Workshop working tolerances

I machined all the castings and barstock gear blanks for my 2" Fowler and just had the actual gear teeth cut, that way I could control the fits as much as possible. Might be the way to go if you need some doing again.

Can you not alter the gears you have ? For example the crank gear you mentioned which is bored 2 thou over, maybe bore it out much larger, and turn up a new centre to size, which could be a press fit or welded/soldered in.

I doubt a gear being 1/16 undersize in width will make any difference to anything.

.

This doesn't qualify as a recommendation, Dave, but it may be worth giving Leek Gears a call: **LINK**

http://leekgears.co.uk/take-a-look/

Family name = Leek

Location = Leek

Always looks 'proper' when I drive past, but I've not yet had the need to use them.

MichaelG.

Generally speaking it's not actually that bad to control sizes, especially if you already have what you want to fit it to, to hand.

For example, you're machining a bar to fit into a cylinder of sorts.

You don't really even have to be concerned with numbers too much at that point. Just creep up on the nominal size that you know the other is and keep checking it by hand against the matching part after each and every light cut.

You can then go from either a force fit to a push fit to a sliding fit, if it feels a bit tight to begin with, just let it wear into it for a short while.

This is why you didn't have to be a walking calculator to successfully manage sizes years ago.

It obviously changes and gets tricky when you want to make lots of bits for parts that you don't have to hand, that repeatability is what industry continually wrestles with.

There would be people who really don't like that idea but in actual fact this is the way craftsmen used to work, by feel and intuition. 

Michael W

Edited By Michael-w on 03/09/2018 12:15:53

Tolerances are for making spare parts interchangeable or special fits such as force fits where a critical difference between parts is necessary so actual precise dimensions are unlikely to be essential on models. Colin

It's how Rolls Royce used to build Merlins until Packard rejected their tolerances as too broad.

Contrary to the urban legend, RR were actually grateful for Packard's input.

Neil

Builders work to the nearest CM.

Furniture makers to the nearest MM.

Boat builders to the nearest boat.

Just as an update and also possibly help to others.

I eventually found a company nearby that was prepared to take on the cutting of the 2 gears for me.

They were ready for collection within 2 days and finished to a very high standard. The service and assistance given was superb as was the price.

The company is a medium sized business and is happy to carry out small / one off jobs.

I'm not sure if the advertising will be approved by the moderators but the company is Northern Broaching Services in Radcliffe, Manchester. Contact details are available on the web.

Hopefully this may be of some help to others in the future for gear cutting or more complex machining.

I have no connection with the company other than that of a very satisfied customer.

Take bigger cuts, but measure how much is removed for x divisions on the machine scale. Each cut of the same size will remove the same amount, so at least the last two cuts need to be about the same number of divisions on the dial, taking into account whether one or two divisions more, or less, for that last cut.

If you need it any closer than 0.01/0.02mm, cut slightly oversize and finish with abrasive.

With practice, and confidence, it works.

Even with a dro, one cannot expect a 0.02mm (or less) cut to be accurate immediately after a 0.2mm cut, or more, if your tolerance is really tight.

Edited By not done it yet on 25/09/2018 19:43:55

Richard

Best way to work to chosen size is sharp tools, trust the feed dials and take a decent finishing cut. Anything under 5 thou / 0.1 mm is too little. Very much in the lap of the gods as to exactly what the cut will be. Especially on a small and, inevitably by absolute standards, floppy machine with a tool of unspecified wear.

Accurate sizing requires a stable cut which can only be got with significant load on the tool. To quote Dennis Turk lathe rebuilder extraordinaire "Everything is made of springs.". If your finish cut is too much smaller than your main working cut changes in load can alter the relationship between dial and actual feed. I habitually cut at 50 to 100 thou for main metal shifting and consider 20 thou a decent finish cut. Thats on a Smart & Brown 1024, a massive beast by home shop standards and very stiff. Even so I'd not put money on a 1 thou cut taking 2 thou off to within a couple of tenths. Usually doesn't mean always. On a 20 thou finish cut if I'm more than a few tenths out either something is wrong or I've been careless.

Clive.

PS :- "not done it yet" types faster!

Edited By Clive Foster on 25/09/2018 20:04:23

If you want to finish with a small cut, run the cutter past again on the same setting after the last big cut. This 'works out the spring in the tool'.

I prefer to take a large cut but not so much as to compromise finish.

HSS and ground finish carbide (GT) are good for fine finishing cuts. Moulded carbide (MT) gives the best finish for cuts of at least about 2/3 the tip radius.

Neil

Thousands of years ago when I was getting my first formal machine tool training I was always taught to plan the cuts in the lathe so the last cut was at least 20 thou, right to finish size. The instructor was always yelling "don't take skin cuts!" meaning cuts of a few thou. He was an ex British Army armourer, and could make anything from forged chain to a pocket watch, he was extremely skilled.

Mind you this was on large Colchester and Hardinge lathes, with tremendous rigidity. On smaller less rigid lathes you might need to reduce cut depth slightly, or reduce feed per rev. I have always followed the advice of "no skin cuts" and got good results most of the time. Some steels are such that it is very hard to get a good finish though, and in that case a grind or hone or lapping op may be required to reach desired finish at size. In free machining steels, take a big final cut right to size.

For a beginner I'd suggest learning with HSS tools, then much later, experiment with carbide tools, particularly on small lathes.

Just my $0.02 worth, I'm sure many dissenting opinions will follow as usual. Don't be afraid to try various tools and steels, draw your own conclusions, and practice practice practice.

Best advice I can give is rough with one tool & finish with another nicely sharpened tool, you won’t get a decent size & finish just by using one tool for both tasks, and this applies to milling also.

I also don’t agree with large finish cuts using HSS, carbide is a different story, but hey what do I know?

Tony

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For a long time I advocated the HSS first approach.

I now use inserted carbide for almost everything, and only use HSS when I haven't got a carbide tool the right size, such as screwcutting.

HSS is less stressful for beginners as you don't need to work it hard to get good results.

The downside of this is that when you change to carbide you either get poor results or chip inserts due to pussyfooting around with them. They do like high speeds and generous cuts. Some people won't persist long enough to make the transition and go back to HSS.

My experience of uncoated CCGT inserts on aluminium, brass, stainless and steel has changed my mind. These can cope well with HSS-style cuts but also allow you to turn up the heat. I recently bored out apart in 6082 using 2mm cuts for roughing and finishing to size with less than 0.01mm cuts with the same insert. That's a 200:1 cut depth ratio. The light cut swarf came off like a spider's web.

I would now advise beginners to consider starting with the CCGT-type inserts, either from the start or to transition from HSS.

Once you are confident with carbide, CCMT inserts are great for ripping up steel and inserted carbide parting tools are great too if you have power feed or a steady hand and a steady nerve to keep up the feedrate.

Neil