When using any machine tool, accuracy is everything. That’s why getting your tools set up and maintained to specific tolerances from the off is vital.
Every machine tool works by holding the cutting tool against the workpiece in a controlled manner. To resist the cutting forces put on it, the machine is designed to take these forces and be strongest where maximum force exists. Therefore the slideways on each tool must be adjusted precisely so they can slide smoothly, without showing slackness under stress.
The Milling Machine
This is especially important on a milling machine as the pressures from cutting can come from all directions, so the slideways have to resist these forces. On a lathe however, most of the cutting forces will be pushed down onto the bed.
On the milling machine there are, like the lathe, three main slideway movements to maintain. These run on the three axis;
- The X axis (side to side)
- The Y Axis (back and forward)
- The Z axis (up and down)
Additionally there is the main rotary movement, which is the spindle.
Adjusting The Mill
Similarly to the lathe, all the slideways on the mill are adjusted using jib strips. These are parts of the slideway that can be moved to reduce clearance. This is all done using screws, which are tightened until the axis is stiff and then backed off slightly, to achieve the correct clearance.
X and Y Axis
Both of these slideways are adjusted in similar ways. The X axis slideway will have a row of screws and locknuts underneath the main table on the front of the machine. It’s best to start with the outer screws on here as they give better control of the slideway alignment. Loosen the locknut and then whilst winding the handwheel tighten the screw until you feel resistance. Back off 5 degrees at most before tightening the locknut.
The Y axis requires a similar adjustment but this time the screws are to the side and underneath the main table. Once again wind the handwheel whilst tightening the screw until resistance is felt. Back off again very minimally and then retighten the locknut.
Perhaps the hardest of the slideways to adjust, the Z axis is also the most important. It is the most important due to its direct effect on the alignment of the spindle.
On the Z axis you have a screw on the underside and topside of the machine head, close to the column. These are attached to a tapered strip that increases/decreases the slideway clearance. Firstly you will have to loosen the bottom screw. Then whilst winding the handwheel, tighten the top screw until resistance is felt and back off approximately 1/2 a turn. Now re-tighten the bottom screw to push the jib strip up to the top screw.
The Machine Spindle
There are two things that you can do to check the accuracy of your spindle;
- Check the spindles slackness – This is done with a dial gauge pressed against the side of the spindle. You then proceed to try and move the spindle side to side – checking the amount of movement. Unfortunately if there is any amount of movement there is little you can do except send the machine back to the company’s servicing department.
- Establish if the spindle is square to the other axis (only required on tilt head machines). To do this the dial gauge needs to be fixed on to the spindle itself (as shown below) with the needle touching the main table and set to zero. The spindle then needs to be turned 180 degrees, and the figure on the gauge noted.
If the reading is slightly out, this can be rectified. Just slacken the clamping bolts and then lightly tap the machine head, noting the dial gauge as you go. Once it’s back to zero, clamp again and retest, just in case the reclamping has moved the head slightly.
Alongside regular adjustments, periodic lubrication is very important. Slideways and leadscrews require lubrication to prevent premature wear and excess friction.
These adjustments to the slideways and spindle will help you get the very best from your milling machine. It should be noted that continued checking of the slideways should be done on a regular basis (depending on how much the machine is used). This is due to them being worn away by the constant movement.