|Ken Johnston||29/01/2012 11:33:13|
|9 forum posts|
Hi Guys, my first post.
I have had my ML7 for years but only used it occasionally.
Never enough time. Now I am going to try to be a bit more professional.
Moved up a bit, I'm heading for 15mm and don't like to take huge cuts. Relieved the cutting edges, drill goes in maybe 15mm and tightens up. Dragged it out. It is as if the drill is conical and it seems to have polished the sides of the bore. OK, try another, same result. Try another, same. Tried with coolant on, which I see is not recommended. Also WD40, oil can. Brass is coming off in chips so not free cutting. Have I picked 1 OK drill and 3 worn ones. Do drills in fact wear out in that way. I made one really negative rake, no joy. They drill steel OK tho. All MT1 drills old but Sheffield made. I could not find any thread which mentions this problem.
Edited By Ken Johnston on 29/01/2012 11:34:25
Edited By Ken Johnston on 29/01/2012 11:57:26
|Ian Hewson||29/01/2012 11:52:57|
|322 forum posts|
Drills can wear on the side of the flutes, if they have been used on hard materials or loaded to produce heat. You can sometimes see this by eye on the edges of the drill.
Gives the symptoms you describe.
I would try a new drill, as sharpening the cutting edge will not improve matters unless you grind down past the wear.
|Ian S C||29/01/2012 12:34:05|
7468 forum posts
Yes they do wear on the side of the flutes, I know, I'v got a number of them, 'fraid all my large drills are second hand. I imagine that a tool and cutter grinder could fix the problem by taking the size down. Ian S C
|419 forum posts|
Tend to agree with the last post, you are most likely using bronze or gun metal, which if you've ground a negative rake on the cutting edges will certainly jam up, but these will make better wearing bushes than brass which is far too soft. Be sure to turn both the I/D and O/D at the same setting to ensure concentricity. on the other hand are the drills metal drills which have a side cutting land, or wood working drills which don't.
|Brian Dickinson||29/01/2012 17:46:41|
62 forum posts
We have loads of copper alloys machined at work. The engineers take the edge off the 'cutter' almost as if to blunt it. works a treat thought.
|Andrew Johnston||29/01/2012 18:25:05|
6601 forum posts
Interesting comments, seems to mirror my limited experience. I find that drilling brass is ok, if you keep the feedrate up. I don't take the edge off the drill, as it means I'll then have to resharpen it. It used to be possible to buy straight flute drills for brass. You can still buy flat drills for brass, essentially a single flute drill, but I don't drill enough brass to justify the expense.
Now, as for bronze, and gun metal! I've had just that experience, the material seems to close on the drill. Bronze doesn't seem to ream very well either, same problem. I'm about to drill and bore the gun metal crankshaft bearings for my traction engines; wish me well!
|jason udall||29/01/2012 18:45:24|
|2031 forum posts|
|41 forum posts|
Maybe this is total rubbish and a "grade 1" wind-up but a very old and experienced machinist once told me to use milk as a lubricant for drilling and reaming gunmetal and PB. Something to do with emulsified fat particlesI in suspension apparently. I have never tried it but would be interested if someone has any experience of this.The chap who told me this produced some of the best work I have ever seen so I'm really not sure!
|1017 forum posts|
Like Andrew, I believe that one must keep the feed rate up. If you feed slowly, the drill can catch and pull forwards. If you feed faster than the drill is trying to feed itself , then there is a resistance to forwards movement, and the drill (and parting tool since the same rule applies) won't grab - it cant, because there is a reaction resisting movement, so it isn't going to jump forwards. Times to watch are when you start the hole into a centre hole, or at the end on breakthrough. Both times are when one should advance the drill very smartly to reduce effective rake.
(The faster you feed, the less the effective rake of course. )
All that is happening is that one is pushing the drill - so its pushed hard against the feed nut. Allow that forwards pressure off, the drill will try to self feed - , it was against the feed nut thread, but now its free to jump into the clearance in front of the feednut, And so it does, grabs a large chunk and presto - one jam up or a jump forwards out of the taper.
Andrew - big holes in bronze - same rule. Don't drill a pilot - just a minimal centre with a centre drill to make sure it starts right - make sure you have to push hard and apply loadsa coolant with a razor sharp drill (4 facet) off the Quorn. Never jammed yet, always cuts to size. Like you I don't like reaming bronze - I drill close to size, and then use a very sharp new ground boring tool, and that seems to work perfectly.. Personally I always use coolant in bronze, (and always with brass to keep those chips under control. Just makes the machine a lot easier to clean)
I used to drill pilots and the things jammed, unless one backed off the drill (aka blunting it) and since I don't use a pilot, I take great care to ensure my drills are ground exactly symmetrical on the grinder, so they cut straight and to size.
879 forum posts
Are you sure it is brass and not phosphor bronze but whatever it is take it very slowly and lots of cutting oil.
|Andrew Johnston||29/01/2012 22:34:24|
6601 forum posts
Michael, mgj: Thanks for the hints and suggestions; I drfinitely need all the help I can get!
The story so far, and it's not pretty. Finished size of the bores are 1.125", and for the first set of bearings just over 2" deep. I set up the bearings, in the housings, mounted on a jig in the 4 jaw chuck. Centre drilled and drilled through 5.9mm at 270rpm, no problem. Followed that by a ~9mm drill which turned out to be kangaroo drill, ie, did exactly what mgj said, digging against the tailstock feednut, backing off, digging in. Sorted that out by dropping the speed a bit and just nipping up the barrel lock enough to add some drag. Next drill, about 1/2", on a Morse taper no problem. Next drill, a bit under 3/4", pulls out of the Morse taper and spins like a top. Tried smaller drills, dropped the speed right down to 40rpm, always the same, as soon as the drill bites it pulls out of the taper. So I've now retired hurt for the evening to consider my options. Once I get the hole to about 3/4" I can use a boring bar. Some of the options are:
1) Slow helix drill specifically for bronze - 20mm Guhring slow helix drill £287+VAT
2) Flat drill for brass - 20mm version about £180+VAT
3) Insert style spade drill - Morse taper holder + insert approximately £200
4) Sort something out at home
Option 4 it is! The idea of using a ball nose cutter is interesting. If I could go through with a 3/4" cutter, given I already have about 1/2" hole, that would be affordable. Anybody got any thoughts. Or I could modify one existing drill to remove the positive rake at the cutting edge. I wonder if a home made 'D' bit would work?
For the next one (4 to machine in total) I'll follow mgj and try a 3/4" drill straight off.
|Ian Hewson||29/01/2012 22:45:08|
|322 forum posts|
|Just a thought, why can't you use a smaller boring bar?|
You say that you can use a bar at 3/4 inch bore, so buy or make a smaller bar for a fraction of the cost of the options suggested.
You will need smaller bars on a regular basis anyway.
|1017 forum posts|
Trouble is Andrew, you now have a pilot, so the most resistanceful part of the bit (the chisel point) is not helping you by slowing the advance of the drill. You are cutting on the flute edges only.
I would suggest you just bore it to size. You may not have a boring bar, but I bet you have a slot drill or end mill that will go in the hole. Mount that in the toolpost and bore with that - set up to cut on one tooth. Cheapest small dia boring tools around (not my original idea unfortunately), but it works a treat with indexable or ordinarly slot drills/ end mills.
The other thing is that with biggish (3/4 drill bits) jamming and jumping in the work is that you can score the MT in the tailstock, or "disalign" the work in the chuck, because forces are quite high. However, if you have to keep drilling, then now you MUST back off the drill. Slower is not necessarily better - the greater the angular velocity the less the effective rake, so by going very slow you are "sharpening" your drill and increasing the bite at each rev, which increases the forwards thrust. Get the speed up, back off the drill and use the quill lock to apply a drag. If thats the route you want to go, then counter intuitively, its a case of more bottle, more throttle, so long as the drill is set right. .
Might be best to get the hole to about .005 undersize, and then with a shed load of coolant to stop any temperature rise and "side/flute" grabbing, ream like that. I always believe that its very unwise to ask a reamer to take more than a few thou, esp in a bronze where temps can rise and cause that sort of grabbing.
All the best - I shall be starting my 4" LS short, so I have this to come.
|Nicholas Farr||30/01/2012 01:04:59|
3360 forum posts
Hi way back in the 70's I once had to make what was called a temperature graduation block out of brass, This consisted of a brass block about 50 mm wide by 25 mm thick and about 400 mm long. In the 25 mm side and starting at around 150 mm from one end, a series of holes had to be drill towards the other end at an angle of about 5 degrees and roughly 40 mm deep across the 50 mm width with a 6.5 mm drill. Drilling the holes with a 6.5 mm drill after a small pilot hole just big enough to clear the chisel point presented no problems, but the holes then had to be counter drilled to approx 15 mm deep and threaded to take a supporting bush which supported mercury thermometers. Drilling the 6.5 mm holes out bigger is where the problem started, with the drill bit jamming and sceaching in the chuck or as Andrew has discovered pulling the chuck out of the morse taper. Ball nose cutters as Michael suggests were just not available (we didn't have any, and the company was not going to get any, "get over it") The turner suggested to me to just touch the cutting edges on the stone to put the smallest of a flat on the them down the flutes, which I did, and hey presto no more problem, counter drilled every hole without any jamming or pulling the chuck out.
Ken, as has been said if it is brass that you are drilling and it seems to be sticking, then the lands on the drill bit are probably worn, it doesn't have to be by very much. Or as also been said it could be a bronze, I used to have to drill the odd bit of aluminium bronze at one time and even with a brand new drill bit it would have the tendency to bind, it felt as if is was kinda sticky.
1936 forum posts
How about a 'D' bit. Homemade, no rake, no helix and relatively cheap. Gives an accurate, straight hole, reamed finish!
157 forum posts
|????? He already has the hole out to 29/64" and no one has said 'boring bar'? That's certainly what I'd use.|
1936 forum posts
The use of a boring bar was suggested by Michael Williams in the third (option 2) reply to the original poster. The original question was not about how to proceed with the boring, but about the possibility and nature of wear on drills. Folk probably assumed that Ken would be boring the bush anyway as he had mentioned that was his intention.
157 forum posts
|OK Other-Tel, missed that!|
|David Littlewood||30/01/2012 11:31:30|
|533 forum posts|
Why a ball-ended mill? I find a 2-flute slot drill works just as well - and the OP is far more likely to have one of those. In fact, as MGJ suggests above, if so much has been removed already, a 4-flute end mill will also do just as well.
I find a slot drill is invaluable for starting holes in rough lost-wax castings or on sloping surfaces.
|1017 forum posts|
Sorry - to be sure - I wasn't suggesting using an endmill in rotation. Specifically Andrew said he had no boring bar of a size. So when I want a little boring bar and I don't have one to size, I just take an endmill, mount it in the tool post and use one tooth as a boring bar.
Endmill isn't turning - its just the unwitting supplier of a tooth. You have to offset the toolpost a touch to give a whisker of clearance.
This will probalby work in Andrews case because the job is a bearing shell, so the hole will be fairly short. Unless one has a long series endmill of a size and hte job warrants such a length.
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