Boring bars

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Thanks, Duncan

By my reading that makes the torsional effect small but non-trivial [as a source of chatter]

... ‘though, of course, your loads do seem unrealistically high
[who could work with 3.29mm deflection of a tool, I wonder?]

The real question is how would the same amount of material, arranged as a square section [or any rectangle you like to choose], compare ?

MichaelG.

Edited By Michael Gilligan on 09/09/2020 22:50:27

The magnitude of the load doesn't change the comparative result, I said it was picking numbers out of fresh air.

A square bar with the same area as a 10mm round bar would be 8.86mm square, its bending second moment of area would be less than the round bar as discussed previously, but it's Torsion Constant J would also be less, round is 981 mm^4, square is 867 mm^4 **LINK**

And anyway, in a given hole you can get more area of round bar than square bar.

Edited By duncan webster on 09/09/2020 23:07:09

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I do realise that, Duncan ... but given that your result for the torsional deflection was already small, I thought it worth mentioning, because in a practical application it would be even smaller.

MichaelG.

The reason i asked in the 1st place was only because in my limited brain power i thought a square bar would have less spring than a round one. .

Steve.

I just bought a set of Sumitomo anti vibration dampening boring bars.

They have a tungsten weight that counteracts against the vibration being produced . They do work very well. There was an introductory offer on them out here, so got a set.

Other companies also make anti vibration boring bars as well. Some like Mitsubishi ,have their dimple bar series, and others have various ways of adding tension to vary the bar frequency.

No one has spotted it yet, (well Michael sort of did) I got my gigas and megas mixed up, which means that the calculated values are 1000 times too high. I've corrected the above. The deflections are very small, but so is the applied load.

Edited By duncan webster on 10/09/2020 05:21:53

Edited By duncan webster on 10/09/2020 05:52:33

Generally ratio's for boring bars are kept under a 5d ratio. So a 10mm bar will be kept to under a 50mm length of working area. As the length to diameter ratio gets longer, the more difficult it becomes to get a good finish etc. Lots of things come into play as well. Sharpness of the tool, the radius of the tool and the depth of cut. Sometimes boring the full length is not the right option.

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I would not have been so presumptuous as to question your calculations, Duncan

... but in my simple ‘arm-waving’ analysis of your example, I knew there was something wrong and guessed it was the loading.

I realised this morning that my guess was wrong ... when I remembered that one Newton is approximately the force exerted by a small apple

Thanks for the clarification

... I blame the system of units : no-one should be expected to work with giga-anythings !

MichaelG.

Boring bars are sold by their diameters, but it is most common for a bar with an indexable insert to have flats top and bottom. Having a mill in your workshop has its benifits as oversize boring bars can have their flats increased slightly. I have milled a little off the backs of 20mm bars so they fit better in the limited toolpost depth, (not height).

The 25mm Sandvik bar that I bought recently is round shank and will have 1.5mm taken off the top and bottom, and 2.5mm off the back to fit perfectly in the large four way toolpost which will be attached to the Smart & Brown model A cross slide. The toolpost height will be set to 17.53mm to match the existing one. The big Sandvik won't need any shimming.

I wondered whether they could be milled down. Was not sure how hard the steel was.

Steve.

I usually mill down the shanks of indexable lathe tools with a 50mm Ceratizit shell mill which has 5 inserts of 12mm diameter. They are economical as they index 8 times, but I would only expect to get one index when milling a 20mm turning tool shank to 17.53mm. This is partly my own fault as the inserts are for stainless, and the steel I use it on is harder. It can be cut with a hacksaw, but it's hard work. The cheap APHT type shell mills would do the job with Chinese inserts. I would not recommend using a solid carbide cutter as the average person cannot sharpen them.

If you are talking about one that looks like a Borg's hand with the circular cutters. I think i have one of those. Is this the one.

If so that could be good as a friend i spoke to tonight says he may have some 16mm bars from when he was in engineering, so i guess they will be top quality. If i can narrow them down to 14mm or so they will fit my holders.

What would the best holding method be. I do not have a collet block to hand to hold them.

Steve.

Stevie, the bars are unlikely to be any harder than those QCTP holders you cut down so the face mill with the inserts for hardened steel that you used on those should do the job. A lot depends on the quality of the bar, from ones I have modified they seem to get harder as the quality goes up so a cheap far eastern one is more likely to be easier to cut than a name brand one.

Hold in your vice supported on parallels and reduce one face than flip it over and do the other. But you may well find that a 16mm bar is already 14mm clamping size.

I knocked the bar up below to bore the CI cylinder for my economy engine, the round part is about 22mm and my lathe takes 16mm shanks. My finished bore was 42mm dia so I could have gone bigger but that was all the stock I had to hand. Basically the bigger the better provided the swarf can get out and it doesn't rub.

I also have a set of tipped bars from the well known international broker from 8 to 10mm that work fine and several other home made bars. The one in the pictures show how I reduced it to fit the lathe but only at the tool post end.

I just hold boring bars in a milling vise on parallels making sure the line contact is a safe ammount below the top of the jaws. First you need to measure the height of the tip above the bar base to find out whether your toolpost/ toolholders minimum height call for any modification. Then measure the thickness of the bar relative to the clamping height. When the bar has had its height/thickness adjusted, hold it in the toolholder and check whether the clamping depth allows the screws to hold the bar securely. If the bar looks like it is not held securely, then a little milled off the back will allow it to seat deeper.

That 63mm shell mill has exactly the same type of inserts as my Ceratizit, the cutouts on the rear of the inserts are the indexing positions. You have to slacken off all the screws, and then hold each insert in turn while further slackening off the screw until you can turn to the next position. Remember to turn them all the same way. As soon as the insert is re indexed, fully tighten it. When you run out of slack screws, you know you have re indexed and retightened the lot.

I have just noticed that the inserts in your shell mill are not a matched set.

Edited By old mart on 11/09/2020 18:26:04

Yes i see now that they are not all indexed the same. This is how they came to me. Would i be best to remove them all & reinsert them all at 1st pos. Then note any wear. If 1st spot worn then move to 2nd & so on. Making sure i turn them all the same. Hopefully finding a good set. I did use this just once doing the tool holders. It worked well with no sparking , but was quite noisy over the square tipped one i have.

Steve.

What I noticed about your shell mill inserts are the inserts are not a matched set. Sone have 8 indexing notches, and others have 4. WNT and Cetatizit both make the particular insertsfor these shell mills, likely RPHX12 if they are 12mm diameter, RPHX10 if 10mm.

The advantage of mine is that it takes half a box of inserts to fill it, but yours can run balanced with 2, 3, 4 or the full six inserts.

Edited By old mart on 11/09/2020 19:59:23

Well i ended up buying a used Tizit 16mm round type bar that takes the triangular inserts. I think i may have some inserts that will fit. The bar has a flat on the top, but is still too big for the toolholder. 1st job will be to mill the top down until it fits the holder. I think i may also need to take a cut along the side too, to make sure it is in the holder enough. edit. I know it is a through coolant version , but at the price it was very cheap & a good quality. I have also ordered a cheap 12mm one from a supplier , in case the 16 is too large. Will then have a 8mm - 12mm - 16mm - & a few HSS ones that came with the lathe.

Steve.

Edited By Steviegtr on 17/09/2020 19:18:55

I'd take a bit off the bottom, then it will sit on a flat rather than an arc

Can you not get or make something like this?

gwAAOSwcjdfV3kT">https://www.ebay.co.uk/itm/Mecanizados-Huesca-25mm-Boring-Bar-Holder/143726614078?hash=item2176c6163e:ggwAAOSwcjdfV3kT