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Post by Tim Bayliss on Aug 25, 2004 16:19:13 GMT
Until quite recently all my turning was done with either HSS or indexable tools. A couple of months ago I decided to purchase some brazed tip TCT tools for internal threading as I had a couple of projects in the pipeline. Because the threads I needed to produce were quite large I purchased a set of 1/2" boring & threading tools from a reputable supplier. Incidentally, when I thread internally, especially if it's to a closed end, I always turn the spindle by hand so that I can feel any resistance and also take comparatively small cuts.
Thread 1 was a 2 1/4"- 8 tpi unified R.H. and thread 2 was an M27x1,5p. L.H. both in mild steel with the tool being reground between.
In both cases of course I used the 60 deg. internal threading tool and in both cases at about 3/4 through the job the very tip of the tool broke off with a 'crack'.
For thread 1 I offset the top slide as it was quite a large thread but for thead 2 I just fed straight in.
Is this problem common with this type of tool or do I have a duff one? If it is a common problem how can it be overcome?
I finished thread 1 with a 60 deg. indexable boring bar as the hole was big enough to get it in at the correct angle, thread 2 I had to finish with the 55 deg. tool that came with the set and 'adjust' the angle afterwards with the aid of the male thread test piece I made first and a 1 1/2 lb ball pein thread angle adjuster.
Many thanks in anticipation.
Tim.
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SteveW
Elder Statesman
Posts: 1,393
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Post by SteveW on Aug 25, 2004 22:40:50 GMT
My only experience with screw cutting was trying to make an ER32 collet chuck for a 7B as per EIM a while back. I started by making a dummy Myford nose both as an exercise but mainly as a try-piece. This was my first ever. I used the cheap (Chronos) TCT 55 degree brazed tips for the successful nose and less successful internal thread.
The tools came without any rake and I reasoned that some would be necessary. I also reasoned that as the tool would be going leftwards it would need more side clearance, there was virtually none. Having read up on the subject and remembering demos from my school days I opted for setting the top slide over to just under 55/2 degree so that the major cut would be on the leading (left) edge of the tool leaving a scrape on the right side. I also ground the top surface to give me cutting rake for leftwards travel, i.e. cutting on the left 'sharp' edge and polishing on the dull right edge. Also remembering the thread root (and crest) are near semi-circular I rounded the tool tip before the material did it for me and thus avoid an in-situ tool regrind.
I also reasoned that in the case of the Whitworth nose thread the later cuts would get very heavy so calculated a number top and cross slide values so that I could rough out the bulk of the waste before the final cuts. If you think about it, if you start off at datum with the tool just touching and then back off the cross slide, by the time the top slide (at 55 over two degrees) advances the tool to the metal it would have traveled left towards the left flank of the final thread. After a couple of passes removing the majority of the waste I returned to datum and attacked it with finishing cuts of minimum loading contact on left/right flanks. At one point I disengaged the halfnut too soon and was very surprised to see everything continuing as normal with the tool being driven left by the right flank of my new thread. Phew!
As on previous contributions I recommend the diamond laps for tarting up the TCT tips and 'adjusting' the default geometry to something that works well on lower powered kit.
Another tip I picked up was on measuring the thread form. It's relatively easy to measure the outer thread diameter and the core can be deduced. Ensuring the width of thread form needs thought. The tip was to place two appropriately sized drills into a master thread, displaced 180 degrees, and then measure across the outer surfaces of the drill shanks. Then it's just a matter of getting this same dimension on the virgin thread. With more metal to take off the drill shanks stick out more. QED.
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Post by Tim Bayliss on Aug 26, 2004 9:46:28 GMT
Steve,
As you seem to be fairly new to the internal threading game I would like to give you a couple of words of advice/warning concerning grinding up tool tips.
Firstly if you grind on a radius it MUST be exactly correct for that thread, which is very difficult to obtain in an amateur environment, because if you `quess' and get the radius too small you won't cut the thread deep enough initially and have to adjust it, not too bad of a situation.
But if you grind the radius too big you will most likely cut the thread too deep and end up with a sloppy thread which can be a project wrecker.
Secondly the angle of the tool nose is important as this produces the angle of the thread. If you grind reliefs onto the tool you can in effect alter the angle of the tool and therefore the thread angle with the 'compound' grinding.
In my opinion it is best to stick with a flat topped, pointed tool and take small cuts, offsetting the topslide for all large threads.
The information for correct thread machining depth, internal diameters, etc., for a pointed tool is readily available in many publications but I particularly like the little book Drills, Taps and Dies by Tubal Cain (no. 12 in the workshop practice series). It's got loads of easy to understand threading info.
Incidentally my point in my original thread was why does my TCT tool point snap when I've never had this problem with HSS?
Regards,
Tim
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Post by ChrisCrosskey on Aug 26, 2004 11:09:14 GMT
I think the problem with the tip might just be down to how brittle carbides are.... they are hard as hell but shatter easily if the loading is intermittant. Sometimes you get away with it, I use a bar from an el cheapo indexable lathe tool set in my flycutter and I've not had problems, equally I do a lot of repeat jobs on 303 stainless at small diameters and I'm fairly certain that the rather vibratory action of a single phase motor on my ML7 is responsible for most of the tip degradation..... doesn't matter if you'r eonly roughing out but I can see it being a big deal on threading and finishing.... Haven't done much threading on mine, but looking at the tips I got in a batch of assorted tips a while ago I'd recommend leaving them alone, they'll have enough problems with being easy to shatter as it is without adding rakes etc...... Carbide is wierd stuff to work with and a lot of it seems to go against common sense (all the negative rake stuff for starters), I get the feeling that we really aren't using machines powerful (or fast) enough to get the best out of it..... Certainly when I take tools into college and use them on the bigger faster machines there it seems to work much better.... I tend to work to a rule of twice as fast as for HSS and leave the depth of cut and feed-per-rev the same..... HSS gets a better finish most of the time but I think that is down to us not using carbide properly as we are unable to rotate the work fast enough..
my 2p worth..
chrisc
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SteveW
Elder Statesman
Posts: 1,393
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Post by SteveW on Aug 26, 2004 22:50:29 GMT
I take your point on the TCT tip geometry. The original tip was way too pointed for the Myford nose thread I was cutting and the radius I used was still sharper than the required thread core. This was easily demonstrable on presenting the tool to the original nose thread.
As for my top rake I made sure that the left cutting edge was all but original as can be achieved using the diamond laps. I went for top rake in the direction of driven cut with the edge on centre height. The Myford doesn't have the stiffness to use TCT properly so I was tending towards emulating HSS tool geometry on steel. The right flank of the thread was a function of half of 55 degrees top slide offset so my point ran down the correct line (or close enough).
As for TCT being fragile I have lost too many indexable bits to a clumsy swip against the job and have had more luck with the cheapo brazed type especially in the flycutting role on cast iron where I've got away with giving them a hell of a bashing.
To the orignal question 'why do they break?': I've lost most of the tips through being clumsy (that's how I break 'em). As has been said above TCT is very hard. maybe there is a problem when they are loaded in a direction they're not stressed for. It could a suport issue, i.e. too much over hang leading to chatter and vibration and they hammer themselves to bits. Maybe the tool has too little front clearance so the load is just below the cutting edge providing an upward thrust vestor. If the tool is set just too high deformation of the metal is what is allowing the tip to cut but again providing an upward vector to lift the tip away. I always heard that TCT needs to be worked hard (faster speed and feed). However, to pull this off you need a grown up and very rigid lathe.
Another thought is you've got a duff batch of tools made either on 'Friday afternoon' or 'Monday morning'.
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Post by Tim Bayliss on Aug 27, 2004 16:42:13 GMT
Many thanks lads,
You learn something every day! Having no experience with TCT tools before I didn't know about using them at higher speed and did my internal threading at 'hand' speed with the spindle handle, as I've always done with HSS tools.
I do have a substantial lathe and I use my indexable tools (external turning and boring) at a good lick but I prefer to cut internal threads by hand, so I'll just go back to HSS and regrind the TCT internal threading tools to boring tools.
Regards,
Tim
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SteveW
Elder Statesman
Posts: 1,393
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Post by SteveW on Aug 30, 2004 16:42:33 GMT
Tim,
It's just occured to me that driving the lathe by hand will cause cyclic loading on the tool at the end of each pull. As the jobs stops the tool will want to twange back, even more so with a boring bar. This coupled to an uncertain re-start could be what's stuffing your tooling.
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Doug J
Active Member
Posts: 20
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Post by Doug J on Sept 9, 2004 9:39:20 GMT
I believe most of the problems with carbide tooling on light machines comes down to the type of tips used. I have tried all sorts of tips in the past with results varying from useless to brilliant. When used in a light machine positive rake TEXT edged tips really do make a huge difference. For some time I have been using high positive rake very sharp tips (meant for aluminium but work a treat on freecutting steel and 303 stainless ) which can produce superb results when run at medium to high speeds.
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Doug J
Active Member
Posts: 20
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Post by Doug J on Sept 9, 2004 9:44:33 GMT
I believe most of the problems with carbide tooling on light machines comes down to the type of tips used. I have tried all sorts of tips in the past with results varying from useless to brilliant. When used in a light machine positive rake TEXT edged tips really do make a huge difference. For some time I have been using high positive rake very sharp tips (meant for aluminium but work a treat on freecutting steel and 303 stainless ) which can produce superb results when run at medium to high speeds.
For screwcutting you can get HSS coated tips that are more robust than carbide ones and perform better at slow speeds, especially when rotating the work by hand. I find that using a mandrel handle makes screwcutting a pleasure when there is not much room for stopping the cut.
Doug Johnston
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Doug J
Active Member
Posts: 20
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Post by Doug J on Sept 9, 2004 9:48:42 GMT
For TEXT in the above message read SHARP. I used the underline feature with the correct word and got the result you saw!
Doug
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Post by Tim Bayliss on Sept 25, 2004 12:16:49 GMT
Many thanks for the replies, folks. I appreciate you taking the time to help out.
I visited the suppliers of the said tool the other day (Chronos in their new premises) and asked if they had any reports of similar problems.They said they hadn't but gave me a new tool for my trouble anyway, which was really very good of them.
I've come to the conclusion that it must be the stop-start action of turning the mandrel by hand that's causing the tips to chip because if I use the tools under power there's no problem.
Good luck and happy turning,
Tim Bayliss
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