screw cutting speed and angle

[mick70Participant @mick70]

what sort of rpm should i be looking at for screwcutting?

its a clarke cl430, and at lower end i can have 160, 250 or 400 rpm.

just treat myself to some hss tools from arc so want to have a go.

also do i have tool at 90deg to steel or slight angle?

any other tips appreciated.

thanks.

[mick70Participant @mick70]

[Keith LongParticipant @keithlong89920]

Unless you've got the reactions of a top fighter pilot (and then some) you'll need to slow the lathe down by a factor of at least 6 from it's lowest speed! Screw-cutting under power is normally done in back gear at about 25 rpm unless you've got a fancy clutch on the machine which will disengage the drive automatically.

You need to have a look and find a design for a handle to fit to the lathe mandrel so that you can rotate the mandrel by hand. The loads during screw cutting aren't particularly high so you can easily swing the lathe by hand. The problem with the speeds that you've got is that even at the lowest speed and cutting say a 16tpi thread the tool will be travelling across the work at the rate of 1/6 inch per second – and you'll have to retract the tool BEFORE you crash into the shoulder, flange, chuck or whatever is in the way when the thread stops!

Edited By Keith Long on 02/04/2015 17:15:26

[BazyleParticipant @bazyle]

While you are thinking of making the handle you can turn the chuck by hand using the key for a lever. It is only a few revolutions normally though tedious still do-able

The tool is at 90 degrees but the topslide can be normal or at half the thread angle depending on preferance. Much discussion on the forums about this already so no point in adding to it. Check the angle of your tool tip. I expect it is 60 degrees so suitable for metric and UNC american threads but will be a poor fit for Whitworth.

[Keith LongParticipant @keithlong89920]

You'll get a lot of information about the pros and cons of tool angle as well as just about all you'll want to know from reading "Screwcutting in the Lathe" by Martin Cleeve. Published in the Workshop Practice Series – number 3. It's a very good source of information and covers change wheel set up in detail including the "approximations" for cutting metric threads on an imperial lathe and vice versa.

Edited By Keith Long on 02/04/2015 17:44:22

[Thor 🇳🇴Participant @thor]

I second Keith's advice about about WPS #3, if you doesn't have that book in your collection there are some websites that may be of interest:

*** Link *** (downloadable PDF) or

*** Link ***

Thor

[Danny M2ZParticipant @dannym2z]

Posted by naughtyboy on 02/04/2015 16:43:25:

what sort of rpm should i be looking at for screwcutting?

its a clarke cl430, and at lower end i can have 160, 250 or 400 rpm.

just treat myself to some hss tools from arc so want to have a go.

also do i have tool at 90deg to steel or slight angle?

any other tips appreciated.

Get yourself one of these – worth their weight in gold.

**LINK**

* Danny M *

[pgk pgkParticipant @pgkpgk17461]

Don't be totally put off by the above. Setup your lathe at it's slowest speed without stock or tool – for a finer pitch thread – say m4 fine pitch 0.5mm. At 160 rpm you'ld be cutting 8cm a minute.

Now a minute is a long time. 10 seconds is quite long when on the ball – only 1 and a bit cm. Get used to how fast the carriage travels etc starting well away from the chuck.

For actually having the satsfaction of cutting a thread for real choose a long bar, fine cuts, tail stock support if needed or a large diameter bar and ignore pitch theory for the diameter you chose. Give yourself a good length to drop your thread end into too. In other words set everything up to be favourable.

For a newbie like me there is satisfaction just in cutting a thread.. even if the piece I cut is of no use for any project – just a practice piece. Reflexes and muscle memory apply. As your body gets used to following the rhythm of that carriage moving towards the bar and starting cutting it will get in tune and you can cut a short length of thread. Don't be afraid to stop short of the end of the thread and do the last few turns by hand.

[Clive FosterParticipant @clivefoster55965]

If you have adjustable feed dials on your lathe the best screwcutting technique in my view is the one I know as the Zero to Zero method in which the top slide is set at approximately half the thread angle off perpendicular to the work and the lathe feed dials used to calculate the required in-feed. Probably the major advantage, especially for the novice worker, is that you know exactly what you have done and where you are when finished so if the thread doesn't fit or is too loose, regrettably all too common occurrences when starting out, its relatively easy to figure out what needs to be changed. You finish with both cross and top slides on zero having started by setting to book values of thread depth so all the clever stuff is done before you start. No tedious keeping track of where you are as you go or remembering what funny number to finish on. When you hit zero on both slides and have worked out all the spring you are done. Its also inherently tolerant of set-up errors, the top-slide angle off merely needs to be somewhere close, mine lives at 25° angle off for various reasons but does both 55° and 60° just fine without moving.

Basic method is:-

1) Turn work to specified thread outside diameter

2) Set topside to approximately half the thread angle off perpendicular to the work

3) Use thread gauge to set tool perpendicular to the work

4) Bring tool tip up to touch the work ensuring that you have adequate travel left on both top and cross slides.

5) Set both cross and top slide dials to zero

6) Move the saddle sideways to clear the work

7) Feed the cross-slide forwards by the specified thread depth

8) Reset cross-slide dial to zero

9) Draw back the top-slide enough to just barely touch the work

10) Make a light witness cut leaving a scratch just plain enough to verify that you have set the right thread pitch or TPI

11) Withdraw the cross slide sufficiently to clear the work

12) Run the saddle back to the start point

13) Set the cross slide back to zero

14) Apply a suitable cut using the topside

15) Take cut

16) Repeat steps 11 to 15 until both slides read zero and successive spring cuts at the same setting stop removing material

17) Test fit of thread.

If the new thread doesn’t fit properly and no silly errors have been made odds are its due to the tool tip shape not being exactly as to the book. Book thread depths assume that the tool tip is either rounded or squared off exactly as specified for that thread TPI or pitch. Usually home worker tools are too sharply pointed to the tool will be a bit too long and needs extra indeed to compensate. Such compensation is best done by adding extra feed on the cross slide leaving the top slide set on zero. Take successive cuts with it moved steadily further past zero until the thread size is correct. If you have several threads to do probably best to adjust the zero to the new setting automatically incorporating compensation.

Clive

[Neil WyattModerator @neilwyatt]

I usually cut at about 100 rpm under power.

Neil

[Neil LickfoldParticipant @neillickfold44316]

Since I have installed the VFD, with decelleration at 0.1 seconds, I now screwcut at 200-400 rpm. depending on pitch. The 0.5 to 32tpi is around 400 rpm, but 1mm and 1.25 I slow down to 200 rpm or so. I always have a runout area when threading. I look at the hand wheel position, and turn it off when I see the reference mark. It is very consistent system for me. I often stop within 0.2mm on the wheel. I dry practice if I have not threaded in a while just to get the timing right,especially on internal blind holes.It is not for everyone not looking at where the tool is to stop a lathe I must admit. I had an idea to put a microswitch onto the saddle. Have all the bits, just have not mounted and installed it yet.Neil

[colin hawesParticipant @colinhawes85982]

I cut screw threads at the fastest speed I comfortably can within my reactive ability. This means I can use a higher speed for fine pitch threads than for coarse ones. If I have to cut close to the bottom of a hole I mount a dti on the bed, usually on a mini magnetic base, and finish the thread cut by stopping the motor before the end and handwindind the countershaft pulley to the dti set at zero. Colin

[Author]

Posts