Grinding HSS Threads

[awemawson]

I acquired an inserted tooth end mill, 32 mm shank, 4 inserts on a 40 mm diameter. Great thought I, I have a large Acramil collet chuck, I just need to get a 32 mm collet and backplate

[awemawson]

Then the horrible truth dawned on me - it hasn't got threads for the backplate to screw on  How I overlooked this is a mystery probably attributable to the passing of years and death of grey cells   

Never mind - I vaguely remember an article showing threads being ground on an end mill shank on a Clarkson T&C grinder. If I remember correctly whoever was doing it used a slave thread and a shoe as a follower.

But surly you could do this using conventional threading gearing on a lathe if you could mount a grinding wheel correctly dressed and presented. A thin disk could be form ground to the thread angle and off you go.

Well, I have a hand held 3" 'cutoff saw' intended for car bodywork - perhaps I could mount it up and try an experiment? Then I remembered that years ago I'd made a holder for a DeSoutter air drill to mount in my Dixon tool post - perhaps it could be adapted:

[awemawson]

It wasn't a perfect fit, but for a quick test 'it'll do'

[awemawson]

I hand dressed the abrasive disk to the thread angle using a diamond dresser, checking with a thread gauge, set the lathe up for 20 tpi, selected a 1" shank victim from my box of large endmills (I wasn't going to experiment on the real thing  )
and set to cautiously taking a thou a time off, having covered the ways with workshop tissue. Lathe was turning at it slowest speed of 30 rpm.

[awemawson]

Now this got frightfully boring, as my 1 thou was really half a thou as my DRO is set for diameter. Thread depth for 20 tpi is 32 thou, so 64 thou indicated on my set up. I thought what the heck, and increased the in feed to 1.5 thou per pass. It was obvious that the grinding disk 'point' is going to suffer, and I expected very rounded troughs on the thread. In fact is wasn't as bad as I expected. From a theoretical cut of 64 thou I actually went to 67 thou for a reasonable thread fit to my collet back plate

[awemawson]

Now when doing this sort of thing in the lathe you have to be very careful to catch all the stuff off the cut or you will slowly grind your lathe into oblivion.

Here is what I caught :

[awemawson]

Now much of that will be high speed steel, but I bet something like 5% is abrasive off the cut off wheel.

Overall a successful experiment that gives me enough confidence to attack the inserted tooth endmill. In an ideal world I'd try and rig a method to dress the wheel during grinding, and also tilt the wheel to the thread helix angle - but as is - it works 

I now just need to work out how to mount the inserted tooth cutter for grinding. For the experiment I just used a three jaw chuck as I wasn't much bothered about concentricity (in fact it was out by only a few tenths) but the cutting bit of the mill could fit within the chuck jaws. For the inserted tooth cutter I want it between centres, but the tailstock fouls the grinder, so some other arrangement will have to be cooked up.

[Manxmodder]

Hi Andrew,That's turned really nicely.
I've often considered the possibility of grinding threads in the lathe with a die grinder on the toolpost,and your little experiment here proves it's a very practical method.

I also believe that making a couple of metal discs of 60 and 55 degree forms and getting the outside margin diamond coated could be done for reasonable cost.

These would never need dressing and guarantee the correct thread form is maintained at all times.

OZ.

[awemawson]

Thanks Oz.

It certainly worked out far better than I'd expected. No doubt the thread form wouldn't pass any sort of inspection, but it's fit for my purpose so that's all that matters in this case

[Jonny]

I acquired an inserted tooth end mill, 32 mm shank,

Just turn it down what evers largest you can hold.
Its only the last half that's usually hardened.

[awemawson]

Jonny, that was my first intention, but the shank is HSS for the full length according to my file testing I also have considered grinding it down, and may still as I have a cylindrical grinder. But this was an experiment that was triggered by this tool - the interest was to see if it was a feasible approach.

[krv3000]

hi good job

[John Stevenson]

Special Acme or trapezoidal taps in HSS ?

[Manxmodder]

Special Acme or trapezoidal taps in HSS ?

Hi John, If your question is could this be a method of making your own Trapezoidal or Acme taps then I would think it very feasible and extremely useful....OZ

[Meldonmech]

 
    Hi Andrew
                         Very interesting post, I have never considered thread grinding even though I have a cylindrical grinding attachment for the lathe. I was under the impression it was a much more difficult procedure, but having seen you efforts, now realize it is quite feasible. Thanks for showing.

                                                                                Well done
                                                                                                     Cheers David

[awemawson]

Thanks David,

I think that in process dressing of the disk is needed for critical threads where accuracy is called for, also some way of tilting the disk to set the correct helix angle.

However I also think that Oz's suggestion of diamond coated forms is worthy of investigation. A bit of googling did reveal a few companies world wide selling suitable disks.

However considering I just literally eyeballed the thread angle onto a disk intended for cutting car bodywork whilst the grinder was held in the vice then roughly clamped it in place it's come out amazingly well. Especially for such a Heath Robinson set up - I was well chuffed.

Even so I'm probably going to grind down the shank of the cutter that started this investigation in my cylindrical grinder, as the bulk of the larger collet chucks like the large Acramil often gets in the way, and if I bring it down to say 25 mm it can go in something much more compact.

[Jonny]

Andrew got a few using those tips, they cut extremely good 
Intrigued should one come up what make or brand was the cutter? Turned down plenty of Ceratizit, Kennametal, Seco and the like, theyre just tough and not through hardened the last half.
Decent tips to turn down.

Good idea though grinding the thread, last time I tried similar must have been 20 odd years ago rigging angle grinder on cross slide of an ML7 not good.

[Manxmodder]

I have a couple of cubic boron nitride wafer tips  of the DCMW 11T 308 shape that will easily cope with machining High Speed Steel to size.

I haven't  seen any profiles suitable for thread forming and I don't think CBN would cope well with the slow speed/high loads often associated with screw cutting on a non specialised lathe.

They do cut hardened materials such as heat treated carbon steel,case hardened and HSS with ease,though there is a bit of a firework display from the swarf if used dry.

Link below to show Carbide tip with Cubic Boron Nitride wafer cutting edge:

http://www.shop-apt.co.uk/cbn-turning-inserts/dcmw-11t308-cbn300-cbn-turning-insert-for-hardened-steel-45-65-hrc-interrupted-cutting.html
OZ.

[awemawson]

Sadly those tips are discontinued according to APT

[Manxmodder]

Sorry Andrew,I made a cock up there.

I've just been out to the workshop and double checked and the ones I have are in fact the ones with a .8 mm nose radius rather than the .4mm rad I originally linked to.(I'll go back and edit the error in previous post)

Link to continuous cutting tip here:
http://www.shop-apt.co.uk/cbn-turning-inserts/dcmw-11t308-cbn2100-cbn-turning-insert-for-hardened-steel-45-65-hrc-continuous-cutting.html

And link to interrupted cutting tip here:
http://www.shop-apt.co.uk/cbn-turning-inserts/dcmw-11t308-cbn300-cbn-turning-insert-for-hardened-steel-45-65-hrc-interrupted-cutting.html

I think they may have dropped the smaller nose radius version as it may not be as popular a seller as the .8mm....OZ.

[awemawson]

In the end I decided to shove it on the Jones & Shipman 1300 EIUR Cylindrical Grinder and take the shank down to 25 mm. I rarely use this machine, so there's a bit of a learning curve each time I do. Actually using it is easy, but getting work ground parallel is another matter. As it's working to tenths of a thou you only have to breathe on it and things get out of kilter. Work is mounted between stationary centres, and the work rotating mechanism spins round the centre in the headstock.

Mounting work involves moving the tailstock, which returns 'almost' to where it was, requiring table corrections. Years ago I made up and laminated a table of error corrections, which I dug out today and it got me out of trouble

It's a question of taking a cut, measuring both ends of the work, applying a correction, then starting again. Easy with this shank as I was reducing 32mm to 25 mm so room for lots of iterations !

[awemawson]

It came out ok - I think I have about half of a tenth of a thou taper in 3" which is far from perfect but much better than the collet it's going in 

[awemawson]

It never ceases to amaze me how this cylindrical grinder can sit unused for sometimes years, then step up to the mark and produce good work. It's a big lump to have taking up space but has got me out of lots of holes in the past.

[Pete W.]

Thanks David,

I think that in process dressing of the disk is needed for critical threads where accuracy is called for, also some way of tilting the disk to set the correct helix angle.

SNIP

Hi there, Andrew,

I seem to have come late to the party so maybe the following has been overtaken by events:

Would it be easier to set your grinding disk to match the required helix angle if it was addressing the work-piece from above rather than from the front?  That way, you'd set the helix angle in azimuth rather than in tilt.  There might be an excess overhang to cope with but it might also let the air hose clear the work better. 

[awemawson]

Pete,

It's just begging for a special tool post adapter to be made, with inbuilt  spindle belt driven electrically from the 'outer' direction to give good clearance, and able to accurately tilt in a calibrated fashion. Needless to say it would also need dressing facilities on board at calibrated angles. Would make a fairly easy project. Vacuum extraction would be sensible, or at least a suitably mounted catcher and spigot. Bells and whistles would be a microscope to inspect results !