|Chris TickTock||27/07/2020 21:29:01|
|605 forum posts|
I have a void in my understanding of wheel cutters. I do not understand why there is no side relief on a wheel (clock wheel) cutter. I am told otherwise you could not sharpen the cutter. Can anyone explain. My issue is side relief means the cutter hits the brass with less area so cuts easier. But the cutting edge which hits the brass is key....help required??
|Martin Kyte||27/07/2020 22:28:40|
2130 forum posts
Not my understanding Chris. Wheel cutters are constant profile (at least Thornton cutters are). This means that for each tooth of the cutter the same profile exists behind the cutting face but at a smaller radius from the centre. If you draw the cutter tooth profile face on and then draw the same profile inside it but down a little (which is what you would get after sharpening) you will see that there is top relief and side relief.
Imagine the above rotated edge on.
|Brian H||27/07/2020 22:32:28|
1864 forum posts
Hello Chris, if there was side relief on a cutter then the actual cutting size would get smaller and smaller with each sharpening.
The side relief has no bearing on the size of cut, the only factor is the length of cut. Side relief makes the cutting easier but on this items such as clock wheels this is of little consequence and is nowhere near as important as the ability to sharpen the cutter without loss of width.
I'm sure this could have been worded better but I hope that you can follow the argument.
|Kiwi Bloke||28/07/2020 07:23:31|
|483 forum posts|
Gear cutters are 'form-relieved' (as Martin says), so sharpening is possible. However, the radius described by the tip of each tooth has to be carefully controlled, so each is equal, thus sharpening isn't entirely straightforward.
I don't know about clock wheel cutters. Since they are generally cutting thin sections of brass, perhaps relief isn't essential. No relief would certainly make sharpening easy. Can anyone confirm they are not relieved?
|John Haine||28/07/2020 07:31:41|
|3428 forum posts|
As the Thornton link shows, high quality clock wheel cutters are form relieved as are involute cutters. They are made for manufacturing wheels in quantity and will need sharpening. You can make cutters without side relief and they will work but couldn't be sharpened much other than careful honing without changing the profile. The relief on a cutter is not to "decrease the area hitting the brass" but to stop the side or end of the tooth rubbing.
|Clive Steer||28/07/2020 09:45:44|
|27 forum posts|
All the clock/watch wheel and pinion cutters I've come across are form relieved and I have examples of those that have been sharpened. For very fine watch pinion cutters this must have been very difficult to achieve accurately and was probably done by the companies that made them as a refurbishment service. There are complicated machines, mostly Swiss made, that were solely designed to automatically cut the cutter profiles. I bought a box of machine parts at an auction to get the Schaublin vice in the lot and it took me about 10 years before I eventually found out what machine the parts belonged to and I still don't know which Swiss company made them but possibly Safag.
There is an excellent book by J M Wild on Wheel and Pinion Cutting in Horology and section 8 shows how the cutters are made and also how simple home made cutters can be made.
I have an early Petermann automatic pinion cutting machine and this would have three cutters mounted on the same spindle. The machine would automatically make gashing , profiling and finishing passes over the pinion blank for a fully manufacturing cycle and some machines had the capability to automatically load pinion blanks from a cassette. To see the machine check out Petermann on the lathes.co.uk site.
|Russell Eberhardt||28/07/2020 10:30:07|
2600 forum posts
Some other fantastic machines in the Petermann range. Lathes with tool setting thimbles calibrated to one micron diameter!
|Chris TickTock||28/07/2020 20:10:06|
|605 forum posts|
I really appreciate all posts..thank you.
My lack of understanding I feel may derive from the fact that a metal cutter shears metal as opposed to cutting it. Unfortunately I find this term 'Shear' ill defined. On a single point wheel cutter you are effectively hitting the metal with a profile having sharp perpendicular edges on the side s od the cutter, the front must have relief. These edges force the metal out. But if the single point cutter were to strike the metal flat it would be an effective stamp. As it is rotated the bottom touches the brass first at an angle and breaks / shears the brass.
Is this right or not?
|John Haine||28/07/2020 21:33:24|
|3428 forum posts|
Not. A good single point cutter just has one tooth like a tooth on a multi tooth cutter and cuts in the same way but slower. No different from a lathe form tool really, you wouldn't expect a lathe tool to cut say a thread form without rake on the front and sides. Why not look for Shaw on Metal Cutting Principles online? Whether it's a hacksaw, file, lathe or milling tool, they all shear metal in the same way.
|Chris TickTock||28/07/2020 21:59:39|
|605 forum posts|
John, I am still struggling to understand but I agree that front relief of a single point cutter is needed but if you have side relief surely if you attempted to sharpen you lose profile. The biggest point I was missing in my understanding was initially thinking that a 'cutters' sharp edge acts as a knife dividing the metal rather than shearing by force of the sharp edge. Thus is my knowledge at this moment in time???
|Paul Kemp||28/07/2020 23:01:41|
|580 forum posts|
Maybe try to imagine it as the form being tilted back so as you grind back the cutting face the form remains the same but the cutter OD diminishes, the actual OD of the cutter is unimportant, the relief is provided by the tilt in the form. Not sure if I explained that very well!
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