Boring bar toolpost.

For quite some time I have been meaning to make a toolpost to take a large boring bar, to be used in place of the topslide and its toolpost for a far more ridgid setup.

I finally completed one during the past couple of weeks, starting with a block of 80mm square black bar for the body, and some 15mm thick steel plate for the base. The body and the base were both machined on all faces then dowelled and bolted together. The hefty lump was then bolted to the lathes tee slotted crosslide and the bore for the boring bar was first drilled and then finished with a between centres boring bar gripped in the lathe chuck.

...............The base is secured to the body with five high tensile M8 cap screws, plus two dowels. A key is also fitted to locate in the tee slotted crosslide. The lathe is a Warco 1330 but the topslide is a Myford 280 item that I modified to fit several years ago.

...............The completed toolpost after slitting one side and fitting three pinch bolts to clamp the boring bar. I was originally going to use a slitting saw in the mill but in the end I used my 7x12 bandsaw. The boring bar will be used to bore the cylinder on my BSA B50...........

Edited By Lathejack on 01/11/2019 19:54:40

Edited By Lathejack on 01/11/2019 19:56:40

Were you aware that the recommended diameter to length ratio of a steel boring bar is 4:1, solid carbide is 6:1, the most expensive bars can go to 10:1, and they cost somewhere in four figures. That cylinder really needs a 3" diameter bar, and a much bigger lathe.

Well there lies a possible snag, I had it in my head that the ratio was 5:1 so this boring bar would be right on that limit.

I had been looking for a bar of 40-50mm diameter, but all the used ones I have looked at so far have been worn away in the area below the cutting tip.

While the bigger the better for this job is true, it doesn't require going as far as a 3" bar. The 1330 lathe is heavily built and more than up to the job of holding and machining the light alloy cylinders bore out by 0.5mm, once I have made a suitable holding fixture.

Edited By Lathejack on 03/11/2019 10:15:03

I have used my Warco BH600 to rebore several motorcycle cylinders. I use a heavy 1 3/4" Dia boring bar mounted in couple of see blocks with strap clamps over the top of the bar to hold everything in place. nAll seemed to work well for me.

Read Vee Blocks not see blocks. Predictive text rules.

Would you not be better making a near 3”boring bar to be supported between centres on your lathe?

Holding the cylinder in a fixture mounted on the headstock spindle will ensure that it is bored accurately on the centre line, and also bored square to the cylinder base mating face.

I have come across several cylinder barrells that have been bored off the original centre line, even though they were machined by firms using equipment made for the purpose.

I've bored several motorcycle cylinders from about 50mm to 90mm diameter using a 25 mm boring bar. By the looks of it the same bar as the OP is going to use (25mm diameter), vibration isn't a problem even though the bar is hanging out of the tool post rather more than might be ideal. Slow speed, 300ish rpm and a brisk feed works for me. I leave a 0.002" for honing.

Boring the cylinder on the original cylinder centre line isn't that important, a few thou out is not going to matter. What is important is that the bore is exactly perpendicular to the cylinders mounting face. If at all possible I always take a light cut across the mounting face at the same time as boring the cylinder.

John

I can't think of a single time I have ever used a boring bar that large in diameter in relation to its length. Not once. A half inch diameter boring bar will cheerfully bore a hole four inches deep if used carefully. Not under high-speed mass production conditions perhaps, but ok on one-off jobs.

The boring bar I have is 32mm. The bored out cylinders I've had in the past were upto 20 thou off centre, so not ideal. I'm certainly not risking my precious BSA cylinders being done the same way.

i have also bored out motorcyle cylinders on the lathe before, including two stroke cylinders with all their ports. The lathe does a perfectly good job whatever the bore being machined is for, as long as the tool is ridgid enough and the lathe cuts parallel.

Assuming the rigidity and resistance to chatter is dependent on stiffness of the tool (a cylindrical beam).

After some digging, it seems deflection is proportional to the cube of length over the fourth power of diameter.

In practice I've never had problems with a 3/8" bar extended by well over 1 1/2" taking decent cuts on steel.

A 1" bar will be 50 times as stiff as a 3/8" bar of the same length (fourth power of the difference in diameter), and so can be 3.70 (cube root of 50) as long to be similarly stiff.

3.70 x 1.5 = 5.55"

The 1" boring bar should be fine, and you can always halve the depth of cut to halve the load.

Neil

<edit> I think we are all familiar from practice, if not calculation, that stiffness of a bar decreases rapidly with length, and many of us know the cube root relation (e.g. a 1" long bar is eight times stiffer than a 2" one). We also have an intuitive understanding that stiffness also increases with diameter, but is suspect that most of us are less aware it is a fourth power relationship (e.g. 2" diameter bar 16 times as stiff as a 1" diameter bar). I'd come across this before but it had fallen off then end of my memory until I looked it up. It's hard to find a simple answer on line as most websites use the "area moment of inertia" in the calculation so it applies to any shape. For a circle, this is proportional to radius to the power of four. (Yes I looked all this up)

Edited By Neil Wyatt on 03/11/2019 14:05:05

GHT covers this point in his Manual and is well worth reading generally on the subject of boring.

This may be a wildly foolish idea, but if there is any uncertainty over deflection, could you put gussets under and behind the bar?

How much is there between the outer face of the bar and the point of the tip? You could shrink or loctite on a sleeve that increases its effective diameter. Even an eccentrically-bored one so that the under and back get more meat than the front.

No it's not a foolish idea. Here are two examples of an old 25mm boring bar that has been lengthened and reinforced by a length of thick flat bar at the back, and it cuts cleanly without the slightest chatter.The bores being machined on these full size Traction engine parts are deeper and larger in diameter than my BSA cylinders. I won't be doing this to my nice 32mm boring bar, but I have already started to make a sleave to stiffen it up when extended to 5x its diameter.

The topic on hand is interesting in terms of the boring bar size and usage, but the thing that really grabs me is the T slotted cross slide on a GH1330; mine's plain.

Bill

If I was going to attempt that boring of the cylinder, I would not use that tool, unless there are inserts suitable for aluminium available for it. The aluminium grade inserts are sharp, and can be used with very small depth of cut, which would significantly reduce the forces on the bar. I would get one of these which use WNMG 08 inserts. I have some cheap Chinese inserts for aluminium which are surprisingly good when used on steel and CI with small cuts, right down to 0.0005".

**LINK**

The tee slotted crosslide shown on my Warco 1330 lathe is a Myford 280 item that I bought and slightly modified to fit my 1330 several years ago. I had planned to machine some Myford size tee slots in the 1330's original crosslide, but when I removed it I found that they have been cast with a large cavity on the underside, so machining it didn't look to be a good idea.

in my original post I mistakenly refer to the topslide being from a Myford 280, when I actually mean the crosslide.

The cylinder Barrell is light Alluminium Alloy, but it is fitted with a cast Iron liner that is to be bored out by 0.5mm. In any case the cutting tip is simply what was fitted when I bought it.

I know what a B50 barrel is made of. I didn't like the tool because of that first generation style of insert in it. Times have moved on and there are much improved indexable tools on the market. How much metal do you intend to leave in the bore for the honing?

Cheers for that, and sorry for taking you off topic.
I did read your album posts before committing to buying mine (second hand) , but forgot about the top slide conversion.
The first thing I did after having it delivered, apart from moving it indoors, was to see if there was any sand included free-of-charge.

Bill