Reference Charts
Interactive, filterable tables for tap drills, drill sizes, ISO 2768 and ISO 286 tolerances, and surface roughness — bookmark and use on the shop floor or in CAD review.
Precision Manufacturing Platform
Resources& Engineering Charts
Free tolerance references, tap drill charts, DFM guides, material catalogs, and interactive calculators — built to help you design better parts and quote CNC machining faster.
Written and reviewed by PREMSA's CNC engineering and production team in Monterrey.
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Everything You Need to Know
Whether you are selecting a material, checking a tap drill, defining ISO 2768 or ISO 286 tolerances, or optimizing a part for CNC — this library covers the references engineers and buyers use most.
Guides & References
In-depth guidance on CNC design, tolerances, holes and threads, materials, and inspection — aligned with how PREMSA reviews RFQs.
Engineering Calculators
Free online calculators for tap drill sizing and ISO 286 fit clearance—thread holes, mating limits, and assembly clearance.
Material & Finish Catalogs
Browse 60+ metals and plastics plus post-machining finishes — with the same filters used in PREMSA's online quote flow.
Articles & Help
Manufacturing articles, quoting guides, and answers to common CNC, material, and lead-time questions.
Featured guides
Most Popular Resources
The references engineers and buyers open most often before submitting a CNC quote.
Tap Drill Size Calculator
Calculate hole diameter for M6, 1/4-20, NPT, and other threads. Compare cutting vs forming taps and clearance drills.
Use CalculatorISO 2768 Tolerance Chart
Look up linear, angular, and geometric tolerances for f, m, c, v and H, K, L classes by nominal size.
View ChartCNC Design Guide
DFM principles, wall thickness, corner radii, hole depth, thread design, and inspection best practices for machined parts.
Read GuideSurface Roughness Chart
Convert Ra, Rz, RMS, and ISO N-grades with typical values for milling, turning, grinding, and laser cutting.
View ChartISO 286 Fit & Tolerances Chart
IT grades and preferred hole-basis / shaft-basis clearance, transition, and interference fits per ISO 286.
View ChartFit & Clearance Calculator
Calculate H7/g6 limits, min/max assembly clearance, and fit type for any ISO 286 hole–shaft pair.
Use CalculatorQuick reference
Key Machining Specs at a Glance
The numbers engineers ask about most when reviewing a CNC RFQ with PREMSA.
ISO 2768
General Tolerances
mK is a common default for undimensioned features. Use our chart for f, m, c, v and H/K/L classes.
H7/g6
ISO 286 Fit
Precision sliding fit on mating bores and shafts. See the ISO 286 chart for IT grades and preferred combinations.
Ra 3.2
Standard Finish
Typical as-machined Ra in µm. Tighter finishes available for bearing surfaces and sealing faces.
60+
Materials
Aluminum, steel, stainless, titanium, copper alloys, and engineering plastics in the catalog.
Live Tooling
Turning Capability
Cross-drilled holes, flats, and slots on CNC lathes in one setup — rotational parts without a second chucking.
Monterrey
Production Base
CNC milling, turning, and secondary ops with local supply chain in Nuevo León.
1 → Production
Order Quantity
Prototype through repeat production. Quote from a single part or pilot lot online.
DFM
Engineering Review
RFQs include manufacturability feedback on tolerances, materials, and thread callouts.
EN / ES
Bilingual Tools
Charts, calculators, and this library available in English and Spanish.
Tap & drill guide
How to Use the Tap & Drill Size Chart
A step-by-step guide for engineers and machinists
A tap drill chart lists the recommended hole diameter before threading to achieve the target thread engagement — typically 75% with cutting taps in ductile materials. The right drill prevents undersized holes (tap breakage) and oversized holes (weak threads). The chart covers metric, UNC, UNF, and NPT threads in one filterable place.
Step 1 — Read the thread callout on the drawing
Start with the exact specification: metric (M6 × 1.0, M8 × 1.25), inch UNC (1/4-20, 5/16-18), UNF (1/4-28, 5/16-24), or NPT for fittings. Note thread class if shown (2B/3B), blind vs through depth, and whether the process requires a cutting or forming tap. Cutting taps evacuate chips and need a smaller hole; forming taps displace material and need a larger hole.
Step 2 — Confirm tap type and thread percentage
Many drawings assume a cutting tap at ~75% engagement — enough for typical fasteners without overloading the tool. Lower percentages mean a slightly larger hole and easier tapping; higher percentages increase torque. If the print does not specify percentage, align with your CNC vendor during DFM review before releasing toolpaths.
Step 3 — Look up the tap drill in the chart
Open the Tap & Drill Size Chart and filter by thread family. Use the tap drill column — not the clearance column. Common references: 1/4-20 UNC is often near a #7 drill; M6 × 1.0 metric is near 5.0–5.1 mm. NPT threads follow pipe tables, not the straight-thread formula.
Step 4 — Match to a stocked drill
Charts return a calculated diameter; shops stock number, letter, fractional, and metric drills. Round to the nearest available drill that meets engagement, usually rounding up slightly for easier tapping. Cross-check against the Drill Bit Size Chart so programming and inspection use the same label.
Do not mix tapped holes with clearance holes: clearance lets the bolt pass without cutting threads in the part; tap drill sizes the hole before tapping or thread milling. If the screw passes through, use clearance values; if the part needs internal threads or an insert, use tap drill values and call out tap type on the drawing.
For non-standard threads, forming taps, or comparing 65% vs 75%, use the Tap Drill Size Calculator. It returns diameters in mm and inches plus the nearest standard drill.
Diameter is not the only variable: blind holes need drill depth below the thread floor; in stainless or titanium a slightly larger drill can reduce tap breakage. Before quoting, align thread notes across model, PDF, and BOM — inconsistencies cause DFM holds. Pair this guide with the CNC Design Guide and ISO 2768 chart, then request a quote with STEP, quantity, and material.
The correct tap drill matters more than most teams expect. Too small risks tap breakage; too large reduces clamp load. Bookmark the interactive chart for UNC, UNF, metric, and clearance reference in one place.
From the blog
Latest Manufacturing Articles
In-depth articles on CNC machining, materials, quoting, and production planning.
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CNC resources FAQ
CNC Machining Resources — Frequently Asked Questions
Answers to the questions engineers, buyers, and programmers ask most about PREMSA's free tap drill charts, ISO 2768 and ISO 286 tolerances, surface finish conversion, CNC design guide, material catalogs, and online quoting workflow.
- What free CNC machining resources does PREMSA provide?
- PREMSA publishes a full CNC engineering library at no cost: interactive tap drill and drill bit size charts, ISO 2768 and ISO 286 tolerance charts, a surface roughness conversion chart, a tap drill size calculator, the CNC Design Guide, 60+ material and finish catalogs, and a manufacturing blog. No account, login, or paywall — bookmark and use them for quoting, CAM programming, and DFM review.
- What is a tap drill size chart and when should I use it?
- A tap drill chart lists the hole diameter you drill before running a tap — for metric (M6, M8), UNC (1/4-20), UNF (1/4-28), and NPT pipe threads. Use it when a drawing calls out threaded holes and you need the correct drill for ~75% thread engagement with a cutting tap. PREMSA's Tap & Drill Size Chart is filterable; for non-standard threads or forming taps, use the Tap Drill Calculator.
- Where can I look up ISO 2768 general tolerances for CNC parts?
- Open PREMSA's ISO 2768 Tolerance Chart to find linear, angular, and geometric tolerances by nominal size for classes f, m, c, v and H, K, L. ISO 2768-mK is a common default on machined drawings when features are not individually dimensioned. Match the class on your print before releasing a quote or CAM program.
- Where can I look up ISO 286 hole–shaft fits like H7/g6?
- Open PREMSA's ISO 286 Fit & Tolerances Chart for IT grade tables and preferred hole-basis / shaft-basis combinations—H7/g6 (sliding), H7/h6 (locational clearance), H7/p6 (press). For instant limits and clearance, use the Fit & Clearance Calculator. Pair ISO 286 on mating bores and shafts with ISO 2768 on general dimensions.
- How do I convert surface roughness between Ra, Rz, RMS, and ISO N-grades?
- Use the Surface Roughness Chart to convert Ra (µm and µin), Rz, RMS, and ISO N-grades — with typical values for milling, turning, grinding, EDM, and laser cutting. Ra 3.2 µm is a common as-machined default; sealing faces and bearing journals often need tighter callouts. Specify finish on the drawing so quoting and inspection align.
- What is the PREMSA CNC Design Guide used for?
- The CNC Design Guide covers DFM for machined parts: wall thickness, internal corner radii, hole depth ratios, thread design, tolerance strategy, material selection, and inspection notes. Engineering and purchasing teams use it to reduce rework, tap breakage, and quote delays before uploading STEP files to PREMSA's online quote flow.
- Which resources should I check before submitting a CNC quote?
- A practical order: (1) CNC Design Guide for manufacturability, (2) ISO 2768 chart if general tolerances apply, (3) ISO 286 fit chart for mating bores and shafts, (4) tap drill chart or calculator for threaded features, (5) materials catalog and finishes catalog to match the online quote selectors. Then request a quote with STEP, quantity, and drawing.
- What is the difference between the tap drill chart and the tap drill calculator?
- The chart is a browsable reference table for standard thread callouts and clearance drills — ideal on the shop floor. The calculator computes hole diameter for a specific thread, tap type (cutting vs forming), and thread percentage, then maps to the nearest stocked drill from the Drill Bit Size Chart. Use both when validating drawings and toolpaths.
- How do I choose a CNC material from the PREMSA materials catalog?
- Browse the Materials Catalog by alloy family — 6061-T6 and 7075-T6 aluminum, 304/316 stainless, 1018/4140 steel, Ti-6Al-4V, PEEK, Delrin, and more. Each page lists mechanical properties, machinability notes, and typical applications. Pick the same material in the online quote form so RFQ data, DFM review, and production planning stay aligned.
- What post-machining finishes are listed in the finishes catalog?
- The Finishes Catalog documents options such as as-machined, bead blast, anodize (Type II/III), passivation, black oxide, powder coat, tumbled, and other cosmetic or protective treatments. Finishes affect cost, lead time, and inspection — select the finish in the quote flow when it is required on the drawing.
- Are PREMSA engineering charts and calculators available in English and Spanish?
- Yes. Charts, calculators, guides, catalogs, and this resources hub are published in English and Spanish with localized URLs (e.g. `/resources` and `/recursos`). Teams in Mexico, the United States, and Canada can share the same technical references without switching tools.
- Does PREMSA review drawings for DFM when I request a CNC quote?
- Yes. When you submit a quote with STEP and drawing, PREMSA's engineering team reviews manufacturability: unreachable features, tight tolerances, thread callouts, material/finish feasibility, and setup risk. Use the resources on this page to resolve common issues before upload — it shortens quoting and reduces revision cycles.
- Where does PREMSA manufacture the CNC parts referenced in these resources?
- PREMSA operates CNC milling, turning, and secondary operations from Monterrey, Nuevo León, Mexico, with supply-chain reach across North America. The charts and guides reflect how our shop programs holes, threads, tolerances, and finishes in production — not generic textbook values.
- How do I find a specific drill size or thread callout quickly?
- Use the site search or open the relevant chart directly: tap drill for thread holes, drill bit sizes for number/letter/fractional/metric drills, ISO 2768 for general tolerances, ISO 286 for hole–shaft fits, and surface finish for Ra/Rz conversion. Bookmark this hub as your single entry point.
PREMSA publishes a full CNC engineering library at no cost: interactive tap drill and drill bit size charts, ISO 2768 and ISO 286 tolerance charts, a surface roughness conversion chart, a tap drill size calculator, the CNC Design Guide, 60+ material and finish catalogs, and a manufacturing blog. No account, login, or paywall — bookmark and use them for quoting, CAM programming, and DFM review.
A tap drill chart lists the hole diameter you drill before running a tap — for metric (M6, M8), UNC (1/4-20), UNF (1/4-28), and NPT pipe threads. Use it when a drawing calls out threaded holes and you need the correct drill for ~75% thread engagement with a cutting tap. PREMSA's Tap & Drill Size Chart is filterable; for non-standard threads or forming taps, use the Tap Drill Calculator.
Open PREMSA's ISO 2768 Tolerance Chart to find linear, angular, and geometric tolerances by nominal size for classes f, m, c, v and H, K, L. ISO 2768-mK is a common default on machined drawings when features are not individually dimensioned. Match the class on your print before releasing a quote or CAM program.
Open PREMSA's ISO 286 Fit & Tolerances Chart for IT grade tables and preferred hole-basis / shaft-basis combinations—H7/g6 (sliding), H7/h6 (locational clearance), H7/p6 (press). For instant limits and clearance, use the Fit & Clearance Calculator. Pair ISO 286 on mating bores and shafts with ISO 2768 on general dimensions.
Use the Surface Roughness Chart to convert Ra (µm and µin), Rz, RMS, and ISO N-grades — with typical values for milling, turning, grinding, EDM, and laser cutting. Ra 3.2 µm is a common as-machined default; sealing faces and bearing journals often need tighter callouts. Specify finish on the drawing so quoting and inspection align.
The CNC Design Guide covers DFM for machined parts: wall thickness, internal corner radii, hole depth ratios, thread design, tolerance strategy, material selection, and inspection notes. Engineering and purchasing teams use it to reduce rework, tap breakage, and quote delays before uploading STEP files to PREMSA's online quote flow.
A practical order: (1) CNC Design Guide for manufacturability, (2) ISO 2768 chart if general tolerances apply, (3) ISO 286 fit chart for mating bores and shafts, (4) tap drill chart or calculator for threaded features, (5) materials catalog and finishes catalog to match the online quote selectors. Then request a quote with STEP, quantity, and drawing.
The chart is a browsable reference table for standard thread callouts and clearance drills — ideal on the shop floor. The calculator computes hole diameter for a specific thread, tap type (cutting vs forming), and thread percentage, then maps to the nearest stocked drill from the Drill Bit Size Chart. Use both when validating drawings and toolpaths.
Browse the Materials Catalog by alloy family — 6061-T6 and 7075-T6 aluminum, 304/316 stainless, 1018/4140 steel, Ti-6Al-4V, PEEK, Delrin, and more. Each page lists mechanical properties, machinability notes, and typical applications. Pick the same material in the online quote form so RFQ data, DFM review, and production planning stay aligned.
The Finishes Catalog documents options such as as-machined, bead blast, anodize (Type II/III), passivation, black oxide, powder coat, tumbled, and other cosmetic or protective treatments. Finishes affect cost, lead time, and inspection — select the finish in the quote flow when it is required on the drawing.
Yes. Charts, calculators, guides, catalogs, and this resources hub are published in English and Spanish with localized URLs (e.g. `/resources` and `/recursos`). Teams in Mexico, the United States, and Canada can share the same technical references without switching tools.
Yes. When you submit a quote with STEP and drawing, PREMSA's engineering team reviews manufacturability: unreachable features, tight tolerances, thread callouts, material/finish feasibility, and setup risk. Use the resources on this page to resolve common issues before upload — it shortens quoting and reduces revision cycles.
PREMSA operates CNC milling, turning, and secondary operations from Monterrey, Nuevo León, Mexico, with supply-chain reach across North America. The charts and guides reflect how our shop programs holes, threads, tolerances, and finishes in production — not generic textbook values.
Use the site search or open the relevant chart directly: tap drill for thread holes, drill bit sizes for number/letter/fractional/metric drills, ISO 2768 for general tolerances, ISO 286 for hole–shaft fits, and surface finish for Ra/Rz conversion. Bookmark this hub as your single entry point.
Ready for your next project with PREMSA?
Upload your STEP file, set material and quantity, and get DFM review with clear lead times and an online quote.