Drilling gun metal bearings

[Mark Gould 1Participant @markgould1]

Gents,

My Dad and I are building a Stuart Triple Expansion. We have made all the crankshaft bearings and these are fixed in the soleplate. Our (tentative) plan is to drill them all in 1 go, vertically on the milling machine (FB-2)and only if we have the z-axis travel. The soleplate would be fixed to an angle plate. If this is a bad idea please tell me. We have completed a Stuart No. 1 and a Stuart Beam without too many issues but are still beginners.

Our second question is regarding the actual drilling of gun metal. I have had destroyed several gun metal parts when drilling and am not sure why. The material seems to be quite “chewy” and is not a pleasure to drill into like cast iron or brass. Any tips are very welcome.

 

Thanks in advance, Jon and Mark

[DiogenesParticipant @diogenes]

Hello Mark, good to see you are still at it..

Partly by way of a ‘bump’ back up the list, with apologies for being a bit general, but there are others much better qualified than me to comment on the specifics of this particular job..

I think with ‘grabby’ material of this kind it sometimes pays to stone the cutting edges of the drill back a bit, so that they are less acute / aggressive – er, in the manner of lathe tools, make the ‘rake angle’ less positive and more ‘neutral’, if you see what I mean..

..perhaps also avoid using small steps when drilling, use a small pilot and then go straight to size – which I guess could be ‘reaming/boring  size’ here?

I’d have thought that if you have capacity on the lathe, bolting the soleplate onto the cross-slide table might give a better ‘bed’ for operating on, more rigid and with better control of feed?

[Ramon WilsonParticipant @ramonwilson3]

Mark, whilst ‘line drilling’ of bearings can be carried out as you describe it is one of diminishing precision unless the set up and machine are rigid and accurate. The bearings in such an engine should not just be drilled either but bored to a good fit and surface finish to take the shaft with the right tolerance. If the bed can be set up on the lathe and the bearings pre drilled then a between centres boring bar could do the job with much more ease and accuracy throughout the four bearings.

Personally, given that the housings for the bearings in the bedplate are in line and uniform and the bearings a good fit in the housings, I would do each bearing individually on a fixture to a set dimension and fit separately tweaking until a good running fit is acquired. This is the method I have done on the marine engine described elsewhere – that just has three bearings though.

There’s a lot at stake so although more work initially, to get this part as accurate and as smooth running as possible will pay off further down the line as the build progresses.

[not done it yetParticipant @notdoneityet]

As Ramon.  Line bore.

[Ramon WilsonParticipant @ramonwilson3]

Further to my previous post Mark the drilling of gunmetal, indeed any of the lower end bronzes really, requires razor sharp tools for the best result so a freshly ground drill is the first thing in order for success.

If a tool is nothing less than sharp even if only used once before and particularly so if previously used on on steel. It will not cut very well but tend to ‘push’ the material – achieving a decent surface can be quite a fraught exercise if the cutters are not really sharp.
It is far better to bore bronze for bearings either in the lathe or using a boring head on the mill, again with freshly ground HSS tooling for best results

Gunmetal itself, as no doubt you’ve previously found is easilly distorted so it requires holding with the minimum pressure to achieve the machining involved, inserting a slip of copy paper between work piece and table/ vice , etc will improve grip considerably for a given clamping force

Hope the above is helpful to you

[JasonBModerator @jasonb]

I don’t recall lineboring any actual bearings, I will quite often line bore the bearing housings to take turned bearings. Or in a case like the tripple tend to mill the rectangular pockets and insert the individually machined bearings.

Most of the housings that I have linebored tend to be larger, with the tripple being 3/8″ bore you start o get into the realms of quite long ands lender between ctrs boring bars where chatter and deflection start to become an issue particularly once you have a hole for an HSS cutter and a locking screw in what can only be an 8mm bar.

Another option to consider is machining them out individually to reaming size and then fitting to the sole plate for final reaming, a hand reamer would be the best as it’s taper will guide it through.

When it comes to drilling, I don’t stone my drills but I do go straight in with the reaming size after the spot drill and nip up the quill lock or better still lock the quill and us ethe head or table to feed the drill as if there was a snatch there is less chance of things being pulled into the work which can happen with an  unrestricted quill. Same if I’m going to bore, I go straight in with the clearance drill an dno pilot holes

[Mark Gould 1Participant @markgould1]

Many thanks for your input gents. I’ll post pics as we go, fingers crossed. Good tips and we’ll make sure our drills are sharp.

Thanks again,

Mark

[bernard towersParticipant @bernardtowers37738]

Would D bits help?

[old martParticipant @oldmart]

I have had no trouble drilling SAE660 leaded gunmetal using decent sharp drills, I found it easier than brass and copper. I would drill undersize and finish bore unless I had a solid carbide drill that was the right size. If possible use a mill or a lathe for the drilling which give more control than a pillar drill.

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