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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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Geo. Thomas rear parting tool holder
I've been musing on rear-mounted parting tools again. The late renowned
British model engineer George Thomas wrote knowledgeably about them. I have his book "The Model Engineer's Workshop Handbook". In that book he recommends grinding some complicated angles on his HSS parting tools. I find I can scarcely comprehend what he's talking about, which is normal for me when reading British publications despite speaking English. Sigh. It *appears* he suggests grinding the wider face of the business end (what would be the top were it mounted normally) with an angled groove with an included angle of 140° so in cross section its top looks like 'M' (a capital emm), back maybe 1". He also suggests grinding awat the front of the tool, top to bottom, so that it has an angular tip with an included angle of about 160°. He has line drawings in his book but I can't figure them out. Realizing how confused I am, I think it is likely that anyone reading this will be even more confused so I will describe how these angles are ground, maybe that will help. He has a 1/8" wide wheel which he dresses so it is wider in its center than on either side. He mounts this on his T&C grinder spindle. He mounts a parting tool on the table of the T&C grinder so its top surface is the wider one, and is flat, then he grinds away an angular groove in about 1" from the cutting point. Then he removes the parting tool (HSS) from the T&C grinder to his tool grinder, which has a table rest which can be set to an angle. He drops the table 10°, then, using a cup wheel, he holds the parting tool so its top surface is parallel to the axis of the cup wheel but lowered by 10°, and grinds away half of the very end of the blade. Then he turns the blade over and grinds away the other half. Now the parting tool is longer in its center than on either side, because the front sticks out like the prow of a ship. He claims the grind on the front helps with digging in on deep parting, and he claims the grind on the top (where the chips slide along) tend to "break the back" of the chips so they won't bind in the cut. OK: thanks for reading all that. Can anyone confirm that I understand his topology correctly? Has anyone tried this? End of first question. On to the second one. Mr. Thomas also asserts that a parting tool should have about a 7° top rake, i.e. the part of the parting tool where the chips slide should not be horizontal, rather canted by 7°. He used to grind this into the front of his horizontally-mounted parting tools but then he realized he couldn't correctly sharpen his parting tools without completely removing the top rake portion and starting over, which is tremendously wasteful, so his toolholder design holds the entire blade at 7° so it can just be sharpened on its end. I have an MLA rear parting toolholder which I'm about to machine, and I thought I might design it to accommodate this feature. The MLA toolpost holds an Armstrong toolholder. It doesn't directly hold the parting tool the way GT's does. It wouldn't be hard to modify the MLA design to hold the toolholder at a 7° angle. Does anyone think this is worth it? Whew. Next I'm going to complain about the lack of information on parting tool angles when you buy them. On many parting tool blades - but not all - the top and bottom are not parallel, rather they are at a slight angle. These would seem to be difficult to hold in many toolholders like the Aloris AXA-7, but they seem to work in the Armstrong type holders. Yet no tool catalog I am aware of does a good job of describing all the blade topologies in enough detail so you can buy these (expensive) little blades and know they will work. Grant Erwin (posting from Karen's account since my hard drive died) -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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