Screwcutting

[Speedy Builder5Participant @speedybuilder5]

I was making some 3/8" x 32 tpi screw thread in phos bronze the other day, Top rake 0 degrees, plunge cut, cutting speed very low (as I was threading up against a shoulder). All was going well using no lubricant, then for the last couple of cuts, I used a threading compound and the surface finish went from 'not bad' to 'sharp as blazes' and torn.
Fortunately I was able to clean the thread up after washing off all the cutting fluid and going back to cutting dry.
What would others have used?
BobH

[Speedy Builder5Participant @speedybuilder5]

Screwcutting Phos Bronze

[roy entwistleParticipant @royentwistle24699]

I would have cut it dry

[Neil LickfoldParticipant @neillickfold44316]

I suggest to cut dry with an air blow, or flood coolant. You need to remove the micro swarf away, so that it does not get to try and jam between the tool and work surface. That is the most likely cause for the bad surface finish. A lot of industrial machines, cut from the back and the tooling is effectively upside down, Gravity aides is getting the chips away from the cutting edge. Normal home hobby lathe set up, gravity has the chips staying on the cutting tool surface.

Neil

[Anonymous]

I screwcut a load of internal and external threads when making my water pumps in brass, gunmetal and bronze; both 32 tpi and 19 tpi:

All cut dry and no problems with surface finish, even when taking spring cuts. All threads were cut using insert tooling and at 260rpm.

For HSS tooling I'd add a little top rake for bronze.

Andrew

[JAParticipant @ja]

I was never a manufacturing engineer but I know what would be said by those I worked with, "what ever works best". They frequently used trial and error to find out how to machine difficult materials. In other words not all of industry know the best way to deal with such things.

If in doubt try a trial or more first.

JA

[HopperParticipant @hopper]

I never use lube or fluid on brass or bronze. You might find it helps too if you don't plunge cut but either set the top slide to slightly under half the thread angle, or keep the top slide in the parallel to lathe axis position and advance it by 50 per cent of the depth of cut each time.

[Author]

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