Another Comber / Coomber build

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DaveH:
Arnold,

You can get them here in SA :D
:beer:
DaveH

arnoldb:
Thanks Dave :headbang: - Aye - Somta makes them, but I nearly had a heart attack when I priced one for delivery here... I love their tools. In fact I exclusively buy their drills as they are reasonably priced and very good quality. But the spotting drills are off the charts, along with their milling cutters. I'd love to buy them, but economies of scale just does not allow it :doh: Have you found any others ? - I'd be happy to try them :D

This afternoon I had a good solid 4 1/2 hours of shop time :D

Started off on the base, and poked in the mounting holes for the columns:

I needed to mill a 2mm wide slot 6mm deep - and re-cutting chips is a problem, so I used a bit of wire to pipe in air from the compressor:

That slot would need 10 passes at 0.6mm deep per pass, so I dragged a chair closer and got comfortable. The first pass nearly done:

And a little while later, all done without any drama:

Then I flipped the base, and countersunk the column mounting holes, and also drilled two 3.3mm holes 10mm deep to tap M4 for the mounting the base to wood later:

A trip to the bandsaw, and I got rid of some excess from the base:

And milled the rest down to size:

I thought I was done, then remembered I still needed holes to mount the cam to - those followed in short order - 2.5mm to tap M3:

When it came to tapping the holes, a problem raised its head - my taps are too short:

That's not the first time I've had this kind of problem with my M3 taps... So I diverted a bit and made a quickie tapping handle for such cases:

It works a treat ;D - I just have to shorten the grub screw a bit:

I was thinking of making cast-iron bushes to press into the columns - to give good bearing properties combined with the brass that the cylinder block and shafts are made of. Unfortunately I could not find any in a suitable size, and while I do have some 25mm CI I could turn down, that seems a bit of a waste for such an expensive commodity... My next best choice was phosphor bronze, which I have quite a bit of and can get at a reasonable price, but research indicates that brass and bronze are "similar" metals and not good for bearings... Well, I could use steel then; it would be better... And then my eye fell on the 1m length of 17mm round PTFE I bought more than a year ago at a whim (and very good price).

Well... Why not?... I don't see plastics used a lot on model engines, but it's supposed to be as slippery as snot, so should make good bushes for this application. This engine will never be run on steam, so the issues with heat and water porosity shouldn't be a problem... There are two problems though; a) Would it be acceptable to use a synthetic material in a model like this ? and b) I've never tried to machine PTFE - not to any sort of accuracy anyway.

As to a) - It "feels" wrong, but then again, I wonder what the Verburgs, Duclosses and Spareys would have done; Some of their build specifications seem to use cutting edge materials in terms of what was available when they built their models, so I'm not going to feel too bad to add a bit of new technology...

And as to b) - Well, there's always a first time :)

Decision made, I started; I just used my regular HSS tools that I use on everything; I just made sure all the cutting edges were honed sharp. Cuts easily, though I had some chatter as I didn't add tailstock support right from the start, but even parting off is a breeze ;D - small diameter bush done:

Bigger diameter bush in-making:

Both bushes were turned on the OD to a press fit in the columns. I'm surprised; it was easy to accurately turn the plastic and get fairly acceptable finishes; I expected to struggle a bit with that, but it turned out rather well. Pressing the inserts into the columns made a fairly significant change on the ID's of their bores.
So I decided to try out my hand reamers on them; the 6mm one for the small shaft went well, and easily cut the bush ID to size again without any complaints.

The 12mm reamer was another matter; it wanted to chatter - severely... I was trying dry cuts, so I decided to try a little lubrication... But what :scratch:. I settled on a squirt of the synthetic oil/water mixture I use when parting off steel; it worked a treat and the reamer smoothed right out and cut nice even chips out of the hole :ddb:

For today's work, I ended with this lot:

I'm surprised by how well the PTFE bushes work :D:

The bush on the thin axle looks like cr@p though, so I think I'll make an aluminium insert to press in with a smaller bush that will be hidden by the flywheel hub later on. I can make the ali insert nearly invisible and things should look nicer.
:beer:, Arnold

arnoldb:
Today's four hour stint was done on the cam ring; it's not finished, but at least some progress.

I printed out the list of machining diameters (Many Thanks goes to Gail Graham for making up the spreadsheet I used), jotted down some notes of things to keep in mind on it, and sawed a bit of 2mm brass plate from some stock I have:

The plate was smeared with some permanent marker ink, and laid out for the most important bits:

On to setting up for machining, and oh dear; it's too big for my rotary table:

But it will fit on the lathe's face plate if I mount that to the RT :D:

I found a bit of wood off-cut from last year's kitchen renovations, and mounted the workpiece to it with some self-tapping screws through the center bit where the screws will be inside the cutting tool path. After that, I drilled holes through the corners where excess will be, but spaced to match up the face plate's slots when the workpiece is centered:

Mounted the face plate on the RT, and centered the RT on the mill as well as zeroing the X and Y handwheels - making an additional note on the cutting chart as to which direction I was moving the table when zeroing the handwheels:

Then I mounted the workpiece on the face plate, lightly tapping it till I could get it as centered as possible. The mill's X and Y wheels were left strictly alone on zero. The idea is to center the workpiece to the center of the RT here:

I also tried to get the base reference as parallel to the X travel of the mill as possible by eye.

Once I was happy that the workpiece was centered, I cranked on X to check on the rotational orientation:

I'd eyeballed the rotation to less than half a degree, so I cranked the RT to bring the line on center and zeroed its dial.

One disadvantage of having screw-on chucks/faceplates is in a scenario like this. In this case, the face plate screwed on with it's slots offset by about 30 degrees, so I couldn't use the RT's own degree scale:

Oh well, a permanent pen can make a new scale; I'm going to work without any cutting fluid, so the markings won't be washed off:

Next I bodged up a DRO - One day I'll get proper DRO's but for the most part I don't mind reading the handwheels:

:lol: - I did say "bodged up" :lol:

A final check of everything later, then a cup of coffee, pulled up a chair, and started... I'd calculated the cutting path for a 12mm cutter, so I drilled a 10mm hole to start things at the first coordinate after locking both X and Y axes on the mill and the RT's table. The hole is to give the cutter less material to cut away for a start - and prevent things moving:

Then I started with the 12mm cutter - same location and drill it out to size, mark the first coordinates on the sheet done. Then the roundabout began... Unlock RT, feed 2 degrees, lock, unlock X, dial in the new coordinate, lock, drill new location, mark as done on the sheet. Repeat. About 60 degrees later:

I stopped for the day at 178 degrees:

It takes about 45 minutes to do 90 degrees and a LOT of concentration (maybe I'm just a slow worker :lol:). One mistake, and I'll get to start all over :coffee: And it is a bit of a mind-numbing process, so better to carry on when I'm fresh. That's why I put a new battery in the caliper; so it can "remember" the last reading, and why I meticulously marked off each position as it was done and the state of the mill when I started; at least I can stop and pick up from there later.

:beer:, Arnold

arnoldb:
After a hectic work week and most of my weekend spent on getting an additional data center going for my company I finally got some shop this afternoon.

Finished off the last 180 degrees of the cam:

Drilled the two 3mm mounting holes:

I nearly forgot to clamp down the outer bit to the wood block before removing the bolts holding everything to the face plate... Fortunately I realised just in time and added a couple of screws through the mounting holes to keep it in place. The whole lot needed to be moved on the face plate to the outer ring center to finish things off, and if it moved it would have lost its place on the center piece that has the center position marked! :

Of course, last weekend in a blinding flash of idiocy I didn't drill additional mounting holes offset with the required 8mm from the initial ones, so no mounting holes could align with the face plate slots once I moved the workpiece to the new center :lol: - So I grew a forest of clamps to finish the rest of the machining operations:

That meant I had to stop during machining to move clamps around to get out of the way - here is one of the stoppages to move a clamp:

Finally - all machining done on the cam three hours into the shop session:

Not a miss-step or anything, and that's the hardest part of this build done :ddb: - just some clean-up left to do.

A close-up of the cusps left by the machining process on the inside of the cam:

They look worse than they are; a little very light and careful filing with a 250mm fine half-round file brought the roughness down very quickly till all the cusps were barely visible and even in size right around the ring. Then I wrapped some 320 emery around a bit of 50mm pipe, and in careful circular motions around the ring removed the last visible signs of the cusps. All this was done very carefully; things needed to just be smoothed out; the shape mustn't change.

After a bit of file work around the outside of the ring to get rid of tool marks, and a rub over some emery and a quick rub with scotch-brite to give a brushed finish, the cam is done:

What do you know? - things fit together and is starting to look like an engine:

That's the engine half-way done then; so only 90% to go :)

:beer:, Arnold

sbwhart:
That cam ring realy look the buisness, cracking bit of machining Arnold.

:clap: :clap: :clap: :clap: :clap: :clap:

Stew

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