Reamers and Gun Metal

[Steve WithnellParticipant @stevewithnell34426]

I've just carefully drilled out to 6.3mm a piece of gunmetal bar as part of making the conrod bearing for the James Coombes. The intent was to ream to 1/4 inch.

I seem to have completely taken the edge off my reamer! Is there a particular process for reaming gun metal, or was my reamer made from chinese rice wands?

[Steve WithnellParticipant @stevewithnell34426]

[Mick B1Participant @mickb1]

I think it'd be unusual for gunmetal bar to have significant abrasive inclusions. I recently screwcut two dozen gunmetal components with a thread needing over 20 passes each with an HSS point, and it didn't blunt noticeably.

Can you touch a file to the clearance flank of a flute on your reamer?

Edited By Mick B1 on 30/08/2022 21:40:31

[HopperParticipant @hopper]

Unless you were running the lathe at 3,000rpm or something, or accidentally had it running backwards, that should not happen with gunmetal. Are you sure it is gunmetal and not one of the harder bronzes that requires much lower rpm? Otherwise, might be a duff reamer that could be tested as Mick says with a file.

[Anonymous]

Reamers need to cut, 0.05mm is far too small an allowance, even if the drill cuts to size. i'd use an allowance of 0.2mm assuming a machine reamer, or 0.15mm for a hand reamer.

When reaming gunmetal and bronze the material can move slightly rather than cut if the allowance is to small. That results in fudged cutting edges and the materilal gripping the reamer tightly.

Andrew

[JasonBModerator @jasonb]

Same with Andrew at those sort of sizes I have stub drills 0.2mm smaller that I use for holes to be reamed.

With the thin wall of the bushes on the JC and other Stuart engines the soft bearing material can also stretch particularly if using hand rather than machine reamers so you would hardly have been taking a cut.

I've certainly had inclusions in GM castings but would have thought you were supplied bar in the kit not a casting. If using your own bar then Culphos in the smaller sizes and SA660 once it gets above 1/2" seem to be the best to machine.

Things can also get hot quite quickly with some of the bronzes so apply some subs to the reamer to keep it cool, not the bearing as it can contract back down onto the reamer.

[Mick B1Participant @mickb1]

Posted by Andrew Johnston on 30/08/2022 23:10:47:

Reamers need to cut, 0.05mm is far too small an allowance, even if the drill cuts to size. i'd use an allowance of 0.2mm assuming a machine reamer, or 0.15mm for a hand reamer.

When reaming gunmetal and bronze the material can move slightly rather than cut if the allowance is to small. That results in fudged cutting edges and the materilal gripping the reamer tightly.

Andrew

Now you mention that, it's a point.

When I was doing the screwcutting I mentioned above, I ran a die nut down each thread once I'd got to depth, to take out any flank angle or crest/root radius errors in my tool grind or setting, or small pitch errors that must exist in the 11TPI geartrain on a metric WM250V. I found it was sometimes tight beyond easy hand-twist even though it was cutting little or nothing. It could also be tight on unscrewing.

Since the die had had little use, I wonder if it was deforming the material slightly and sticking on the compressed flanks.

Using a 6,3 drill could've left the bore very close indeed to size, and perhaps the reamer burnished the bore to its own detriment?

[Steve WithnellParticipant @stevewithnell34426]

Thanks, lots of helpful stuff in the comments as usual. I'm going to leave it alone now! It was the material that came in the Stuart kit, but bear in mind I have had the kit a long time, maybe ten years since I started work on it and just decided it was time to get it done.

As aside, Stuart supplied 1/2 x 3/8 inch bar for the connecting rods, whereas the drawing shows 1/2 x 5/16ths. Not sure why they did that, Maccmodels stock 1/2 x 5/16ths and I used their material.

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