Reaming vs. Drilling Propellers - RC Groups
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Nov 29, 2010, 03:59 PM
Registered User

Reaming vs. Drilling Propellers

Hello all,

Can someone explain to me why reaming R/C propellers is preferable as compared to drilling them?

I have a Saito 220 glow that has a prop shaft size of 8mm x 1.25mm. 8 mm shaft diameter is fairly close to 5/16 inch, and I have a set of made-for-wood drill bits and a drill press. I do not have a reamer in this size and I would have to go buy one.

I am curious as to the differences in the cutting edges of these two different tools and why the hobby shop owner keeps telling me to "only ream propellers to the correct size"?

Thanks for your help.

John T.
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Nov 29, 2010, 04:05 PM
Gas Only
Fokker DII's Avatar
A drill will "walk" to one side due to the twist effect and any change in density of the material being drilled, especially wood. A reamer will keep your prop hole centered and not lose the balance..
Nov 29, 2010, 04:11 PM
Registered User
earlwb's Avatar
Drills rarely make a really straight and circular hole. The drill bit will walk some causing the hole to be more oblong in shape. Plus it is difficult to get the hole to be an exact size. Say you need a 1/4 inch hole for the prop to fit the shaft of the engine. if you used a 1/4 inch drill bit the hole will wind up oversize and offcenter.
Now using a reamer, makes for extremely accurate hole sizes and shapes.

I have both a special 8mm drill bit for drilling prop shafts and a 8mm reamer, and the reamer always wins when you want the hole to be 8mm in size. it really sucks to ruin a nice large prop because the hole winds up being off center slightly or a little too large in size. So I much prefer the reamer.

of course you can give it a try and see what happens. Some people tend to get lucky and wind up with usable props anyway.
Nov 29, 2010, 04:17 PM
Registered User
Broken Wings's Avatar
The propeller reamers are "piloted" or stepped. This allows you to follow the original location of the hole put there by the manufacturer.

I've drilled smaller less exspensive $$$ propellers using a drill press. I've never tried it using a spade or brad point drill.

The trick is to try and maintain the original hole "location". If you drill the oversize hole off center its going to run-out and vibrate real bad. If it was me I think I'd just buy the correct reamer. Saito 220 propellers can't be cheap....
Nov 29, 2010, 04:20 PM
Registered User
Twist drills designed for general drilling are far too aggressive for the soft materials propellers are made from. I had to enlarge the diameter of 18" props for my big O.S. and used a 12mm drill. The drill cutting edges had to have a "flat" ground onto them to prevent them from diving into the prop. (Remember there is an existing hole that just makes this worse).

My end recommendation? Use a reamer if possible for the roundest hole and best results.
Nov 29, 2010, 04:33 PM
Registered User

Thank you

To all who replied, thank you. I will be buying an 8 mm reamer.

John T.
Dec 28, 2010, 05:43 PM
Boring holes in sky & runway
Waterbug's Avatar

Ream it!

Good discussion. Sorry I'm late.
First and foremost, the OP said he'll get an 8mm reamer..,. you actually want a Stepped propellor reamer for the piloting of each size with the next smaller size ahead of it, John T... just to be clear. If you got an 8mm "one size" reamer you'd have no piloting.
And now for the unnecessary details....
Most drills that we purchase have 2 cutting flutes, and they are not at the same height axially, i.e. nothing's perfect. This makes one tooth invariably hit first, bite in an dkick the drill in a direction 90 degrees from that tooth- where there is no support from the cylindrical OD margins or even a cutting tooth. The prop reamers that I have seen are all 4 fluted- which helps this problem a lot. This is probably at least as important as the piloting reamer effect listed above. There are actually 4 flute "core drills" used in industry to open holes up but they are not priced/available for hobbyists- but they are hard to distinguish from reamers! All of this discussion is predicated on the assumption that the mfr. put the hole in the right place... which I understand APC does particularly well, based on the actual balance of each prop after moulding... i.e. follow their hole and balancing required will be minimal if any. Good reason to invest in a reamer right there!
Also, if you have an APC with a hole that looks like it is not centered, just close your eyes and follow it (or return it)!
That being said, If you MUST use a drill, (and I often do for those plastic GWS props on electrics) use a drill press or other squaring device and go lightly on feed- almost holding it back sometimes as a sharp drill will tend to bite and try to screw itself in. When this happens, stop, and unscrew it, pull out the chips, run the drill backwards a few revs while feeding to knock down the bite marks left when it screwed in. Then you can proceed,even lighter feed than last time or it will just bite in again!
I also like to drill an oversize hole (1/64" bigger) from the back for some length, depending on where the pilot section of the cranshaft ends. You still want some (~1/8" min) of that pilot to hit the proper size (front-drilled) hole if possible. Thread OD doesn't pilot the prop as well as the solid part of the crank. The key here is that the hole has to be dead square to the back of the prop, or oversize, or back-bored. I choose backboring 1/64" oversize because nothing is perfectly square.
This backbore (predrilling from the back) also eliminates splintering on the back at break-thru.
Last edited by Waterbug; Jan 07, 2011 at 04:19 PM.
Dec 29, 2010, 10:27 AM
Registered User
flypaper 2's Avatar
I use these stepped drills for props. Won't grab like a twist drill. Makes nice round holes self centred. Great for thin stuff too, like thin ply,fibreglass and thin aluminum without leaving a ragged hole.

Dec 29, 2010, 10:51 AM
Registered User
No one has mentioned a tapered reamer. I have two 100 year old multi-flute reamers that I use. They are a little dull but I work slow anyway. When reaming out a prop I cut a couple of turns and try it, cut a few more turns and try it, until the prop will just screw on the crankshaft with out any play. These reamers are long with very little taper.
I also use the stepped reamers but if I remember right the standard reamer works better for the metric sizes than the metric one does.

Dec 29, 2010, 01:11 PM
Registered User
pilotpete2's Avatar
Hi Gord,
Thanks for posting that link, I think I'll treat myself to a set
I bet they're great for cutting round holes in fiberglass cowls!
For an 8mm prop hole, do you just go the next SAE size down, then ream?
Dec 29, 2010, 02:23 PM
Registered User
flypaper 2's Avatar
The 5/16 is only a couple of thou. smaller than 8 mm so a little rotating may do it. Next size up will be to big.
Google may show where to find metric ones.

Dec 30, 2010, 08:42 AM
Registered User
You can get the ASE and Metric step reamers from Fox Manufacturing:

I have used them for many years.

Dec 30, 2010, 09:59 AM
Registered User
Why not center the prop up then clamp it then use a end mill to inlarge the hole . If you use a lot of the same size props a fixture could be made to speed things up.

Mike 1484
Jan 07, 2011, 04:23 PM
Boring holes in sky & runway
Waterbug's Avatar

I wish!

Because not all of us have acess to a good spindle and table (Drill press spindles are so-so for such work. I would LOVE to have a nice Bridgeport in my basement... but it would never fit because of all the plane stuff!
Jan 07, 2011, 08:27 PM
Curmudgeon at large
oskartek's Avatar
Waterbug, Having an expensive piece of machinery will not make your problems go away if you can't use it properly.

Using a motor driven cutting tool to size a hole in a prop is simply overkill in my opinion.

I hand ream all my props with a stepped prop reamer.
If the hole is off center, I put the prop in a home made fixture made to fit on a DuBro prop balancer and center it, then hand cut the re-centered hole using an end mill in a drill bushing then hand reaming to final size.

This comes from experience and a trash can full of props.

By the way I am a toolmaker with over forty five years experience and obtained my tool and die certification in 1978.

"It ain't what you can make it's what you can save."

Happy flying, Oscar
Last edited by oskartek; Jan 07, 2011 at 08:35 PM. Reason: content

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