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Member postings for Steve Crow

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

Thread: Help wanted with piston design
28/11/2021 17:37:47

Hello, I'm 16 months into building a 1/12 scale V8, powered by compressed air/gas. The bore is 7.2mm and the stroke is 5mm. I will not be using rings. There is thread on here about it but I haven't updated it in ages.

I've made nearly all the parts but I haven't even started designing the pistons yet. I made the con rods and the crank nearly a year ago!

I haven't been able to find any help on the net as regards proportions of the pistons. I'm sure somebody here can help.

Here's a diagram to help me explain:-


My questions are-

Is there a rule of thumb for the proportion of D to x? I realise that having a bigger x will help keep the piston parallel.

Is it more advantageous to have the centre of the gudgeon pin (p) between a and b as in the first diagram or is it ok below as in the second?

Grooves as shown on the left. Are they a good thing?

Finally, materials. I haven't made the bore liners yet and have been thinking of using brass. For the pistons, I have no idea. I would like to keep the weight down, how about ali with a steel sleeve for bore contact?

Any ideas, advice or experience would be very welcome.



Thread: Boring Head - Thread Quality
17/11/2021 17:32:57

Thank you Brian, I thought that they'd be rolled. Should be a better surface finish than cut as well.

I'm going to go for the commercial screw, even though it feels a bit like cheating.

M3 sounds really small but it is a tiny boring head, copied from this one but with an MT1 Sherline shank.



17/11/2021 16:46:43

I'm making a small boring head with an M3 "leadscrew".

I can single-point cut the thread but I've considered using an M3 screw as I can retain the hex socket on the head for adjustment.

My question is, in practical terms, would give this measurably inferior results than a single cut thread?

I know this depends on the accuracy of leadscrew on the lathe doing the cutting.

On a side note, are commercial screw threads cut or rolled/formed?



Thread: Spur gear diff rotation.
07/11/2021 11:30:57

If I understand it, the two pairs of "contra rotating" pinions are not actually in mesh.

Hi Peter, this animation shows the pinions in mesh. **LINK**.


07/11/2021 10:46:18

Thanks for you help. I am ok with a bevel diff, (here's one I made earlier - **LINK**), I'm just finding it hard to get my head around the spur gear type. It's the fact that there are two gears counter rotating transmitting the power rather than a single bevel gear.

Using the picture above. If cage B is fixed and shaft A is rotated once clockwise, I presume shaft C will rotate once anti-clockwise, the same as a bevel diff?

If so, would putting an idler gear between the two small pinions reverse that?



31/10/2021 15:33:40

Thank you Chris but I don't understand why the rpm is doubled.

31/10/2021 14:46:58

A bit of help needed.

Here is a picture of a spur gear differential I nicked off the 'net.


My question is, if shaft A is held stationary and cage B is rotated clockwise, which direction would shaft C move?

Logic and intuition tell me clockwise but I may be missing something!



Thread: Depth Stop for Collet
23/08/2021 14:07:25

I thought that this little device might be of use to someone.

I have made a start on the valves, guides and tappets for my model V8 and there are 32 of each of them, plus spares. To ensure repeatability, I decided to make a depth-stop system for 8mm watchmakers collets.

I started by making M4 tapped inserts for the back of the collets, in this case 2.6 and 3.0mm.


I then cut of the head of a long M4 screw and drilled 2.5mm holes at each end. In one end I "loctited" a M3 allen head screw turned down. The other hole accepts the actual depth stop.


I then made a couple of lock nuts from a scrap spur gear (hence the splines) turned down so they fit inside the drawbar. I then made a long and short depth stop for different purposes.


Here is the complete unit.


Some of the parts I have to make are very shallow so the long stop should be a great help.


The collets can be used normally when I don't require the stop as the clearance in the M4 insert is greater than the collet size so long stock will pass through.


Thread: Scale gearbox (or how I leaned to love aluminium)
15/07/2021 19:48:35

"It was made from a scrap of 1/12" x 5/8" bar 30mm long. It weighed 52g originally but it is 10.6g now."

Sorry, that should read 1 1/2".

15/07/2021 19:14:51

This is a housing for a 1/12 scale Hewland gearbox to mate with an engine I'm making.


This is my first attempt at machining aluminium. I've drilled and tapped it in the past but that's it.

I really disliked the stuff and thought it was the Devil's metal but I wanted to keep this light as it's hanging off the rear of the engine so I bit the bullet (billet?).

It all went so much better than expected and I got a great finish.


I've rubbed down flat external areas but all curved and internal bits are straight from the lathe. It tapped nicely as well with a drop of WD40 which was just as well as I had 33 hole to do, nearly all M1.2.


It was made from a scrap of 1/12" x 5/8" bar 30mm long. It weighed 52g originally but it is 10.6g now.

I've just got to make a few more parts then I can drop in my differential.



Thread: Nickel Electroplating
19/06/2021 22:52:55
Posted by Jouke van der Veen on 19/06/2021 14:38:59:

Nickel electrolytically deposited on the substrate is delivered by the nickel present in the bath. One of the bath constituants is often nickel sulphate. Of course, some ions dissolve from the anode but this hardly contributes to the coating.

I should of clarified. I was considering making the initial bath using nickel silver.

19/06/2021 17:41:41

Thanks for your replies.

Dave(s), I didn't consider the atoms migrating in order! The result sounds unpredictable and substandard.

I only asked because I've got plenty of bits of nickel silver. I'll just have to shell out for some pure nickel.



18/06/2021 16:48:01

Quick question.

Has anybody tried using nickel silver as anodes for electroplating copper or brass?

It's copper, nickel and zinc which can all be plated so I can't see why not.



Thread: Model differential
26/04/2021 14:13:43
Posted by Ady1 on 26/04/2021 12:58:05:

Beautiful work as usual

edit: does the bar poking out on the left side serve any purpose?

Edited By Ady1 on 26/04/2021 13:00:07

The bar will be cut to length eventually. I've just left it long to make it easier to assemble and take apart for testing.

26/04/2021 10:22:17

I've completed the parts for a differential for a 1/12 scale gearbox I'm making.


Here it is assembled next to a penny.


The central pin will be cut down so it's flush with the cage and slotted for a screwdriver. This is the first time I have been able to test the bevel gears I made for this and I'm very happy. Everything meshes smoothly.


Thread: Home made screws
03/04/2021 10:48:45

Hi Phil

Glad it worked for you. Are you using mild steel?

It makes threading a joy. I wouldn't have contemplated making all those screws using any other method.

Also, if you inspect the thread under magnification, the finish is so much better than a cut thread. there is no tearing and the surface has been burnished by the rollers. It's stronger too as the "grain" of the metal has been "folded" rather than cut. I believe it work hardens the material as well.

I also threaded my screws under power even though they are only 2.5mm long. I have to reverse off manually though.

These Russian tools are great value. I've got a few sets of involute gear cutter from the same supplier. I might do a post about them at some stage.

Nice lathe you have there, by the way.


02/04/2021 13:11:35

Hi Phil

I think that extra couple of thou is critical. If the blank is only slightly larger than the pitch diameter, there is too much metal and nowhere for it to go so it jams and breaks.

It's a bit like tapping a slightly undersized hole - the tap's going to break.

I'm sure that you could go down a bit less than 1.35 mm for safety.

I've done all my threading dry so far. My thinking was that I'm not actually doing any cutting so there is no advantage. I might be wrong on this.


02/04/2021 10:06:13

I have just been experimenting with those thread rolling dies with very little success.

Any advice would be welcome.


Hi Phil, maybe the problem lies in the diameter of the blank you are threading.

The theoretical diameter for thread rolling is the pitch diameter of the thread. This makes sense if you think about it as the tool deforms equal amounts of material in and out.

Pitch diameters :- M1.2 =1.038, M1.4 =1.205 and M1.6 =1.373.

In practice the blank should be slightly less. I've only tried the M1.2 die and I turned down the screws to 1.02mm. It works fine on 1mm rod as well.

For M1.6, I'd give 1.35mm a try.

The rollers don't revolve. The alignment of the wheels forms the thread. Did you get a little threaded rod with each tool? When assembling the tool after a breakage, this is used to ensure the rollers are in the correct alignment when tightening everything up. If you can screw this in and out easily, everything is in the right place.

I hope this is helpful, let me know how you get on.


01/04/2021 22:34:48
Posted by Brian H on 01/04/2021 18:07:23:

Steve, for slotting, do you hold by the thread or do you have a threaded collet?


Edited By Brian H on 01/04/2021 18:07:39

Hi Brian, I just use a 1.2mm collet clamped on the thread.


01/04/2021 17:52:54
Posted by Ian Parkin on 01/04/2021 16:48:56:

Can you talk us through your mass production process and what tools fixtures you used?

Hi Ian, I can try.

First of all, I used a Sherline lathe and milling machine to make them from 1/16" silver steel.

I did it in three stages.

First I fixed a turning tool (I used 0.2 radius "ali" type inserts) and a rear mounted parting tool to the lathe carriage about 20mm apart to leave room for the threading.

The material is held in a watchmakers collet and turned to desired diameter and length. Set Y dial to zero for future turning.

Use jewellers cup burr in pin vice to lightly chamfer.

Bring forward the tailstock with thread rolling die/head attached and form the thread. (I bought a Russian made M1.2 thread roller on ebay for £13.01 inc. postage from St. Petersberg. Fantastic value for money.)

Repeat with the cup burr.

Bring in the rear parting tool, index from the screw shoulder and part off.

Withdraw some more rod from the collet and start again.

For the subsequent screws, your Y Is already indexed to zero and you can do it in one cut.

The next stage is just facing the heads after parting. Again you can just set your X and do them all in one cut.

The third bit is the slotting.

I used a homemade square collet block held in a mill vice and a 0.3mm slitting saw.

I hope this is helpful. If you want more clarification, just ask.


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