Can't get the hang of HSS!

Hi everyone,

Another 'what am I doing wrong' question, I'm afraid.

Everything I read suggests that carbide tips are difficult to work with and HSS very forgiving. If that's right, why can I get the finish in photo A using carbide, and no matter what I try, get the result in photo B with HSS? I must be doing something very wrong indeed...

Photo A

Photo B

The lathe is a Warco WM240B - with belt drive. The stock is 20mm dia. brass, with about 2 inches out of the chuck. Carbide produces this finish on a first-pass at either 620 or 1,000 rpm. HSS makes the mess in photo B at all available speeds from 240 - 2,0000 rpm. The only thing I haven't tried is the lowest speed of 120 rpm. The HSS is a right-handed pre-ground knife straight out of the bag from Arc, and I've tried it square on to the stock and at slight angles - in both directions! Hand feeding or running off the gears at 0.1mm / rev creates the same result.

It's brass, a new knife - not home ground, The more I read, the more I think it should be a great finish and easy...obviously beyond me at present!

If anyone can point me in the right direction, I would be extremely grateful - and I could stop depleting the world supply of brass trying to get it right!

Can you show us the HSS tool please? Maybe that will help with diagnosis. Don't lose hope!

+1

Almost certainly something wrong with the tool or setup. The only caveat is what type of brass? The most common type, CZ121, should be easy with carbide or HSS, but some alloys can be problematic.

Andrew

HSS tools for brass usually have zero side and back rake. The pre-ground HSS usually have a groove ground in the top that makes them too "sharp" for brass. What type of Carbide are you using? The brazed tip type usually have little or no rake.

HTH,

Rod

Radius the tip and travel faster might help. I think that looks like a very pointy tip has been travelling too slowly and has cut a very fine thread.

Martin C

Hi everyone,

Here's the HSS knife I've tried - I've also included the neutral tool I had a go with briefly.

The carbide tips are CCGT specifically for non-ferrous, and the brass is off-the-shelf stuff from metals4u.

The one on the right has a bit too much top rake to be ideal for brass and out of the bag could so with the end rubbing with a diamond stone.

The one on the left could do with the top and two 45deg faces rubbing on a diamond stone as it does not look sharp which brass likes.

If you have a bench grinder I would suggest having a go at grinding the other end of the RH one. leave the top alone and just grind the end and LH side to 5degrees

I would not radius the tip of that RH tool as the groove means you will get even worse rake and it will be hard to set to ctr height

Edited By JasonB on 13/07/2021 16:30:36

Radius the tip and try different feed rates might help. I think that looks like a very pointy tip has been cutting a very fine thread. The carbide tip probably has a 0.2mm or 0.4mm radius viewed from above and you need something similar with HSS to get a nice finish. If you use power feed try something like half the tip radius per revolution as a starting feed.

Martin C

The tool on the right is too sharp and pointed for brass.

I would use a round nose, maybe 1/2 mm radius with zero rake on the top and left side and about 10 degrees on the front.

I do agree that in the second picture it looks like a very fine thread !

This poster from Steve's Workshop might help:

The page on Steve's website has a good description of the process and details of his sharpening guide.

John

Edit: Add Link

Edited By Journeyman on 13/07/2021 16:52:12

Thanks Martin - what sort of radius? I can just ease the point with a diamond credit card-style stone, or put a slightly bigger turn on the tip using a bench grinder. If I know what I'm aiming at, I can try it tonight.

Strange - I use that type of knife on brass without bother.

Roderick and Martin may be on to something; a sharp tool, taking a shallow cut at low feed rate, might scratch the surface and nip swarf into the grooves. Try taking a deeper cut and increase the feed rate.

Also, check the change gears if they've been messed with recently: I've been known to put them back wrong. I hesitate to admit it but last time I produced a finish like that, the lathe was in reverse. Blush.

Carbide may be doing better because its bluntness smooths out the furrows. There's no reason HSS shouldn't cut as well as carbide.

Dave

I think I've just got the answers to my 'what sort of radius' question!

Lots to think about and try - I especially like the poster - pictures are always worth it.

I'll give it a go and let everyone know how I get on. Got a couple of HSS blanks, so can always start anew if I make a mess of the grinding.

Thanks everyone - great to know everyone is so friendly and helpful.

Might I suggest checking Sparey ? I know he,s regarded as Old Hat, but he does cover this subject fairly well.

I dont use much brass [too dear to play around with ], but I do know that it likes a higher speed than BMS, along with really sharp tools.

Just my pennyworth.

An outside possibility is backlash from an easy-rotating handwheel that can roll back when the knob's near the top, opening up the risk of tool 'flutter', where the tool's randomly dragged in and flicked away from the surface. It does look as if, apart from the 'screw thread', your tool point is digging a rapid series of tiny circumferential burrs. Try locking the crossslide.

I'd also turn the toolpost to open up the plan trail angle a bit on the RH tool to avoid trapping swarf against the cut surface, and stone a rad of about 0,5mm with a medium India oilstone on the point to improve tool finish.

Edited By Mick B1 on 13/07/2021 20:18:53

After an evening of experimentation, I've made some progress - definitely better than my earlier HSS attempt, but not there yet.

I haven't tried increasing the feed rate yet - I've only got two choices, and changing means swapping over the gears each time. Something to try if 0.1mm / rev really is too slow. I've eased the tip of the knife in the photo, and tried it on the various diameters of brass you can see at a speed of 620 rpm, and with various cut depths, ending up at about 0.2mm as indicated on the handwheel. Adjusting the knife angle as Mick B1 mentions produced a slightly better finish, although I suspect I am not cutting deep enough yet to make this a significant factor. Maybe my next step should be to follow the poster from Steve's Workshop and try grinding my own. Need to get an india stone as well - this easing was done with a diamond credit card.

The sooner I get this sorted, the sooner I'll stop wearing my fingers out moving the belt every few minutes to try different speeds. This is where I really wish I'd got a variable speed machine!

Your diamond credit card might be as good as my India oilstone, or better. Just saying what works for me. A polished finish on the tool tip helps if other factors are right.

Some HSS is better than other for tools. I have a knife tool that looks like yours that has a sort of brittle-crumbly microstructure, and I don't get on well with it. I prefer straight 1/4" square-section HSS blanks ground to suit, on a packing-piece I made to set them on centre-height.

Experimentally, just to test the hypothesis wind the saddle over fast by hand rather than driving it with the lead-screw. If finish improves, it could be due to the faster feed by hand, or because the saddle is being rocked by by a bent lead-screw or the half-nuts not engaging properly. Not sure how to prove the saddle is rocking, because the movement will be be tiny. Perhaps run a DTI mounted on the saddle along a rod in the chuck. Can't try myself because my workshop's filled with empty boxes pending daughter moving house soon!

Another thought, if the saddle has been crashed into the headstock, the pin protecting the lead-screw may have sheared and is acting as a slipping clutch, ie the cut progresses by fits and starts. With the lead-screw engaged and turning try holding the saddle to stop it moving: if the lead-screw stops, the shear-pin needs to be knocked out and replaced.

Dave

I had a rummage about in the box of toolbits and dug out this one which used to be kept in my apron at school so that I always had a half decent tool rather than the abused ones that were usually stuck in the toolposts. I gave it a quick rub on the top, end and side with a small diamond slip and took a cut.

First 5mm are with a sharp corner, next 5mm with the corner slightly rounded with the diamond slip. Speed 1000rpm, 16mm dia, 0.005" (0.13mm) Depth of cut, feed 0.0025" (0.06mm) / rev. No slides locked. You can see that the first cut gives a constant satin finish and the cut with the rounded corner a brighter more polished finish. Click for larger images

View of tool from above, end and side are a few degrees less than 90

From the side facing the chuck, you can see this actually has a few degrees of negative top rake (points down towards work)

View of the end, the slightly brighter areas are where the diamond slip has made contact, the darker is where the slight concave face from the grinder has not been touched. Important what stoning a tool that you do not get convex faces as they will rub (except the rounded corner)

Quick swap to a more rounded tool did not make a lot of difference.

Edited By JasonB on 14/07/2021 18:35:02

Edited By JasonB on 14/07/2021 18:35:50

HSS tools Start with zero top rake,then clearance ground on the front and side, then with an india oilstone polish out the grinding marks on the side and front clearance,then stone a small rad on the front of the tool then just place the stone on the flat top of the tool,and rub the tool for a few strokes to remove any burrs,No top rake is required, if the side/front clearance has been ground hollow due to the radius of the wheel then only polish the top i/2 mm to get a good edge on the toolbit. Beginners tend to not keep the stone flat agaist the surface to be stoned and let the stone roll and spoil the cutting edge. Dont worry about self act feeds, wind the saddle along by hand and get a feel for the tool doing the cutting if its hard work/poor finish then the tool is blunt. When I started my training the first lathe I used was a Lorch plain lathe,no self act feed just learn to wind the handles steadily,then came the Boxford,self act feed was rarely used,the engagement clutch was a pain and it was quicker,with hand feeding the saddle.Then there was a German large plain lather,when finishin turning microscope eye piece tubes,it was a case of top speed of around 3000 rpm on plain bearings and spin the top side quickly,8 1/2 hour day you soon learnt to keep the tools sharp as there was also the chance that your next job could be polishing the tubes on the polishing mop, and the better the turning the easier it was to polish.Horible job polishing. Not a carbide tool in sight in the late 1950s and no need for one on brass.