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IndustryArena Forum > MetalWorking > MetalWork Discussion > Question on compound rest angle in lathe
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  1. #1
    Join Date
    May 2013
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    455

    Question on compound rest angle in lathe

    Hey everyone, pretty new here.

    I have been trying to learn how to properly use the lathe, and watching lots of videos and reading.

    One thing I always see, but don't understand is that it seems like people always use the lathe with the compound tool rest at an angle other than 0.

    Can someone explain why this is?

    It seems like if your just making a face cut or a turning cut, having the cutting tool at 90 degrees to the stock would be more efficient, but I almost always see it at an angle.

    Also, can someone explain if there is a standard angle people use, or is this personal preferencs?

    Thanks!

  2. #2
    Join Date
    Dec 2003
    Posts
    24216
    One reason is when threading for single face cutting as the tool is advanced into the thread at the thread angle.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  3. #3
    Join Date
    Feb 2009
    Posts
    6028
    You already have an x axis cross feed, and Z in feed, so no reason to have it parallel with those axis. I believe 29.5 degrees was standard set up for cutting threads, so your in feeding to keep the front of the tool cutting, not dragging the back of the tool by plunging straight in. Some people like a 45 deg for chamfering stock.

  4. #4
    Join Date
    Jan 2005
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    15362
    AVRnj
    One thing I always see, but don't understand is that it seems like people always use the lathe with the compound tool rest at an angle other than 0.


    If you use a bed-stop, you want it set at 0 deg, the reason is you can then ajust to cut the correct length to a shoulder, the only time you need it on a angle is to help with thread cutting, if you have a good lathe & good skills, the set angle for thread cutting is not needed as well

    To be safe machining on a manual lathe you should always use a bed-stop, this way you will never crash into your chuck

    Some lathes are poorly designed, this is another reason that you see people using a lathe with the compound slide set on an angle, is the Tailstock will crash into the compound slide, when set at 0 deg, with it set on an angle, this allows the Tailstock to get closer to the work piece without interference of the compound slide

    I always use the compound slide set at 0 deg , This is the normal & correct way it was meant & designed to be used
    Mactec54

  5. #5
    Join Date
    Dec 2003
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    24216
    Quote Originally Posted by AVRnj View Post
    I have been trying to learn how to properly use the lathe, and watching lots of videos and reading.
    Thanks!
    If you pick up one of the many reprints of 'How to Run a Lathe' by SouthBend lathe works it explains quite a few reasons and methods for using the compound other than 0°.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  6. #6
    Join Date
    Apr 2006
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    3206
    Quote Originally Posted by mactec54 View Post
    AVRnj

    I always use the compound slide set at 0 deg , This is the normal & correct way it was meant & designed to be used
    Oh really.

  7. #7
    Join Date
    Feb 2009
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    I agree, and your putting all the tool pressure on an acme screw 1/3 the size of the z axis. I've run hundreds of manual lathes, and have NEVER needed a bed stop to keep from hitting a chuck. Infeed angle on threading is very important, even modern CNCs have infeed patterns.

  8. #8
    Join Date
    Jan 2005
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    15362
    underthetire
    NEVER needed a bed stop,

    Then you have never done any serious work on a lathe then

    Infeed angle on threading is very important,

    Not at all, it depends on your machine & how good you are as a machinist, CNC are a different story, we are talking about a manual machine

    It's not so you don't hit the chuck,It is used so you can do repeat cuts at rapid rates, & not have a problem with having a crash, I have one lathe that is always used with the compound slide at 0deg it was made in 1962, I can take up to .5 cuts in steel with this machine , it still has the original screws, & holds .0001 very easy on any job it does, (51 years & still perfect )

    So this has no affect on the screws at all

    All good quality manual lathes have a bed-stop as a standard part for them
    Mactec54

  9. #9
    Join Date
    Sep 2006
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    6463
    Hi, been a Fitter & Turner for 50+ years and I ALWAYS use the compound slide at the 0 deg position.

    Over the years I have never seen anyone canting the compound slide over for normal turning to give the tailstock more room, although it would be practical if you feel the need to do so and your tools were set very close to the (4 way) tool post face.

    The ability to feed the tool in a few hundreths of a mm with the compound slide is one reason why it's not set over, and setting the bed stop for everyday turning is tedious, bad practice and not practical also......swarf tends to get between the bed stop face and the saddle stop face.

    At the same time, I also do not set the compound over for screw cutting, being trained in the "English way" and being able to cut threads with a tool that has positive top rake for roughing and a form tool to CORRECTLY cut the thread to size for finishing.

    This can turn into a long essay on lathe working preferences, so in the end it comes down to mainly personal preference......no advantage apart from tailstock clearance either way, and for screw cutting, the canting over method at half the thread angle was always written as the "American way".

    BTW, if you are using a QCTP (quick change tool post) you won't want to have the compound slide canted over ever.
    Ian.

  10. #10
    Join Date
    Jun 2013
    Posts
    1041
    I use manual lathes on a regular basis and have for atleast 15 years. I almost never set my compound at 0 degrees. It is usually set at 45 for almost everything. The only time I change it is to cut specific angles as needed. To be clear my compound is at 45 but my qctp is rotated so it is 90 degrees in relation to the bed. I use carbide insert cutters and boring bars. The reason for 45 is so I can throw a boring bar on for quick chamfers. Also good carbide threading inserts don't require you to feed in with your compound. I really don't think it matters what angle you set the compund to since I rotate my qctp to 90 from the bed anyways. And I also use a dro so I don't ever set a hard stop I just pay attention to what I'm doing. I have never had a crash. Sometimes ill set a mag base dial indicater on the bed to double check the dro. Almost all the work I do requires tolerances of less than .0005 and it is never a issue to hold that. I think a lot of times preference is more important than opinion.

    Ben

  11. #11
    Join Date
    Sep 2006
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    6463
    Hi, whatever it takes, the end justifies the means.

    When you turn for a living, the options become more rarefied, but the preferences remain with the user.

    I still have leather flat belt drive on my lathe........LOL...... but then I'm retired and just do some turning when I feel like it.....big difference to the commercial arena.

    To put the original posters mind at rest, there is no one way to use a lathe, and whatever you decide to adopt, if it's within the bounds of engineering proper, go for it.......you'll find out by experience what works and what shouldn't, no matter what they tell you.
    Ian.

  12. #12
    Join Date
    Sep 2006
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    6463
    Hi again, it is a little known fact that the compound slide is the root cause of many crack ups when it comes to turning.

    Many people do not have the gibs on them tight enough to resist canting over under load.

    The compound slide places the tool on top of a column that cantilevers the cutting forces down and against the left side of the crosslide dovetail, and causing the right dovetail to lift......working towards the chuck......and so causing deflection.

    You can prove this by placing a dial indicator on the saddle and against the toolpost and applying a side force to the tool post with a block of wood against the chuck.

    This deflection is what occurs when the machine is under load and the tool needs to be held rigid, which is not happening.

    You won't find a compound slide on a turret or capstan lathe, and with my 1-1/4" Taylor turret lathe I use to plunge into the bar material at 400 rpm with a form tool that was 25mm wide.....from the front tool post, and the chip that came off was also 25mm wide, but working with leaded and free cutting steel this soon broke up into small chips.

    Some years ago a friend of mine demoed a parting tool approach he "invented", that had zero front clearance, zero side clearance and zero top rake, 3mm thick and 25mm wide.

    The parting tool was held in a solid block toolpost....the compound slide being removed completely......and the tool at centre height was fed into the job with the auto feed engaged.

    The actual parting tool blade was made from a HSS power hacksaw blade.

    All the cutting forces were directed down onto the saddle through the crosslide dovetails, and no leaning back or sideways was encountered due to the tool holder being a solid block 50mm square with one slot cut in the side for the parting blade.

    A lathe of modest proportions can benefit greatly by removing the compound slide for normal turning operations and having a solid block for the tool post, possibly with QCT holders for convenience.
    Ian.

  13. #13
    Join Date
    Mar 2008
    Posts
    240
    .0001" wow. You must have a miracle lathe or you are a absolute genius. One "zero" to many?
    As far as the angle on the compound goes - up to you. If you do a lot of straight work and you need to adjust by taking an extra few 1/1000" or a extra 2/100mm than keep it on zero. I use a bed stop with a micrometer dial and keep my compound on 29 1/2 deg for threading (the 60 deg thread is determined by the 60 deg threading tool) and 29 deg or 29 1/2 deg from the "straight in feed" is the preferred way of cutting thread. Any 45 deg chamfer (small) I use a hand held tool. Nothing but a 1/4 tool bit in a file handle. If the chamfer is large than I turn my Multifix or set my compound to cut 45 deg. No rule - all depends on your situation and preference. Btw. - if "0" was the "designed" way - than why go thru all the expense of making a compound that can be turned at different angles?

  14. #14
    Join Date
    Mar 2008
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    240
    Handlewanker you have some good points. Experience counts. However I do not agree with the "English way" of cutting a thread which btw. is also used in Germany. They like to feed straight IN and move the top slide a little forward and backward each cut to get the tool bit to cut on one side.They also do not believe in thread dials even so they are available for metric. Rule #1, you never disengage the halfnut. I can still hear those words.
    If your compound is in a fairly good condition than there should not be any problem with cutting at 29 deg.. If your compound is "loose" than there could be a problem at "0" degrees. However all these are personal preferences. What ever makes you happy. Happy Retirement! (same here).

  15. #15
    Join Date
    Sep 2006
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    6463
    Hi Juergen, for screw cutting I normally use a HSS hand ground tool with a top rake, so that it cuts only on the left side as you go towards the chuck, and feeding in with the compound a few hundredths mm for each pass, also with the half nuts always engaged and winding the saddle back in reverse......speed is 100 rpm.

    Once the thread is almost down to depth, I replace the tool with a carbide form tool, flat top, to give full form and feed the tool straight in and a few hundredths mm left and right to clean the thread......speed for finishing is 50rpm with coolant to polish the thread.

    The carbide form tool just cuts a few hundredths mm off each side and you get a "chrome plated" thread finish, dead to form.

    Many years ago I made up a set of thread form tools with carbide tips brazed on, one each for BSW 55 deg left and right hand and one each for 60 deg right and left hand.

    I've never had to do large quantities of threads by manual screwcutting in the lathe, so although it takes a bit of time, the threads are almost perfect.....measured with 3 wires etc.

    Once I did a test to see how fast a thread could be cut manually, and running the lathe (DSG) at 500 rpm with a carbide full form tool it took 3 passes.......scary......you need quick reflexes to drop the half nuts out, and quicker getting them in again ..LOL.
    Ian.

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