Boring bars

I have a 8mm round boring bar , but am looking at buying a more beefy one. I converted the Super 7 to the 250-100 2nd size QCTP some time ago. So i can accommodate up to 14mm dia at a pinch. 13mm definately. Due to the flex with the little one , which should i go for. A round one or square section. Thinking that a square would have less flex in it. Any suggestions appreciated.

Steve.

Bigger is better for rigidity but has to also be small enough to go in the hole that you are boring. Therefore you need more than one. I recently bought a set of 4 (7,8,10,12mm) with replacable tips for not much money on ebay and they work well.

I only have a 4-way type tool post on my lathe which will hold a 16mm tool but sometimes use a 25mm dia boring bar with flats machined on the top and bottom so it will fit into the 16mm slot in the tool post

Paul..

Hi Steve,

As Paul says, a bigger boring bar is more rigid. I too use several sizes and made a toolholder for my QCTP that will take round boring bars up to 14mm dia. I usually use boring bars with HSS toolbits.

Thor

Edited By Thor on 09/09/2020 05:29:40

Hi Steve this picture of my boring bars. I prefer HSS tools but I have some carbide tip bars brazed on and one changeable tip tool bar.

The small HSS boring bars can bore down 4 mm and 5mm holes to make sure the holes are running true after drilling and before reaming.

David

Edited By David George 1 on 09/09/2020 07:05:45

Rigidity is critical in removing metal with a lathe bit, boring bar, or a milling cutter. The typical boring bar will be steel. Hardened steel is more rigid than mild steel but solid carbide is even stiffer. Look for the solid carbide boring bar.

The rigidity of the bar is governed by two properties: the second moment of area (I-value), a geometric property, and the Young's modulus (E-value), a material property.

So which of these change after hardening please?

Don't know the change but the solid carbide bars that take inserts are all sold as being able to work at about 40% more stick out than the average and they are a lot more rigid.

Thanks for the replies. Still not sure though if round & square are the same strength.??

Steve.

https://www.model-engineer.co.uk/news/article/a-heavy-duty-boring-bar-holder/23221

Steve, one of these may be worth making if you want to do some heavy boring, any quick change toolpost and compound slide will cost rigidity. It’s not so handy for boring a taper though.
Mike

Edited By Mike Poole on 09/09/2020 13:02:28

Not sure if it was just another one for the wall, My reasoning was i had turned some tapered cones & there was a lot of spring in the little bar i had 8mm round. So i was going to buy a couple of larger ones for the same sort of work.

The cones i have been doing are in mild steel & the holes were about 2" deep. Small dia about 7/8" & large end finished at 1-1/4".

I was looking at the insert type bars. It was just a thought if square had more rigidity than round section. I was not thinking of hss bars.

Steve.

Stevie the problem with square ones is that they tend to catch the bottom corner on the hole you are cutting unless it's quite large.

Not only are the insert bars approx a round section but they also tilt the insert so that the bottom does not rub so you can get them down a smaller hole.

I have insert bars @5,6, 8, 10, 12 and 16mm dia and would suggest the 12 as a good step up from your 8mm. if you want a second than look closely at the tool height of the 16mm bars as many will actually be 14mm due to having flats top and bottom so will fit your QC holders

Tungsten carbide has a Young's Modulus of 98,000,000 psi compared to steel at 30,000,000. As DC31k says, hardening steel doesn't change its stiffness

Edited By duncan webster on 09/09/2020 13:37:31

Thanks for that. I did wonder about square catching the hole. Now i know what to order . Many thanks.

Steve.

For the same weight of material a round section is stronger than square, unless the square is bent across the diagonal. Which is unusual.

In practice the strength difference may not matter much so convenience reasons for shapes kick in. Like David George (see photo above) a couple of mine have square shanks and a round section that goes into the hole. Others are round throughout apart from flats, and I've got one which is square except one quadrant has been rounded so as not to foul the hole.

They all perform, but the square ones are quicker to get in and out of my 4-way tool post. The round ones are easier to get into small holes before boring starts. Useful to own a selection.

Possible to spend really big money on boring bars. Solid Carbide bars well out of my price bracket aren't top of the range. High-tech examples contain complicated engineering such as adaptive anti-chatter technology.

Dave

Is it? Bending stiffness is the important factor, and for the same material we need to compare the I values (second moment of area).

For a round bar this is pi*d^4 / 64, which I make to be area^2 / (4*pi)

For a square bar it is b^4/ / 12, which is area^2 / 12

4 * pi is greater than 12, so area for area square is stiffer, not by a lot.

To put some numbers in, a bar 0.886" square has I = 0.0514, a 1" diameter round bar (same area) has I = 0.0491

I found a solid carbide 10mm insert boring bar on ebay for £23 EDIT Link Removed see CoC

I've no idea how good or bad this is, but it's not stunningly expensive

Edited By JasonB on 09/09/2020 19:21:13

The ebay 10mm solid carbide bars seem to be ok, I bought one to try it out. I would recommend a shim above the bar to reduce the screw loading on the brittle shank. I have made several myself, by getting carbide ground rod and silver soldering the end of a similar size steel bar on. I have 3 different 12mm bars and also a 7mm one. I had a pile up and broke the end off the 7mm one at the silver solder joint. I have also made threading and boring bars out of Densimet, which is a tungsten alloy, not carbide. This is not as hard, so milling and threading can be carried out, avoiding a steel end. I recon Densimet is not as brittle as carbide, which makes it better, I have 1/2" and 12mm stock.

They must have sold it. The link does not work.

Apart from being totally baffled by your arithmatic, i think a 12mm or maybe slightly bigger is going to be much better than the little 8mm one i have.

Steve.

Edited By JasonB on 09/09/2020 19:22:08

More likely the link is a broken one. Have a look on epay for yourself?

.

With due deference to your expertise, Duncan ... May I suggest that torsional stiffness is another important factor for a boring bar ; and this would most probably make a round section the optimum.

Happy to be persuaded otherwise, if you can explain it to me.

MichaelG.

well you asked for it!

just taking some numbers out of fresh air: diameter 10mm, overhang 100mm, tip load 1 N, use property values for steel

Twisting

Shear modulus = 79.6 N/mm^2

tool tip to centre of bar = 6 mm

Torque = 1 N * 6 mm = 6 N.mm

angular deflection = 32 * 100 * 6 / (79.6 * pi * 10^4) = 0.0077 rads **LINK**

This gives rise to a tangential deflection of 0.0064 * 5 = 0.032 mm

Bending Modulus = 207 N/mm^2

Second moment of area = pi * 10^4 / 64 = 490 mm^4

end deflection = 1 * 100^3 / (3 * 207 * 490 ) = 3.29 mm

With a much shorter bar the difference would be smaller, if the length was about 10mm they would be the same, but I don't think we'd care. If anyone wants this sum in SMath studio send me a pm, I'll not hold my breath

Edited By duncan webster on 09/09/2020 22:19:46