Boring bars

I am building the Stuart beam engine and having trouble finding a boring bar long and rigid enough to machine the cylinder which is bolted to an angle plate on a faceplate of a ML7 lathe any suggestions would be appreciated.

I can not seem to find a bar long enough which would fit in the toolpost.

There are plenty of articles on the internet on making a home made one. The simplest to make has a round piece of HSS in a hole drilled at a suitable angle with a grub screw to secure the bit. This will allow you to make a boring bar to suit your job to get the best rigidity.

Martin

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Is it the Stuart beam with a 2" stroke or it's big brother the Major beam.?

If it's the latter, a line boring solution set up may be your best bet.

Nick

Edited By Nick_G on 16/02/2016 14:29:57

How long does it need to be, and what is the bore? I have made a couple of thick boring bars, IIRC one is 5/8 and the other 3/4, that fit in bored holes in a MS block that fits instead of the toolpost. These can be quite long since they are large diameter so very rigid.

.

This guy's solution maybe what you need.

You should be able to get away with a simple homemade one. Take a piece of 1/2" or 12mm BMS bar, cross drill about 5mm in from one end 4mm to take a round HSS toolbit. Drill and tap in from the same end for a M4 grubscrew. I think I used this same bar many years ago when I did my stuart beam. As you can see it does well over 3x dia which is the often quoted max overhang.

If thay had them in stock then this set from Tracy would be as cheap as making them.

In true Blue Peter style this is one (or two) that I prepared earlier from 1/4" upto 1.5"

Edited By JasonB on 16/02/2016 16:06:38

I was taught to make boring bars from silver steel,lot more rigid than mild steel.I prefer a rigid bar rather than a line boring bar between centres, they do tend to chatter. Though the one big advantage of a line bar between centres is that it maintains a constant diameter(though subject to tool wear and deflection) whereas a bar held in the tool post is subject to the accuracy of the machine,i.e. it may bore tapered. Also a bar between centres has to be more than twice as long as a bar held in the tool post .

I think mild steel and silver steel have the same Young's Modulus. Hardening it affects the modulus less than you might think.

Russell

Hi Martin,

If you want a more rigid boring bar, buy one that is bigger than you need and machine it to suit your tool holder as shown below. Bought this one on %-bay for $10. (it was originally round)

Paul.

or make a special holder. This one uses standard welded-on carbide boring bars in a big extension.

Edited By Paul Lousick on 17/02/2016 07:51:13

When the length to diameter ration starts to get greater than 5 to 1, like a 10 mm bar , boring 50 to 60 mm deep, have a small radius on the tool helps immensely. My boring bar inserts are all 0.2mm radius for that reason. They also need to be sharp. Sometimes having the boring bar above centre slightly can help with getting a better surface finish. If it digs in a little, the bar deflects to the centre line and reduces the cut depth. But when an on centre bar deflects , it digs in more as at goes further below the centre line, leaving a series lines or score marks.

If you can afford them, solid carbide boring bars do make boring longer holes a lot easier. For very deep bores, as been mentioned, line boring is the way to go.

Neil