T slot cutting in mild steel

[VicParticipant @vic]

I may need to cut some T slots in some mild steel. I’ve only used HSS cutters in aluminium alloy before and that went well. I’m just wondering what sort of speed for a carbide tipped 16mm x 3mm in mild steel? Options on my mill are:

190, 270, 480, 560, 1000 etc up to 2100.

If it was a HSS cutter I’d probably make a test cut at 190 but I’ve not used carbide T slot cutters before. The job will be done in several passes. TIA.

[Martin ConnellyParticipant @martinconnelly55370]

I think you should be able to use 1000rpm without problems.

Martin C

[SillyOldDufferModerator @sillyoldduffer]

My rule of thumb (10000/dia in mm) suggests 625rpm for HSS, so I’d try at least double with carbide, say 1200 or faster.

The sum puts RPM in the right ball-park, but says nothing about the feed rate, which depends on how rigid your mill is and the motor power.

So I always experiment, judging the feed rate by look and sound.   I adjust feed until the motor sounds as if it is working, but not hard, and certainly not labouring.   The cutter must be seen to be cutting, producing ribbons or chips, not tiny shavings.  Likewise, the work, tool, and mill itself mustn’t be seen to be bending – it means the machine is being asked to do too much.

If the cutter can’t be made to work properly by adjusting the feed-rate, try increasing or decreasing RPM.   When HSS is struggling, often best to reduce RPM.  When carbide is in trouble, often best to increase RPM.   Not the law though – a great deal depends on the machine, material, lubrication, the operator, and cutter type and condition.

The skill part of machining is recognising when feed-rate, rpm and all the other factors are in your machine’s sweet spot, which means everything is ‘just so’. Some lucky folk have a natural talent for finding it – not me!

Trapped swarf is likely to cause problems in a T-slot, so take care to remove it.  T-slots are one of the few jobs I use flood coolant on, because it washes swarf away as well as cooling and lubricating.

Dave

[Author]

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