Brazed carbide boring bars

[lensman57]

Hi,

Just to add a further point to my OP, I am not quite sure if the geometry-design of a boring bar differs from a lathe where the tool is rigidly held but only moves along a single axis to a boring head on a mill where the work is stationary and the boring head and tool rotate and plunge at the same time.
The moments of inertia of the boring head and the tool must put a great deal of load on the mill head, bearings and tool shank therefore the design of the boring head becomes even more important in particular with reference to the dynamic balance of the boring head.
I would be grateful for any advice.

Regards,

A.G

 

[JLeatherman]

I've long had a very love/hate relationship with this style of boring bar. Two things I have found they are very sensitive to. The most important is rake. Since they are round you have to set the rake by hand, and if you mount them in the holder wrong the rake can end up way off and produce a terrible quality of hole even with a sharp bar.

The second is feed rate. Being a small boring bar they will flex a surprising amount. If you don't feed at a constant and proper rate the flex can cause chatter. I try to feed these with the power function whenever possible and make sure to take a few spring passes at the same crossfeed setting when you are getting close to your intended diameter.

 

[goldstar31]

Far, far too complicated. I am guessing that you simply do not understand the geometry of a cutting tool.
Can I suggest you rig your mill or lathe with a bit of faced chip board as the workpiece. At the spindle end, you drill a small dimple with a drill chuck- as a reference point. You then leave the workpiece where it is and remove the drill chuck and replace it with a boring head. Don't put the boring bar cutter into the boring head- but draw a line from the point. Then insert the boring bar tool and align the top edge of the cutter with your line. That is going to be the reference- for subsequent adjustments of the diameter of the cut. You do NOT disturb the alignment. You can now- manually turn boring head in a cutting motion to scribe a circumference.
Now look at where you have got. Does any part of the cutting tool touch inside the circumference- other than the tip of the cutting tool? If it does, you have the wrong tool shape.

All that I have described- but differently-is the action of a lathe tool or a twist drill.

Try it- tell me your discoveries.

 

[lensman57]

HI ,

I have actually done that this afternoon, prior to that I dressed the tip as best as I could and reduced the length of the shank to just enough to fit in the small holder to reduce the flex and chatter. I did bore the hole to exactly 15.85 mm that I wanted and the finish as not bad with the exception of the bottom of the hole. This was not a very deep hole to bore but I am glad it worked out. BTW the tips really do need serious sharpening with these bars, they must have sharpened them to bore in to "set custard" at the factory.

Regards,

A.G

 

[goldstar31]

Of course, this is a long and expensive journey into the realms of boring holes.

From the information- just recorded, you were boring nothing more than a hole of 5/8th of an inch.
If I had to do this, I would not have gone to all this rigmarole of boring bars and boring heads - and Uncle tom, Cobley and All-- and bought a bit 5/8ths in cheap carbon steel or a 13" piece of 5/8ths silver steel-- and made a D bit from enough of the rod turned it down to 15.85mm and halved- or almost halved it. I would have put a 5 degree cutting edge on it- heated it on the gas cooker-- and bunged it carrot red into a raw potato and worked from there.

This- I have to say- is 'engineering' and cheap at that.

If you disagree- re-read George Thomas- you have the book. Page 94 Figure 7.11 refers.
----------------------------------------------------------------------------------------------------------------------
I made up his small boring head from the scrap box - which is really for bores of up to 50mm and I made up the two boring bars- inline for the Quorn- and a funny one to do the rotary table.

 

[lensman57]

Hi Goldstar,

The size of the hole ,15.85 mm , was incidental as it was the the diameter of the pyrex test tube that arrived from a vendor on ebay, I bought a parcel of 10 of them and evryone is different from each other, therefore I needed to bore it to the exact size of the tube. If I break this glass during cutting then I'd have to choose the next one which is about 15.89 mm, these were sold as nominal 16 mm tubes, and the bore has to be enlarged, so the idea of making my own reamers , although appealing is not quite practical, infact I bore the adjacent hole of 20.03 mm on my lathe using a 6" 4 jaw independent. At the time it seemed like a good idea to try and do the small hole on the milling machine using a boring head and then proceed to make the bolt pattern to finish the job.

I will at some point make my own reamers for small holes of about 5 mm and under.

Regards,

A.G

 

[gus]

For my part, I am beginning to think my spindle speed was not high enough. I will be trying again at the weekend, looking forward to making more swarf.

I am turning at 600-rpm. Bore finish looks OK. I did improve grind the BT.

Will use the BH and test bore with the BT.

 

[Propforward]

Hm. That's what I was turning at, on a 3/8 bore. Going by what you say, and also others, I will see about dressing the bits up first, and see if that helps.

 

[goldstar31]

At the risk of boring everyone else, doing a 3/8th bore is a task for a simple D bit. If you want a mirror finish, you bore undersize and make up a hone to do the last bit.

 

[Propforward]

If you say so.