boring head vibration

Hi.

I'm using a boring head to enlarge a hole in a aluminium pulley from 11.5mm to 12mm diametre. The goal is getting a tight fit on the shaft.

I'm taking off what I thought it was a small amount in each pass: around 0.1mm increments in diametre.

The cut starts ok but once the boring bar is cutting about 1mm deep into the hole the whole mill head starts vibrating noticeably. I believe the boring bar is flexing and not taking the material off.

I;ve tried to slow the Z axis feed but to no avail

The boring bar is carbide tipped and almost new, could it be that it is blunt already?

thanks for any help

Sounds like the back of the cutting tool is rubbing on the cut. A lot of the off the shelf sets of boring bars and those supplied with the heads need some grinding if they are to work in small holes.

J

Edited By JasonB on 21/05/2014 12:47:36

Hi JCC

I have been digging through a box of custom made High speed steel boring bars made by a family friend who passed away last year. He did a lot of fine work. Some of the bars were only 30 or 40mm in total long the smallest being about 2mm in diameter (Not for the entire length) but with a working length of maybe 20mm. All of these tiny bars... there is a tobacco tin full of them in with different grindings to fit their intended purpose.... and have razor sharp cutting edges some so small they are hard to see.

No doubt he used them to precision bore tiny holes prior to lapping or burnishing.

High speed steel obviously does not have carbides ability to remove metal at high speed. However most carbide inserts have a rounded cutting edge it is not sharp. unless you have purchased specially ground inserts. carbide in a way works like a plough rather than a knife.

But a boring bar is long and thin, it flexes. So we have a plough on the end of a stick! not Ideal.

If you are using a boring head, and have purchased the generally available Asian boring bar set for it you may find that the carbide quality is a little lacking (Well on the two sets I have it is a soft grade) I actually use them quite a lot so this is not a complaint just an observation. As commented earlier you will need to re grind the boring bar tip to provide proper clearances, and make sure the tip is not touching along the side of the hole for more than a mm or so, always a good way to encourage chatter.

When finishing to size a blunt cutter can just burnish the work ok if you are at size but if you are still a little under so add a little more feed which causes the cutter to actually cut due to the increased pressure you can get a bigger cut than you dialled in, possibly ending up oversize. spoiling the work.

Sharp is the key.

When I get near size I like to make a second cut at the same dial setting to work out any spring in the bar, it is often enough.

I still have a preference for high speed steel for precise hole work in mild steel and aluminium because of its ability to take really fine cuts.

No doubt there will be commercially made boring bars out there that would change my mind but unfortunately they would not mind my change, too much of it in fact. Anyway making your own tooling is a lot more fun.

Regards
John

.

Hi
As John says use HSS and make sure there is clearance at the back so it don't rub. a spring cut is always a good thing to try . sound silly but sometimes cutting dry works on finishing cuts on MS
Nobby

Hi.

thanks for the replies. good point about the clearance angle. I'll try to regrind the tip and see how it goes.

Hi JCC,
I agree with the comments about the tipped tool rubbing and also the advice to try HSS. One of the main items on my "to buy" list for Harrogate was some second hand long 12mm shank milling cutters to re grind to use as boring tools to fit my boring head. I have also made an adapter to allow me to use 1/4" square HSS boring bars that I made for use in my lathe. I will try to find it and post a picture. If the tipped tools are rubbing then rotating them in the boring head MAY solve the problem.

Les.

Here are the pictures of the adaptor.

Note the 12mm shank that fits into the boring head is offset from the main body in such a way that the top of the tool (When in the lathe.) is on the centre line of the adjustable part of the boring head. The slot is quite a lot bigger than than 1/4" to allow packing pieces to be put under the tool if the top face has been ground to bring it back to the centre line. Also note the flat you can see in the second picture was filled in error.. The correct flat is at 90 Deg to the one you can see.

Les.

Edited By Les Jones 1 on 21/05/2014 15:36:54

Hi.

just a quick followup to say that regrinding the angle worked out brilliantly. as a side effect the cutting tip is now more sharp. it's like cutting butter.

these boring bars do come less than sharp out of the box. I had not a clue.

it took a while to grind it since I only have a tormek (i'm a woodworker). i used the grindstone meant for hss planer blades.

thanks again

Edited By JCC on 23/05/2014 16:58:18

Edited By JCC on 23/05/2014 16:59:12