Nalon Viper 2.5cc CI Engine

[xpylonracer]

That's what model making is all about IMO, makes a change to make some tooling and gives great satisfaction when it works as intended. As said good quality tooling is expensive and the cheaper tooling may not be accurate or of a quality to do all the cuts needed.

 

[Tim Wescott]

I got curious and looked. I didn't find any either -- you could maybe take a 1" diameter, thicker cutter and grind it thinner, but the shank might get in the way, and you'd need a toolpost grinder or a really dab hand at a bench grinder. Or, there seems to be a good selection of 1" diameter slitting saws. I didn't see any 3/32" saws on MSC Direct, but you could either stack a 1/16" with a 1/32" (I don't know how much of a Machinist's Sin that is), or you could take two cuts with a 1/16" saw for a 3/32" wide slot.

 

[pat_pending]

I got curious and looked. I didn't find any either -- you could maybe take a 1" diameter, thicker cutter and grind it thinner, but the shank might get in the way, and you'd need a toolpost grinder or a really dab hand at a bench grinder. Or, there seems to be a good selection of 1" diameter slitting saws. I didn't see any 3/32" saws on MSC Direct, but you could either stack a 1/16" with a 1/32" (I don't know how much of a Machinist's Sin that is), or you could take two cuts with a 1/16" saw for a 3/32" wide slot.

Yeah I really *did* have a good look about. My other thought was that the width of the cutter is critical here with respect to position of the top of the port but probably not the diameter. I was thinking you could probably get away with a slightly smaller diameter and adjust the port width through the depth of the plunge cut. I'd be reluctant to go with a larger diameter cutter as the edge of the port might get too thin/glow red when running/cause a raft of other issues.

It was at this point that I rolled up my sleeves and started trying to make my own cutters. I have some HSS round bar coming soon also so will give the boring bar/fly cutting technique a try.

 

[pat_pending]

OK, I'm back in business. The headstock bearings have been done and, my word, what a difference that made. It's like a completely different machine. I'm really looking forward to getting going on those cylinder liners on Monday.

I posted the nitty-gritty of the bearing replacement on a separate thread if anyone is interested it's here.

https://www.homemodelenginemachinist.com/threads/sieg-sc6-headstock-bearing-replacement.33159/
So in the end I'm measuring 0.007mm runout measured in the spindle taper and a tailstock alignment of 0.003mm over 60cm. Probably the best I'm going to get on that machine. I reckon I'm good to go.

P

 

[Billitmotors]

Gday Pat
if you imagine a very short HSS parting tool with an increased front clearance so that the heel of the cutter does not rub, and minimum side clearance on both sides. I will find my cutter on the weekend and take a photo and post it here. When cutting the ports I I fed it in slowly and gently by hand and it worked just fine.

G’day Pat here are some photo’s of the tool that I made to cut the exhaust ports on my Naylon Vipers as well as one of the one of the spare cylinders I made

 

[Billitmotors]

G’day Pat here are some photo’s of the tool that I made to cut the exhaust ports on my Naylon Vipers as well as one of the one of the spare cylinders I made

 

[xpylonracer]

That's much simpler to make than a multi-tooth cutter and as long as it does the job a very good alternative.

 

[pat_pending]

Thanks Billit. I'm going to give that a try too. Cheers for the photos.

P

 

[pat_pending]

Perhaps jumping the gun a bit here and at the risk of opening up an old debate, could someone recommend a prop for running in these Vipers? I used a 9x5 on the Boll Aero which it turned 6.5K RPM and now understand was probably a bit too much 'work' for it. I've seen people talk about 8x4 on a bunch of the forum threads only I cant seem to find props of that size anywhere in the U.K (other than funny-shaped plastic ones for drones/quadcopters). Anyone in the U.K with a source would be greatly appreciated.

Thanks,

P

 

[sniffipn]

Perhaps jumping the gun a bit here and at the risk of opening up an old debate, could someone recommend a prop for running in these Vipers? I used a 9x5 on the Boll Aero which it turned 6.5K RPM and now understand was probably a bit too much 'work' for it. I've seen people talk about 8x4 on a bunch of the forum threads only I cant seem to find props of that size anywhere in the U.K (other than funny-shaped plastic ones for drones/quadcopters). Anyone in the U.K with a source would be greatly appreciated.

Thanks,

P

8 x 4 IC by APC

 

[pat_pending]

8 x 4 IC by APC

Thanks. Have ordered a couple. P

 

[pat_pending]

Afternoon all. I managed to stick my head in the shed a bit this morning and put my mind to those pesky cylinder liners. Rather than ruin 4 blanks I thought I'd take a pause after the first to see if I've made any obvious gotchas. I'm quite happy with the result but the inside of the liner is full of burs as expected. I'll need to get some quality time with the needle file and magnifying glass to remove the worst before lapping.

The drilling operation was a bit iffy. The undersized slot drill (2.5mm) seated nicely after starting v, v gently but following with the specified #33 jobber drill didn't feel right until the last mm of the plunge or so. I will have a look whether such a thing as a #33 slot drill exists as this would allow me to do the operation in one and possibly lave a nicer hole.

I also made a new cutter this morning with 'backed-off' teeth. This seemed to cut OK-ish but I still have a few problems preventing t from cutting cleanly which I will try to remedy before making liner #2. I have some ideas.

Anyway here's how it went.

The new cutter before hardening.

Turn down the bottom of the liner and check fit with the crank case. Bore out as smooth as possible to save time with the lapping later. I took a lot of care to get the dimensions spot-on here as i would be using this as the reference surface for the ports.

Expanding mandrel #4 ... getting a lot quicker at making em. I realised I should have made a box of M6 and M5 tapered screws in batch when I last had the lathe set up to use in the future.

Mount on mandrel and finish the outside and final length of the liner.

4 blanks ready for action.

Scribe the line for the start of the transfer port drill hole.

Mount in rotary table and cut the exhaust ports.

Set the head of the mill over at 25deg and drill the transfer port hole. I located on the line with the needle point attachment from my wiggler set

 

[Ramon]

Pat - looking at your image of the cutter- 'draw a line' from the gullet of the tooth bottom left hand side to the gullet of the tooth above. I would think you need to remove the land out side of that line on each tooth (both sides) as I would guess that's possibly rubbing at depth.

As said before - a sacrificial, tight fitting, aluminium plug inside the bore will help minimise 'break through' burring inside the bore - just tap the plug out after machining.

Looking good - certainly sold on the idea of the scribed line set up.

BTW following a discussion on a control line forum be aware that the Kavan flexible yellow nylon 8x6 props have been known to shed a blade and particularly so on diesels. Not happened to me but I pass that on in the interests of safety - not only to your self but to the engine too!

You'll be fine with the APC though but do lightly sand the rough edges of the blades before use - particularly the TE as they are as sharp and hard as a razor otherwise. Believe me, skin always loses

Tug

 

[pat_pending]

Pat - looking at your image of the cutter- 'draw a line' from the gullet of the tooth bottom left hand side to the gullet of the tooth above. I would think you need to remove the land out side of that line on each tooth (both sides) as I would guess that's possibly rubbing at depth.

As said before - a sacrificial, tight fitting, aluminium plug inside the bore will help minimise 'break through' burring inside the bore - just tap the plug out after machining.

Looking good - certainly sold on the idea of the scribed line set up.

BTW following a discussion on a control line forum be aware that the Kavan flexible yellow nylon 8x6 props have been known to shed a blade and particularly so on diesels. Not happened to me but I pass that on in the interests of safety - not only to your self but to the engine too!

You'll be fine with the APC though but do lightly sand the rough edges of the blades before use - particularly the TE as they are as sharp and hard as a razor otherwise. Believe me, skin always loses

Tug

Thanks Tug. To make sure I understand correctly, i should continue the filing right to the bottom of the gulley shown in the red line here and bring it right up to the edge (blue line) removing that land?

 

[pat_pending]

Hi my 8mm HSS round blank arrived yesterday, I will have a go at the approach you suggested too. I'm a bit rubbish at shaping HSS tools and don't even have a grinding rest yet so this might be a bit of fun

Thanks,
P

 

[Ramon]

'Morning Pat,

The teeth only need enough clearance to achieve the depth of cut desired. Also bear in mind it's only the very tip of the teeth that define the cut dimensions so anything behind is potentially rubbing. This will be even more important with your single point cutter if you go that way. It's always best for finish and dimension to ensure the cutter behind the cutting edge has clearance. About the only commercial cutter that features straight sides and no clearance that I can think of is a woodruff cutter which has a ground flat surface either side. Unfortunately these can't be used because of their large (relative to cutter dia) shank diameter and very short minor shank length

Hope that helps - Tug

 

[pat_pending]

Pat - looking at your image of the cutter- 'draw a line' from the gullet of the tooth bottom left hand side to the gullet of the tooth above. I would think you need to remove the land out side of that line on each tooth (both sides) as I would guess that's possibly rubbing at depth.

As said before - a sacrificial, tight fitting, aluminium plug inside the bore will help minimise 'break through' burring inside the bore - just tap the plug out after machining.

Looking good - certainly sold on the idea of the scribed line set up.

BTW following a discussion on a control line forum be aware that the Kavan flexible yellow nylon 8x6 props have been known to shed a blade and particularly so on diesels. Not happened to me but I pass that on in the interests of safety - not only to your self but to the engine too!

You'll be fine with the APC though but do lightly sand the rough edges of the blades before use - particularly the TE as they are as sharp and hard as a razor otherwise. Believe me, skin always loses

Tug

Thanks on the advice RE props. Incidentally, i went down a Youtube 'rabbit-hole' the other day looking at control line speed flying championships. Fascinating stuff. Turns out that the fastest flyers only have one wing and one propeller blade and go 200+ MPH! I suspect a specially designed setup vs a blade firing off unexpectedly at 13,000 RPM are very different propositions .

P

 

[Ramon]

Pat - You would be right but don't dismiss that info lightly. Have a look here Barton Model Flying Club :: View topic - Nylon Props - post 6 is from someone I have known for many years - a seasoned, very knowlegeable and highly experienced aeromodeller. The potential is there with these specific props - not necessarilly will but might - if it does then personal safety and that of the engine/model is of concern. I tend to work on the basis that if you don't use them the problem doesn't exist

BTW If you really want to see an engine working (maybe that should be hear!) at it's best try You Tube - 'tethered model car championships'

 

[xpylonracer]

Same problem with Topflite white nylon props many years ago, advice slip with each box was to keep the props in damp atmosphere because the problem arises when the nylon dries out, whether that advice still stands today I don't know.

Marcus

 

[pat_pending]

OK, 4th and final attempt on the cutter today. I think the result is quite decent (by my dodgy surface finish standards anyway) and the cut seems much less violent on the part. I reckon tomorrow I will make some tight-fitting plugs as suggested by Tug, touch-up the cutter with a stone and go for the 3 other liners . I will try the boring bar method once I have sorted out a grinding rest but have the HSS bar and boring head on standby ready to go.

So this time round I remade the arbor to solve the ridiculous runout issue I had before. I made a register on the arbor thats a tight fit with the cutter hole rather than relying on the bolt for anything other than pulling force. I had another go at 'backing-off' the teeth and think the results are far better. Might give the teeth one more lick with the diamond file tomorrow before I crack on.

Anyway, here are a couple of pics of the new cutter for those insomnia sufferers out there

New arbor with a register for the hole in the cutter

I took a tiny skim to make sure all teeth were the same length taking out any runout vs the hole (only reduced the diameter by 0.05mm or so)

Backing off the teeth. I went as close as I could without touching the tooth behind. Engineers blue (uh-herm, blue marker) was useful to see when you get to the edge.

Profile view. Perhaps I went a bit extreme with the backing-off!

Hardened and sharpened. I will do a bit of a touch up on the sharpening tomorrow before I attach those liners as that one tooth looks a bit blunt!

It seems to cut OK and with a lot less effort than before and there are less 'lines' on either the bottom of the cut or the sides.

Again, nice little rolls of swarf and no little crumbs. .