Lash Out! - How to Make a Milling Machine More Accurate

Play is good, but not on a milling machine. There is nothing more frustrating than working on a milling machine, but having too much play in the hand wheels making it almost impossible to do accurate work.

Play in the hand wheels is called "lash". In this Instructable I use a Sherline 5400 mill to demonstrate how to remove lash completely. I will provide a basic overview of the disassembly and reassembly, including adjustment and calibration for zero-lash precision milling on a Sherline 5400 milling machine, which can be easily applied to other small machines as well.

The Sherline has accuracy to 4 decimal places (0.0001") with the correct adjustment, making it a powerful tool - as long as it is adjusted correctly. Eliminating lash is also the first and most important step if you are interested in converting an existing milling machine or lathe to CNC, which requires zero lash.

Step 1: Get Disassembling!

When I purchased mine, both the lathe and milling machine were covered in a thin layer of rust, and were completely unusable due to being badly out of adjustment. I used the machines in this condition, as many small metalworking machines are used, and then I realized it could be better. Here is what I did to make it a lash free and full precision mill again:

Step 1.0: remove carriage slide shim by loosening the small set screw on the side of the carriage.

Step 2: Remove Slide Shim

Remove slide shim by gently prying the loosened shim retainer pin, using a small tipped slotted screwdriver, and being careful not to mar the machine. This will slide the shim and retainer pin out of it's groove.

This part, for those who are not familiar, is to keep the carriage precision slide sliding smoothly, without any play. As the machine is used, and possibly gets wear, the tapered shim can be pushed in farther into the groove to compensate for wear. It needs to be removed for what we are doing today.

Step 3: Remove Crank Handle on Y-axis Leadscrew

Here we are going to find the hidden hole in the base of the hand wheel, under the zero-adjusting red micrometer tube. Rotate the red part separately from the larger wheel until you see the hole appear under the red micrometer indicator tube. Here is a set screw which you loosen to slide the whole hand wheel off the leadscrew.

Step 4: Slide Leadscrew Out

Here we are going to slide the leadscrew out of the retainer by pushing the whole assembly back, and now we can pull the carriage slide up and off the base of the mill.

Step 5: Disassemble the X and Y Axis Carriage

Here you can see the anti-lash screw, retainer, and removing the hand wheel of the X-axis. There is the main barrel nut, a brass round nut in the center of the slide. This is the main nut, and is held in by a set screw. When properly tensioned and adjusted, this nut sets up tension on the inside of a thread of the leadscrew, and the anti-lash nut on the outside gives a pull on this main inner nut. This pushes on the other side of the threads as the inner nut pulls, causing all play to be removed from the leadscrew assembly. If this is not clear, the pictures should help. Read on:)

Step 6: Remove Shim Block From Carriage Slide

Remove the shim from the slide as before. Only 2-3 turns of the set screw are needed to allow the retaining clip for the shim to be gently pried out with the screwdriver as before.

Step 7: Remove More Parts and Continue to Disassemble the Carriage

Before the X-axis slide can come off the main Y-axis carriage, you need to remove the little indicator barrel. It has the small tick mark that you take measurements from on the main big carriage edge. It also doubles as a carriage lock, if you tighten down the screw that holds it in all the way. That way you can do single axis work without worrying if that will creep as you work.

Step 8: Remove Anti-lash Nut

Here we loosen the phillips screw holding the retaining clip, to let the anti-lash nut get loose. Note: relatively frequent adjustment may be necessary to keep zero-lash performance on your mill. Tightening the anti-lash nut and then retightening the phillips screw and retaining clip is all that is needed. But here we are taking all of the assemblies apart to check wear, clean, and reassemble in tight adjustment.

Step 9: Push Out Main Barrel Nut

Now it is important not to rush this step as you might bend the leadscrew, and that would be very very very bad. Well pretty bad at least. ;)

Push the end of the leadscrew into the table, after you loosen the setscrew for the main barrel nut of course. This will push the rod and the barrel nut out the top. It may be a bit of a bugger if you have not cleaned your machine in a while, as chips and gummy dirt may build up in the hole. Copious amounts of WD40 is an excellent idea. Drown it with the all wonderful liquid, and things are sure to be smooth and easy.

Repeat for the other leadscrew, making sure to loosen the setscrew for the main barrel nut.

When the nuts are out, you can continue to remove completely, or just soak with WD40 to wash any dirt off, and reassemble.

Step 10: Reassemble Carriage Table and Slides!

Once you clean the main barrel nut and leadscrew threads, and are ready to reassemble, the barrel nut needs to be positioned a few inches onto the shaft. Put the anti-lash nut on, and in the correct orientation before you put on the barrel nut! Once cleaned, the main inner barrel nut should slide down it's hole by hand easily, and seat at the bottom of the hole right under the set screw. You can remove the set screw to positively identify the seating of the brass barrel nut.

Tighten the setscrew with positive seating pressure on the leadscrew, holding the barrel nut inside as tight as it will seat as you tighten the setscrew.

Step 11: Anti-lash Nut Tensioning*

Here is the heart of this Instructable: How to tighten the anti-lash nut. I could've made this Instructable just this, but where is the fun in that? Also, I feel a good quality Instructable is not only thorough, it helps you learn what you need to know to apply the knowledge to other things ( like other CNC machines or 3D printers with lash problems)

Tighten the anti-lash nut up into its hole by hand, seating it as hard as you can without tools.

Now, push on one of the teeth of the star with a slotted tip screwdriver, to further tighten it and remove slack in the threads.

What this does is pull against the main barrel nut, causing the anti-lash nut and the barrel nut to pull against each other, keeping the threads always tensioned. When the leadscrew turns, it is either pulling on the anti-lash nut or the main barrel nut. There is never that slack when going from pulling to pushing anymore!!!:D

Now tighten the phillips screw and retaining clip into one of the star teeth in the anti-lash nut to keep it from loosening.

Repeat on the other leadscrew. Remember where the long leadscrew goes compared to the short one.

Step 12: Reassemble Carriage and Slides

Clean with the almighty fluid once again, and then reassemble in the reverse order of disassembly.

Put smaller slide on bigger slide, aligning the long leadscrew with the long main carriage.

Push leadscrew end through the retaining bushing, seating the slide on the side that makes it straight.

Step 13: Put Shim Block Back in Slides and Carriage

Here you can see the shim block, made from a tough, mostly hard plastic. Since it is tapered, as the slides wear, you can push it further into the slot, compensating for wear. Here as you insert the shim block, you want to feel for slide resistance as well as twist play in the carriage. Push the shim too far in, and it will be very hard for the slide to move. Not enough and the slide will have side to side play. So feel it out, and when it is tight but easy to slide, tighten the set screw down on the retaining clip.

Step 14: Assemble the Carriage Back on the Main Mill Base

Put the guide rail on as you took it off, making sure the X-axis hand wheel will come to the right of the machine, and you can see the micrometer tick marks on the carriage. That means it is not on backwards.

Tip down, and seat the carriage in the mill's base grooves. Push the whole carriage to the left, making space for the shim block in the left base v groove.

Install shim block as before, checking for twist play and ease of sliding.

Finally, tighten set screw on retaining clip to keep the shim block from sliding out.

Step 15: Put on Indicator Barrel

Put the indicator barrel on where it came from, but don't tighten down all the way or it will lock the carriage.

Also note the ends of the leadscrews are tapped, with a small screw installed where the CNC adaptor goes instead of the hand wheel. A future Instructable will be on converting this mill to CNC. Stay tuned!

Step 16: Z-axis System Maintenance

So far we have dealt with the X and Y axis bed, but the Z-axis (depth of cut) is still with lots of lag and play in the screw...

So, loosen the setscrew, pull shim out, and remove the drive head and motor. One simple screw does this, so watch it doesn't drop off.

Step 17: Remove Z-axis Slide

Here there are one big screw that attaches the brass threaded nut plate to the Z-axis slide, and two smaller set screws for alignment of the slide with the nut plate and threads. One side tightening will twist the threads of the nut plate on the vertical leadscrew, and that is how it removes slack and lash in the threads instead of having two separate nuts. Watch for reassembly where you can clearly see the adjustment bring slide into alignment.

Step 18: Remove Hand Wheel and Head Bearing

Again remove the hand wheel, and underneath lies a nylon captured ball bearing assembly as well as two smooth races. To remove the ball bearings and races you need to pop the leadscrew end out of the bearing, downward. Sometimes wiggling (and doing the hopscotch) can help.;)

Remove the bearing and races, wipe clean, and reinstall with good quality grease of you favorite brand. to remove the leadscrew you can take the main vertical bar off the mill, or the bearing housing, but for our purposes, that is not necessary. Just let the wonderful liquid rain down on it (WD40) and it should be as sparkly as your teeth, which is good enough.

Step 19: Assemble Z-axis

Here you see the bearing and new grease, leadscrew back up into bearing, and the slide going back on. Remember the washer on the leadscrew!

Step 20: *Adjusting the Z-axis Nut Plate**

Here is another important part: It's what makes zero lash on the Z-axis.

When you put the slide back on the nut plate, make the main attaching bolt snug but not tight all the way, to leave room for the adjusting setscrews to adjust.

Putting the top setscrew tighter will tilt the whole nut plate DOWN, and putting the lower setscrew tighter will tilt the assembly UP, so it is important you carefully look at the slide where it hits the rail for alignment.

Step 21: Finishing Up!

Put the shim block back in the Z-axis, and do the usual wiggle test.

Step 22: *Hand Wheel Adjustment****

Combined with the other important parts, this is the other thing you may have to do from time to time to keep the machine zero lash.

As you put the hand wheels on, you will have to pull the carriage towards the wheel, seating the end bushing of the leadscrew, so when the wheel goes on there is absolutely no play.

I don't know why the wheel doesn't have some kind of mechanism for keeping it tight all the time, so that will be another Instructable as well, to invent something better.

For now, just push the wheel in, and pull the carriage out at the same time while tightening the set screw that holds the hand wheel on. Wiggle check for play.

Turn the hand wheel, and there should be absolutely no easy turning before the carriage moves ever so slightly. If there is, repeat last step. Once you get zero free play of the hand wheel, you're good!!