Boring a cylinder

I have attempted to machine a cylinder of 25,mm bore on my Myford ML7 lathe. Using a boring bar with a separate bit secured by an allen screw the result is a bore of 1.5mm less at one end than the other.

I suppose the obvious answer is a reamer. Unfortunately they are morse taper 3, the lathe is morse taper 2. Also, upwards of £40 for is lot to spend for one job!

Any ideas how to solve the problem. (The length of the cylinder is 50mm).

Either lapping or honing will sort it out.

HTH,

Rod

Eeek, that's not right! How are you traversing the slide? For example, if you use the top-slide to move the boring bar it will cut a taper unless it's exactly at a right-angle to the spindle.  1.5mm in 50mm is about 1.72 degrees out.

Dave

Edited By SillyOldDuffer on 18/11/2017 19:22:54

Use a high speed steel bit in the biggest diameter bar with the overhang set to the minimum required. When making the final sizing cuts repeat the cuts without moving the bar out, until no material is cut. For final finish hone to remove machining marks.

As Dave said do not use the compound to do the boring. Use the carriage & lock the compound if you can. Use a fine feed with small cuts as the boring bar can bend under pressure.Even a reamer is not made to take out that much material.

regards

If you are using the saddle movement to make the cut, that suggests the tool is either blunt or rubbing,

You may be surprised how much clearance boring tools need.

Neil

I'll second what Neil says, that is a lot of taper and sounds like the tool is being pushed off the work.

Unless your cross slide is very slack and the handwheel undid the cut, should have the cross slide locked

What you havnt mentioned is the material you are boring. Hard spots in cast can make a right mess of small boring jobs.

I used the carriage to move the tool, not the top slide. The boring bar is about 3/8" thick with a 1/2" square section to clamp it in the lathe. The cutting edge seemed sharp enough, it was taking long shavings off. I assume that it was being pushed off deeper into the cylinder.

My first intention was to drill through with a 31/32" drill and ream to finish. The drill had the same problems of fit, i.e. morse taper 3 and price. A drill and reamer would have been in excess of £60, even if they did fit!

Set your tool a frogs hair above center, be sure it is absolutely sharp, use as big a bar as possible and maybe sloww your machine down a wee bit.

Boring is the right way to do the job I think; expensive drills and reamers shouldn't be necessary.

1.5mm is a lot to deviate over 50mm whilst boring. Which is the wider end, inside or out? Given that you have a hefty boring bar, I can only think it's not cutting properly, either not centred or blunt.

Bit of a mystery! Any chance of a photo & more details.

Dave

+1

There's something fundamentally wrong here. There's no way a HSS boring bar is going to deflect that much.

Andrew

Seems to me that most of the issues mentioned above would result in bell mouthing rather than a taper. So, is the result really a taper or is it just that the ends are different diameters?

Rod

Check the back and particularly the bottom of the boring bar clear the pilot hole as it enters, and only the cutting edge makes contact. This much taper says the tool is moving, possibly being pushed aside as it enters the pilot hole.

If the boring bar is too big (or the hole too small) you can find that the leading edge cuts Ok because the frohnt clearance of the  tool guarantees thatt the cutting edge makes contact first. Result, nice pretty sweetarf and all is well until the tool enters further, now the bottom of the tool rubs and the cutting edge is no longer the only touch point of the tool with the work.

Give away symptom is an absolutely horrible surface finish in the hole, tapered so the hole is narrower the deeper it goes.

If you've got the opposite taper, do let us know. No theories yet, but I'm working on it

Edited By Simon Williams 3 on 18/11/2017 23:31:44

Edited By Simon Williams 3 on 18/11/2017 23:32:40

Sorry about multiple edits, predictive text.......

Edited By Simon Williams 3 on 18/11/2017 23:34:00

I agree with fizzy that chills in cast iron are a swine.My cylinder became oval and it took me hours of honeing to get it true.That said I think that your tool could be blunt or ground to the wrong angle.

Frank

Sounds like something is moving , check that your toolpost and compound slide are clamped down tight .

The toolbit could be moving in the boring bar , if it is the type that the toolbit sits in a square hole and retained by a grub screw i would look at investing in an insert type boring bar i had nothing but trouble with the cheap boring bar that i had .

You can get an adapter that has a mt2 male on one end and a 3mt female on the other so you can use your 3mt tooling on a machine that has a 2mt tailstock barrel .

I think we can rule out CI as Stephen says the tool was "taking long shavings of"

Also sounds to be reducing in bore towards the headstock "being pushed off deeper into the cylinder."

Honing or lapping won't cure 1.5mm difference that is 1/16" you would be there all week.

As well as a photo it would help to know material, depth of cut, how many passes at final tool setting, did the tool cut as it was withdrawn, hand or power feed, etc.

Problem solved. Having read all the replies, (thanks everyone), I have made a new tool from an HSS blank.

The cut was excellent and the sides parallel. All that I have to do now is lap the piston in. It seems to me that the tool I was using must have been flexing as it cut despite being a professionly made, (I assume), item.

Glad you got it fixed - well done.

You have to be a bit careful setting up boring. Simon Williams comment about making sure only the cutting edge is in contact reminded me I got that wrong last month. I was using my largest boring bar, an insert type, in a hole that was slightly too small. After a while I realised that the insert holder was doing the cutting, not the insert! Cut remarkably well too considering it was just the rounded end of the bar, but much inferior to the insert once I'd sorted it out.

Dave