|Mick Henshall||15/07/2018 20:11:03|
|483 forum posts|
Trying to make one for my lathe to use hss blades.question is whether to angle the slot for holding the tool ( 7 degrees seems to be the appropriate angle ) to give back rake, or no back rake at all and machine the tool slot horizontal?
|larry phelan 1||15/07/2018 21:01:27|
|375 forum posts|
I made one for my lathe using two pieces of 3 " x 3" x 1/2" with a spacer 2"x 2" between them. Screwed the whole lot together,never bothered about rake one way or the other,and have no trouble with it. If I were to make another,I might think about machining it from a solid block,just to avoid the bother of drilling/tapping,lining up the pieces ect.
Do make it solid enough,because you can,t beat having a bit of bulk there to deal with parting off.
That,s my tuppence worth anyway !
|Mick Henshall||15/07/2018 21:10:08|
|483 forum posts|
|Chris Evans 6||15/07/2018 21:12:31|
|1360 forum posts|
I made one to take a tipped parting blade from ARC Euro. Nice solid piece of steel seems to absorb any vibration. Make one and you will never look back, parts 40mm stock at 800 rpm and power feed at 0.05mm per rev. Just have to use flood coolant if doing that though, most times hand fed and a bit of lube from a brush.
|not done it yet||15/07/2018 21:44:27|
|2643 forum posts|
I prefer the cutter slot to be horizontal in the toolholder. Otherwise, if any adjustment is needed for shallow or deeper parting cuts, the centre height will change - which is a pain in the posterior
|duncan webster||15/07/2018 21:53:23|
1910 forum posts
I'm on the brink of making a rear toolpost for mine, but the brink is getting crowded. If you are using the tapered HSS blade, then as you grind the top to get back rake, the cutting edge gets thinner. Seems to me better to angle the whole blade then you only grind the front edge. However this is all theory as I haven't made one yet.
I think it is in accordance with GHT articles from the 70s/80s
Edited By duncan webster on 15/07/2018 21:53:48
|Mick Henshall||15/07/2018 22:10:58|
|483 forum posts|
On reflection the type in my photo is the one to make, the centre height is easily adjusted and looking at various YT videos most use no back rake it would seem to me that back rake improves the chance of a dig in and can lead to excessive grinding of tool to maintain thickness of taper as Duncan points out, will post a photo of my effort when completed
3289 forum posts
That's my rear toolpost. Glad you like it! But sadly I can't tell you much about it. It was apparently made by a previous owner of my Dad's old Drummond M-type. Looks to have been well made, possibly on a larger milling machine (foreign order at work maybe?).
It is based on the concept of the patented Norman tool post that came standard on the M-type, with the large diameter upright post and the toolholder that slides up and down it and clamps in position. I also have a Duplex/GHT-style rear toolpost topped with a fourway toolholder that holds the same parting tool and it works just as well.
But the old one does work very well, as the pic shows. I don't grind any back rake on parting tools, ever. Just use them flat. I do very lightly grind the top (er, bottom in this case) of the parting blade nice and flat as they sometimes come with a slight angle on that edge. That way, the tip is the same width as the rest of the blade. I do this with all of my several parting tools, all of which are mounted level with no back rake. They all work well.
I am currently (slowly) making a Duplex/GHT-style rear toolpost for our Myford that will have the parting blade held directly in the toolpost, just to simplify things. Using a T section blade from Eccentric Engineering. That will also be mounted level.
The search for the ultimate parting tool continues...
2255 forum posts
I made one for my WM250, it works a treat on larger diameters. It was made from a castings kit but I found that the angled slot altered the centre height. I adapted a tool holder to use in the straight slot on the other side which improved it a lot. have a look in my album at 'Rear tool post' for build pics.
|Ian S C||16/07/2018 13:41:24|
7224 forum posts
My rear tool post is a stretched version of one in an old ME, its welded up from scrap steel.
|Howard Lewis||18/07/2018 21:19:30|
|1762 forum posts|
Made one for my ML7, and never looked back!
My Far Eastern machine now has a four way indexing one, made to match the front toolpost. The, at least, 20 year old secondhand, holder, with original blade, has some flexibility built in, and I am too tight to grind away good tool steel for a top rake. Dig ins are very rare, and rarely damaging (unlike replaceable tip parting tools in the front toolpost when I want something narrower than 3/32"!).
+1 for make the toolpost heavy and rigid. If it flexes, you will have problems.
The current one, for 6" a centre height lathe, is 75mm square steel, held down with two M8 nuts. The Myford ML7 one was made from 2 inch box section with plates welded on to bottom, centre and top.
|Mark Rand||18/07/2018 22:57:13|
|657 forum posts||
It could be worth using the other technique presented (I think) by Geo Thomas that gives the same effect as the carbide inserts:- Grind a shallow groove on the top of the tool. This causes the swarf to be narrower than the slot and stops it from binding. You still need a bit of lubrication for the sides of the tool, but swarf clearance is better.
I'm ashamed to say that I've got a set of Hemingway rear parting tool castings half machined, but since I rebuilt the Hardinge I no longer find parting to be an issue. The bloody top slide's got more metal in it than the S7 cross slide .
|439 forum posts|
is a rear tool post better than the front sort......I was under the impression that it was better for the smaller lathes.....
IAN's SC photo shows a monster lathe with 1........
I'm still not happy with the parting off results on my Student 6" sqaure top lathe.........have tried tipped as well as HSS, best results so far are with a Co/Hss with a slight groove on the top edge like Mark Rand.......
to the point if it's bigger than 35mm I'll use the band saw and then go back to the lathe, the small stuff just use the hack saw.......
and before u ask all the gibs are tight and well adjusted, saddle locked, high speed and plenty of pumped coolant.......I make high speed the next speed up in the gearbox from the turning speed.....ie. if I turn at 500 the next up may be 800rpm........the high speed parting seems more reliable.........
just sick of buying new tipped tool holders.....did make my own parting tool holder that sit's right next to the mounting stud for the tool block......but it's a faf to take it on and off all the time.......main tool holder is the Dickinson type....always thought that the parting tool with the Dickenson holder was to far out, away from the body.......
be interested on ur thoughts.........
|Mick Henshall||19/07/2018 10:07:10|
|483 forum posts|
Thanks for comments thus far,all very interesting. Started my own version,base is 3" diam x 1" thick, pillar is 1" dia with end let into 3/4" hole in base. The base will be secured with two cap heads situated each side of pillar which in turn will be held with an M8 through stud to another t nut, the toolholder probably 2" x 2" square of bms, still to design. I have made as chunky as I can Mick
|3058 forum posts|
I just use a standard bladed tipped tool in a Dickinson main tool holder on the top slide standard block, without problems on both S7 and M300 lathes. I do not have the problems others seem to, but cannot offer any particular answers that may help them.
381 forum posts
Like Duncan I had making a rear tool post holder for my lathe. The problem was I didn’t have the benefit of a T slot cross slide like the Myfords. In the meantime, I gingerly used my parting tool in the front toolpost.
Then I had a eureka moment. My lathe has a D1-4 chuck and can run in reverse. I put the parting tool in upside down and haven’t looked back.
One of the pitfalls of being new to the game, and doing my apprenticeship on the internet, is that you don’t always get to hear about the tricks the old guys would passed on, on the shop floor.
3289 forum posts
Many of the old guys' tricks are written down in the old books by LH Sparey, Duplex, Ian Bradley, GH Thomas and Tubal Cain (the English one, pre-YouTube). All still in print from model engineering suppliers such as Tee Publishing. Well worth the investment of the few pounds they cost, and many hours of pleasant reading to boot.
|Martin Johnson 1||20/07/2018 08:52:00|
|106 forum posts|
The only problem with the single post type tools in this thread is that all the tension is on one bolt and a fairly small length of T slot. Rear parting is less prone to "dig ins" - but not immune. I will leave you all to work out what is going to happen if you do get a dig in.................
GHT thought that one through with a "toe" on his rear tool post and two bolts to hold it down.
|Mick Henshall||20/07/2018 09:30:57|
|483 forum posts|
Fair point Martin, the base and pillar will be held down with 3 bolts into a 3" long t nut, but I am considering adding a 1/4" toe plate the toe of which will tie into the middle t slot, I just like the idea that means centre height can be adjusted easily, if this works then great but if not I have plenty of hacksaw blades
3289 forum posts
My single bolt rear toolpost has been working since before my old man bought the lathe circa 1954. The Drummond does have larger T slots and bolts than the Myford though. From memory it's a 3/8 stud up through the centre of the post. I doubt there would ever be enough force to tear lumps out of the T slots. Mostly when T slots get damaged it is when there is no flat plate directly above the T head, which is not the case with this design.
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