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Brazed tip screwcutting. Tip radius, for what pitch question?

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Ian P30/01/2012 15:10:57
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I need to cut internal and external LH and RH threads and was considering using ready ground indexable tips until I investigated how many tips and holders I would actually need, too many! so gets very expensive. Since I am unlikely to have an ongoing requirement I am now looking at using brazed tip tools.
 
I do not have the skill to grind the correct profile but several suppliers have 60 degree tools in their catalogues but I presume I will have to create the tip radius to suit the intended pitch?
 
In my case I want to cut a 3mm pitch threads but I suppose the tools when new are supplied with the tip left as a sharp point? Once radiused to suit a specific pitch the tool then becomes dedicated to that pitch unless it is reground?
 
Is my understanding correct? Buying one internal and one external brazed tip tools is a small fraction of the cost of indexable tools.
 
Ian Phillips

Edited By Ian P on 30/01/2012 15:11:27

Clive Hartland30/01/2012 16:56:20
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I am thinking that i would cut the internal thread with the internal threading tool untouched and then turn/thread the male to fit allowing for the flat crest of the male thread.
The diameter of the male part will before screwcutting be less the allowance for the flat crests.
Someone else will come up and explain this better.
 
Clive
David Littlewood30/01/2012 17:15:40
533 forum posts
Ian,
 
If you buy them in sets the price can be quite reasonable, for example here
 
There are other sellers around - I have an internal and external set and toolholders to suit both lathes.
 
By the way, Clive, the specification for ISO metric threads includes the flat within the nominal diameter; it can optionally be rounded within that profile, which makes it nicer to handle. This means that, to screwcut a metric thread, you start with the shaft at exactly nominal diameter. Dont know what the postion is for other thread forms, suggest you check.
 
David

Edited By David Littlewood on 30/01/2012 17:17:21

Edited By David Littlewood on 30/01/2012 17:18:48

chris stephens30/01/2012 17:41:30
1049 forum posts
1 photos
Hi Ian,
Insert tips come in two main types, full form and partial form. The full form will cut both root and crest of the thread to a high degree of accuracy and yes you need a different one for each pitch.. The partial on the other hand will only cut the root, and the flanks of course, but as you have already realized the tip radius is set for a small pitch thread. This is no problem if you keep your top slide where it is meant to be,ie parallel to the lathe axis and not set to an absurd and quite un-necessary half thread angle. Cut your thread , using cross slide for in-feed, to the correct depth given in the books for your pitch thread, and then widen the thread, using your top slide only, by an amount to compensate the small tip radius.
If the above sounds difficult to comprehend, don't worry I know what I mean.
Someone will chime in saying you must have the top slide set at half the thread angle, but this is complete baloney as anyone who has tried knows but the myth still persists.
chriStephens
Tony Pratt 130/01/2012 18:32:08
2319 forum posts
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Hi Ian, give it a go as per Chris Stephens method and please report back! I do use the half angle approach method but I haven't used full or partial form screwcutting tips at home so I can't really comment in this context.
Tony
Ian P30/01/2012 18:41:56
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I called a company today (in Glasgow) that has indexable holders and tips that seemed very competitively priced. I explained I wanted to cut RH and LH inside and outside threads. The person I spoke to was very knowledgeable and did not try to sell me anything, in fact he recommended I by brazed tip tooling!
 
I found out that the holders to cut LH and RH are different and need different tips. We discussed 'laydown' tooling and how 'shims' are needed to tilt the carbide insert. All in all it seemed to get quite involved. I doubt I will need to cut threads like these very often so dint want to invest a fortune in tools that wont get much use.
 
Clive,
I do understand your explanation. I want the male thread to be right (and look right) and conform to the ISO standard for M48x3, not having a test gauge or anything of that thread the ready profiled tips do appeal.
 
David,
I can see me getting the Chronos kits (but not necessarily for this job) which look incredible value. I presume they are only for RH threads though but it does not state it on their website.
 
Chris,
I have only ever used the cross slide for feed and had no problem so will do the same again.
 
Ian
David Littlewood02/02/2012 16:33:06
533 forum posts
Ian,
 
Sorry, I missed the question until now - this site is not very good at flagging replies (I don't think signing up for e-mail notification is an acceptable alternative).
 
I had a good look at the tips, and as far as I can tell without a microscopic examination the V-tips seem to be set absolutely vertical. The flanks also taper towards the base, so they should not bind on a normal ISO thread (if you want to cut something with an unusually high thread angle, such as a two-start worm, then you're on your own!). However, I have not tried cutting a LH thread with them, so don't take my word for it - I suggest you try asking Chronos, they should know.
 
David

Edited By David Littlewood on 02/02/2012 16:33:52

Edited By David Littlewood on 02/02/2012 16:34:16

Ian S C03/02/2012 07:10:35
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Unless its a particulary dificult material, ie., a high grade steel, or stainless, I would tend to stick to HSS. No great problems with grinding up a tool, and making a boring bar to suit. You can use HSS square, but for internal tools, old center drills, or broken HSS taps, or old warn out end mills are ideal. Ian S C
Ian P03/02/2012 08:38:00
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Posted by Ian S C on 03/02/2012 07:10:35:
Unless its a particulary dificult material, ie., a high grade steel, or stainless, I would tend to stick to HSS. No great problems with grinding up a tool, and making a boring bar to suit. You can use HSS square, but for internal tools, old center drills, or broken HSS taps, or old warn out end mills are ideal. Ian S C
 
 
The male thread is 48mm diameter and 3mm pitch on EN24T. I have ordered a brazed tip tool and will titivate the radius with a stone and magnifying glass. I am hoping to cut the LH and RH threads with the same tool.
 
I am doing the same for the internal threads in aluminium, carbide is not ideal so may make one in HSS which I've done before but in much finer pitches.
 
I have also bought internal and external ready profiled carbide tips of the correct pitch and will investigate making my own holders.
 
Ian P
David Littlewood03/02/2012 14:35:57
533 forum posts
Ian,
 
You might find it instructive to look at the MSC/J&L catalogue, pages 513 onwards:
 
 
Contains quite a useful discussion of using full form threading tips. Seems you can buy shims of different angle to suit unusual helix angles - not cheap, but you could probably make your own. Also have a look at Greenwood Tools:
 
 
They supply toolholders with the option of changeable shims.
 
David
Ian P03/02/2012 15:08:45
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David
 
I had seen the MSC notes on screwcutting a while ago and it is a good guide. Greenwood mention the shims but do not show or price them on the website but having just seen the prices of their tips I can imagine the shims are really expensive,
 
I ordered some ER and NR16 tips from APT in Glasgow and they were under £5 each. To my eye they look OK but the proof will be when I use them.
 
On the Greenwood site it easy to see how the insert is tilted quite a bit (to give the correct top rake?). The loose shims must then have a sideways tilt to suit the pitch. I get the impression that toolholders have an inherent tilt for pitch which is why they are designated to cut LH or RH threads. To me it would seem logical to supply the holder with no tilt for pitch but maybe thats not good for business!
 
Ian P
Clive Hartland03/02/2012 16:18:44
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2929 forum posts
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I think the term LH and RH in regard to the these tips is because the leading part of the tip are only part thread form so you need both for LH and RH threads.
 
Clive
Ian P03/02/2012 19:10:24
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Clive
 
Some manufacturers tip holders are definitely handed (helix angle wise) so in addition to internal and external the range of tooling is immense. The business corner of these triangular tips is on the left or right depending on whether its for internal or external use but I was not thinking of that when I mentioned LH/RH.
 
 
Ian P
 
 
 
 

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