Need a perfect 8mm hole - to ream or bore?

[BillTodd]

Cheap 'jobber' bits can be OK. I used a one to drill a 5/6mm 8" through a steel spindle (the 5mm was riveted into the end of a 6" long 6mm rod - I didn't want to wreck a good bit). Just check how they cut and make sure the end is ground correctly.


Boring bars should only be used to cut forwards (make sure you don't drag them back on the final pass).

As Robin said, you should take nice even cuts up to the finished diameter. The finishing pass should be the same depth (since the bar will always bend while cutting; if you try to take finer cut the bar will bend less and you'll take off more than you thought you were going to and probably over shoot the diameter)

My guess your new boring bar is only ground to an approximate shape, so it's ready to be sharpened (since its final shape is determined by the material you are going to cut)

The tool angle should be correct: the tool should cut with the very tip of the tool: the top, front and underside should all slope away from the part.

When you cut, the material has to shear apart and slide along the face of the tool, The material will cut easier if this shear face is at an acute angle. However, as the shear angle gets more acute, the tool gets weaker. The cutting tool shape is a compromise between shear angle and strength (and cutting force).

Soft/weak materials (ones that shear easily) will cut with a low force, so the tool can be acute. Stronger/harder materials need more force to shear, so will need tools the a more obtuse angle (for strength). Beware strong materials that are also soft (e.g. copper and its alloys) they will grab the tool.

You can cut strong/hard materials like steel with a knife like tool - you just need to take a very very fine cut.

The tool should be sharp: I.e. the cutting faces should be smooth, flat and at the same angle all the way 'till the last molecule :naughty: Don't confuse the words 'sharp' (meaning the edge is not rounded) and 'acute' (a tight, narrow angle).

Bill

[HankMcSpank]

Thank for you valuable input.

Here's my strategy for now...

1. Order up an MT2 Tailstock ER32 collet holder (on account I already own a full ER32 collet chuck & collets for the spindle....not essential I realise, but they're only £20 inc shipping from China & will be highly useful for some of the small intricate things I want to finally make)

2. Check tailstock alignment!!!

3. Order a full set of centre drills (the one I had was a bit piddly)

4. Order up some new 8mm drills (ie abort the concept of drilling a smaller hole & boring until I become more lathe savvy!)


thanks once again!

[HankMcSpank]

Just to give some closure here.

Last night I finally got around to mounting/using my ER32 collect chuck on my mini lathe.

I flipped the workpiece around since taking that photo above (it's 20mm diameter where I've turned it down...which just fits into my largest ER32 collect nicely).

Anyway, my larger centre drill arrived yesterday so I used that & then followed up with an 8mm drill......it's turned out just fine for my needs.

So it was all much ado about nothing rally ...no drilling undersize & then borring needed. (I still need to revisit why the boring tool seems woeful at erhm....boring - for another day though!)

Coming back to why the drill bits wandered - bit of a mystery ...I'd have to think it was something to do with the tailstock not being fully secures &/or centred (I made doubly sure the tailstock nut was tightened extremely well this time...possibly moved during the drilling process last attempt?) & then using the larger centre drill seems to have helped.

And wow...what a difference using an ER32 collet chuck is on my lathe - it's made the whole experience quick & accurate & enjoyable. I no longer have to think "B0ll0****...there's a turning job I need to do, but that *&$*ing 4 jaw....euugh"

[miopicman]

That's what engineering is. i.e. whatever is adequate for the purpose. There are pros and cons for all methods.

I have a similar engineering situation and I will be using an undersize drill followed by a reamer. The pros for this is that it is quick and can be used over and over again with consistant results. Like a twist drill the reamer can be used more than once. Also a reamer will make a consistantly parallel hole; this is something to be aware of when boring deep/long holes - is the lathe quality good enough? You don't want to end up with a hole that is ok at one end and tight or slack at the other.

This my part. It's a chopper disc for speed feedback on a spindle motor.