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| vtsteam:
Thanks Pete, Will! :beer: Will, In this case I need to tap the casting. It's a little different than the usual method of a cross slide leadscrew nut. Gingery style. Anyway, can't cast for this particular job, and impractical to internal thread on the Craftsman with the amount of overhang, so a tap, either made or purchased is the right tool for the job. Later I can cast nuts with threads, and will do that for sure with the half nut. Pete, thanks. Those taps have a really long taper, which is probably a very good way to do it, I'm thinking. The problem yesterday was, I believe, too short a taper, and too shallow a rake angle -- which is a result of the particular unusual way of making a tap. Apparently it is a very old way of doing it -- but I just think t's not good for a coarse acme thread, and maybe my practice piece of aluminum wasn't th best choice to try it out on, or maybe these need a big clearance hole and have low thread engagement. I don't know what was wrong in my case, because I do believe that the person who recommended it was successful in cutting acme threads. One of those cases where you have to see the tap and operation to know what was different about it from what you are doing, rather than just read a word description. Well today, more tap experiments....... |
| vtsteam:
Oops, missed your replies Andrew and Pekka -- thanks for the suggestions -- and as I said this isn't tapping a nut, but the operator end of the carriage casting itself. Pekka your tech description is helpful. I just remembered that I do have an acme tap, a 1" x 6 TPI (I think) , and it works fine, so I'd like to take a close look at that, check the rake angles, taper angle, relief, etc. since it cuts great. I made a drive nut for my horizontal mill with it in steel, just hand tapping. Seems like 1/2" x 10 should be a piece of cake by comparison, with the right tap. |
| vtsteam:
Here is one site I was looking at for tap making info. http://mckgyver.pbworks.com/w/page/20654129/AcmeTap Notice that the chart shows the tap rake angle as low as 0-3 for cast iron, and relatively low for brass (just as we typically reduce top rake angle for lathe tools, and flatten drill lips for that material) but relatively high rake angle for aluminum and zinc die castings. That makes me think that the first tap I made, following the old style 4 bevel end on a coarse thread produces a very low rake angle, and while it might work for finer pitches in brass (as shown on the forum post I saw) or cast iron, would definitely not work with coarse acme in aluminum. Since the ultimate use will be on zinc, I'm going to say a 10 degree tap rake angle would be minmum, according to that chart. |
| Pete W.:
Hi there, Steve, The tap-making whose link (another Andrew!) I posted earlier was for a square thread, I guess acme ought to be easier. I don't think he intended to cut many threads with his taps - his project is steerinng gear for a pair of traction engines (but quite big ones!!!). Did you specify Right-hand or Left-hand, or did I miss that? If all else fails, there's a firm here in the UK that sells Acme taps of feed-screw sizes to the Model Engineering market, I'd be happy to act as 'go-between'. |
| vtsteam:
Thanks Pete, will try making one before buying -- pretty sure I can get them here, if need be. Right hand, so not too unusual. I've been playing around with a drawing and one way of cutting the flutes to get a 10 degree angle might be to just offset an end mill from center and cutting to a specific depth -- at least it works out on "paper." In this case, bring the edge of the mill in .155 from center, then mill to .293 depth. That should yield 10 degrees tap rake angle: (Interestingly, the .293 depth determines the rake angle, the .155 just determines how much clearance you get -- I just guessed .094" might be enough for the tooth depth. Tapering the end of the tap just reduces clearance, but doesn't alter the tap rake angle -- it is determined throughout by that .293 cut.) There is no backed off relief in any of this -- hoping it will work well enough wihout it -- fingers crossed.... |
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