Eccentric Control Rod Continued.

Earlier I showed the making of the eccentric strap, the eccentric sheave and the control rod for the valve control eccentric.  However I had to pack up my workshop and store my equipment etc in a container supplied by the builder who had to resurface the unsatisfactory floor following the fire I suffered in 2010.

I have made a little progress on the engine but not as much as I would like, but I have finished the valve control mechanism.  Here is a little article showing the finished piece.

Connecting rod made from 'scrap' material as described in earlier posting

This component was made from 'scrap' 2mm thick sheet steel.  I try to reuse so called scrap wherever possible, not because of cost necessarily but for a variety of reasons including convenience - I may have a suitable bit handy - or for environmental reasons - less waste.

Clevis

This was simply made by cutting and filing from a short piece of solid bar. It has to be finally finished to size and some final polishing.

Eccentric strap showing construction

complete valve control rod assembly with eccentric sheave

Rotary Table Indexer - Overview - 1

Some more pictures and description of functions.

All images are clickable to enlarge

The indexer has various functions:

1. Jog;
2. Division;
3. Degree;
4. continuous;
5. Program; and
6. Setup.

Jog:

The Jog command allows the table to be moved by a fixed angle according in either direction i.e. clockwise (CW) or counterclockwise (CCW) depending on which of 8 function buttons are pressed. 

Division:

The Division command will step the table a step at a time when the command button is set.  Awkward divisions are easy, simply type in the number of required steps and use the command button to step the table as required.  Difficult divisions such as 127 are no problem.

Degree:

This command is self explanatory, simply type in the number of degrees required and the command button will step the angle one at a time.  E.g. 60o degrees for a hexagon.

Continuous:

Again self explanatory - one commenced the rotary table will turn continuously at a predetermined speed.

This function could also be used to operate an X or Y table.

Program

Up to 10 user defined programs can be stored.

Setup:

This allows user parameters to be set up such as worm - wheel ratios, trigger settings to allow for different Stepping motor controllers, plus many others outlined in the documentation available on the forum.

The cost of building is relatively low, mine cost probably less than £20.00 to build but I did search around for inexpensive sources of components.  The most costly items were the PIC chip on which I embedded the supplied firmware, the display, and the case.  The latter was an inexpensive buy from Maplin though.  The keypad was built up from a simple circuit board and PCB buttons for about £2.00.

Cost could be reduced with a home made case.  For the practical ones amongst us, I made another case from some surplus oak planking, a bit of 3mm ply for the back and some coated steel sheet from a defunct desktop computer case for the front.

Stepper Motor and Controller.

Disassembled Device to Show Controller and Motor and home made keypad

The controller was an inexpensive Chinese one from an eBay source and has proved to be quite satisfactory so far.

The stepping motor was again a buy from an eBay source and is a 200 step, 439 oz in motor, which is more than powerful enough to run my small rotary table.

At the moment I am using a 12V power supply salvaged from an old laptop computer, I will probably upgrade this to a more powerful PSU at a later date.

Sourcing components carefully can find some good equipment at decent prices, I use eBay etc where possible and avoid the extra expense of using dedicated suppliers but it needs a knowledge of what you need for the job in hand.  It is well worth carrying out some of research before buying.

Rotary Table Indexer - Overview

Description of Indexer

I have been working on a rotary table indexer to use on my milling machine.  The indexer is one designed by 'Kwackers' aka Steve on the CNC zone forum.  Several members of the forum offered suggestions for variations and I have incorporated some of these in my own indexer.

The Assembled Indexer

Basically it is an electronic device with a pre programmed chip which operates a stepping motor which in turn is connected to the rotary table in place of the handwheel.

I have built the circuit, programmed the PIC chip with the firmware (program to drive the stepper), purchased an inexpensive Chinese stepping motor drive unit and a suitable motor.  All I need to do now is to connect the motor to the rotary table.  However the table needs a bit of simple modification to ensure accuracy but that should not be too expensive.  It will also require a mounting adapter making as well as some kind of connector between the motor shaft and drive shaft of the table (Oldham Coupling or similar to allow for slight misalignment).

Inside the Box

I have taken quite a few pictures of the process so I will write a few posts on the making of the device in sections so that anyone could follow the process.  Where possible I have used simple processes and techniques in order to make the making of the unit accessible to most.

As well as driving a rotary table it could also be used for other purposes such as controlling the table of the milling machine or operating other devices.  I have thought of using it to drive an equatorial mount for a telescope and there must be many other uses for such a device, especially if you are able to program the PIC for your own purposes and there must be suitable open source programs available.

Centering work on the Lathe.

Odd shaped or square work is often set on the lathe for boring or facing etc. and is often difficult to centre.  There are several ways of doing this depending on the work itself.  If you have a centre punch mark, say where a hole is to be drilled, then you can use a dial or 'finger' indicator in conjunction with  a centering rod.  The centering rod (my term) is made with a length of accurate, ground round bar.  I use a shaft from an old defunct printer.  One end is accurately centre drilled and the other end given a point, I use 60 degrees included.  The work to be centred is mounted on the faceplate or in the 4 jaw independent chuck, depending on the work, and the centre punch mark to be centred is set approximately on centre using a good dead centre in the tailstock.

Centering Work On the Lathe

The tailstock is withdrawn and the 'centering rod' is mounted between the dead centre and the centre punch mark on the work.  if the work is rotated by hand the rod will probable show a wobble, if not you are lucky and the work is centred.  I have no photograph at the moment so a rough sketch will have to suffice to show the idea.

You can now use an indicator of some type to measure the offset of the centre and adjust until the centering rod runs true.  You will then have an accurately set job in the lathe.

I have seen various contraptions used including spring loaded devices, but I have never needed such complications but can see the logic so I may get around to designing and making one some day.  Often another dead centre with rear centre drilled hole (of course), is substituted for the rod.  Takes your choice it's up to each individual to use what is convenient.

Just a point, if you are using a 4 jaw independent chuck, set one opposite pair of jaws accurately first, then concentrate on the other two.  Trying to set all four at the same time will have you running to the psychiatric ward before long and you will take up some other easier hobby such as building a fusion reactor or harnessing dark energy.

Rear Parting Toolpost on Small Lathes.

Large diameter, large vertical force v, small horizontal force h

There has been a lot of debate about the pros and cons of using a rear mounted tool post for parting off on small(ish) lathes. Some advocate the rear mounted while some argue that they are not necessary. This post tries to look at the advantages of the rear mounted tool post in a (hopefully) simple and clear way.

Small diameter still large vertical force v1 but much larger horizontal force h1

Every lathe has some play in the headstock bearings, no matter how little, it is there, and there will be some flexibility in the tool (absolute rigidity is impossible) as well as some flexibility in the workpiece, metal unfortunately is flexible. it may be minute movement but the effects of all these are cumulative.


As I see it, the front mounted tool will tend to be forced downwards while the work is trying to climb the tool. Even if the tool is set exactly on centre before parting these effects will tend to force the tool below centre as the work progresses. There will then be a forward component to the forces h and h1 above) acting on the tool pulling it into the work and the backlash present in the cross slide screw will allow the tool to tend to be pulled forward, into the workpiece.

As the diameter of the workpiece decreases the forward pull (h and h above) increases as does the tendency of the work to climb and the tool to be forced down. Eventually the tool tries to submarine below the work into a restricted space between the work and the cross slide. It jams, causes problems and then the 'bang' we have all dreaded, a broken tool and possibly damaged workpiece as a consequence.. That is why large diameter work such a 100mm dia tubing etc is relatively easy to part off, because these effects are minimised at the larger diameter, it is as the cut progresses and the work gets smaller that the effect increases. I.e. the smaller the diameter the greater tendency to climb and the forward, pulling force acting on the tool increases and the tool submarines.


With a rear toolpost as the flexibility allows the tool to be forced upwards it is pushed away from the work due to the position of the centre of rotation of the toolpost. hence less jamming. It is also mounted directly onto the cross slide and not on the compound and the latter allows more movement due to the two sets of dovetails.

Rear Parting Toolpost on Small Lathes.

There has been a lot of debate about the pros and cons of using a rear mounted tool post for parting off on small(ish) lathes. Some advocate the rear mounted while some argue that they are not necessary. This post tries to look at the advantages of the rear mounted tool post in a simple and clear way.

The issue of parting off with Myfords (and other small lathes) is well covered on the Myfords website here. Their theory coincides with my own except the position of the rotation centre of the front mounted toolpost but that's another story. I also have an extension of the idea due to the forces involved and a little analysis will make clear my ideas. I hope. I apologise for the length of the post and hope that it is not too boring.

Every lathe has some play in the headstock bearings, no matter how little, it is there, and there will be some flexibility in the tool (absolute rigidity is impossible) as well as some flexibility in the workpiece, metal unfortunately is flexible. it may be minute movement but the effects of all these are cumulative.


As I see it, the front mounted tool will tend to be forced downwards while the work is trying to climb the tool. Even if the tool is set exactly on centre before parting these effects will tend to force the tool below centre as the work progresses. There will then be a forward component to the forces acting on the tool pulling it into the work and the backlash present will allow the tool to tend to move forward, into the workpiece.



As the diameter of the workpiece decreases the forward pull increases as does the tendency of the work to climb and the tool to be forced down. Eventually the tool tries to submarine below the work into a restricted space between the work and the cross slide. It jams, causes problems and then the 'bang' we have all dreaded, a broken tool and possibly damaged workpiece as a consequence.. That is why large diameter work such a 100mm dia tubing etc is relatively easy to part off, because these effects are minimised at the larger diameter, it is as the cut progresses and the work gets smaller that the effect increases. I.e. the smaller the diameter the greater tendency to climb and the forward, pulling force acting on the tool increases and the tool submarines.

With a rear toolpost as the flexibility allows the tool to be forced upwards it is pushed away from the work due to the position of the centre of rotation of the toolpost. hence less jamming. It is also mounted directly onto the cross slide and not on the compound and the latter allows more movement due to the two sets of dovetails.

If you have difficulty visualising, imagine a 100mm cube of wood as a toolpost, knock in a nail towards the top of one side so that 20 mm are protruding to represent the tool, and then press down on the end of the nail and imagine the movement, this is the action of the front toolpost, now lift the nail at the end and this is the action of the rear toolpost. Both are greatly magnified of course, but it doesn't take much imagination to understand the consequences of these movements.

Eccentric Sheave

View of Machined Sheave Showing Locating Rib

The next part to be made was the eccentric sheave.  This was a relatively straightforward turning job but there was some query about the eccentric centre in relation to the centre of the sheave.  I have been referencing the valuable little book by Andrew Smith in which he describes the manufacturing process.  When drilling the sheave for the eccentric, he says to set the centre at 11/64" (4.37mm), however the drawing shows the distance as 9/64" (3.57mm). making a difference of 1.6mm on the throw of the eccentric.  Obviously that would make quite a difference in the valve movement on such a small model.  An enquiry on the Model Engineer magazine forum returned the answer in a very short time.  It turns out that the drawing is correct at 9/64" which goes to show that it is important to double check even the best of source material.  It is well known that there are often mistakes in published drawings and text so it is o=important to check and double check.  The internet interest forums are such a valuable resource these days, no longer are we working alone and in the dark, there is always knowledgeable person willing to offer suggestions and information.

The most tricky part of the sheave was the rib which had to be machined accurately to match the groove in the strap.  Initially it was a plain cylindrical machining job. A piece of FCMS (free cutting mild) steel  with a generous chucking allowance was turned to the overall diameter of the rib.  Left hand and right hand lathe tools were then used to turn the smaller diameter to fit the main internal diameter of the strap.

The now established centre of the eccentric was marked out, centre punched.  This was set to centre  (for the method of doing this see the page - 'Centring work in the 4 Jaw') in the 4 jaw independent chuck on the lathe and the boss turned to diameter.  The drilling to 7/16" (11.1mm) was done by step drilling with a succession of drills in the drilling machine.  That was quite accurate enough, no need for reaming.  To complete the eccentric a hole was cross drilled in the eccentric boss and tapped 5BA for a fixing grub screw.  Now for a bit of finishing bling with wet 'n dry.