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[ianb]

I have to drill out 2 brass props and retap them for a larger prop shaft. Sounds simple, until I tried to figure out how I would hold the prop so that the holes are absolutely vertical for drilling.

Only have a drill press (pillar drill?), no lathe.

The only idea I can come up with is to put the prop on a sheet of styrofoam, push down until all the blades touch the surface, and drill very slowly holding everything by hand.

There must be a better idea out there. Would certainly appreciate any advice on how to do this in a safer and more accurate manner.

Ian

[Proteus]

simplest way is to turn the end of the shaft down and re tap that, if it goes wrong you only have to buy a bit of stainless and not two props.

Proteus

[dreadnought72]

Top marks, Proteus, for the lateral thinking. I wouldn't've thought of that. 

Andy

[BarryM]

Only have a drill press (pillar drill?), no lathe.

Ian

Back to the drawing board?

Barry M

[wideawake]

Proteus has a good bit of lateral thinking but still needing a lathe.    It seems to me that there are two issues if working on the props themselves is the only answer.  First how to hold the workpiece and second how to line the hole up dead vertical.   Both are tricky with a simple drill press.   Getting the hole vertical is, I think, a matter of either getting a bit of straight rod sized to fit snugly in the present hole or even better sacrifice an old prop shaft.   screw it into the prop nad put the other end in the drill chuck then clamp the workpiece (prop) on the drill table without flexing the shaft.  Slack off the chuck, unscrew the stub shaft and replace in the chuck with the tapping drill for the new thread.   You can also, with care, start the tap from the drill chuck to get that vertical as well.

Holding the prop boss firmly is another matter.   I'd be tempted to make some sort of special fixture out of a bit of square material I think.   Drill it to be a tightish fit on the prop boss and cut through one side into the drilled hole.   If you now put the prop boss in the hole and clamp the fixture in your drilling vice so that the cut closes up it'll hold the prop firmly.  Obviously leave it slightly slack until you've done the "getting it vertical" bit.   Then clamp the vice to the worktable and drill and tap.

HTH

Guy

[Vintage]

In the absence of a lathe & assuming a 3 bladed prop with the boss protruding back beyond the blades.

Drill a hole in a piece of mdf / ply the diameter of the prop boss so that the boss fits through it & allows the blades to rest on the surface.

You can line up the prop centre accurately beneath the chuck using a small length of studding / sawn off bolt (same as the existing prop thread) screwed into the prop with the protruding end held in the chuck & the whole brought down into contact with the drill bed - you could actually temporarily epoxy the prop boss into the hole (as it's brass) to keep it lined up.

Once you've done that - hold / clamp the prop & board firmly in place, remove the studding, replace with the relevant drill & begin drilling.

I'd drill out using a number of consecutive drills very slowly to avoid the prop "biting" & causing damage - I'd always wear protective leather gloves as well...

Mark

Edited to add: Must type faster - essentially what has been said by Guy above 

[wideawake]

I'd drill out using a number of consecutive drills very slowly to avoid the prop "biting" & causing damage - I'd always wear protective leather gloves as well...

Mark

Edited to add: Must type faster - essentially what has been said by Guy above 

Nah you added a very useful point Mark.   I should have mentioned drilling out using a series of drills and feeding the drill very carefully.  It just goes to show - I was trying to write a step by step note and carelessly took it as read that he wouldn't go straight to the tapping drill.   Step drilling will also help to avoid the final drill going oversize.   As you say, drilling into brass or gunmetal it's all to easy to get a "snatch" and a ruined workpiece.

Guy

[boatmadman]

You should modify your drill bit before precision drilling in brass, otherwise it may 'grab' and pull the bit into the brass and damage the boss.

This website tells you how:

http://yarchive.net/metal/brass_drilling.html

Ian

[wideawake]

You should modify your drill bit before precision drilling in brass, otherwise it may 'grab' and pull the bit into the brass and damage the boss.

This website tells you how:

http://yarchive.net/metal/brass_drilling.html

Ian

Very true Ian.   I didn't mention it partly because, though I know it's what you're supposed to do and I've read descriptions many times, I still can't get my head around what you actually do     I've not had too many problems if feeding the drill slowly, though I must admit that's mostly been with the mini-mill so using the slow feed not a pull down lever.

Guy

[tigertiger]

Unless you are really hell bent on doing this, without proper tools.

Have you considered the cost of a new propshaft, balanced against the cost of stuffing up and replacing a screw?

[ianb]

Well, as always on this forum, I have learned a lot from the experts.

Having no access to a lathe I couldn't turn down the shaft and re-thread it, so that option was not available unless I bought and learned to use a lathe. Shudder to think what SWMBO would have to say about that!

So, I held the prop down on a piece of hard styrofoam using gloves, (thank you for that good bit of advice) and drilled out the holes very slowly and carefully with lots of oil.

Used incremental drills and then tapped the blind holes.

The method worked, much to my surprise, and the props don't wobble on the shaft.

But, I really wouldn't suggest that anyone try it. Too many things could go wrong, and I wouldn't ever want to do it again. Hopefully, with better planning on my part I won't have to.

Thank you all for your good advice and ideas.

Ian

[More Coffee]

When I worked in for a silk screen outfit ..we used Bondo ..or automotive body filler to set an object..

made a little box like structure  that was roughly or as close to the height of the object that had to be printed on filled it with filler ..then using a square or straight edge ..set the piece in postion in the putty ...let it set up .. get rid of any excess,and there ya go..ready for printing ...

I think the same could be used to drill a light ,iregular shaped ojbect in a press..

[Bee]

I know it's a bit late but:

Put a short length of threaded rod of the original size into the prop and hold that lightly in the drill chuck. Now you know it is in alignment.
Feed it down into some warmed plasticiene or even plaster of paris and allow to cool/set a bit, even putting it in the freezer to harden in summer.


With regards stoning the edge of drills for brass. We all know the books mention it but seriously who actually has 2 sets of drills or is prepared to keep resharpening their single set. If worried about snatiching just make a D bit.