How do I cut a 0.75mm pitch thread when the lathe offers 0.7 and 0.8mm...

[joenoci]

In making my toolpost grinder set I opted for an ER11 collet setup on the high speed internal grinder. To this end I purchased an ER11 collet nose nut to save the pain of making one. Now the collet chuck spindle side is complete, and ready to cut the nose nut thread, and behold, the thread is a 13 X 0 .75mm pitch. My lathe, an Emco Maximat V10P, has a nice screwcutting gearbox which neatly cuts either side of that thread, 0.7mm and 0.8mm, but not what I want! Is there any simple way out of this trap? I suppose I am going to have to make my own nose nut....
Joe
Swakopmund..

[steamer]

You will need, at the least, to add a pair of gears in the train that has a ratio of 0.9375.   This could have many solutions, but ...say 60/64....or   30/32....and set the lathe to cut 0.8 and you will get .9375*0.8 = 0.75.    Now how to do this, I would need to see the train and the gears in the current chain.

Dave

[arnoldb]

Hi Joe

Can you change any gearing on the left hand side between the spindle and the QC box ?
For example, if the QC box can be set to 1.5mm pitch, and you can halve the gear ratio between the spindle and the QC box, you've got your 0.75mm pitch...

Kind regards, Arnold

Just had a thought - you can finish of everything on the spindle except the threads - and pop it on Nampost Courier to me; I'll happily cut the thread for you

[Si_UK]

You could get 13x0.75mm die and make a tail stock holder for it.

Good luck!

Si

[joenoci]

Thanks everyone for all the suggestions - I guess this one was very much an Ostrich Approach - I more or less knew all the suggested options but the mere thought of implementing any of them scared me silly! So, head in the sand, and ask the question - Ah Well..
Thanks again to all -
As I do not fare well sitting contemplating the problem, I climbed in and made a nose nut, threaded everything 0.7mm pitch. All worked out ok, but that took all of 7 hours!

The nose nut does not have the collet release half ring - that was to much to do, especially on an ER11 collet nose nut - it is so small there is no space for the eyes to see anything when the cutting tool is in there!
Threading the internal went well - I have a single phase to three phase variable speed invertor driving a 3 phase motor on the lathe. The invertor has all sorts of programmable features, one of them being a stop - brake. This can be programmed to any number of seconds, from almost 0 , ie , instantaneous, up to 20seconds. I used en ER30 collect chuck in the lathe nose so inertia is low, and set the brake to 0.3sec. Marked a point on the lathe bed where as the carriage got to it, I hit the stop button, and the spindle stops within a half turn of my press. Takes the Poo out of internal threading to a shoulder..

Arnold, Thanks for the offer for doing it for me! Had I seen that mail before I started, I may have just done that!
My lathe does have those gears on the side, on a Banjo, but I have the set that came with as supplied, and no others - also looking in the handbook and catalogue, there do not seem to be others available.  Such a pity Emco stopped making these series of lathes - they are really good.( except for 0.75mm threads..)

Arnold, or anyone, I seem to have a regular mind tussle with spindle ( and tool) runout.  On my PCB CNC Engraver, the spindle is a Perske hi-speed spindle, around 48,000rpm at 400herz invertor feed. I use 3mm carbide tools, ground with a D flat, and a 60deg cone. The tip does the work and since my PCB tracks are 0.2mm wide, with 0.5mm spacing ( typically) , there is not much space to engrave the copper away to make the track. So runout is a premium here. And the Perske spindle delivers - They claim 0.0015mm in the collet taper, and with the special collet supplied, 0.003mm on the 3mm tool. I cannot read the runout with my simple 0.01mm dial...So I understand that one, but what is good enough for say the business end of en endmill? and what is ok for the toolpost grinder?  On the internal unit I and doing, I made the main spindle up, and then set it up in the lathe with the fixed steady holding the end bearing section, in its own housing. I then machined the ER11 collet in place, so there is 'no' runout.

After the nose nut saga, I then fitted a 6mm ER8 collet ( make is Gaia - seems to be east european - claimed less than 0.01mm TIR..?) and fitted a 6mm Carbide endmill into it and tightened up. The cutter has a long solid shank that I hoped is straight. I then set the dial gauge against this shaft and measure 0.03mm hi to lo...!! I removed the collet and measure the runout in the taper with a limited range dial gauge - does 0.5mm max, but has a long tip to go inside the taper. I seem to get around a third of a dial division , 0.003mm or so? - it varies as the bearing does a few revs ( not rocket bearings, plain garden variety..) and I have to estimate the reading as the gauge is in 0.01mm markings.

So the collet seems to have around 0.025mm or more runout. I guess this is typical of collets, esp those of hobby quality?
So what is ok? Is 0.03mm typical, and acceptable? I know the stories of one tooth cutting on the endmill, etc, but none of our hobby machines have exceptional runout performance. Or do they?

The internal grinder is to be fitted with various mandrels that have the different internal grinding wheels on them. They would be say 70mm to 90mm long, tapered from ER11 diameter to a diameter to suit the wheel at the other end. The ER11 end has a male ER11 taper, and an M4 tapped hole. This then fits into the ER11 spindle end, and a drawbar pulls it all up through the spindle. So I made one mandrel, between centers, etc, to get the best concentricity I can. After fitting it to the Spindle and pulling up on the drawbar, I read 0.02mm to maybe 0.025mm runout at the grindstone end of the shaft. At the ER11 collet end I read just less than 0.01mm.  And I was so careful!!

Is this a problem? This thing should spin at about 10,000rpm to 12,000rpm - what will the 0.02mm runout do? I will true the stone, but is the thing going to vibrate around the shop?? Rotating the mandrel in the spindle collet does not change the reading - the reading stays on the same place on the mandrel - I guess this means its the mandrel that is not concentric to its male ER11 taper..

I need to make up 4 or 5 more mandrels, but this recipe does not work - or am I overreaching here?

Thanks for any comments!

Regards
Joe

[arnoldb]

Hi Joe

I don't have much experience of high-speed spindles - except for my Dremels, so hopefully some of the other members can chime in.

I'd think the size and weight of the grinding stone would be a major factor on the end of your long adapters; a small light stone shouldn't be a problem, but as they get bigger, it might become an issue...

0.03mm run-out for your PCB engraving shouldn't be that bad - if you use, let's say a 0.4mm toolbit, the groove will end up at 0.43mm...  There's a niggling feeling in the back of my mind that you might actually be able to use the "run-out" to make the D-bit cutter cut a narrower groove by rotating it so the "flat" faces in the direction of run-out  - Some experimentation needed.

My ER11 collets are of the "hobby" variety (This links to them), and is specified at 0.015mm tolerance - which they appear to keep...  I have found that I need to be super careful on keeping them clean to maintain tolerance though - any bit of swarf in the spindle, the collet's slits, or even in the closer nut will throw it out a lot.  And it's as if CTC is reading minds (or perhaps forums?)...  I'm on their mail list, and they sent through a 10% discount code on 0.008mm run-out ER11 collets today.   Needless to say, right now I'm flipping around the old "plastic money" trying to decide if I need to buy myself a late birthday present.  With shipping and the current exchange rate, it'll end up at about N$700...

It's always nice to work precisely, but it gets to a point of diminishing returns.  Like mentioned, I can't speak for high-speed spindles.  What I have found though, is that in my hobby environment, 0.05mm (or two thou) is more than close enough for accuracy in most cases.  Below that, even the ambient temperature starts to play a role; some of my engines that were built with 0.01mm piston fits will run well on hot days, but are stiff on cold ones.  The same happens to the machines; If I'm working in the shop and it's above 30 degrees, the lathe and mill is very "loose" compared to working with them when it's 12 degrees.   That is most definitely due in part to oil and grease changing viscosity at different temperatures, but still, there's a lot to do with metal expansion.  The same happens to spindles - from what I've researched, well-built spindles are designed in such a way that they get more accurate once they heat up a bit.

As to your mandrel that got out of whack - Did you turn it in one session ? - If you did, it could be due to internal stresses getting released as you turn away metal.  It might be a good idea to turn the spindles down to near-size leaving about a mm for finish-turning, and let them lay around a couple of days/weeks before finish-turning.  Or have a couple of friends over for a braai, make a decent fire, and chuck the spindles-to-be amongst the coals and let them cool down in there over-night 

With regard to your lathe's screw-cutting - it's got a banjo, so it's not rocket science to machine additional gears that you can use if you can't buy them - just a bit of effort fun. 

Kind regards, Arnold

[John Hill]

Arnold,  this is the easiest way I know to make a gear..



http://madmodder.net/index.php/topic,3487.msg37703.html#msg37703

... but I am unsure if plastic would be good enough for your change gear.

It took me only a few minutes to make the first gear  but unfortunately I didnt take the idea any further although I did think it would be an easy way to make a clock I never got around to doing it!