how to machine hard steel with an old lathe

My lathe is an old Holbrook with max speed of 400 rpm. min 15 rpm the finest cut is 112 per inch about 9 thou or 1/4 a mm
I need to make some cylinders about 1 1/2 inches internal diameter from EN19T which is a hard steel. I have done a fair amount of machining stainless steel with HSS tools at a very low speed 50-80 rpm.

The cylinders need to have a closed flat end but a small 5-10 mm hole in the centre would not be a problem. Initially I though to buy an indexing boring bar but now not sure they would work in my lathe.

Does anyone have a suggestion. It is a job for myself so time is not a consideration even if it took a week to make each one. Cost of tips or other tooling could be.

I need help .......

Not hard unless its been heat treated. USA equivalent AISI 4140 which can vary from ANNEALED to reasonably hard - depending ENTIRELY on thermal processing applied

It has a low machinability index. For home, I'd probably make a stout boring bar, 3/4" or larger, that could hold a brazed carbide tool in the end. Slow to avoid chatter and a good cutting oil. More time and money and I'd use an insert. If it's been hardened you have a different problem but maybe a similar solution if the lathe is rigid enough!

The T at the end means it has been hardened and tempered ready for use. It says machine as for stainless, but I think this might be a bit harder than 316L and lathe not fast enough for indexing tools

It's equivalent to 4140 and according to this the T does not mean it's already hard, it looks like T condition is about 300HB. I'd use brazed on carbide tooling. Hand grind for sharp edges so you don't load up that rattly old machine, don't think you'd have any problem.
https://www.thamesstock.com/assets/pdf/steels/en19-709m0t-datasheet.pdf

according to this

Click to expand...

I like the part about carbide can be used for high finishes. Sounds rather ordinary and certainly not very hard. Its low limit on hardness is 248 Bhn

woodfarmer said:

My lathe is an old Holbrook with max speed of 400 rpm. min 15 rpm the finest cut is 112 per inch about 9 thou or 1/4 a mm
I need to make some cylinders about 1 1/2 inches internal diameter from EN19T which is a hard steel. I have done a fair amount of machining stainless steel with HSS tools at a very low speed 50-80 rpm.

The cylinders need to have a closed flat end but a small 5-10 mm hole in the centre would not be a problem. Initially I though to buy an indexing boring bar but now not sure they would work in my lathe.

Does anyone have a suggestion. It is a job for myself so time is not a consideration even if it took a week to make each one. Cost of tips or other tooling could be.

I need help .......

Click to expand...

Does that machine have rolling bearing spindle or plain?

Unless your Holly's knackered you shouldn't have a problem, it's the sort of job those lathes were built for, ……..as others have said Sharp brazed carbide tools, and make it have it!

IMHO carbide positive inserts at 400rpm will work just fine in that, its not dissimilar to cut to EN24t, kinda a nice material IMHO and nothing like 316 stainless, except you can't spin it stupid fast, which won't be a problem for you. the 9 thou feed needs sorting though to work with most smaller positive inserts that will cut that nicely, you really want to half that, any chance of swapping any gears pre the quick change threading box to get lower?

don't know much about Holbrooks, but certainly there is an apron reduction for the feeds??

Plain white metal bearings about 4-5 inches in diameter. I noticed this when I recommissioned it some years ago.

The gear train starts from the main lathe spindle with a 42 tooth straight gear wheel. This powers a pair of "rocking gears which allows using the back gear to drive the feeds and threading forwards and the front gear drives them backwards. Forwards allows RH Threads and backwards LH threads from 28 to 2 TPI on the apron is a dial that allows me to do multi start threads. The bottom of the rocking mechanism drives a big gearwheel which is used to drive the input gear for the feeds/leadscrew 42 teeth same as lathe spindle. The big gear wheel is on a swing frame and could be moved to allow use of a bigger feed gear. But when I got the lathe I didn't get any other gear wheels. Once I want to do a metric thread and was advised to make a 127 tooth wheel for the feed/leadscrew but I was not so sure that would work could not reconcile 42 and 127 as being right.

I used to know a guy who ran an old plain bearing lathe. It had a lot of bearing clearance. He used to do a lot of turning 'upside down' as it were, so as not to have the spindle lift out of it's favored spot in the bearings. This is because cutting forces lifting the spindle amounts to a lot of chatter.

I don't know that you'd be spending a fortune getting an indexable bar and some cheap inserts. I'd go for Triangular shape, and get a high positive rake insert such as would be used for machining aluminum. They'll be uncoated, but so will brazed carbide. The nice thing about an indexable bar, is that the insert tip is on bar centerline height, and you can often face the bottom of a hole with it, almost to center. Hard to do with a home made bar, because you just run out of room for the bar and the bit as you try to face the bottom of the hole. You can do it with home made bars, of course, if you are willing to stop and reset the tool bit once or twice, and very carefully, return to where you left off at the bottom of the hole.

A lot of popular boring bar sets use C inserts (which are a slight diamond shape) and those will face the bottom of a hole very well, too. But they tend to chatter and deflect a lot when boring, and if your machine isn't nice and tight, you'll be annoyed by that.