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Cylinder boring

Bore finish

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Martin Green 305/11/2018 11:06:03
5 forum posts

I am boring my first cylinder in cast iron using my mill. I have machined all the surfaces and the finish looks fine. However when I take a cut in the cylinder bore using my standard boring head the surface finish is awful. It looks as though the tool has juddered. I have tried slowing the cutting speed. I am running at about 500 rpm and the head is moving very slowly.

I am using an indexable tool and the bore finish size is 1 3/4in and cylinder length is 3 1/4in.

Can anyone suggest a solution?

JasonB05/11/2018 11:31:07
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What mill are you using? What type of indexable holder is it a lathe tool or one made for a boring head?

I suspect the main issue is the length of the tool sticking out the bottom of the head, try an insert made for aluminium as these tend to be sharper and therefore don't get pushed off and chatter.

Other option is to put it on the lathe cross slide and use a sturdy between ctrs boring bar.

Edited By JasonB on 05/11/2018 11:31:30

Paul Kemp05/11/2018 11:38:02
289 forum posts
9 photos

Martin,

I would use an HSS tool, nice sharp point with perhaps a very small stoned radius and go slower. It's all very well sticking to theoretical cutting speeds but they assume ideal conditions in terms of tool geometry and machine rigidity. So sometimes you have to deviate from the recommended by quite a bit. With HSS I would try about 100 rpm, I would say your 500 even with a carbide insert is a bit fast. With an HSS tool you have the opportunity to also play with the geometry by grinding on more top rake, clearance or even adjust the angle of attack!

I have just finished doing the boiler saddle radius on a 6" scale traction engine cylinder (6 1/8" radius, cast iron cylinder, machined on a horizontal borer). Took me three attempts for the finishing cut to get the finish I wanted, final adjustment was increasing the top rake on the tool bit. Don't be afraid to experiment, cast iron can be a very variable material even within the same casting!

Paul.

Martin Green 305/11/2018 12:18:21
5 forum posts

Thanks for your thoughts. The mill is a Chester mill and seems to perform all the other functions accurately. The indexable tool is in an adjustable boring head and is about 6in long when inserted in the head. I have an insert for cutting aluminium and will try that.

From the comments above it would seem that very light finishing cuts at slow speed with a very sharp tool is the solution.

Would you agree?

Fowlers Fury05/11/2018 12:22:12
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Would endorse Jason B's recommendation "....... put it on the lathe cross slide and use a sturdy between ctrs boring bar." Adequately secure the casting on the cross slide after squaring it against the face plate. The boring bar designed by Geo Thomas is ideal as it is comparatively easy to adjust and measure the tool tip swing. Make it as big a diameter as you can. (Can point you to the source if required)
boring bar.jpg

However, as Paul K comments " cast iron can be a very variable material............." I've just finished boring out CI cylinders to similar dimensions as yours, sold by one of our suppliers and the CI has machined much less cleanly than previous castings I've purchased. Surface finish even after much honing is relatively "rough".

006.jpg

Martin Green 305/11/2018 13:00:45
5 forum posts

I am coming to the same conclusion as all of you. Although the boring bar is 12mm in diameter it can flex easily when stressed. The lateral movement can be substantial.

However I will try a light cut using my aluminium cutting tool and if that works then fine. I suspect I will have to go to the boring bar solution.

Fowler, I think you should have an apostrophe in you name...!!! Thanks for the photo they are very informative.

Can you point me to Geo Thomas's boring bar design?

Gas_mantle.05/11/2018 13:23:52
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I sometimes have similar trouble with those round shank brazed carbide bars but I find by rotating the bar in the socket to adjust the cutting angle I can get a decent finish. It takes a bit of trial and error with a few test cuts but after a few goes I can usually find the optimum.

JasonB05/11/2018 13:46:57
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If you can pic up a microbore insert second hand they make setting the tool a doddle, got this one for about £7

Even with the 35mm dia bar I got a bit of chatter due to the length needed for the 310mm cylinder but changing from the CCNT shown to a CCGT had it cutting the 42mm bore sweetly

Fowlers Fury05/11/2018 15:03:18
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323 forum posts
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"Can you point me to Geo Thomas's boring bar design?"
Sure - it's available from 2 sources:-
1. GHT's original article in Model Engineer. But I'm afraid the 3 xls index files to M.E. don't provide me with the reference despite trying several keywords. Maybe someone else can provide it?
2. That most valuable compendium of GHT's articles - "The Model Engineers Workshop Manual (vol 1)" 1992. Pub: Tee Publishing, ISBN 1-85761-000-8. It's on page 92.

Copyright of course, but herewith GHT's introductory paragraph. His comment about the normal, 90 deg tool resulting in flexure of a boring bar was well made IMHO. By putting the tool in the bar at an angle, that effect is very much reduced as well as resulting in much easier ability to measure the swing with a micrometer.

PM me if I can help further. wink 2

ght art ext.jpg

Edit: Have since found the original ME article.
Model Engineer, 3rd June 1977 p. 615 (Vol 143, No. 3562)

 

Edited By Fowlers Fury on 05/11/2018 15:24:06

Martin Green 305/11/2018 15:44:07
5 forum posts

I have just been in my workshop trying out the aluminium cutter in my boring head. The finish is substantially better than using the standard indexable cutter. However it is not really smooth.

I note from Fowlers Fury says

"However, as Paul K comments " cast iron can be a very variable material............." I've just finished boring out CI cylinders to similar dimensions as yours, sold by one of our suppliers and the CI has machined much less cleanly than previous castings I've purchased. Surface finish even after much honing is relatively "rough"."

The milled surfaces of the castings although smooth when finished with emery are not as good as the wheel casting finishes. They look as if there are slight pits in them. My wheels look lovely. I would add that the castings for the wheels as over 30 years old and the cylinder casting are less than 6 months old.

My problem is I do not know if I can expect a better finish.

I will hone the cylinders but has anyone any suggestions as to the finish I should be able to achieve?

Andrew Johnston05/11/2018 16:21:10
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4724 forum posts
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Posted by Martin Green 3 on 05/11/2018 15:44:07:

I will hone the cylinders but has anyone any suggestions as to the finish I should be able to achieve?

I've just measured the surface roughness of the bore of one of my traction engine rear hubs. It comes out at 5µm Ra. On the face of it that isn't great, but the bore feels pretty smooth using a Mk1 finger. Comparison of the bore with a Rubert scale using the fingernail suggests more like 2µm Ra. Although it's the rear hub I'd be happy using the bore as is for a steam engine cylinder.

The hub is 4" wide and was bored in the lathe using a 16mm boring bar sticking about a bit more than 4". I used an ordinary insert, not an alumimium specific one. I can't remember the feedrate but it may well have been greater than the normal 4 thou per rev that I use as standard. Particularly with shallow finishing cuts a boring bar can chatter with too fine a feed rate.

Andrew

John MC05/11/2018 16:29:39
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If the OP cannot get a good enough finish (for piston rings?) perhaps having it honed to the final size by his local engine refurbishers? A typical honing allowance for a small automotive cylinder would be -0.002/0.003".

John

Bazyle05/11/2018 16:51:01
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How much of the 6 in extension is the bar and is it including the head ie is it tip to bearings?

It is quite a large diameter for only a half inch bar, even 4 in long. If your head has multiple bar holes in the 'top' of the moving part, lots have 3, you can put more bars in those holes and make a joiner so they back up the working one.
After setting the depth of cut for a pass do you lock the slide?

Think about the geometry of the cut. Is it such that there is a radial force and can you reduce that by changing the insert angle? The advantage of HSS is that you can grind to ensure almost zero radial force though you to want a tiny bit to keep everything under a little pressure.

A 'sharp point' was mentioned above. As with outside turning you want a tip width > rate of feed or you just get a very fine screw thread.

Fowlers Fury05/11/2018 16:58:03
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"any suggestions as to the finish I should be able to achieve? "
Though no metallurgist, I've assumed that a poor surface on CI is due to poor foundry work where the carbon has not been effectively distributed through the melt for whatever reason. With one of those aforementioned (above) poor castings, I've cut off several extraneous bits with a fine saw and the surfaces all showed the same pitting even after a fine diamond file.
You don't state Martin what material you'll use for the rings but whichever, it's not going to ever generate a mirror-like bore. I don't believe the surface finish, when bedded down by the rings, is going to represent any problem. In fact, the small pits could be a +ve advantage in retaining oil.
As for the age diffeence in your 2 sets of castings, I was given some 60 y.o. Stuart Turner castings a while ago and they machined to a superb finish. I've read iron castings should be left outside to rust before machining but can't understand how that would change the deeper structure.
"Nuff said" in the words of Curly ?.

SillyOldDuffer05/11/2018 17:18:27
4538 forum posts
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Posted by Martin Green 3 on 05/11/2018 15:44:07:

... and the CI has machined much less cleanly than previous castings I've purchased. Surface finish even after much honing is relatively "rough"."

The milled surfaces of the castings although smooth when finished with emery are not as good as the wheel casting finishes. They look as if there are slight pits in them. My wheels look lovely. I would add that the castings for the wheels as over 30 years old and the cylinder casting are less than 6 months old.

My problem is I do not know if I can expect a better finish.

I will hone the cylinders but has anyone any suggestions as to the finish I should be able to achieve?

I think you need to experiment. Can you polish the outside of the cylinder to see how good a finish can be got without having to worry that inside boring and honing that you can't see are going wrong?

I suspect the problem is the metal, and only you can find out what you've got.

As a material cast-iron varies wildly. Quite a lot of cast-iron is rough muck, and it always has been, whatever grandad told you! The sort of iron used to make cheap non-critical items like street furniture, ornaments and weights can be whatever the foundryman throws in the melt. At the other end of the scale cast iron is an expensive and carefully controlled engineering material. But the 'quality' alloys are race-tuned for a particular purpose and not all of them take a good finish or machine well. Whatever the grade, even in a well-regulated foundry, castings can go wrong resulting in hard skin, inclusions, porosity, or blow-holes.

Apart from the black mess my experience with cast-iron has been good with two exceptions. A weight was truly horrible to turn, and, although the blank I used to make a chuck backing plate cut well, it has a coarse grain that won't take a polish. Luckily for me it doesn't matter! But if you can't get a good finish on the outside of the cylinder, you're wasting your time on the inside.

Dave

Martin Green 305/11/2018 18:18:51
5 forum posts

Everyone has given me really good ideas I will pause for a while on this post and experiment. From the support I have been given I know where to come when I want more help.

Thanks again.

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