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Thread cutting with carbide inserts

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Nige28/07/2018 21:09:34
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370 forum posts
65 photos

I have a holder and carbide inserts for thread cutting on the lathe. It struck me that the usual way with carbide insert tools seem to be to drive them hard and fast but that is not how we cut threads on a lathe where we take lighter cuts at MUCH lower spindle speeds so how are carbide tools used in thread cutting on the lathe in our sort of workshops and with machines like the SC4.

richardandtracy28/07/2018 21:52:51
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938 forum posts
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I have carbide inserts fror thread cutting and hand wind the spindle on my Warco WMT300/1. Feed in 0.025mm every cut. Works on steel, acrylic and aluminium. Thread like this with the work smeared in Rocol RTD cutting paste. No complaints so far from the tip or work.

Regards

Richard.

Leszek Delag28/07/2018 21:58:02
13 forum posts

I also have an external thread cutting tool with carbide inserts and from my own limited experience have found that these are not really suitable for non-CNC lathes. You are quite right about ‘fast’ but not about ‘hard’, though doubtless those more experienced will have found a way.. If your tool inserts are of full external thread form, they are best used by applying the cut down half of the included angle, using multiple, equal cuts at high speed. For say 1/2” diameter BSF threads, you would need about 20 passes or more at about 750 rpm with coolant to prevent the metal tearing and giving a rough finish. The big problem will be with leadscrew disengagement at high speed. Cutting into a shoulder might be courting disaster! If there is a big enough area beyond the threaded part, to give you a safe disengagement, these tools are fine, but you really have to be on the ball when using them manually. Sorry I cannot be of more help.

duncan webster28/07/2018 22:33:12
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2232 forum posts
32 photos

I use HSS for screwcutting, and much else. Carbide seems like a waste of money for a lot of work, fine if you want big cuts at high speed, but then you need lots of power and rigidity, which most hobby lathes don't have.

Even using HSS I wouldn't take anything lie 20 passes to cut 1/2" BSF, it's only 60 thou deep

pgk pgk28/07/2018 22:53:20
1453 forum posts
282 photos

How about the option of inverting the tool/reverse the lathe and running the thread away from the headstock... whatever speed you like.

pgk

Neil Lickfold28/07/2018 23:08:16
568 forum posts
102 photos

Full form carbide thread cutting inserts are just great. If turning the lathe by hand, like some do, it is essential, that at no point do you move it backwards while it is engaged in cutting the thread. I just have a small run out area and use now use a micro switch on the VFD to stop the spindle when the tool is in the relief area. I also cut the area to be threaded about 0.1mm bigger on diameter. Us a marker pen for colour. I keep cutting until the marker pen is gone, measure the OD of the threaded area, eg 10.05mm and then make another pass at 0.1mm so it makes the OD of the form 9.95mm for a M10X1.5 thread. Works perfect every time. On threads smaller than 6mm I use -0.03mm and on threads smaller than 3mm I make them -0.01mm, so M2 and M2.5 I make them -0.01mm , for M3 -0.02mm , M4-M6 -0.03mm, above M6 , make -0.05mm , all are on diameter references. I buy the full form ground inserts for home. They will cut as hard as 58Rc and will cut all materials I need to cut, The inserts I use for threading Titanium, I only use on Titanium and nothing else. Thread cutting at 90 to 300 rpm is just fine, on hobby lathes. Use a good cutting fluid suitable for the material you are cutting also makes a difference. I either cut slow and dry , or with a flood of fluid to wash away the chips and it gives a good thread form finish. There are charts for carbide inserts for the number of passes in thread cutting. It is like about 4 to 6 passes, depending on the thread depth and pitch etc.

JohnF28/07/2018 23:08:29
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864 forum posts
102 photos

I use both HSS and full form tips, both work very well and the carbide is fine at around 100rpm, one thread I cut a great many of is 1/2 UNF never counted but nowhere near 20 passes - never broken a tip YET !! I also cut quite a few metric specials all 1mm pitch all using a full form tip.

Having said that tips would for sure be better at much higher speeds but don't be afraid to use them, you will get good clean cut threads and there is no need whatsoever to cut at half angel a straight plunge cut is fine and you will get a good finish. I usually finish with a couple of 1 thou cuts and maybe a spring cut plus a calliper gauge or thread mic for size.

Pics in my albums workshop stuff John

Pete Rimmer28/07/2018 23:15:38
424 forum posts
18 photos

There's no compulsion to use carbide at high speed if the circumstances prevent it. Thread as slow as you like, but beware you might suffer broken edges on the tip of the insert especially if you stop the spindle with the tool in the cut or don't have a groove to run into. I've been cutting 1.5mm threads today at 200rpm with a hand-ground carbide tool (ground from a busted end mill) and I've had to re-grind it several times after breaking the edge. HSS wouldn't stand up to it at any speed in this material. The task doesn't allow for a groove so I've been manually retracting the cross-slide and a couple of times I've missed-timed it then off came the tip.

Mark Rand29/07/2018 01:34:40
761 forum posts

The Hardinge HLV manual suggests that one limits the lathe to no more than 1000 rpm for threading. I've found that fits with the threading I do. I use partial form and full form carbide inserts for nearly all threads, they are cheaper than the time needed to grind an HSS tool for a given thread. The single tooth clutch between the lathe mandrel and the gearbox, controlled by adjustable stops, combined with a retracting topslide, make threading a matter of trained reflexes in putting successive cuts on rapidly rather than worrying about crashing into a flange or chuck.

For those with a Myford and a bit of spare time, it's well worth looking into the work of Martin Cleeve and Graham Meek on single tooth screwcutting clutches.

One thing that really winds me up with thread cutting inserts is that the internal inserts and their holders are pretty much all far too big to cut any standard thread, even a fine one.

John Reese29/07/2018 04:03:24
782 forum posts

I have many HSS bits ground for screw cutting. Some are 60* V and some are 29* Acme in several pitches. I also have holders for on edge carbide inserts as well holders for lay down inserts. I use the on edge inserts the most for external threads. It is a matter of convenience. I use the lay down inserts for internal threading if the hole size is large enough. I have form ground small boring bars for threading smaller holes. There are some instances where I have to go back to HSS . Examples are 3/4-4 Acme internal threads or 1 1/2-8 and 1 1/4-7 internal V threads.

JasonB29/07/2018 07:07:15
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16269 forum posts
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Run it out in reverse with the tool behind that way you don't have to react too quickly when you get to the end of the cut as there is only the tailstock to worry about.

Think I did this one at 5-600 rpm

not done it yet29/07/2018 07:14:06
3358 forum posts
11 photos
Posted by pgk pgk on 28/07/2018 22:53:20:

How about the option of inverting the tool/reverse the lathe and running the thread away from the headstock... whatever speed you like.

pgk

Running in reverse is OK, as long as the chuck is not threaded on the spindle.

Old School29/07/2018 07:19:23
254 forum posts
8 photos

JasonB. Where do you buy your carbide cutters and holders from ?

Raymond Anderson29/07/2018 07:20:10
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723 forum posts
140 photos

Vargus GENius software is available [ free ] for either thread turning or thread milling. Tells you all you need to know about your chosen profile. You can specify the number of passes you want to make and it gives you all the depths ect Choice of Flank infeed or Radial infeed, cutting speed, rpm, ect ect. Although it is aimed at CNC it is so versatile it can be used for manual machines. I use it constantly, wouldn't be without it.

JasonB29/07/2018 07:30:57
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Posted by Old School on 29/07/2018 07:19:23:

JasonB. Where do you buy your carbide cutters and holders from ?

That one came from JB Cutting Tools at one of the shows.

Can also be done with threaded chucks if you have a retention method fitted.

HOWARDT29/07/2018 08:36:55
452 forum posts
14 photos

Sandvik recommend using a normal full form tip then finishing with threading insert. This reduces the chance of chipping the tip, alternative is to take multiple small depth cuts. Initial material diameter should be a thou or so over size to finish the full form.

Old School29/07/2018 08:49:29
254 forum posts
8 photos

Jason, Thanks for the info, I won't have a problem with the chuck unscrewing Myford 254s. I am going to try and make some taps to hopefully solve a problem with leaking taper seat glow plugs on our racing. The glow plugs threads are undersize and this might be part of the problem. I am going to make undersize taps as part of the solution.

Neil Lickfold29/07/2018 09:19:20
568 forum posts
102 photos
Posted by Old School on 29/07/2018 08:49:29:

Jason, Thanks for the info, I won't have a problem with the chuck unscrewing Myford 254s. I am going to try and make some taps to hopefully solve a problem with leaking taper seat glow plugs on our racing. The glow plugs threads are undersize and this might be part of the problem. I am going to make undersize taps as part of the solution.

The problem you have is that the thread needs to be concentric to the taper seat in the head insert. If you make your heads, bore the inner diameter for the thread form, and turn the taper seat angle as well. Then use a small threading tool to screw cut the thread form. When it is mostly thread cut, you can use a tap to act as a thread chaser. If the tap is run just straight down the drilled hole, often the tap does not start concentric to the hole. I also found, that if the taper seat angle is made a very small amount, tighter on the front, over the nominal angle. I make the Nelson taper seat 34.75 deg instead of 35 deg. With the Turbo plugs, I make the taper 60.5 deg included angle. With bearing blue, the plugs will seal on the very front of the taper seat. Some glow plugs the seat is not concentric to the thread form. Those plugs will always have sealing issues. You can also correct a head, by making a precision thread form and use that to get the head insert concentric to the thread. Then rebore the taper seat.

SInce setting up the myford S7 with a VFD and a micro switch to stop the spindle, it has made doing internal threading quite easy, with confidence to internal thread cut.

John C29/07/2018 10:00:56
243 forum posts
87 photos

Nick L - please could you give details of your microswitch/ VFD set up? It sounds interesting.

John

Brian H29/07/2018 10:52:50
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1222 forum posts
92 photos

A friend of mine used to work at the local Dormer factory but then it closed down. Anything that was not new, unused and boxed was thrown into a skip!

My friend rescued some master threadcutting equipment that had been used many years before to manufacture tap & dies and he gave them to me.

The 3 tool holders, including the round home made one, hold the 'inserts' which have a common spigot but different forms on the cutting end.

The boxes of chasers include UNF, MM, light bulb threads, bottle threads and something called GKN threads, which seem to be Whitworth form.

I don't use them very often but they are VERY useful at times. I dread to think how much they cost to make but they cannot have been cheap.

Brian

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