Machining EN24T

Machining basically a cylinder in EN24T. Have been using a HSS tool for turning the outside which went OK . However, come the bore and it's a different story. Bored it out to 1/2 inch diameter then attempted to bore it out to just over 3/4 inch diameter using a HSS boring bar and the bore has 'humped' in the middle - I've tried boring from both ends. It seems as if the boring bar is just deflecting instead of cutting, despite being sharp using the same depth of cuts and speeds as usual. I have come to the conclusion that perhaps the material has work hardened. Or perhaps it just has a hard lump in the middle!

So what speeds should I be using for EN24T both for rotating and feed using HSS tools and again if using and indexible tip tool?

Chris

Edited By ChrisH on 12/02/2017 19:06:37

Hi Chris,

EN 24T and similars

Surface speed for HSS about 45ft/min.

Feed rates

Roughing for HSS about 0.005/0.006" per rev

Finishing for HSS about 0.002/0.004" per rev

At least 3 times or more faster for good quality inserts tooling.

Home workshop speeds here, not industrial

Edited By MalcB on 12/02/2017 20:23:49

I have machined en19t, en24t and pre hardened p20 tool steel at work, you have to be pressing on and not letting tools rub. Would you not try carbide tipped cooling ?

Hi Chris In the last few years I became aware of hard spots in some pieces of material especially P20 and EN 24T On investigation I believe that the material in question was from the far east and not from our usual steel maker due to the cost. Sometimes we could grind out the hard spot with a small air grinder or use carbide tooling. Sometimes it was easier to replace the component part and start again.

David

If theres hard patches quite likely on quality steels from within Europe the last 4 decades the cutter should still cut.

I would have a closer inspection on tool and tooling used.

EN 24 (817M40) is notoriously sensitive to heat treatment control and wouldn't be my choice of steel for home shop work when we are typically buying in small quantities without being able to ensure batch traceability. Its unfortunate that you have found a hard spot but I don't find it remarkable having had similar issues myself with materials supplied for a job. I think the guy thought EN24T would be better than EN16T or EN19T because it was more expensive.

Far as I can see the only reason to choose EN 24T over EN19T (709M40T) or EN 16T (605M36T) is to get T tensile range in larger diameters. Ruling section, largest diameter that can be heat treated to a specific tensile range, is 250 mm for EN24T but only 100mm for EN19T and 65 mm for EN16T. Other properties are much of a muchness with EN24T coming out to the top of the range and EN16 towards the bottom. Machinability being very much t'other way about. Not that we ordinary home shop guys have any business messing with things where the difference is important.

Besides speed and feeds all three steels are sensitive to tool tip radius, especially when boring, and you must keep the tool cutting. Years since I bored any so I don't have helpful figures, probably tip radii in the 0.5 mm, 1/64 th region, razor sharp and nicely polished. On external turning a carbide insert properly specified for such materials gives an awesome finish when run at book speeds and feeds but such are too high for most folks machinery. Many inserts do just fine at home shop machine friendly speeds and feeds but finding out which one can be difficult as the usual "choose something sharp with positive rake intended for Aluminium" advice may come up with selections that, whist fine for ordinary black and bright bar, aren't really strong enough for extended use on these tougher steels.

Clive

I machine a fair bit of EN24T/EN16T and P20. Most times using indexable tooling. Rare to find hard spots now although P20 from 40 years ago was dreadful stuff from piece to piece. As stated above for finishing the "GT" aluminium cutting inserts are very good. Your 1/2" diameter bore limits your choice of tooling to suit what you have available, a sharp tool with a small nose radius should suffice. Have you checked the tool clearance to ensure it is not rubbing ?

MalcB - thanks for those figures, from them I may have been going a tad, not much, faster.

The tools are as sharp (and honed) as I can get with only a very small radius and checked it wasn't rubbing. It cut the first half inch no probs then seemed to just start to deflect the tool away.

David and Co on hard spots - yes I was wondering if I had a lump with a hard spot and found your comments very interesting. I was using EN24T because that's what the plans specified - it's a sleeve in a sleeve valve engine - but had pondered on EN16 instead. If the indexible bit doesn't work I have one more length and if that screws up I will try either EN16 or 19.

Many thanks for all the replies - always highly valued.

Chris