firstly I always sand the edges of my APC fibreglass props after buying them with 240 grit paper. Otherwise flick starting engines or even just turning over can give cuts. Fibreglass props usually have a sharp seam on the side the edges that needs finishing . Secondly just like Ron says above I have found the best way of reaming props is putting a drill bit in my vice and turning the prop, gradually increasing bit size until correct diameter reached . I find that much more accurate than hand held reamers and turning the prop on the bit is easier. Obviously you need a good set a sharp drill bits.

Edited By Tim Flyer on 24/11/2019 08:27:34

Edited By Tim Flyer on 24/11/2019 08:30:10

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Thank you for the good wishes and the suggestions. It looks a bit rough today but seems to hold together reasonably.

I like the simplicity of putting the drill in the vice and turning the prop so I'll try that first and if unsuccessful make up a jig on the lines of Cymax' suggestion.

I've not been very successful with a reamer but never tried a stepped one as Nigel suggests, I guess that the cutting angle is reduced so likely to be noticeably better and less likely to produce 3 cornered holes.

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i12fly, sincerely hope your hand is soon ok again.

One other little concern I might have about your OP though is the fact that the prop seemed to break with such relative ease in the first instance. I’d have thought that normally it would take some considerable effort to break a blade off at the boss, although if it were a wooden one that may be a bit different.

Indeed you really do have to be careful; there is also yet another way a prop can bite. A modeller decided to run his model at home; the engine being a medium sized four stroke. Like me, he uses exactly the same starting procedure as he does at the field; hopefully this helps to automatically keep his actions the same every time. So the model is in it’s restraint, he’s holding the fuz behind the engine with his left hand and applying the starter with his right. It starts instantly, at a high throttle setting, he instinctively lets go of the fuz and the model moves very sharply backwards with the result that the back of the prop struck his left hand before he had time to move it out of the way. The blow injured his hand to the extent that the ambulance crew decided that rather than take him to a nearby A &E they would take him straight to a specialist hospital equipped for physical types of injuries further away. It’s all healed up now, but he’s left with no sense of feeling in his index finger and thumb; the surgeon told him at the time that some nerves were too badly damaged to repair.

His immediate surmisal of the situation was that he’d reversed the starter lead 4mm banana type plug connections into his flight panel; which of course makes perfect sense because of the speed at which the engine started. As always with hindsight it’s easy to say that a polarised connection would have prevented this, but who would have considered it happening in the first instance anyway? Certainly now it’s apparent that it’s another good reason for the more recent technique of having a lipo permanently attached to the starter, though!

Hopefully not too far off topic…

Take care,

PB

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Posted by Peter Beeney on 25/11/2019 13:29:08:

One other little concern I might have about your OP though is the fact that the prop seemed to break with such relative ease in the first instance. I’d have thought that normally it would take some considerable effort to break a blade off at the boss, although if it were a wooden one that may be a bit different.

Some designed for electric motors have very lightweight hubs - perhaps it was that type?

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Yes indeed, Martin, I certainly take your point. I once tried an APC electric propeller in a test for best performance, the current flow was down, therefore the revs were up so I gave it a test flight. After a short time, seemed like only a minute or so, one blade sheared. Maybe I was perhaps a bit too enthusiastic and the revs were too high but I wouldn’t have thought so. I’ve only ever used APC i/c types ever since.

Having said that, the carbon fibre folders we used a good few years back now on the ‘hotliner’ powered gliders of the day were very successful, I don’t ever remember having any problems with those.

PB

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I found that enlarging prop mounting holes with a drill was a bit hit-and-miss. It seems that it's quite easy for the drill bit to bite and go through the prop hole not dead centre, resulting in off centre holes.

I now enlarge all my prop mount holes with a tapered reamer. This gives a cleaner hole that has a better chance of being concentric with the original.

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The key to enlarging a hole through material and keeping it straight is by only increasing the diameter in small steps each time. That is why some claim people prefer reamers( the have a number of small steps). It’s exactly the same as using a drill press in the workshop. You do not increase a 5mm hole to 9mm by immediately reaching for a 9mm bit! If you put a drill bit in the vice and step up diameter in small increments you will have success. Obviously you need a decent set of proper drills. The diy bargain drill sets are mostly rubbish and do not have narrow increments. Many of the modelling reamers are also poor quality and rather blunt.

Edited By Tim Flyer on 29/11/2019 08:52:38

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I took the advice of putting the drill bits in a vice and turning the prop by hand onto the drill. So easy!

I increased the hole size in steps of 0.2 or 0.3mm (depending on the drill sizes I have), going from 6.5 to 8.2mm with good quality drills. I've done 3 props that way and the results have turned out better than any I've done before. By eye there is no loss of concentricity.

Previously I've gone up in 0.5mm steps normally using the bench drill, but will not be using that method again!

The finger is knitting together nicely now, thank you embarrassed

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