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how to set up my QCTP?

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Investigator

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#1
I have my new Bostar AXA QCTP in hand. Tommorow I will be installing it. I am aware of how to cut the T-nut to fit. But how does it need to be set up? Do I set it square with the work? Square with the compound? Does it matter?
 

Bob Korves

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#2
I have my new Bostar AXA QCTP in hand. Tommorow I will be installing it. I am aware of how to cut the T-nut to fit. But how does it need to be set up? Do I set it square with the work? Square with the compound? Does it matter?
Whatever works best for what you are doing. One of the beautiful things about QCTP is how easy they are to swivel to the position that works best for you. How the tool actually will be cutting at different angles is something you will need to study. Make sure that the t-nut cannot pull up until it hits the bottom of the QCTP. It will not be clamped to the compound rest if it is like that. With the tool post tightened down to the compound rest, you should be able to see light between the top of the t-nut and the bottom of the tool post.
 

Dabbler

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#3
Some people position the compound at 0 degrees, 90 degrees and 30 degrees...

The same is true for your QCTP. For an AXA type, most of the time it will be 90 degrees to the centre of rotation, but on some of my tooling, I move it to a 5 degree offset to facilitate better surface finish... Expermentation and experience will teach you the correct setup for your particular lathe. be conservative, be careful, and experiment!
 

wa5cab

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#4
Unless you are doing something that needs a different setting, the normal default setting for the compound is (if you are using carbide or pre-ground HSS cutters) usually 30 degrees. The QCTP is normally set with the CCW clamping face toward the chuck and the CW face parallel to the spindle axis. This is easy to set accurately by temporarily chucking a piece of round bar and advancing the TP against it with the cross feed, while the clamping nut on the TP is just good finger tight. The TP will rotate until its front surface is parallel to the axis. Then you use the wrench to tighten the clamping nut before you back out the cross slide and remove the bar. Because when facing, the carriage should be locked, so you use the compound to advance the cutter. And as SIN 30 = 0.500, if you need to take off 0.010", you advance the compound 0.020". So you can do both turning and facing accurately without needing to adjust either the compound or TP angle.
 

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#5
CW? CCW? I am unfamiliar with these terms, but I think I understand what you mean.

Also, I'm going to have to learn some math. I recognize the term done, but would be unable to define it or determine what it is.
 

Dave Paine

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#6
The two instances when I check that the tool setting is when parting and threading.

For parting the tool needs to be 90 deg to the axis or parallel to the face of the work. You can adjust the QCTP or the compound to get this correct. I adjust the QCTP.

For threading the QCTP also needs to be 90 deg or parallel to the face of the work. If single point threading the compound is set to 30 deg to the face of the work, some prefer 29.5 deg. If you are plunge cutting with the carriage then the tool needs to be 90 deg to the face of the work and the compound just needs to be locked.

I normally have my compound set to 29.5 deg. I have a block of wood with magnet to set the angle since the scale on my compound only goes to 55 deg.

A picture of my block. Note the 29.5 deg is measured from the face of the work. The compound scale on my lathe "0" is 90 deg to the work, so on the scale, I would set this to 60.5 deg - if my scale went this far and was readable.

Angle_block_to_set_threading_angle_7616.jpg
 

wa5cab

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#7
CW? CCW? I am unfamiliar with these terms, but I think I understand what you mean.

Also, I'm going to have to learn some math. I recognize the term done, but would be unable to define it or determine what it is.
Sorry.

CW = ClockWise.
CCW = Counter-ClockWise.
SIN means Sine.
COS means Cosine.

The Sine of an angle varies from 0.000 to 1.000 as the angle varies from 0 to 90 degrees. And the Cosine varies from 1.000 to 0.000 at the same time. At 45 degrees, they are equal. If you start with your compound pointed straight across the bed with its dovetails parallel to the cross slide dovetails, on some lathes (including all Atlas machines) the witness mark on the right side of the compound swivel will line up with 0 on the scale engraved on top of the cross slide. And the scale engraved around the right side of the cross slide compound pivot point would read from 90 on the back side nearest the line connecting the center of the spindle to the center of the tailstock (variously called the axis, spindle axis, machine axis or work axis) down to 0 at the right side witness mark and then back up to 90 on the side nearest the operator. Dave's machine is apparently different, but if you advance either the compound or the cross slide 0.010", the cutter will move 0.010" toward the axis.

If you rotate the compound CCW (counter-clockwise,) so that its back side moves around toward the headstock, and stop and lock the compound when its witness mark lines up with 30 on the scale, advancing the the cross slide 0.010" still advances the cutter toward the axis by 0.010". But advancing the compound by 0.010" moves the cutter both toward the axis and toward the headstock. For 0.010" compound advance with the compound set at 30 degrees as just defined, the cutter will move toward the headstock by 0.00500" and toward the axis by 0.00866".

The practice of setting the compound around to 30 degrees grows out of the fact that the included angle of UNF and UNC and some other threads is 60 degrees. So each side of the thread is laid over 30 degrees from the cross slide dovetails. If you were cutting BSF or BW/BSW (Whitworth) threads, the magic numbers would be 55 and 27.5 degrees. Anyway, only the finest of threads are cut in one pass. So after you have zeroed everything and are ready to start cutting a thread, you advance the compound (not the cross slide) by however much is appropriate for the material and the cutter that you are using and take your first pass. At the end of the pass, you disengage the carriage drive, stop the motor if you want to, back the cross slide out enough that the cutter won't drag on the work, and crank the carriage back to the right, return the cross feed to zero, and advance the compound by however much you are using. And take the second pass.

The reason for setting the compound around to 30 or 29.5 degrees is that the carriage drive system inherently has back lash. Even on a new lathe, if you engage the carriage drive with the motor not running and push and pull on the carriage, it will move a little (or if badly worn, a lot). When turning toward the headstock, only the left side of the cutter is engaged. It takes force to make the cutter bite into the work and the reaction takes all of the backlash out toward the tailstock. By rotating the compound around to 30 or 29.5 degrees, all or most of the cutting is being done by the left side of the cutter, again taking out all of the backlash toward the tailstock. If you advanced the cutter into the work using the cross feed, an equal amount of material would be removed from both sides of the cut. This would be an unstable condition and you could expect chatter and poor surface finish. The reason for using 29.5 (or even 29) instead of 30 degrees is because the scales and witness marks aren't always that accurate. So you want to be removing a little bit on the tailstock side of the "V" to ensure that the thread finishes up as a symmetrical "V".

I see that as usual, I got carried away. Most of you probably know all of this. But I spent the time to write it, so I guess I'll post it. And hope that I didn't make too many mistakes. :apologize:
 
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