index plate materials such as brass

Hello,

After a quick search for this using google to search the forum, the best material has come up before but machining cast iron or steel don't suit my circumstances right now, and to be fair, brass would also look pretty in any publicity pictures I may take of the setup.

I need to make some index plates to cut watch or clock gears on my bench lathe. Has anyone used brass, and how well does it hold up, say for occasional use?

My lathe has a spindle brake that engages when not running, that will reduce the load on the plate. I also considered 10mm acrylic sheet.

4140 or cast iron would be nice but I've only got access to my pantograph at present - so I don't fancy those options.

Brass will be okay for occasional use, especially with a lock - which all dividing heads should have and be used.

This will only be occasional use, thank you

If it is for occasional use you can use plexiglas. I use it all the time for occasional use. 1/4" thick and how ever big in diameter you want. You can put as many hole circles as you want on a disc. Easy to drill, won't rust, and are really durable. Go to a glass/sign shop and ask for off falls of larger sheets.

Brass vs Steel?

My first thought is I really do not see any difference in the difficulty level for using either of those two materials. The very same drill bits that can drill holes in brass will also drill them in steel. In fact, due to the tendency of standard twist bits to grab in brass, drill in steel is perhaps easier and prone to fewer mistakes. I have a complete set of drill bits that are resharpened to not grab in brass or plastics.

I need to make some index plates to cut watch or clock gears on my bench lathe.

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That seems to indicate that you are going to use direct indexing. That is, the plates will be used without any kind of indexing or dividing head. And it is for gears for clocks and watches so a decent amount of precision will be needed. I am not going to go beyond that statement as I do not know the consequences if a tooth on a watch or clock gear is off by any particular amount. At the very least, you would not want the movement to stop due to bad gear teeth.

Brass in general is softer and will wear more rapidly than steel. That may not be true for every combination of alloys, but in general it is probably true. So we must consider just how much use these index plates will get and just how much wear is a tolerable amount before the plate is no longer useful and must be replaced. I can't answer that, but you should. And even with a completely accurate answer, it will still be hard to say just how long each metal would last.

But I would like to know just how you propose to make these plates. If you have a rotary table or an indexing head, then you have the perfect way to make them. I have often written about just how some almost perfect hole circles can be made with those devices. This is due to the fact that the worm mechanism in those devices functions as an "accuracy amplifier". With a 40::1 worm a second generation plate will have errors that are only 1/40 as large as the first generation plate used to make it. And a third generation plate will be another 40 times better than the second generation one. So, by making three generations of plates, the first one can be laid out in the most crude manner possible and by the time you make the third generation plate from it, the errors will be below the level of the errors in the rotary table or indexing head that you use.

If you are going to use the plates for direct indexing, then a third generation one would be needed. But if you are going to do the actual work with a rotary table or indexing head, using the plates you make, then the second generation plate is all you need because the second amplification of the accuracy will take place when the plate is used and your parts will be as accurate as your rotary table or indexing head is capable of.

However, I have seen many other methods for making indexing plates. Some of these use rather questionable pattern pieces, like circular saw blades. Others are just laid out by hand, using dividers. Many people do it this way, but in my mind, it is questionable. It is especially questionable for direct indexing. And it is especially questionable if you are going to be making gears.

Still other plates are made on milling machines with DROs (digital read outs) using coordinates generated by those DROs or by a CAD program. These can be satisfactory if care is used in drilling the holes: and that is another subject. Still more have been made with milling machines with CAM or computer assisted machining capability. Those are also very acceptable, again with care in how the holes are drilled.

But back to the basic question. I would have to say that steel would be my choice and for indexing plates I would probably choose a harder alloy, like 4140, prehardened. That alloy will give longer service while not being excessively difficult to work with. If you decide to go with brass, I would recommend a hard brass, like Marine Grade 485.

When I spoke above about taking care in drilling the holes, I meant things like this:

1. Starting each hole with a spotting bit. That is a very short bit and carbide would be best. This is to ensure that each hole is located at the desired location and there is no sideways drift as the first penetration is made.

2. Use screw machine length drill bits to actually drill the holes. Same reason as above and again, carbide bits would be best.

3. Drill the hole a bit undersized at first. The initial drill bit's flutes will become packed with chips and those chips will create scratches in the hole. This is not desirable.

4. At this point you should use a larger spotting bit or a countersink to create a small chamfer at the entrance to the hole. This will help guide your indexing pin into the holes and not have their edges becoming damaged by that.

5. Then drill the hole to it's final size. If you choose to use a reamer (step 5 below) than this would be a few thousandths smaller than the actual final size.

6. A final step would be to ream the holes to their final size. This will ensure accurate diameters and a nice, scratch free finish in them.

This is a lot of steps, but it will ensure accurately finished holes. At the very least, this should be done on a good drill press. A vertical milling machine would be a lot better. Oh, and do use a good quality cutting fluid for all of the above. One that I like is Tap Magic but there are others, many others.

I think brass would be just fine, occasional or regular use. If you cut tapers in the plate and use a matching tapered pin, the wear will be minimal and for the most part self-compensating. In this case the pin would need to be spring-loaded with sufficient pressure to hold for the operation.

Thank you to everyone for taking the time to respond. I didn't want to make the first post too heavy but will go in more detail and answer some questions.

I have a decent milling machine but it's going to be stuck in storage until the pandemic is stable. I recently acquired a Taylor Hobson K pantograph so it should be possible to lock the stylus to the table and do some light milling. This limits my slowest speed to 3000 rpm so I'm a bit apprehensive about the practical side of machining any of the good stuff in a decent thickness. I've also acquired an old Grafton 12" rotary table so that I can make the indexing plates 8-10".

The pantagraph should do the job but the spindle is a bit limiting for tool holding.. I'm going to turn an adaptor for a small keyless chuck to go in a 3/8" collet.

My plan was to mill the plates completely from sheet stock on the rotary table fitted to the pantograph - drill the indexing and attachment holes, mill the center hole, then the OD - leaving a square sheet with a large circular hole in it - clamps can be used on the corners with some plywood between the table and work.

I figured that 8-10" plates should be pretty accurate on the lathe.

4140 or EN19T seems like the best option for durability and reduced internal stresses.. I'm just not convinced it's as practical to mill on the pantograph - to cut through the plate with a small cutter, and to do any hole finishing. Brass sounds like it will last long enough until I can get access to my mill.

EPAIII's points about finishing the holes are going to be interesting, I need to check the clearance on the machine to see if there's enough height to switch between drills and reamer with everything fitted - it will probably be close.

I do have a drill press but it won't be suitable for that size/weight of rotary table.

Traditional wheel cutting engines used a large brass plate for indexing. Watch & clockmaking are very different things. What are you trying to do?

How many times will each plate be used? That would seem to drive the choice of material.

Watch parts are my main concern, I don't really like working on clocks. There's also a wheel in the slow speed escapement in a specific pre-war camera which can be a bugger to remove for servicing, it would be handy to have some spares.

Quantity wise, mostly occasional parts for now, and a batch of the camera escapement wheels.. Eventually I'd like to batch produce more watch parts but there are other bits I'd need to source or practice making before going that far.