|James Alford||14/12/2020 07:59:42|
|423 forum posts|
What type of parting-off tool do people recommend for use on a small lathe: I have a Flexispeed? I have a one of those holders which take a flat blade from RDG, but the fixing for the blade is not very good. It tips the blade over at angle and does not grip the blade very well, allowing it to slide in the holder, rather than cutting the metal.
I have tried grinding my own from HSS, but not with any real success.
|not done it yet||14/12/2020 08:16:31|
|5639 forum posts|
There are three pages for a forum search for ‘parting off’. Have you read them as the topic has been covered many times?
|683 forum posts|
I use a HSS Co parting tool with tapered sides in a home made holder clamping the tool in from the side. Works well on the steel I mostly use, just need to watch the slide overhang to reduce the movement under load. I use the tool pretty well as supplied with no additional top rake grinding.
|Martin Connelly||14/12/2020 08:47:41|
1664 forum posts
If the holder is tipping the blade over then it is probably designed for a wedge shaped blade. Are you using a matched blade and holder?
Also if the blade is slipping it implies the blade does not match the holder or it is set too high and rubbing not cutting. As ndiy said parting is a regular topic and will pick out any little error in the process or tooling as it needs to be just about perfect in every aspect. How are you setting the cutting tip's height?
When everything is done correctly parting is not a problem and there is very little pressure pushing the blade backwards towards the operator.
|Oven Man||14/12/2020 09:00:10|
106 forum posts
Consider using a rear tool post for parting off, it makes a world of difference.
|Mick B1||14/12/2020 09:19:30|
|1839 forum posts|
+1 for this. I've been using a 5/16" RDG blade holder tool for decades. The only time I had a persistent problem I found I'd inadvertently - maybe ignorantly - bought some parallel-flanked blades off the Bay. The top-to-bottom tapered ones generally work well.
6878 forum posts
Sounds like you have a bad example of a flat bed holder. Fix or replace.
Unfortunately, expect more trouble because parting off is notoriously difficult. The cut applies a lot of force to the tool, and through it to the tool-post and saddle. The whole lot bends and twists, causing the cutting edge to move far enough to dig in. Even more likely to go wrong when the blade is extended for a deep cut. The cure is rigidity. Minimise tool and job overhang, lock the saddle and all gibs that don't need to move, and snug up the one that must. Ideally, fit a rear-tool post of the Gibraltar type (a solid lump of metal) and run the lathe in reverse. This avoids trouble due to a bendy tool-post and compound slide and grounds cutting forces straight into the saddle. A worn lathe may allow enough movement to make parting-off all but impossible; is the bed near your chuck in good order?
Rigidity isn't the only problem. Generous lubrication is important, and so is the absolute need to keep the slot clear of swarf. Some metals cut better than others - aluminium has a nasty habit of welding itself to the cutter! The tool has to be straight and at the right height. Finally, and perhaps most difficult, the operator has to maintain a steady feed-rate, not too fast and not too slow. Jerky tentative handle twirling is a recipe for disaster. Another operator issue is judder and squealing during parting-off often mean 'go in harder', when the natural reaction is to back off.
Parting off on my mini-lathe was so unreliable I mostly hacksawed instead. My WM280 (a much bigger machine) parts off satisfactorily from the ordinary tool-post, but it's safer to use a rear-tool post. Either way, the lathe's auto-feed is better at parting off than I am - it applies constant cutting pressure, whilst I wobble and dither!
I don't know how good or bad a Flexispeed might be at parting off. I hope a Flexispeed owner will comment. Given it's size and age I'd expect it to be sensitive rather than straightforward - the operator has to compensate, which means practice, practice, practice.
Edited By SillyOldDuffer on 14/12/2020 10:08:09
Edited By SillyOldDuffer on 14/12/2020 10:09:18
|old Al||14/12/2020 10:14:44|
|180 forum posts|
I use the tapered blade type parting off tool. It always looks wrong as it cranks the blade over so one side is vertical and the other side has all of the taper. Been meaning to change it for the last 25 years
|1801 forum posts|
You seem to have already identified several problems with your parting tool holder/blade - so you need to fix these before you go any further. If the blade is not vertical it will most likely foul. If it is not firmly held, then it will not cut correctly. This seems to me to be a straightforward mechanical problem.
So do you have the correct blade for the holder (silly question perhaps - but is it in upside down?) and/or can you slightly modify the clamp to grip the tool better? If you cannot fix these problems, then either try fitting a new blade (perhaps check with RDG what the right blade is first) or get a new holder. Once you have sorted the obvious problems, then we can talk about how best to use it.
|Rob McSweeney||14/12/2020 11:59:39|
|25 forum posts|
I would question Mr Duffer's advice to 'run the lathe in reverse' - this is generally advised against as it can cause the chuck to unscrew from the mandrel under load. Normal practice with a back toolpost is to mount the tool upside down and cut with the normal direction of rotation.
1917 forum posts
Here is one i made some time ago. It works, but i always struggle parting off. Agree with SOD's comments. I think my main problem is i do not go agressive enough since i caused a total jam up previously.
|not done it yet||14/12/2020 13:10:58|
|5639 forum posts|
SOD is absolutely correct in what he posted. Do read that word “ideally”.
Yes, those with screwed-on chucks need to invert the parting tool and run the lathe in the normal forward direction. That is FAR better than parting from the front on a hobby lathe, but there will still be tendency to lift the carriage, if at all possible - so the downwards direction is the ideal situation for most of us hobbyists.
My lathe has the carriage lock on the rear, so that is close enough and perfectly satisfactory for my particular circumstance. But reverse and rear tool post is the ideal situation. I’ll defend
|not done it yet||14/12/2020 13:35:55|
|5639 forum posts|
A good start, but next time, do make one such that the parting blade is horizontal. One should always part off with the minimum tool extension, which (with your design) means likely needing to reset the cutter on centre height if parting off a larger diameter workpiece.
I have discarded the parting tool holder which came as part of a kit - I would buy the QCTP tool post and necessary holders separately, if I were ever to buy another. The only advantage of an angled cutter, as far I can see, is that it can be sharpened at a right angle to its length - not something that particularly bothers me.
Yours looks as though it is extended enough for a 50mm diameter bar. Parting from any QCTP is going to cause consternation for many, like that, as the cutter is both displaced longitudinally from the cross slide and has lots of overhang and leverage on the tool post holder, tool post, top slide, cross slide and carriage... And that would be true for even a new machine.
I had occasional aggravation with my previous lathe while parting off, but none with the current one which came with a rear tool post already fitted.🙂
Edited By not done it yet on 14/12/2020 13:38:12
|2005 forum posts|
Someone wanting to experiment might consider Eccentric Engineering's Front or Rear parting off tool holder.
An advantage of rear/forward is that the hoped-for curly chips can drop out of the cut and the flow/dribble of coolant has an unimpeded journey to the tool point.
Edited By ega on 14/12/2020 14:13:01
6878 forum posts
Good point - cutting in reverse isn't a good idea if the lathe has a screw on chuck! Reverse cutting is best left to owners of lathes with other fixing systems - bolt-on and cam-lock etc.
|Andrew Johnston||14/12/2020 14:44:41|
5933 forum posts
Plus one for Camlock. But of course with a lathe so equipped one won't need to faff about with rear toolposts and inverted, or otherwise, tooling.
|Howard Lewis||14/12/2020 14:50:15|
|4455 forum posts|
If it is possible to fit a rear toolpost, parting off will be far less of a problem, than in a front toolpost.,
Another vote for the tool having no top rake, to reduce the risk of dig ins. Front clearance should not be excessive, to maximise strength and rigidity, and again, to reduce the risk of dig ins.
Keep the tool sharp!
+1 for minimising overhang, to maximise rigidity.
Leaving gibs slack tends to encourage dig ins.
A light, steady feed, preferably with lubrication, if only a slow drip feed, is the way to go.
The blade needs to be square to the lathe axis, mounted with the cutting edge on, not above, the centre line, and ground so that the cutting face is square to the blade.
With a parting tool in the front toolpost, mounting the tool inverted and running in reverse is not to be recommended on a lathe with a screw on chuck; unless there is some means of preventing the chuck from unscrewing.
(My lathe, fortunately, has two dogs behind the chuck for this purpose, but not every one has this feature. )
Grinding the front of the blade at angle, may reduce the "pip" on the work, but increases the width of the swarf, making it more liable to jam in the cut, and produces a sideways thrust on the tool, meaning that the faces are not likely to be flat (More likely to be slightly curved or tapered ).
Remember that you are effectively taking a 2 or 3mm deep cut, with you, or the machine, providing the feed rate.
|James Alford||15/12/2020 07:45:59|
|423 forum posts|
Thank you for all of the replies, which are helpful. I shall have another good read, look for a new tool and try to distill the suggestions and apply them.
A few general comments in reply to specific posts.:
|not done it yet||15/12/2020 08:20:20|
|5639 forum posts|
Yes, I did do a search before posting, but I did not find what I was looking for, hence my question.
I fear you will not have found the answer you require in this thread, as everything here seems to have been covered somewhere in the multitude of previous postings.
The lathe has a screw-on chuck, so I assume that a rear or upside-down tool is not advisable
As your lathe has a screw on chuck an inverted cutter fitted to the rear would be the best option, if possible to arrange sensibly.
I have a tool height gauge and an adjustable height tool post and set the tool to a smidgeon below centre height.
Tool height gauges are useful, but, while not worthless, cannot be assumed to be perfect because if the tool is immediately (upon engaging with the workpiece) deflected by the cutting forces (resulting in a real cutting height of lower than “assumed&rdquo the original setting will have been altered in/by practicality. Ie to be a good, reliable indicator, one would require a totally rigid lathe arrangement - only possible in the ‘ideal’ situation. Think here of excessive cutter overhang from a ‘less than perfectly rigid’ tool, holder, tool post, top slde, cross slide and carriage. They are all summative errors.
I personally only ‘assume’ anything after I have searched, and satisfied myself of no reason for doubting the validity of making that assumption. Assumptions can so easily be completely wrong.
|Michael Gilligan||15/12/2020 09:23:20|
17338 forum posts
Assuming this to be the lathe in question:
A ‘Gibraltar-style’ rear tool-post, used in the manner described by Rob McSweeney, would seem very practical.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.