Touching down tool to work when turning

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

I have a friend who , quite late in life , has bought himself a lathe and taken up home workshop activities .

 

When making a stepped shaft recently he was turning a longish parallel bit and was very near to size when he retracted the tool in order to sharpen it for the final cut . When about to re-engage the cut he realised that he did not know how to touch down the tool to the work so that it just met the surface ‘dead’ – ie with no gap and no cutting in .

 

This accurate touching down problem actually arises quite often when using techniques like ‘ turning to a witness’ and in screwcutting .

 

I showed him one way of doing it but it occurred to me later that I have never seen this problem mentioned in the magazines .

 

Any thoughts ???

 

 

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

[Steve GarnettParticipant @stevegarnett62550]

I have questions – is there actually an ‘approved’ method of doing this? On material I didn’t want to mark, I’ve used a feeler gauge against the work and then advanced the crossfeed by the feeler thickness, but is this good enough?

 

One thing I would like would be an accurate way of taking a tool off, sharpening it, and then being able to return it to a quick-change holder so that it projected the same amount (this is all to do with DROs…) .The only way that seems to be obvious is to mount a vertical surface on the lathe bed, run the tool up to it, leave the cross slide where it is, remove the tool from the holder, sharpen it and put it back so that it aligns against the vertical surface again. Or is there a better way?

[Jeff DaymanParticipant @jeffdayman43397]

Shut the lathe off and use a cigarette paper between work and tool. Wiggle the paper as you move the tool up SLOWLY, as soon as it grips, you are .001″ to .0015″ away from the work. The paper will prevent scratches as you get close.

 

Steve G what you are talking about is tool setting – in industry this is often done in an off-machine purpose-built setting jig. It consists of a toolholder mount just like on your machine and a setting block that is fixed or partially fixed so the tool or the setting block can be moved up and touched then tool clamped in the correct position. A bit beyond the usual needs of ME, but certainly could be done if you wanted one for use with DRO or CNC control.

 

JD

[chris stephensParticipant @chrisstephens63393]

Hi Micheal,

I have to ask why your friend needed to sharpen his tool at such a critical point in the proceedings. If he was taking heavy cuts first, why did he not sharpen the tool when he was, say, 10-20 thou away from final diameter so he could still measure to check how much more was needed to remove. It would be much better to not sharpen the tool, and rely on the graduations on the dials. The accepted way to do the job would be to turn down till , as I said, about 20 thou over size, measure the job and dial in half the amount expected to be removed then measure again and dial in the actual amount needed. If you do it this way you are taking into account such things as spring and backlash. If you have a properly set up DRO, you can ignore most of the above, as MGJ will no doubt testify.

 

As for methods of touching, I go with a wet ciggy paper stuck to the job, advance to tool, with the lathe running, till the paper snags, and as has been mentioned above you still have a thou or so to go. Another method is to use a permanent marker, coat the job and again advance the tool till it just cleans the ink away. The trouble with doing the job this is way is that there is no load on the lead screw, unlike the first paragraph, so you can not be totally such how your cut will work. You will almost certainly have to put more of a cut on than you would expect.

 

This is one of those techniques that are easy to explain and show in a few seconds in a one to one situation but takes many words to do.

chriStephens

[mgjParticipant @mgj]

HiChris- I think perhaps you are crediting DROs with more magic than they posess.

 

Finishing cuts- for me I always still use your 50% rule, in case there is any lack of rigidity, or whatever other gremlin it is that lurks. Then you are setting up more or less identical contitions on the final cut. Depends on how critical doesn’t it? If you want to be within a thou on dia, you can be fairly lax. If you want to be within a couple of tenths then you have to pull the stops out?

 

I’m entirely with you about the time to change or sharpen tools. A bit of forethought and planning and you can avoid most of these critical tool changes.

 

Ran perfectly on air this eve. 7 months to the day.

 

 

[mgjParticipant @mgj]

Sorry Chris – I missed your point. Yes, once one has the job sorted, they are very accurate on repetition, but I don’t think that absolves the operator from sound practises..

[WeldsolParticipant @weldsol]

Just one thing to add when using DRO’s lock your topslide as any movement in this will not change the DRO

I found this out the other day, as I always have the topslide set over ( to enable the centre to do its job without fouling) I had several parts to make which needed to be the same. So set up a blank set the DRO put in the shaft machined it spot on put in another shaft machined it tried in the mating part and it was a mite tight checked it with a mic, and it was a tad over but the DRO was reading right.

Went through it all then realised the topslide had taken up the tool load via its backlash.

Locked it all up then everything was right.( after resetting the tool against the readout)

 

Paul

Edited By Weldsol on 12/02/2011 21:18:00

[Clive FosterParticipant @clivefoster55965]

If using a QC system it shouldn’t be too hard to come up with a poor boys version of the industrial tool setting system. One workable way would be to make an L shape carrier deep enough and wide enough to take the complete tool holder having closed ends sufficiently far apart to contain the maximum tool projection you anticipate using. By firmly pushing the tool holder into the channel, interposing suitable spacer block(s) between the tool holder and one closed end then sliding the tool along until it touches the other closed end before nipping up the clamping bolts it should be possible to control tool projection adequately closely. It doesn’t matter exactly what the projection is so long as its repeats so a couple or three spacer blocks should do fine. I’d probably use bar stock 1/4 or 6 mm (ish) thick milled on the edges and screwed together to ensure accurate right angles.

Of course the correct way of using a QC system is to have a roughing tool and finishing tool set-up in separate holders ready to swop over at the appropriate point on the job. One reason I’m unconvinced over QC systems for hobby users, novices in particular, is that you need a decent number of holders to exploit the system. Which comes expensive, at the last count I had 16 holders! QC systems for smaller lathes really aren’t large enough for optimum stability and the extra joints inevitably reduce stiffness. Small lathes are flexy enough at the best of times without adding potential problems. Much to be said for keeping the top slide for special jobs when its really needed and doing most work with a spacer block instead, ideally with a heftier tool post.

If you have a 4 way, or other type of block tool post, a U gauge in either drop over the top or push against the front format ought to be workable. Push against the front with the two channel arms against top and bottom of the post looks preferable but the job may get in the way.

If you use Armstrong tool holders and HSS bits, or any similar system which holds the tools sloping upwards, setting the tool tip to centre height after sharpening automatically sets the projection too. The optical type centre height gauge makes this easy. In my view interchanging tool bits in Armstrong holders mounted via inexpensive shop made tool blocks is a much more appropriate QC system than commercial offerings for the hobby user given the (usually) restricted financial constraints.

Clive

[Steve GarnettParticipant @stevegarnett62550]

Posted by Jeff Dayman on 12/02/2011 17:47:44:

 

Steve G what you are talking about is tool setting – in industry this is often done in an off-machine purpose-built setting jig. It consists of a toolholder mount just like on your machine and a setting block that is fixed or partially fixed so the tool or the setting block can be moved up and touched then tool clamped in the correct position. A bit beyond the usual needs of ME, but certainly could be done if you wanted one for use with DRO or CNC control.

 

Had a feeling that there would be something like this for more industrial situations than our prototyping really is. I might consider trying to arrange something, along the lines that Clive suggests.

[adyParticipant @ady]

you should have sorted it all out30 years ago.

[mgjParticipant @mgj]

I did – on the Myford I just run the tailstock up and use the quill extended. Put a piece of 1/8 in between to protect the quill. Don’t know whether you can do that with a standard topslide, but I use the Radford one, which gets rid of all the tailstock clearance problems, and its much more rigid.

 

With tipped tooling I just turn the tip round to get a fresh face.

Edited By mgj on 13/02/2011 01:14:30

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by MICHAEL WILLIAMS on 12/02/2011 16:55:32:

I have a friend who , quite late in life , has bought himself a lathe and taken up home workshop activities .

 

When making a stepped shaft recently he was turning a longish parallel bit and was very near to size when he retracted the tool in order to sharpen it for the final cut . When about to re-engage the cut he realised that he did not know how to touch down the tool to the work so that it just met the surface ‘dead’ – ie with no gap and no cutting in .

 

This accurate touching down problem actually arises quite often when using techniques like ‘ turning to a witness’ and in screwcutting .

 

I showed him one way of doing it but it occurred to me later that I have never seen this problem mentioned in the magazines .

 

Any thoughts ???

 

 

What was your method?

 

Regards,

Tony

[DustyParticipant @dusty]

C’mon guys why are you making this so b****y complicated. I use a method very similar to Jeff Dayman and his cigarette paper, but I keep the lathe running. The secret is do not get the ‘fag paper’ wet. Once this position has been established I then paint the job with layout blue. This must be allowed to dry, I use layout blue as opposed to felt tip as it is a thicker film. I then advance the tool until it shows a witness in the blue. The method I use for advancing the tool is known as the ‘spanner method’ I was taught this by a very experienced turner. The method is as follows Take the cross slide handwheel and remove any backlash, now holding the handwheel in this position you tap the hand wheel with a spanner (in my case a 5″ length of 1/2″ brass bar) with a little bit of practice you can advance the tool by a tenth of a thou or two. Try it you will be suprised how easy it is.

Tip of the day. Tear the cigarette paper in half lengthways, it is easier to control, has less drag and most of all it saves you money.

[David Clark 13Participant @davidclark13]

Hi There

I usually leave the job overlength so I can get the size correct on the end of the bar.

When it is correct I can run all the way along the bar.

I bought a packet of fag papers yesterday for use in the workshop.

They cost 25P, how much would I save by tearing the papers in half, not a lot.

regards David

 

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

My friend only got into trouble with this due to inexperience – certainly it is better to retract and sharpen tool when at least several thou left on the work and then exact touching down doesn’t matter so much .

 

The real interest in this is when you need to touch down to a reference diameter which already exists such as the pre turned OD of a shaft which is going to be screwcut .

 

Like everything in engineering the method needs to suit the accuracy needed :

 

(1) Within one thou accuracy just do it by eye – with a bit of contrasting paper behind the work it is quite easy to judge when the gap disappears . Also the same accuracy is achieved if you traverse the tool along the work whilst it is turning and watch for the first whisker fine turnings to appear .

 

(2)There are lots of ways of doing it super accurately .

 

One example : apply much thinned blue to the touch down area and with the lathe running and the tool being traversed along the work advance the tool until the first fine scratch mark appears in the blue – with a sensitive touch this method has an accuracy of one tenth of a thou or better .

 

Another method : called the secondary witness method where a short bit of the work is used just for tool setting and turned away afterwards . First the reference section is just turned true to any convenient diameter and left alone . As each tool being used is put back after sharpening its is used first to take a very fine cut along the reference section which is then measured . This measurement effectively tells you exactly where the tool tip is relative to the index dial reading and you have an exact setting . Knowing the diameter of the actual work area as well means that you can touch down again very accurately just using the index dial .

[DustyParticipant @dusty]

Sorry David, the” it saves you money “was a bit tongue in the cheek.

[Ian AbbottParticipant @ianabbott31222]

I use the same method as David and leave some waste at the end to get the final cut right. If it’s a face cut, then a bit extra on the diameter.

 

Ian

[VersabossParticipant @versaboss]

Posted by Dusty on 14/02/2011 09:36:19:

…. The method I use for advancing the tool is known as the ‘spanner method’ I was taught this by a very experienced turner. The method is as follows Take the cross slide handwheel and remove any backlash, now holding the handwheel in this position you tap the hand wheel with a spanner (in my case a 5″ length of 1/2″ brass bar) with a little bit of practice you can advance the tool by a tenth of a thou or two. Try it you will be suprised how easy it is. 

 Dusty, can you please explain this method a bit more detailed? Where do you tap the hand wheel? At 12 o’cl, 2 o’cl  or any time &#160? in which direction? Axially or radially? Do you try to hold the wheel stationary with the left hand while you tap it (assuming you are right-handed ) ?

I would like to try that, if I would know exactly how.

I had never much luck with the color band method; I think when I see the color disappearing then the cut is often already too deep.

Greetings, Hansrudolf

[DustyParticipant @dusty]

Hansrudolf

Hold the wheel with your left hand, keeping a slight pressure on the feed screw, it is really only maintaining contact. I tap the handwheel at about 1 o’clock on the edge of the wheel.The tap is a vertical hit on the hand wheel, you are not trying to knock the handwheel round. I would mention that it does not work if you are using tip tools to scrape the last few tenths from a job. Tip tools are not designed to cut such small amounts. H.S.S. properly ground and set on centre height is the answer. I suppose that ‘the spanner’ method is using kinetics.

[VersabossParticipant @versaboss]

Dusty, I am glad to report that your method works a treat! One learns something every day; I never saw that  in the many books I own.

Both my lathes have friction dials (no ‘hand wheels’), so I tapped instead (very lightly, with a small brass rod) the center of the ball handle. Giving a light bias with the index finger in the desired direction, I could see the index lines creep forward in minuscule steps. Say about 20 taps for 1/100 mm.

I put then some cuts with a sharp HSS tool on a piece of scrap free cutting steel, and was able to reduce this several times in repeatable steps of 1/100 mm on diameter. The finish was perfect, and the  ‘swarf’ was a bit of black powder! The tool was possibly not even the best; I think even smaller cuts would be possible with a better one.

Graham, I don’t think that this qualifies as  ‘striking a machine tool with an implement’. Using the palm of the hand does not work. It must be a tap which gives a vibrational (or call it sonic) pulse to the leadscrew.  My interpretation, I admit. But I am absolutely sure it does no harm to the machine.

Greetings, 

[chris stephensParticipant @chrisstephens63393]

Or you could just set the compound slide at 6 degrees, which will give you one tenth of the movement inwards as shown on the compound dial. This method is much more predictable than clobbering your lathe with a lump of metal.

chriStephens

[Ramon WilsonParticipant @ramonwilson3]

You know guy’s I’m surprised that no one has mentioned using a magnifying glass.

 

If you are in doubt about bringing the tool too close set it as said with a ciggy paper – you’re within a thou or so at that point then hold an eye glass (loupe) close to the tool point – it will give you quite an advantage. Lot’s of light too – helps no end

 

Regards – Ramon

[VersabossParticipant @versaboss]

Well Chris, on SOME lathes (like one of mines e.g.) you need the topslide to turn parallel. In most cases I don’t need or want a 12 deg. cone. .

Greetings, Hansrudolf

[Jeff DaymanParticipant @jeffdayman43397]

Ramon, you make two good points, the light and the magnification do work wonders. I have gotten into the habit of automaticlally putting on a headband magnifier and using task lamps at each machine. I also have a small aluminum body LED flashlight (torch) I keep handy for extra light when needed. Very handy on mill setups close to the vise when the head throws the cutter into shadow.

 

JD

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