PARTING ON THE LATHE
The planer or planing machine is a machine tool, which like the shaper produces flat surfaces in horizontal, vertical or inclined plane. The fundamental difference is that the planer operates with an action opposite to that of the shapers, i.e., the work piece reciprocates past one or mare stationary single point cutting tools. Slotting or keyway making is the operation generally carried on a shaper machine but by using Slotting attachment, this operation is possible on lathe machine. You can read the Slotting attachment project here.
Introduction
Parting on the lathe is simple operation in theory, but requires close attention to detail for success.Failure to heed the following tips can result in destroyed parting tools, ejection of the part from the spindle, and permanent damage to the X-axis power feed on the lathe, so when practicing, pay very close attention to what you are doing and have Mike or a senior TA supervise the first time you try.For those who like to take notes as you read, here’s a condensed pdf version of this document.
Step 1: Part-off Tool Selection
As shown in figure 1, there are generally two types of part-off or grooving tools: HSS and tungsten carbide.As usual, HSS is cheaper, tougher, and can be reground by hand once damaged.Carbide can tolerate much more heat, but once the insert is damaged it must be replaced with a new one.
Figure 1: HSS part-off tool (left) versus indexable part-off blade (center) and carbide insert (right).
There are many different styles of part-off inserts depending on the type of material being cut and insert effectiveness.Most styles are available in left hand, right hand, and neutral axial rakes depending on where you want the inevitable little cylindrical “pip” to remain (on the work clamped in the chuck or on the drop).In addition, carbide part-off inserts are available with high temperature coatings for use cutting ferrous and abrasive materials.
Figure 2: Carbide parting inserts are available with different rake angles and coatings like titanium nitride (TiN) for use cutting ferrous materials.
Step 2: Part-off Tool Inspection
Inspect the parting tool closely before using.If using HSS parting blades, grind and/or hone the cutting edge to ensure it’s sharp.If using an indexable part-off tool, check that the cutting insert is in good condition.
Figure 3: Video on how to properly sharpen and use HSS parting blades.
Step 3: Maximize Cutting Tool Stiffness.
Always maximize cutting tool stiffness by minimizing the length the blade sticks out beyond the toolholder block.For parting larger diameters or stronger workpieces, it will be necessary to adjust the stickout multiple times during the parting operation, as the tool cuts deeper into the material.For the same reason, always part-off as close to the chuck jaws as possible (typically within 1/8”) to maximize part stiffness during parting, as illustrated in figure 7.
Figure 4: Minimize the distance the part-off blade sticks out of the toolholder to increase tool stiffness during parting.Note the clearance between the toolholder and the part is also minimized.
Figure 5: Compared to Figure 4 the part-off blade is sticking out farther than necessary for this 2” round stock, decreasing tool stiffness.The blade should be adjusted shorter.
Figure 6: To adjust the part-off blade length for a particular cutting depth, use an Allen wrench to loosen the screw in the back of toolholder which loosens the wedge at the front of the toolholder and allows the part-off blade to be repositioned at a different length.
Figure 7: It is better to perform part-off operations closer to the chuck (right) because of the decreased deflection when parting. Within 1/8” of the chuck jaws is preferable.
Step 4: Align Part-off Tool
To function, the part-off tool must be perfectly aligned with the X-axis on the lathe.This can be done several ways; the most common two are using a 1-2-3 block or a dial indicator, as shown in Figure 8.
Figure 8: To ensure the parting tool is perpendicular to the face of the chuck you can use a 1-2-3 block (whose surfaces are ground to be parallel to one another).Loosen the tool post, align the edge of the toolholder flush with the 1-2-3 block, and re-tighten the tool post.Be careful to not drop the 1-2-3 block.
Figure 9: To ensure the parting tool is perpendicular to the face of the chuck you can alternatively use a dial indicator.Attach the dial indicator to a Noga holder found in the metrology cabinet, and place gently on the guideways (DO NOT slide the holder along the guideway; simply set it down in place!)Position the contact tip against the toolholder and move the cross slide along the X-axis while checking to see the change on the indicator.+/- 0.001” over the length of the toolholder or part-off blade is acceptable / desirable.
Step 5: Check and/or Set the Vertical Height of the Tool
Like all tools used on the lathe, the part-off tool height must be checked and adjusted so it is as close to the vertical spindle centerline as possible.Never assume the tool is setup properly just because it was in the lathe cabinet for the machine you’re using!If the parting tool is not on vertical centerline it will fail to cut properly, and will be damaged in use.
Figure 10: Just like the turning/facing tool, the parting tool has to on the vertical centerline using the lathe gage (bubble level) or ruler technique.
Step 6: Feed by Hand with Lots of Oil
When parting on a manual lathe, NEVER use the automatic feed (because you can’t feel poor chip evacuation like you can by hand), and be sure to use lots of oil because the parting process has a wide width of cut and generates a LOT of heat.For this reason, the part will also be VERY hot after parting.
Like drilling, use pecks to break up the chips and apply more oil to the tip of the parting tool.
Figure 11: Ensure that throughout the cut oil is being used to reduce the heat in the cutting zone (left), if the chip starts to coil or “crunch”, back out of the cut then reengage the workpiece.This should break the chip (center).The parting tool should leave a fairly smooth surface finish (right).
Miscellaneous Notes
Part-off Surface Speed
When parting on a manual machine, the parting tool should generally be run at approximately 60% of the recommended surface speed for the same workpiece/cutting tool materials (like all manual machining operations).The Feeds and Speeds resource document has an example of parting that will be repeated here for convenience:
Calculate the speeds for parting off 1” diameter aluminum and 1” diameter mild steel workpieces on the lathe using a standard carbide part-off inserts.
First, lookup the recommended surface speeds in Table 1 for a HSS part-off tool (VALUM ≈ 250 ft/min, VSTEEL ≈ 100 ft/min).It’s often better to use the HSS value even when parting with carbide inserts on a manual machine because parting is a problematic operation for inexperienced users.
Next, calculate the spindle speeds:
NALUM [rpm] = 12 × V / (π × D)
= 12 in/ft × 250 ft/min / (π × 1 in/rev)
≈ 950 rpm
NSTEEL [rpm] = 12 × V / (π × D)
= 12 in/ft × 100 ft/min / (π × 1 in/rev)
≈ 380 rpm
Note: since applying oil manually, scale the speeds back to 60%, so NALUM ≈ 570 rpm and NSTEEL ≈ 230 rpm (final answer).Note these are MAXIMUM values and lathe chuck safety must take precedence; spinning the lathe chuck at 570 rpm is about the upper limit of what we safely do in the lab, so for smaller workpieces, do not exceed 600 rpm, regardless of the calculation results.
Catching Parts
Figure 12: Parts with thru-holes can be safety caught using an undersized drill bit.NEVER try to catch a part by hand!Be careful not to run the part-off tool into the drill bit.If there is no hole in the part, place a rag under the part to soften its landing.
Here is a good video on parting tool basics using HSS blades.
Various operations are performed in a lathe machine other than plain turning. These are:-
- Turning
- Straight turning
- Step turning
- Drilling
- Reaming
- Boring
- Tapping
Facing
Facing is the operation of machining the ends of a piece of work to produce flat surface square with the axis. The operation involves feeding the tool perpendicular to the axis of rotation of the work.
Turning
Turning in a lathe is to remove excess material from the workpiece to produce a cylindrical surface of required shape and size.
Straight turning
The work is turned straight when it is made to rotate about the lathe axis and the tool is fed parallel to the lathe axis. The straight turning produces a cylindrical surface by removing excess metal from the workpieces.
Step turning
Step turning is the process of turning different surfaces having different diameters. The work is held between centres and the tool is moved parallel to the axis of the lathe. It is also called shoulder turning.
Chamfering
Chamfering is the operation of bevelling the extreme end of the workpiece. The form tool used for taper turning may be used for this purpose. Chamfering is an essential operation after thread cutting so that the nut may pass freely on the threaded workpiece.
Grooving
Grooving is the process of cutting a narrow goove on the cylindrical surface of the workpiece. It is often done at end of a thread or adjacent to a shoulder to leave a small margin. The groove may be square, radial or bevelled in shape.
Forming
Forming is a process of turning a convex, concave or any irregular shape. For turning a small length formed surface, a forming tool having cutting edges conforming to the shape required is fed straight into the work.
Knurling
Knurling is the process of embossing a diamond shaped pattern on the surface of the workpiece. The knurling tool holder has one or two hardened steel rollers with edges of required pattern. The tool holder is pressed against the rotating work. The rollers emboss the required pattern. The tool holder is fed automatically to the required length.
Knurls are available in coarse, medium and fine pitches. The patterns may be straight, inclined or diamond shaped.
The purpose of knurling is
- to provide an effective gripping surface
- to provide better appearance to the work
- to slightly increase the diameter of the work
Undercutting
Undercutting is done
- at the end of a hole
- near the shoulder of stepped cylindrical surfaces
- at the end of the threaded portion in bolts
It is a process of enlarging the diameter if done internally and reducing the diameter if done externally over a short length. It is useful mainly to make fits perfect. Boring tools and parting tools are used for this operation.
Eccentricturning
If a cylindrical workpiece has two separate axes of rotating, one being out of centre to the other, the workpiece is termed as eccentric and turning of different surfaces of the workpiece is known as eccentric turning. The distance between the axes is known as offset. Eccentric turning may also be done on some special machines. If the offset distance is more, the work is held by means of special centres. If the offset between the centres is small, two sets of centres are marked on the faces of the work. The work is held and rotated between each set of centres to machine the eccentric surfaces.
Taperturning
Taper
A taper may be defined as a uniform increase or decrease in diameter of a piece of work measured along its length.
Taper turning methods
- Form tool method
- Compound rest method
- Tailstock setover method
- Taper turning attachment method
- Combined feed method
Form tool method
A broad nose tool is ground to the required length and angle. It is set on the work by providing feed to the cross-slide. When the tool is fed into the work at right angles to the lathe axis, a tapered surface is generated.
This method is limited to turn short lengths of taper only. The length of the taper is shorter than the length of the cutting edge. Less feed is given as the entire cutting edge will be in contact with the work.
Compound rest method
The compound rest of the lathe is attached to a circular base graduated in degrees, which may be swiveled and clamped at any desired angle. The angle of taper is calculated using the formula
The compound rest is swiveled to the angle calculated as above and clamped. Feed is given to the compund slide to generate the required taper.
Tailstock setover method
Turning taper by the setover method is done by shifting the axis of rotation of the workpiece at an angle to the lathe axis and feeding the tool parallel to the lathe axis. The construction of tailstock is designed to have two parts namely the base and the body. The base is fitted on the bed guideways and the body having the dead centre can be moved at cross to shift the lathe axis.
The amount of setover – s, can be calculated as follows
The dead centre is suitably shifted from its original position to the calculated distance. The work is held between centres and longitudinal feed is given by the carriage to generate the taper.
The advantage of this method is that the taper can be turned to the entire length of the work. Taper threads can also be cut by this method.
The amount of setover being limited, this method is suitable for turning small tapers (approx. upto 8°). Internal tapers cannot be done by this method.
Slotting Operation On Lathe Machine Operator
Taper attachment method
Slotting Operation On Lathe Machine Tools
The taper attachment consists of a bracket which is attached to the rear end of the lathe bed. It supports a guide bar pivoted at the centre. The bar having graduation in degrees may be swiveled on either side of the zero graduation and set at the desired angle to the lathe axis. A guide block is mounted on the guide bar and slides on it. The cross slide is made free from its screw by removing the binder screw. The rear end of the cross slide is tightened with the guide block by means of a bolt. When the longitudinal feed is engaged, the tool mounted on the cross slide will follow the angular path as the guide block will slide on the guide bar set at an angle of the lathe axis. The depth of cut is provided by the compound slide which is set parallel to the cross-slide.
The advantage of this method is that long tapers can be machined. As power feed can be employed, the work is completed at a shorter time. The disadvantage of this method is that internal tapers cannot be machined.
Combined feed method
Feed is given to the tool by the carriage and the cross-slide at the same time to move the tool at resultant direction to turn tapers.
Metal Lathe Operation
Thread cutting
Thread cutting is one of the most important operations performed in a lathe. The process of thread cutting is to produce a helical groove on a cylindrical surface by feeding the tool longitudinally.
Basic Lathe Machine Operation
- The job is revolved between centres or by a The longitudinal feed should be equal to the pitch of the thread to be cut per revolution of the work piece.
- The carriage should be moved longitudinally obtaining feed through the leadscrew of the
- A definite ratio between the longitudinal feed and rotation of the headstock spindle should be found Suitable gears with required number of teeth should be mounted on the spindle and the leadscrew.
- A proper thread cutting tool is selected according to the shape of the It is mounted on the toolpost with its cutting edge at the lathe axis and perpendicular to the axis of the work.
- The position of the tumbler gears are adjusted according to the type of the thread (right hand or left hand).
- Suitable spindle speed is selected and it is obtained through back
- Half nut lever is engaged at the right point as indicated by the thread chasing
- Depth of cut is set suitably to allow the tool to make a light cut on the
- When the cut is made for the required length, the half nut lever is The carriage is brought back to its original position and the above procedure is repeated until the required depth of the thread is achieved.
- After the process of thread cutting is over, the thread is checked by suitable gauges.