Compound / feed ratio

[JimM]

Hi all

Is there a formula for working out how much the tool will move when the compound is set at a given angle. ie if my compound screw is 10TPi and I have the compound set over at 27.5degrees, how much will the DOC increase with each full revolution of the screw

Thanks

Jim

 

[bearcar1]

Jim, I am no good at math but MKLOTZ (Mike) has several excellent formulas on his website that will perform these functions for you. I own a MAC and cannot run the files but if you have a BigBlue machine (DOS based), have a look see and hopefully you will be calculating the missing variable in your equation. (you already know two of the three sides of the trangle)

BC1
Jim

 

[gbritnell]

If your compound is set at 30 degrees the amount it moves will be 1/2 of the dial reading.
gbritnell

 

[dsquire]

gbritnell

If a 90 degree setting moves it at 100% of the dial reading, then

Would not a 45 degree setting move it at 1/2 the dial reading?

I would think that a 30 degree setting would move it at 1/3 the dial reading.

I would think that a 60 degree setting would move it at 2/3 the dial reading.

Now I could be wrong here and if so someone please show me the error of my ways. ???

Cheers

Don

 

[mklotz]

The name of the program is COMPOUND, although you hardly need the program if you have a scientific calculator (every shop should have one).

If you want the ratio of infeed to compound advance to be R, then you must set the compound angle to:

angle = arcsine (R)

Thus for R=0.5 (George's example):

angle = arcsine (0.5) = 30 degrees

Another popular ratio is R=0.1 so one division on the compound is 0.0001" infeed. Then:

angle = arcsine (0.1) = 5.739 degrees

 

[dsquire]

Maryak mklotz

Thanks for the quick reply and setting me straight. After I replied I started to wonder but by then it was too late. Guess that's what happens when you open mouth and insert foot. :-[

Cheers

Don

Edit: When you're having a bad day sometimes it just gets worse!

 

[dsquire]

If your compound is set at 30 degrees the amount it moves will be 1/2 of the dial reading.
gbritnell

gbritnell

I must apologize for even questioning your answer. I guess I just started answering before engaging brain. A man of talents has no doubt used this feature many times.

Cheers

Don

 

[Maryak]

Marv and George,

If the compound slide is set as below.

Then IMHO the infeed is the cosine of the angle 27.5 deg, 0.887 in the above setting or 0.866 at 30 deg.

Best Regards
Bob

 

[mklotz]

I measure the angle of the compound relative to the spindle axis of the lathe since that's the only line that is well defined.

You can puzzle that out on your own by looking at my equation.

If you want to set the infeed ratio to zero (i.e., no infeed when you crank the compound), the compound must be parallel to the spindle axis. From the equation, we have, with R=0:

angle = arcsine (0) = 0 deg.

so zero deg is parallel to the spindle axis.


Similarly, for an infeed ratio of unity, we have:

angle = arcsine (1) = 90 deg.

so the compound must be perpendicular to the spindle axis, which is, of course, what's required for 1:1 infeed.

Since there's no apparent consistency in the marking of the protractor scales on various lathe compounds, the program outputs the angle as well as its complement, although the latter is easily found with a scientific calculator.

 

[Bernd]

It's amazing how fast this simple answer has become more complicated than it needs to. In Guy Lautard's book Bedside Reader #1 he has a section called "How to put on a mighty fine cut"

He states "The basic dodge is simple and well known: slew the topslide to an angle of 5 degrees, 44 minuets, 21 seconds with the lathe axis: then .001" on the topslide dial will give you .0001" in-feed of the tool tip." So how many have such fine settings on their degree setting of the topslide? He states to set the top slide to a full degree setting to make it easier.

His example of dia. reduction is taking a piece that measures .5447" dia. . The dia. should be .5311" dia. So .5477" minus .5311 = .0136 . Divided by 2 = .0068" Set the topslide at 4 degrees off parallel. Then he recommends taking a very fine cut and remeasure. So now the dia. of the piece is .5439" dia. So .5439 - .5311 = .0128/2 = .0064. Take the sin of 4 degrees that the topslide is set to which is .069,756 and divide it by .0064 which equals .091,748. So feed in the topslide .091" and some. Take a cut and you'll have a final dia. of .5311".

The same can be done on a facing cut except the angle to the topslide is closer to perpendicular with the axis of the lathe and you use the cosine instead of the sine.

Regards,
Bernd

 

[Maryak]

I measure the angle of the compound relative to the spindle axis of the lathe since that's the only line that is well defined.

I am just curious why you think the spindle axis more well defined than the x slide axis. ???

Best Regards
Bob

 

[IanN]

Hi,

You do not need to do any calculation to get the compound infeed required for correct thread depths. Just use this simple method:

1 ) Use thread tables to get the correct major diameter for the thread and turn the OD of your work to this size.

2 ) From thread tables read off the depth of the thread - if this is not in your tables the thread depth is:

Thread Depth = Major Diameter - Pitch Diameter

or

Thread Depth = (Major Diameter - Minor Diameter) / 2

3 ) Set the top slide (compound) over as you are currently doing.

4 ) Adjust the cross slide and the compound so the threading tool tip just touches the work (so it is on the major diameter). Set both slide micrometer collars to zero.

5 ) Move the tool towards the tailstock until it is clear of the work, then advance the cross slide to the full depth of the thread - read this directly off of the cross slide micrometer collar. The tool tip is now in the position it will be in when the thread is finished - the compound collar reading zero and the cross slide collar reading the full thread depth.

6 ) Reset the cross slide collar to read zero - The tool tip is now in the position it will be in when the thread is finished and both the compound collar and the cross slide collar reading zero.

7 ) Leaving the cross slide at zero, back out the tool using the compound until it just clears the work to be threaded. You are now ready to start cutting.

8 ) For each cut, advance the compound a few thou and make the pass down the thread. Back the tool out using the cross slide and return the saddle to the start point, then return the cross slide to the zero position. Advance the compound and repeat. The end point is when you do a pass with both the compound collar and the cross slide collar reading zero.

This method requires no calculation based on the thread angle (so this system works for any thread form). No need to keep track of the depth of feed (just reset the cross slide to zero each pass). No need to remember odd target readings - the end point is when all the dials read zero.

Ian.

P.S. - Regarding Bernd's point about the accuracy of angular settings:

I have a 5 inch sine bar and a set of slip gauges with tenths as the smallest increment - in theory these allow an angle of about 5 degs, 44 mins to be set to an accuracy of 4 seconds of arc:

sin(5 degs 44 mins) = 0.0999 approx.
For 5 inch sine bar, height difference = 5 * 0.0999 = 0.4995 approx

With sine bar set with height difference = 0.4995 inch:
arcsin(0.4995/0.5) = 5.733412

With sine bar set with height difference = 0.4996 inch:
arcsin(0.4996/0.5) = 5.734563

Angular difference = 5.734563 - 5.733412
= 0.001151 deg
= 0.06906 minutes of arc
= 4.144 seconds of arc.

As I said, this is just theory - there are far more significant errors in my sine bar, lathe, etc, and my workshop is not sufficiently thermally stable to attempt such angular precision.

Ian.

 

[mklotz]

It's amazing how fast this simple answer has become more complicated than it needs to.

I fail to see how anything has gotten complicated. The OP asked a simple question...

Is there a formula for working out how much the tool will move when the compound is set at a given angle.

and was told that the ratio of infeed to compound advance (R) is given by a very simple equation:

R = sin (angle)

What's so complicated about that?

 

[Bernd]

What's so complicated about that?

Not a thing Marv. It was just my choice of words.

Bernd

 

[Maryak]

Hi,

You do not need to do any calculation to get the compound infeed required for correct thread depths. Just use this simple method:

Ian,

Bloody brilliant. :bow: :bow: Gets a tad more complicated if you don't have adjustable dials on your cross and top slides.

Best Regards
Bob

 

[IanN]

Hi Bob,

Gets a tad more complicated if you don't have adjustable dials on your cross and top slides.

As the slides are advanced to a target rather than being used for measurement, this is easy to solve:

1 ) Turn the OD of the job to the major diameter.

2 ) Turn a short length of the job at the tailstock end to the minor diameter.

3 ) Set over the cross slide and mount the tool for threading.

4 ) Advance the tool tip to touch the minor diameter section of the job - the tool is now in the finish position.

5 ) Place a piece of tape across the feed wheel and body of the cross slide.

6 ) Slice tape along edge of cross slide body with a razor blade.

7 ) The edges of the tape form the zero reference.

8 ) Repeat process with top slide.

You now have two zero marks.

Ian.

 

[Maryak]

Ian,

Your not just a pretty face are you.

:bow:

Best Regards
Bob