Fitting new chucks

December 2014 (updated September 2015)

The lathe came with an 80mm 3-jaw, and a short 100mm 4-jaw. Unfortunately both had problems: the 3-jaw had inside jaws only and so was limited to holding ~32mm material, and the threads on the 4-jaw were very worn and pitted. These items look very similar to ones I have seen in pics of the lathe, so I presume they are stock.

I completed this little project using my 150mm digital vernier caliper, and a set of inside calipers. The calipers were very handy for quick measurements, although I don’t have the feel or skill to use them properly yet.

Choosing chucks

A browse around my favourite suppliers (RDG Tools, Chronos, and ErcEurotrade) and some measurements of the chuck boss showed that a stock-size chuck would not be available.

Mounting chucks on the MD65 is not very convenient… The spindle backplate arrangement means that if you want to use chucks of different sizes, you need to

After a great deal of online shopping and comparing chuck boss sizes, I settled on a Vertex 3-jaw from Chronos (£73, ref: VSC3AFB), and a budget HBM 4-jaw from RDG (£54, ref: 89743SAD). Both are front-mounted which suits the MD65’s inconvenient chuck mounting arrangement.

3 chucks
The Good, the Bad, and the downright Ugly. The new HBM 4-jaw, the stock 3-jaw, and the stock 4-jaw with the pitted threads. The new Vertex 3-jaw was busy and unable to attend the group photoshoot.

The 3-jaw is mounted with three bolts, allowing me to take the chuck off the lathe and mount it in my Vertex rotary table (which has three T-slots). Gear-cutting here I come!

The 4-jaw is not a shortie like the stock one, but I could not find one online and decided to live with the reduced distance between centres – reasoning that long materials could be held in the 3-jaw or some other arrangement.

4 jaws
New (left) and old (right) 4-jaws. The old one is a mere slip of a chuck, while the new one I have christened The Chuckernator.

I also purchased a 100mm cast iron blank backplate from RDG (£18.50, ref: 7879876). I had also bought an 80mm backplate from Chronos (£15.84, ref: BP80), however I found it to be too thin to be of practical use, and quite rough compared to the one from RDG – a rare fail from Chronos.

Machining cast iron chuck backplates

I did some research online and found that machining chuck backplates is reasonably straightforward. I found this article particularly useful.

The chuck needs to be a snug fit on the register, to within 0.005mm or better. This level of accuracy is beyond my ability to measure, so the final few cuts should be ‘cut and test-fit’ rather than trying to cut to size by measurement.

To achieve very fine cuts, I planned to use the top-slide set at a 1:10 ratio (about 6 degrees) – meaning if I advanced the top-slide by 0.1mm then the tool should advance 0.01mm towards the work. Instructions on how to do this are in Harold Hall’s excellent book Lathework: A Complete Course. I set the angle of the top-slide using the index marks on the top-slide, reasoning that precision in setting the angle would be an academic exercise as I was unable to measure the resulting cut.

I also found that I need to machine a small recess next to the shoulder of the register, to ensure that the chuck has good contact with the backplate around its perimeter.

Lastly, the outer layer of cast iron blanks should be machined off in one pass, This layer is comprised of brittle iron and some silica from the casting process and is very abrasive. I found that a 0.3mm cut successfully removed this outer layer on the blanks that I used. I used the slowest speed on the MD65 for cast iron – any faster and the tool was inclined to dig in.

Mounting the 3-jaw

The register on the 3-jaw is 54.90mm, which meant that I could simply turn down the chuck boss from it’s stock diameter. By chance, the existing chuck boss already included holes that suited the Vertex 3-jaw, so no new ones needed to be drilled.

This was a relatively easy job, I took it very slow not wanting to ruin the spindle backplate by taking too much off. I took about 10 cuts in total, test-fitting after each of the last 5 cuts. This was a painless process (unlike the 4-jaw) as I could just pick the 3-jaw up and offer it up to the spindle backplate.

Machining a new backplate for the 4-jaw

This proved to be a much more difficult and time-consuming job than I expected. This is mostly down to my inexperience in turning cast iron blanks and miscellaneous cock-ups. I think it should take just 6 hours or so to complete.

I mounted the cast iron blank in the 3-jaw using the outside jaws. This mounting arrangement was helped by the fact that the blank was quite smooth, and not rough like the 80mm blank from Chronos.

I then machined away the outer layer – this took a couple of hours of machining, mainly because I had not set the blank in the chuck accurately enough. The large face I was machining was ~0.3mm out across its diameter. This would not normally be a problem as I could just face it flat, however the outer layer of the cast iron is extremely abrasive, and kept blunting my HSS tools as the cut transitioned from cutting iron to skimming the outer layer… lots of practice sharpening.

Next I drilled the center and bored it out to 23mm, which is the size of hole in the middle of the stock 4-jaw backplate.

I then roughed out the cut for the register recess – the target being 54.90mm I machined to 53.90mm (measured using my digital vernier calipers). The register in the chuck boss was 2.30mm high, so I machined to 2.50mm depth.

Next I honed my tool to ensure I could take very fine cuts for the finishing passes. I took a few passes with the cross-slide locked, advancing the top-slide which I had set to 1:10.

To test-fit the backplate I removed the 3-jaw by undoing the three nuts behind the spindle backplate, marking the 3-jaw and chuck boss to make sure I returned the assembly to its original position. With this precaution, and provided I didn’t remove the backplate from the chuck jaws, I was able to remove and replace the chuck and backplate while maintaining concentricity.

Initially I thought that this part of the process would be the most time-consuming, but after a couple of times on and off I got the hang of it, and in the end it was not as laborious as I anticipated. I made sure to clean the faces and shoulders of both registers before test-fitting.

backplate in 3-jaw
Machining the spindle plate register – here is the 3-jaw after being removed from the lathe so I could offer the machined recess up to the spindle plate. This is casting #1 which was destined to become a paperweight.

I am not entirely sure what happened next, I suspect that I mistakenly advanced the cross-slide rather than the top-slide, taking too big a cut. I test-fitted the backplate and it was loose – after the trauma of facing the backplate this was very disheartening. There was 0.1mm play. A quick call to the Amateur Machinists Helpline (my mate Mike), proved what I already knew: the backplate was scrap.

Two days later, the replacement arrived from RDG. I repeated the steps above, this time taking care to set the blank up in the 3-jaw as accurately as possible, and taking a 0.3mm first cut. I removed all of the hard outer layer in one pass – result. It took five test-fits (removing and replacing the 3-jaw chuck) but at last I had a snug fit.

My plan from here was to drill the holes for mounting the chuck backplate to the spindle backplate, mount the backplate to the spindle and finish machining on the chuck side (machine a register for the chuck). Rather than trying to drill through ~30mm of cast iron with my 10mm counter-bore bit and potentially end up with counterbore holes that were not aligned to the 5.5mm through holes, I decided to drill 12.7mm holes (1/2 inch to some of you) to depth less than the protruding face of the backplate.

Next I turned up a transfer punch to locate the inner three mounting holes from the stock 4-jaw backplate. I then center-punched the witness marks, centre-drilled, then all the way through with 4mm then 5.5mm drills. I did this from the spindle side of the backplate, to ensure that even if the drill wandered a bit over the 30mm depth of the backplate, at least the important ends of the holes would be accurate. This proved a wise precaution…

marked for drilling
The old 4-jaw backplate was a mess, with holes drilled through the shoulders of the registers on both sides, some thread repair (top hole in the pic), and holes so close together that the threads cut in one hole were visible in the other.

drilled and ready to machine
Mounted onto the spindle backplate and ready to start machining. That’s a whole lotta iron to turn away. I saved some of the filings to give to Mrs Naut as an xmas present.

Cock-up #3 soon became apparent as I drilled clearance holes for the bolt heads with 7.5mm then 10mm drills. My knackered and not-very-good-even-when-new drill Fermi drill press has a bit of vibration, and the tilting table is prone to tilt of its own accord now matter how hard I graunch up the locking bolt (I bought it many years ago before I knew what I should be looking for). I had leveled the table before starting drilling, but had not noticed as it did its usual thing and moved a little. The result was holes that did not line up with the pilot 5.5mm through holes.

clearance hole misalignment
The one on the right is especially bad – the 10mm counterbore would hit the sides of the existing hole before the counterbore guide tip reached the 5.5mm through hole.

The misalignment was so bad that the plan of going in with the 10mm counter-bore needed re-thinking, as the counter-bore would need to cut into the wall of the clearance hole. I decided to remove all of the remaining outer layer and as much material as possible, and then remove the backplate from the spindle and mill the offending clearance holes away, leaving only the 5.5mm through holes.

correctional facilityWhen good backplates go bad: the Nauty Backplate Correctional Facility. I took as much material off as I could in the lathe. The hole top right didn’t need relieving.

I was pleased to find that what seemed like a desperate measure actually worked quite well. The mill removed enough material for the counterbore guide tip to reach the 5.5 through hole, and I drilled the counterbore holes with no problems.

It looks like a mess, but the counterbore is good and the rest of it will soon be turned away into dust and chips.

The rest of the process was reasonably straightforward. Turning the register on the chuck side was the same process as for the spindle side, with the significant improvement of being able to test fit the chuck without removing it from the lathe.

My final cut on the register was 0.002mm, and the fit is snug. This last part was very satisfying after overcoming so many (self-inflicted) problems.

Cinema verite style, hand-held phone while turning.

Seemed like a hell of a lot of work just to get here… but the end result is very satisfying.

The last job was to drill and tap the holes to mount the chuck to the backplate. The chuck came with hefty M8 bolts, which make the M5 bolts in the picture above look pretty weedy. I figure that the register carries most of the load during cutting, and that the bolts are just there to hold the two faces together.

I turned up a transfer punch out of silver steel, and hardened the tip by simply heating and quenching. I clamped the chuck to the backplate and marked the holes, then onto the mill table for drilling and tapping. Lastly I turned down the four M8 bolts so that they are slightly recessed behind the backplate.

After install I checked for runout and was very pleased to find no measurable wobbling.

4-jaw itemsWeedy-looking M5 bolts for mounting the backplate to the spindle, compared to chunky M8 ones to mount chuck to backplate.

4-jaw mount holes

Chuck mounted to backplate, showing recessed ends of the M8 bolts.

4-jaw installedInstalled at last. Shown with jaws fully extended, just clearing the ways. I think 100mm is a good sized chuck for this lathe.

4-jaw in use

…The Chuckernator in action, gripping a giant block of solid steel in its vice-like jaws.

Leave a Reply

Your email address will not be published. Required fields are marked *