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Stepperhead 2

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jackary:
Stepperhead 2
What to do in the Covid lockdown? I wish you the best of health and hope you are coping with this trauma. In my case I have tried to deal with these long lockdowns by trying to design a smaller version of the Stepperhead lathe that I built and displayed in 2008 ( http://www.lathes.co.uk/stepperhead/ ).   I managed to get access to Solidworks and intended to teach myself in the process. You may have  seen that the Model Engineers Workshop magazine has recently published an article on this design. So this is a follow on from that and I am sticking my neck out here venturing for opinions from the forum members. I have added a few screenshots and a basic description.
I set myself a design brief as listed below:
1.   Size - A table top lathe suitable for the home workshop; or even indoors - with senior management approval of course! The machine is to be mounted on a baseboard of 750mm wide x 500mm deep. Of course the size can be easily increased, but a design for a limited footprint is more demanding in the use of the space.

2.   Concept – Mini lathe size - similar but smaller than the first Stepperhead design but with changes and improvements to simplify construction and operation.  A cast iron one-piece bed. The spindle drive motor and all the electrical components are housed in a box at the rear of the lathe which will also form the splashback or chip guard. The raising and lowering of the head, overarm and tailstock will be done by a vertical screw. similar to a milling machine table. This keeps everything above the base board.

3.   A Camlock spindle nose fitting. This is such a boon enabling easy chuck changes and the safe ability to rotate the spindle, under power, in either direction.

4.   The spindle motor is a 0.37 kW three phase motor, frame size 63, driven by an inverter also mounted in the splashback box. The motor is pivoted at its lower front so it can move as the head and tailstock is raised. This uses the motor weight to tension the belt. The motor & spindle pulleys have three steps for a wide speed range. The largest spindle pulley enables a low back gear speed range at the spindle. The belt is accessible and can be easily, and quickly, changed to the various pulleys. A polyvee belt drives either the spindle or a shorter length belt is used for an overhead drive shaft mounted in the upper section of the splashback box. The motor pivot point enables both drives to be adjustable. The pivot may benefit from some friction damping adjustment.

5.    The spindle runs on taper roller bearings and has a 26mm bore. The bore is as big as can be made without substantially increasing the head block size of 80mm square. It uses the same Camlock chuck mounting arrangements as Stepperhead. I tried to design a slightly smaller Camlock fitting but there was little to be gained with keeping the mandrel bore size and ER32 collet fitting.
The drive from the spindle stepper motor is via a worm wheel mounted on the spindle inside the head block. The stepper motor worm engagement is spring loaded when moved to into this wormwheel to drive the spindle. This gives a backlash free drive to the spindle. The worm drive is positively locked out when not required. This is to ensure that cannot be accidentally engaged. The motor belt can also be disconnected at the motor for additional safety. The stepper motor mounting box also incorporates an optical sensor and its circuit board for screwcutting etc.

6.   New ideas – I have incorporated some new features I have been experimenting with over the last few years. There are always reader comments about gibs and their settings for smooth shake free sliding. It is a fundamental requirement that dictates the performance of the whole machine. This requires accurate slideways and careful adjustment and is very difficult to achieve on less than perfect fits.

So here I would like to propose my oval gib idea to replace the conventional gib arrangements throughout. It seems to me such an improvement and simplification and has become subject to a patent application. It eliminates the adjustment and locking screws at intervals along the gib length. It contacts, solidly, both the sliding and fixed guideways the full length of the moving slideway. It is simply rotated to set the close sliding requirements by setting and locking an eccentric stop. If the gib is further rotated away from this stopped position it will lock the full length of the moving slideway solidly to the fixed slideway. Rotating it back to the stop will return it to the set close sliding position. It is also easy to make.

The oval gib itself can be solid or it can have a surface that is segmented into slightly springy zones. This will spread the loading and be of great assistance for smooth operation. It will also enable it to adapt to, and make the best of, less than perfect surfaces due to local wear or less than perfect machining. Of course it cannot compensate for non-parallel sliding surfaces and poor fitting, but it will tend to slightly deflect over the bumps and troughs, so to speak, and promote smooth shake free movement.  It is also easy to remove and replace for inspection and can be made from a variety of materials. The oval gib system has also been used to guide and lock the vertical column in the radial and vertical planes, avoiding the triangular gib which works well but was difficult to make.

7.   Tightening and undoing ER style collets seems to be a subject of contention, especially for the smaller diameters. So I have suggested a method of tightening the collet nut without having to lock the spindle, or use a second spanner to hold the chuck body. The castellated mandrel mounting body has fifteen notches and the nut has sixteen holes so there is always an appropriate zone for the locking/unlocking spanner. The collet nut has two hardened discs positioned to enable the collet to be withdrawn rather than an eccentric diameter.

8.   Vertical parting tool - I have shown a design which mounts on the front of the lathe and rests solidly on the cross slide, preventing any downward deflection and allowing free chip flow. Inserted tips could also be used in a suitable mounting. With the topslide locked by its oval gib it becomes a virtual Gibraltar mounting point because the upper and lower slides are locked together. The cutting tool height can easily be set even when the machine is in operation.

9.   The upper part of the splashback box provides a mounting for a horizontal secondary driveshaft in a similar manner to watchmaker and earlier instrument lathes. A cover panel at the centre of the splashback front panel can be either, slid to the left or just removed to reveal a rectangular cut out exposing this driveshaft with a pulley drive for round belts. The pulley is positioned along the shaft to provide the overhead drive. To fit the round belt, the shaft is moved to the left enabling the belt to be fitted over the end of the shaft. A locked collar retains the shaft in the righthand end bearing. The spindle drive belt is removed and a shorter length polyvee belt used to drive the shaft.

This makes use of the main motor with its inverter control, enabling a wide speed range for a milling spindle mounted on the topslide for example. The arrangement I used on Stepperhead with a milling head driven by a DC motor with electronic control worked but had a limited speed range around 2000 rpm. It had limited power and got quite hot, its bulk was also restrictive. The problem with the high speed was that a carefully shaped carbon steel cutting tool would overheat and lose its hardness halfway through the operation, say when cutting a gear, especially in steel. High speed steel cutters solve the problem but are difficult to make and shape compared to carbon steel. So to be able to slow down the cutting speed without power loss would be of great benefit.  Round belt pulleys and belts of various diameters can be used. This would enable a wide use of operations say, using the stepper driven spindle via manual or CNC control for a myriad of operations, including decorative ornamental designs etc.
 
10.    The milling spindle has a bore for No 2 Morse taper collets and a locking/eject drawbar. It has bronze self-lubricating bearings and uses a double taper similar to a watchmaker’s lathe headstock design. Ball bearings have been avoided to keep the overall diameter down to 40mm.
Comments welcome
Alan

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vtsteam:
I really admired your original Stepperhead -- which was mentioned during my lathe build thread here on MM. Wonderful looking machine, and it's really nice that you've posted this new version here. Looks like there are some new innovative features in this one.

Do you think you will be building it, now that you've done the CAD design work?

MetalMagus:
That's really impressive.

Hope you build it and create a series in here for us to follow.

On the chuck, have you considered using 5C collets with a closer mechanism out the rear of the headstock for fast part change over. I know Taig have produced a 5C headstock, and have seen similar set ups on a Sherline lathe.

Regards

Sean

vtsteam:
I see the new gib style on the lathe bed. I do have a question -- how do you scrape something like that into bearing? Would you need a special profiled straight-edge?

philf:
I'm confused about the 'oval' gib too.

Surely you can't ever practically have more than a line contact with an oval gib. If it was manufactured perfectly with the correct clearance built in then it would be feasible but as soon as you rotate the gib the curves won't fit resulting in line contact.

Or am I missing something?

Phil.

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