What size boring bar for a Sieg C2 lathe : 8mm or 10mm ?

[Brian JohnParticipant @brianjohn93961]

I will be buying a Sieg C2 lathe when they are back in stock in a few weeks. I will be starting with 8mm indexable cutting tools using the DCMT 070204 inserts. But the boring bars have a flat on them so should I buy the 10mm boring bars rather than the 8mm ?

NOTE : I will not be using a QCTP in the beginning, perhaps later.

[JasonBModerator @jasonb]

Really comes down to what size holes you want to bore.

The larger the bar the larger your drilled starting hole needs to be, typically an 8mm bar may need a 10-11mm diameter hole before it fits without rubbing. You do not say if you are using the same DCMT inserts for the boring bar as they tend to need a larger hole than one that uses CCMT so they could push it upto 12 mm starting hole

The length of the hole is also a factor, the deeper the hole the more tool you need sticking out which will start to flex and chatter so using the thickest bar possible will reduce that.

I would say 8mm based on the type and size of engines you have previously made.

Personally I would not get DCMT inserts although the included ones when bought are likely to be those. Get a box of DCGT 070202 which will work a lot better on the small SC2. Though personally I would go with CCGT inserts and suitable holders both for general turning and boring then you only need keep one shape of insert.

[HollowpointParticipant @hollowpoint]

Get the 8mm

The 10 might be too high in the tool post. I had an SC2 and if I remember correctly the maximum tool size was 8mm.

Also, I agree with Jason. CCMT or CCGT tips for both tools makes sense.

[Brian JohnParticipant @brianjohn93961]

But the boring bars have a flat on them, if I remember correctly. So isn’t the 10mm boring bar actually 8mm when measured from flat to flat ?

In the mean time, I while look for CCGT tools.

[JasonBModerator @jasonb]

A 10mm bar is typically 9mm across flats. Also as the tip is half way up that 9mm you will need more backing below a boring bar then a standard turning tool so the total toolpost capacity needed would be something like 12mm to fit a 10mm bar.

[David George 1Participant @davidgeorge1]

I needed to cut a large thread size in some new main spindle replacement bronze bearings and as the thread inserts would only fit into a boring bar that was 20mm diameter I decided to make a new tool post to suit. I also found that the centreline of the boring bar was the centre cutting height of boring bars tip and so if I bored it with a boring bar or drills and ream in the chuck the tip height would be on perfect height and if I used a smaller boring bar in a concentric bush it would still be on centre height. I eventually just made a block which bolted down to the cross slide which was simple to swap and but in two holes of different diameters side by side with a line of clamping screw holes in each hole.

I started with the single tool holding hole till I decided it could be made to take smaller boring bars as well.

 

Just my thoughts of using larger size and more solid boring bars.

David

[JasonBModerator @jasonb]

Not being a big fan of making tools I took a different option, the toolpost make a good Vee block to clamp the shank to. In this case a 25mm shank and the hole in the cylinder liner was just over 160mm deep.

[PeteParticipant @pete41194]

As others have already said, there’s really no one size fits all needs and requirements for the tooling when single point boring on a lathe. A good example, and of course light years beyond a Seig C2 lathe though. https://youtu.be/xjNHjjanehc?t=41 But I’d guess his finished boring bar outweighs a C2 lathe by a lot. 🙂

Mentioned as well already is the bore depth and diameter. Buying one single sized boring bar might still handle a lot of tasks for most of us. And I think the OP should have a good starting point. Especially any cylinder or hole boring within the diameter / bore length range and suitable for the lathe and bar size being used.

However I think it’s worth adding the OP should also understand that there’s fixed and maximum limits to bore lengths depending on the material used to produce the boring bar shank and the boring bar diameter being used. The rigidity of the lathe and it’s parts are also a factor, but we’ll ignore those for now. A general rule of thumb is that steel shank boring bars are limited to around 3-5 times the bore length verses there diameter. In other words, an 8 mm steel shank boring bar is going to have that absolute maximum of at most 40 mm bore depth. Solid carbide boring bars which are usually far too expensive for most hobbyist’s from 6 – maybe 8 times the bar diameter when pushed to there depth limits. So when the bore diameter allows there use and the bore length is deep enough, larger diameter boring bars may not be optional. Depending on just how sharp and well honed the tool edge happens to be? Those numbers might sometimes be fudged enough to cheat the physics slightly with very light depths of cut, but there’s still fixed limits for what’s really possible.

An extended boring bar or any extended and unsupported tool length increases the chances of tool chatter. The second rule of thumb when that happens is to try reducing the rpm and / or increasing the tool feed rate. Rarely or so far I’ve never found an instance where going the other way works, I’ve read of others that have though.

So why does that lower rpm / higher feed work? Your changing the tool or parts natural harmonics level to hopefully something outside it’s natural frequency of vibration range and enough to work. Any extended tool or even part length in reality becomes much like what a tuning fork does. There are a few tricks that might be tried, surprisingly and sometimes something as simple as a fat elastic band wrapped and taped tightly to the boring bar shank, wrapping soft solder wire tightly to the tool shank, or even a lump of window putty or plasticine adhered to the tool shank can sometimes be enough. The position of any of this on the boring bar can also have an effect for how well it may or may not work. It can sometimes work because those materials help to deaden, absorb or change those natural frequency vibrations. For some part cross sections, the same is also true. Just learning this cost me a new set of lathe spindle bearings I didn’t need.

 

 

[Clive FosterParticipant @clivefoster55965]

An inevitable issue with insert type boring bars is the fixed length of shank extending from the toolpost. This fixes the effective stiffness so you are always working with a relatively weak bar, most especially so with a smaller size. Which can be a pain when doing a relatively short hole.

A handy standby for short bores is a damaged two flute milling cutter. If only one edge has gone the good edge will bore just fine if the tool presented to the work pointing very slightly towards you so there is clearance behind the cutting edge. Probably best to trim the dead side back a bit to ensure clearance on the backside too. Important is you are working on a bore only just big enough. I use a similar angled presentation trick to minimise grinding on HSS tool bits.

Heel clearance is a nasty little gotcha when trying to get the biggest, stiffest, possible boring bar into a smaller hole. I persistently left a bit too much metal towards the base of the heel when grinding my own tools. Eventually I did a bit of CAD work to see how much clearance and what angles I should be grinding under the cutting edge to ensure clearance. The results surprised me. Needed to shift more metal than I expected if the front of the bottom was to be safely clear.

Using that data I trimmed a bit off the heal of my smallest insert bar making it much better behaved at the bottom end of the recommended hole range. Turned out that, as supplied, the heel would interfere until the hole got to about 2 mm diameter above the suggested minimum. Taking very little metal off cleared the interference and pushed the minimum hole diameter down by about 0.5 mm. Actually seeing the small amount of interference on the machine was impossible.

Maybe visible with one of the modern inexpensive borescope devices. If you are good at interpreting the images.

I’m not.

Clive

[Howard LewisParticipant @howardlewis46836]

Bear in mind that a C2 is a relatively small, and low powered lathe.

So the size of boring bar selected will determine how small a hole can be used as a starting point, and how deep a cut can be set – but it will flex rather than take the full cut.

A 6 mm bar will be more flexible but will fit into a 10 mm hole.; a 10 mm won’t!

Allow for flex in the bar, so that as you approach finished size, the depth of cut needs to decrease, and “spring” cuts are needed to accommodate flex in the bar.

(I’ve known it take SIX spring cuts before the bar stops removing metal)

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