new gear for Fortis lathe

I have found that I have a number of teeth missing from one of my feed screw gears (aprox 6.25" 93 teeth) the lathe seems to be well worth a replacement, so I am looking for a machine shop to make one to pattern. Any advice would be appreciated. Thanks Vic Donnelly

Are the missing teeth in one group or singly? Singles are easy to repair, groups a little more difficult (but not impossible).

Have a look on HPC gears website, they may have a standard or be able to make you one. They are based in Chesterfield UK.

Don't know the lathe, (According to Lathes UK, it is based on the Clausing ) but 93 teeth and 6.25 diameter looks like 15 DP, if Imperial.

15 DP would be 6.33333 diameter 16DP would be 5.9375 diameter. 14 DP would be 6.785 diameter

Measure the OD of the gear(s ) with which it meshes, and count the teeth OD + (N+2 ) / DP, so DP = (N+2 ) / OD

If the gears are Metric, they will be Mod. so that Mod = Pitch Circle Diameter / N.

Where PDM is in mm, and N = No of teeth.

I fear that buying a gear may be expensive, and it will then need modification to fit onto the lathe...

Are you able to turn up a blank and cut the gear yourself? You will need a No.2 Brown and Sharpe cutter to do this. Someone like RDG or Tracy Tools could probably provide one for around £25.

Stating the obvious, to ensure concentricity, the blank should remain on the arbor when it is moved from the lathe to the Milling machine for cutting the teeth.

If not, is there a member of your club who would do this for you?

Howard

I see that GHT's dividing tools will do 93 divisions.

So will a Vertex HV6 Rotary Table. You need 30 holes on a 31 hole circle on the B plate, per division (tooth ).

Howard

Hi Vic

I had a couple of gears cut for 'my' Fortis a few years ago by Glasgow engineers, straight forward and painless. I cannot remember the DP and so on I worked out but will see what I have left after my recent tidy up. Let me know if Glasgow is suitable or within posting range.

M

Vic are you sure that it's 93 teeth and not 98? A stamped 8 often looks like a 3.

A 98 tooth 16DP gear would be 6.25" diameter exactly.

Are you sure it's 93. - count again. Introducing a prime of 31 into a gear chain makes no sense at all. If the number 93 is cast onto the gear it might just be the part No, I have some gears like that just to confuse.

I assume it is the same as the pictures on Lathes.co.uk so this gear meshes with a smaller one on the gearbox input and they would be best as a 4:1 ratio. 96 or 92 are much better options. If 92 then the other two gears can be a 2:1 ratio for which 38:76 fits nicely.

PM from OP revising tooth count to 95 and OD leads me to believe that it is 16 DP. But what pressure angle? 14.5 like Myford or 20 degrees?

Does anyone know for certain?

Howard

Howard,

These lathes were produced in the 1940s and 50s so I would imagine them to be 14.5 degree pressure angle. Furthermore, they were I believe based on the US Clausing of the 1940s which would almost certainly have been made to that PA.

I think it a fair bet that the 'clone' would be as little changed from original as possible.

I hope that helps

Regards Brian

Thanks Brian,

You have confirmed what I suspected, for a machine of that era.

So together with raw materials for an arbor, and a gear blank, a No 2 Brown and Sharpe Gear Cutter; 16 DP,

14.5 degrees Pressure Angle is what is needed to make a replacement gear.

Howard.

If you don't make your own you can get a suitable 2nd hand gear, chain drill a ring about 1/2" inside the teeth then chuck and bore the drilling holes away leaving you with a ring gear.

Chuck you busted gear and turn the teeth off to close ish fit to whatever you bored the ring to. You need room in the joint for Loctite so not too tight a fit. Once set drill and tap two M6 holes on the joint line for bolts if belt and braces appeals.

This keeps all the existing gear center keyways, bore size etc etc

I've done this on a back gear it's not that hard.

That's neat idea David, I might vary it by using a holesaw to cut off the rim as you suggest rather than flog through the dozens of chain drilled holes. I have made big holes through both steel and cast iron using those.

Regards

Brian

The use of holesaws was discussed in a thread here some time ago when the interesting point was made by someone that (with the right setup) no pilot drill is necessary. I favour rotabroaches for this kind of job but they are much more expensive, particularly in the larger sizes.

I needed to remove the centre from a 4" diameter by 1" thick cast iron blank the other day and successfully achieved this with a 64mm holesaw ending up with a useful piece left over. The operation was done in the lathe, reversing the work at the halfway point to avoid hitting the chuck jaws although it looked to be possible to get through at one setting. A lot of torque is needed for this and I had to use a strap clamp to release the keyless chuck.

My impression is that this method is less successful with steel where the chip is less easily cleared from the cut.

Hello ega,

Light cuts and slow speeds with some coolant are the secret, I have made holes through steel with the holesaw arbor held in nothing more that a standard drill chuck. The arbor did have a six sided grip for that purpose

Brian

Edit. Frequent chip clearance is needed, especially when the saw teeth have cut into the material and buried the gullets

Edited By Brian Wood on 23/11/2019 14:25:22