Myford Super 7 Large Bore depth stop

I recently had to turn a number of items on my lathe and wanted them to be consistent in length. These were long items, too long to reference with my home made Morse Taper depth stop.

In desperation I raided my Sherline CNC Rotary Table hardware kit for my coaxial tailstock clamp. This is based on the Myford tailstock winding handle but without the handle. It clamps the lathe bore to the CNC table for cutting items needing indexing such as clock wheels. Wheel cutting is quite a demanding application. There is no tolerance for slippage or you will get to the last tooth and have only half half the material that you expected left to cut … and the box of shame looms.

The clamp is quite simple – the main body is an expanding collet, this mates with an expansion cone and a centre threaded rod. The cone angle is 5 degrees. There is a knob to tighten the assembly and squeeze the cone into the body to expand its arms. The body has four slots that two pins on the cone engage with to stop rotation.

Having finished the particular job in hand I put away the Sherline clamp and thought it would be useful to make a second version of the clamping collet dedicated as a lathe depth stop. I was rummaging in my metal stock for material to make this when the light bulb came on …. why not 3D print it? When the clamp is used as a depth stop there is no rotational stress. It just acts as a stationary reference stop that sits down the lathe bore.

I copied the dimensions from the Sherline version and input this into Fusion. I printed the body, the cone and a knob (with an M8 embedded nut). The centre rod is M8 studding which passes through the collet body and then screws into the cone and then protrudes onwards to set the depth in the lathe bore. You would think that there needs to be a nut to lock the studding to the cone but the cone cannot rotate as it has the two locating pins. Providing the studding is stopped from rotation while the knob is tightened, the depth should be set solidly. A lathe depth stock is rarely an accurate setting tool but just a repeatable reference. The accuracy of the final cut is catered for by the tool position set and the various lathe DROs.

Printing this was a lot easier than cutting it from metal. No messing setting cutting angles etc. Here is the Fusion pictorial view and cross section.

After using it a couple of times I ended up putting a spring and two M8 washers between the body and the knob to maintain a slight pressure on the cone. The spring keeps the locating pins engaged in the slots on the body as the assembly is pushed in place down the spindle bore.

I also added a ‘top hat’ on the end of the M8 rod (not shown above) to give a larger surface area for the work piece to bear against. This would also stop wobble of the threaded rod as the spindle rotates.

Attached below is a ZIP file with the body, cone, top hat and knob STL files. You will need some M8 studding, an M8 nut and some short M3 stubs for the anti rotation pins in the cone. If you add the spring then two M8 washers will be needed. All the threads are modelled in the STL files but will need cleaning up post printing. I printed in PLA+ and had the prints set to four perimeters. I superglued the two M3 studs in place. The tops of the studs should not protrude beyond the unexpanded body surface otherwise they will bind on the inside surface of the spindle bore.

Things now went a little bit off piste …. I had forgotten that the end of the lathe spindle has a threaded section that mounts the bearing pressure collar. The exposed thread is normally protected with a plain collar. The thread is M35 x 1.5, something very easy to model and print via Fusion 360. Below is the relevant exposed thread with the protector collar removed.

Another light bulb came on (getting to be competition to Blackpool you might think). Why not make things very much simpler?

I modelled a boss to screw onto the M35 thread with a central core to fit down the spindle bore (26mm) and with a central M8 tapped hole. I also printed another through hole M8 knob, a M8 top hat and a new blind M8 knob. I now had a much more simple depth stop.

Here is the Fusion image of the boss. This would have been very difficult to cut as one piece in metal but very simple as a 3D print.

Here is a close up view of the final assembly with the top hat out of shot at the far end of the M8 studding but shown in the second image. The knob on the left grips the end of the studding to allow easy adjustment of the position of the top hat. The inner knob locks the studding in place against the boss. Both knobs have embedded M8 nuts. (Note the bend on studding is a photographic distortion).

Two solutions for a depth stop. The second option is peculiar to the Myford and the first solution more universally adaptable to other lathes. The following download ZIP file contains the STL files for both versions and there is a second ZIP which has the Fusion 360 model for those wanting to tweak. Either version will make a useful addition to the workshop tooling.

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Tangential Lathe Toolholder for Myford Super 7

Yet another design to add to the plethora out there

The concept of a tangential lathe tool holder had passed me by until Blondie Hacks mentioned it in one of her YouTube posts.  This resulted in an hour or more lost down an internet rabbit hole.   The conclusion seemed to be that I was very late to the party.  There are many designs out there which include the Eccentric Engineering commercial item.

I quite liked the look and simplicity of Mike Cox’s design. With the knowledge gained from my unintended meandering internet research, I tweaked it slightly and committed it to Fusion 360. This is shown in the pretty picture below.   Note my design was to be mounted in a QCTP on my Myford Super 7 and is intended to hold a 3/16” HSS tool. Here is my Fusion 360 image of the model.

The following ZIP file contains my full write up along with the Fusion 360 model for the toolholder and an associated 3D printed sharpening jig.

Update : This tool is very good. It can tolerate decent depths of cut and leaves a nice finish both turning and facing.

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Hemmingway Sensitive Knurling Tool

A rite of passage ?

You may have watched James at Clough42 and Brandon at Inheritance Machining both make the Hemmingway Sensitive Knurling Tool (HK# 1115) and shared their experiences. In the past I have made a few Hemmingway kits and always enjoyed making them and the associated learning process. I decided to add the knurling tool to my collection.

Like James and Brandon I found the drawings beautifully presented but a little dyslexic with imperial dimensions showing as a mix of fractional and decimal units accompanied by metric equivalents. To get more intimate with the design and to circumvent the fraction and decimal imperial dimension issue I resorted to redrawing the design in Fusion 360. Fusion lets you set an overall dimension standard for a drawing but you can enter an individual dimension in any format or standard. Fusion will then display the individual dimension at the default drawing standard. This allows you to enter any dimension in mm, fractional Imperial or decimal Imperial. I find this a convenient way to better interpret the Hemmingway drawings. Redrawing in Fusion also highlights the potential problems in the rounding of metric dimensions from the imperial entries.

James and Brandon both resorted to traditional methods for the construction. This involved using a rotary table for the curves (of which there are many on the two jaws). I will now add further to my reputation of being a lazy engineer. Having redrawn the design in Fusion it was a logical step to produce the kit with CNC assistance.

While CNC might make the construction appear easier, it still creates headaches on work holding. This is magnified by the fact that the kit is supplied with ‘final size’ material. Brandon took this further by skimming all the supplied stock but then fell foul of the subsequent impact of this on some parts.

The CNC machining of the side plates, jaws, adjuster plates, spacer block and mounting block all went well to my CAM processes. This required sacrificial mounting plates and lots of checking before hitting ‘GO’. I am pleased and relieved to report no dings resulted from the CAM.

The majority of the remaining parts are lathe activities with, where needed, a liberal use of a collet block for cross drilling . I cheated on the adjuster thread and used M8 threaded rod with appropriate dimension changes to its mating components.

I had one post CNC ding on the lower jaw leading to an additional M3 hole. The lower jaw nut was the most probably the most tricky part. That aside all went to plan.

I used the semi finished tool to knurl the adjuster knob and pressure lever knob and was very pleased with the clean knurls that resulted. Here is my finished knurling kit mounted on a Myford QCTP holder.

Would I recommend the kit ? Yes – but it has some slightly advanced activity needing careful planning and thinking. The drawing dimensioning can catch you out. I think redrawing in Fusion helped highlight and clarify these issues. Measure twice and cut once is the order of the day particularly with respect to shaping the jaws. If you have CNC then this is a much easier way to go once you have got your head round the necessary work holding.

Note – the kit does not contain the knurling wheels so you need to order them separately.

Overall this has been an interesting and challenging project. I now have a very useful asset added to my workshop tooling.

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Workshop air compressor problems

Understanding what was happening and why

I have a Bambi PT50 air compressor in my workshop. I chose it on the basis of being relatively quiet and having the capacity to provide sufficient air for my Fogbuster coolant system while working within the compressor’s specified duty cycle. The PT50 has dual compressor motor pumps.

The PT50 works very well. It is an oil free compressor and only needs the moisture bleed as a regular maintenance activity.

Of late I ran into an issue with air leaking from under the Condor MDR1 controller. This led to an investigation of how the compressor control process works, something I had not given much thought to while the unit was working fine.

The process, when working properly, is that the compressor motors push air into the air reservoir (tank) via a non return valve (NRV). When the pump stops the NRV seals the pump feed into the tank. The air contained can then only exit the tank via the MDR1 controller. When the air pressure in the tank drops due to demand and to a defined level (set by the controller) then the pump kicks in again and tops the tank up. A simple looping process.

The controller has a rubber diaphragm that expands and retracts subject to the pressure level in the tank. This diaphragm pushes on a hinged plate that acts as the trigger for the electrical switch that turns the pump motors on and off.

There is a secondary process to this.

When the pressure level in the tank reaches the point where the pumps need switching off, the NRV closes but there still remains a high pressure in the feed pipework from the pump to the NRV. This pressure will put stress on the pump motor when it next tries to start up. In the extreme this back pressure could cause a pump motor to stall.

To overcome this there is a tiny bleed valve in the controller that gets triggered to open simultaneously with the point where the pump switches off. The bleed is triggered by the same hinged plate mentioned above. The bleed valve is connected via a small tube that tees off the NRV in the feed from the pump motor. When triggered the valve vents the high pressure in the pump feed pipe to the NRV. This cause the sudden burst of air you hear when the pump switches off at its high pressure trip point.

So onto the problems that might occur.

The NRV seal can become damaged or worn or contaminated with debris from the reservoir tank. This means it will not seal when the pump switches off and the air in the tank will vent back out past the NRV, pass along the thin tube feeding the bleed valve and manifest as a continuous rush of air similar to the ‘switch off burst’ mentioned above. To summarise the fault – the pump switches off and you hear air continuously escaping. Solution – NRV valve damaged or contaminated and therefore not sealing. Check, clean and / or replace the valve.

The second problem is where there is air escaping from under the controller but only when the pump is running. (It is important to note that this does sometimes occur naturally when the pump is first switched on with an empty tank. In this situation there will perhaps not be enough pressure at switch ON to close the NRV. Once a few seconds of pumping has occurred the NRV should close). If the air continues to leak from bleed valve under the controller with the pumps running then this means the tiny needle that activates the bleed is either jammed with debris or the diaphragm / hinge plate in the controller is damaged. Note that on the MDR1 you can remove the valve to check it. The valve is enclosed within the black body of the bleed pipe mounting gland. It is twist locked in position.

In summary therefore : –

Air escaping from under the controller when the pump is not running suggests the NRV needs checking and / or replacing as needed.

Air escaping from under the controller when the pump is running suggests a problem with the needle bleed valve or the diaphragm / hinge plate in the controller. This is a more serious problem. It will cause the pump motors to overheat while trying to deliver air that is continuously escaping from the bleed valve. The MDR1 is not user repairable and is a crimped sealed unit. The picture below shows the MDR1 internals and the NRV.

The first glance the cost of replacing the NRV or the MDR1 controller will appear similar but do take time to shop around. Replacing either of these will involve disconnecting various associated pipe fittings. As ever, take a picture before you start disassembling. Thoroughly clean all the parts before re-assembling. Use a sealant such as Loctite 577 to achieve quick and easy air tight seals.

Finally a word of warning – compressed air is a dangerous medium. Before undertaking any remedial work on a compressor you must ensure that the reservoir is fully vented and empty.

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Replacement Cowells Chuck Key (Part 2)

Sintered Tool Steel Experiment

Some while ago I posted about my experiment in reproducing a Cowells ME lathe chuck key as it seemed there were a few owners with broken teeth on their keys.

Since then I have received a number of requests for replacement keys which I have tactfully declined.  They are not the easiest of things to reproduce and a potential for broken tooling.

I use PCBWay for my production PCBs and I recently noticed that they can produce sintered 3D objects from a STL file.   They offer various materials for this process.  Out of curiosity I sent the STL file for the chuck key to them and ordered 5 pieces in sintered tool steel.  Twenty days later I received 5 chuck keys.   They look good and they match my dimensions and more to the point they work in the chuck.  I have no experience on how strong sintered tool steel might be but this video from My Tech Fun is quite impressive.

My Tech Fun

The downside of course is that they were not cheap (~£50 each, heavily thumped by courier costs) but if you have a Cowells and the frustration of a chuck key with a broken tooth maybe this is a reasonable price to pay.  Let me know quickly if you are interested in buying one otherwise I will put them on EBay.

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