mtt.tr
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You are probably right, total target fixation.
Now we would say they were on the spectrum.
- Thread starter Pete.
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You are probably right, total target fixation.
Now we would say they were on the spectrum.
mtt.tr
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And now we have servos and encoders.
Seadog
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And all done on a drawing board!
Pete.
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Tonight I stripped off the gear train plate that holds the gear sets for the feeds and differential gears. I wasn't going to remove this but after spending quite a considerable time with light oil and penetrating fluid I just couldn't get the feed release lever moving without feeling horribly gummy. In the end, I took it all off, stripped it down and now it works with a light touch and I'm glad I did because most of the gumminess was due to old oil in and on an internal sleeve which held the slidiging dog gear and no amount of lubing or trying to force oil into the oil nipple was going to clear it out properly.
Here is the plate on the outside with the banjos removed. The drive for the feeds comes in via the bottom middle drive spindle which has a spiral gear that engages with a 5 start worm on the silver shaft behind the plate. This spindle has gears which mesh with the shaft on the right (the one with the square drive) which then has a bevel gear on the back that drives a vertical shaft for the downfeed for hobbing gears and a horizontal shaft for infeed for hobbing wormwheels. There is also a neat automatic knock-off system that trips out the feed in either direction using sliding stops which you can see in the pic.
Here's the feed shaft with the 5 start worm wheel at the bottom of the pic. Also seen are the two bevel gears that provide downfeed and infeed power.
The internal gearing. Showing the spiral input gear and the sliding dog that engages the bevel gear with the feeds gearbox.Also see the three gears which carry power to the differential banjo. All the time that the machine is runnng, the feeds gearbox and differential gearbox are also turning all together.
It's important to note that because the differential gearbox is driven from the feeds gearbox, at a 1:1 ratio you can change the feedrate for optimum cutting without affecting the differential ratio.
Here is the plate on the outside with the banjos removed. The drive for the feeds comes in via the bottom middle drive spindle which has a spiral gear that engages with a 5 start worm on the silver shaft behind the plate. This spindle has gears which mesh with the shaft on the right (the one with the square drive) which then has a bevel gear on the back that drives a vertical shaft for the downfeed for hobbing gears and a horizontal shaft for infeed for hobbing wormwheels. There is also a neat automatic knock-off system that trips out the feed in either direction using sliding stops which you can see in the pic.
Here's the feed shaft with the 5 start worm wheel at the bottom of the pic. Also seen are the two bevel gears that provide downfeed and infeed power.
The internal gearing. Showing the spiral input gear and the sliding dog that engages the bevel gear with the feeds gearbox.Also see the three gears which carry power to the differential banjo. All the time that the machine is runnng, the feeds gearbox and differential gearbox are also turning all together.
It's important to note that because the differential gearbox is driven from the feeds gearbox, at a 1:1 ratio you can change the feedrate for optimum cutting without affecting the differential ratio.
julianthegypsy
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With log tables and slide rules!
Agroshield
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Because everything is geared and thus every gear has an integer number of teeth*, all the 'mechanical maths' is strictly ratios of integers (fractions). Log tables are not so helpful there. What is needed is a very good grasp of the maths of continued fractions (where you can find a ratio of two integers that are arbitrarily close to any number you like).
*(none cut by me, as a few I have tried to make have had 37 1/2 teeth)
Pete.
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For the indexing of teeth that is true but for the differential calculations it absolutely does (did before the invent of calculators) require log table calculations and there's an example in the manual in fact. The four-gear train used to set the helical gear compensation is rarely perfect, you're expected to work to at least 5 decimal places and choose four gears for your train that produces the closest possible helix.
There are books which contain hundreds of pages of four-gear ratios (A/B x C/D) and I need to get my hands on one of them so if anyone has the Pfauter/Gleason book I will pay for a copy of it). The magic number is called the machine constant which is (thankfully) given in the supplied manual and I'm glad it is because without that one single number I would not be able to do the calculations at all. I would have to find some way of discovering the constant by measuring and I shouldn't like to have to attempt that.
julianthegypsy
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I remember in college a farmer brought a huge pivot shaft for a big reversible plough, snapped in half. Usual rules, a nice donation to the college "scrap" fund in return for a shiny new one. The metallurgy lab sorted out what the steel was, and why it failed, then metrology started measuring. Then there was a very large tapered section. Out came the calculators, except for one old lecturer, who pulled out a set of ten figure log tables. He had it nailed in no time and no one else came close to his accuracy.
The shaft was then machined and ground to size and is probably the only part of that plough still serviceable forty five years later. Our college was something else in those days!
The shaft was then machined and ground to size and is probably the only part of that plough still serviceable forty five years later. Our college was something else in those days!
Agroshield
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I think this is some attempt to do that:
It is a good site to skim through as there is some obscure stuff there.
Do you know the title of the Pfauter/Gleeson book?
Pretty much what I did to get to the gearset needed for my Cooper pinion. I still haven't back calculated to see why it didn't come out exactly according to the theoretical lead though, but I will.
Pete.
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It's called "Change Gear Tables" by Becher and Koerner. There are other similar publications but this is the one I find most referenced.
Pete.
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Yes it is. I am right now installing a program someone has made for the PC, which I hope will do the same job but with added functionality over a mere look-up table.
Pete.
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Ok, thanks to a very clever guy called David Malphrus who has written software and made it freely available on his website:
I have this calculator installed on my PC. It's super-simple, you just put in your list of changewheels, and your desired ratio, and it calculates numerous 4-gear trains to choose from along with calculated error and how many decimals of accuracy. For helical gears you are supposed to work to at least 5 decimals. I put in the number from the example given in the manual for the machine and it not only calculated 80 different 4 gear setups but the first 22 results were smaller error than the actual example given. The reason for this is because even though there are a huge range of combinations given in the traditional book tables there are literally millions of actual iterations which are too many to calculate and put into print. the power of computers removes this restriction by perfoming millions of calculations in an instant (the results actually update as you type in the number).
I have this calculator installed on my PC. It's super-simple, you just put in your list of changewheels, and your desired ratio, and it calculates numerous 4-gear trains to choose from along with calculated error and how many decimals of accuracy. For helical gears you are supposed to work to at least 5 decimals. I put in the number from the example given in the manual for the machine and it not only calculated 80 different 4 gear setups but the first 22 results were smaller error than the actual example given. The reason for this is because even though there are a huge range of combinations given in the traditional book tables there are literally millions of actual iterations which are too many to calculate and put into print. the power of computers removes this restriction by perfoming millions of calculations in an instant (the results actually update as you type in the number).
Pete.
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By the way, anyone who operates a lathe with change-gears can make good use of that calculator, because the 4 gear train on a hobber is no different at all to the 4 gear train on the end of a lathe. So, with a simple bit of maths (basic TPI- inch or mm pitch conversions), putting in your list of gears and the decimal ratio you need to achieve, this calculator will give you all of the options of gear combinations you require to set up any lead required. It's true that the average gear set for a lathe doesn't lend itself to such a wide variety of ratios (they usually go up in fives) but if you threw in some extra wheels you could be cutting inch, millimeter, module, DP, BA threads down to a theoretical accuracy that is barely possible to measure. Or you could just do one good enough to screw a nut onto a bolt.
Dr.Al
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I used the same principle (just try every available combination) for the various change wheel calculators on my website.
There's a verified and published one for the M250 and the mini-lathe and some unverified and unpublished ones for an M300 and an LZ4P. The latter two were created after people got in touch to ask for them; they provided the required information & I made the calculators for them. I asked them to check they were right and never heard from them again. Ungrateful s*ds
Anyway, if anyone is interested, they're here:
CGTK - Mini-Lathe Change Gear Calculator
CGTK - Harrison M250 Change Gear Calculator
CGTK - Harrison M300 Change Gear Calculator
CGTK - Boley & Leinen LZ4P Change Gear Calculator
Pete.
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I'm in awe of your mathematical and program writing skills Al. I wish I was as adpet I'm sure that it would be possible to plumb in the data for my particular hobber and have it spit out a soluton. I've been sitting here for several evenings just to get this far.
Now I've found the change gear program (which annoyingly won't save my gear selection) I'm going to try to get my head around the formula to get to that ratio number from the DP and helix angle. For that my challenge is going to be figuring out how to measure the helix angle of an existing gear accurately and then expressing it in a suitable manner for the calculation. the example above has it expressed in degress and minutes but I imagine it needs expressing in degrees decimal for this purpose. I need to figure out the best method for making actual measurements.
Now I've found the change gear program (which annoyingly won't save my gear selection) I'm going to try to get my head around the formula to get to that ratio number from the DP and helix angle. For that my challenge is going to be figuring out how to measure the helix angle of an existing gear accurately and then expressing it in a suitable manner for the calculation. the example above has it expressed in degress and minutes but I imagine it needs expressing in degrees decimal for this purpose. I need to figure out the best method for making actual measurements.
Agroshield
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There are a couple of methods to do this depending on what you have. Easiest is a mating pair of gears, where you can measure the centre distance. The formulas in 'Gear Design Simplified' then lead you to the helix angle (see: https://www.practicalmachinist.com/forum/threads/calculating-lead-for-a-helical-gear.383428/)
If you only have the one gear, there are ways of using span and overpin (overball) measurements. See attached.
Closely related to this is the overpin measurement stuff in the back of the Van Keuren Handbook. For identification (as opposed to inspection), you'd know the measurement and use it to calculate the helix angle. If inspecting, you would know the helix angle, use it to calculate the overpins measurement and then see how close the manufactured article was to that measurement.
Pete.
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Thanks for that but the maths is too heavy for me.
On the plus side I find tables of natural sines from an old Brown & Sharpe publicationg long since downloaded right here in the literature folder on my PC (and this has reminded me to back this stuff up). That's one hurdle crossed, I can look up the sine/cosine of any angle in degrees and minutes.
On the plus side I find tables of natural sines from an old Brown & Sharpe publicationg long since downloaded right here in the literature folder on my PC (and this has reminded me to back this stuff up). That's one hurdle crossed, I can look up the sine/cosine of any angle in degrees and minutes.
