RIGHT THE STUFF

PART TWO MORE OF BOB’S TIPS FOR YOUR METALWORKI­NG LATHE

The Shed - - Metal Work - By Bob Hulme Pho­to­graphs: Bob Hulme

I f you have fol­lowed our Metalworki­ng Lathe 101 se­ries in pre­vi­ous is­sues of

The Shed you will have a grasp of the ba­sics, so here are some help­ful tips to im­prove your lathe ex­pe­ri­ence and make those projects a bit eas­ier to do.

Chuck­ing

Quite of­ten the ma­te­rial or item we need to hold in the chuck is del­i­cate, ei­ther due to a fine fin­ish that we do not want to put chuck-jaw marks on or due to it be­ing thin walled. For jobs with a sur­face fin­ish that you need to pro­tect, it is handy to have some strips of alu­minium to put be­tween the chuck jaws and the job ma­te­rial. These are mostly used when hold­ing in a four­jaw chuck, as the job will need to be ‘clocked up’ us­ing a dial in­di­ca­tor to get it run­ning true.

The thick­ness of the alu­minium strips can­not be re­lied on to be con­sis­tent, as they squish up a bit with the tight­en­ing of the chuck jaws, so when us­ing a three­jaw chuck the auto cen­tring ef­fect is not so good. If you want to use a three-jaw chuck, then in­stead of the alu­minium strips, use an alu­minium soft-drink can. Just cut the top and bot­tom out, slit the

cylin­der and cut it to suit the size of the job you are go­ing to hold in the chuck. Be­ing quite thin, the alu­minium layer will not ad­versely af­fect the cen­tring with the three-jaw chuck ac­tion all that much. Three-jaw chucks do not cen­tre ma­te­rial per­fectly any­way and are a good ap­prox­i­ma­tion only.

For hold­ing thin-walled ma­te­rial such as tube, you will need to turn up a man­drel or plug to snug­gly fit in­side it to pre­vent it from col­laps­ing in­wards when the chuck jaws are wound down on it. The plug does not need to be very long, just long enough for the length of the chuck jaws. If you don’t use a plug then the tube will go out of shape and it will not be held well enough to pre­vent slip­ping against the jaws when you are ma­chin­ing. Part­ing off is par­tic­u­larly dif­fi­cult with thin-walled tube (3mm or less) and a plug or man­drel is ab­so­lutely es­sen­tial then. Be sure to make your part­ing-off cut as close to the end of the plug as pos­si­ble to pro­vide the best sup­port for the tube.

The fin­ish

Achiev­ing a fine fin­ish can be a chal­lenge, par­tic­u­larly if you are work­ing with a lathe that is a bit old and worn or if it is lightly built so that rigid­ity is less than de­sir­able. Ide­ally, we would all be us­ing top-of-the-line lathes built from heavy cast­ings with high-pre­ci­sion slides, etc. How­ever, bud­get and shed space con­straints are the re­al­ity we all have to fit around, so here are some ideas on how to achieve a good fin­ish on those jobs where it is needed.

If you are us­ing tung­sten car­bide insert tool­ing to do the cut­ting then you are part of the way there. These have been de­vel­oped over many years and after much re­search as dif­fer­ent man­u­fac­tur­ers strive to be ahead of each other in a com­pet­i­tive mar­ket. You can spend less money by us­ing high-speed steel (HSS) tool bits and grind­ing them your­self, but it does take skill to get the ge­om­e­try right and the edge life is less than for tung­sten car­bide. (I should just qual­ify this com­ment, as I do find that hand- ground HSS cut­ting tools are still the best for ma­chin­ing ma­te­ri­als such as plas­tics and alu­minium.) As a rule of thumb for se­lect­ing tung­sten car­bide in­serts when turn­ing on a lathe, se­lect a nose ra­dius that is larger for rough­ing (say 0.8mm di­am­e­ter) and smaller for a bet­ter fin­ish (say 0.4mm di­am­e­ter).

Cor­rect speed

Speeds and feed rates have an im­pact on sur­face fin­ish too. A coarse feed is best for rough­ing and re­mov­ing ma­te­rial quickly and the op­po­site is true for achiev­ing a fine fin­ish. How­ever, it’s not so clear cut for speed (rpm). Too fast a speed and you will get chat­ter­ing on the sur­face. This will also be ap­par­ent from a high-pitched, squeal­ing noise when ma­chin­ing. Too slow a speed and the fin­ish will be rough due to the ma­te­rial tear­ing in­stead of cut­ting cleanly.

It’s a bit like us­ing a ro­tary lawn mower. It re­lies on the blade spin­ning around fast to whack the grass fast enough to cut it rather than chew it. The grass does not have time to bend over. Clearly some ex­per­i­men­ta­tion is needed to find the op­ti­mum speed for the lathe you are us­ing and the type of ma­te­rial you are cut­ting. Start off by us­ing rec­om­mended cut­ting speeds from charts or ref­er­ence books such as the En­gi­neers Black Book. Use the rough­ing cuts to ex­per­i­ment so that when the last cut is to be done you will have it sussed!

Some clean­ing ad­vice

So the last cut has been taken and the fin­ish is not too aw­ful, but it could be bet­ter. There are ways to fix this, but be aware that the size (di­am­e­ter) is go­ing to get a lit­tle smaller as a re­sult. One of those plas­tic scour­ing pads that are used for clean­ing pots and pans can be good for run­ning over the sur­face to give it a shine and take off the tops of the rough­ness. How­ever, be very care­ful as it can grip on the spin­ning job and snatch your hand too. Un­der no cir­cum­stance use a lathe wear­ing gloves!

Achiev­ing a fine fin­ish can be a chal­lenge

It’s im­pos­si­ble to pull your hand out of a glove once it tries to wrap it­self around a spin­ning job in a lathe (or a drill for that mat­ter).

You could also use emery tape to im­prove the fin­ish. The same safety pre­cau­tions ap­ply. Push from one side only and do not wrap the tape around the job. It will grab in a flash and pull your hand in too. Clean­ing your lathe after us­ing emery tape is a pri­or­ity, as there will be abra­sive grains dropped onto the slide sur­faces that will ac­cel­er­ate wear if not cleaned off.

Save time

It is a myth that you can take your time when work­ing on a project for your­self in your home work­shop. At least that’s what I have found. Time is al­ways a valu­able com­mod­ity no mat­ter what you are mak­ing or fix­ing. There are al­ways plenty of other things that need to be done (in­clud­ing time to do noth­ing!)

When turn­ing a job on the lathe it’s of­ten the case that the piece of ma­te­rial you have avail­able is much larger than the item you want to make, par­tic­u­larly when you have to work with what­ever you have avail­able at the time in your shed. So, ma­chin­ing away all that un­wanted ma­te­rial needs to be done as quickly as pos­si­ble.

Ma­chin­ing the di­am­e­ter down (‘turn­ing’) is al­ways go­ing to be faster than cut­ting over the end face (‘fac­ing’) of a piece of ma­te­rial in the lathe. This is be­cause deeper cut­ting depths are pos­si­ble and most of the cut­ting is done at the largest di­am­e­ter where the cut­ting speed (me­tres/min­utes) is achieved. With fac­ing, every cut has a chunk of time spent when the cut­ting tool is close to the cen­tre of the end face where cut­ting speed is nearly zero.

Clear­ing the swarf

When drilling deep holes into the cen­tre of a job, it is nec­es­sary to keep re­tract­ing the drill to clear the swarf from the hole, and the deeper the hole gets, the more fre­quently it needs to be cleared. Fail­ure to do this will re­sult in the drill jam­ming in­side the hole due to all the swarf jam­ming around it.

Don’t keep wind­ing the tail­stock hand wheel out to clear the drill. In­stead, re­lease the lever that clamps the tail­stock to the lathe bed and slide the whole tail­stock back. Once the swarf has dropped away, slide the tail­stock back again, be­ing care­ful not to bang the drill too hard into the end of the hole.

Time is al­ways a valu­able com­mod­ity no mat­ter what you are mak­ing or fix­ing

Feed the drill to cut the hole deeper with the tail­stock hand wheel, then re­peat the process un­til the full re­quired depth is reached.

Know your oils

Use an ap­pro­pri­ate cut­ting oil when ma­chin­ing, as this will im­prove cut­ting­tool life and sur­face fin­ish. It takes time to stop and re­sharpen cut­ting tools or to undo and in­dex around tung­s­ten­car­bide in­serts. This can be frus­trat­ing as well as in­volv­ing ex­tra cost.

As a gen­eral-pur­pose cut­ting oil, a sol­u­ble oil type mixed with wa­ter is good, as this lu­bri­cates as well as cools ef­fec­tively. Ide­ally, if you don’t have a lathe with a pump unit for coolant/ cut­ting oil you can buy sep­a­rate units as an add-on.

The sol­u­ble type of cut­ting oil is es­pe­cially good for ma­te­ri­als such as stainless steel, which is not a good con­duc­tor of heat, so the area where the cut­ting is tak­ing place gets very hot and the cut­ting tool soft­ens and wears more quickly if not cooled. The best sol­u­ble cut­ting oils are a mix­ture of min­eral and syn­thetic oils.

Straight (non-sol­u­ble) cut­ting oils are good for tap­ping threads, screw cut­ting, and there are some very good oils purely for alu­minium. Brass is happy when machined dry.

A down­side of sol­u­ble cut­ting oil is that it can stag­nate and smell. This is caused by anaer­o­bic bac­te­ria — bac­te­ria that grow in the ab­sence of oxy­gen. The best way to com­bat this is to sim­ply pump the oil/wa­ter mix­ture around for about half an hour each week. The splash­ing oxy­genates it and the bac­te­ria do not flour­ish. Some peo­ple in­stall a fish aquar­ium aer­a­tor in the tank and this is very ef­fec­tive.

I hope these tips in­spire you to do more with your home work­shop lathe. Con­tact us here at The Shed if you have any as­pects of lathe work you would like us to delve into or you have any ques­tions.

Alu­minium soft-drink cans are ideal for pro­tect­ing the part

Alu­minium strips pro­tect against chuck jaw marks

Use a scour­ing pad to en­hance the fin­ish

Us­ing a man­drel in­side a thin-wall tube job

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