Grinding A Cuting Angle For A Fly Cutter On 1/4 Square

[Round in circles]

I'm now into making my fifth thread counter gear that turns on the screw shaft of my Sphere cum Atlas lathe.

I tried to faithfully grind the tooth profile on the 1/4 " sq HSS cutter using a bench grinder , daylight and the old Mk 1 eyeball to get the shape , then used my lathe as a shaping machine to gently scrape away the unwanted metal.

This resulted in the tips of the gear being about as thick as a razor blade and getting stripped off within seconds of setting it up in the thread counter body axle & seeing it if worked.

Second version was to use my Dremel to make a micro adjustment to the cutter profile and I still got the same end result of razor sharp fins . The depth is exactly the same as an unworn part of the gear wheel . So I conclude it is the angle of profile of the cutter that too obtuse . I noticed that the sides of the cut teeth are not symmetrical, one side is a tiny bit to one side and slightly hollow along the side . Being central to the point of axis appears spot on . ( I can repeat that alignment easily enough ) .

Again with the third trial ..... not a very good result it engaged whilst the saddle travel was not engaged but came free of the screw thread as soon as I've engaged the feed lever ..
Perhaps this suggesting that it is not in deep enough engaged or it's teeth are at the wrong angle ( Your thought appreciated here please )

My hands and arms are now beginning to fatigue somewhat from all the hand turning of the hand wheels these last few days so I've decided to try to use my boring bar to make a small fly cutter and to mount the gear blank in the boring bar tool holder of the QCTP. I also tried to tidy up the tip of the fly cutter with the Dremel
I've made the fourth wheel .
I'm getting good at using a 32 toothed wheel to cut the 16 tooth wheel using the boring bar holder in the QCTP , made a very pointed pointer that fixes to the height adjuster of the boring bar holder . When set up , it throws a needle point shadow that's easily seen on the top edge of a tooth on the 32 tooth wheel ...so the spacing of the teeth is reasonably accurate .
Clamping solid is also very effective now I've used new real high tensile steel clamping screws not the ones made out of cheese that came with the thing. ( One sheared off with just a two finger tightening pull ) .

I cut the full set of teeth as things were looking very good however on getting round to cutting the last tooth I found it was out by about 20 thou and as a result has shaved the last tooth away to yet another razor thin thickness. I've put this last tooth problem down to not rotating the master wheel in the same direction all the time., as when I cut the first three teeth I found there is a slight play in the fit of the shaft in the QCTP holder ( I'll be making a better one tomorrow ) .
I also found that the profile of the teeth is not as good a shape as I thought it would be.
The test /trial run has again resulted in the gear riding out of the acme screw thread which again indiacted I've got the wrong tooth profile /angle angle .
Looking at the original gear tooth profile with a magnifying glass , it's indicated that the teeth sides are straight sides of a triangle ,the tips are very slightly rounded at a 1 mm radius and the tooth root floor is flat


Are any of you ingenious guys using some sort of simple hand made device that can grip or guide the HSS blank to give me a symmetrical 60 degree angle on the blank when I use it on the grind stone ?
OR
Have any of you guys and gals got any tips of how to do it ?
OR
Have you any other suggestions as to how to make the gears ?
Bearing in mind I only have a lathe , a drill press ,some hand files , an angle grinder , a Dremel , a 4 "table top mounting vice & spanners ( aka . wrenches )

 

[jocat54]

David,
I am no where near qualified to answer any of this--so please keep this in mind.
I have only cut a few gears for my SB 9A and only after several tries were they okay. I did cut my gears on my mini-mill using a gear cutter.
My first thought after reading your post was that you need a positive way of indexing your gear blank it needs to be dead on the money. I think you can grind a cutter close enough to work.

 

[Round in circles]

I slept on it .
This morning I woke up knowing exactly how to get my tool tip angle symmetrical & with clearances .
A couple of months ago I went and purchased a £15 ( $20 USD ) digital mitre gauge that give degrees in tenths of a degree from zero right round to 359.9 degrees .
I set it to what looked like a matching angle . Scribed the angle on some bright galv plate and then using my 100 mm angle grinder with a stainless steel 2 mm thick slitting blade , cut on the waste side of the scribe lines , giving it a final kiss with the grinder to take the angle to the scribed marks.


On trying it as a bench fit in the old gears it was found to be slightly out , instead of an exact 60 degree tooth , on the third adjustment and the third cutting I found it's as near as dammit 58 .5 degrees.

After that it was a simple case of using one straight machine cut edge of the galv plate as the reference point and making the angle of 58.5 . Then putting down a bisector angle of 29.3 . also then scribing a guide line either side of the bisector line 1/8 of an inch away so they were both parallel with the bisector line . Then aligning the HSS blank using a magnifying glass to get it as near perfectly aligned on the guide lines & clamping it securely in place having very little overhang at the apex of the angle & rechecking everything after I'd done the clamping up.

It was then easy to use the grinder & thin blade to cut on the waste side about 1/32 nd of an inch away from the scribed angle lines and then gently ease the grinder blade up to the line , once the angle was made I tilted the grinder to give the clearances on the sides with out altering the 58.5 angle .

To get the nose of the tip at the right root I turned up a bit of steel concave dished the end face to give a 90 degree vertical block when its rubbed over my carpenters/ cabinet makers diamond honing block , bored it through so the hole would just take the square bar . I then slipped the cutter bar in the guide , set it on the hone added a few drops of water and gently rubbed it over the block .
Five strokes later the root and angle fitted almost perfectly & symmetrically, even though the angle centre line is a fraction off the central axis of the blank . I noticed a microscopic bulge on one of the side , so gave it a free hand rub on the hone block .
On re checking the profile & fit using a torch as background light then inserting the cutter in various teeth I found it was a near perfect fit in almost every tooth I checked on the two damaged small gear wheels . Yipeeeee!

Some of the feint scribed lines on the plate & my first two test/trial cuts to get the angle right .
Note the finished angled toot tip in the boring bar that will become my fly cutter device .




The chavelled up /damaged gears of the thread counter that I want to replace with homemade ones .




This is a very very handy digital mitre gauge .


The plug on the left is a 34/4 " diameter , the middle shows one of the damaged/worn gears & the gear on the right is the 32 toothed one I'll be using to make the 16 tooth replacements



Tomorrow is the crunch day as I've got to modify the original holding devices & set things up with something that is a greater precision fit, but more of that tomorrow.

 

[Round in circles]

Well I've successfully made the thread cutting gear wheel , it's not as good as I hoped but it works reasonably well .
Using the boring bar as a fly cutter is not easy to get it aligned on to the axis of the blank gear , mine is a tad out so the teeth are not 100 % in alignment which makes one side of a tooth look a smidgeon longer than the other .

First I ran several test pieces to try and get things accurately set up before I decided to make the complete replacement gear . I found that using as high a spindle speed as possible allowed me to gently feed the blank over the fly cutter with the least vibration.
I also discovered that everything that should be locked down needs it doing , including locking the gibs on the compound for it tries & often succeeds to wander a few thou if you don't ... making some teeth quite thin and skinny .
The odd thou here & there changes things dramatically when the rod of gear blank is poking 85 mm out of the boring bar holder on the QCTP.

Tomorrow , after I cut a gas thread on some steel & make an adapter for a pond cleaner to fit it on the high pressure water side of my power washer, I'll try using the lathe as a shaping machine to see how another cut gear or two turns out.
Now I have the angles about right & have knocked up some various forms or carriage stops from bits of scrap metal things should be a little easier this time round..

 

[joshua43214]

Forgive me if I missed something important.
Are you trying to machine a gear with teeth cut at a 60 degree angle with straight sides? You will have grief and broken teeth if this is the case.

Regardless, the normal way to cut a gear is to cut one face of each tooth, lower and rotate the work the correct amount and cut the other face. You do not normally cut both faces of the tooth at the same time. This could still be done one the lathe, you will just need to figure out how to reindex the work when cutting the second set of faces. I would only try to grind one face of the cutter correctly, and flip the cutter for the other face.

Really though the best way to do this is just hob it. Cut yourself an Acme thread to match what you have from tool steel between centers (you can make this with out the thread counter, just leave the half nut engaged, and turn the work in reverse by hand to the start of the thread). Grind a bit, and use it to hob the replacement gear on the lathe. There are some excellent videos on YouToube for this.