Drilling through bronze

I want to make slightly larger piston valves for my 5" gauge loco. The piston "bobbins" are1.25 inches long by 0,750 inches diameter with a No 10 hole running through the centre. In the past I have struggled to drill a straight hole through bronze. In the end the bobbins always "wobble" on a straight 3/16 spindle. What am I doing wrong ? I was thinking about getting slightly over sized bronze and making a mandrel, Then machining the OD to size using a tail stock centred mandrel.

Before using the number 10 drill use a slightly smaller Stub Drill. The shorter drill and stiffness will help.

Make it out of Colphos....machines like a knife through butter

You could leave the OD a little larger than finished size and mount the drilled bobbins on a makeshift mandrel and skim the OD

Can we can we assume sharp drill with even length flutes? Piston valves should be able to float a bit on their spindles anyway so the valves can self centre if the spindle is not dead parallel to the bore. I made a split collet for my 7/8 piston valves so I could end to end them without marking the surfaces, and used 4 jaw and DTI for these critical jobs.

If you are having a problem Peter - 20mm stock, drilled through 4.7mm, then 3/16th - use a 3/16th mandrel (Shellac'd on) with male centres (use a pivot runner) before turning between centres to final outside diameter.

"Low Cost, High Precision" - Don Gordon (ME Jan 1980) is a great reference. If you would like a copy, then please just PM me.

Regards,

IanT

Hi Peter I drill under size first about 1/2 inch deep and then with a small boring bar skim the hole to size which makes the hole run true then when you continue the drill to size it follows the true hole.

David

David G. Thats a wee wee boring bar to suit a #10 drill ?

That's actually quite a nice article

I've put it into here

Just use a small boring bar like one on right to bore small holes. Made from a piece of HSS ground by hand.

David

For weeny little boring bars don't forget silver steel, it can be bent/filed/machined to size then hardened and finally ground to get it sharp. You're unlikely to run it fast enough to give carbon steel a headache. Most of my round inserts are broken CS taps

If you want to drill a straight hole through anything, always use a BRAND NEW QUALITY drill. Preferably split point. A new solid carbide drill is even better, but most people aren't willing to buy them. Starting the hole with a centre drill, or a spotting drill and using a stub for a short depth also helps.

Edited By old mart on 20/12/2020 15:17:17

A 3/16" hole through 1 1/4" of metal is going to be difficult, it is 6.666:1 ratio, only a solid carbide boring bar could manage that, and an undersize hole would have to be drilled first. If you have a 4 jaw independent chuck and can dial in the od to 0.0005" and parallel and the work allows it, then drilling from both ends seems a better way. I would use a solid carbide drill from APT, that type are stiffer and drill to size without reaming, but care would have to be taken to avoid clogging the flutes with swarf.

Proper deep hole boring rotates the drill and the work in opposite directions and really deep holes have very special drills which are supplied with cutting oil at 1000psi or more.

Edited By old mart on 20/12/2020 17:31:02

Edited By old mart on 20/12/2020 17:56:41