grinding a 60 degree symmetrical point on 1/4 " sq HSS blank

[DAPPH]

I guess that the title really tells it but first you need the sob story .
Here goes , hankies and beer a the ready lads .
I've set up my lathe to turn it into a milling machine so that I can mill out gear teeth with a fly cutter in aluminium
( if this works well I'll repeat it in Dur-alumin or phosphor bronze .

It works only occasionally , the counter often fails to stay located in th acme thread or the aluminium gear is chaveled away in seconds by the clearance groove along the length of the feed screw ( wrong tooth angle ? ) .
The profile of the teeth appears to be a symmetrical one that is a 60 degree tooth & I'm having problems in getting the 60 degree right .

I've tried free hand grinding the cutter tip but keep getting it slightly acute and off to one side , also the sides are not perfectly straight as they are a tad concave.
I'm now on my fifth attempt at getting it right .. Every time I've re ground the fly cutter tip I've made a new gear wheel .

There are 16 teeth on the 19 mm dia wheel , it is for the thread counter wheel that runs in the acme thread on the feed spindle so it has to be accurately cut.
I use a 32 or a 64 toothed gear that I have to locate & lock the cutting positions accurately using a turned spindle of aluminium set in the boring bars tool holder on my lathes QCTP for the blank to be fly cut

Do any of you guys & gals have any tips or suggestions as to how I can accurately grind this 1/4 " sq HHS blank ..with clearance angles in to a symmetrical 60 degree point .

All I have is a bench grinder and a 4 inch whizzer , half inch square MS , 50mm equal 5 mm angle iron , 50 x 50 mm sq aluminium bar and some 15 foot of 1" free cutting round steel plus a fish box of various bits of " valuable scrap " & a stack of UK Racers bolts & screws etc.

 

[optima21]

you can get screwcutting gauges that have a 60 degree V in them. with a bit of care you may be able to use one to help you grind it freehand.

http://www.ebay.co.uk/itm/272000144219

 

[DAPPH]

Thanks optima ..I've been using my thread cutting gauge like you suggest , but I'm afraid I'm not getting it as symmetrical as needed .. I've even resorted to trying to sculpt it better with my Dremel especially for the clearance angles . Using the gauge and checking the angle in daylight against the sky it looks like it's the correct angle .
If I weren't already bald , I guess I'd be pulling out my hair by now

 

[optima21]

how wide does the groove need to be? can you buy a threading tool thats wide enough, and you'd get the clearance angles too

http://www.ebay.co.uk/itm/330930690044

 

[711jrp]

Maybe you could make a grinding jig that runs across the edge of your grinders rest something like this

 

[AndyStobbs]

Can you put up a picture?
Might be possible to use a multitooth round cutter and grind that to your angle.

 

[GrahamGKD]

Do you have adjustment of the cutter tip once it is fitted? What I would normally do with screw-cutting on the lathe (not that I have done any for years) is to use the screw-cutting gauge to get the angle of the tool correct and then use the gauge again to set the tool in the lathe so that the correctly angled tool tip is square to the work?

Failing that use the screw-cutting gauge in conjunction with a square to get the angle correct and symmetrical?

 

[migmug]

Get a copy of Workshop Practice Series 17: Gears and Gear Cutting by Ivan Law (ISBN: 9780852429112) less than £10, give loads of info you will need.
Check out the section on the round fly cutter you make it on the lathe so nice and easy to get the right shape, but you will need some tool steel.

 

[DAPPH]

Well folks I woke up this morning with the solution tohand . No not that one .....The one that's way to get the fly cutter angle right .
On my way out and about this morning i popped into a new " All your engineering needs for tools & fasteners here." place I didn't hold out too much hope as most places of these little industrial unit tool carry very little and it's usually total tat to boot.
On going inside I was very very chuffed to find that it is as advertised, better still they run to two adjoined units ( I think ) . Most of the gear I saw on display seems to be quality brands as well .
I purchased a £12 digital adjustable angle finder ..a bit like the old mitre guage . I can home and set it to 60 degrees locked it up . Then I scribed the angle on some bright galv plate , then used the super thin stainless steel slitting disc on my 100 mm angle grinder to chop the plate to an exact 60 degree point & checked it against the sky , t'was a good fit with straight sides .
On doing the first bench fit test on the old gears I found I had to take about 0.25 of a mm off the point to get the angle plate to sit in between the teeth. " Oh b *01l**s " said I ... it slopped about like a sausage in the Blackwell tunnel it was not 60 degrees after all .

Two more attempts and it turns out to be 58.5 degrees for the tooth angle with a 0.03 mm flat root and a 0.5 mm looking tooth tip
I then re jigged things making a 29.3 angle from the reference edge , scribed it , then the 58.5 .

After extending the 29.3 line I set the tool steel blank as accurately as I could to run along a new scribed line that was 1/8 inch to the side but parallel line to the 29.3 line
Getting it precisely clamped up solid took several attempts with me eventually using F clamps and my trusty FACOM self gripping pliers . Once it was clamped and re cheacked for alignment I used the slitting disc and cut from the front to follow the angles lines going backwards .. at first I cut it to about 0.4 of a mm to the wast side , then gently eased the grinder to a clearance angle and gently trimmed the whole thing to the 58.5 angle line .

No body was happier than me at the moment when I checked the work with a small set square for squareness and again with the angle finder , this time with a torch light switched on behind it .
It needed only the merest of catscockhairs taking off to remove a microscopic bulge along one of the cutting edges. . After giving it a gentle run on my wet diamond hone stone it was flat as a pancake
I then had to think of a way of getting the nose of the cutter right .. In the end I turned up a small stub of bar ,centre drilled it then bored it out so that it would just take the dimensions of the cutter , cut back most of the central part so that the outer edge became a rim 90 deg to teh sides & bored hole , slid the cutter in point first and rubbed it back and forth a couple of times on the diamond hone till I got the correct root and then with the angle grinder gave it a tiny bit of clearance.

Tomorrow will be the big day when I set it all up and see how well it works and what other adjustments I'll need to make to get the teeth at 90 deg & symmetrical to the axis .

Piccies to follow .. after I've had my evening meal .

 

[graffian]

I don't use a screwcutting gauge for grinding toolsteel, I use a protractor. Set the
protractor at 30º [or 150º, depending how you look at it] and grind a side at a time.
I can easily see errors of less than a degree. You should have a nice symmetrical tip
And you can align it with a gauge on the side of the steel.

 

[DAPPH]

Thanks qraffian , where where you when I needed you yesterday ?

Basically that's what I did in the end .

As promised here are some pictures:-

The first attempts ...... cuts and scribed lines on the profile cutting galv plate .. Note the symmetrical cutter tip in the boring/ fly cutter bar .



Two of the worn /damaged gears I want to replace with my own homemade ones .



The digital mitre /protractor that I purchased a couple of months ago . I't worth its weight in gold from now on .
I got confused in an earlier post and said I got it yesterday ..I lied !
It was other man cupboard goodies and essentials instead ....HONEST !



left to right A 3/4 " dia steel cone for size comparison , the offending knackered gear & on the right a 32 tooth gear that I will be using for indexing the 16 tooth wheels I'll be making.




I'll put up some pictures of how it's all been set up and the end results in the next couple of days for others in the same boat as me .