Re: Tips on machining pulley grooves
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All of what you say can be improvements.? The root problem is a lot of cutting surface all at once on a large diameter in a small minimally powered machine.
Going in at two different X positions as you mentioned will help to mitigate this.? And, also, to an extent the chip formations from the left and right sides of the tool will, to some extent, both try to move into the same airspace above the tool bit.? They effectively collide there making things worse.? (The is the reason why single point thread cutting is ideally done at a 29 of 29.5 degree angle so most cutting is only on one edge.? Because for thread cutting this collision of the "left" and "right" chip can be severe.)
You also mentioned what accounts to first roughing the shape.? This can be done with a narrow square end tool (such as a cut off tool because it takes a lot of grinding to make a narrow tool from a standard size tool bit) to make a number of cuts resulting in a stair step shape to approximate the final.? You still have the issue of a large diameter in a too small machine.? But will have the advantage of having a much smaller cross section of the cut.? Then finish with the forming tool.? Or it might even be possible to use a round file that is, or has a region that is, the correct diameter.
Beyond that, it is important to have really sharp tool bits, a slow speed, and a lot of patience.? A cutting fluid for aluminum might help.? But with minimal removal rates, it might not make much difference.? And you probably aren't going to be able to get the part so hot that you have to worry about tool bit temperature.? But you never now.
This is also an example where just because something fits the envelope of a machine, it is not necessarily comfortably within the machine's capability.? There are parts that might need a large swing to clear, but a small diameter is actually being worked on such as boring a hole or machining a smaller diameter feature.? That's different.
Sometimes the easiest solution is to use a larger machine to make the part.? And if you don't have one, find somebody who does.
Charles E. "Chuck" Kinzer
On Saturday, September 16, 2023 at 01:12:27 PM PDT, Peter Brooks <peter@...> wrote:
As part of my 24v re-motoring exercise I¡¯m making new pulleys from aluminium. I¡¯m not finding it easy to machine the groove for the belt and would appreciate any tips (apart from ¡®buy them online¡¯ and ¡®do it on a bigger lathe¡¯ - I just have the U3).
I use ¡®O¡¯ rings for belts, they have a 5mm cross-section diameter. The finished inner diameter at the bottom of the groove will be 38mm. The pulley body is 9mm thick to allow for the flange portions either side of the groove.
For a cutter I first tried a 6mm drill bit cut off at 45 degrees and then milled clean. Blimey a drill bit is hard to saw with a hacksaw. Two blades later¡ the endmill had no problem however. ?The finished ¡®cutter¡¯ didn¡¯t cut very well though.
Next I ground a cutter (my first!) from a 6mm blank. Semi-circular, with a 6 degree angle, and some top relief (as recommended for aluminium I believe). This was moderately successful but it is doing more of a scrape than a cut. It is literally ¡®stop start¡¯. It did however produce some beautiful swarf, almost like snowflakes, or frost.
It is making pulleys - but painfully slowly. I have more material to remove and it is not working well at the moment.
I think I should be taking out excess material in smaller chunks first then finishing with the shaped tool, but a plan to do this is puzzling me given the finished shape. It¡¯s taken a long time to get this far from a large chunk of aluminium and I don¡¯t want to ruin anything!
So far I¡¯m going in at one position. I did wonder if alternating cuts between two slightly altered positions (kind of left and right) would be beneficial - the cutter would (or might) only be using 50% of the cutting edge.
I¡¯m making three pulleys in all - one for the motor and one each for the lathe and mill.
As usual I have very little idea what I¡¯m doing so am looking to tap into folks wealth of knowledge and experience. Any tips gratefully received. Thanks!
(Sorry for the overlong post - the quick version would be ¡®How would you make a pulley groove?¡¯ :-)
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