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Straight Cut Knurling Problem and Plans For A Bigger Lathe (Colchester Master 2500)

Beauvais

Plastic
Joined
May 15, 2022
Just end of last year I bought my first lathe, a chester db11vs. After the 2nd day of trying it out I knew something was up as the chatter was terrible. I found some videos and a forum post talking about this type of (chinese) lathes cross slide/top slide design being at fault. I didnt know how I was going to solve the problem at first apart from sending some designs for someone to make me, after many months I knew this wasnt going to happen, so I bought a Centec 2a and a Colchester Bantam cross/top slide assembly, along with all the various end mills and drill bits to go about modifying the slide to fit. I had to cut a 60deg dovetail angle in the cross slide and make new gibs, I went up to an m5x0.8 and drilled/milled out the cross slide to suit the slightly bigger bolts, reason being I snapped 2 bsw taps trying to make the gibs, In hindsight I could of drilled the starting hole bigger perhaps but its done now. It was a quite difficult on the mill due to the lack of travel, I made some fixture plates and made the cut in 2 ops, I made sure the 'miss' was that the final smaller cut would taper away so the dovetails mostly had a good mating, It was extremely close in the end so worked out great.

Just thought Id do a write up as some context and it might help someone if they are also having problems with their lathe of similar design, I can now make cuts with a square insert from 0.5-2mm doc (2.5mm triggers the breaker in the unit...) with a perfect finish in en19t, so the colchester parts have done the trick!

But heres the problem, it has a lack of torque, lack of bed length and lack of head rigidity as it will deflect the part and taper say 0.04mm even with it only sticking out the chuck 3 1/2", thats with both scmt and vbmt type inserts, although bigger scmt cuts will do it more which is understandable. With a live centre its almost none although It could still be 0.01 or so, its hard to tell with the calipers. I can with a lot of care, make 8mm round cuts which is something I need to be able to do reliably, the lathe wants to stall as I have to run the rpm quite low to lessen the spindle humming, It has stalled a couple times but I have better feel for it now.

With all this mind and the not so dear price of an older bigger lathe, Im thinking of just going for it and buying a Colchester Master 2500, the reason being over other lathes is the 2500 rpm, the current lathe goes up to 2500 also and with the 1" and below parts, the carbide inserts I like calls for those speeds.

So my question is, why shouldnt I buy one? It seemingly ticks all the right boxes, long bed, high rpm, 3ph 5hp motor, a lot heavier than the current lathe. I know there could be things that could do with some tidying up being an old lathe but I dont mind having to do that as long as it will give me many years of turning. I appreciate any advice given and thank everyone that have contributed on this forum already as the info has really helped me to get to where I am with machining.

Here are some pics of the setup. Briefly with the knurling, no matter what I do, it will not cut deep at all, It seems to form and not cut. The tool height I know is perfect, the angle is 2deg, I use coolant, it just seems to deflect when I try to plunge regardless of support or not. Ive tried changing the angle, the rpm, feed rate, same result everytime..
This is also why I want a bigger lathe as I feel the head or spindle bearings arent rigid enough, its a Quick 1.2mm wheel with a chinese tool, I dont know how it could be the tool being a 20mm shank, but someone might know.

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technocrat

Cast Iron
Joined
Feb 9, 2009
Location
Oz
The quick change tool post is not helping. The tool will be hanging out on a big lever without much rigidity in the rest of the lathe. You can send it to me if you want.
 

technocrat

Cast Iron
Joined
Feb 9, 2009
Location
Oz
Also consider dumping the top slide on light lathes unless you need it for a specific operation. But overall the Colchester will have the metal you need for rigidity.
 
Joined
Apr 19, 2006
Location
Manchester, England
“ Colchester’s “ are good little lathes, especially the modern “ Student”. I worked on one of the slightly older ones with the square headstock and it would take 3/16” a side off BDMS without much trouble as long as you kept the revs down and didn’t get too ambitious with the feed rate.

That was with 3/8” HSS toolbits ground by hand. I think people are a bit too keen to use carbide inserts because they don’t know how to grind a lathe tool. I did a test once on a big lathe with an ammeter. Positive ground brazed carbide tool against a negative carbide insert tool. Same depth of cut, feed rate, revs etc. The negative rake tool was pulling twice the amount of amps the brazed carbide tool was.

Regards Tyrone.
 

Beauvais

Plastic
Joined
May 15, 2022
Thanks for the advice guys.

Re the top slide, as its from a Colchester Bantam and the great surface finish I get, I didnt think it was any of the problem, but I havnt tested it for deflection yet so will hook up an indicator at the end of the tool perhaps and take a cut to see.
 

Beauvais

Plastic
Joined
May 15, 2022
The quick change tool post is not helping. The tool will be hanging out on a big lever without much rigidity in the rest of the lathe. You can send it to me if you want.

I wanted to buy more tool holders and was thinking of buying the shorter length 75mm rather than the 90mm that I have to bring the tool closer to the centre. But I didnt think it was much a problem, Im using 20mm shank tools as well. For example, the smaller of the 2 parts I posted was cut using an 0.8mm radius vbmt insert with 0.9mm doc, zero chatter.
I will give the top slide a test when Im next at the unit
 

Beauvais

Plastic
Joined
May 15, 2022
Your cut knurl does not seem to be cutting.
Am i reading this wrong?

smt<----has several cut knurls and always looking at tips for using them

It doesnt seem to be cutting no. With a 1mm straight knurl with considerable feed I was able to get a more complete knurl, but its still not how it should. Picture was a 1.2mm
 

NewGunPlumber

Aluminum
Joined
Dec 18, 2019
I have a Colchester master 2500 and am very happy with it. Admittedly I'm not an expert like many here but I'm doing a bit more than your average home shop guy.

The only really complaint I'd have with it is parts can be very expensive so make sure it comes will all the accessories and is in good condition or budget accordingly.

I wouldn't exactly agree with some of the statements above. If its worn out then I'd probably rather have a new Chinese machine then a industrial machine that you can't hold tollerences with.

I've found that a lot carbide can still run well at sensible speeds. I'm rarely in the top range and do most of my general turning in the mid range which is about 430 - 770 from memory. Once your in top range you need to have your work and chuck balanced well. Depending on your inserts I'm finding depth of cut and feed rate has more impact on preventing birds nests and getting good finish with carbide than running at high RPM.
 

technocrat

Cast Iron
Joined
Feb 9, 2009
Location
Oz
I wanted to buy more tool holders and was thinking of buying the shorter length 75mm rather than the 90mm that I have to bring the tool closer to the centre. But I didnt think it was much a problem, Im using 20mm shank tools as well. For example, the smaller of the 2 parts I posted was cut using an 0.8mm radius vbmt insert with 0.9mm doc, zero chatter.
I will give the top slide a test when Im next at the unit
It's not so much the shank size etc, but the entire flex from tool edge, through the toolholder, top slide, cross slide and carriage down to bed flex. When testing with an indicator, work from the chuck or headstock, not any part of the carriage assembly. That is what the work will see. Carriage lift is a likely source of deflection.
On a constant cut, flex can get you into the zone, as well as throwing you out, depending on tool height. A good test for rigidity is deep power feed parting, that will quickly show you how much flex you have.
 
It doesnt seem to be cutting no.

I was under the impression the cut knurls would be easy to cut

Again, i check posts like this because i'm still learning and have not had a reason to practice much with the different pairs of Hardinge cut knurls found in the bottom of an auction box lot.

However, my impression from print and online images is that a cut knurl actually does make chips. It does cut, when working properly.
For my few experiments, i still need to work on set up/geometry, but i think my knurls are dull and need sharpened.

smt
 

Beauvais

Plastic
Joined
May 15, 2022
It's not so much the shank size etc, but the entire flex from tool edge, through the toolholder, top slide, cross slide and carriage down to bed flex. When testing with an indicator, work from the chuck or headstock, not any part of the carriage assembly. That is what the work will see. Carriage lift is a likely source of deflection.
On a constant cut, flex can get you into the zone, as well as throwing you out, depending on tool height. A good test for rigidity is deep power feed parting, that will quickly show you how much flex you have.

I emailed accu trak and they said they same thing about carriage lift likely being a problem.. Im going to test the lathe this evening.
 

DDoug

Diamond
Joined
Oct 18, 2005
Location
NW Pa
I emailed accu trak and they said they same thing about carriage lift likely being a problem.. Im going to test the lathe this evening.
Not just lathe flexing, but a scissor type would also fix the problem of "part flexing".
Your part in the pix is O.K. (short & stout, supported with tailstock as well) but you might do a job with a skinnier/longer part that would flex (and cause problems) that even the stoutest lathe cannot fix.
 
Not just lathe flexing, but a scissor type would also fix the problem of "part flexing".
Your part in the pix is O.K. (short & stout, supported with tailstock as well) but you might do a job with a skinnier/longer part that would flex (and cause problems) that even the stoutest lathe cannot fix.

All standard practice/cure for compression (or "bump") knurling.
Should not be necessary for cut knurling.
No one is addressing that point.

Or is the OP's tool not a cut knurl, as listed?

smt
 

DDoug

Diamond
Joined
Oct 18, 2005
Location
NW Pa
All standard practice/cure for compression (or "bump") knurling.
Should not be necessary for cut knurling.
No one is addressing that point.

Or is the OP's tool not a cut knurl, as listed?

smt
It would help anyways, by reducing the tool pressure,
so the OP could use both bump or cut.
 








 
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