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Member postings for John Shepherd

Here is a list of all the postings John Shepherd has made in our forums. Click on a thread name to jump to the thread.

Thread: Connecting a pair of motor controllers.
04/06/2019 16:07:08


Have you considered using two single pots and connecting the shafts with gears so that one is connected to the control knob and the other is a slave?

The advantages are:

  1. you can make mechanical adjustments to reduce the difference (if any) in the two pots.
  2. two single linear pots will be easier to source
  3. there are plenty of cheap plastic gears around that will do the job.


04/06/2019 15:46:55


Have you considered using two single pots and connecting the shafts with gears so that one is connected to the control knob and the other is a slave?

The advantages are:

  1. you can make mechanical adjustments to reduce the difference (if any) in the two pots.
  2. two single linear pots will be easier to source
  3. there are plenty of cheap plastic gears around that will do the job.


Thread: 3D Printer Engineering
21/11/2018 13:43:01

OK moving on from astrophotography and stepper motors, one of the other issues that receives a lot of comment on printers is the standard of wiring and connectors used.

My solution was to move away from the Arduino plus RAMPS controller board that I had used previously and use a Duet3D WiFi board. Not only does this use suitable screw terminal blocks but has Molex KK style connectors that lock in place in one direction only. The board is larger than the RAMPS so that makes room for neater, more reliable, wiring.

I have a lot of admiration for how the RAMPS interfaces with the Arduino but a custom board with several additional features designed for the job, although not without challenges, is a big improvement. Changing over to the Duet was like moving from a ZX81 to a Pentium PC.

The board can be powered by 12 or 24 volts and I decided to opt for 24v. This is a safer option because of the reduced current and the wiring to the heated components can be reduced in size. Consequently, the wiring is easier to terminate and more flexible when it needs to be. Heated beds are normally dual voltage and the cost of replacing the extruder heater and the fans is low. Also, I needed to replace the PSU anyway.

Wiring the print head can be a bit of a challenge with a stepper motor, two fans, a thermistor, and in my case, the IR Z probe. A small PCB and removable multiway connectors made wiring much neater and maintenance easier. The heater uses a separate ‘Deans’ connector often used as a battery connector on model cars etc. though.

Good quality crimping tools made for the connector in use are a must and I have learned the hard way by skimping on these. Some people recommend soldering crimps, but I believe there are good reasons not to do this and prefer to use them as intended, besides if crimp connections are good enough on aircraft, they are good enough for my printer.

Has anyone got any alternative solutions?


17/11/2018 09:25:07

Picking up points about the Z axis that MAY make a difference for better or worse.

First re Steps/mm – my M92 Reprap Gcode values are X80 X80 Z400 E417 ; set steps per mm. using an Acme 4 start lead screw

I have seen a lot of comment about fitting bearings on the free end of the lead screws and as I have not experienced much in the way of wobble have followed the example used on my lathe cross slide and other x y tables where the free end is left to float. Perhaps someone could explain the pros and cons of this?

I used a plum connector to connect the leadscrew to the stepper motor, not particularly because it provided better alignment, in fact the axial movement is quite stiff, so I doubt it moves much with the loads applied to it, but because it has a clamping arrangement on the two shafts. My previous coupler was just a straight one held in place with grub screws. I didn’t like the idea of grub screws on shafts with no flats or indentations because of the possible damage that might make disassembly and reassembly difficult. I also tried the flexible couplings with spiral slots in them, but these introduced more wobble and artefacts on the vertical part of prints, probably due to poor manufacture or perhaps design. Again it would be good to have a definitive view on the use of these.

In the past I have tried the sprung anti backlash nuts on the lead screws and these were useless, in any event the weight of the carriage on the Prusa takes care of that. I now use Delrin nuts that are smooth and don’t have much play.


16/11/2018 18:31:24

As promised, here is my printer:


16/11/2018 15:43:30

Ok Here goes.

I will feed in just one or two mods/issues at a time to hopefully keep the thread focused.

My printer is based on a Prusa i3 and uses a Duet WiFi board (perhaps more on that later). It runs on 24v, has a laser bed level compensation system, heated bed and a glass print surface. ( I will add an image later)

The issue that raised some response was using an Acme leadscrew (or a ball screw) instead of threaded rod as used on the Z axis in the original Prusa design. It was suggested that using a Ball screw (and I suppose an Acme thread), threw away resolution. My printer came with Acme threads on so can’t comment on if there is any improvement in print quality but it seems to make sense to use properly machined components made for the job rather than studding that because of how it is made may not be straight or have a great deal of dimensional accuracy.

At the risk of getting the math wrong, I won’t go into the calculations, but my z axis parameter is 400 full steps for 1mm movement. The TMC 2660 driver on the Duet has 256 micro stepping and I can get Z movements of 0.05mm with repeatability by using the function on the interface. I don’t think that is any sort of compromise. Smaller value ‘Baby step’ offsets can also be made on the fly and can be useful in making adjustments to squash down a first layer on difficult prints, it is also useful for setting up the nozzle height above the board, but I have not found any other uses for it.

Connected with the Z axis is the laser module that is used at the start of each print to adjust one of the two Z axis motors to eliminate any tilt of the bed and then to measure the relative height of each corner of the bed so that software corrections can be applied if needed. It also acts as an ‘endstop’ and allows precise setting of the nozzle to bed height.

That all for now, but there is more!

16/11/2018 09:44:45

Following on from an exchange of views I had with Neil in another thread about how many of the popular 3D printers are engineered, is there any interest in a new thread about the subject?

In a thread about cheap printers, I complained about what I consider to be poor practice such as bearings held in place with cable ties, poor rigidity, 'all threaded' rod used for construction and for lead screws etc. and outlined some of the modifications I have made.

Neil expressed the view that the loads and demands on a 3D printer are a lot less than on machine tools and there is no need to employ the same engineering standards.

My stance is that there are many variables in 3D printing and it is good practice to reduce as many as possible. Besides, I am more at ease using a better engineered printer just as I would be when using any other well made machine.

I realise that a lot of printers are supplied as kits and many users do not have engineering experience to build a complex design or the machining facilities to improve them, but if that route gets them into 3D printing that is good.

There are a lot of modifications to 3D printers featured on the internet and many of them are ill founded, so perhaps it would be good to get some input from the wealth of knowledge on this forum?

If there is an appetite for it, I will be happy to put my head above the parapet and put my thoughts on the firing line.


Thread: Cheap 3D printers
15/11/2018 10:49:49


I take your point about some of my mods not making much difference but take issue on the 'mistake of replacing threaded rod with ball screws' . A machined ball screw can provide greater precision and at least equal resolution as long as you also upgrade the stepper driver.

There are many variable when 3d printing and I think it is good practice to reduce as many as you can for piece of mind if nothing else!


09/11/2018 11:31:18

I was grateful to receive a Prusa 3D printer from my Son when he moved onto another design.

It performed ok but what annoyed me was the poor engineering :

1. Threaded rod and plastic parts to form the base for the bed.

2. Linear bearings held in place with cable ties (later models used U bolts on the bed bearings but that was not much better IMO.

3. Printing surface mounted on springs so that it could be adjusted to make it level. I accept that levelling adjustment is needed, particularly as the bed base plate cannot be relied on to be flat, but this introduces instability and side to side movement.

4. Stepper motors coupled to Acme lead screws with simple joints using grub screws (no flats on either as well).

5. The vertical frame was not braced in the original design.

6. The bed runs on 8mm dia rails that can only be supported at each end. This is not as rigid as it could be.

I accept that some of these issues have been addressed in later models but my mods so far include:

1. New base using 30 x 30 extrusion and aluminium end plates for the base.

2. Replaced all linear bearings with ones enclosed in housings that have mounting holes and made an aluminium X carriage to replace the plastic one.

3. Printing surface mounted with silicone spacers to give some adjustment, but with much reduced side to side movement.

4. Used Flexible Plum Couplings with a clamping action rather than grub screws to connect stepper motors to lead screws.

5. Braced the frame using aluminium supports.

6. I am about to replace the round rods and bearings on the bed with flat linear rails that can be supported along their entire length.

There are several other minor mods including those to wiring, belt fixings and tension adjustment etc. and I do get prints I am satisfied with.

I suppose my point is that a cheap printer is just a kit of parts as a base for development, like some of the cheaper Chinese machine tools judging by the number of modifications that appear in MEW etc.


Thread: What's the best way of telling a poster he's wrong?
25/01/2017 14:31:38

Oh dear! A perfectly reasonable opening post and we drift on to debates about rotten fish and tightening chucks.

Now that has made my mind up about not contributing in future.

24/01/2017 18:09:40

Mark C

I am sorry I don't understand what you mean by 'fast'.

Am i right in thinking you mean 'fact' (not being pedantic just need to make sure). If so, yes I agree.

Edited By John Shepherd on 24/01/2017 18:10:38

24/01/2017 17:10:25

Thank SOD for raising this, I share your concerns.

Another thing that annoys me is when there is a post asking for advice on a particular subject or machine and they get a reply such as "I know nothing about it/ don't have such a machine, but..." and what usually follows is of no use at all.

1. Expecting moderators to take action, unless there is hostility or obvious bad practice, can be difficult for the moderators and can escalate the problem if my experience of other forums is to go by.

2. Unless I am sure of my facts or have recent experience, I would rather sit back than offer an opinion. Many things I did in my early career are now looked on as bad practice and I would rather learn from that than say 'we always did it that way and no harm resulted from it'.

3. Why must people get on the bandwagon as soon as Health and Safety is mentioned? Most of the time we are not concerned with not playing conkers, just helping with good practice so that many of us can continue with our hobby with our hands and eyes intact.

4. A recent thread on Consumer Units was a good example of some misguided information. A good case if there was one of understanding the subject before replying and a need to point to some independent up to date evidence and regulations.


5. If you don't agree with a post or the advice it offers, perhaps it is best to explain why and offer evidence rather than wading in with an opinion or comments like ' in all my years as a professional xyz we never did it that way'.

6. Humour can be a good let out but often it is misread, so use it carefully, especially if it is directed at an individual.

7. From John Reese: 'There are those who hold strong opinions that are not based on fact or experience. In that case, the best approach is silence.' This is the stance I usually take and it results in non-contribution rather than taking part, but if we had a more open forum it would be better for everyone.

8. I have been close to canceling my subscription to this forum because of some of the comments, so please can we all realise what is wrong and make sure that the tremendous amount of good information is not diluted and make it a happier place to visit.

John Shepherd.

Thread: Bench Grinder
06/12/2016 15:49:17


Have you actually checked the threads to see if they are R or L handed?

Another reason for the instructions stating that a grinding wheel should not be fitted is perhaps the lack of large dished washers. The brush mounting may not need these because of its metal hub but the grinding wheel needs them to spread the load.

Thread: Ferrari Symbol
30/11/2016 11:07:49

Went to the Haynes motor museum a few years ago and they had a special Ferrari exhibition. There was a document on display that showed the permitted sizes and proportions of the logo for a variety of uses, the tolerances were very tight as I recall.

Thread: RE: Tony Hicks Electrical Safety Letter in MEW 248
05/11/2016 11:23:28

My experience with H&S seems to be at odds with some others. I worked in an environment where we had to do a risk assessment for all major works because if we didn't get it right, there was a serious risk of injury and/or loss of life and it would stop us doing similar beneficial work in the future.

Our H&S team never stopped us doing anything. They suggested how we could look at problems and minimize the risk. Once we showed that the risks were acceptable against the outcome and that control measures were in place, without making the job impossible, we were good to go.

I don't get upset about legitimate H&S issues but I do object to the publication of bad practice and poor uninformed advice. Fortunately, we don't get too much of that in the MEW magazine but when it is perceived that there is an issue, there is often a massive over-reaction as in this case.



Thread: Audi recall
24/09/2016 15:59:40


Just heard that the recall has been extended to all cars made in Germany.

(My Audi was made in Hungary so it is not included)wink

Thread: Possible new ideas for Model Engineers' Workshop
05/09/2016 10:20:18


A vote for 3D projects as long as they are directly related to engineering projects and do not stray into general 3D printing. If you are considering this, you may wish to think about how the 3D printer files might be shared?

Don't mind articles on repairs as long as they are about innovative techniques and not specific to a product few of us own. JS posts on how he carries out repairs are a good example of the former, mending frying pans is not IMO. (apologies to Des Bromilow).

Other ideas:

Producing a model from a full-size item when no drawings exist. I marvel at the work done by the likes of Cherry Hill but wouldn't know where to start.

Finishing projects. There have been articles on painting etc., but getting a good surface finish and how to best use paint and other coatings eludes many of us judging by requests for help on this forum from time to time.

Hope that helps.

Thread: 3D Printed Laser Centre Finder
15/08/2016 17:06:15


I used a 20deg angle for the laser and it is very sensitive for a vertical bar. The ring almost snaps into the horizontal when X & Y are aligned but I need to use it a bit more to give a definitive answer on accuracy. I don't find finding the centre line of a horizontal bar so easy, though.


15/08/2016 15:43:36

Recently my son made me a 3D printer, I am still on a steep learning curve with both the printer and drawing in 3D so I decided to make a rotary laser centre finder as a project. I will not re-open the debates on 3D printers or laser centre finders already aired on this site, but I can say I found the exercise rewarding and as a bonus I have a useful tool I would not otherwise have made. If anyone wants further details please ask.


Thread: Rivet Squeezer
15/08/2016 14:54:28

I have a similar set made from castings as mentioned in the previous thread. I can't remember how much the castings cost but if the rest of my tools have increased in value by the same amount as the prices charged by 'these people' perhaps this model engineering hobby has not kept me as poor as I thought.

( I believe castings are still available from Polly?)



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