Member postings for JasonB

I'm not sure it is any better than an eccentric driven slide valve, if anything it is a bit more restricting as you can't alter the timing by moving the eccentric on the crankshaft. There are quite a few different arrangements for operating the slide valve with the links in various places relative to the cylinder's point of rotation but I've not seen many with this particular one.

I would think it is better than an oscillating engine where the valve arrangement is done by positioning holes in the two faces where cylinder meets trunions as it is easier to seal the shaft of the trunion and not have to worry about the flat faces.

As oscillator is also generally a shorter engine than one with trunk or bar guides for the cross head and a separate conrod but a bit harder to construct as the trunions need to be perfectly square to the cylinder/piston rod.

Edited By JasonB on 26/09/2023 18:59:59

Firstly I must say sorry to all those who eagarly clicked on the thread title that were looking forward to another off topic gardening thread

This one is actually about building "Digger" a slide valve oscillating steam engine from a set of castings that were at one time offered by B & R Tite. This seems to be a very rare engine the only previous mention of it on the net was buy the owner of these castings asking for information about it and a small entry in one of the "World of Model Enginnering" mags. I was contacted by said owner recently asking if I would consider building it for him as he felt it was beyond his ability and being so rare a mucked up casting would not be replacable particularly as he only wanted a limited amount of paint so any substitute materials would also have stood out out and he even hoped I'd be able to make an engine that ran well

The only image of a completed model was on the rather poor photocopied detail sheet and I was unable to track down the example in the Science Museum that it is said to be based on. If anyone has an image of the full size engine I'd love to see it.



I asked the owner for some images of the drawings and picture sof the castings and after looking through them and seeing that the castings looked to be of good quality I decided to take on the challenge. They are all Gun Metal which is not my favorite visually and it can also be a bit soft depending on what was boiled up in the pot. The one proviso for taking it on was that I could substitute metric fasteners in place of the BA ones shown as I prefer my spiral flute taps, stub length tapping size drills and it's easy to use cap head screws to hold it together during construction.





While waiting for the castings to arrive I quickly drew up the engine in Alibre to my often used 24mm bore which was an easy conversion from the models 3/4" bore using the ratio of 1/32" = 1mm and was able to determine a couple of missing dimensions and check that it all went round and round as it should. I may make one of my own at some time but will probably go for 18 or 20mm bore as the 24mm is a bit big unless I want to start adding more detail.

As Bill says I term a bolt as having some plain shank under the head, a screw is threaded for it's full length.

UN threads below 1/4" use the number system as the increments become smaller than fractional equivelents

It's the same, just ignor the third dimension. 9.3mm up in Z and 3.68 in X.

Alibre puts it in the same position and if I extrued equally either side of the sketch it just gives the Y position as zero

I've been able to get rid of that now by sketching the helical cut on a different plane

David, I may be possible with a bit more work or positioning the sketches differently but I was getting a "step" in the bottom of the groove where the straight one met the helical.

By effectively filling the bottoms those steps become part of the solid

Yes I created a plane at the end of the first angled cut then projected the sketch to be extruded for the next part.

Here is a rough one done as described above, the three angled parts of the slot are helical cuts, the rest a revolved cut. Fillets added to internal and external corners though the external need looking into some more a sthey are not right. All options that are available in ATOM. I suggest making the groove deeper than needed and then extruding a solid cylinder to bring the bottom out to the correct depth.

Pitch of the helical cuts needs to be pi x outer diameter assuming 45deg angles. When sketching the rectangle for the helical cuts make it 1.404 times the width of the straight groove again assuming 45 degree angles. I simply used 14mm and 10mm in the example and a 100mm dia with 314mm pitch to the helix

Edited By JasonB on 26/09/2023 14:54:19

A work around in Atom would be to treat each part of the groove as a separate operation applying different cuts to each. The main straight groove could simply be sketched onto a plane and then cut. The ones where the grove runs at an angle would need to be done with a helical cut for each angle so you would need to do that in three operations.

Once you have these done then as David says add the fillets as a separate item.

Although the mamods run at a low pressure they struggle with the skin off a rice pudding so as Noel says what do you want to use the boiler for, little point in making a low pressure one if it won't run a wobbler than has been built a bit on the tight side. Or you overheat and melt the tube while trying to learn how to solder.

The thicker walled tube of known parentage will not be a big percentage of the overall cost by the time you have added in silver solder, the other tubes and platework and your time. Even less if you only have to make the one boiler not two.

Watch what you are buying on e-bay a lot of the 108mm tube will be "plumbing black lable" which at 1.5mm wall is getting quite thin for a boiler assuming that is what you intend to u yellow gaugese it for. And the thinner wall will obviously affect the price as you will be getting less copper than say 10swg wall tube.

Edited By JasonB on 26/09/2023 10:42:09

Mine are mostly ARC with a couple of YG-1 for the smaler sizes ARC don't do.

Negative geomerty really applies to HSS tooling, the CCGT inserts are positive and cut brass and other non ferrous materials very well. Try ARC or APT These will fit your existing holder

I'll see if I still have the broken bits in the morning.

I've found they snap at the side of the hole long before there is any sign of bending. Though the one I made at school is still going strong, just a couple of bought cheapies that are in bits now.

Edited By JasonB on 25/09/2023 18:50:39

I almost always use machine reamers these days with plain shanks they will ream to within about 1mm of the bottom of a blind hole

I thought it was hidden surcharges that often got applied at the last moment before checkout that were banned. By making it obvious from the start that they will charge you more to cover their costs that is not breaking any rules.

The way to measure the tool size is to put the MT4 centre into the headstock and then measure from th ebottom of the slot in the toolpost to the point of the ctr and that will give the maximum height you can fit

If using insert tools sometimes it may pay to come down a size so you can use smaller inserts which works out a bit cheaper so a 10mm sq shank would tend to take CCGT 06 but a 12mm sq shank CCGT 09. Ditto with HSS as it is a lot easier to grind the smaller sizes rather than the max and you will only really need the max shank if you have excessive tool overhang. I mostly use a mix of 8 & 10mm on my 280 which suits the work I do.

I did search for it yesterday and found it fairly quickly.

This Thread

Make sure the gear cover on the left is making proper contact with the interlock switch, the metal tab can get bent.

Also the big red safety switch on the left needs to be pulled out and rotated to activate it

Edited By JasonB on 24/09/2023 18:51:14