Lathe is one of the most basic machining tools.
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Tool bit, height setting importance
Tool bit needs to be corretly positioned when it comes to height or you can encounter following issues:
Tool bit is too low, and therefore it can't reach the center of the work piece.
Tool bit's cutting blade was too high, so tool bit was rubbing the work piece instead of cutting it. This made the work piece heat up and work hardening happened to it.
Tool bit height setting in practice:
One can either put spike to where the work piece normally goes ( spindle nose ) and make sure that the cutting blade is in excatly same level.
Or one can machine the end of work piece and see when adjustment of tool height makes the end flat.
Or one can use live center that is put into tailstock to see the rotation axis ( as live center should/has to be in same height as the spindle nose ).
Using tailstock so see correct position.
Take the tool bit so close to the work piece that it touches the piece:
you will see shining line forming to the piece if work part surface is bit darker. You can zero Z here if you can't take material out from the piece but then just be careful that the tool bit barely touches the part. Otherwise your dimensions will be faulty.
Use X axis to take the tool bit off from the work piece.
Take approximately 0,5mm chip out from the work piece to straighten the end of the work piece, and zero Z to this position.
Moving the tool bit 0,5mm to get material off, so the surface will be aligned with machine coordinate system.
Taking the material off. This new surface is fully straightened and works as new point zero for Z-axis.
This is identical to Z axis, just use other axis. After taking material off, do not touch X-axis and measure the newly cut dimesion. Put this diameter as X axis dimension value.
Here one can see how tool bit has touched the material as there is shining silver line. This position is 40mm for this specific workpiece as the part diameter is 40mm. Zero point is in the middle of the part, and when working with lathe is best to use diameter definition as it makes easier to understand the dimensions and checking the measures. Other way would be to zero the X at the surface of the work piece but that would require constant calculation of how much one has to remove material and how much material is left. One can also use radius dimension but this is not typical except maybe when doing chamfers.
*Holes and threads: *
Start making hole with center drill:
Center drill is short and thick, so it is really stable and therefore accurate. Drill it so deep that about half of the chamfer goes into material. This creates nice centering geometry for the next drill to go into the material, meaning that the drill will be centered because of the shape in the material.
Next use smallish drill, then bigger one and so forth.
When making hole alwas remember to break the chips: take the drill little bit back, so the chip breaks, then drill bit more, take drill back and so forward.
When making really deep holes, take the drill bit really out so that the chips come out from the hole.
After the final hole is done, it's good to chamfer the hole edge with chamfer drill. Remember to always use really small speeds with this, so the chamber drill lasts longer.
Internal threads with tap:
Note: make first chamfer to the hole before using thread tap! This way it aligns and centers better.
With internal threads one can use lathe's tailstock to get the tool aligned with the axis:
Put the lathe gearbox to neutral, so spindle can spin freely. Attach the thread tap to tail stock chuck. Tailstock must not be locked. If the tailstock is lightweighted and moves easily, you can push it towards the work piece gently while also rotating the spindle. If the tail stock is heavy, use feed screw for the move but be careful to not push the tap with too much force.
After the tap is starting to cut material properly and there is no danger for it to get misaligned, deattach the tailstock from the tap, and manually rotate the tap.
For making external thread, the part end should look like this and have narrower part after the needed thread. This is because the tool for making the thread can then create good looking thread as it can go past just the needed thread for the narrower part doesn't touch the tool blades.
Another smaller part that is ready for external thread making.
Most important thing with threads is to get them straight and aligned. In lathe one can use tailstock with keyless chuck to have straight surface to which the thread tool can lean to. This process requires really gentle rotating that keeps the thread tool straight but doesn't forcefully push it directly into the material.
So, one needs to keep the chuck in contact with the tool by rotating tailstock's feed screw and pushing the chuck forward while manually rotating the tool to make the thread.
After the tool is deep enough in the material and thereis no longer danger that the tool would get misaligned, one can push the tailstock to side and just keep manually rotating the thread tool to create thread.
Note: Always use cutting oil when making threads!
Note2: Always break the chip by rotating the thread tool a bit and then rotating it a bit back.