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Threading Inserts

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Hugh Gilhespie08/06/2011 19:03:48
130 forum posts
45 photos
Hi, I have it in mind to make a swing up toolholder for external threading. I want to base this around using insert tooling. Looking at threading inserts for ISO coarse threads, say from M3 to M12, there are separate inserts for each pitch. My question is how important is it to match the insert to the thread pitch? If you buy an ISO external threading tool from say Glanze, you only get one type of insert so presumably provided the angle is correct, you can cut any pitch you want within reason.
 
Confused of Brixham
chris stephens08/06/2011 19:21:56
1049 forum posts
1 photos
High Confused,
There are two types of inserts, full form and partial form. Clearly a full form only cuts the pitch thread it is designed for, while a partial will cut any pitch. Sounds simple doesn't it, just buy the partial and away you go. If only!
On second thoughts it is that simple if you don't mind only 95% perfect threads. I won't go into the whys and wherefores of why they are not perfect, but let us just say you have to cut the thread slightly deeper than the books say or alternatively you have to widen the thread slightly.
 
Now if you can afford to buy full form inserts you WILL get a nicer thread and be able to do it quicker, than partial. Today I have been experimenting with different inserts and I found that to be the case.
 
If you make a holder for the inserts from Glanze, (which are actually quite good despite their country of origin) remember to angle the insert down by 10 degrees and cant it side ways by about 3.5 degrees(to allow for an average helix angle)
 
Hope all this helps, there is too much to the subject for a reply here, it really requires an article or two to cover the all the things it is useful to know. If you have specific questions please ask.
christephens
WALLACE08/06/2011 19:37:59
304 forum posts
17 photos
I'm sure a lot of people a lot more knowledgable than be can answer this better- but I think the major advantage of the 'fully profiled' inserts is that they can be used to turn to the blank thread to size as well as cutting the thread in the same pass.
 
W.
 
 
 
PekkaNF09/06/2011 17:25:15
96 forum posts
12 photos
Hi,
 
I can do external thread and most of time also a internal thread acceptable.
 
However, I'm having trouble with finer threads. I'm not confident that I'll get the threadfrom exactly right. I use partial form insert, plunge it straight (no rubbing), but even with all the initial external measurement, loupe + some eye balling + trying out with a nut and "close" enough I have this nagging feeling that it's not all that great.
 
I probably could afford thread gauges for few most used thread sizes, but the ones that gives me most trouble are typically something like M13*0,75!
 
What about wire method, would that get me out of trouble?
 
Old books recomend thread chasers to finnish up the work. Can you buy those anywhere? Probably I would be better of with simple HSS inserts shaped like carbide threading inserts.
 
Pekka
KWIL09/06/2011 19:15:14
3681 forum posts
70 photos
The commercial toolholders for thread inserts have an inbuilt 1.5 degree pocket angle and as Chris says are 10 degrees downwards. This for RH threads, cut towards the chuck. LH threads require an added negative anvil to tilt the insert over for the correct Helix Angle for whatever thread pitch you are cutting.
 
You can use a single Coventry Diehead cutter for a chaser if you wish.

Edited By KWIL on 09/06/2011 19:16:16

WALLACE09/06/2011 20:26:47
304 forum posts
17 photos
Or partially cut the thread on the lathe and finish the profile with a die ? (although I must admit that I have made the odd drunken thread this way - possibly due to NOT using a tailstock die holder but doing it on the bench in the 'usual' way !).
 
 
w.
 
 
mick10/06/2011 08:57:20
421 forum posts
49 photos
Not matter what type of tool you use for screw cutting, beit inserted or hand ground always take several "spring" cuts at finial depth, this will completely clean out the form. Full form are the best, but with all screw cutting the depth of cut must be applied incrementaly, getting smaller as you approach finial depth. Thread chashers are for finial forming of the thread and were used when all threading tools were hand ground. The chasher must be supported on a block of flat bar mounted in the tool post. By far the best method of finishing the thread form is with a fixed die, or die nut.
Roderick Jenkins10/06/2011 12:33:30
avatar
2376 forum posts
800 photos
Full form inserts will give you a thread with the correct root and crest radius for their particular thread. With a general insert you will, in general cut a V form thread. Since the strength of a thread comes from the flanks and a 60% engagement gives you pretty much full strength then the only real advantage of the "correct" thread form is to avoid having stress raisers at the root of the thread and a flat at the crest. So for 99% of model engineering applications the V form is perfectly satisfactory. My prefernce is to run a tap or die down the thread if I have one, but really only to avoid the trial and error cuts at the end of the process neccessary to get a matching thread to fit.
 
I have to say that my experience with recently acquired TC thread cutting tips is that it is a much pleasanter processs than I have ever had with my own hand ground HSS tools.

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