Hi all, happy new year. I am converting my S7 to Poly-V drive and have found an 11 tpi Whitworth chaser as per John Stevensons' advice (lucky really it's the only chaser I have). My question being new to these things is, "do I tilt the chaser in the tool post to remove the helix angle and approximate the centre height at the centre of the chaser or just ignore it and cut the grooves with the chaser square". The problem is I only have one piece of material for each of the pulleys and don't want to produce scrap in the learning process.
I suspect it may not be as critical as others (Hemingway) make out but the Polly-V-belt conversion calls for a 40 degree groove, not the 55 degrees that would be formed by a Whitworth chaser.
Having made one (or a pair of these) in the last few days (a Christmas project), I, personally, would not take the risk of deviating from the original design. If I assume that you have done all of the boring, recessing, keyways and so on prior to cutting the v-slots, you have already spent some hours on this project (and about 3 vacuum cleaners full of swarf?). Grinding a 40 degree single point tool is easy and relatively pain free, the only challenge is to accurately and repetitively advance the tool by the required amount.
In reality, the critical aspect is to produce a tool with adequate strength (bearing in mind that it needs to be quite narrow to allow it to cut the groove nearest to the step) and so it needs as much depth as you can muster or else chatter becomes your enemy.
I found it slightly challenging cutting all of the grooves without having to resharpen the cutting tool but the cutting depth is not as critical as the drawing makes out in the Hemingway kit (i.e. resharpening is possible part way through if necessary). The key aspect is to advance the tool to point where there is no 'land' on the outside diameter (i.e run the tool into the work until a sharp point is formed at the apex of the grooves but no further). The sharp points should then be machined away as specified by the drawing.
The chaser sounds like a neat solution but I would be weary of cutting the grooves at 55 degrees as opposed to the 40 degrees required by the belt. As I say, I have just finished making a pair of these and the level of grip and smoothness of the transmission has to be seen to be believed when compared with a conventional V-belt (I'm thinking of adding a similar conversion to the motor drive end as well).
I hope it helps,
Mark
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True there are differences in the angle but we are talking rubber belt here and only a few thou deep when compared to normal vee pulleys.
The one saving grace I have with using an 11 pitch chaser is that I have done many of these. Some have been fitted to production machinery in 1999 that run for about 8 hours per day over a 3 to 5 day week depending on production. Running 0.75 Hp at 9,000 rpm smallest pulley speed Belt life is usually about 2 to 3 years.
I have wondered about converting my S7 for some time now.
Are you guys making the conversion set using a Hemingway kit o starting from scratch ? I would appreciate a copy of suitable drawing and part number for the correct belt etc.
I bow to your experience, this is my first and only experience of using these on a lathe. I suspect that the 55 degree angle will aid belt life as there is slightly less surface contact between the belt and the pulley but I also suspect that there might be less grip if really pushed - although I don't know that it would be noticeable in reality. The grooves are surprisingly deep being in the region of 100 thou (2.7 mm) IIRC (they certainly seemed bloody deep when machining 48 of the things with a single point tool!).
Mark
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Thanks for your comments I must admit that I was all enthusiastic and hadn't given the whole job too much thought. I managed to get a couple of bits of material large enough and was off without any logical plan. I see now that things might not be that simple. My idea was just to replicate the Myford pulleys as near as possible but leave them solid. Now that I have filled my tray and floor with swarf, other "little" issues like the provision of lubrication for the bushes when in back gear have come to mind. I'm now thinking I should have invested in the Hemingway drawings at least.
I am a bit concerned by both the depth as you mention but also by the spacing of the grooves as the two will obviously interact. This is where I thought the major advantage of Johns' idea lie.
The one concern I had, apart from my angle question, was that the chaser is not "full depth" however, your comment on taking of the peaks leads me to believe that it might not be a major issue after all?
Mark thanks, I very much appreciate your post as without it I would have ploughed on and only added to my scrap bin. At least now I have a couple of "blanks" ready to complete when I have sorted myself out.
Phil, you will see from the above I was working from "pattern" and do not yet have the necessary drawings. I think Mark has obviously been more professional in his approach. I hadn't seen a major problem with the belt as there is provision for some adjustment on the countershaft. I was going to cut the pulleys and then measure my needs in case I made a "bo**$ks" of the grooves.
Thanks very much for the kind offer but when my wife left earlier this evening to help sort out the local guide group she took my order for a set of drawings and a belt from Hemingway. She doesn't forget much (particularly once it's on my "to do" list) so I expect that it will already be in the post. I hadn't noticed that their very reasonable drawing cost also included a belt so I thought I would treat myself for once (or is it twice now?). Anyway thanks again for your information and kind offer.
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