Some (daft?) boring bar questions plus related one on drilling.

[Clive FosterParticipant @clivefoster55965]

I think we have a failure to communicate!

Hopper is using Depth of Cut correctly to describe how big a bite the tool tip is taking out of the material.

You seem to have gotten DOC mixed up with the length of the hole.

Very easy to get confused if you don't have someone to explain things. So Hopper believes you are trying to take unrealistically deep and difficult cuts for a small machine.

Although both my lathes are full on industrial machines well able to utilise the full power of their I rarely find it worth taking heavy cuts. But I do have drills out to the largest 3 MT size so generally I can get pretty close.

Frankly most of the time I'll be taking no more than 25 to 50 thou (0.5 to 1 mm) out of the bore. My longest boring bar is 1" diameter with a reach of almost 12" from the tool post. When needs must it worked just fine. I was impressed with how trouble free it was.

As we mostly do onesey-twosey-threesy jobs there is little benefit in pushing tooling hard with large depths of cut. Quicker to take a couple or four extra cuts rather than wrestle with getting the maximum metal removal per pass. Boring bars do deviate. Its the nature of the beast. Working wth heavy cuts you are likely tp spend more time taking out the deviation induced taper than you would going down in lighter cuts.

Gotta remember that sometimes the Hare wins, sometimes the Tortoise wins. Hafta know which coat to put on for which job. No way am I going to confess on a public forum the time when being "Mr Hare" has created "problems".

Clive

[JasonBModerator @jasonb]

Hopper I suspect the OP is a 3/16 and 5/16" shank solid HSS bar, I certainly have these sort of sizes.

By DOC he means depth of hole those 3x and 5x figures as multiples of the shank diameter.

All perfectly possible with a mini lathe, have even done it on my old Unimat 3

 

How deep you need to bore down into the hole is what generally determines bar diameter, I have upto 28mm and that only comes out for holes that are 150mm deep or more, tell a lie I do have a 35mm one but that only gets used between ctrs.

Diameter is not so important. Take this afternoons efforts, 185mm dia but as I was only going 18mm deep I used a 12mm insert bar as it's the biggest that fits the QCTP so less hassle to just slip it in as it stays in a holder all the time 

Lathe;s power liminted the cutting depth (amount removed per pass) so it did not break the swarf into chips but got an impressive sized birdsnest of swarf in various colours.

Edited By JasonB on 07/01/2023 15:53:58

[Martin KyteParticipant @martinkyte99762]

Here is my rendition of one of G H Thomas' Boring bars.

Has all the advantages of the square tool holder as has been mensioned but with the additional advantage of being able to clamo round sections larger than the tool post slots. It's also easier to extend the bar without having it come lose in the toolpost.

regards Martin

[Mick B1Participant @mickb1]

Posted by Clive Foster on 07/01/2023 13:23:33:

I have, probably 2 or 4 boring bars of the type shown by Mick. Various makes, not identical but they all do the same job.

Rule 1 for boring is always to use the biggest bar that can be finagled into the hole. Which means very short tool bits. To my mind the one Mick shows is way too long. I recon less than 1/4" stick out at the front, nearer 1/8" for smaller ones, and nowt out the back. Theoretically it wastes tool steel because you only have a few re-grinds before it is too short to hold. But I have a couple of baccy tins full of itty bitty bits

….

Clive

Yes – for boring, of course it's too long, Clive!

I was last using it to flycut a flat on the bottom of a titanium toy car for the grandkids!

Time before that I was cutting some 1 7/16" x 12 TPI internal threads for the railway, using a different toolbit ground to fit the pitch gauge.

Versatility was my point.

Edited By Mick B1 on 07/01/2023 17:46:35

[Mick B1Participant @mickb1]

Posted by JasonB on 07/01/2023 13:43:02:

You can still buy those bars, Tracy Have them for starters

Thanks, Jason, I hadn't thunk of them. RDG and Chronos don't appear to do em.

[Andy StopfordParticipant @andystopford50521]

To return to the maximum drill size, when I had a mini lathe, 16mm was about the limit. Any bigger and you were moving into the realm where the speed had to be reduced so much to prevent chatter that the motor would stall unless the feed was so small that the drill would rub instead of cutting.

You could probably go a bit bigger if the drill was very carefully ground, but I long ago lost the knack of doing that.

[Anonymous]

Posted by Clive Foster on 07/01/2023 13:23:33:

Andrews increase speed and feed suggestion works well…

Not quite what I wrote.

I increase feed and depth of cut. I tend to leave speed as is. Although I may move speed up or down if the chatter is particularly persistent.

Andrew

[samuel heywoodParticipant @samuelheywood23031]

Yes Hopper & Clive, I meant hole depth, mommentarily forgot DOC has a more precise meaning in engineering.

Hobby machining has taught me to be a lot more precise than i used to be,still working on it~ guess it hasn't quite rubbed off on my language useage yet.

Hopper ~ the 3/16" hss IS the boring bar in this instance. & it's not hard to spot the flex.

I'm sure you've seen twist drills bend at some point in the past? …maybe only the really tiny ones if you're doing things correctly.

Edited By samuel heywood on 07/01/2023 23:16:05

[HopperParticipant @hopper]

Posted by samuel heywood on 07/01/2023 23:10:35:

Yes Hopper & Clive, I meant hole depth, mommentarily forgot DOC has a more precise meaning in engineering.

Hobby machining has taught me to be a lot more precise than i used to be,still working on it~ guess it hasn't quite rubbed off on my language useage yet.

Hopper ~ the 3/16" hss IS the boring bar in this instance. & it's not hard to spot the flex.

I'm sure you've seen twist drills bend at some point in the past? …maybe only the really tiny ones if you're doing things correctly.

Edited By samuel heywood on 07/01/2023 23:16:05

Ah that makes much more sense now. Thank you.

A 3/16" HSS boring bar and a 2" hole should not be mentioned in the same sentence. As you have found, the one is not suitable for the t'other. So you do need a bigger boring bar, but going to 1" diameter is overkill on a mini lathe, unless you are going to bore 2" hole 12" deep or some some such madness on a mini lathe.

Those small 3/16 HSS shank boring bars, commonly refered to as micro-boring bars are more for small holes about 1/4 to 1/2" diameter and maybe 1/2" deep. They are very good for that. But they are designed for CNC work where the hole is drilled nearly to size first and the micro bar is used for a light finishing cut. Not for serious metal removal.

But for your larger holes, all you will need is one or two of those boring bars that Mick B1 posted above, with a steel shank with square hole in the end to hold some 3/16 or 1/8" HSS. They come in varying shank sizes so if you get a 5/16, 3/8 and 1/2" shank set, you will have most jobs covered.

Now, grinding the toolbits for a boring bar so they cut cleanly and chatter free is a bit of a dark art and perhaps not for the raw beginner without someone there to give hands on instruction, so you might consider buying a set of carbide insert boring bars like the one in the picture I posted way above boring the 2" in a red block of steel. I bought a cheap set of four with shanks ranging from about 7mm to 12mm . The tooling code for them is SCLCR 06 and then the shank size. The standard inserts are CCMT060204 but if you get the ground inserts CCGT060204 they have a sharper cutting edge and will work better in a small lathe like yours.

They work very well with minimal fuss. The shanks seem to be made of alloy steel and are claimed to be hardened and tempered, and are very rigid for their size. I think the 12mm one will have capabilities that far exceed those of your mini lathe so no need to go any bigger.

[Martin KyteParticipant @martinkyte99762]

Going back to small boring bars in the 3/16” range, I use them to true up drilled holes and then finish to size with a reamer. This makes any taper or poor surface finish due to slight flexing irrelevant.

regards Martin

[HopperParticipant @hopper]

Posted by Martin Kyte on 08/01/2023 09:09:35:

Going back to small boring bars in the 3/16” range, I use them to true up drilled holes and then finish to size with a reamer. This makes any taper or poor surface finish due to slight flexing irrelevant.

regards Martin

+1 on that for small holes where the job is critical. Drilling for rapid but rough metal removal. Boring for concentricity and location and parallel. Ream for final size and finish.

[Martin KyteParticipant @martinkyte99762]

My point was I almost never use use small bars to finish bore anything below 1/2” except on the rare occasion I need to produce a hole to an odd size.

[samuel heywoodParticipant @samuelheywood23031]

May thanks to all who've contributed advice & expertise.

I shall attempt to make my own boring bar~ I'll start with a 1/2" one & see how that goes.

If it's any good I'll post a picture in due course~ don't hold your breath.

[HopperParticipant @hopper]

Good move. A half inch bar should do everything you need in a mini lathe.

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